Tektronix - Biophysical Measurements (1970)

BiophysicalMeasurements

ECGl" ,

MeBsurement Concept Serias

BIOPHYSICALMEASUREMENTS

BVPETER STRONG

Illustrationsby

DOROTHY FREED

MEASUREMENT CONCEPT:

FIRST EDITION, FIRST PRINTING NOVEMBER 1970062-1247-00

© 1970, TEKTRONIX, INC.BEAVERTON, OREGON 97005ALL RIGHTS RESERVED

CONTENTS

CAUTION

SCOPE 1

SECTION 1 PHYSIOLOGY AND GENERATION OF BIOMEDICAL POTENTIALSWITHIN MAN 5

1 THE CELL AS A BIOELECTRIC GENERATOR 7

1.1 The Source of Internal Cell Potentials 71.2 Cell Stimulation and Stimulus Threshold 161.3 Current from a Single Cell and the Resultant Externa11y

Recorded Action Potential 161.4 Extemally Recorded Action Potential from a Group of

Cells, the Travelling Wave of Depolarization 18

2 THE HEART AND THE CIRCULATORY SYSTEM 23

2.1 The Cardiovascular Circulatory System 232.2 The Heart 262.3 Electrical Potentials Generated Within the Heart -

Generation of the Electrocardiograrn Wavefonn (ECG) 26

3 MUSCLE ACTION - AND THE SENSORY SYSTEM 313.1 The Motor Unit 313.2 Muscle Action 323.3 The Muscular Servo-mechanisrn 333.4 Reflex Response 343.5 The Potential Generated During Muscle Action 343.6 The Sense Receptors 363.7 The Potential Generated by Sense Receptor Stimulation 36

4 THE BRAIN AND THE CENTRAL NERVOUS SYSTEM 39

4.1 Nerve Cells in the Central Nervous System andin the Brain 39

4.2 The Brain 404.3 Excitation and Inhibition Potentials 424.4 Evoked Potentials 454.5 The Electroencephalograrn (EEG) 45

SECTION II MEASUREMENT TECHNIQUES 47

5 ELECTROCARDIOGRAPHY (ECG) 495.1 Electrocardiographie Planes 495.2 Frontal Plane ECG Measurements 505.3 Bipolar Limb Lead Frontal-plane ECG Measurements 525.4 Unipolar Limb Lead Frontal-plane ECG Measurements 545.5 Frontal-plane Axis Derivation 575.6 Frontal-plane Electrode Positioning 575.7 Transverse-plane ECG Measurements 605.8 Sagittal plane ECG Measurements 635.9 ECG Instrumentation Requirements 635.10 Interpretation of the Electrocardiogram 65

6 VECTORCARDIOGRAPHY (VCG) 676.1 The Spatial Vectorcardiogram 686.2 Electrode Placement 696.3 Frank Electrode System 706.4 Polarity Convention 736.5 Other Electrode Systems 746.6 The Normal Vectorcardiogram 746.7 Timing and Direction Reference 776.8 Instrumentation Requirements 77

7 FETAL ELECTROCARDIOGRAPHY 81

7.1 The Existence of the Fetal ECG 817.2 The Normal Fetal ECG 827.3 Subject Preparation 827.4 Electrode Placement 837.5 Electrical Interference 857.6 Recording Techniques 867.7 Interpr.etation of the Fetal ECG 89

8 BLOOD PRESSURE AND FLOW 93

8.1 Direct Blood Pressure Measurement 938.2 Indirect Blood Pressure Measurement 1018.3 Indirect Relative Blood Pressure Measurement 1048.4 Blood Flow Measurement 1088.5 Cardiac Output 1128.6 Blood Volume 113

9 RESPIRA TION AND TEMPERATURE 115

9.1 Physiological Considerations 1159.2 Respiratory Activity 1189.3 Respiratory Air Flow 1229.4 Respiratory Volume 1259.5 Temperature 129

10 ELECTROENCEPHALOGRAPHY 131

10.1 The Characteristics of the Normal EEG 13110.2 EEG Electrode Considerations 13410.3 EEG Recording Instruments 13610.4 EEG Recording Modes 13910.5 Unusual EEG Display Modes 14010.6 Intra-cranial Electrode Placement 14310.7 Applications of the EEG 14410.8 The Characteristics of the Abnormal EEG 14510.9 Intentional Modification of a Subject's EEG 147

Il EVOKED CORTICAL RESPONSES 149

11.1 Evoked Action Potential 149Il.2 Microelectrode Technique 150Il.3 Input Neutralized Amplification 157Il.4 Noise Reduction - Averaging 159Il.5 Typical Evoked Responses 16411.6 Stimulation 16611.7 Sterotaxic Instruments 166Il.8 The Electroretinograrn - ERG 169

12 STIMULATION - ELECTROMYOGRAPHY - NERVE CONDUCTION 171

12.1 Stimulation 17112.2 Stimuluslsolation 17512.3 Strength/Duration Curves 17612.4 Myography 18012.5 Electromyography - EMG 18212.6 Electromyography with Voluntary Muscular Action 18212.7 Electromyography during Electrical Stimulation 18512.8 The H Reflex 18712.9 Nerve Conduction 18912.10 Repetitive Stimulation 19212.11 Smooth Muscle Potentials 193

13 GALVANIC SKIN REFLEX - GSR 195

13.1 The Autonomie Nervous System 19513.2 GSR Measurement by Resistance Change 19713.3 GSR Measurement by Potential Detection 19813.4 Electrical Skin Resistance 199

14 ULTRASONOGRAPHY 20114.1 Ultrasonic Systems 20314.2 HA" Scan Ultrasonography 20414.3 "Time-motion" Mode Ultrasonography 20714.4 Ultrasonic Scanning 20914.5 Doppler Ultrasound 211

SECTION III INSTRUMENTATION 213

15 INSTRUMENTATION SUMMARY 215

15.1 Biomedical SignaIs 216

16 ELECTRODES 21916.1 Electrode Offset Potential 21916.2 Electrode Offset Potential Characteristics 22516.3 Other Electrode Characteristics 23216.4 Reusable Surface Electrodes 23316.5 Disposable Surface Electrodes 24016.6 Needle Electrodes 24016.7 Microelectrodes 24316.8 Electrode Application 24516.9 Stimulating Electrodes 24716.10 Comparison of Electrode Types 247

17 GROUNDING - SAFETY 249

17.1 Grounding 24917.2 Induced Ground Currents 25217.3 Electrostatically Induced Currents 25517.4 Electric Shock Current Thresholds 25517.5 Instrumentation Safety Considerations 25917.6 Electrical Service Grounding 26217.7 Operating Room Isolation 26417.8 Electrocautery and Defibrillation 265

18 TRANSDUCERS - TRANSDUCER SYSTEMS 269

18.1 Resistive Transducer Concepts 27018.2 The Unbalanced Wheatstone Bridge 27318.3 Practical Transducer Systems using the

Unbalanced Wheatstone Bridge 27618.4 AC- and De-bridge Systems 27818.5 Displacement Transducers 28318.6 Force, Pressure and Acceleration Transducers 28818.7 Transducers for Nonmechanical Quantities 291

19 AMPLIFIERS 29319.1 The Differential Amplifier used with Electrodes 29519.2 Common Mode Rejection 29519.3 ln put Resistance 30019.4 Input Guarding 30219.5 Input Current 30619.6 Dynamic Range, DC Offset and Recovery 30819.7 Noise and Drift 31019.8 Specialized Amplifiers 317

20 SIGNAL PROCESSORS - OPERATION AL AMPLIFIERS 319

20.1 Optimum Bandwid th for Physiological Signals 31920.2 Amplifier Noise Reduction by Bandwidth Limiting 32120.3 Amplifier Low Frequency Response Limiting 32520.4 Line Frequency Rejection 32520.5 Noise Reduction by Signal Averaging 32620.6 Operational Amplifiers 32620.7 Operational Amplifier Applications 33020.8 Integration and Differentiation with Operational

Amplifiers 334

21 OSCILLOSCOPES 337

21 .1 Oscilloscope Vertical Amplifiers 33821.2 Oscilloscope Horizontal Amplifiers/Sweep Generators 33921.3 Oscilloscope CRT Displays 34121.4 Typical Oscilloscopes 34221.5 Rate Intensification 34921.6 Slave Oscilloscopes 351

22 PULSE GENERATORS AND STIMULATORS 361

22.1 Stimulus Systems 36322.2 Tektronix 160 Series Pulse Generators 36422.3 High E or 1 Output via a Power Operational Amplifier 36722.4 High E or 1 Output via a Stimulus Isolation Unit 36922.5 Cardiac Pacemakers 37022.6 Cardiac Defibrillators 373

23 DISPLAY DEVICES AND INDICATORS 375

23.1 Tektronix Display Units 37723.2 Resolution 38323.3 Large Screen Expanded-sweep Displays 38523.4 Indicators 387

24 OSCILLOSCOPE CAMERAS 389

24.1 Conventional CRT Photography 39124.2 Photography of Curved Faceplate CRT's 39324.3 Continuous Motion Cameras 395

2S GRAPHIC RECORDERS 399

25.1 Basic Recorder Mechanisms 40325.2 Recording Formats 40525.3 Writing Principles 40725.4 Commercial Galvanometric Recorders 40925.5 Special Purpose Recorders 410

26 MAGNETIC TAPE RECORDERS 413

26.1 Direct Magnetic Tape Recording 41526.2 Indirect FM Magnetic Tape Recording 41726.3 Tape Transport Mechanism 41926.4 Other Magnetic Tape Recorder Considerations 420

27 DATA TRANSMISSION AND PROCESSING 423

27.1 Data Transmission via Shielded Cable 42327.2 Data Transmission via a Telemetry Link 42627.3 Time Division Multiplexing 42927.4 Data Processing with an Analog Computer 43027.5 Data Processing with a Digital Computer 43227.6 Data Processing Applications and Software 435

28 INTENSIVE CARE CONCEPTS 437

28.1 Physiological Functions to be Monitored duringIntensive Care 438

28.2 Intensive Care Instrumentation 440

SECTIONN APPENDIX 447

29 CUSTOM INSTRUMENTATION 449

29.1 Type 410 Modification for Fetal ECG Use 45029.2 A BNC Input Adapter for the Type 410 Monitor 45329.3 A Signal Output Adapter for the Type 410 Monitor 45329.4 A Resistive Transducer Adapter for the

Type 410 Monitor 45429.5 A Thermistor Pneumograph for the Type 410 Monitor 45729.6 An Input Neutralizing Adapter for the Type 3A8

Operational Amplifier 45829.7 An Absolute Value Adapter for the Type 3A8

Operational Amplifier 46029.8 A Low Speed Gating Adapter for the Type 3A8

Operational Amplifier 46329.9 A Resetting Step Generator Adapter for the Type 3A8

Operational Amplifier 46529.10 A Self-contained Resetting Stairstep Generator 46629.11 A Frank Network for Vectorcardiographic Use 46929.12 A Current Limiting Adapter for Protection from

Electric Shock 47029.13 A Lo-pass Filter for Physiological Signal Processing 47129.14 A Pulse Shaping Circuit Simulating the Action Potential 47329.15 A Constant Current Pulse Source for GSR and

Other Uses 474

30 DEFINITIONS 475

INDEX 489

REFERENCES TO TEKTRONIX PRODUCTS 499

ACKNOWLEDGMENTS

Professor Harold W. ShiptonBioengineering Resource FacilityUniversity of IowaIowa City, Iowa, U.S.A.

Dr. David J. DewhurstDepartment of PhysiologyUniversity of MelbourneMelbourne, Vic., Australia

The author gratefully acknowledges the assistance provided byDr. Dewhurst and Professor Shipton in the writing of this book.Professor Shipton's contributions are particularly evident inchapters four and ten dealing with the electroencephalogram.Dr. Dewhurst's extensive contributions, specially to the firstfour chapters on physiology, were invaluable.

CAUTION

Engineers and nonmedica11y qua1ified personnelshou1d not perform biophysica1 measurements on humansubjects un1ess such measurements are conductedunder professiona1 supervision. Many of themeasurement techniques described in this book, whileincorporating medica11y accepted procedures, are notentire1y free from risk. These risks may beminimized by fo11owing the precautions out1ined inChapter 17, "Grounding - Safety."

One or more "current 1imiting adapters forprotection from electric shock" as described inChapter 29, Section 29.12, should be used betweenhuman subjects and e1ectronic instrumentation toprotect the subject from e1ectric shock shou1d afailure occur within this instrumentation.

SCOPE

It is the intent of this publication to familiarizeengineering personnel with electronic measurementsassociated with the biophysical sciences. As such,a developed knowledge of electronics and electronicmeasuring techniques is assumed, but biological andphysiological aspects are presented from firstprinciples and simplified to a level consistentwith the understanding of the basic principlesinvolved.

This book will be found to differ somewhat fromother publications on "medica1 e1ectronics." Manyof these publications are authored by, and directedtoward, medica1 personnel. As such, engineeringpersonnel may find the e1ectronics aspects somewhatoversimplified and the physio10gical aspects takenfor granted. The opposite approach has been takenhere. The fol10wing books are suggested as generalreferences on Biophysica1 Measurements. Specificreferences are given, where appropriate, in thetext throughout this book.

Dewhurst, PhysicaZ Instrumentation In Medicineand BioZogy, London: Pergamon, 1966.

Dickinson, EZectrophysioZogicaZ Technique,London: E1ectronic Engineering, 1950.

Dona1dson, EZeatronic Apparatus For BiologicaZResearah, London: Butterworth, 1958.

Geddes and Baker, PrincipZes of AppliedBiomedicaZ Instrumentation, New York: Wi1ey &Sons, 1968.

Kay, Experimental BioZogy, London: Chapman andHall, 1964.

2

Stacey, Biological and Medical Electronics,New York: McGraw-Hill, 1960.

Suckling, Bioelectricity, New York: McGraw-Hill,1961.

Yanof, Biomedical Electronics, Phi1ade1phia:Davis, 1965.

Engineers interfacing with the medica1 professionare encouraged to 1earn as much as possible aboutmedica1 and hospital practice and in particularabout the physiology of the human body. It is onlyby gaining such an understanding that they cancommunicate inte11igently with members of themedical profession. One is often approached by adoctor with the request, "can you supply me withthis item of equipment?" The tactful but firmreply must be, "what do you want it to do?" Veryoften the requirement, once understood, can be metfar more effectively by some more modern developmentthan that embodied in the original request. lnthis way a rea1 service to the medica1 professionis provided and the engineer becomes a colleaguerather than a graduate technical assistant.

Information is separated into the followingcategories:

SECTION l -- Physiology and generation ofbioelectric potentials within man.

SECTION II -- Measurement techniques required toperform various biophysical measurements.

SECTION III -- Instrumentation required toimp1ement the measurement techniquescovered in Section II.

A reader wishing to study, for example,electrocardiographic techniques should thus firststudy Section l, Chapter 2, to determine the sourceof the bioelectric potential referred to as thee1ectrocardiogram; he should then study Section II,Chapter 5, to determine the measurement techniquesnecessary in recording this electrocardiogram and,finally, he should study Section III to determinethe instrumentation necessary to implement thesemeasurement techniques.

3

Wherever possible, the biophysical measurementscovered assume man as the subject. Although, inpractice, many of these measurements are performedon laboratory animaIs, such as rats, cats, dogs,and monkeys, the principles involved and the resultsobtained relate directly to man. No division isattempted between biophysical measurements performedin a clinical environment and biophysicalmeasurements performed in a research environmentas they are, from the electronic engineeringviewpoint, essentially similar. ln general, onlycommonly accepted and widely used measurementtechniques are covered in this publication as itwould be impossible to document the infinite varietyof unique, specialized, biophysical measurementsbeing performed by different clinicians andresearchers in the biophysical sciences. Thispublication should not be interpreted as a generaltext on medical electronics; it is, as its titleimplies, limited to measurement techniques.

5

SECTION 1

PHYSIOLOGY AND GENERATION OFBIOELECTRIC POTENTIALS WITHIN MAN

The following four chapters (1-4) cover the basicphysiology of the cell, the heart, the muscularsystem and the brain and the generation of electricalactivity within these physiological systems.

The intent of these four chapters is to familiarizeengineering personnel with the physiological aspectsassociated with the various bioelectric generatorswithin the body. ln general, the information ispresented in the simplest possible form. Thus,while satisfying the intent of this publication,the professional physiologist may find the subjectmaterial somewhat oversimplified.

Interested readers will find the following textsinvaluable for further study:

Brazier, The Bl.ecta-i cal: Activi ty of the NeruoueSystem, Baltimore: Williams and Wilkins, 1968.

Eccles, The Neupophysiological Basis of Mind,London: Oxford, 1953.

Eccles, The Physiology of Nepve Cells, New York:John Hopkins, 1957.

Hoffman and Cranefield, Electrophysiology of theHeart, New York: McGraw-Hill, 1960.

Katz, Nerve3 MUscle and Synapse, New York:McGraw-Hill, 1966.

7

THE CELL AS ABIOELECTRIC GENERATOR

The ce11 is the basic source of a11 bioe1ectricpotentia1s. A bioe1ectric potentia1 may be definedas the difference in potentia1 between the insideand the outside of a ce11; in other words, thedifference in potentia1 existing across the ce11 wallor membrane. A cel1 consists of an ionic conductorseparated from the outside environment by asemipermeab1e or se1ective1y permeable ce11 membrane.Many different types of ce11s comprise any onespecies of living matter. Human ce1ls may vary from1 micron to 100 microns in diameter, from 1mil1imeter to 1 meter in 1ength and have a typicalmembrane thickness of 100 Angstrom units. Onemicron is equa1 to 10-4 centimeters and one Angstromunit is equa1 to 10-8 centimeters. Bioe1ectricityis studied both from the viewpoint of the sourceof e1ectrical energy within the ce11 and also fromthe viewpoint of the 1aws of e1ectro1ytic currentflow relative to the remote ionic fields producedby the ce1l. ln e1ectrophysiology we may, in somecases, penetrate a cell to investigate its interna1potentia1. More commonly, however, we makemeasurements externa1 to a group of ce1ls whi1ethese ce11s are supplying e1ectro1ytic current f1ow.

1.1 1lffi SOURCEOF INTERNALCELL POTENTIALS

Experimental investigations with microe1ectrodeshave shown that the interna1 resting potentia1within a cell is approximately -90 millivolts withreference to the outside of the ce11. Thispotential changes to approximately +20 millivoltsfor a short period during ce1l activity. Cellactivity results from some form of stimulation asdescribed later in this chapter. The Hodgkin-Huxleytheory, initia11y postulated during the 1950's, isgenera11y considered to give the best exp1anationas to the source of these potentia1s and providesequations that give an emperica1 mathematica1 fit toexperimental data. This theory is brief1y describedas fo1lows.

8

chemicalgradient

Nernstrelation

The interior of a cell primarily containsconcentrations of sodium and potassium ions. Theseconcentrations within a cell differ markedly fromthe concentrations of these ions in the spaceoutside the cells (see Fig. 1-1). Elementary ionictheory states that, under suitable conditions, anyuneven distribution of ionic concentration in anaqueous solution will result in a potentialdifference between the regions of differentconcentration. If, for example, solutionscontaining unequal concentrations of ions areseparated by a membrane semipermeable to these ions,a potential will be found to exist (see Fig. 1-2).This potential, which is referred to later as thechemical gradient, is given by the Nernst relation:

Potential (rnV)=

61.6 Lo Concentrat~on, one side of membraneg Concentratlon, other side of membrane

Or, for uni-univalent ionic solutions, the Nernstrelation simplifies to:

Potential (mV)U - V= 61.6 U + V

where U = Mobility of the negative ions(anions) through the membrane

V = Mobility of the positive ions(cations) through the membrane

Referring to Fig. 1-2, for a 10:1 activity(concentration) ratio at 37°C, the relativemobilities of the chloride and sodium ions are 65.4and 43.6 respectively. Applying these values to theNernst relation gives:

Potential (mV) = 61 6 65.4 - 43.6. 65.4 + 43.6

= 12 mV

This can be confirmed with a voltmeter as shown inFig. 1-2. This potential will, of course, run downas diffusion proceeds, unlike that of a living cell.

9

VOLT METER(READS 12mV)

+

CONCENTRATEDSOLUT ION OF --+-I-/.._j

SODIUM CHLORIDEDILUTE SOLUTION

"'__'__- OF SODIUM CHLORIDE

•••.Na+ IONIC1MOVEMENT

---t •••.~ CI- 10NIC----I,...~MOVEMENTCI- MOBILITY THROUGH THE MEMBRANEIS GREATER THA Na+ MOBILITY.THUS CI- IONS ON LEFT OF MEMBRANEAND Na+ IONS ON RIGHT.

Fig. 1-1. Potential generated by ionie concentration difference betweentwo solutions.

RELATIVE INTERNALCONCENTRATIONS

SODIUM 1 : POTASSIUM 30

RELATIVE EXTERNALCONCENTRATIONS

SODIUM 10 : POTASSIUM

SODIUM IONS DIFFUSEINTO CELL

K+ POTASSIUM IONS DIFFUSEOUT OF CELL

SEMIPERMEABLE MEMBRANERESISTANCE TO 10NIC FLOW ISINVERSELY PROPORTIONAL TOPERMEABILITY OF THE MEMBRANETO THAT ION.

Fig. 1-2. Typical concentrations of sodium and potassium ionswi thin a cell.

10

INSIDEOF CELL

MICROELECTRODEINTERNAL TO

CELL +

VOLT METER(READS -90mV)

ELECTRODE EXTERNALTO CELL

LARGE ETSODIUM GRADIENTACROSS MEI~BRANE

SODIUM IONIC CURRENT

SMALL NETPOTAS S 1UM GRAD 1ENr.-~...;._,:A"Nwvs,....,-f--""ACROSS I~EMBRANE

POTASSIUM 10NIC CURRENT

(A) IONIC CURRENTS INVOLVED lN A POLARIZED CELl

INSIDEOF CELL

EXTERNAL

SODIUM 10NIC CURRENT PRODUCEDBY A LARGE NET Na+ GRADIENTAND LOW PERMEABILITY IS BALANCED BY A SMAlLER NET K+GRADIENT AND A HIGHERPERMEABllITY.

+

VOLT METER(READS +20mV)-.

LOW NETSOD 1UM GRAD 1ENT .•..-~:..;,.,II,j{J'Ii.I\N~~-ACROSS MEMBRANE

SODIUM IONIC CURRENTHIGH PERMEABILITY

LARGE NETPOTASS 1UM GRAD 1ENT-----j;,..;.;;.;;WVvil.Ar.~-__...

ACROSS '~EMBRANEPOTASSIUM IONIC CURRENT

MEDIUM PERMEABlll fY

(B> IONIC CURRENTS INVOLVED lN A DEPOLARIZED CELL

Fig. 1-3. Cell ionie currents.

SODIUM IONIC CURRENT PRODUCEDBY A SMALL NET Na+ GRADIENTAND HIGH PERMEABILITY IS BALANCED BY THE POTASSIUM IONICCURRENT PRODUCED BY A LARGENET K+ GRADIENT AND MEDIUMPERMEABILITY.

netgrad ient

cellconcentrations

ce Il inrestingstate

netgrad ient

Il

The ionic current produced by ion movement througha semipermeable membrane depends on the permeabilityof the membrane and also on the "gradient" thatforces the ion through the membrane. This gradient,referred to as the net gradient, consists of botha chemical gradient and an electrical gradient. Achemical gradient is formed due to a difference inconcentration producing a potential gradient as givenby the Nernst relation. An electrical gradient isformed as a result of a potential that may existacross the membrane due to some other source.

Experimental investigations have shown that a markeddifference in concentrations of both sodium andpotassium ions exists across a cell membrane. lnmammalian nerve cells as shown in Fig. 1-1, theconcentration of potassium ions is in the vicinityof 30 times higher inside the cell than in thefluid external to the celle On the other hand,sodium ions are approximately 10 times moreconcentrated in the fluid external to the cell thanin the fluid within the celle

Consider a cell in its resting or polarized state(Fig. l-3A). ln this state the membrane ismoderately permeable to potassium ions, that is,potassium ions can pass fairly readily through themembrane as the membrane offers medium resistance.This membrane is, however, almost impermeable tosodium ions and, thus, offers a high resistance tothe passage of these ions. A large net gradientaffects the movement of sodium ions into the celleThis net gradient consists of a chemical gradientproduced by the 10-to-1 concentration differencebetween sodium ions on each side of the membrane anda 90 mV electrical gradient produced by the standingpotential within the celle The net gradientaffecting the movement of potassium ions out of thecell is considerably less than the net sodiumgradient. This gradient consists of a largechemical gradient due to the 30-to-l concentrationdifference across the membrane; however, thischemical gradient is opposed by the electricalgradient produced by the 90 mV standing potentialwithin the celle Thus, although the membrane isalmost impermeable to sodium ions, the net sodiumgradient is high. Conversely, although the membraneis moderately permeable to potassium ions, the netpotassium gradient is low. The net result is that

12

-90 mVrestinglevelpolarizedcell

ce Il afterstimulus

depolarization

repolarization

the sodium and potassium currents are equal; thesodium current balances the potassium current witha resultant current of zero. Since the net currentthrough the membrane is zero, the cells internaIpotential will not change and will remain at its-90 mV resting level. Indeed, this -90 mV restinglevel is determined by the internaI cell potentialrequired for sodium and potassium current bdlance.

When the cell receives a stimulus from an outsidesource, the characteristics of the membrane at thepoint of stimulation will be markedly altered and,thus, the ionic currents will also change. Afterstimulation, the membrane permeability to potassiumions is unaltered but the permeability to sodium ionsis increased. A much lower resistance is offered tothe flow of sodium ions, thus increasing the sodiumionic current. This increased sodium ionic currentcauses more positive ions to pass into the cell thanare passing out of the cell, causing the internaIcell potential to drop from -90 mV in an attempt toachieve sodium current and potassium current balance.As this potential decreases, the net sodium gradientacross the membrane decreases and the net potassiumgradient across the membrane increases, causing thecurrents to decrease and increase, respectively.This process continues until current balance isagain obtained, at which time the internaI cellpotential is +20 mV. The cell is then referred toas being in a depolarized state.

By the time the cell has fully depolarized thecharacteristics of the membrane have begun to revertback to their prestimulus state. This causes thesodium ionic current to be considerably lower thanthe potassium ionic current; the internaI cellpotential thus begins to go negative with theprocess continuing until the -90 mV restingpotential of the cell is once again obtained.

cellelectricalanalogy

13

An electrical analogy to a cell membrane is shownin Fig. 1-4. This circuit cannot strictly bereferred to as an equivalent circuit as theelectronic current flow in an electrical circuitand the ionic current flow through a cell membranecannot be said to be equivalent. After assigningresistance values inversely proportional to therelative permeability of the membrane and assumingpotassium and sodium concentration ratios, then theintracellular potential for both a polarized celland a depolarized cell can be determined. Thevalues assumed are analogous to actual values foundin a celle

INSIDE OF CELLEc

CELLMEMBRANE

OUTSIDE OF CELL

E = INTRACELLULAR POTENTIAL WITH RESPECTC TO THE OUTSIDE OF THE CELL.

E = NERNST POTENTIAL DUE TO THE POTASSIUMK ION CONCENTRATION DIFFERENTIAL ACROSS

THE MEMBRANE

ENa = NERNST POTENTIAL DUE TO THE SODIUM IONCONCENTRATION DIFFENRENTIAL ACROSS THEMEMBRANE.

RK = RELATIVE PERMEABILITY OF THE MEMBRANETO THE FLOW OF POTASSIUM IONS THROUGH IT.

= RELATIVE PERMEABILITY OF THE MEMBRANE TOFLOW OF SODIUM IONS THROUGH IT WHEN THECELL IS POLARIZED.

Rd = RELATIVE PERMEABILITY OF THE MEMBRANE TOTHE FLOW OF SODIUM IONS THROUGH IT WHENTHE CELL IS DEPOLARIZING.

R a

C = CAPACITY OF THE CELL.

Fig. 1-4. An electrical an alogy to a cell membrane.

14

cell actionpotential

Assume:Relative values for RK, RNa and Rn are 1 kn,150 kn and 0.35 kn respectively.Potassium ion concentration ratio of 30:1 insideto outside.Sodium ion concentration ratio of 10:1 outside toinside.

Then:EK 30= 61.6 Log - = 91 mV

110= 61.6 LogT = 62 rnVin opposite polarity

-- by Nernst relation.

to EK -- Nernst.

For a polarized cell:net potassium current + net sodium current = 0

net potassium gradient + net sodium gradient0=RK RNa

potassium potassium sodium sodiumchemical + electrical chemical + electricalgradient gradient + gradient gradient = 0

RK RNa

EK + E -E____________________ c~ + NaRK

Ec = 0

91 x 10-3 +--------------~----~--+

1 x 103

-62 x 10-3 + Ec-------",...--~ = 0

150 x 103

E

Solving:E = 90 x 10-3 = -90 mV (polarized)c

Similarly, for a depolarized ce11, RNa is replacedby Rn.

E = +20 mV (depolarized)c

If a microelectrode were inserted into the cell asshown in Figs. l-3A and l-3B and a stimulus wereapplied to the cell, the output of themicroelectrode would appear as shown in Fig. 1-5.This waveform is known as the "cell actionpotential." It should be noted that the currentsinvolved in bioelectricity are unlike the currentsinvolved in electronics. Bioelectric currents are

sodiumpotassiumpump

15

due to positive and negative ion movement within aconductive fluide As these ions possess finite massand encounter resistance to movement within thefluid their speeds are limited. The cell actionpotential, thus, shows a finite "risetime" and"falltime."

The ionic concentration gradient across the cellmembrane is maintained by virtue of metabolicenergy expended by the cell in "pumping" ionsagainst the ionic gradient formed by the differingionic concentrations between the inside and outsideof the celle This action has been referred to asthe "sodium-potassium pump."

+50mV

~ DEPOLAR 1ZAT 1ON •••1... REPOLAR 1ZAT 1ON --...j~------------2ms--------------~

DEPOLARIZED CELLPOTENTIAL +20mV

REGENERATIVEBREAKDOWN

-50mV

THRESHOLD -60mV

POLARIZED CELLRESTING POTENTIAL -90mV

-100mV

STIMULUS ULTIMATELYDECREASES CELL POTENTIAL

TO THRESHOLD

CURRENT 1STIMULUS

1

1

1'"

MINIMUM WIDTH OF CURRENT STIMULUS REQUIRED FORACTION POTENTIAL GENERATION ASSUMING VALUE OF

STIMULATING CURRENT REMAINS UNCHANGED

Fig. 1-5. Cell action potential (internally recorded with microelectrode).

16

1.2 CELL STIMULATION AND STIMULUS THRESHOLD

stimulusthreshold

refractoryperiod

A cell may be stimulated, or caused to depolarizeand then repolarize, by subjecting the cellmembrane to an ionic current. This current may beproduced by other cells, it may be produced byionic currents existing as nerve impulses, or it maybe artifically produced by sorneexternal currentstimulus. A cell will be stimulated whensufficient positive ions are added to the inside ofthe cell to cause its resting potential to bedecreased from its -90 mV level to approximately-60 mV. Once this threshold level is reached, thecell depolarizes without requiring the addition ofany further positive ions to the inside of the cellfrom the stimulus source. Unless a stimulus abovea certain minimum value is received, known as thestimulus threshold, the cell will not be depolarizedand no action potential will be generated. Thestimulus required to exceed the threshold is afunction of both current and time; the thresholdmay be exceeded by a short, high-current pulse orby a longer, lower-current pulse (see Fig. 1-5).

Since the energy associated with the actionpotential is developed from metabolic processeswithin the cell itself and not from the stimulus, afinite period of time, known as the refractoryperiod, is required for metabolic processes withinthe cell to return the cell to its prestimulusstate. This refractory period has been observed inmost cells found in the nervous system. Closerstudy reveals that the refractory period has twoparts: The first in which no stimulus, howeverstrong, will cause depolarization (the absoluterefractory period) and the second whendepolarization occurs only if the stimulus is ofmore than normal threshold strength (the relativerefractory period).

1.3 CURRENT FROM A SINGLE CELL AND THE RESULTANT EXTERNALLYRECORDED ACTION POTENTIAL

We have previously discussed the depolarizing andrepolarizing action of cells and the resultingpotential existing within the cell. As statedearlier, this potential may be recorded with amicroelectrode; however, in most bioelectric

externalelectrodes

externalactionpotential

depolarizationcurrent

17

measurements, this potential is recorded byelectrodes external to the actual cell. Theseexternal electrodes typically would record the netaction of many hundreds of cells, but for the timebeing, we will consider only a single cell. Whenrecording with external electrodes, an actionpotential is produced between these electrodesduring periods of current flow; that is to say, nopotential exists when cells are either in theirdepolarized or repolarized state. A potentialexists only while the cell is changing from onestate to another. As the external action potentialis generated by the external current that flowsduring cell activity, the shape of the actionpotential is related to the variation of this currentwith time.

The external potential field rises to its maximumvalue sometime during the regenerative breakdownphase of the membrane. The external actionpotential that is recorded from the cell is somewhatsimilar to a mathematical time derivative of thetransmembrane potential. This potential is detectedwith maximum amplitude when one electrode is placedas near as possible to the active area and the otherelectrode is located in a completely inactive orremote area. It is detected with reduced amplitudeas the electrodes are placed closer to each otherso they intercept smaller elements of potentialdifference.

Consider a single polarized cell; the inside of thecell is negative with respect to the outsideenvironment which may be regarded as a reference.As stated previously, the net ionic current flowacross the membrane is zero; thus, ionic currentflow to and from the cell is zero. Should theseconditions be altered due to the presence of astimulating current through the cell membrane, thenregenerative membrane breakdown will occur and thecell will depolarize. During the depolarizationprocess the net current through the membrane is notzero; there is a net positive ionic current into thecell through the cell membrane. This current may bedetected as a potential difference between twoelectrodes placed in the vicinity of the cell withthe potential difference being produced across thefinite resistance of the fluids external to the cell.

18

repol arizationcurrent

This current continues until the cell is fullydepolarized, thus, the potential will appear betweenthe two electrodes while the cell is undergoingdepolarization. For the short time that the cellexists in a depolarized state, the net currentthrough the membrane is once again zero, thus, nopotential will appear between the electrodes.

Almost immediately after depolarization the cellbegins to repolarize again. During repolarization,positive ionic current flows from the cell membrane,that is, in the opposite direction to ionic currentflow during depolarization. During this period ofcurrent flow, a potential will be detected betweenthe electrodes and, since this current is in theopposite direction to the depolarization current,the potential produced between the electrodesduring repolarization will be of the oppositepolarity to the potential produced duringdepolarization. The area included by the .depolarization and the repolarization potentialwaveforms is the same since the quantity of currentinvolved in each process is the same. It should benoted that the process described above is somewhattheoretical as it describes the external actionpotential generated by a single cell. ln thefollowing discussion we show that single celldepolarization invariably results in depolarizationof the adjacent ceIls and, hence, this externallyrecorded action potential will be the net summationof the results obtained from these cells.

1.4 EXTERNALLY RECORDED ACTION POTENTIAL FROM A GROUP OF CELLS,THE TRAVELLING WAVE OF DEPOLARIZATION

synchronousdepolarization

The preceding discussion must be modified to allowfor many cells in close proximity to one anotherand to allow for the appreciable length of many ofthese cells. Consider a group of cells in closeproximity to one another as shown in Fig. 1-6 and1-7. Under certain conditions of stimulation thesecells may aIl depolarize at the same time(synchronous depolarization); however, therepolarization process is random. Repolarization ofthe individual cells will occur at different times.The resultant externally recorded action potentialis shown in Fig. 1-6. Once again, the area includedunder each wave is the same since the quantity ofcurrent involved in each process is the same.

19

STIMULATION OF ALL CELLSWITHIN A GROUP OF CELLS

STIMULUS

1 1~

\7 +

21 /\en V_J<C-t-

Az

31UJt-0 VD....zenO_J- _Jt-UJ

41 Il00<C -o V'_J<CUJ=:>Iot-

>l.L

51 /\-00z V-

61 r-:V

SUMMATION OFINDIVIDUAL ACTIONPOTENTIALS 1 THROUGH 6

A AREA A = AREA B

l-c ~I" ~IDEPOLARIZATION REPOLARIZATION

Fig. 1-6. External action potential produced by a group of cells alldepolarizing as a result of one stimulation (synchronousdepolarization).

20

STIMULATIO~ OF ONE CELLWITHI~ ~ GROUP OF CELLS

1 1/\._

\.T +!

l/) 2~_Jce SUMMATION OF 1-,_ /\_ INDIVIDUAL ACTIONz

3/w POTENTIALS 1 THROUGH 6>-0 VQ_

Z lnO_J- _J

41 Il T II~E>- wuuce

V_Jcew;:JI0>--

si 1\> •...-0

DEPOLARIZATION REPOLARIZATION0z V-

61(\_-V

Fig. ]-7. Externa) action potential produced by a group of cens depolarizingdue to stimulation by adjacent cells (asynchronous depolarization).

r>:\\,\~

DEPOLARIZEDAREA TRAVELSDOW~ CELL

10NIC

lJEPOLARIZEDEND OF CELL

ELECTRODE A IS +WITH RESPECT TOREMOTE ELECfRODE

TIME '" to

POTENTIAL OF AWITH RESPECT TO

REMOTE ELECTRODE

A

DEPOLARIZED \,\ a\AREA TRAVELS \FURTHER DaWN ~\CELL

ELECTRODE A IS 0WITH RESPECT TaREI~OTE ELECTRODE

T IME tl

ELECTRODE A IS -WITH RESPECT TOREMOTE ELECTRODE

TIME = t2

+

Fig. 1-8. "Traveling wave of depolarization" resulting from depolarizationof successive section of a long cell.

asynchronousdepolarization

travellingwave withincells

21

Under other conditions of stimulation, the group ofcells described previously will not aIl depolarizeat the same time (asynchronous depolarization).The stimulation may result in one cell depolarizing;the action of this cell depolarizing will then actas a stimulus on its adjacent cell causing it todepolarize also. This chain reaction would proceeduntil aIl cells in a particular area havedepolarized. The resultant externally recordedaction potential would appear as shown in Fig. 1-7.ln practice, combinations of synchronous andasynchronous depolarization occur in a group ofcells.

ln the same way as the depolarization of one cellcauses adjacent cells to depolarize, depolarizationof a localized area of an individual cell willcause depolarization of other parts of the samecelle Thus, the depolarization process will appearto travel along the length of the cell causing atravelling wave of depolarization as shown inFig. 1-8.

As most bioelectric potentials are recorded asexternal cell action potentials, the resultsobtained are a summation of the action of many cells.The action potential waveform may be modified by thenumber of cells, the shape of these cells, and thetype of stimulation applied.

myocardialinfarction

23

THE HEART AND THECIRCULATORY SYSTEM

Coronary heart disease is the greatest single causeof death in most developed countries. ln the UnitedStates alone, over a million persons die every yearfrom heart disease; the most frequent cause ofdeath is the acute myocardial infarction. Thus, amajor percentage of medical effort, both in theresearch and clinical fields, is directed towardunderstanding of cardiovascular system and thesubsequent prevention of heart disease.

2.1 THE CARDIOVASCULARCIRCULATORY SYSTFM

cellrev ita 1ization

ln a complex multicellular animal with a highdensity of cell population and a relatively smallvolume of tissue fluid bathing these cells, acontinuous and rapid revitalization process musttake place. This revitalization is the fundamentalpurpose of the cardiovascular circulatory system.The previous chapter discussed the potentialproduced within a single celle This potentialexists as a result of continuous metabolism withinthe celle This metabolism process requiresnutrients and excretes waste products; thecirculatory system provides these nutrients andremoves these waste products.

The major component of the circulatory system is theheart which supplies the power required to circulateblood throughout the body. The heart is two pumpsin series; the smaller, right-hand section providesthe power required to force blood through the lungs,and the larger, more powerful left-hand sectionprovides the power required to force blood throughthe body. A simplified block diagram is shown inFig. 2-1.

24

RIGHT HAND SIDEOF HEART

LEFT HAND SIDEOF HEART

RESISTANCE TOBLOOD FLOW

FLOW5 LITERS/M 1N

PUMP A PUMP

r LUNGS

PREJUREPRESSURE

! 20mm Hg 7mm Hg

.AA

~HIGH PRESSURE100mm Hg

LOW PRESSURE-",3mm Hg

DIRECTIONOF FLOW

CAPILLARIES

Fig. 2-1. Simplified block diagram of the circulatory system.

VE OUS

LEFT LUNGOXYGEN DIFFUSES1NTO BLOOD ANDCO2 REMOVED

RIGHT LUNG

ARTERIAL BLOODOXYGENATED. "RED"CONTAINS HEMOGLOBIN

CO TAINS CO2"BLUE" ~=--i+-- CELLULAR AND TISSUE

METABOLISM REMOVESOXYGEN A D NUTRIENTSAND ADOS WASTE PRODUCTS

Fig. 2-2. The cardiovascular circulatory system.

blood flow

25

E1aborating on Fig. 2-1 and referring to Fig. 2-2,the blood flows from the heart into the aorta. Theaorta curves in an arch up from the heart, downalong the back bone and into the abdomen; from itother large arteries lead to the head, the digestiveorgans, the arms and the legs. From these arteriesbranch the smaller arterioles (or small arteries)and from these, branch billions of tinycapillaries. By the time the blood has reached thecapi11aries it is moving slowly along channels thatare only about 10 microns in diameter. Here theblood discharges its load of dissolved food andoxygen to the body cells. These cells in turndeposit waste materials such as carbon dioxide intothe blood stream. ln yielding oxygen and taking onwaste, the b100d turns color from bright red to du11red or "blue." The blood now sta'I"tsback to theheart passing from the capillaries into the venules.The venules converge into larger veins and theninto the two largest veins just above and below theheart known as the vena cava. The blood emptiesinto the right atrium, is pumped into the rightventricle and then moves out through the pulmonaryartery to the lungs. The lungs supply the bloodwith fresh oxygene The blood passes from the lungsto the left atrium, then is pumped into the leftventricle and passes out via the aorta to begin thecirculation process again. This general flowthroughout the body is known as the "systemiccirculation;" the flow to and from the lungs isknown as the "pulmonary circulation." Localcirculations within the systemic system include therenal (to the kidneys), the hepatic portal (to theliver), the cerebral (to the brain) and thecoronary (to the heart itself). The waste productscontained in the blood are removed by the kidneysand liver (see Fig. 2-2). The average quantity ofblood in man is about five liters and is completelycirculated through the body in approximately oneminute.

26

2.2 THE HEART

cardiaccyclediastole

systole

The heart itself weighs less than a pound, is aboutsix inches long at its maximum dimension and liespointed downward in the chest cavity to the leftof the mid-center body line. The walls of the heartare made entirely of muscle; within these walls arefour hollow chambers, a left and a right receivingchamber (atrium) and below them a left and a rightpumping chamber (ventricle).

The cardiac cycle is characterized by the followingmechanical events. Between beats, the heartmechanically rests and this is known as the periodof diastole. During diastole the heart assumes itsmaximum size and fills with oxygenated bloodreturning from the lungs and venous blood returningfrom the body. The heart's period of mechanicalactivity is known as systole. The onset of systoleis initiated by contraction of the musclessurrounding the atria which propels additional bloodinto the ventricles. The ventricles then begin tocontract, thereby causing a rise in pressure withinthe ventricles. This increased pressure shuts thetwo atrioventricular valves and, with furthercontraction, the pressure continues to rise. Oncethe pressure of the systemic and pulmonarycirculations are exceeded, a phase of ventricularejection is begun. The aortic valve is forced openand blood is squeezed into the aorta and thence intothe systemic circulation. Likewise, the pulmonaryvalve is forced open and blood is supplied to thepulmonary circulation. After the ventricularcontents are partially ejected, the musclessurrounding the ventricles relax and theventricular pressures fall. As soon as thesepressures fall below the pressures sustained in thecirculatory systems, the aortic and pulmonaryvalves close, signalling the onset of diastole.

2.3 ELECTRlCAL POTENTIALS GENERATED WITHIN THE HEART -GENERATION OF THE ELECTROCARDIOGRAM WAVEFORM (ECG)

The preceding section covers the mechanical activityof the heart and states that mechanical activity isinitiated by contraction of the muscle surroundingthe atria. The detailed relationship between cell,nerve and muscle producing this contraction iscovered in Chapter 3. It is sufficient at thisstage to state that muscle contraction is initiatedby stimulation.

27

MUSCLES FOR_k--....-- VENTRICULAR

CO"lTRACTION

LEFT ATRIOVENTRICULAR(MITRAL) VALVE

SINOATRIAL NODE (SA)BUNOLE OF HISPURKI JE SYSTEM RIGHT ATRIOVENTRICULAR (TRICUSPIO) VALVE

LArEDIASTOLE

Fig.2-3. The heart.

VENTRICULAREJECT ION

---

ATRIALSYSTOLE

ISOMETRIC VENTRICULARCONTRACTION

\\111

ISOMETRIC VENTRICULARRELAXATION

Fig.2-4. Mechanical cycle of the heart.

28

MUSCLEACTION

DIASTOLE "RELAXATION"SYSTOLE "CONTRACTION"

ATRIALSYSTOLE

VENTRICULARSYSTOLE

VENTRICULARDIASTOLE

120nYllHg - - - - - - - - - -

ARTERIALPRESSURE

100mm Hg-

1'" •• 1ATRIALDIASTOLE

ARTERIALD IASTOLICPRESSURE

80mm Hg - -

100 90 80 70 60 50BEATS/min

ECG

t---------0.5mV ~i__20mm/mV

5Omm/s

R

P WAVE - ATRIAL DEPOLARIZATION

TA WAVE - ATRIAL REPOLARIZATION MAY OCCUR

PR INTERVAL - ATRIO-VENTRICULARCONDUCT10N TIME 0.12-0.225

ORS SEGMENT - VENTRICULAR DEPOLARIZATION

T WAVE - VENTRICULAR REPOLARIZATION

ST SEGMENT - ISO ELECTRIC

OT INTERVAL -

o S1 1

-.j f-+- O. 15 MAX

Fig. 2-5. The ECGwavefonn and related heart action.

U WAVEUNKNOWN ORIGIN

sinoartrialnode

atrioventricularnode

muscleactionsequence

29

The right atrium contains a bundle of nerves knownas the sinoatrial node (abbreviated SA node). Thistype of nerve cell is found nowhere else in the body.Its function is to start the heart beat and set itsrhythm or pace. The electrical and mechanicaloutput from the heart is initiated by stimulationfrom this node which results in contraction of thevarious heart muscles. Although the basic rhythm ofthe heart is self-sustaining through synchronizationfrom the SA node within the heart, this rhythm ismodified by certain nerve fibers external to theheart that affect the SA node. These nerves have afunction in the normal control of the heart rate torespond to increased or decreased demand for bloodby the body.

Impulses generated by the SA node stimulatecontraction of the muscles comprising the atria.These impulses also travel along conducting fibersin the atrium to the atrioventricular node or AVnode, stimulating depolarization of this node(Fig. 2-3). Stimulation of the atrioventricularnode causes impulses to be sent to the myocardium ormuscle comprising the ventricules via the bundle ofHis and the Purkinje conducting system resulting incontraction of this muscle. Thus, the muscularcontractions necessary to maintain the heart'spumping action are initiated by depolarization andrepolarization of the SA node and thendepolarization and subsequent repolarization of theAV node.

These depolarizations and repolarizations generateexternal action potentials which can be recorded atthe surface of the body as covered in detail inChapter 1. These external action potentialsgenerated from within the heart are known as theelectrocardiogram or ECG. It is common, also, torefer to this waveform as the EKG, derived from theGerman spelling of electrocardiogram. The ECGwaveform (Fig. 2-5) is shown in relationship to themechanical action of the heart and the resultantarterial pressure.

30

heartelectrica 1•activityECG

optimumelectrodepositions

Electrical activity of the heart is, as statedearlier, initiated by depolarization of the SA nodeand a resulting contraction of the musclessurrounding the atria. The resulting external actionpotential is known as the P wave. Immediate1yfollowing this depolarization, repolarization of theatria occurs; however, for,so~e reason, this doesnot generate a pronounced action potential. Thispotentia1 is known as the TA wave and is rare1yobserved in practice. Electrical activity producedby depo1arization of the SA node travels throughfibers within the atrium to the AV node. The timetaken for this e1ectrical stimulation to trave1from the SA node to the AV node is known as theatrioventricular conduction time and is typica11ybetween 120 ms and 220 ms. When this stimulationreaches the AV node, this node depolarizes and thedepolarization is conducted down through the bundleof His to the myocardium muscle causing ventriculardepolarization. The external action potential isreferred to as the QRS complexe Immediate1yfo11owing this depolarization the ce11s concernedrepolarize which resu1ts in ventricu1arrepolarization or the T wave. Many ECG waveformsalso show an additional wave occurring after the Twave. This is designated the U wave and its originis unknown.

The ECG waveform is recorded at maximum potentia1when one e1ectrode is p1aced slight1y above theheart and to the right and the other electrode isplaced slight1y be10w the heart and to the left;thus, the potentia1 output of the heart can be saidto be generated along this axis. Chapter 5discusses variations in the ECG waveform withdifferent electrode positions.

31

MUSCLE ACTION -AND THE SENSORY SYSTEM

Fundamental relationships exist between musclefunction, the sense receptors, the brain, thenervous system and the peripheral nerves. Thediscussion of muscle function that follows includesa discussion of the physiology of the nervous systemand the peripheral nerves. The brain is, at thisstage, regarded as a processor and is considered inmore detail in Chapter 4.

3.1 1HE MOTOR UNIT

motorend-plates

The motor unit, as the name implies, is thebiological unit of muscle function. A motor unitconsists of a motor nerve arising from motoneuronsin the brain-stem or spinal cord and branching intovarious motor end-plates. These motor end-platesare each connected to an individual muscle fiber;stimulation of these motor end-plates causescontraction of the single muscle fiber attached toit, as shown in Fig. 3-1. The number of motor unitsvaries between the different muscles of the body;

MUSCLE FIBERS

./1

1

~ SENSORY NERVE FIBER, (TO BRAIN)

SENSE RECEPTOR

ONE MOTOR UNIT

-------- ....j,

_~ MOTOR END-PLATE \11

- - - - -l~:R/N:VE FlBERS(FROM BRAIN)

'.~~~~~~~~-----+----

"- ..•....

Fig.3-1. Relationship between nerve and muscle.

32

generally speaking, the larger the muscle, the moremotor units will be found in that muscle. The sizeof the motor unit, that is, the number of musclefibers activated by the same nerve fiber, can bequite different for different muscles; in man, onemotor unit may contain from 25 to 2000 muscle fibers.The force developed by a motor unit may range from0.1 to 250 grams weight. The muscle fibers of amotor unit are not clumped together in one part ofthe muscle, but rather the muscle fibers ofdifferent units are interlaced as shown in Fig. 3-1.

3.2 MUSCLE ACTION

movementsmoothness

musclemodulation

Chapter 1 stated that a cell can only exist in itspolarized or depolarized state; that is, it is abistable device and intermediate potential levelsare not stable. Motor nerves are also cellular innature, thus, any individual motor nerve can onlyexist in a polarized or depolarized state and willtransmit only two potential levels to the motorend-plates causing a bistable "on-off" action ofthe muscle fibers. Thus, the individual musclefibers of one motor unit can exist in only twostates, a relaxed state and a tensed state. Normalmuscular activity is characterized by smoothnessof movement, steadiness and precision. Thesecharacteristics are due to the large number of motorunits comprising any one muscle. If a smallmuscular effort is required, only one motor unitwill be called into action; as increasing musculareffort is required, many more motor units are calledinto action until the muscle is providing maximumeffort at which time aIl motor units connecting tothis muscle are being used. ln this way, sornesmoothness of movement is obtained.

Additional smoothness of movement is obtained bymodulating the number of muscle fiber contractionsper unit time. Although an individual motor unitcan result in only one level of muscularcontraction, the number of times that thiscontraction occurs per unit time (the number ofdepolarizations and repolarizations executed by themotor end-plate cells) will effectively increase thepower of these muscle fibers. Thus, the smoothnessof movement of a muscle is controlled both by thenumber of motor units activated and by the rate atwhich these motor units are being activated.

33

MOTOR NERVEMUSCLE t

1

1 EMERGENCY BRAIN1 "GATE"1

1 SENSORY NERVESENSE RECEPTOR

FEEOBACK FROM SENSE RECEPTOR CONTROLSMUSCLE AS lN SERVO SYSTEM

Fig.3-2. Block diagram of the nervous system.

3.3 THE MUSCULARSERVO-MECHANISvi

braincontrol

A highly simplified block diagram of the nervoussystem controlling muscle action is shown inFig. 3-2. The system is similar to a servomechanism control system: A sense receptor ortransducer produces a position or velocity signalwhich is sent to the brain via the sensory nerve.The brain in turn initiates an "error" or controlsignal by comparing the measured position with thedesired one stored in the memory. This signal issent via the motor nerve to the muscle to controlits action.

150 msreflexresponse

This servo-system can be demonstrated by thefollowing experiment. When onets finger is placedonto a cool object, the sense receptor in the fingersenses the temperature and relays this informationto the brain. The brain interprets this signal ascoming from a cool object and, thus, does notnecessarily initiate any signal to the motor nerves.If the finger is then placed on a warm object, thebrain will interpret the information received fromthe sensory nerves as relating to a warm objectand will activate the motor nerves controlling themuscles in the arms and the hand causing the fingerto be lifted from the warm objecte There is a timedelay of several hundred milliseconds between thetime that the sense receptor feels the warm objectand the time that the finger is lifted from thewarm object. This delay is governed largely by thedegree of attention that the subject is paying tothe warm objecte Now, if the finger is placed on ahot object, a reflex response is obtained and thefinger is removed from the hot object in about150 ms. This reflex is active at aIl times althoughit is most marked for the hot object.

34

3.4 REFLEX RESPONSE

emergencygate

An emergency gate has been shown in Fig. 3-2; thisemergency gate is not markedly involved in normalreceptor/muscle operation. When a reflex responseis called for, however, this emergency gate bypassesthe signal path to and from the brain and initiatesa reflex response. This emergency gate is usuallylocated within the spinal cord. Thus, a reflexresponse results from a "large" signal, i.e., ahigh repetition rate signal, being received from asense receptor. This signal bypasses the brain inthe initiation of muscle action. These reflexresponses protect the body from serious damage.

3.5 THE POTENTIAL GENERATEDDURINGMUSCLEACTION

recordingactionpotential

Depolarization initiated within a sense receptortravels to the brain along the sensory nerve fiberas a series of traveling depolarization waves. Thebrain then initiates another series of travelingwaves of depolarization along the motor nerves tocause a series of depolarizations of the motor endplates. Depolarization of the motor end-platesdepolarizes cells within the muscle fiber causingcontraction of these fibers. The actual internaIcell potentials involved here are the normal cellpolarized potential of -90 mV and the normal celldepolarized potential of +20 mV.

ln dealing with muscles and nerves, it is unusual touse microelectrodes to record the action potentialwithin individual cells. More commonly, needleelectrodes are used to record the net result of anumber of cells such as one motor unit, or surfaceelectrodes are used to record the results of manymotor units. If a microelectrode were inserted intoa muscle cell to observe the depolarization andrepolarization process, the total process wouldoccur in less than one millisecond. If, however, aneedle electrode is placed near these muscle cells,it will detect current flow from many fibers of thecorresponding motor unit. These fibers are beingfired at their motor end-plates at practically thesame instant by the branching nerve. The differentfibers of a motor unit do not, however, develop theiraction currents simultaneously; small timevariations between the different fibers occur. These

35

varying delays are due to the varying lengths of theterminal branches between the motor nerves and themuscle. Thus, the excitation as it travels alongis slightly ahead in sornefibers compared withothers. The result is that current flow in anysmall area from the cells comprising one motor unitlasts from 2 to 5 milliseconds, which is severaltimes the duration of the current from any singlemuscle fiber. This asynchronous action helpsproduce smoothness of muscle action.

EMG

With anything other than microelectrodes, the basicelectrical activity detected in any muscle is thesingle motor-unit action potential. These potentialsfrom single motor units are best detected byconcentric-needle electrodes or by insulated-needleelectrodes as shown in Fig. 3-3. Single motor unitactivity may sometimes be detected by placing a smallelectrode on the skin over the muscle. The volumeof muscle influencing such an electrode is, however,considerable and the actual recording is typicallythe total activity from many motor units havingrandom relationships to one another. The potentialproduced by muscle action, whether recorded by needleelectrodes or by surface electrodes, is known as theelectromyogram or the EMG. Measurement techniquesfor electromyographic recordings are covered inChapter 12.

ms

MaTORNERVES

ACTIVITY FROM ASINGLE MOTOR UNIT

4

+10OmVhMICROELECTRODE 0~--jl\~r----.----.-

ACT 1V 1TY FROM A \ ~ ~SINGLE CELL \_••• _

-10OmV

+1~VFSURFACE ELECTRODE

TOTAL MUSCLE 0 ..••••••ACTIVITY 2

-lOmV

:p=4

Fig.3-3. The EMGobtained with various electrode types.

36

3.6 THE SENSE RECEPTORS

logarithmicresponse

If a sense receptor cell or "transducer cell" isactivated by a physical distortion, a temperaturechange, or sorneother effect, a series of nerveimpulses are produced. The time between theindividual nerve impulses become greater and greateras various factors come into play. After a while,despite continuation of the physica1 stimulus, nonerve impulses are generated. It is believed thatthe sense receptors generate discrete potential1evels which are more or less logarithmicallyrelated to the stimulus strength. These potentialsin turn produce a series of nerve impulses in thenerve fibers. Impulse repetition frequencies fromsensory elements vary from a few impulses per secondto abqut 1000 per second; rates above 50 per secondare most unusual in human subjects. Since the sensereceptors generate an approximate logarithmicresponse to the stimulus, they can respond over anenormous range (1:107 or more) of stimulus energylevels.

3. 7 ras parENfIAL GENERATEDBY SENSE RECEPTOR STIMULATION

detectingsensereceptoractivity

It is almost impossible to detect the electricalactivity associated with most of the sense receptorson the body; they are small and are not located inclusters of sufficient size to allow detection ofthe electrica1 activity associated with a group ofreceptors. Most sense receptors are not electricalin nature; we measure the electrical activityassociated with them only because we do not have thetechniques available to us which enable us to observeunderlying biochemical mechanisms. There are twogroups of sensors that do produce detectableelectrical activity: The middle ear and the retina.Potentials associated with the hearing mechanismand the sight mechanism can be detected in the middleear and on the retina, respectively, because a largenumber of sensory cells are packed closely togetherand can be stimulated simultaneously. Only by suchan arrangement will enough depolarization currentbe produced to make detection possible.

earresponse

electroretinogram

37

When a sound reaches the ear, it causes vibrationof a membrane within the inner ear known as thebasilar membrane. Vibration of this membrane inturn stimulates a large number of sense receptorsknown as hair cells. If the sound reaching the earis fairly loud, a large number of these hair cellswill develop action potentials at the same time andthe current produced will be strong enough to bedetected in the middle ear. Electrical potentialsprecisely following the shape of the stimulatingsound waves can be detected by an electrode in themiddle ear. This part of the inner ear acts as atransducer and generates electrical activity fromsound.

If a bright light is projected onto a substantialarea of the retina, many light-sensitive cells withinthe retina will be stimulated simultaneously and willdevelop a considerable synchronous response which canbe detected from the outside of the retina. Theelectroretinogram (the external electrical responseto light stimulation) may be detected by an electrodeconsisting of a small, flat silver plate fitted tothe inner surface of a small contact lens asdiscussed in Chapter Il, Section Il.8.

ONEBRAIN CELL

TE~INALARBORIZATIONOF 1HE AXON

fig.4-1. Brain cell interConnections- highly siroplified.

39

THE BRAIN AND THECENTRAL NERVOUS SYSTEM

Chapter 3 discussed sense receptors, muscle actionand the potentials associated with the peripheralmotor and sensory nerves. To simplify thepresentation, Chapter 3 referred to motor andsensory nerves as connections to the brain; strictlyspeaking, however, most of these nerves connect tospinal nerves in the spinal cord and these spinalnerves carry information to and from the brain.

4. 1 NERVE CELLS lN 1HE CENfRAL NERVOUSSYSTEMAND lN 1HE BRAIN

cellcomponents

Although nerve cells are located throughout the bodyand carry information between aIl body locations,most of the nerve cells are located in the brainand in the spinal cord. The cells comprising thespinal cord and the brain are simi!ar; the cell bodyis known as the soma and surrounds a region knownas the nucleus. The celi body has an extensionknown as the axon and this may comprise part of thespinal cord and interconnect with sensory and motornerves or may branch into many small fibers knownas the terminal arborizations of the axone Braincells consist also of projections of the cel! bodyknown as the dendrite which sense information fromadjoining celis. The terminal arborizationstransmit information to adjoining cells eitherdirectly in spinal cells or via the dendrite inbrain cells. These cell interconnections (synapticjunctions or the synapse) allow electrical impulsesto flow throughout the brain and the central nervoussystem; one cell acting as a trigger to influenceneighboring cells. See Fig. 4-1.

40

The mechanism by which information is carried isexceedingly complexe As we saw in Chapter 1, themigration of potassium and sodium ions is a primaryfactor. However, there are many other chemicalinterchanges involved which cannot be discussed inthis text which is specifically concerned withelectrical phenomena. It must be emphasized,however, that we measure the electrical activityonly because of its convenience; it is not thefundamental forrnof nervous activity.

4.2 THE BRAIN

neurogl ia

Only a very cursory treatment of the brain is givenin this chapter. It is a complex structure,comprised of very large numbers of nerve cells whichare interconnected among themselves and which alsoreceive data from the various sensory organs. ln aloose sense it can be called a supervisory controlunit for it can override most reflexes and directthe body to act in a coordinated manner. Untilrecently it was thought that most of the functionsof the brain resulted from the interaction ofnervous impulses at the synapses of the cells (asdiscussed in Section 4.3) and many analogies withdigital electronic computers were formulated. It isnow known that this is only part of the story. Forexample, the substance in which the cells areembodied, the neuroglia (from a Greek word meaningglue), is known to play an important part in brainfunction and may be vitally concerned with memory.It is thought that oscillatory electrochemicalmechanisms are responsible for short term memory butthat these impulses gradually modify the chemicalstructures of parts of the brain so that thepermanent storage of information is chemical ratherthan electrical.

controlunit

memory

Anatomically, the brain is divided into severalparts with the general appearance shown in Fig. 4-2.When sectioned along the midline, the appearanceis as in Fig. 4-3. Speaking generally we may saythat the deeper structures of the brain, that isthose nearest the spinal cord, are those whichevolved first and are largely responsible for themore primitive and less easily controlled parts ofhuman behavior (anger, fear, etc.). The cortex(outer structure) is the part of the brain which ishighly developed in man and which has enabled him todominate aIl other species.

41

MaTORCORTEX CRAN1UM

LEFT CEREffiALHEMISPHERE

RIGHT CEREBRALHEMI SPHERE

SPINALCORO

Fig.4-2. Anatomical sections of the brain.

CORPUSCALLOSUM

Fig.4-3. Section of the brain.

42

•Sorneareas of the cortex serve specifie parts of thebody. For example, in Fig. 4-2, sensory input ishandled in the area rnarked sensory cortex whilemotor output proceeds from the motor cortex.Likewise, vision is handled at the rear of the brainin the visual cortex. It is less easy to defineareas which serve intellectual functions althoughthe frontal areas are at least in part responsible.There is a large bundle of fibers which interconnectthe left and right hemispheres; this is known as thecorpus callos.umand its location can be seen inFig. 4-3.

4.3 EXCITATION AND INHIBITION POTENTIALS

cellpotentials

The following discussion is a simplified analysis ofexcitation and inhibition from one scientific schoolof thought. The properties of excitation andinhibition are, however, highly conjectual and, asstated previously, tell only part of the story.There is no general scientific agreement on thesubject.

ln Chapter 1, cells were referred to as bistabledevices existing in only two states; a polarizedstate of -90 millivolts and a depolarized state of+20 millivolts. Intermediate potentials wereregarded as having little significance. Nerve cellsin the central nervous system and the brain existin a polarized state of between -70 millivolts and-110 millivolts and the exact cell potential withinthis range appears to be significant as itdetermines the cell's vulnerability to a stimulusthat would result in regenerative breakdown.

inhibitoryeffect

excitatoryeffect

frequencyis aparameter

43

Consider a group of cells producing zero activity;aIl cells within this group would rest at the normalpolarized level of -90 millivolts. Assuming thatone cell is depolarized from an external stimulus,this depolarization will affect adjoining cells viasynapses, causing a change in the resting potentialof these adjoining cells. The effect on theseadjoining cells can be either to raise or to lowertheir resting potential depending on the type offiber involved in the synaptic junction. If theresting potential is increased, breakdown of the cellmembrane will not occur and the impulse can bethought of as having had an inhibitory effect. Ifthe impulse arriving at this synapse decreases theresting potential, the cell is then more susceptibleto depolarization and the impulse can be thought ofas having had an excitatory effect. With repeatedexcitatory stimuli at a sufficiently high rate, themembrane will eventually fully depolarize and a newimpulse will be propagated. Any single cell isinfluenced by synapses from many other cells; someare excitatory and some inhibitory. The play ofthese two opposite effects on any area may or may notcause depolarization of the cell; the decidingfactor is the number of impulses being received perunit time and the balance between excitation andinhibition. Although the effect at a synapse of anysubthreshold impulse dies away rapidly, it does notdie away immediately. This allows almostcoincident impulses from different sources or arapid rate of impulses from a single source to buildup their effect; thus frequency becomes animportant parameter. Fig. 4-4 shows the effect ofvarious excitation and inhibition signaIs on any oneindividual cell, finally resulting indepolarization of the cell once the -70 millivoltthreshold is obtained.

44

+IOOrnV

-JOrnV

-IOOmV

EXCITATION

DEPOLARIZATION

CELL1 HIBITEO

tCELL

INHIBITED

t THRESHOLO

RESTINGLEVEL

CELL ItHER'IALPOTENT IAL HIGH PRF EXCITQRYSTIMULI CAUSES CELLDEPOLARIZATIO~ WHENTHRESHOLD IS REACHED

INHIBITION STIMULUS APPLIED TO CELL VIA THE SYNAPSES

Fig.4-4. Stimulus changing cell resting potential may resultin depolarization of the cell.

LOW AMPLITUDEHIGH FREQUENCY

ANXIOUS ANDEXCITED

RELAXEDBUT AWAKEEYES CLOSED

L1GHT SLEEP

REDUCED ALPHA, SOME DELTA

DEEP SLEEP

OPTIMUM DELTA RHYTHM

COMA 5011V

1-- 1 SECOND-l

Fig. 4-5. Physiological states and the resultant EEG.

45

4.4 EVOKED POTENTIALS

producingevokedpotentials

As shown in Fig. 4-2, parts of the surface of thebrain have been found to be associated with varioussensory systems. If an appropriate stimulus isapplied to a sense receptor, the correspondingsensory area of the brain responds by producing anelectric potential known as the evoked potential.The evoked potential, as it appears at the surfaceof the brain, is the integrative result from theaction of many cells. When recorded externally onthe scalp, the evoked potential is of the order of10 ~V or so. It is not yet known completely whichcortical elements are responsible for these evokedpotentials. Often they are camouflaged by theelectroencephalogram, thus it may be necessary toremove the electroencephalogram by an averagingtechnique when attempting to record the evokedpotential. Averaging is discussed in more detailin Chapter Il.

4.5 THE ELECTROENCEPHALOGRAM (EEG)

alpha anddeltarhythms

As briefly discussed in Section 4.2, the brain canbe regarded as a highly developed biochemicalfactory. The electrical activity which results fromso much chemical change is known as theElectroencephalogram (EEG, see Chapter 10) and is ina sense a useful byproduct of nervous action sinceit allo.wsus to make nonmutilating measurements onan organ which is singularly resentful of externalinterference. The electroencephalogram, as recordedfrom the surface of the head, consists ofrhythmical, slow sinusoidal waveforms between 10and 100 microvolts in amplitude. Theelectroencephalogram varies in both form, amplitudeand frequency; the basic frequency of around 10 hertzis known as the alpha rhythm. Should a subjectlapse into deep sleep, the alpha rhythm will alsodisappear and will be replaced by a slower highamplitude signal known as the delta rhythm. Theelectroencephalogram produced by a subject invarious conditions is shown in Fig. 4-5.

47

SECTION Il

MEASUREMENT TECHNIQUES

The following chapters (5-14) cover some of the morecommon measurement techniques used in thebiophysical sciences. It is intended that thismaterial be used by engineering-oriented personnelas a basis for developing measuring techniquesthat are uniquely suited to particular requirements.As these requirements are defined by biomedicallyoriented personnel who will operate the measurementsystems, cooperation between the engineer and thesepersonnel is necessary for refinement of thetechniques. A working knowledge of electronicmeasurement techniques is assumed; this materialsimply applies standard electronic measurementtechniques to physiological situations. Typicalresults are shown for normal subjects and littleattempt has been made to define the limits of thisnormality or to present results that may representabnormality.

48

SAGITTALPLANE

FRONTALPLANE

Fig. 5-1. Electrocardiographie planes.

TRANSVERSEPLANE

49

ELECTROCARDIOGRAPHY (ECG)

Generation of electrical activity within the heartis discussed in Chapter 2. Electrocardiography isthe art of analyzing this electrical activity bymeasuring potentials at the surface of the bodyresulting from this electrical activity within theheart. This is achieved by applying electrodes tocertain positions on the body and recording thepotentials generated between various combinationsof these electrodes with an amplifier and CRTdisplay or strip chart recorder.

5.1 ELECTROCARDIOGRAPHIC PLANES

measuringcardiacpotential

The heart can be regarded as an electrical generatorenclosed in a volume conductor, the torso. As thiselectrical generator is completely enclosed by thetorso, a direct measurement of the generator outputvoltage is impossible without resorting to surgery.The electrocardiographer measures the potentialexisting between various points on the surface of thevolume conductor and uses the information obtainedto determine the clinical condition of the heart.The art of electrocardiography is simplifiedsomewhat by considering that at any one time thecardiac potential is projected along axes existingon each of three reference planes: The Frontalplane, the Transverse plane and the Sagittal plane,as shown in Fig. 5-1. This projection of thecardiac potential is referred to as theelectrocardiogram (abbreviated ECG or EKG). Varioustechniques are used to measure the projection of thecardiac potential along axes existing on each of thethree planes; these techniques are consideredseparately.

50

s. 2 FRONTAL PLANE ECG MEASUREMENTS

cardiacvectors

Einthoventriangle

cardiacvector atR wave peak

The electrical potential generated within the heartas projected along axes existing on the frontalplane of the body is known as the frontal-planecardiac vector, as shown in Fig. 5-2. As withvector measurements in the physical sciences, therelative amplitude and angular position of thisvector at any instant cannot be measured with asingle measurement. Two separate measurements arerequired and the results of these two measurementsare plotted on a vector diagram to determine therelative amplitude and angular position of thevector. ln the physical sciences the twomeasurements usually determine the projection ofthe vector along two axes at 90° to one another.ln electrocardiography the projection of thisvector is recorded along two axes at 60° to oneanother as it allows the limbs to be used for theattachment of electrodes; the results obtained arerelatively independent of where on the limbs theleads are placed.

Although only two measurements are theoreticallynecessary to determine the relative amplitude andangular position of the frontal plane vector, it iscommon electrocardiographic practice to record atleast three projections of the frontal plane vectoralong three axes at 60° to each other. The triangleformed by these three axes is known as the Einthoventriangle as shown in Fig. 5-2. Willem Einthoven, aDutch physiologist, pioneered many of theelectrocardiographic techniques in use today andduring the first quarter of this century hedeveloped this vector approach to electrocardiography.Einthoven's law states that the vector sum of theprojections of the frontal plane cardiac vector atany instant onto the three axes of the Einthoventriangle will be zero. This is a well-knownphysical law; however, Einthoven realized that itdid in fact also apply to cardiology. Although theabove applies to the cardiac vector at any instant,cardiologists are usually only interested in thecardiac vector at the peak of the R wave. Thuswhen the term cardiac vector is used, it implies"Cardiac vector at the peak of the R wave."

51

aV L

r--~+

- ~ -

I~ \ FRONTALPLA E+ / /:'~~ ECG VECTOR ~ j~-

.--JlA__+ +k

LL-

~l+

Fig. 5-2. The Einthoven triangle.

52

Burger'sdistortedtriangle

LA

RA

LL

EINTHOVEN

LL

BURGER

Fig.5-3. Einthoven's triangle assuming a homogeneous torso, andBurger's triangle with the effects of the lungs and spine.

More recent advances in electrocardiography havepointed out the weIl known, but previously ignored,fact that the human torso is neither homogeneous nortriangular and that this leads to a distortion ofthe electrical field. Thus, the angle of the vectordeduced from Einthoven's triangle would be in error.Burger attempted to allow for the inhomogeneities ofthe torso by introduction of the distorted triangleshown in Fig. 5-3 which compensates for the effectof the lungs and the spine. Although Burger'striangle is perhaps a more accurate representationof the frontal-plane cardiac vector, it is notwidely used and cardiologists still prefer Einthoven'sapproach. They realize that Einthoven's vectors areonly an approximation but use them largely forconvenience. The clinical interpretation of ECG'sis quite empirical in practice, being done byreference to the enormous number of records whichhave been correlated with known cardiac disorders,usually at autopsy.

5.3 BIPOLAR LIMB LEAD FRONTAL-PLANE ECG MEASURfMENTS

The three potential measurements commonly used todetermine the frontal plane vector in conjunctionwith the Einthoven triangle are:

1. Potential between the right arm and leftarme

2. Potential between the right arm and leftleg.

53

3. Potential between the left arm and leftleg.

These three ECG measurements are known as thestandard frontal-plane or bipolar limb leadmeasurements and are more commonly referred to asLead l ECG, Lead II ECG, and Lead III ECG as shownin Fig. 5-4.

r

STANDARD SIPOLAR LIMS LEADS

U 'IPOLAR LIMS LEADS

III

Fig. 5-4. Electrode positions - frontal plane ECG.

54

5.4 UNlPOLAR LIME LEAD FRONTAL-PLANE ECG MEASUIŒMENTS

augmentedvectors

aVR, aVL,aVF

The unipolar limb-lead ECG measurernents are asomewhat recent addition to electrocardiography andpermit the unipolar registration of the electricalpotential in each of three extremities (the leftarm, the right arm and the left leg) by creatingan "indifferent electrode" or "central terminal"by combining the signal obtained from the othertwo extremities. The three unipolar limb-leadmeasurernents are referred to as augmented vectorright, augrnented vector left and augmented vectorfoot (abbreviated aVR, aVL and aVF, respectively).Augrnented measurements provide the same waveshapebut 50% more potential output than the now unused,nonaugmented, unipolar limb-lead measurements, VR,VL and VF. The VL, VR and VF measurements wereoutdated and replaced by aVL, aVR and aVF whenrecording equipment sensitive to voltage changesrather than current changes becarne available, asshown in Fig. 5-5. The unipolar limb-lead electrodeconfigurations are simply a projection of the samefrontal-plane cardiac vector enta three different60° axes rotated 30° from the Einthoven configuration,as shown in Fig. 5-6. The three unipolar leads beara direct vector relationship to the three bipolarstandard limb leads.

aVR = l + II2 aVF = II + III

2l - III

2aVL =

The aVR unipolar measurement refers to the potentialat the right arm using the left arm and left leg toforrothe indifferent electrode. The aVLmeasurement refers to the potential at the left arrousing the right arm and left leg to forrotheindifferent electrodes and aVF measurement refersto the potential at the left leg using both arms toform an indifferent electrode. It should be notedthat in these three measurements the indifferentelectrode is formed at the negative input of theamplifier in aIl cases. The ECG waveform will thusbe positive for aVL, positive for aVF, but negativefor aVR. Fig. 5-4 shows the electrode positions forrecording aIl frontal plane ECG's.

OLDER, NONAUGMENTED UNIPOLAR CONFIGURATION:

55

POTENTIAL INPUT B _ (A + ~ + C) = 2B - ~ - CTO AMPL 1FIER

AUGMENTED UNIPOLAR CONFIGURATION:

~n-----------------~

POTENTIAL INPUTTO AMPLIFIER () 1

SAME ABC RAT10 BUTB - A; C = 2B - A - C 50% MORE POTENTIAL

2 THAN NONAUGMENTED

Fig.5-5. Equivalent circuit ofunipolar measurements.

,/

< ...•..<,

<,

THE ALGEBRAIC SUM OF THERAND S WAVE FOR EACH OFTHE SIX FRONTAL PLANEELECTRODE CONFIGURATIONSIS TRANSPOSEO TO THISVECTOR DIAGRAM CONSI$TINGOF TWO 60° TRIANGLES.

ALGEBRAIC SUM 1$ A - B

Fig.5-6. Relationship of frontal plane ECG's to the frontal plane axesof the QRS complex.

56

Fig. 5-7. Extremityelectrodes: for fully dressed subjects,somewhat "noisy" at times, used in surgery.

Fig. 5-8. Shoulder electrodes: preferred in most applications,noise free , used in surgery.

57

5.5 FRONTAL-PLANE AXIS DERIVATION

cardiacvectorversusP, QRS andT waves

As stated earlier, enly two ECG measurements arenecessary to determine the axes of the frontal-planecardiac vector. It is, however, common to recordaIl six frontal-plane ECG measurementconfigurations (1, II, III, aVR, aVL, and aVF). Atrained electrocardiologist utilizes aIl sixrecordings when analyzing the ECG waveform as eachECG recording will differ in shape as weIl as inamplitude; certain complexes within the ECGwaveform are more clearly visible in oneconfiguration than in another.

To this stage we have simply referred te theelectrical output of the heart when recordingfrontal-plane ECG's as "the frontal plane cardiacvector." This electrical activity is, however,composed of three discrete waveforms as discussedin Chapter 2. These are the P wave, the QRS complexand the T wave. The relative amplitudes of eachof these waveforms can be measured separately fromthe six standard frontal plane ECG measurementsand plotted on a vector diagram to determine theelectrical axis of a particular segment of thefrontal-plane cardiac vector. There is usually adifference of 30° or more between the frontal planeaxis of the P wave, the frontal plane axis of theQRS complex and the frontal plane axis of the T wave(see Chapter 6, Vectorcardiography).

5.6 FRONTAL-PLANE ELECTRODE POSITIONING

electrodeplacement

Hitherto, the three electrode positions used torecord the frontal-plane ECG have been referred toas right arm, left arm and left leg. ln actualpractice the electrodes can be placed anywhere onthe limb or, in fact, anywhere in the general areaon the torse near the limbe When recording theECG of a fully dressed subject, as is u8ually thecase in a doctor's clinic, it is convenient to usethe wrists and left ankle as shown in Fig. 5-7.This configuration does restrict movement. Thus, ifthe subject i8 to move his body or if the electrodesare to be left in place for severa! hours, it ismore convenient to place the arm electrodes on theupper arms near the shoulders as shown in Fig. 5-8.

58

"noise" onwaveform

If the subject has no upper garment, they are placedon the shoulder blade. Accurate results are alsoobtained with the electrodes placed in an Einthoventriangle configuration on the chest as shown inFig. 5-9. This "chest cluster" configuration isused extensively for subject monitoring in intensivecare wards and when the subject is exercising duringrecording. The right leg electrode shown in Fig. 5-7,5-8 and 5-9 is a ground electrode as discussed inSection 5-9.

ln general, the closer the electrodes are to theheart the better, as any muscle between theelectrodes produces "noise" on the ECG waveform.Thus if possible, it is desirable to use the chestconfigurations to avoid the "noise" generated bythe powerful arm muscles. A typical series offrontal-plane ECG measurements is shown in Fig. 5-10.

Fig. 5-9. Chest cIuster electrodes: for intensive care or exercisingsubject, difficult to apply in many cases.

.,..

.L

II

III

59

aVR

aV F+

VERTICAL O.5mV/DIV tHORIZONTAL O.2s/DIV

Fig.5-10. Typical frontal plane ECG's (recorded with Type 410PhysiologicalMonitor).

60

5 •7 TRANSVERSE-PLANE ECGMEASlJRRvŒNTS

electrodeplacement

We have previously discussed the measurement of thefrontal-plane projection of the cardiac vector; itis common electrocardiographic technique to alsorecord the transverse-plane projection of thiscardiac vector. The transverse-plane is shown inFig. 5-1. Many of the unipolar limb leadmeasurement techniques discussed earlier app1y totransverse-plane electrocardiography. Anindifferent electrode is formed by summing thepotential at the right arm, the left arm and theleft 1eg and the measurement consists of recordingthe potential between this indifferent electrode anda chest electrode placed at various positions on thetorso. Transverse-plane ECG measurements are knownas "the V lead measurements;" VI, V2, V3, V4, V5and V6. The subscripts 1 through 6 refer to theposition on the torse of the chest electrode asshown in Fig. 5-11. Although the standardtransverse-plane ECG configurations are VI to V6many other transverse-plane configurations are used.The more common of these other configurations areV7, V8 and V9 with the chest electrode placed onthe left-hand side of the subject towards the back.If the chest e1ectrode is placed in a positionsomewhat above or below a standard V position, thenthe configuration is subscripted H or L; thus, ifthe chest electrode were placed on the midline atthe base of the rib cage, it would be referred toas V2L' Occasionally V configurations are recordedwith the electrodes on the right-hand side of thesubject. Typical transverse-plane ECG's are shownin Fig. 5-12.

VI V2

yr

~V6Vs {\

•V4 •

Fig. 5-11. Transverse plane ECG.

61

62

V2

+VERTICAL O.5mV/DIV tHORIZONTAL O.2s/DIV

Fig.5-12. Typical transverseplane ECG's (recorded with Type 410PhysiologicalMonitor).

63

CATHETER INSERTEDINTO THE ESOPHAGUSVIA THE NOSTRIL

ESOPHAGUS

RA LA LL

ELECTRODE

Fig.5-13. Electrode positions - sagittal plane ECG's.

5.8 SAGITTAL PLANE ECG :MEASUIŒMENTS

esophageallead

The measurements of the cardiac vector projection inthe sagittal plane are referred to as "the unipolaresophageal lead ECG measurements" or the "E lead ECGmeasurements." Although the unipolar esophageallead ECG is rarely recorded these days, it can berecorded by attaching an esophageal lead to thepositive input of the amplifier and forming anindifferent negative electrode using both arms andthe left leg as with the transverse-plane V leads.Esophageal lead measurements are recorded fromwithin the esophagus using a catheter, or finerubber tube, threaded with a wire with an electrodeattached to its tip, as shown in Fig. 5-13.

5.9 ECG INSTRUMENTATION REQUIREMENTS

The previous discussion has been concerned withelectrode placement and electrode configurations.The potential appearing at these electrodes mustbe amplified by a differential amplifier andrecorded on either a strip chart recorder, acathode ray tube or, occasionally, magnetic tape.

64

displaydevices

electrocard iograph

CRTdisplays

Commercial instruments usually include electrodeselection, amplification and either a CRT displayor a strip chart recorder. These instruments arecommonly referred to as ECG monitors orphysiological monitors if they utilize a CRTdisplay and as electrocardiographs if theyutilize a strip chart recorder display. Thecommercial electrocardiographs preceded commercialphysiological monitors by 50 years or so; thus,many of the features on physiological monitorsare carried over from electrocardiographs eventhough they may not be optimally suited for usewith CRT displays.

Electrocardiographs almost invariably use graphpaper with horizontal and vertical lines at1 millimeter intervals and a heavier line at5 millimeter intervals as shown in Fig. 5-14. lnroutine electrocardiography the recording speed is25 millimeters per second; thus, the 1 millimeterhorizontal intervals each represent 0.04 secondsand the 5 millimeter intervals represent 0.2seconds. The sensitivity of electrocardiographs istypically 10 millimeters per millivolt.

When referring to CRT displays, it is normal practicein the electronics industry to refer to millivolts/centimeter vertically and seconds/centimeterhorizontally. However, due to the historicalinfluence of electrocardiographs, the reciprocal ofthese dimensions is used in electrocardiography;i.e., millimeters/millivolt and millimeters/second.

1 mY

0.2s

COMPUTER INSTRUMENTS CORP. HEMPSTEAD, NEW YORK RP·120-

Fig.5-14. A typical electrocardiograph tracing.

right leggroundelectrode

subjectprotection

65

Physiological monitors commonly use twice thevertical sensitivity and twice the sweep speed ofelectrocardiographs; that is, 20 millimeters/millivolt and 50 millimeters/second.

As high input impedance differential amplifiers arealmost invariab1y used in modern electrocardiographsand physio1ogical monitors, some ground referencemust be maintained between the subject and theamplifier. This is usually accomplished by attachingan electrode to the subject's right leg andconnecting this electrode to the ground of theamplifier. For reasons covered in Chapter 16(Electrodes) and Chapter 19 (Amplifiers), amplifiersused to record the ECG should have a common modeinput impedance of at least 10 megohm per input andshould have a common mode rejection ratio of atleast 10,000:1. Frequency components within the ECGextend from .05 Hz to approximately 80 Hz; mostcommercial amplifiers will exhibit adequate highfrequency response, however many amp1ifiers do notexhibit a low frequency response to .05 Hz or better.

It is highly desirable that any amplifier used torecord ECG's, or used to record other potentialsfrom the human body, should incorporate internaIcircuitry to protect the subject against electricalshock should the amplifier fail. Protection circuitsare covered in Chapter 17 (Safety).

5.10 INTERPRETATION OF THE ELECTROCARDIŒRAM

Interpreting the results obtained from the variousECG measurements discussed previous1y is an art initself and should only be atternpted by suitablytrained rnedical personnel. Although it is not withinthe scope of this publication to cover ECGinterpretation, it is desirable that the reader havesorneconcept of what is considered normal and whatis considered abnormal and appreciate the meaningsof the more important terms used by cardiologistswhen discussing the shape of the ECG waveform.

The "normal" ECG has come about by observing thedistribution in many thousands of healthy subjects.The ECG's presented in Fig. 5-11 and Fig. 5-14 are"normal" ECG's from a healthy adult male. Any

66

variation from the normal rhythm of an ECG is knownabnormal ities as an arrhythmia. As arrhythmias may be more

evident in certain ECG measurements, a completeset of frontal-plane and transverse-plane ECG'sis usually required for a complete analysis of theelectrocardiogram.

A fast heart rate in excess of 100 per minute isknown as tachycardia; a slow heart rate of less than60 per minute is known as bradycardia. Anexcessively large and continuous ECG with norecognizable P, QRS or T waves is a sign ofventricular fibrillation, a condition where theventricular muscle goes into local oscillation ina "circus movement," waves of depolarization goinground and round the ventricular tissue. A randomand changing phase relationship between P and QRSand T indicates adioventricular rhythm, in which acomplete bundle block occurs in the bundle of His,and the auricles and ventricles are beatingindependently, with the ventricles slower.Arrhythmias also appear as extra beats, known asectopie beats, on an otherwise normalelectrocardiogram. A good reference for the studyof cardiac arrhythmias is PrincipaZs ofEZectrocardiography by J. Goldman or Coronary CareUnit Nursing, Part I by H. A. Braun and G. A.Diettert.

derivedfram ECGmeasurements

67

VECTORCARDIOGRAPHY (VCG)

The electrical activity of the heart as projectedalong various axes on three planes of reference isreferred to as the ECG and is discussed inChapter 5. Vectorcardiography is the art ofanalyzing the electrical activity within the heartby obtaining ECG's along three axes at right anglesto one another and displaying any two of these ECG'sas a vector display on an X-y oscilloscope. Thisdisplay is referred to as a vectorcardiogram orVCG. The three axes chosen are designated X, y,and Z as shown in Fig. 6-1. The planes bounded byany two of these axes are also shown in Fig. 6-1.

% +REAR/ -FRONT

+HEAD Y-FEET

TRANSVERSEPLANEX,%

FRONTALPLANEx,y

SAGITTALPLANEy,%

Fig. 6-1. VCG planes.

68

The electrocardiogram displays the electricalpotential in any one single axes, however thevectorcardiogram displays the same electrical eventssimultaneously in two perpendicular axes. Atpresent, vectorcardiography is used only in medicalcenters and by specialty medical groups. Its valueis limited for routine clinical practice due to theexpense of the necessary equipment, the timeinvolved, and because no definite standard as to thebest lead system to be used or as to what is"normal" has been established.

6.1 TIffiSPATIAL VECTORCARDIOGRAM

The spatial vectorcardiogram may be defined as therecord of the time variations of the instantaneousvectors which represent the electrical activity ofthe heart. Fig. 6-2 shows the spatialvectorcardiogram and the resulting projections ofthis spatial vectorcardiogram onto the frontalplane, the transverse-plane and the sagittal plane.The projections are referred to as the frontal VCG,the transverse VCG and the sagittal VCG.

69

+y

t

SPATIALVECTORCARDIOGRAM

TRANSVERSE PLANEPROJECTION

Fig. 6-2. VCGprojections.

6.2 ELECTRODEPLACEMENT

absoluteaccuracydifficult

The location of electrodes on the body to allowaccurate recording of the ECG's along the X, Y andZ axes is extremely important. To obtain anaccurate vectorcardiogram, the electrodes must beplaced in positions that permit accurate recordingof the three vectors. The body and the heart,however, exhibit characteristics which cannot becircumvented for practical purposes. This makesabsolutely accurate recording of thevectorcardiogram almost impossible as any systemof electrode placement is somewhat imperfect.

70

The vectorcardiogram can be recorded by obtainingthe potential between two points located under thearms for the X or left to right component, betweenthe chest and the back for the Z component andbetween the head and left leg for the y component.Simply displaying any two of these three signaIs onan X-Y oscilloscope with the same sensitivityfor both the X and the y amplifiers would result inconsiderable errors as the potential generated withinthe heart is attenuated by different amounts in theX, Y and Z directions and, as the torso is nothomogeneous, these signaIs are generated along axeswhich are not exactly at 90° to one another.

6.3 FRANK ELECTRODE SYSTEM

sevenelectrodes

compensatingnetwork

Various attempts have been made to compensate forthe above attenuation and torse inhomogeneities.The Frank system of vectorcardiography attempts toachieve isotropy as weIl as orthogonality by usingseven critically placed electrodes and a resistivenetwork. The Frank electrode positions and theFrank attenuation and compensation network is shownin Fig. 6-3.

71

FRANK ATTENUATION ANDCOMPENSATION NETWORK

y 75kn

x

130kn

z

x

LL -FOOT

y

~

10knH +HEAD• •

RL LLRL GROUND

Fig. 6-3. Frank electrode positions.

72

y VERT ICALX HORIZONTAL

FRONTAL

y

t

---t--1

1

111

THREE TRACE DISPLAYS SHOWING THE THREEQRS COMPO E TS OF THE VECTOR DISPLAYSSHOW BELOW.

.z VERT 1CALX HORIZONTAL

···ù::. . . . , .. . .., ••• 1 •

, ., .TRA SVERSE

X

Fig. 6-4. The Frank vectorcardiogram.

X

THE X, Y AND .z OUTPUTS OFy THE FRANK ELECTRODE NETWORKS

.04s/DIVO.5mV/DIV

y VERT ICAL.z HORI ZONTAL

SAGITTAL

y

t

73

6.4 POLARITY CONVENTION

+x to leftside, +Y tohead, +Z toback

Fig. 6-3 shows a positive direction for X towardsthe subject's left-hand side, a positive directionfor Y towards the head and a positive direction forZ towards the back. Since aIl modern oscilloscopesadopt the convention of a positive potential changedeflecting the beam upwards and towards the righthand side of the screen, it is necessary to adoptthis polarity convention for the three axes topresent the three vectorcardiograms superimposed onthe human body as viewed from logical viewpoints;i.e., from the front, from the top and from theleft-hand side. The polarity of the ECG waveform inthe X and Z direction will thus appear "positive,"however the polarity of the normal ECG waveform inthe y direction will be inverted, as shown inFig. 6-4. Frank's original network suggested apositive Y direction towards the feet which resultedin the three separate ECG components beingrepresented as "positive" signaIs. This resulted ininversion of the frontal and sagittal projectionsof the vectorcardiogram, making correlation to theactual position of the heart somewhat difficult.The conventions adopted in this chapter are nowadaysregarded as standard.

74

6.5 OTHERELECTRODESYSTEMS

Fig. 6-5 shows three other electrode configurationsin common use, the axial electrode placement, thetetrahedron electrode placement and the cubeelectrode placement. Although none of these threeconfigurations are considered as accurate as theFrank system of vectorcardiography, they do notrequire complicated resistive networks and canutilize two standard ECG monitors and an X-Yoscilloscope to obtain satisfactory results.Indifferent electrodes are formed by summing theeffects of various electrodes in the same way asfor electrocardiography as covered in Chapter 5.

6.6 THE NORMALVECTORCARDIOGRAM

QRS looppredominant

Vectorcardiograms obtained from a healthy, maleCaucasian subject using the Frank system are shownin Fig. 6-4. The upper CRT display shown inFig. 6-4 represents the QRS segment of the threeseparate ECG's used to forrothe vectorcardiogramsshown in the lower CRT photographs. Carefulinspection will reveal that the three QRS segmentsare not exactly in phase; if they were, thevectorcardiogram loops resulting from the QRSsegment would aIl be straight lines. Although eachvectorcardiogram should theoretically exhibit threeloops showing the vector orientation of the P wave,the QRS axis and the T wave, the loop obtained fromthe QRS complex predominates and an increase inhorizontal and vertical deflection factors isnormally necessary to adequately view the loopsresulting from the P wave and T wave. It should benoted that the VCG is very dependent on the phaseof the respiratory cycle in which it is recorded.The posture of the subject is also very important.

y

A 22k

22k -R IGHTA'

X

l • IOk o +LEFTw..

X

10kB •.•----'w....-----<O +REAR

LL--------O -FO'n

yH ---------0 +HEAD

(A) AXIAL VECTORCARDIQGRAM

RL---------0 GROUND

y

(Cl CUBE VECTORCARDIQGRAM

CIOk

-"~33k

33k -.33k /10k ~

22k

22k

-RIGHT

X

75

y

y

X

+LEFT

-FRONT

+REAR

LL -FOOT

R~--------O GROUND

(B) TETRAHEDRONVECTORCARQIQGRAM

•RL

B" -RIGHT

X

B ~ +LEFT

X

C -FRO'lT

%

+REAR

'--------0 -FOOT

yB' ---------0 +HEAD

RL---------O GR')UND

Fig.6-5. Axial, cube and tetrahedron electrode positions.

RA+HEAD

LA

76

DIRECTIONOF TRACEMOVEMENT

1NTENSITYMODULATIONWAVEFORM

PERIOD!-4 2.5ms •• 1

PULSE 1 Ii' 0 ••.1• msPULSE 2 ...-t ..._

.25msGATED FOR 1.0s~------------ ••• 1

Fig. 6-6. Sagittal VCGwith 2.5-mstime-duration markers.

ELECTRODESTO SUBJECT

~

410 SIG OUTPUTMON 1TOR ADAPTERx'

FRA K -RESISTIVE Z' PASS 1VE~

SWITCHINGNETWORK y'

~y- ---'--

410 SIG OUTPUT 611 STORAGEMONITOR ADAPTER r----- X DISPLAY UNIT Z -

CAL 1BRATESIGNAL TRIG

PULSE ANDATTEN ~ WAVEFORM

GENERA TOR BLANKING PULSE TO MONITOR

ECG

TR IG FR<J.1410

BLANKINGPULSE OUT __'

DELAY RAMP GATED FORDURATION_OF DISPLAY

Fig. 6-7. VCGsystem using two monitors and incorporatingintensity modulation.

77

6.7 TTIMINGAND DIRECTION REFERENCE

intensitymodulation

The vectorcardiograms shown in Fig. 6-4 give noinformation as to the direction of movement of theCRT beam when inscribing the vector loops, nor dothey provide any indication of the time taken toinscribe the 100ps. As this information is oftendesirab1e, some form of intensity modulation isusually incorporated into VCG instrumentationto provide a time sca1e and direction reference.The single sagittal vectorcardiogram shown inFig. 6-6 includes intensity modulation at 400-Hzrate to provide a timing reference of 2.5mi11iseconds between each consecutive brightenedarea on the display. Two discrete intensitymodulation levels are used for each intensifyingpulse providing an "arrowhead" effect to indicatethe direction of movement of the CRT beam wheninscribing the vector 100ps. As with the VCG'sshown previous1y in this chapter, only the QRS loopis clearly displayed. Some VCG systems utilizesawtooth intensity modulation in place of the twostep intensifying pulse.

6.8 INSTRUMENTATION REQUIREMENTS

A complete vectorcardiography system is shown inFig. 6-7. Many of the ECG instrumentationtechniques discussed in Chapter 5 are used whenrecording the VCG. VCG measurement requires twoseparate ECG measurement systems as we1l as a Frankresistive network, an X-y storage oscilloscopeand intensity modulation circuitry.

The circuit of the Frank resistive network is shownin Fig. 6-3. Further details of this network aregiven in Chapter 29. The passive switching networkselects the correct combination of the X, Y and Zsigna1s from this Frank network for displayingeither the frontal, transverse or sagittalvectorcardiogram. The two ECG's se1ected aredisplayed on individua1 410 monitors and the outputof each of these monitors is coupled via passiveoutput adapters to the 611 storage display unit.

78

410 with611

5031

The output adapters are necessary to decrease thesignal output of the 410 to the level required bythe 611 and to allow calibration of the system froman external calibration signai. For details ofthese adapters refer to Chapter 29. A combinationof Tektronix pulse and waveform generation modules(Refer to Chapter 22) is triggered by the triggersignal available from either of the 410 monitors toprovide blanking pulses to the CRT. This blankinginformation adds time and direction information tothe vector display. These modules also provide acalibration signal which is fed to the passiveswitching network via an attenuator. A C-lO cameramay be used to record the VCG from the 611 displayunit.

The Tektronix 5031 storage oscilloscope operated inthe X-y mode is ideally suited to vectorcardiology.The 5031 may be used as a complete vectorcardiographsystem by using the cube electrode placement givenin Fig. 6-5. Normally the internaI time base of the5031 is disabled when operating in the X-Y mode.However, if this disabling circuit is removed byremoving pin Z from the sweep circuit board, theinternaI sweep circuitry can be triggered from thecalibrator and used to provide a multilevel intensitymodulation as shown in Fig. 6-6. If desired, aFrank resistive network as detailed in Chapter 29could be used ahead of the 5031. Any oscilloscopecamera may be used to record the results.

X'Z'

• VERT~..•HORIZ

FRANKRESISTIVENETWORK

5031 STORAGEOSCILLOSCOPE

X-Y MODEy'

Fig.6-8. SimplifiedVCGsystem.

5618 or5648 with3A74

410 subjectprotection

79

Many other vectorcardiograph systems are in use.One of the more common alternate techniques is touse a 56lB or 564B oscilloscope with 3A74 four-traceunits in both the vertical and horizontal channels.When used in conjunction with three preamplifiers,aIl three projections of the spatial vectorcardiogramcan be displayed simultaneously. The display canthen be photographed and the photograph enlarged forconvenient viewing. Additional information on thecomponents required for these VCG systems are givenin various chapters in Section III. The custominstrumentation required for the above systems arediscussed in Chapter 29.

It should be appreciated that the Tektronix 410monitor is the only Tektronix product incorporatingsubject protection features. While other Tektronixproducts include sorneprotection and are inherentlyhighly reliable, current limiting adapters asdescribed in Chapter 29, Section 29.12, should beused when using these instruments in conjunctionwith human subjects. Safety is covered further inChapter 17.

80

1142/min16IJV

2150/min

3IJV

3144/min

5IJV

Fig.7-1. Fetal ECG's at 16 weeks on several subjects.

17 WEEKS160/min

5IJV

19 WEEKS141/mln

4IJV

21 WEEKS140/mln12IJV

23 WEEKS140/min35IJVIJ

25 WEEKS134/mln45IJV

29 WEEKS125/min10IJV

36 WEEKS125/min

5lJV

Fig. 7-2. Fetal ECG variations with gestation time.

81

FETALELECTROCARDIOGRAPHY

Fetal electrocardiography is the art of recordingthe ECG of a fetus in utero by placing electrodeson the mother's abdomen and using recordingtechniques similar to those used for normalelectrocardiographic recording. The procedure iscomplicated by the existence of the mothers's ECGand muscle action potentials and by the severeattenuation suffered by the fetal ECG when recordedat the mother's abdomen. The fetal ECG is normallyabbreviated F-ECG.

7.1 THE EXISTENCE OF THE FETAL ECG

best trom16 to 24weeks

The fetal ECG may be recorded as early as the llthweek of gestation in some subjects and can berecorded in almost aIl cases after 16 weeks gestation.Fig. 7-1 shows the variation in fetal ECG amplitudethat may be expected at 16 weeks. By the l8th weekthe fetal ECG is invariably pronounced and continuesto become more evident to about the 24th week. Atthis stage, amniotic fluids begin to build up in themother which reduces the amplitude of the fetal ECG.This reduction slowly continues for most of theremaining weeks of pregnancy. Just prior to deliverythe amniotic fluids are released and the fetal ECGcan again be prominently recorded. The variationin the fetal ECG obtained from a subject at variousgestation times is shown in Fig. 7-2.

82

MOTHER'S R-R INTERVAL62 BEATS/MIN

'S ORS COMPLEX

--------IDENTIFiCATION OF 2OR MORE GRATICULELINES lN THE PHOTOGRAPHHELPS DETERMINE THE EXACTCAMERA MAGNIFICATION

PHOTOGRAPHED WITH A C30ACAMERA FROM A TYPE 410PHYSIOLOGICAL MONITOR

•1 •1 FETAL ECG CAMOUFLAGED'--- ....L..._ BY MOTHER' S HEART ACT 1VITY

Fig.7-3. A typical ECGdisplay at 18 weeks.

7 •2 TIIE NORMAL FETAL ECG

maternai ECGversusfetal ECG

only R wave

As previously stated, the fetal ECG amplitude variesgreatly with gestation time and also varies from onesubject to another. A typical fetal ECG obtainedat 18 weeks is shown in Fig. 7-3. The mother's ECGis clearly evident and, being many times greater inamplitude than the fetal ECG, may camouflage thefetal ECG. The fetal ECG can clearly be observedduring the isoelectric period between the mother'sECG. The techniques used to record this fetal ECGare covered late in this chapter. From thisrecording, it is evident that the mother's heart rateis 62 beats per minute and the fetal heart rate is158 beats per minute. ln the early stages ofpregnancy the fetal heart rate is normally between140 to 160 beats per minute. If the mother isrelaxed during the recording of the fetal ECG, it isexpected that her heart rate would be under 80 beatsper minute. Due to the low amplitude of the fetalECG, the fetal R wave is normally the only part ofthe fetal ECG complex that is evident.

7.3 SUBJECT PREPARATION

As the electrical potential generated by muscularactivity within the mother's abdomen will clearly bepresent using abdominal electrodes when attempting

83

relaxedstateimperative

to record the fetal ECG, it is imperative that themother be situated comfortably and in a state ofcomplete reste A bed or couch is normally preferredto a clinical examination table. To reduce themother's muscular activity in the abdominal area asmuch as possible, it is desirable that the subjectdoes not eat for several hours prior to recordingand that the subject empty her bladder shortlybefore recording.

7.4 ELECTRODE PLACEMENT

Many electrode positions have historically been usedto record the fetal ECG. It is now generallyaccepted that abdominal electrodes are preferableto most other configurations. Due to the variationexpected in the fetal position, many electrodesshould be applied to the abdomen and the fetal ECGrecorded between various combinations of theseelectrodes until a positive recording is obtained.The most common fetal ECG lead configuration is the

810ndheim Blondheim configuration shown in Fig. 7-4. Thisconfiguration configuration consists of three electrodes (A, D and

F) placed at the vertices of a 60° triangle and anadditional three electrodes (B, C and E) placed atthe vertices of an inverted 60° triangle. Fig. 7-4also shows two electrodes on the back of the subject(G and H). It has been found that, during the earlystages of pregnancy, a recording from one of thesetwo back electrodes to any of the abdominal electrodesproduces more reliable results. It is important thatcare be exercised when applying the electrodes.

USE ONE OF THESE 8 ELECTRODESAS "RL" OR GROUND OR AFFIX ANADDITIONAL RL ELECTRODE ANYWHEREON THE ABDOMEN.

BACK MIDU NEELECTRODE

BACK LLH ELECTRODE

Fig.7-4. Fetal ECGelectrode positions.

84

TWO BACK ELECTRODES AFFIXED ANDADAPTER CABLES FITTED TO THEM

ELECTRODES AFFIXEDTO ABDOMEN

ADAPTER CABLES FITTED TOABDOMINAL ELECTRODES

Fig.7-5. Fetal ECG electrode placement.

lowresistanceattachmentnecessary

85

Electrode placement techniques providing low sourceresistance, as covered in Chapter 16, must be used.The abdominal and back electrodes affixed to asubject are shown in Fig. 7-5. It has been reportedthat, during early stages of pregnancy, betterresults may be obtained by exerting considerablepressure onto the electrodes so as to move theseelectrodes closer to the actual fetus.

7.S ELECTRICAL INTERFERENCE

away frompower lines

short leads

Particular care must be taken 'to eliminate linefrequency interference from the fetal ECG recordings.Adequate instrumentation can eliminate any linefrequency signal appearing as a common mode signal,however it is impossible to eliminate line frequencyinterference if it appears as a differential signal.Filters cannot be used, as the prime frequencycontent of the fetal ECG is between 20 Hz and 80 Hz.Line frequency interference can usually be eliminatedby recording the fetal ECG with the Type 410Physiological Monitor operated from its internaIbatteries and by completely disconnecting otherpower-line-operated devices within the near vicinity.Such devices should be completely disconnectedrather than simply turned off to e1iminateelectrostatic interference if the switch is locatedin the power-line neutral lead. Interference canbe eliminated in many cases by orienting the patientin a particular position within the room. If thisis not successful, it is normally preferable tolocate another recording site rather than to investin costly electrostatic and electromagneticshielding. The fetal ECG recording may still showevidence of fetal ECG activity even though linefrequency interference is present. The leadsconnecting the patient to the monitor should be asshort as possible, preferably no longer than twofeet.

86

7.6 RECORDING TECHNIQUES

electrodecombinations

using the410

Electrodes should be placed on the mother's abdomenas shown in Fig. 7-4. If it is intended that thefetal ECG from many patients be recorded, it isdesirable that some form of custom electrode selectionswitching unit be built to eliminate the necessity ofhaving to make separate connections to the recordingequipment for each electrode configuration. Atypical set of fetal ECG's obtained by recording aIlcombinations of the electrodes shown in Fig. 7-4 isshown in Fig. 7-6. Normally it is not necessary torecord aIl possible combinations of these electrodesand the procedure is usually considered completewhen a fetal ECG has clearly been observed in two orthree configurations.

ln early stages of pregnancy the following electrodecombinations appear to give the greatest possibilityof observing the fetal ECG: C-F, B-D, A-F, A-D and,if the Gand H electrodes are used, G-F, G-D andB-H. ln later stages of pregnancy when the fetalECG is pronounced, it is normally only necessaryto record from either of the three sets ofelectrodes forming the 60° triangles of theBlondheim configuration, that is, electrodes A, Fand D or electrodes B, C and E. As the amplitudeof the fetal ECG is normally between 5 and 50microvolts, and, as it is preferable to use batteryoperated equipment to record the fetal ECG forreasons discussed in the previous section, mostcommercial electrocardiographs or physiologicalmonitors are not compatible with fetal ECG recording.lt is common practice to use an EEG recorder or aphysiological monitor operated in the EEG positionto monitor the fetal ECG. This technique isrecommended when recording the fetal ECG with theType 410 Physiological Monitor. The EEG positionon the function selector switch is selected and thefetal ECG electrodes are connected to the EEGposition on the patient cable. A standard 410, asweIl as any other commercially available EEGrecorder, may give satisfactory results, howevermost of these instruments have a low frequencyresponse extending below .1 Hz. Muscle activitywithin the mother's abdomen will probably result insevere interference causing the trace to be offscreen most of the time. This problem can be

87

+C-G

+C-E

+C-F

+C-H

+0-G

+0-H

+E-F

+E-H

+F-H

+G-H

Fig. 7-6. Fetal ECG variations with various electrode positions(23 weeks gestation).

88

alter lowfrequencycutoff

photography

compensatingfor nonl inearphoto

overcome by altering the low frequency cutoffcharacteristics of the monitor. A modification thatmay be added to the 410 monitor to increase the lowfrequency response in the EEG position from below.1 Hz to approximately 1 Hz is given in Chapter 29.This modification also reduces the gain in the EEGposition from 50 to 100 microvolts per centimeter.The gain can, however, be increased with thevariable control by a factor of 3 to give a maximumgain of 33 microvolts per centimeter. AlI fetalECG photographs shown in this chapter were recordedat a sensitivity of 33 microvolts per centimeter andat a sweep speed of 50 millimeters per second.

As detailed analysis of the waveform is oftennecessary to detect the fetal ECG, particularly inthe earlier stages of pregnancy, it is desirablethat a photograph be taken of the informationappearing on the screen of the 410 monitor.Although no rigid mounting bezel is available toadapt the 410 monitor to a Tektronix trace-recordingcamera, a Tektronix C-30A camera can be held againstthe faceplate of the 410 monitor and satisfactoryresults can be achieved. AlI photographs shown inthis chapter were recorded in this manner. Whenphotographing from the 410 with the C-30A camera, theshutter should be in the B position and openedmanually for approxirnately 2 seconds. Optimumresults are obtained using an aperture of f/16 anda magnification of .9X. Certain nonlinearities existwhen photographing the curved faceplate CRT used inthe 410, however these nonlinearities can becornpensated for by examination of the trace toidentify two or more graticule lines in thephotograph. Once these graticule lines areidentified, the ratio of the spacing between thesegraticule lines to the spacing between the fetal ECGR waves gives the fetal ECG heart rate as given bythe following formulas:

89

-- for fetal ECG spacings measurednear the center of the screen usinga sweep speed of 50 millimeters persecond.

Fetal heartrateBeats per min.

= 300 x graticule mark spacingfetal ECG R wave spacing x .9

-- for fetal ECG spacings measurednear the edge of the screen using asweep speed of 50 millimeters persecond.

Fetal heartrateBeats per min.

= 300 x graticule mark spacingfetal ECG R wave spacing

Due to the nonlinearities mentioned earlier, acorrection factor of .9 is necessary for informationat center screen and no correction factor isnecessary for information at the edge of the screen.Chapter 24 discusses photography of the 410 in moredetail and gives the reasons for these correctionfactors.

7.7 INTERPRETATION OF THE FETAL ECG

fetalECG versusstethoscope

When attempting to record the fetal ECG, positiveresults give a positive indication that the fetaiECG does exist; however negative results arenonconclusive and give no indication as to theviability of the fetus. Although the fetal ECG canbe recorded as early as the llth week and can berecorded in almost aIl cases by the 16th week, itis generally accepted that fetal heart activitycannot be detected by a standard stethoscope earlierthan approximately the 20th week. Thus the fetalECG can give a positive indication of the existenceof a live fetus several weeks earlier than can beobtained with a stethoscope.

90

mother'sECG vector

fetal ECGvector

multiplepregnancy

ELECTRIC FIELDFROM MOTHER 1 S HEART ---

Fig. 7-7. Fetal cardiac vector vertex presentation at full term.

Fig. 7-7 shows the orientation of the electricalaxis of the fetal heart compared to the orientationof the electrical axis of the mother's heart. Asthe electric field generated by the mother's cardiacvector produces "mother's ECG" activity whenrecording the fetal ECG, the axes of this "mother'sECG" activity will be tangential to this field andnot necessarily parallel to the direction of themother's cardiac vector. Within a few weeks afterthe fetal ECG is first detected, its amplitude isgreat enough for accurate measurement and theresults can be plotted on a vector diagram as shownin Fig. 7-8 to determine the electrical axis of thefetal heart·. Fig. 7-8 shows the electrical axesobtained from the recording shown in Fig. 7-6. Asonly two fetal ECG's along two axes are necessaryto plot the fetal ECG vector, the other four vectorsshown on Fig. 7-8 can be used to determine theaccuracy of the measurement technique. An errortriangle is obtained by plotting the intersectionof aIl vectors; the head of the fetal ECG vectormust lie somewhere within this error triangle.

Fetal electrocardiography is perhaps most usefulwhen attempting to diagnose multiple pregnancy.X-ray techniques are known to be harmfui to thefetus; however, fetai ECG techniques are completelyharmless and can detect multiple pregnancy as earlyas the l6th week. Fig. 7-9 shows a fetal ECG

91

ALGEBRAIC SUM OF + AND - COMPONENTS OFTHE FETAL ECG SHOWN lN FIG. 6 PLOTTEDON A 60° VECTOR DIAGRAM FORMED BY THEELECTRODE POSITIONS SHOWN lN FIG. 4.

A

FETAL~'__--~,------ CARO 1AC

VECTOR

ERROR TRIANGLE"HEAD" OF VECTOR CAN BE ANYWHERE

WITHIN THIS TRIANGLE

Fig. 7-8. Vector determination of fetal position.

Fig.7-9. Twin gestation - fetal ECGat 21 weeks.

recording of twins at 21 weeks. As the fetal Rwaves are of opposite polarity, it is obvious thattheir electrical vectors are in opposite directions,i.e., one twin is breech and the other is cephalic.Fetal R waves in the same direction as the mother'sQRS complex represent the breech twin and fetal Rwaves in a direction opposite to the direction ofthe mother's QRS complex represent the cephalic twin.

The fetal ECG is often recorded during fetalde1ivery to indicate whether the delivery will befrom the vertex or the breech position and to givean indication of fetal distress during labor.

93

Œj"

BLOOD PRESSURE AND FLOW

Blood pressure measurements can be c1assified intothree discrete groups: direct, indirect andrelative. Direct blood pressure measurementinvolves gainlng access to the circulatory systemand measuring the pressure in the system direct1ywith sorne form of pressure transducer. Indirectb100d pressure measurement involves application ofpressure externa1 to the circulatory system andobservation of the effect of this externa1 pressureon the system. Relative blood pressure measurementsare uncalibrated indirect measurements usuallyperformed with simpler, and inherently moreconvenient, instrumentation. Blood f10w is derivedfrom the measurement of blood velocity whichnecessitates gaining access to one or more of theprimary arteries within the body using surgica1techniques. Information on blood pressure and bloodflow is often adequate for analysis of thecirculatory system, however a complete hydrodynamicanalysis of the circulatory system can ouly beaccomplished by also measuring blood volume.

8.1 DIRECT BLOOD PRESSUREMEASUREMENT

Direct blood pressure is measured by inserti~g apressure transducer somewhere within the circulatorysystem: Figs. 2-1, 2-2 and 2-3 of Chapt.e.r 2 referto this system. Sornetransducers are designed tobe inserted directly into the circulatory system,however it ls more common to connect to thecirculatory system with either a catheter or ahypodermic needle and to record the pressure with apressure transducer attached to the catheter orneedle.

94

ca rd iaccatheter

PRESSURETRANSDUCER

CONNECTS HERE

OPENING

HOLLOW TUBE5mm OUTER DIA

Fig.8-1. A 5mm cardiac catheter.

FILLING AND FLUSHINGSYSTEM MAINTAINS SODIUMCHLOR 1DE SOlUT ION 1N .----------,\:HETER. RECORDING

INSTRUMENT(OSCilLOSCOPE)

PRESSURE TRANSDUCER

CATHETER INSERTEDINTO BRACHIAL ARTERY

Fig, 8-2. Direct blood pressure measurement.

A cardiac catheter consists of a rubber, teflon orpolyethylene tube with one end formed to a smoothbu11et-like shape to a110w its easy introduction intoveins and arteri.es. A sma11 opening Ls fo rtaed about1 centime ter from this smooth end as shown inFig. 8-1. The catheter shown in Fig. 8-1 is·approximately 40 centimeters long and 5 millimetersin diameter. Catheters are generally availablefrom 1 to 10 millimeters in diameter and in variouslengths. Cardiac catheters are also often describedby their cLrcumfe.rence ,

entry tocirculatorysystem

card iaccatheterization

arter ialpressure

transducer

95

With the catheter/pressure-transducer configurationshown in Fig. 8-2, pressures can be measured inalmost any portion of the circulatory system. Bygaining entry to the circulatory system at either thearms or legs a catheter may be manipulated throughoutthe circulatory system. It should be noted that thepressure measured will be the pressure at the bluntend of the catheter and not the pressure at the pointwhere the catheter enters the circulatory system.

It is possible, with careful manipulation, to actuallyintroduce the smooth end of the catheter into theright atrium via the inferior vena cava. Cardiacdefects, including holes in the heart, can then beexplored. This procedure is referred to as cardiaccatheterization. Investigation of the leftventricule by catheterization via the aorta, referredto as arterial catheterization, almost always causestrouble with arterial damage and leakage into thetissues.

The most common direct pressure measurement isarterial pressure which is usually measured byintroducing the catheter into the brachial artery atthe elbow of either arme

A typical blood pressure transducer for use with acatheter is shown in Fig. 8-3. As little differencein pressure is encountered throughout the arterialsystem, it is unnecessary to introduce the catheterany great distance into this circulatory system.

STATHAM INSTRUMENTS INC. - MODEl P23DelENGTH - 2.5"OUTPUT WITH 7.5VRMS EXCITATION = 75~VRMS/mmHg

FOR USE WITH CATHETERS.

Fig. 8-3. A conventional blood pressure transducer.

96

,TRANSDUCER NEEDLE

STATHAM INSTRUMENTS INC. - MODEL P37TRANSDUCER LENGTH = 1.2"OUTPUT WITH 7.5VRMS EXCITATION = 37~VRMS/mmHg

TRANSDUCER CONNECTED AT BRACHIALARTERY PUNCTURE SITE.

Fig.84. A needle blood pressure transducer.

STATHAM INSTRUMENTS INC. - MODEL SP25OVERALL LENGTH := 9"OUTPUT WITH 7.5VRMS EXCITATION = 37~VRMS/mmHg

Fig.8-5. A syringe blood pressure transducer.

needle/transducer

dri ftcatheters

catheter tiptransducer

97

The catheter/transducer combination may be replacedby a needle and transducer as shown in Fig. 8-4.The needle/transducer combination is also availablein a syringe configuration as shown in Fig. 8-5.These combinations, using needles rather thanflexible catheters, are obviously far easier tointroduce into the subject's circulatory system andare thus preferable for direct blood pressuremeasurement. They are normally introduced into thefemoral artery near the groin as this artery isclose to the surface at this point. The syringeconfiguration allows administration of an arterialinjection while recording arterial pressure.

For measurement of blood pressure within the heart,very small diameter (1 millimeter) catheters, knownas drift catheters, are required so as not tointerfere with the normal operation of the heart.These small catheters are inserted into the venoussystem and "drift" with the blood flow, eventuallyreaching the heart. They may actually be passedthrough the heart into the pulmonary circulatorysystem. The primary disadvantage of these smallcatheters is the damping effect they have on thepressure pulse waveform which limits the highfrequency response of the system and gives erroneousreadings. To avoid this problem, the catheter tiptransducer shown in Fig. 8-6 incorporates a smallpressure transducer built into the end of thecatheter, the combination being less than 2millimeter in diameter. This allows introduction ofthe actual transducer into the heart and thus avoidsthe damping problem inherent with small catheters.

STATHAM INSTRUMENTS ING. - MODEl P866TRANSDUCER TIP - .065" DIAMETEROUTPUT WITH 7.5VRMS EXCITATION = 5~VRMS/mmHg

Fig. 8-6. A catheter tip pressure transducer.

98

bridge-typetransducers

microelectrodepressuretransducer

measurementterminology

Recording from transducers is covered in more detailin Chapter 18. The transducers shown in thischapter are aIl bridge-type transducers and, whenexcited at their maximum rated excitation voltage,they produce an output in the low millivolt ormicrovolt region, the smaller transducers in generalproducing less output. While it is possible to useDC excitation and record the output directly usinga high sensitivity amplifier, superior results areobtained using a carrier amplifier system such asthe Tektronix 3C66 plug-in.

Extremely small direct blood pressure "transducers"have recently been developed for research work.These transducers have an effective diameter of 10microns. The transducer consists of a 10 micronmicroelectrode filled with an electrolyte having adifferent resistivity than that of blood or bodyfluids. A pressure system is connected to themicroelectrode and the impedance between the centerof this electrode and the body is monitored. Aservo control system contraIs the pressure in themicroelectrode driving pressure system to keep theimpedance between the electrode and the body constantby exerting a pressure on the electrolyte in themicroelectrode equal to the pressure external to themicroelectrode tip. This pressure exerted on theelectrolyte is therefore a measure of the bloodpressure at the tip of the microelectrode.

Arterial pressures are normally measured asmillimeters of mercury, mm/Hg, and venous pressuresare usually measured as centimeters of water, cm/H20.These units are derived from older blood pressuremeasuring techniques using either mercury or watermanometers. A typical arterial blood pressurewaveform is shown in Fig. 8-7. The peak pressure isreferred to as the systolic pressure and the minimumpressure is referred to as the diastolic pressure.

99

It is common termino10gy to refer to an arteria1b100d pressure of, say, 120 mi11imeter systo1ic and80 mi11imeter diasto1ic as "120 over 80" or 120/80.As stated in Chapter 2, the pressure in the arteriesis many times greater than the pressure in the veins,typica1 arteria1 pressure being 120/80 and typica1venous pressure being 9/5 mm/Hg or, in the morecommon termino10gy, 12/7 cm/H20.

Whi1e direct b100d pressure is re1ative1y easy tomeasure, it does require an incision in an arteryfor introduction of a catheter or insertion of ahypodermic need1e. Since this often resu1ts in thepermanent 10ss of the artery used, it is regardedwith sorne disfavor in most countries except theU.S.A. Resu1ts accurate enough for c1inica1 useare usua11y obtainab1e by indirect methods.

SYSTOL lePRESSURE131mmHg

=-~ - -- ;;;--iiil ~--

!!!!.:;;; --- i.---

••

11 fJ ilIl~ 1I~u

1••••Iiiii ..,:=I-I a

rJ-- ~\, •JI Il

VERTICAL 10mmHg/cmHORIZONTAL .2s/cm

100mmHgDIASTOLICPRESSURE

82mmHg

• • • 1.1 ;.DICROTIC NOTCHREPRESENTS A REFLECTED PULSEFROM THE FAR END OF THE TAPERINGGREAT ARTERY SYSTEM.

Fig. 8-7. Arterial blood pressure waveform .

100

SPHYGMOMANO ETER-- ./"..... ---PNEUMATIC PRESSURE RELEASE HANDCUFF GAGE VALVE PUMP

~ J

STETHOSCOPE PLACEDAT THE ELBOW OVERTHE BRACHIAL ARTERY

Fig.8-8. Indirect blood pressure measurement with a sphygmomanometer.

101

8. 2 INDIRECT BLOOD PRESSUREMEASUREMENT

sphygmomanometer

Korotkoffsounds

By far the most common forroof blood pressuremeasurement is the indirect measurement using thefamiliar pressure cuff, hand pump and pressure dialdevice, used by aIl physicians, referred to as asphygmomanometer. The sphygmomanometer, as shownin Fig. 8-8, incorporates a pneumatic cuffencircling the upper arm. An inflatable section ofthis cuff is inflated by a small hand pump and thepressure in the system is indicated by a mechanicalpressure gauge or, in sornemodels, a mercurymanometer. The cuff is inflated to a pressuregreater than the blood pressure in the largebrachial artery of the arm. This pressure thuscollapses the artery and occludes (cuts off) bloodflow to the arm. As the pressure in the cuff isgradually released using a release valve built intothe hand pump, a point is reached where the cuffpressure and the peak or systolic arterial pressureare the same. At a pressure slightly below thislevel the peak arterial pressure slightly exceedsthe cuff pressure and blood is able to squirtthrough the compressed segment of the brachialartery. This squirting blood results in turbulencewithin the artery creating sounds known as"Korotkoff" sounds. These sounds are usuallydetected with a stethoscope placed over the brachialartery. As the pressure in the cuff is furtherdecreased, Korotkoff sounds continue until a pointis reached where no further turbulence is producedas no constriction exists in the brachial artery.This point represents the diastolic blood pressure.As it is somewhat difficult to detect the pressurewhere the Korotkoff sounds begin and cease, thissphygmomanometer technique cannot be relied upon toproduce an accuracy of much better than about10 millimeters of mercury. While the technique isinaccurate, it is simple to perform and very littlediscomfort is felt by the patient. ln the hands ofa skilled operator highly repeatable results areobtained and, since the clinician is usually moreinterested in trends than exact numbers, thetechnique is entirely appropriate.

102

automatiemeasuringequipment

using anose i 1loseope

PUMP MAY BE OPERATED FOR A SINGLE CYCLEOR MAY BE PROGRAMMED TO CYCLE AT PRESETINTERVALS.

PIEZOELECTRICCRYSTAL PRQVIDES

HIGH OUTPUT LEVELS

\A CONTACT CLOSURE DURING INFLATION CANBE USED TO ERASE A STORAGE OSCILLOSCOPE.

E & M INSTRUMENT CO. INC. 92-201-70

E & M INSTRUMENT CO. INC. 95-300-70

Fig.8-9. An automaticcuffpump.

Fig.8-10. A Korotkoffsounds, microphone.

The sphygmomanometer technique may be automated byreplacing the hand pump with an automatic cuff pumpas shown in Fig. 8-9. The automatic cuff pump maybe operated by pushing a panel-mounted button toproduce a single cycle of inflation and deflationor it may be set for repeat cycles at variousintervals for continuous monitoring of blood pressureover long periods of time. The stethoscope may bereplaced by a Korotkoff-sound microphone as shownin Fig. 8-10. This microphone consists of a smallpiezoelectric transducer specifically designed toefficiently reproduce Korotkoff sounds. Thepressure indicating dial may also be replaced witha pressure transducer similar to the transducersused for direct pressure measurement.

The subject's indirect blood pressure may beconveniently recorded using the system shown inFig. 8-11. The vertical channel of an oscilloscopedisplays the output from a Korotkoff sound microphonewhile the horizontal channel displays the outputfrom a pressure transducer. A typical displayproduced by this system is also shown in Fig. 8-11.The horizontal axis is calibrated in pressure; thepoints at which vertical information appears andthen disappears are the systolic and diastolicpressures. This system is capable of somewhatgreater accuracy than the conventional cuff/stethoscope as it removes human judgment indetermining the presence of Korotkoff sounds. lnsornecases clearer displays have been obtained byrejecting aIl information from the microphone below150 Hz using a high-pass filter between themicrophone and the oscilloscope vertical channel.The p'rimefrequency content of this informationappears to be between 400 Hz and 500 Hz.

103

\\ -,"<, , e-,,.

,"',\ -\

// /

/ //' 1

/' /.... / /....- ,/"/ (/

PNEUMATICCUFF

5648 STORAGE SCOPE

VERT HORIZ

2A63 3C66VERTICAL AMP CARRI ER AMP

AC 50mV/OIV 10mmHg/OIV

200-- ~STRAIN/OIV -+-T

PRESSURETRANSOUCER

'- -

\.. - - -- - -- - - - - - - - --,).' . • 143mmHg

CRTOISPLAY

• :' --,=.-=.._-:==- --III' ---- ----- :===

,1:1111

'IUI,I!1

m•••• 1HUll UI u

t50mV/OIV

AC COUPLEO

60 100

•• 10mmHg/OIVHORIZONTAL - PRESSURE

160

-""

11

1111111111

1

1

-----------.)REMOTE ERASE FROM CONTACTCLOSURE WITHIN THE CUFF PUMP

EITHERAUTOMOTIVECUFF PUMP

OR

HANO PUMP WITHRELEASE VALVE

VERTICAL - OUTPUT FROMKOROTKOFF SOUNOS MICROPHONE

Fig.8-11. Automatic indirect blood pressure measurement.

104

8. 3 INDIRECT RELATIVE BLOOD PRESSURE MEA.SUREMENT

plethysmography

PulseSensor

TektronixPulse Sensor

ln many cases, it is unnecessary to measure theabsolute value of arterial blood pressure; aIl thatis required is an indication of blood flow throughoutthe body. If blood flow throughout the bodydiminishes for sornereason, the principal areas ofthe body deprived will be the fingers and the toes,thus monitoring the presence of blood flow in theseareas will insure that blood is indeed flowingthroughout the body. The simplest technique ofrecording the presence of blood flow in theperipheral arteries is to use a plethysmograph.Plethysmography is the art of monitoring thephysical changes in size of part of the body asmodified by the flow of blood within it. Varioustechniques are used to detect this change in size.The Pulse Sensor utilizes a photoelectric technique.Other techniques, such as impedance measuring, arealso used to indicate relative change in size.

The Pulse Sensor uses a light source andphotodetector to record a change in opacity of theflesh as blood is purnpedthrough it. This PulseSensor relies on light being reflected by sornereflecting medium such as bone, etc. For adequateoutput potentials td be obtained, a fairly highconcentration of arteries near the surface isrequired. The Pulse Sensor produced by Tektronixfor use with the Type 410 Physiological Monitor isideally suited for application to the finger and isless suitable for toe, ear or nose use. If thesubject is in a drugged state, or if he isparticularly cold, then vasoconstriction will occurwhich tends to limit blood flow to the limbs. Underthese conditions it may be preferable to tape thePulse Sensor to the forehead of the subject. It isimperative that the Tektronix Pulse Sensor be usedwith resting subjects and, as it detects changesin physical size, it is definitely not suited toexercising subjects. The Tektronix Pulse Sensor isshown in Fig. 8-12, and is applied to the fingerand forehead as shown in Fig. 8-13. A typicalplethysmogram recorded with this Pulse Sensor andthe Tektronix Type 410 Physiological Monitor isshown in Fig. 8-14.

PHOTORESISTOR

LOW POWERL 1GHT SOURCE

RESISTOR AND LIGHT SOURCEARE SEEN THROUGH "WINDOW"lN THE PULSE SENSOR

105

L 1 GHT FROM THESULS IS REFLECTEDSY SONE TO THEPHOTO RESISTORTO

••AMPLIF 1 ER11--_+-__+-(

VOLTAGEREGULATOR

Fig. 8-12. The Tektronix plethysmograph (pulse sensor).

PLETHYSMOGRAPH HELD lNPLACE BY TAPE OR BY ANELASTIC STRAP AROUNDTHE HEAD.

PLETHYSMOGRAPH HELD lN PLACEBY THE TEKTRONIX FINGER HOLDER.

Fig. 8-13. Placement of the Tektronix plethysmograph.

RATE 78/MIN

VERT 1 CAL 2mm/mVHORIZONTAL 50mm/s

---'DICRQTICNOTCH

410 TRIGGER CIRCUIT TRIGGERS OISPLAY.

Fig.8-14. Plethysmogram obtained with Tektronix Pulse Sensor(attached to finger) and 410 Monitor.

106

564B STORAGE SCOPE

VERT HORIZ

3C66 2B67CARRIER AMP TIME BASE

.05lJF 100lJSTRAIN/DIV 0.5s/DIV

(2 EXTERNALR .05lJF ARMS)

RAND C BALANCE THE SUBJECT'S RAND CLOAD BETWEEN THE ELECTRODES.

VERTICAL 100lJSTRAIN/DIVHORIZONTAL 0.5s/DIV

SOME 8ASELINE SHIFT DUETO IMPEDANCE VARIATIONSWITH RESPIRATION. THE.05lJFCAPACITORS SHOWNABQVE HELP TO MINIMIZETHIS SHIFT.

PERIOD .85sRATE 71/MIN

Fig. 8-15. Impedance plethysmography with Tektronix 3C66 Plug-In.

impedancemeasuring

107

As stated above, impedance measuring techniquesmay also be used to indicate relative changes insize resulting from blood flow. If the impedancebetween the arms is monitored, it will be foundthat the impedance changes due to the action ofthe heart and due to respiration. Extensive studieshave shown that the sensation level of arm to armelectrical current increases with frequency: sornesensation may be felt with 1 mA of DC currenthowever, at about 25 kHz, ten times this current isrequired to produce sensation. Thus audio frequencycurrents of at least an order of magnitude less than1 mA are used to monitor impedance. Thesefrequencies and low currents also avoid ceIlstimulation. It has been found that when measuringthe impedance between the arms, cardiac actionprimarily changes the resistive component of thisimpedance and respiratory action primarily changesthe capacitive components. Thus, to detect cardiacaction, one must use an audio frequency systemsensitive only to resistive changes, to avoidrespiratory effects.

Such a system can be incorporated using theTektronix Type 3C66 Carrier Amplifier with a storageoscilloscope and time base as shown in Fig. 8-15.With this system capacitive impedance changes arealmost completely eliminated by paralleling theimpedance to be measured with a large fixedcapacitor. A similar capacitor is required tobalance the bridge, as is a balancing Rand C tosimulate the Rand C of the subject. The 3C66 isoperated in a Wheatstone Bridge configuration withtwo of the arms of the Wheatstone Bridge externalto the instrument, the other two internaI. Atypical impedance plethysmograph recorded with theabove technique is also shown in Fig. 8-15. Theslight change in base line evident in this recordingis due to the influence of respiration. Furtherdetails on impedance measuring techniques are givenin Chapters 9, 13 and 18.

108

FLOWMETER 564B SCOPE

O v = MEAN FLOW VELOCITY(METERS/SECOND)

B = FLUX OENSITY OFMAGNET 1C FIELD

[J["CEP 1 (WEBERSISQU", MElERS)2867 Z = SPACING 9ETWEEN

SENSING ELECTROOES(METERS)

Vf = VOLTAGE BETWEEN\;;::;;;=====0 ELECTRODES

(VOLTS)

F = FLOW(CUBIC METERS/SECOND)

a = INTERNAL CROSS SECTIONALAREA OF BLOOD VESSEL(SQUARE METERS)

:Z IS CONSTANT FOR A SYSTEM.

oRANGEo

ZERO

Vf = si»

F = vaMAGNETCURRENT

:. F •• Vf

ELECTROMAGNET

TYPICAL VALUES: 8 = .02Z = .01

a = .05 x 10-3

Vf = 20~V per cm/s VElOCITY

Vf = 66~V per 100 cc/min FLOW

•••

SIGNALSENSINGELECTRODES

BLOOD VESSEL

Fig. 8-16. Electromagnetic flowmeter - theory.

8.4 BLOOD FLOWMEASUREMENT

electromagneticvelocitytransducer

Blood flow is measured by placing. a mean velocitytransducer in an artery having a known crosssectional area. Blood flow is the product of themean velocity and area. Many types of meanvelocity-sensitive blood flowmeters have beendeveloped; the electromagnetic blood flowmeter isnow extensively used. A diagrammatic representationof an electromagnetic blood flowmeter is shown inFig. 8-16. The theory of electromagnetic flowmetersis based on Faraday's law. When a conductive fluid,such as blood, traverses the lines of force of amagnetic field, an electromotive force is generatedin the fluid which is perpendicular to both themagnetic lines of force and the direction of motionof the fluid. This electromotive force is directlyproportional to the intensity of t~e magnetic field,the distance between the sensing electrodes and thefluid velocity.

flow-ratemeasurements

flowmeter

ACexcitation

sinewave

squarewave

signal-tonoise ratio

powerrequirements

frequencies

109

The electromagnetic blood flow transducer consistsof an electromagnet to generate a magnetic fieldand two electrodes to sense the flow signal. Theyare encapsulated in epoxy in a form to allow themto fit around the blood vessel. The lumen or insidediameter fixes the cross-sectional area of thevessel, changing the transducer to a flow-ratemeasuring instrument although basically it is avelocity transducer. Electrodes make contact withthe vessel wall. The flow transducer is connectedto a flowmeter. This flowmeter supplies energizingcurrent to the electromagnet, amplifies the flowsignal, discriminates it from artifacts and makesit available for display on an oscilloscope.

The above theory assumes a DC magnetic field whichwould produce a DC flow signal. Since it would beimpossible to differentiate this signal fromelectrode offset potentials, amplifier drift, etc.,commercial blood flowmeters use AC excitation;either sinewave or squarewave. When revising the

above theory for AC signaIs a ~~ component appears

in the output voltage formula. Commercial systemsmeasure the output voltage only during the period

h h· 6B . 1 l'w en t lS 6t component lS equa to zero. n Slnewave

6Bexcitation, the 6t component is effectively zero only

for a relatively short time at the peak of theexcitation wave. The voltage sensing circuit mustgate on during this time and correct adjustment ofthe gate is critical. ln the squarewave excitation,

6Bthe 6t component is always zero except during short

switching periods, thus larger variations in gatecharacteristics are permissible.

Signal-to-noise ratio is proportional to the peak-topeak value of the excitation voltage. For sinewaveand squarewave excitation of the same peak-to-peakvoltage, the power required for sinewave excitationis only 50% of the power required for squarewaveexcitation. The chief disadvantage of squarewaveprobes is, therefore, that they are larger and mustoperate at higher temperatures than sinewave probesto maintain an acceptable signal-to-noise ratio.Both systems require excitation frequencies highenough to permit effective sampling, typically from200 Hz to 1000 Hz.

110

commercialflow probes

commercialflowmeters

Sornecommercial electromagnetic blood flowmeters areshown in Fig. 8-17. The flowmeters are of theintracorporeal type which have a slot opening toallow placement around the blood vessel. Theseopenings are usually fitted with a cover to maintaina constant blood vessel size. Extracorporealtransducers have tubular sleeve or cannula extensionsand are applied by cutting the blood vessel andinserting the tubular sleeve in series with thisvessel. Blood is in direct contact with the sleevelumen in extracorporeal units.

Blood flowmeters suitable for use with the abovetransducers are produced by several manufacturers.ln general, these instruments provide either sinewaveor squarewave excitation at between .1 and 1 amp andprovide an output for use with an oscilloscope orother recording device of between .1 volt and 1 volt.This recording device should have a response from Deto 100 Hz. The instruments are calibrated to readin cubic centimeters per minute and usually cover arange from 1 to 100,000 cubic centimeters per minute.Blood velocity in the pulmonary artery may reach100 centimeters per second. As this artery has across-sectional area of about 1.8 square centimeters,this veloci.tyresults in a peak flow of 100,000 cubiccentimeters per minute. Blood flow systems may becalibrated by allowing blood to escape into agraduated flask for a known brief period or by usingan external calibration system providing a knownflow. A zero flow baseline may be obtained byoccluding flow for a brief periode A typical bloodflow waveform is shown in Fig. 8-18, and a completemeasuring system is shown in Fig. 8-19.

111

9mmLUMEN D 1A'METER

5mm LUMEN~ ~DIAMETER

MOOEL FT-S FT-1 SER 1ES 1NTROCORPOREAL FT-1C SERIES EXTRACORPOREAL

Fig, 8-17. Commercial blood flow meters, (ln Vivo Metric Systems,Los Angeles, Calif.)

VERTICAL 1000cm3/MIN/DIVHORIZONTAL O.5s/DrV

5000cm3/MIN

1000cm3/MIN

.95 PERIODRATE 67/MIN

Hg. 8-18. A typical blood flow record from an electromagnetic flowmeter.

Hg. 8-~9. A commercial blood flow measuring system,

112

other flowmeasuringdevices

Other forms of blood f1ow-measuring devices areoccasiona11y used. The isotherma1 f10wmeter placesa thermistor within the b100d f1ow. The flowingb100d tends to cool the thermistor and a measurementof blood flows is obtained by recording the increasein thermistor excitation required to maintain thethermistor at a constant temperature (constantresistance). The e1ectroturbinometer inserts aminute rotational generator into the b100d f10wsystem; this generator is driven by a sma11prope11er driven by the b100d f1ow. U1trasonicblood f10w measuring techniques are becomingincreasing1y popular; these techniques invo1ve thedetecting of smal1 phase differences betweenu1trasonic signa1s propagated in the direction off10w and in the opposite direction.

8 •5 CARDIAC OUTPUT

B100d f10w may be measured in a1most any part of thecircu1atory system by using the blood flowmeasurement techniques discussed previously. When

direct blood flow is measured in the pulmonary artery ordetermination the aorta, this blood flow, integrated over a cardiac

cycle, represents the total amount of b100d f10wingthrough the heart and is referred to as cardiacoutput. Thus cardiac output may be determined byintegrating the results obtained from conventionalb100d flow measurement at the pu1monary artery oraorta. As this obvious1y invo1ves critica1 surgicalprocedures, cardiac output is more common1y

di lution determined using dilution techniques as discussedtechniques brief1y be1ow. These dilution techniques give total

cardiac output whereas flow measurement shows thediscrete changes in cardiac output over one completecardiac cycle.

The Stewart-Hamilton dye dilution technique involvesintroduction of a known volume of dye into thesuperior vena cava via vesse1s in the arms andwithdrawal of samples of the blood/dye mixture fromthe aorta via a catheter. The dilution of dyecontained in these samples is determined with adensitometer. Modern dye dilution techniques useradio isotopes instead of dye and counters instead ofdensitometers. The output from the densitometer orcounter is coupled to an analog computer whichcomputes cardiac output direct1y. Cardiac output inan adult male is approximate1y 5000 cubic centimetersper minute.

113

8.6 BLOOD VOLUME

Blood volume is measured using a modified dyedilution technique. A known amount of dye isintroduced into the system and allowed to circulatefor many cycles. After several minutes, a bloodsample is taken and the dilution of the dye in thissample is noted. Total blood volume is then aproduct of the dilution ratio and the originalquantity of dye injected. Blood volume in an adultmale is approximately 5500 cubic centimeters or oneand one-half U.S. gallons.

114

TRACHEA

LEFTLUNG

PULMONARYVEINS

PULMONARYARTERIES

Fig.9-1. Respiratory system.

115

RESPIRATION AND TEMPERATURE

9.1 PHYSIOLOGICAL CONSIDERATIONS

respiratoryfunctions

lungstructure

The physiology of the respiratory system is notcovered in Section I; thus the more importantphysiological aspects of this system are discussedbelow. The primary functions of the respiratorysystem are to oxygenate the blood, i.e., to dissolveoxygen into the blood, and to remove carbon dioxidefrom the blood. If blood is not oxygenated due tofailure of the circulatory system, the oxygen contentof the blood will rapidly decrease and after about60 to 90 seconds the subject will become unconscious,death occurring in 4 to 5 minutes.

The lungs are the major component of the respiratorysystem and it is in the lungs that oxygenation of theblood occurs. When the lungs are forced to expandby muscular contraction of the diaphragm andexpansion of the thoracic cage by contraction of therib muscles, air enters the lungs via the bronchiand is diverted to millions of small air sacs knownas alveoli. The membrane comprising the alveoli ismoist and the oxygen contained in the air isdissolved by this moisture. Interspersed with thealveoli are fine capillaries, branching from thecirculatory system, through which blood iscontinually flowing. The oxygen dissolved in themoist surface of the alveoli diffuses into the bloodstream via these capillaries. The carbon dioxidecontained in the blood stream is also diffusedthrough the alveolar membrane to be expelled withexpired air. Some of the principal components ofthe respiratory system are shown in Fig. 9-1.

116

6j TOTAL LUNGCAPACITY

5

INSPIRATORYINSPIRATORY VITAL RESERVECAPACITY CAPACITY VOLUMEt 4

LUNGVOLUME 3 TIDALLITERS VOLUMEOR103cm3 EXPIRATORY

2 RESERVEVOLUME

FUNCTIONALRESIDUALCAPACITY RESIDUAL

VOLUME

o 10 20 30 40 50

SECONDS ____..

NORMALBREATHING

MAXIMUMINSPIRATION

MAXIMUMEXPIRATION

Fig. 9-2. Respiratory changes in lung volume.

lungcapacity

lungvo 1ume

spirometer

pneumograph

117

A grown man at rest inhales about 500 cubiccentimeters of air with each breath and takes between12 and 18 breaths per minute. During periods ofmoderate exercise he may inhale one liter or morewith each breath and take up to 25 breaths perminute. At maximum exercise the vital capacity of4 to 5 liters is reached.

The action of breathing is control1ed by muscularaction causing the volume of the lung to increaseand decrease. During normal breathing the lung doesnot contract to its minimum possible volume, nordoes it expand ta its maximum possible volume.Minimum lung volume is obtained when exhaling withextreme effort to expel the maximum possible amountof air and maximum lung volume is obtained wheninha1ing with maximum effort. Fig. 9-2 showsdiagrammatica11y the changing lung volume that maybe expected for a resting man and the lung volumethat may be achieved during maximum inspiratory andexpiratory effort.

Lung volume is measured by a spirometer and therecording of lung volume changes with time is knownas a spirogram. Instruments that simply detectrespiratory activity are referred to as pneumographsand the resulting recording of respiratory activitychanges with time is known as a pneumogram. Aspirogram is normally on1y required when attemptingto analyze the respiratory system or to detect amalfunction of this system and is rarely used forroutine monitoring. A pneumogram may be used forroutine monitoring and is basically used to indicatethe fact that the subject is breathing. Breathingrate may be obtained from either a spirogram or apneumogram.

118

9.2 RESPlRATORY ACTIVITY

thermistorpneumograph

temperatureconsiderations

Relative respiratory activity may be detected byeither detecting the physical changes in the torsoassociated with breathing or by detecting the flowof air through the nostrils. Since no absolutemeasurements are required, the measurement techniquesinvolved have been simplified to allow easyapplication of the devices concerned to the subject.

Perhaps the simplest form of pneumograph employs athermistor placed in the outer nasal passage todetect the temperature differential between inspiredcool air and expired warm air. This techniquefulfills the majority of clinical needs includingthose of operative and post-operative subjects. Ifthe subject breaths through his mouth, or if hewishes to converse, the thermistor may be placed inthe mouth or in such a position as to detect flowfrom either the nose or the mouth. The thermistorconcerned should be supplied from a constant currentsource at a low enough current to maintainthermistor self-heating below one degree centigradeor so. Adequate sensitivity can usually be obtainedwith 5 milliwatts of thermistor dissipation.Excessive thermistor heating can cause subjectdiscomfort, thus thermistor dissipation should belimited to 40 milliwatts for small bead-typethermistors. A thermistor pneumograph suitable foruse with a Tektronix 410 monitor, together with atypical pneumogram obtained from this system, isshown in Fig. 9-3. Further details are given inChapter 29.

If the temperature of the outside air is the sameas the temperature of the expired air (bodytemperature) the above system is unsatisfactory. lnthis case, enough current should be passed throughthe thermistor concerned to raise its temperaturein still air to a temperature somewhat above bodytemperature but below a temperature that may causesubject discomfort. This can usually be achievedwith thermistor dissipations of between 5 and 25milliwatts. The flow of both inspired and expiredair over this thermistor will tend to cool it, thusproducing a resistance change. Since bothinspiration and expiration decreases the thermistortemperature, the resulting output will be at twicethe respiratory frequency.

119

410 MONITOR WITHSTANDARD PATIENT CABLE

o(

THERMISTORlN HOLDER

CONSTANTCURRENT SOURCE

THERMISTOR PNEUMOGRAPH (FOR DETAILS SEE CHAPTER 29)

RATE 4~ = 22 BREATHS/MIN

VERTICAL AUX 2mm/mV = 5mV/DIVHORIZONTAL 25 mm/s =.4s/DIV

Fig.9-3. Therrnistor pneurnograrn with the Tektronix 410 Monitor.

120

ELASTICFORCE GAGE

f\

564B STORAGE SCOPE

DVERT HORIZ

3C66CARRI ER AMP

50IlSTRAI /OIV

2B67TIME BASE

25/01V

(1 EXTERNALARM)

LEATHER STRAP

~ELASTIC STRAP

12Dn STRAIN GAGE(TYPE SR-4 B.L.H. ELECTRONICS

WALTHAM, MASS.)

TO CARR 1ER AMPPERIOD '"65

10 BREATHS/M 1N

VERTICAL 50IlSTRAIN/OIVHORIZONTAL 2s/01V

Fig. 9-4. Elastic force gage pneumogram with a Tektronix 3C66 Plug-In,

strain gaugedetectschest sizechanges

resistancechanges withchestcircumference

121

Changes in the physica1 size of the torso withrespiration may also be detected to indicaterespiratory activity. A strain gauge attached to apiece of e1astic may be used to form a band aroundthe chest. Since chest circumference varies withthe respiration, the e1astic band will stretchcausing a change in resistance in the strain gauge.This resistance change may be detected by aDC-excited Wheatstone Bridge or, preferably, by acarrier amplifier such as the Tektronix 3C66 system.Such a system and the resulting pneumogram obtainedis shown in Fig. 9-4.

Changes in chest circumference may also be detectedby a rubber tube filled with mercury, fastenedfirmly around the chest. As the chest expands, therubber tube increases in 1ength and thus theresistance of the mercury from one end of this tubeto the other changes. This resistance change maybe detected using a constant-current source in thesame way as the thermistor resistance change wasdetected in Fig. 9-3. The principal disadvantageof this mercury-fi11ed rubber tube pneumograph isits extreme1y low resistance, requiring largecurrents and sensitive detecting instruments. Thesedisadvantages can be 1argely overcome by replacingthe mercury with a conductive solution, such ascopper su1phate solution (with copper plugs in theends of the rubber tube) or with sorneof the 1essviscous types of e1ectrode paste cornmonlyused toapply electrodes to the skin. Commercial electrodepastes vary great1y in resistivity; however, a4S-inch long 1/8 inch diameter rubber tube filledwith e1ectrode paste shou1d have a resistance ofbetween 1,000 and 100,000 ohms.

122

torsoresistivity

(2 EXTERNALARMS)

5646 STORAGE SCOPE

DVERT HORIZ

3C66CARRI ER AMP

50\lSTRAIN/DIV

2667TIME 6ASE

2s/DIV220

R 220

RAD C BALANCE THE PATIENT'S RAND CLOAD 6ETWEEN THE ELECTRODES. (SEE APPENDIX.)

VERTICAL 50\lSTRAIN/DIVHORIZONTAL 2s/DIV

IMPEDANCE VARIATIONS WITHCARDIAC ACTION CAUSESMODULATION OF THE RESPIRATORYWAVEFORM. THE 22n RESISTORSSHOWN ABOVE HELP TO MINIMIZETHIS MODULATION.

1" ~IPERIOD '"5s

12 BREATHS/MIN

Fig.9-5. Impedance pneumogram with a Tektronix 3C66 Plug-in.

Respiratory activity may also be detected bymeasuring the change in resistivity across the torso.This technique is similar to the technique used tomeasure cardiac activity as described in Chapter 8,Section 8.14. However, whereas the resistivecomponents of this impedance are detected to measurecardiac activity, the capacitive component of thisimpedance is detected to indicate respiratoryactivity. A typical impedance pneumograph systemtogether with a typical impedance pneumogramobtained from this system is shown in Fig. 9-5.There appears to be little practical use for thissystem as the variations produced by the heart inthe record prevent a good recording from beingobtained.

9.3 RESPIRATORY AIR FLOW

pneumotach

Respiratory flow is invariably measured with apneumotachograph (commonly referred to as apneumotach), consisting of a hydraulic resistancehead, and a differential pressure transducer. Thepneumotach consists of a one-inch diameter tubecontaining a fine mesh screen, as shown in Fig. 9-6.

measurementsystemca 1 i b rat ion

123

This mesh screen offers slight resistance to airflow; this resistance to flow produces a pressuredifferential across the mesh screen which isproportional to the mean flow velocity. Thispressure differential is detected by a differentialpressure transducer. The sensitivity of the devicecan be varied by varying the size of the mesh screenor by using several mesh screens, however totalairway resistance offered to a subject should neverexceed about 1 cmH20 if normal respiration is not tobe affected by the measuring device. A typicalpressure differential across the screen would be0.09 cmH20 per 10 liters per minute flow using 2inch diameter, 400 per inch, stainless steel gauze inthe head.

The differential pressure transducer can be used inconjunction with a Tektronix Type 3C66 CarrierAmplifier in a Tektronix Type 564B StorageOscilloscope. The complete respiratory flowmeasuring system, i.e., pneumotach, differentialpressure transducer, 3C66 carrier amplifier andoscilloscope, may be calibrated with a known flow.

TRANSDUCER EXCITATIONAND SIG AL OUTPUT TO3C66 CARRIER AMPLIFIERPLUG-I

DIFFERENTIALPRESSURE TRANSDUCER

THE TRANSDUCER MUST BE MOUNTEDABOVE .THEHYDRAULIC RES 1STANCEHEAD TO PREVENT CO DE SEO MOISTURE FROM THE EXPIRED AIRENTERING THE TRA SDUCER.

SECTION VIEWOF HYDRAUL ICRESISTANCE HEAD

TO SUBJECT NOSEPIECE VIA LARGEDIAMETER TUBE

~ AIR INLET

MESH SCREEN TO OFFERSLIGHT RESISTANCE TOAIR FLOW

MAXIMUMINSPIRATORY FLOW

laLiTERS/MIN

VERTICALIOO~STRAIN/DIV = IOLITERS/MIN/DIV*

ZERO FLOWBASELINE" HORIZONTAL 2s/DIV

MAXIMUMEXPIRATORY FLOW

1OL ITERS/MIN*PRECALIBRATED WITH KNOWN FLOW.ZERO FLOW BASELINE OBTAINED BYREMOVING PNEUMOTACH FROM SUBJECT.

Fig. 9-6. Pneumotach air flow transducer.

FROM5648 CAL 18RATQR

" 13mAr--- -- --,

IMn

10kn

10kn

l\lFL __ - _ _j

PRESSURETRANSDUCER lMn-=-

5648 STORAGE SCOPE

PNEUMQTACH [] CAL OUT40V DC

VERT HORIZ

3A8 2867OPERAT10NAL TI E BASE

AMPSIGNAL

7

>--+---- FLOW

>-___..RESP1RATORYVOLUME

EXCITATION

IMn

lknFROM

TRANSDUCER FLOW(INVERTED,lkn STORAGE DISPLAY SHOWING

90TH FLOW AND LUNG VOLUME

VERTICAL (j)FLOW (REFER TO FIG.6l10LITERS/MIN/DIV

HORIZONTAL 25/DIV

VERTICAL (Î)LUNG TIDAL VOLUMEWAVEFORM 1 INTEGRATEDO.2L1TERS/D 1V(PRECALI8RATéD WITHKNOWN FLOW FOR 55. l

>-----i~De LEVE LONE MINUTE AFTERSWITCH IS OPENED

GIVES VOLUME

FROM 5648CALIBRATOR 22MQ

80TH DIODESGE-SE416TEK PIN 152-0324-00

CONFIGURATION FOR MINUTE VOLUME MEASUREMENT.

Fig. 9-7. A spirogram obtained with an integrating pneumotach.

125

Commercial pneumotachs usually provide a calibrationof the differential pressure produced per 10 litersper minute flow. This information, when related tothe sensitivity of the pressure transducer, can beused to provide a calibration factor, as indicated inChapter 18. A typical flow pneumogram is shown inFig. 9-6. A zero base line was added to this displayto provide zero flow reference by simply removingthe subject from the pneumotach system and adding anadditional sweep to the previously stored pneumogram.

Respiratory air flow measurement is frequently usedto estimate a subject's respiratory function. Flowmeasurement also allows respiratory volume to beeasily obtained.

9.4 RESPIRATORY VOLUME

transducersignalampl ifiedthenintegrated

minuterespiratoryvolume

Since air flow is simply a measurement of volume perunit time, respiratory air flow information may beintegrated to provide respiratory volume. Such asystem is shown in Fig. 9-7. A pneumotach anddifferential pressure transducer produces an outputproportional to respiratory flow as discussed in theprevious section. To simplify the instrumentationrequirements, the pressure transducer is operatedfrom a ~13 milliamperes DC source (ocilloscopeCalibrator set to 40 V DC) and the resulting outputamplified by one of the operational amplifiers inthe Tektronix Type 3A8 Operational Amplifier. Theamplified flow signal is then integrated using thesecond operational amplifier in the 3A8 unit. Theoutput from this operational amplifier is thus anindication of respiratory volume. With the abovesystem, the resistors and capacitors associated withthe operational amplifiers can be selected from thefront panel of the 3A8, with the exception of one ofthe 10,000 ohm resistors in the amplifier which mustbe added between the appropriate terminaIs on thefront of the 3A8.

Minute respiratory volume may be measured bymodifying the above procedure. Minute respiratoryvolume is the amount of air that a subject inhalesin a one minute periode It may be measured byintegrating inspiratory fiow only, over a one-minuteperiode Referring to Fig. 9-7, the output of thesecond operational amplifier in the 3A8 unit willregister minute volume if the flow signal is coupledto the integrating circuit via a diode and theintegrator is gated ON for a one-minute periode

CONTAINER~Bfr:~m--If-- (6 LITER CAPAC 1TY)

SPIROMETER - FUNCTIONAL DETAILS

126

PULLEY

TO SUBJ ECT 1 SMOUTH ORNOSEPIECE

FLEXIBLEHOSE

A COMMERCIAL SPIROMETER

INSPIRATION RESERVE VOLUME2.5 LlTERS

TIDAL VOLUME0.6 LlTERS

EXPIRATION RESERVE VOLUME0.8 LITERS

OUTPUT CABLE

HIGH LINEARITYPOTENTIOMETERCOUPLED TO PULLEYPROVIDES RESISTANCECHANGE PROPORTIONALTO BELL MOVEMENT.

BELL

COUNTERWEIGHT

VERTICAL LUNG VOLUME0.5 LITERS/DIVHORIZONTAL 2s/01V

Fig. 9-8. A spirogram obtained with a spirometer.

conventionalspirometer

127

A more conventional spirometer is shown in Fig. 9-8.Referring to this device, inspiration and expirationraises and lowers a counterbalanced bell located ina container full of water. Movement of this bell istransferred to a pulley whose periphery contains acalibration of bell displacement which is, of course,related to bell air volume. Respiratory volume maybe read directly from this calibrated pulley. Thispulley may also be coupled to a high-linearitypotentiometer and the resistance change in thispotentiometer used to indicate respiratory volume.The ~13 milliamperes De output from a 564B Calibratoris used to provide a constant-current source for thispotentiometer; the output voltage will then beproportional to the changing resistance. A spirogramrecorded with this system is also shown in Fig. 9-8.A spirometer is inherently a heavily damped device,containing appreciable hysteresis, so small subtlechanges in inspiration and expiration volumes arenot recorded with this device.

Fig. 9-2 shows theoretical changes in total lungvolume with inspiration and expiration. Neither theintegrating pneumotach or the spirometer can showtotal lung capacity as neither of these instrumentshave the ability to measure the residual volume ofthe lung. This must be measured by gas dilutiontechniques, which are beyond the scope of this book.

128

YELLOW SPRINGS INSTRUMENT CO. TELE-THERMOMETERWITH AN OUTPUT FOR A RECORDER.

VARIOUS THERMISTOR PROBES AVAILABLEFROM YELLOW SPRINGS INSTRUMENT CO.

Fig.9-9. A commercial thermometer for use with thermistors.

129

9. 5 TFMPERATIJRE

thermometerusing athermistor

ln most cases, ternperaturedoes not vary at anyappreciable rate and, thus, rnaybe displayed usinga moving coil meter. Such a thermometer, usingthermistors in conjunction with a moving coil rneter,is shown in Fig. 9-9. For some applications,however, the meter display is inadequate and thus anoscilloscope or chart recorder is required. Themeter shown in Fig. 9-9 provides an output for usewith an oscilloscope. Perhaps the main applicationwhere temperature does vary rapidly is in therecording of respiration, as covered earlier in thischapter.

While other temperature sensing devices, such asthermocouples, are feasible they are rarely usedand small thermistors are used almost exclusively fortemperature detection. These thermistors areavailable in a wide variety of sizes and rnountingstyles as shown in Fig. 9-9.

131

ELECTROENCEPHALOGRAPHY

The following is necessari1y a short and thereforesomewhat incomplete survey of the EEG and EEGmeasuring techniques. Interested readers will findw. Grey Walter's text, The Living Brain, 1953,Duckworth and Co., invaluable for further study.

Electroencephalography, convenient1y abbreviatedEEGy, is the study of the electrical activity of thebrain. Usually this activity is recorded frome1ectrodes placed on the scalp, although sornerelatively rare diagnostic procedures requireelectrodes on or beneath the cerebral cortex. TheEEG has been known for sorne40 years and has mademany contributions to man's knowledge of brainfunction. For reasons which are examined later, ithas been of greater help to neuro1ogy (the study ofbrain function) than to psychiatry (the study ofmental processes).

10.1 THE CHARACTERISTICS OF THE NORMAL EEG

a rhythm

The EEG of a normal adult human, normal being usedin the everyday sense of the word, is relativelyeasily described. When the subject is relaxed, butnot drowsy, a relatively smooth oscillation, whosefrequency is seldom less than 8 Hz or more than 13 Hz,can be recorded from the area of scalp immediatelyover the occipital lobes. (Refer to Chapter 4,Fig. 4-2.) Typically this oscillation, the a rhythm,has an amplitude of 50 ~V peak-to-peak, althoughin rare subjects it may be twice this amplitude andin about 10% of the population it is absent or verysmall. This rhythm is responsive to mental activity;in most subjects attempting a task such as mentalarithmetic will attenuate or abo1ish it.

132

(A) ELECTRODES ARE APPLIED TO THE SCALPAND PLUGGED INTO THE JUNCTION BOX.

EYES OPENED .•..-+--I~EYES CLOSEDTl-YS

YS-OI

(B) A SWITCH SELECTOR ALLOWS THE DESIRED ELECTRODECONFIGURATION TO BE CHOSEN.

21 YEAR OLD MALE

50~V ~ ~ONE SECOND

H2-fI

FI-T4

T4-T.

TI-02

(C) A SEGMENT OF THE RECORD OBTAINED SHOWINGSIX OF THE SIXTEEN CHANNELS RECORDED.

Fig. 10-1. A typical adult EEG from a normal subject.

recorder

frequency

artifacts

133

Most EEGs are recorded using multi-channel ink writingoscillographs, as shown in Fig. 10-1, historicallybecause they were widely available to physiologists.If sornemore sophisticated method of display is usedit is found that more than one generator is involved,that there are generally several differentfrequencies, that there are differences inresponsiveness between the cerebral hemispheres andthat often the frequency of the signal measured in atransverse plane is consistently different from thatobserved with laterally placed electrodes.Frequency information is particularly significantsince the basic frequency of the EEG varies greatlywith different behavioral states. To assist in theEEG analysis, the normal frequency range of the EEG(.5 Hz to 30 Hz) has been subdivided into five bands:

delta (ô) . 0.5 Hz 4 Hz

theta (e ) . 4 Hz 8 Hz

alpha (a) • 8 Hz - 13 Hz

beta (S). 13 Hz - 22 Hz

gamma (y) . 22 Hz - 30 Hz

Various techniques for signal display are discussedlater in this chapter. Although the a rhythm is themost prominent activity in the EEG of healthy adults,it is not seen in very young children and its absencedoes not indicate a lack of mental health or anydeficiency in intelligence.

A segment of an EEG record from a normal adult maleis shown in Fig. 10-1. Six of the 16 channelscommonly recorded by an EEG instrument are shown.The tracing is read from left to right. Initially,the subject's eyes were open but after about 2.5seconds he was asked to close them. The largedownward deflection in leads FP2 - F8 and thesmaller one in F8 - T4 are the "eye blink artifact".The a rhythm can be seen in the occipital channelsT5 - 01 and T6 - 02 after the eyes were closed.Although the subject was completely normal, thea rhythm is somewhat smaller and less persistentthan usual. The high frequency component in the twomiddle tracings is an artifact due to muscle activityand is not from the brain. The EEG shown in Fig. 10-1represents only about eight seconds of recording,however, in practice, a recording may be maintainedfor an hour or more, producing a vast quantity ofinformation for analysis.

134

10.2 EEG ELECTRODE CONSIDERATIONS

inputelectrodes

signalsources

From an engineering standpoint the design of an EEGinstrument and its accessories (electrodes etc.) isnowadays a routine matter requiring little morethan ordinary care and attention to detail. As isso often the case in electronic design, the overallsystem limitations are almost aIl in the inputdevices, (the electrodes) which interface theequipment to the subject, and in the methods ofstoring the output data.

The input electrodes are the most criticalcomponents of the recording chain. To be of use forroutine EEG recording they must be small, be easilyaffixed to the scalp with minimal disturbance ofcoiffure, cause no discomfort and remain in placefor extended periods of time. They must also havesornefairly rigid electrical specifications if thesignaIs are to be recorded with acceptably lowlevels of distortion.

We have noted that many EEG signaIs are of microvoltlevels and it should be remembered that the signalis arising not at the scalp but in the cerebralcortex which is separated from the scalp by thecerebral spinal fluid (in which the brain issuspended) and by the skull. Parenthetically, weshould note that engineers often suppose the skullto be an insulator because they usually see it driedand mounted. The living brain, however, is encasedin living bone which is weIl permeated withconducting fluide The amplifying system thus seessignaIs which arise in generators which have large,complex and variable source impedances. There maybe large electrode offset potentials of the orderof many millivolts developed between the electrodeand the scalp unless a suitable electrode materialis used. The high common-mode rejection ratio ofthe modern EEG amplifier will cancel the common-modepart of this signal but in practice small movementsof the subject's head can cause substantialvariations in the standing potential and if theseare .different in each lead they will of courseappear as differential signaIs.

1i neinterference

electrodeconstructionandconnection

135

A further cause of problems in EEG recording is thepresence in the modern clinic of many pieces of lineoperated equipment so that there are substantialmagnetic and electric fields at the line frequency.The CMMR of the amplifier can, in principle, reducethese signaIs to insignificance but only if theentire system, including the electrode impedances,is balanced with respect to the common (ground)point on the amplifier. Thus electrode resistancemust be reduced as far as possible; with goodtechnique interelectrode resistances of l - 2 kQcan be obtained. The alternative technique ofreducing the line interference by the use of ashielded cage is not generally satisfactory sincethe degree of physical isolation it entails can bean emotionally upsetting experience, especially fora child. A relaxed subject is a necessity if goodrecordings are to be obtained.

The most widely used electrodes are small silverpads electrolytically coated with silver chlorideand attached to the scalp with a quick dryingadhesive, usually collodion. A harness of rubberstraps is also often used to hold the electrodes inplace. Before the electrodes are applied, the scalparea is degreased and cleaned with alcohol and thesurface resistance reduced by the use of aconducting paste. These electrodes are satisfactoryfor most recordings in the range l - 60 Hz. If,however, the low frequency limit is to be extended,which is the case in some research applications ofthe EEG, then electrodes which more closelyapproximate truly nonpolarizable electrodes, such asTektronix Ag/AgCl electrodes must be used. Electrodesare generally placed at standard locations on thescalp to facilitate communication betweenelectroencephalographers. These positions, withtheir usual designations, are shown in Fig. 10-2.The usual abbreviations are: F = frontal,T = temporial, C = central, P = parietal,o = occipital.

136

F - FRONTALC - CENTRALT - TEMPORALP - PARIETALo - OCC 1P 1TA LA - EAR, COMMQN

.Fpl .F7

''1::;::::- ,-.

PERSPECTIVE VIEW OFELECTRODES lN PLACEON LEFT-SIDE FRONTOF THE HEAD.

Fig. 10-2. EEG electrode positions.

10.3 EEG RECORDING INSTRUMENTS

osci 1lographs

systemfrequencyresponse

Turning now to the output end of an EEG recordingsystem, multi-channel ink-writing oscillographs areused for many reasons. The recording material isrelatively cheap, the record is available forinspection as it is being written, theelectroencephalographer can quickly flip through along recording and obtain an "eyeball" impressionof its contents and he may study interesting orcomplex parts of the record for as long as isnecessary. Other media such as magnetic tape do notpossess these properties and have not become popularin the routine clinical laboratory. If visualanalysis is to be supplanted by computer or otherautomated data-processing techniques, then thewritten record must be supplemented by a magneticrecording or a curve reader (e.g., the multichannelhigh-speed curve reader described by Barlow in 1968)must be used. The frequency response of most EEGsystems is limited by the characteristics of therecorder to something of the order of 60 Hz but thisis adequate for most clinical purposes.

137

EEG recording systems are usually self-contained unitsconsisting of electrode switching networks, high-gaindifferential amplifiers and graphic recorders.

multichannel Multichannel recording is almost invariably used,the number of channels ranging between 6 and 32 with8 or 16 channels being the numbers preferred forroutine work.

1inefrequencyfil ter

sensitivity

A multiplicity of electrodes is affixed to the scalpas shown in Fig. 10-2 and the recording channels areconnected to them via a switching network. Theamplifiers are invariably designed to acceptdifferential inputs and their design is usuallyoptimized for low noise and good common-moderejection. The low-frequency response usuallyextends to about 0.1 Hz, however the high-frequencyresponse need not be in excess of about 100 Hz dueto the limited high-frequency response of the graphierecorder following the amplifier. Since most EEGactivity occurs below 50 Hz, a notch filter tuned toline frequency is often included in EEG instrumentsto minimize line frequency interference, but its useis strongly discouraged except as a last resort.

Although the gain of most modern EEG instruments isstable to within a few percent, a 50 ~V squarewavecalibrator may be included in the instrument.Although the sensitivity of the amplifier may beadjusted to suit particular subjects, theelectroencephalographer rarely changes thesensitivity during the recording of an EEG. Heusually selects a gain that makes the initial record"look right" and uses the same gain throughout aIlphases of recording the EEG. Since theelectroencephalographer is concerned with relativeamplitudes between the channels, it is necessarythat each channel has the same sensitivity. It isdesirable to standardize on both sensitivity and~a~er s~eed to achieve aspect ratio consistency,which allows comparison with other EEG's recordedfrom other subjects. No firm standard exists here,however many workers prefer a sensitivity of about7 mm per 50 ~V for adult subjects, a somewhat lowersensitivity for children and a somewhat highersensitivity for the aged. The range ofsensitivities used is usually within the range 4 mmper 50 ~V to 15 mm per 50 ~V.

138

CHART PAPER MOVEMENT30fTVll/S

~Fa

COMMQN REFERENCE

(Al U IPOLAR EEG RECORDING CONFIGURATION

~T6

SUMMI.G RESISTORS

(Bl AVERAGE EEG RECORDING CONFIGJRATION

COMMO REFERE CE

>--..•.~ FP2 - Fa

~T4-T6

(Cl BIPOLAR EEG RECORD 1 G CONFIGURATIO

>----1~~ T - 06 2

Fig. 10-3. EEG recording modes.

ONLY 5 CHANNELSOF A MULTICHANNELRECORDING SYSTEMARE SHOWN.

ONLY 5 CHANNELSOF A MULTICHANNELRECORDING S~STEMARE SHOWN.

ONLY 4 CHANNELSOF A MULTICHA NELRECORDING SYSTEMARE SHOWN.

paper speed

139

Most EEG instruments provide auxiliary outputs fromall amplifiers to be used with other equipment, suchas oscilloscopes, tape recorders or display devices.A paper speed of 30 mm per second (25 mm per secondin sorneinstances) is often used forelectroencephalographic recording. Many EEGinstruments may also add time markers to the EEGrecording on a separate channel and may alsoincorporate electrode contact resistance measurementfacilities.

10.4 EEG RECORDING MODES

unipolarmode

Three modes of recording are used in the routine EEGlaboratory as shown in Fig. 10-3. They are known asunipolar (often improperly called monopolar),averaging reference and bipolar recordings.

ln the unipolar mode one electrode is common to aIlchannels (Fig. 10-3A). Ideally, this commonelectrode is regarded as electrically inactive,however in practice, electrical activity near thiselectrode will appear in all channels and invariablythere are problems in selecting the site for thiscommon electrode. The ear, or both ears connectedtogether, are sometimes used as being generallyclose to regions of the brain with little on-goingelectrical activity. If a subject has a localizeddischarge, for simplicity we will assume a spikedischarge, then successful localization of the spikewill be dependent on its amplitude in the variouschannels. With some loss of scientific vigor wemay say that the amplitude will be greatest in thechannel with its active electrode nearest the sourceof the spike. If the common electrode is near thespike focus, localization is either not possibleor very ambiguous. Although one electrode is commonto aIl channels, to reduce interference andartifacts it is desirable to not ground this commonelectrode and a separate ground electrode is oftenconnected between the subject and the instrumentationground.

140

averageelectrodemode

bipolarmode

ln the average electrode system one input lead ofaIl amplifiers is taken to the common point of asumming network in which equal (high) resistorsare taken to each electrode (Fig. 10-3B). Therecording will now indicate deviations from the meaninstantaneous potential of the electrode system andthus an isolated feature (e.g., the spike) will, ifit is sharply localized, stand out in one, or atworst, a small number of channels. This recordingmode can be loosely compared with the recordingconfiguration used for unipolar ECG's as describedin Chapter 5, Sections 5.4 and 5.7. A resistivesumming network is used to create a common point ineach case.

ln the bipolar mode, the channels are connected inseries between electrode pairs (Fig. lO-3C). Bynoting the change in the recorded EEG between theseelectrode pairs, very sharp localization ofdischarges is possible. The electrode immediatelyover the spike generator will cause a positivedeflection in one recording channel and a negativedeflection in the adjacent recording channel so thatthe electroencephalographer will see an apparent1800 phase difference between them. This "phasereversaI focus" is accepted as the most reliablemeans of localization of discrete phenomena.

10.5 UNUSUAL EEG DISPLAY MODES

The voltage/time graph was used originally to displaythe EEG because oscillographs were available to theelectrocardiographer. There is no certainty thatthese are the optimum ordinates to use in studyingthe EEG and a nurnberof other techniques have beenproposed. Some of these give a forrnof map-likepresentation and allow the potential gradient overthe head to be studied either in a "snapshot" fashionas used by Remond or as a synoptic display over ashort time interval (Shipton). Such devices,

spectrumanalysis

141

genera11y speaking, are good indicators of change inthe EEG activity but poor for quantification,especia11y in terms of amplitude. It can bep1ausib1y argued that amplitude is not the parameterof greatest interest and that what we are rea11yconcerned with is the way that the brain hand1essensory data in terms of their temporal or spatialsuccession. The use of more sophisticated methodsof disp1ay is increasing because the avai1abi1ityof on-line, rea1-time digital computers which havepermitted data transformation in a number ofinteresting ways.

A spectrum ana1ysis system has been occasiona11yused in research applications to present the EEGusing amp1itude/frequency coordinates. Mostconventiona1 samp1ing-type instrumentation spectrumana1yzers are unsuited for direct EEG ana1ysis astheir low-frequency performance characteristics areinadequate and they do not 1end themselves to theanalysis of continuously changing data such as anEEG. The most desirab1e form of spectrum ana1yzersis a "rea1-time spectrum ana1yzer," however, suchinstruments are inherent1y rather expensive.Frequency ana1ysis is rare1y used as a clinica1procedure; it masks much usefu1 information which ahuman operator, using our superb pattern recognitionabi1ities, can see at a glance. Mathematica11y, thedifficu1ty of spectrum ana1ysis of an EEG is thatit is not time-invariant for a period long comparedwith the lowest frequencies present (refer e.g.Broadman & Tukey Meaeuremeni: of Poiaer Spectra).

142

ELECTRODESELECTOR

60INCH/sTAPE LOOP

RECORD EEG

AMPLIFIER

INSTRUMENTATIONTAPE RECORDER

1-7/8INCH/s

~O

EEG FREQ Hz1 1 1 1 1o 3 6 9 12

ANALYZE EEG

RO~U R564B UR564B STORAGE OSCILLOSCOPEWITH 3L5 SPECTRUM ANALYZERAND 2B67 TIME BASE PLUG-INS.

30 Htttttt+----+---t---tttt-----+----+---t--t-----t--j

10

SCALE FACTORS 5REFERRED TOINPUT SIGNAL 0

ZERO Hz t-1ARKER

~~ ..•..DELTA THETA ALPHA

o e CI

480Hz CENTER FREQ

III-1 1 1 1 1 115 18 21 24 27 30

•• ••• ••• ~BETA GAMMA8 Y

EEG RECORDED AT 1-7/8INCH/s, REPLAYED AT 60INCH/s

"SPEEDUP" IS 32 x CENTER FREQ :::15 x 32Hz:::480Hz

Fig. 10-; EEG spectrum analysis.

taperecorder

143

Spectrum analysis of the EEG has been performed withconventional instrumentation and sampling-typespectrum analyzers by utilizing an instrumentationtape recorder as indicated in Fig. 10-4. With thissystem a standard instrumentation tape recorderoperating at 1 7/8" per second records the EEG anda sma11 section of this recorded EEG is formed intoa tape loop. This tape loop is then p1ayed back at60" per second to effectively increase the frequencyof the recorded information and to a1low the use ofa samp1ing-type spectrum ana1yzer, such as theTektronix Type 3L5 Spectrum Ana1yzer in conjunctionwith a Tektronix Type 564B Storage Oscilloscope.The simulated CRT display shown in Fig. 10-5represents two separate spectrum analyses of twoseparate tape loops, one recorded with a subject'seyes open and one recorded with a subject's eyesclosed but with the subject awake. This displayclearly shows the predominance of alpha activitywith the eyes closed and the shift from alphapredominance with the eyes open.

10 . 6 INfRA - CRANIAL ELECTRODE PLACf1\1FNI'

insulatedneedleelectrodes

ln some diagnostic procedures the EEG recordingelectrodes are placed directly on the exposedsurface of the brain. Under these conditions theoutput voltage will be considerably greater than thevoltage obtained with normal EEG electrode placement,thus the gain of the recording instrument must becorrespondingly reduced. Standard EEG electrodescannot be used under these conditions as they arenonsterile and physically unsuited, thus specialelectrodes are used.

During neurosurgery insulated needle electrodes areoften used to place an electrode deep within thesubject's brain. These "deep electrodes" may consistof a needle insulated over its entire length withthe exception of a small area at the tip or they mayconsist of concentric needles of varying length toeffectively provide many electrodes at regularintervals along the length of the needle.

144

10.7 APPLICATIONS OF THE EEG

anestheticlevel

monitoringduringsurgery

alertnessmonitor

The EEG is primarily used in clinical neurology forpartially assessing a subject's neurological state.Such applications are covered in more detail in thefollowing section when discussing the abnormal EEG.

As weIl as its utility in the neurological clinic,there are other uses for the human EEG. Brain cellsare, for example, affected by anesthetic agents andthe EEG is a sensitive indicator of the depth ofanesthesia. Some workers indeed have used the EEGsignal in a closed-loop controller to keep a constantanesthetic level.

ln many surgical procedures involving the heart, theECG waveform cannot be monitored, thus the EEG signalis used as an indication of subject weIl being. Withthese procedures the verification of death can nolonger be related to the activity of the cardiacsystem, thus the presence of EEG activity is, in part,a useful indicator. EEG monitoring during surgerydoes not require the use of multichannel EEGinstrumentation. The EEG signal can be monitoredusing a single channel recorder or, more commonly,by using an oscilloscope. Many surgical monitors,including the Tektronix Type 410 PhysiologicalMonitor, designed primarily for cardiac monitoring,include EEG monitoring facilities. The 410 monitorsimply requires two electrodes placed on the headover the occipital regions for EEG recording andone ground electrode placed anywhere on the subject.

The EEG is also a very subtle estimator of thedifferences between sleep and wakefulness. Much ofour present knowledge of sleep phenomena - andsleep is much more complex than it seems at firstsight - we owe to the EEG observation of sleepingsubjects. A number of states can be distinguished.For those who must be alert at specified times duringa long task, piloting a spacecraft is a case inpoint, the EEG can be made the basis of a reliable"state of alertness" monitor.

stimuliresponses

145

Recently there has been great interest in the slow(circa 0.1 Hz) potential changes in the EEG. Modernamplifier techniques and nonpolarizable surfaceelectrodes have established that these shifts in theDe potential are associated with voluntary responsesto stimuli. It is very probable that such studieswill extend the use of EEG techniques into the realmof psychophysiology; already a number of tentativerelationships between mental state and variation ofthe slow "expectancy waves" have been established.

10. 8 THE CHARACTERISflCS OF THE ABNORMALEEG

ep i 1eps ies

braininj ury

inbornep i 1epsy

ln describing the EEG as it is seen in variousdisease states, it is important to remember that veryfew single clinical tests are by themselvessufficient to make a diagnosis. Thus, the EEG isonly one of many procedures used by the clinicianin assessing the neurological state of a subject.The most common condition in which the EEG isvaluable is epilepsy. Strictly speaking, we shouldrefer to "the epilepsies" since many varieties arefound. The EEG is of great help in forecasting theoutcome of an epileptic illness and is valuable inestablishing the optimum course of treatment.

For example, injury to a specific region of the braincan leave a permanent scar on the cerebral cortex.Such scar tissue is electrically inert but has anirritative effect on nearby healthy cortex. The EEGwill often show a localized spike discharge and willsuggest, among other possibilities, surgical removalof the damaged tissue.

When epilepsy is inborn, and sorneforms of thisdisease are hereditary, the abnormal electricalactivity generally contains signaIs at manyfrequencies in the range l - 50 Hz. There is noconsistent phase relationship between the variouscomponents so that the EEG presents, to the eye, a"noisy" appearance. The signaIs are generally a gooddeal larger than the a rhythm and usually cannot belocalized to any specific region of the brain.

146

"petit mal"

tumors

ln the so-called "petit mal" epilepsy, in which themanifestation of the illness is often a transientloss of consciousness or sorneautomatic motorbehavior, the signaIs are wideband but haveremarkably consistent phase relationship between eachcomponent. The signal is thus seen as a regularpattern in which a sharp spike appears superimposedon a smooth low frequency (1 - 3 Hz) wave. Althoughmany hypotheses have been advanced to account for theremarkable phase consistency seen in this "spike andwave" phenomenon, none are entirely convincing andaIl are outside the scope of this chapter.

When the brain is invaded by sorneforms of tumor, aconsiderable portion of the active nervous tissuemay be displaced by the electrically inert new growth.If this is very large, its presence can be inferredfrom the absence of organized electrical activityfrom the region of the tumor. Usually, however, atumor large enough to be associated with adetectable area of "electrical silence" is largeenough to manifest itself in other ways so that thistechnique of tumor detection is of limited practicalvalue. There are, however, other ways in which theEEG can help in the diagnosis of brain tumors at amuch earlier state of their development. Theexpanding new growth can interfere with the bloodsupply to neighboring areas and the consequentmalfunctioning of nerve cells around it manifestsby a large, slow discharge - the 6 rhythm. Theremay also be significant differences in the electricalactivity in the affected hemisphere, perhaps as aresult of interference with the internaIcommunicating pathways within the brain. The extentto which the EEG is of value in tumor localizationdepends on many factors. Sorneof the more importantare the rate at which the tumor is growing, itsspecial relationship to the recording electrodes andthe skill of the electroencephalographer.

The conditions noted above are those in which theEEG is most frequently used. ln other circumstancessuch as certain toxic conditions and sornepsychological states, the EEG can add to the overallamount of clinical information and be of significantbenefit to the subject and physician.

147

10.9 INTENTIONAL MODIFICATION OF A SUBJECT'S EEG

externalstimul i

Up to this point we have assumed that theelectroencephalographer plays an entirely passiverole and is content to study the brain in its normalphysiologie milleau. ln practice a number oftechniques are used to increase the yield ofmeaningful information; sorneof these apply externalstimuli to the subject and note their effect on theEEG. Because the excitability of various parts ofthe nervous system is critically dependent on theacid-base balance (and thus on the oxygenation of theblood) it is general practice to modify this balanceby asking the subject to hyperventilate, that is tosay breathe rapidly and deeply while at reste lnthe normal subject this procedure produces a nominalslowing of the a rhythm and sornesmall increase inthe overall signal level. If, however, the subjectis an epileptic, the record is dramatically changedto the extent that a seizure may be provoked. Sorneepileptics are markedly affected by lowering theirblood sugar and for this reason many clinical recordsare obtained from fasting subjects.

Another important means of modifying the EEG is theuse of rhythmic sensory stimulation. One or more ofthe senses is stimulated by brief repetitive stimuli;light flashes are the most commonly used partlybecause they are easy to generate and partly becausethe visual cortex is large and the source of thea rhythm. Sensory stimulation of this kind canemphasize latent abnormalities in the resting EEGand help in the interpretation of the tracing.

148

+lrnVrOrnV

-lrnV

+20rnV

OrnV

EXTRACELLULARACTION POTENTIAL

(EXTERNALLY RECORDEDl

INTRACELLULARACTION POTENTIAL

(INTERNALLY RECORDEDl

O.2rns/DIV

CELL ACTION POTENTIAL RESULTSWHEN CELL IS STIMULATEDCAUSING DEPOLARIZATION

ELECTRODE INSERTEDINTO CELL RECORDS THEINTRACELLULAR ACTIONPOTENTIAL OF 110rnV

ELECTRODE ADJACENTTO CELL RECORDS THE

>---l~EXTRACE LLULAR ACT IONPOTENTIAL. 10~V - 10rnV

Fig. 11-1. Cell action potential - internally and externally recorded.

149

EVOKED CORTICAL RESPONSES

The electroencephalogram referred to in Chapter 10is a measure of the over-all electrical activityof the brain with the subject essentially at restaThe electroencephalogram is probably associated withthe computation process continuously active withinthe brain. Evoked potentials are also potentialsgenerated within the brain, however these potentialsresult from a stimulus being applied to the body'ssensory system and are localized to a particulararea of the brain. These potentials are said to be"evoked" by the stimulus.

Il.1 EVOKED ACTION POTENTIAL

As stated above, stimulation of a subject's sensorysystem produces electrical activity in a localizedarea of the brain. When attempting to analyze theeffect of the stimulus, it is necessary to recordthe electrical potentials generated within individual

intracel lular brain cells by using intracellular recordingrecording techniques. These techniques are only used in

research applications on nonhuman subjects. Externalelectrodes will record the electrical activity of acell, however, as many adjacent cells may also beproducing electrical activity, the results obtainedcould not be attributed to any particular cellaUnder certain conditions, particularly when recordingfrom the spinal cord rather than from the brain, itmay be possible to isolate individual cell activity

extracel lular using external electrodes to record the extracellularrecording action potential. Fig. 11-1 shows the action

potential generated by a single cell when recordedwith both internaI and external electrodes and showsthe time and voltage relationship between these tworecording techniques.

150

i ntrace 1 1u 1aractionpotential

REGENERATIVEBREAKOOWN PER 100

~

POTENTIAL CHANGE FROM-90mV TO THRESHOLO ISPROPORTIONAL TO EXCITATION STIMULI STRENGTH

REGENERATIVEBREAKOOWNOCCURS

t-+- ACT 1ON _..,POTENTIAL

Fig. 11-2. Action potential showing threshold.

When recording the intrace1lular potential with aninternaI electrode, about 110 millivolts of signalis generated during a cell depo1arization/repolarization process and this signal is known asthe intracellular action potential. It is, however,possible to record lower amplitude signaIs generatedwithin the cell as a result of excitation andinhibition stimuli as discussed in Chapter 4. Theintracellular action potential shown in Fig. 11-2 isc1early preceded by a period where the cell isreceiving excitatory stimuli which decreases thecell's resting potential at a linear rate unti1 thecell threshold is reached, at which time the rate ofchange of potential increases, indicatingregenerative depolarization. The cell subsequentlyrepolarizes.

Il.2 MICROELECTRODE TECHNIQUE

The preceding discussion covers intracel1ularrecording and shows typical intracellular actionpotentia1s. ln practice, intracellular recordingrequires highly specialized measurement techniques,and results similar to the action potentials shownin Fig. 11-1 and 11-2 are difficult to achieve.

microelectrodes

metal

glass

151

When recording the intracellular action potential itis, of course, necessary to insert an electrode intothe cel1 concerned. If the results obtained are toserve any practical purpose, it is also necessarythat this electrode have a negligible effect on thecharacteristics of the cell concerned. It is thusdesirable to use an electrode with dimensions muchsmaller than the dimensions of the cell concerned;requiring electrodes with tip diameters weIl underone micron (10-4 centimeters). These small electrodesare known as microelectrodes. Both glassmicroelectrodes, often referred to as micropipettes,and metal microelectrodes are available.

The basic difference between these is that in themetal microelectrode the metal is in direct contactwith the biological tissue whereas in the glassmicroelectrode an electrolyte is interspersed betweenthe tissue and the metal electrode. Thus, metalmicroelectrodes have a lower resistance, but theypolarize with smaller amplifier input currents,their resistance may increase, and they may developunstable electrode offset potentials. Unless extremeprecautions are taken, they are, therefore,unreliable for steady-state potential measurements.

The glass microelectrode interposes an electrolytebetween the tissue and the metal electrode whichresults in improved stabi1ity as the metal and theelectrolyte can be chosen so that small, steadycurrents can pass through their junction withoutmodifying the electrical properties of the electrode.The surface contact area between the electrolyte andthe metal is large so that the current-carryingcapacity of the electrode is substantial. The glassmicroelectrode is, therefore, usually preferred.

152

dimensions

electrolyte

resistance

equivalentcircuit

Fig. 11-3 shows typical tip dimensions of a glassmicroelectrode, having an over-all tip diameter ofabout 0.6 microns with an internaI diameter of 0.2microns. The glass microelectrode is formed byheating special glass tubing and drawing it out overseveral stages of reduction. Although the tubing isreduced to less than a micron in diameter, it stillremains hollow. Potassium chloride solution isintroduced into the microelectrode as the electrolyte.As it is not possible to fill the microelectrode bypressure (surface tension) or by capillary action,boiling with or without reduced pressure is oftenused. Ideally, one would like to use an electrolytewithin the microelectrode having the sameconcentration as the potassium chloride within atypical cell (0.1 Normal). It is, however,impossible to use an electrolyte of this concentrationas the resistance of the microelectrode would be weIlover 1,000 megohms. It is, thus, common practice touse 2 or 3 Normal potassium chloride in themicroelectrode. This electrolyte has a resistivityof 3.3 ohm centimeter which gives the microelectrodesimilar to that shown in Fig. 11-3 a typicalresistance of 10 megohms. Special purposemicroelectrodes having smaIIer tip diameters and/orusing a less concentrated electrolyte may have atypical resistance of 100 megohms. As will be shownlater in this chapter, special recording techniquesare necessary to accommodate this extremely highelectrode series resistance.

Fig. 11-4 shows a typical microelectrode insertedinto a cell and the equivalent electrical circuitformed between this microelectrode and a cornmonelectrode elsewhere on the subject. This equivalentcircuit can be further simplified to themicroelectrode resistance with distributed capacity,and an Re load on the microelectrode formed byinterconnection capacity, amplifier input capacityand amplifier input resistance. Since the dimensionsof the microelectrode tip are extremely small, andas most of the microelectrode resistance is locatedwithin one millimeter of the microelectrode tip,this distributed capacities associated with theelectrode resistance are less than one picofarad.

MICROELECTROOE OVERALL OIMENSIONS:

6 5

153

4 3 o cm2SILVER WIRE

~\~~>-GLASS TUBING KCI ELECTROLYTE - 3N CONCE TRATION

MICROELECTROOE TIP OIMENSIONS:

MICROELECTRODE FILLEQ WITH3N POTASSIUM CHLORIDE (KCI)

Fig. 11·3. Glass microelectrode geometry.

APPROX.EOUIVALENT TO

APPROX.EOUIVALENT TO

Rc' Rm' Rf AND

C ARE NEGLIGIBLEm

AMPLIFIER WITH RAND C INPUT LOAO.(C INCLUDES INTERCONNECTION CAPACITY)

,------ ..•.TO AMP

INSIDE

CELL MEMBRANE

R = TIP RESISTANCE OF MICROELECTRODEm

Cmf = CAPACITANCE - MICROELECTRODE TIP TO FLUIDSURROUNDING CELL

Cmc = CAPACITA CE - MICROELECTRODE TIP TO FLUIDINSIDE CELL

R = "RESISTIVITY" OF FLUID INSIDE CELLcE = BIOELECTRIC ACTION POTENTIAL GENERATOR

R = MEMBRANE RESISTANCEm

C = MEMBRANE CAPACITANCEm

Rf = "RESISTIVITY" OF FLUIO SURROUNDI G CELL

Fig. 11·4. Cell-microelectrode equivalent circuit.

154

1/3R m 1/3R m 1/3R m

EQUIVALENT CIRCUIT FROM FIG. 11-4.

INPUTPULSE

E

OUTPUT PULSE FORVARIOUS VALUES OF Rm

10MIl

R

TYPICAL VALUES:C = O.lpFme

Cmf = 0.2pFC

R = 1000MIlC = lpF

R = 10Mrlm

E = -90mV TO +20mV

t = O.lms t = 0.2msr f

DURATION - 0.5ms

R COMPRISES THREEm

RESISTORS - EACH 1/3R m

O.lpFPULSEGENERATOR 0.2pF 1000MIl

MICROEI.ECTRODEEQUIVALENT CIRCUITMODELED WITH IMPEDANCES SCALED BY 103.

Fig. 11-5. Microelectrode model and resulting pulse response.

lpF

circuitdeterm inesfi de 1i ty

155

Referring to Fig. 11-5, a typical microelectrodeequivalent circuit is shown together with typicalvalues for the ~mpedances concerned. Thisequivalent circuit represents a low-pass filter andattenuator and it is necessary to know thecharacteristics of this circuit to determine thefidelity expected from the system when recordingthe action potential. This equivalent circuit canbe rnodeledusing a pulse generator in place of thecell and using discrete impedances in place of themicroelectrode resistance and capacity. Fig. 11-5shows the degradation that can be expected from aninput "action potential" having a risetime of 0.1millisecond and a falltime of 0.2 millisecond forvarious values of electrode internaI resistancewhen using an amplifier having an equivalent inputcapacity of one picofarad and an equivalent inputresistance of 1,000 megohms.

As can be seen from Fig. 11-5, a 100 megohmmicroelectrode used in the above system reduces theamplitude of the action potential by 20 percent anddegrades its risetime and falltime. It is thusdesirable to either reduce the resistance of themicroelectrode to 30 rnegohrnsor perhaps even 10megohms to increase fidelity or, alternatively, toincrease the input impedance of the amplifier andinterconnection network. Since decreasing themicroelectrode resistance usually means increasingthe size of the microelectrode, the latter alternativeis preferable.

156

Ro180

CI<lpF

564B STORAGE SCOPE

D CAL

VERT HORIZ0LO PASl- '- 3A8 2867FILTER OPERATIONAL TIME BASE

AMPLIFIER

1PULSE

SHAPI GINPUT CIRCUIT

NEUTRAL 1ZED

v: MICRO- ADAPTER

;;:,:LELECTRODEf"...........~.~-~:

'<{i~~~t.:;r;~~'~;' COMMON

ELECTRODE WITH CALIBRATOR "OFF", COMMONELECTRODE IS GROUND.W 1TH CALIBRATOR AT "40V", PULSESHAPING CIRCUIT ADOS A 50rnVPULSElN SERIES WITH THE MICROELECTRODE.0.2rns/DIV

~ ~

OUTPUT WAVEFORM FROM PULSE SHAPINGCIRCUIT USED FOR SYSTEM CALIBRATION.

PERFORMANCE

WITH 10MnMICROELECTRODE

FOR CIRCUIT DETAILSREFER TO CHAPTER 29

WITH 100MnMICROELECTRODF

CALIBRATOR "ON", MICROELECTRODEREPLACED WITH WIRE (OOl

+MICROELECTRODE lN PLACE ANDNEUTRALIZATION CONTROLS ADJUSTEDFOR OPTIMUM RESPONSE

tAS ABOVE WITH BANDWIDTH LIMITINGADJUSTED FOR "LO" NOISE WITHMINIMUM DISTORTION

EFFECT OF AMPLIFIER AOJUSTMENTS

WITH 100MnMICROELECTROOE

INPUT RESISTANCE TOO LOWAPPROX. 30Mn

INPUT CAPACITY TOO HIGHAPPROX. 5pF

INPUT CAPACITY OVER-NEUTRALIZEO, PROOUCING RINGINGAND EVENTUALLY OSCILLATIONCORRECT ADJUSTMENT

Fig. 11-6. An input neutralized amplifier and scope system.

157

Il.3 INPUT NEUTRALIZED AMPLIFICATION

feedbackincreasesinput Z

systemcal ibration

ampl ifierinputcurrent

The impedance load on the microelectrode can beincreased by using active feedback amplification toprovide input impedance neutralization. Carefuladjustment of such amplifiers can achieve an inputres~stance in excess of 10,000 megohms withequivalent input capacities of the order of 0.1picofarad. Fig. 11-6 shows a typical microelectrodesystem utilizing a Tektronix Type 3A8 OperationalAmplifier plug-in unit operating in an "inputneutralized" mode. This requires the use of an inputneutralizing adapter in conjunction with the 3A8.Details of this adapter are given in Chapter 29.

This microelectrode recording system also incorporatesa series calibration waveform. By switching the 564Bcalibrator from OFF to 40 v, a 50 millivolt, 1000 Hz,signal is added in series with the microelectrode.This signal has a controlled risetime of 0.1 ms anda controlled falltime of 0.2 millisecond and is usedin conjunction with the two input neutralizationcontrols (-R and -C) to check the amplitude and timecalibration of the system. When the calibrator isreturned to its OFF position, this signal source isessentially replaced by a short circuit. Thiscalibration waveform and the response of the systemfor various settings of the input neutralizationcontrols are shown in Fig. 11-6. ln practice, theinput ~eutralization should be adjusted to the pointat which overshoot and ringing are reduced to lessthan 10 percent. Further reduction of the overshootand ringing will degrade the risetime of the system.

Although the dynamic input resistance of aneutralized amplifier may be above 1010 ohms, thesystem may have excessive amplifier input currentand will thus not operate. Since the dynamic inputresistance of any device is determined by the changEin input current for an incremental change in input

voltage; R = ~~; and as amplifier input current

does not change with an incremental change in inputvoltage, this amplifier input current is notreflected by the input resistance specification ofthe amplifier and must be specified separately. Anyneutralized amplifier should have an amplifier inputcurrent below 10-12 amperes.

158

tangentialnoise

lowpassfi 1ter

The system shown in Fig. 11-6 has an amplifier inputcurrent of approximately 10-13 amperes. Theseextremely small currents are~necessary when using theamplifier for intracellular recording as the currentwill cause potassium ions to migrate from themicroelectrode into the celle This upsets theintracellular concentration, which may change thecell resting potential. Small changes in restingpotential due to extremely small currents cannot beavoided, and may not be significant. However, it isobviously desirable to maintain the current below thelevel required to cause cell depolarization.

The noise in any neutralized amplifier isproportional to the degree of neutralization required;a tangential noise of several millivolts is notuncommon. "Tangential" noise is discussed in Chapter19, Section 19.7. The system shown in Fig. 11-6,when correctly neutralized, produces a tangentialnoise of 1 millivolt when used with an electrodehaving a resistance of 10 megohms and 2.5 millivoltswhen used with an electrode having a 100 megohmsresistance. This noise is insignificant whenrecording intracellular potentials as thesepotentials are in the order of 100 millivolts. Forextracellular measurements however, the extracellu1araction potential may be as Iowa? 10 microvolts andthe amplifier noise will completely swamp anyextracellular signaIs. This situation also ariseswhen using intracellular recording at highsensitivity in an attempt to observe excitation andinhibition potentials. The signa~-to-noise ratiocan be substantially improved by a lowpass filterfollowing the neutralized amplifier. Such a filteris included in the system shown in Fig. 11-6 andfurther details of this filter are given in Chapter29. Even using a filter for extracellularmeasurements, the noise level obtained may stillcompletely swamp the signal and, at best, will beextremely objectionable. Thus, particularly whenworking at sensitivities below 100 microvolts perdivision, some more effective noise reductiontechnique is almost always necessary.

159

Il.4 NOISE REDUCTION - AVERAGING

signalaveraging

A single response evoked by one stimulus is usuallytoo small to be seen above the on-going EEG activityand the noise inherent in the system. However, ifwe assume that the response always follows thestimulus after a fixed delay, "signal averaging"computers can improve the signal-to-noise ratio.Many such signal averaging computers have beenmarketed in the past few years and, while they differin detail, they aIl use the same basic principle.

Signal averaging extracts a wanted signal from abackground of unwanted noise. It can only be usedeffectively if the desired signal, with itsaccompanying noise, can be generated a number oftimes, either periodically or aperiodically. lnaddition, a trigger pulse is required that has afixed time relationship to the desired signal. Thestimulus, or trigger pulse, initiates a scanningdevice which samples the signal at fixed intervals.These time-sequential samples are stored in discretestorage channels or memory locations, each channelcollecting data over a small time segment. Whenthe stimulus is repeated, the responses are added tothe values stored at each location. During readout,each of the channels displays the sum of aIl theprevious samples fed into it. If the backgroundactivity is random with respect to the stimulus,the time-Iocked reSponse will add linearly withthe number of samples (n) while the background willadd only as the square root of n. If the stimulusis repeated 64 times (a fairly common number), thenthe response to noise ratio will have been improved

nby a factor of ru = ru = 8. Filtering to the maximum

permissible extent should always precede theaveraging device.

160

RAW SIGNAL OUTPUT FROMNEUTRALIZED AMPLIFIER

VERTICAL lOOuV/DIV (REFERRED TO INPUT)HORIZONTAL O.2ms/DIV

CONVENTIONAL SIGNAL PROCESSING

AMPLIFICATION WITH BANDWIDTHLIMIT (10kHz) AND GAIN REDUCTION(200~V/DIV REFERRED TO INPUT)

ORIGINAL SIGNAL WITHOUT NOISE.IDEAL SIGNAL PROCESSING SHOULDPRODUCE THIS WAVEFORM.

10~V/DIV O.2ms/DIV

SIGNAL AVERAGER AND READOUT OSCILLOSCOPE

SIGNAL AVERAGING

NUMBER OFSWEEPS

OF AVERAGER

1 SWEEP

4 SWEEPS

r.iI!..•'/' . .-:1\ "'. /' ~\:- . !"'-,••..•. 1 .~\ 1 \f~ 1'.1'- 1P,j'~. E1I1

64 SWEEPS

~

~ pj ,..PlI 1Pa'!!, ,~

il r4: • 1r.I, 1\ .'(I! n' 1'.1' I~' V,..

Il

256 SWEEPS

100~V/DIV O.2ms/DIV

Fig. 11-7. Signal to noise ratio improvement with averaging.

signal-tonoise ratioimprovement

triggeringon signal

averagercharacteristics

161

Typical signal-to-noise ratio improvement viaaveraging is shown in Fig. 11-7. The noise has apeak value about equal to that of the signal,equivalent to an RMS SiN ratio of about 4. Averagingis an ideal technique under these conditions. Forgreater noise levels the signal content is reduced,because the peak noise plus signal must still notoverdrive the averager; difficulty then arisesbecause the signal component alone is not beingdigitized to a sufficient number of bits to givea usable output.

ln many circumstances, averaging can be carried outin the absence of the trigger pulse if it ispossible to use part of the response itself as thestable point. This is an aspect of averaging whichis currently of great interest to physiologists.

The resolution of the signal averager is limited bythe number of addressable channels; commercial unitsmay contain from 50 to 1,000 channels. The "speed"of the system is limited by the length of timerequired to store information into any one channel.Commercial averagers are able to sweep through aIlchannels in less than 5 milliseconds, providing aneffective sweep speed of 0.5 milliseconds percentimeter for the readout oscilloscope. Since thesweep speed is limited by the individual channelaccess time, the use of fewer channels can increasethe effective sweep speed at the expense ofresolution. The channel access time for most modernaveragers is about 10 microseconds; however, sorneearlier averagers designed specifically forbiological use have an access time in excess of 100microseconds.

~lost signal averagers contain the necessary circuitsfor averaging but require a separate X-Yoscilloscope to display the output information.Normally, calibration facilities are incQrporatedinto averagers to allow the monitor to becalibrated for both voltage and time reference.This calibration is particularly important as the"level" of each channel in the averager varies withthe number of sweeps used and aIl absolute amplitudeinformation is, therefore, meaningless unless acalibration system is provided. Sornetime averagersare now available in which the average rather thanthe sum of the sweeps is displayed.

162

usingstimulus astrigger

timejitter

digitalcomputeraveragers

correlatingtwo inputsto detectsignal

averagingevokedresponsesfromunwantedsignais

The signal averager relies on a trigger pulse beingavailable having a known time relationship to thedesired signal. When recording evoked potentials,it will be recalled that these potentials aregenerated in response to a stimulus being appliedto the patient, this stimulus can be used as thetrigger for the averager.

The averaging technique described above canintroduce considerable error if time inconsistencies(jitter) exist between the trigger pulse and theevoked response. The response shown in Fig. 11-8was produced after averaging over 256 sweeps, eachsweep being initiated by light flashes at anapproximate 2 second rate; thus 512 seconds or about8.5 minutes are required to produce the response.During this long time interval, both the shape ofthe evoked response and the timing relationshipbetween this evoked response and the light stimulusmay vary and the simple averager will produce aresponse of a longer duration and shorter amplitudethan will be the case if these time inconsistenciesdid not existe

More sophisticated signal-averaging techniques,using digital computers, although inherently moreexpensive than the techniques described above , aresomewhat more versatile as they also incorporatefacilities to analyze statistically the resultsobtained. The Digital Equipment Corporation Lab 8signal averager is typical of a computer-type signalaverager.

Many other computational aids are in use to improvethe quality of signal recovery. For example, thepresence or absence of a rhythmic component in asignal can be determined by cross-correlating thesignal from one amplifier channel with that inanother. This technique is a sensitive indicatorthat a signal is present but does not giveinformation about its amplitude nor necessarilyabout its phase or time relationship.

Although the signal averager is primarily intendedto detect a wanted signal in the presence of unwantednoise, it can also be used to detect a wanted signalin the presence of other unwanted signaIs. Whenrecording the EEG, for example, it is possible todetect small changes in the EEG when flashing alight at the subject.

163

Using conventional amplification techniques, it ispossible to detect the response of the brain to thislight flash as this response is camouflaged by EEGactivity produced within other parts of the brain.The signal averager can be used to detect thisresponse by regarding the EEG as an unwanted signalwhich bears no time relationship to the stimulus.Fig. 11-8 shows a typical EEG recorded with a lightbeing flashed at the subject at approximatelytwo-second intervals. The EEG shown appears normaland it is almost impossible to detect the effect ofthe light flash. The same EEG signal, when analyzedby an averaging computer triggered from the lightflash, clearly shows the electrical response of thebrain to the light flash as seen in Fig. 11-8.

EEG RECORDED WITH SCALP ELECTRODES DURING REPEATED LIGHTFLASHES OF lms DURATION AT APPROX. TWO SECOND INTERVALS.

RESPONSES TO THE LIGHT FLASHESARE BURIED BY THE EEG AND BYNOISEi

50IlV/DIV

t

LIGHTFLASH

LIGHTFLASH

LIGHTFLASH

THE ABOVE WAVEFORM AFTER ANALYSIS BY A SIGNAL AVERAGINGINSTRUMENT TRIGGERED FROM THE LIGHT FLASH.

25 MICROVOLT RESPONSE CANCLEARLY BE DETECTED AFTERAVERAGING WITH 256 SWEEPS

i5jJV/DIV

T___., ~

0.1s/D IV

Fig. 11-8. Use of averaging to detect a response in the EEG.

164

lms/DIV~ ~

i20mV/DIV

T~ PRINCIPAL EVOKED RESPONSE

~ ~ .•.....•ADOITIONAL RESPONSES

~ENCY PERIOD= O.6ms

STIMULUS

Fig. 11-9. Evoked response showing latency and additional responses.

Il.5 TYPICAL EVOKED RESPONSES

stimulus/responsedelay

A typical evoked response obtained with amicroelectrode on a laboratory animal is shown inFig. 11-9. This response waveform is preceded by asmall amount of stimulus artifact. Although it isdesirable to reduce the level of stimulus artifactappearing in recordings, it is often undesirable tocompletely suppress it as it does provide sornetimerelationship in the recorded evoked potential. Theevoked potential begins sometime after the stimulusis received; this delay is known as latency and isapproximately 0.6 milliseconds in the waveform shownin Fig. 11-9.

Typical evoked potentials obtained with anextracellular microeleGtrode with stimulation of thevisual cortex of a laboratory animal are shown inFig. 11-10. For the reasons covered above, stimulusartifact has deliberately been added to theserecordings to provide a time reference. Fig. 11-10shows the evoked response obtained for a singlelight flash and for multiple light flashes separatedby various periods from one another. The 50millisecond pair of flashes is seen by the subjectas one, whereas two flashes are perceived for the150 and 250 millisecond intervals. Unlike theevoked response shown in Fig. 11-9, these responseswere obtained with extracellular electrodes and showthe effect of many cells within the visual cortex;thus the "time scale" of the response is greatlyincreased as many cells are involved.

165

RESPONSE TO SINGLE AND DOUBLE FLASHES OF LIGHT. EACHFLASH IS 10~s DURATION. (ARROWS INDICATE LIGHT FLASH.)

SINGLE FLASH

SINGLE RESPONSE OBTAINED

DOUBLE FLASH50ms APART

SINGLE RESPONSE OBTAINED


DOUBLE RESPONSE OBTAINED


DOUBLE RESPONSE OBTAINED

10011V/DIV

Fig. 11-10. Evoked potentials from the visu al cortex.

166

Il.6 STIMULATION

bra inexposed

stimuluselectrodes

This chapter has previously discussed potentialsproduced as a result of stimulation, however thetechniques used to provide this stimulation have notbeen covered. Stimulation is covered in detail inChapter 12 when discussing the Electromyogram andNerve Conduction. The principles applied tostimulating the peripheral nerves and muscles alsoapply to stimulating the cortex. It should be noted,however, that cortex stimulation may be carried outwith the brain exposed and with stimulationelectrodes spaced by only a few millimeters so as toprovide extremely localized stimulation. Under theseconditions, a greater percentage of the stimulationcurrent passes through the cells of interest.and,thus, the overall stimulus current required toproduce a response is far lower than is the casewhen stimulating through skin and bone withelectrodes placed many centimeters apart. Generally,when stimulating with the cranium opened, stimulatorscapable of outputs up to ten volts are adequate.Stimulus electrodes are normally needles orconcentric needles -- the concentric needle providesstimulation between two "electrodes" less than onemillimeter apart and also provides sornedegree ofshielding to minimize the level of stimulus artifactappearing in the recorded response. Since electricalstimulation may create stimulus artifact potentialswhich camouflage the response, and since it appearsto be desirable to stimu1ate via a sensory moda1ityrather than to use direct stimulation, lightstimulation or other sensory receptor stimulation isoften used.

Il.7 STEREOTAXIe INSTRUMENTS

electrodeplacement

The instrumentation system shown in Fig. 11-6 simplydepicts a microelectrode inserted into a specimen.ln practice, when recording evoked potentials, theskull is held firm1y in a jig and the microelectrodeis positioned with a micromanipulator or stereotaxicinstrument, usually with the aid of a microscope orX-ray techniques, as shown in Fig. 11-11. Althoughstereotaxic instruments are availab1e commercial1y,many are constructed from precision microscopestages.

167

} STEREOTAXie INSTRUMENT

RIGID SUPPORT FOR ANIMALS SKULL

A TYPICAL STEREOTAXICINSTRUMENT AND SKULLSUPPORT

.•

THE FRY* PROBE AND ELECTRODEPOS 1T ION1NG UN IT

VERNIER SCALES ON THEX,Y AND Z AXES PERMITSPOSITIONING TO O.05mm

*INTERSCIENCE RESEARCH INST.CHAMPAIGN. ILLNOIS

Fig. 11-11. Sterotaxic instruments.

168

CHAMBER OFCONTACT LENSFILLED WITH

SAL 1NE

CONTACT LENS WITHELECTRODE EMBEDDEDBELOW VISION AREA

AMPLIFIER AND OSCILLOSCOPE

COMMON ELECTRODE ONSUBJECT'S FOREHEAD

STROBE UNIT

Fig. 11-12. Contact Jens ERG electrode.

169

Il.8 THE ELECTRORETINOGRAM -- ERG

electrodes

stimulus

mon itor

caution

As stated in Chapter 3, it is almost impossible todetect the electrical activity produced by most ofthe sensory receptors on the body as they are toosmall and are dispersed. The notable exceptions tothis are the potentials resulting from stimulationof the middle ear by sound waves and of the retinaby light. If a bright light is projected into theeye, the retina will be stimulated and will generateaction potentials known as the electroretinogram, orERG, which can be detected by an electrode placed onthe outside of the retina.

The human electroretinogram is recorded with the aidof a silver/silver-chloride electrode imbedded in acontact lens, and a "common" silver/silver-chlorideelectrode placed on the subject's forehead, as shownin Fig. 11-12. Application of the contact lenselectrode is painless and it is thus suited toclinical use. The subject is placed in front of alight source which may simply be an incandescentlight coupled to a photographic shutter or, morecommonly, a stroboscopic light as shown in Fig. 11-12.The General Radio Company Type 1504 P4 Stroboscopeis Ideal for this application. Stimuli are presentedin the form of flashes of light and the responses,which are obtained by means of the contact lenselectrode, are displayed on a medical monitor, suchas the Tektronix Type 410 Physiological Monitor.The 410 is operated in the EEG mode with the "EEGelectrodes" formed by the contact lens and commonelectrode. Careful technique is required in affixingERG electrodes and recording the ERG. It is ofparamount importance that the input current of theamplifier used to record the ERG be extremely low asseries damage can occur to the eye if DC current isallowed to flow through an ERG electrode. Sincestimulus artifact is not involved, AC-coupledamplifiers·can be used and a capacitor may be usedin series with the ERG electrode to eliminate DCcurrents.

170

componentsof ERG

A typical ERG is shown in Fig. 11-13 together witha diagram showing the general characteristics of theERG. Variations in the characteristics of thestirnulatinglight, as well as variations in thelevel of the state of light adaptation of the retina,will affect the response characteristics. Theelectroretinograrn shown in Fig. 11-13 was recordedafter the subject had become adapted to roomlighting. The general characteristics of theelectroretinogram consist of an initial A wave,followed by a positive B wave, a more slowlydeveloping positive C wave (which is dependent uponthe duration of the stimulation) and a D wave oroff-effect.

.....I .•• I----~. 5s----I ••.~IO.is/DIV

---1 1-- C

ERG100].JV/01 V

LIGHT FLASH ASTIMULATION

GENERAL ERG CHARACTERISTICS

Fig. 11-13. A typical ERG.

171

STIMULATION - ELECTROMYOGRAPHY

- NERVE CONDUCTION

12.1 ST~TION

artificialstimulation

Chapter 1 covers the bioelectric potentials generatedwithin individual cells when these cells arestimulated. Stimulation refers to an external forcebeing applied to the cell which results in the celldepolarizing and then repolarizing. Cells in theeye are sensitive to light stimulation, cells inthe ear are sensitive to sound stimulation and nervecells are sensitive to electro-chemical stimulation.AlI cells are sensitive to one degree or another toartificial electrical stimulation: The passage ofelectric current through the cell will cause it todepolarize.

When analyzing biomedical phenomena it is oftendesirable to artificially stimulate a group of cells.Such artificial stimulation is achieved by using apulse generator to pass current though the cellsconcerned for a brief period of time. If, forexample, one wishes to cause the ulnar nerve in thearm to propagate a depolarization pulse, this nervecan be stimulated by passing current through the armwith the aid of two surface electrodes placed so thatpart of the current flow between them passes throughthe ulnar nerve.

ln general, it is impossible to localize stimulatingcurrent to a particular cell or small group of cells.Current is passed through the bulk tissue surroundingthese cells and a finite, but unpredictabZe, portionof this current will pass through the cells. Sinceit is impossibZe to ZocaZize current flow to aparticular cell, it is impossible to know how muchcurrent will flow through it.

PULSE GENERATOR111 TRIGGERED

FROM OSCILLOSCOPEGATE WAVEFORM(384), ETC.

172

CO'JSTA T CURRE'JT STIVULATOR CONSTANT VOLTAGE STIMULATOR

.'?

CURRENT.•..---WAVfFORM

~~!~~'~

;;:.•.•.;/i é,ri] ~~~U.E...... .. .~...'

Re

l

,'. '...' ." ", ',' ~. :': ::'

Re .•..------'

ST l'~ULATOP OUTPUT CURRE T tDIVIDED INTO ~A y PATHS AFTERENTERI G TISSUE.

STIMULATOR OUTPUT VOLTAGE EATTENUATED BY ELECTRODE RESISTANCEAND BY TISSUE RESISTANCE 1 SERIESWITH THE CELL TO BE STIMULATED.

Re

CELL TO BE STIMULATEDRESISTANCE RCONTACT RESISTANCE OFTHE STIMULATI G ELECTRODES

Fig. 12-1. Constant current and constant voltage stimulation.

OP AMPOUTPUT±50V OR ±50mA MAX

PULSEGEN#1

PULSEGEN#2

STIMULUS ISOLATION UNITMAY BE USED TO PROVIDE±100V OR ±50mA MAX AND TOPROVIDE ISOLATION FROMGROUND

STIMULUSISOLATION

POSITIVE OUTPUT PULSEFROM PULSE GENERATOR #1

{

AMPLITUDE OF PULSE #1IS 10 TIMES AMPLITUDEOF PULSE #2

WIDTH OF PULSE #1 15ONE TENTH THE WIDTHOF PULSE #2

NEGATIVE OUTPUT PULSEFROM PULSE GENERA TOR #2

C~BINED BI-PHASICSTIMULATING PULSE OUTPUT

PULSE #2 INITIATED BYFALLING EDGE OF PULSE #1

PULSE #1 INITIATED BYA TRIGGER PULSE

Fig. 12-2. Biphasic stimulation pulse generator.

st imu 1atortypes

depolarization

biphasicstimulation

173

Stimulators provide an output from either a constantcurrent source (Zo > 10,000 ohms) or from a constantvoltage source (Zo < 50 ohms). Both constant currentstimulators and constant voltage stimulators areextensively used and arguments for and againstconstant current or constant voltage stimulation haveexisted since the very early days of physiology.Although this controversy as to the best type ofstimulator for routine use still exists, neither typeallows an accurate prediction of the amount ofcurrent passing through the cells concerned.Satisfactory results can be obtained with either type.Many modern pulse generators intended for tissuestimulation provide both constant current and constantvoltage output characteristics. A pair of stimulatingelectrodes immersed in tissue fluid typically has animpedance of about 500 ohms at the frequenciesinvolved in the stimulus pulse.

If a stimulus current of insufficient intensity tocause cell depolarization is applied to a cell, thecell membrane resting potential will be reduced fromits normal -90 millivolts. If an additional stimuluspulse of the same intensity is then applied to theceIl within the next few milliseconds or so, the cellmay then depolarize, as the cell membrane potentialhas not yet returned to its normal resting potential,as discussed in Chapter 1. The cell would thusappear to be more sensitive to stimulation from thesecond pulse than from the first. It has beensuggested that cell recovery from the effect of astimulus current can be hastened by the passage of anopposing, lower intensity, current for an appreciablylonger period so tha~ the net quantity of electricityis zero. This feature is incorporated in the biphasicstimulator shown in Fig. 12-2. Biphasic stimulationalso neutra1izes recording electrode polarizationwhere silver/silver chloride recording electrodes arenot used and helps to maintain a fixed baseline on DCcoupled recording systems. ln the waveform shown inFig. 12-2, the stimulating pulse is followed by apulse of opposite po1arity, of one-tenth theamplitude and ten times the width.

RECORDINGELECTRODES/ .--------..,

AMPLIFIER

STIMULATOR STIMULATOROUTPUT VOLTS

STIMULATINGELECTRODES EQUIVALENT TO

Z5

STIMULATOR IMPEDANCE TO GROUND(ZERO FOR GROUNDED STIMULATORlIMPEDANCE LEAD TO STIMULATORTISSUE IMPEDANCE BETWEEN STIMULATORAND AMPLIFIER + INPUTTISSUE IMPEDANCE BETWEEN STIMULATORAND AMPLIFIER - INPUT

Zr = INPUT IMPEDANCE OF AMPLIFIER

DIFFERENTIALAMPLIFIERINPUT IMPEDANCE Zr

EQUIVALENT TO

E = OUTPUT VOLTAGE OF STIMULATORe = DIFFERENTIAL SIGNAL BETWEEN

AMPLIFIER + AND - INPUTS

lZ2 - ZdZ + Z.5 1

Z5

SINCE Z2 - Zl WILL NEVER BE ZEROe CAN BE MINIMIZED BY MAKING Z AND Z. LARGE:

5 1TYPICAL Z = 20pF, 1090 AND Z. = 30pF, 1070

5 1

e 0:

e PRODUCED WITH 100V STIMULATION AT 1mV/DIV RECORDED 5em FROM STIMULATOR

e STIMULUS ARTIFACT WITH GROUNDED STIMULATORAND 100MO, 2pF AMPLIFIER

e STIMULUS ARTIFACT WITH ISOLATED STIMULATORAND 1MO, 47pF AMPLIFIER

ARTIFACT WITH GROUNDED STIMULATOR47pF AMPLIFIER

~ ~1ms/DIV

Fig. 12-3. Equivalent circuit ofstimulator, subject and amplifier.

175

12.2 STTIMULUS ISOLATION

reducingstimulusart itact

isolatedstimulator

high inputZ amp 1i f ier

The previous section referred to stimulation.Stimulation may be produced directly from a pulsegenerator or it may be produced via a stimulusisolation unit. The results of stimulation may bedetected with an oscilloscope and it is importantthat the pulse produced by the stimulator does notinterfere with the response obtained on theoscilloscope. It is thus necessary to insure thatthe stimulating current is limited to the areabetween the two stimulating electrodes and thatlittle or no stimulating current appears in theregion of the recording electrodes.

Any stimulating current flowing near the recordingelectrodes will cause a potential difference betweenthese electrodes which will appear as an out-of-phasesignal and thus not be rejected by the differentialamplifier. The equivalent circuit involved instimulating and recording is shown in Fig. 12-3. lnthis circuit, most of the stimulating current flowsthrough the impedance formed by the tissue betweenthe stimulating electrodes. It is apparent, however,that two alternative current paths exist; via thetissue impedances Zl and Z2 and the amplifier inputimpedances to ground. If it were possible to makeZl equal to Z2, then no in-phase signal would beproduced at the recording electrodes. Theseimpedances are not controllable however, and it isimpossible in practice to balance them by carefulplacement of the recording electrodes.

Since it is not possible to balance the tissueimpedances, then one must reduce the stimulatingcurrent passing through these impedances to as lowa value as possible in an effort to reduce thedifferential signal appearing at the recordingelectrodes. To reduce these currents, it is necessaryto either increase the input impedance of thedifferential amplifier or to increase the impedancebetween stimulator and ground. ln practice oneattempts to make both these impedances as large aspossible in an effort to achieve the maximumreduction in the level of stimulus signal appearingat the recording electrodes. With a nonisolatedstimulator, the impedance between stimulator andground is zero, however with an isolated stimulatorthis impedance can be made very high, typically 20picofarads and 109 ohms. A typical differentialamplifier input impedance would be 30 picofarads and107 ohms.

176

acceptableart i facts

amp1if i erover load

AC-coupledamp1if i ers

If a grounded stimulator is used on tissue, theground electrode should aZways be placed between the"hot" stimulating electrode and the recordingelectrodes, to minimize stimulus artifacts. Theupper trace shown on the waveform in Fig. 12-3 showsan acceptable level of stimulator pulse appearing atthe recording electrodes when the recording electrodesare approximately 5 centimeters from the stimulatingelectrodes. This trace shows a 100 volt stimulatorpulse reduced to less than 0.5 millivolts, a reductionof 200,000:1, which is entirely acceptable. Thesecond trace shown on the waveform in Fig. 12-3 isalso acceptable as, although the level of.stimulatorpulse appearing at the recording electrodes issubstantially greater than shown on the upper trace,the amplifier quickly returns to zero after thestimulating pulse returns to zero. The lower traceshown on the waveform in Fig. 12-3 is unacceptable.ln this instance, the stimulating pulse has severelyoverloaded the differential amplifier and theamplifier takes longer than 10 milliseconds to returnto zero. Any response occurring in this 10millisecond period would be either camouflaged by,or distorted by, this amplifier overloadcharacteristic.

ln practice, complete reduction of the stimulatorpulse is rarely achieved and a severely attenuatedstimulator pulse will appear on the oscilloscopetrace. This attenuated stimulator pulse is referredto as stimulus artifact. Stimulus artifact is notaltogether undesirable if it is not of excessiveamplitude as it does provide a time reference onthe CRT trace.

The above discussion assumes the use of DC-coupleddifferential amplifiers. If AC-coupled differentialamplifiers are used, the time constant of the inputcoupling capacitors and the input resistance willdetermine the amplifiers recovery characteristicsafter overload.

12.3 STRENGTH/DURATION CURVES

Strength/duration curves show the excitabilitycharacteristics of muscle or nerve fibers.Stimulation of a cell is achieved by the passage ofa certain quantity of electricity through the cell,

nerve tissuethreshold

177

thus stimulation is dependent on charge rather thanon current. Stimulation can be achieved by passinga large current for a short period or with a lessercurrent for a longer periode Since diffusion withinthe cell tends to oppose the stimulating current, alower limit of current is reached below whichstimulation will not occur, no matter how long thecurrent is maintained.

For any given muscle or nerve fiber a strength/duration curve may be drawn, as shown in Fig. 12-4,which represents the minimum stimulus or thresholdrequired to stimulate the muscle or nerve fiber.Referring to the strength/duration curve shown inFig. 12-4 for a normal muscle, it can be seen thata 0.01 millisecond stimulus pulse of 3.5 amplitudeunits applied in the region of the nerve associatedwith the muscle will cause the nerve to depolarize,causing muscular action. Alternatively, muscleaction can be produced with a 0.03 millisecond widestimulus pulse of 1.7 amplitude units, a 0.1millisecond wide stimulus pulse of 1.1 amplitude unitsor a 0.3 millisecond wide stimulus pulse of 1.0amplitude units. It is also apparent that, for pulsewidths greater than 0.3 millisecond, no furtherreduction in stimulus amplitude will be effective.This amplitude, below which stimulation will notoccur, no matter what the stimulus pulse width is,is referred to as the nerve tissue threshold orrheobase for the muscle. The term is now rarely used.

6,, ,

5

t 4

VOLTS~NORMAL

ST IMULUS 3 MUSCLEAMPL ITUDE

VOLTS NORMAL 1ZED 2 2RTO RHEOBASE FORNORMAL MUSCLE I----'C "

1 f-R = 1 ' 1

\_i DENERVATEu

MUSCLEPARTIALLYDENERVATEDMUSCLE

C'If

f' .~,.3 100 300

,1 3

,10.'01 t .03 .1

0.022ms30

ms~STIMULUS PULSE WIDTH

R = RHEOBASE FOR NORMAL MUSCLEC = CHRONAXY FOR NORMAL MUSCLE

0.22ms FOR EXAMPLE GIVENC'= CHRONAXY FOR OENERVATEO MUSCLE

llms FOR EXAMPLE GIVEN

Fig. 12-4. The strengthjduration or S-D curve.

178

pulseampl itudenormal izedto rheobase

chronaxy

denervatedmuscleaction

Pulse widths of from 0.03 millisecond to 100millisecond in eight steps have been proposed asan international standard for strength/durationcurve determination.

ln the particular example shown in Fig. 12-4 thisrheobase occurred when using a constant voltagestimu1ator set at 30 V. Since, as discussed earlier,this absolute voltage 1eve1 has little meaning,strength/duration curves are normally shown with astimulus amplitude normalized to the rheobase, thatis 30 volts in this instance is equivalent to 1.0unit of stimulus amplitude, 2.0 units of stimulusamplitude being equivalent to 60 volts, etc.

Once a strength/duration curve has been determined,it is desirable to have a technique whereby curvesfor various muscles can be compared. Strength/duration curves may be compared by comparing thechronaxy obtained from the curve. The chronaxy isthe minimum pulse width required to excite the tissuefor a stimulus of twice the amplitude of therheobase. The chronaxy, in the example shown onFig. 12-4, is 0.022 millisecond for normal muscle.

The strength/duration curve for a denervated muscleis a1so shown in Fig. 12-4. A denervated muscleexists when the nerve connection to the muscle haseffectively been interrupted. This may be achievedby impairing the motor end plate action with drugssuch as curare. It can be seen from Fig. 12-4 thata denervated muscle is 1ess sensitive to stimulusthan a normal muscle. ln a normal muscle thestimulating current excites the more sensitive nervefibers which in turn excites the muscle whereas ina denervated muscle the stimu1ating current muststimulate the muscle directly. Thus the strength/duration curve for a normal muscle shows theexcitability characteristics for a single motorneuron, however, the strength/duration curve for adenervated muscle shows the excitabilitycharacteristics of a single muscle fiber.

musclereaction tostimul i

abbreviatedstrength/durationcurves

179

Fig. 12-4 also shows a strength/duration curve for apartially denervated muscle. It can be seen, forpulse widths up to 10 milliseconds, muscle action isdue to stimulation of the nerve whereas for pulsewidths greater than 10 milliseconds muscle action isdue to stimulation of the muscle directly. Thestrength/duration curve is, thus, displaying theexcitability characteristics of the component whichhas the lower threshold for a specifie pulse width.

When preparing strength/duration curves, muscleaction is determined by viewing the muscle concernedin a weIl-lit environment. When stimulating with a100 millisecond pulse, the muscular contractionproduced is brisk and pronounced when the contractionis due to excitation of the nerve fibers and issluggish and wormlike when the contraction is due todirect excitation of the muscle fibers. Whenstimulating with narrower pulses, muscle action ischaracterized by a twitching of the muscle concerned.Muscle action can also be determined by using sorneform of force-gage attached to the muscle concernedto detect muscular movement or, more commonly, byrecording the electrical activity produced withinthe muscle when the muscle is stimulated by thenerve.

ln sorneinstances, the plotting of the strength/duration curve is somewhat tedious and abbreviatedstrength/duration curves are obtained by determiningthe stimulus amplitude required for a 100 millisecondpulse and a 1 millisecond pulse and expressing theseas a ratio. ln a normal muscle this ratio should beapproximately unity, in a partly denervated muscleit will be between 1.5 and 4 and in a fullydenervated muscle it will be between 4 and infinity.Abbreviated strength/duration curves may also beobtained by determining the rheobase and thendetermining the chronaxy directly by doubling thestimulus amplitude and reducing the pulse width toa point where the tissue can no longer be stimulated.This point will represent the chronaxy.

180

USE FORCE GAGE lN CONJUNCTIONWITH TEKTRONIX 5648 STORAGEOSCILLOSCOPE, 3C66 CARRIERAMPLIFIER ANO 2867 TIME BASEAS SHOWN lN FIG. 9-4.

FORCE

50J,l lSTRAIN/OIV 1-

2s/DIV 1 W CONTRACT ION

~ RELAXAT ION

Fig. 12-5. A myrograph using an elastic force gage.

12.4 MYŒRAPHY

straingage

Myography is the study of muscular contractions anda myograph is an apparatus for recording themechanical effects of a muscular contraction. Amyograph may simply consist of a displacementtransducer or a force transducer mechanically coupledto the muscle under investigation. As shown inFig. 12-5, an eLastLc strip is placed around themuscle concerned and a strain gage is bonded to thiselastic strip. Muscular contraction causes a tensionincrease in the elastic strip which results in aresistance change in the strain gage. The muscularcontraction may be initiated voluntarily or producedby electrical stimulation. A strain gage myographmay be used with a Tektronix Type 564B StorageOscilloscope and a Tektronix Type 3C66 CarrierAmplifier as shown in Chapter 9, Fig. 9-4. Theoutput from such a recording system, a series ofmuscular contractions over a 20-second period, isalso shown in Fig. 12-5. Force myographs areparticularly suited to exercising subjects or forthe study of muscular fatigue over prolonged periods.

181

DUAL TRACESTORAGE OSCILLOSCOPE

564BoPICKUP

ELECTRODES

CHIA P

CH2

TIMEBASE

3A72 3B4

3A9 AND 3A8 USED lN A YTEKTRONIX S60 SERIESOSCILLOSCOPE (e.g., 56IB).

ABSOLUTE VALUECIRCUIT - "FULLWAVE RECTIFIER"

INTEGRATOR GATEDWITH TIME BASE

3A8 OPERATIONAL AMPLIFIERWITH ABSOLUTE VALUE ADAPTERAND GATING ADAPTER.(SEE CHAPTER 29 FOR DETAILS)

3A9 DIFFERENTIAL AMPLIFIER SET AT ImV/DIV. PROVIDES A GAINOF 1000X TO THE 3A8 OPERATIONAL AMPLIFIER AND 3A72 CHANNEL 1.

3A72 CHANNEL 1 SET AT IV/DIV PROVIDING A SENSITIVITYREFERRED TO THE ELECTRODES OF ImV/DIV.

3A8 OPERATIONAL AMPLIFIER HI USED lN CO.JUNCTIO WITH AABSOLUTE VALUE CIRCUIT AS SHOWN lN CHAPTER 29.

3A8 OPERATIONAL AMPLIFIER H2 USED lN CONJU CTION W!TH A LOWSPEED GATING ADAPTER AS SHOW lN CHAPTER 29.Zj SET AT ImO, Zf AT O. luF FOR 1 TEGRATION.

3A72 CHANNEL 2 SET AT O. l, 0.2 OR O.SV/cm PROVIDING VARYING1 TEGRATION SENSITIVITIES.

INTEGRATOR CAL 1BRAT ION - OSCILLOSCOPE AMPLIFIER #2 AT 0.2V/DIV

io.sevTr 2 DIV

4mV CALIBRATOR SIGNAL FROM OSCILLOSCOPE CALIBRATORCONNECTED TO DIFFERENTIAL AMPLIFIER.DIFFERENTIAL AMPLIFIER GAIN REDUCED BY 5X (FROM ImV/DIVTO 5mV/DIV) PROVIDING A CALIBRATIO VOLTAGE RELATIVE TOTHE EMG SYSTEM OF O.8mV.CALIBRATOR DUTY FACTOR IS 50%.

CALIBRATOR INPUT SHOWN = 0.8 X 10-3 X I~~ X 0.2 VOLT SECONDS= 80 X 10-6 VOLT SECONDS.

2.2 DIV OF INTEGRATOR OUTPUT - 80 X 10-6 VOLT SECONDS:. INTEGRATOR CALIBRATION IS 36 X 10-6 VOLT SECONDS/DIVWITH OSCILLOSCOPE AMPLIFIER #2 AT 0.2V/DIV.

INTEGRATOR OUTPUT =180 X 10-6 VOLT SECONDS INPUT/VOLTS OUTPUT

Fig. 12-6. An electromyograph system.

182

12 . 5 ELECTRClv1YOGRAPHY -- FMG

musclefiberdepo 1arization

Whereas the myograph records the mechanical effectsof a muscular contraction, the electromyographrecords the electrical effects of such a contraction.Muscular contraction is caused by depolarization ofthe muscle fibers. This depolarization produces anaction potential as covered in Chapters 1 and 3.This muscular action potential is known as theelectromyogram, or EMG. An electromyogram will beproduced in a muscle when the muscle contraction iscaused either by voluntary muscle action or byelectrical stimulation of the muscle.

12.6 ELECTROMYOGRAPHY WITH VOLUNTARY MUSCULAR ACTION

instrumentation

absoluteintegrals

A typical system for recording the electromyographproduced by voluntary muscle action is shown inFig. 12-6. Further details on this system are givenin Chapter 29. The muscle action potential is pickedup by needle electrodes inserted into the muscle orby surface electrodes placed over the muscleconcerned and then amplified by a suitabledifferential amplifier. The EMG can then be detectedaudibly by using a speaker in conjunction with anaudio amplifier. The EMG may also be displayeddirectly on an oscilloscope or may be converted toan absolute integral and then displayed on anoscilloscope. Both direct displays and integrateddisplays are shown in Fig. 12-7.

Referring to Fig. 12-7, the upper trace on the topphotograph shows an EMG produced by a mild voluntarycontraction. The action potential produced by asingle motor unit can clearly be differentiated fromother action potentials. The absolute integral ofthis activity is displayed on the lower trace in thesame photographe The integral displays the quantityof electricity involved in the muscular contraction.The quantity of electricity associated with thesingle motor unit action potential can also bedetermined.

With a more forceful voluntary contraction, as shownin the middle photograph, many motor units areinvolved and the EMG obtained is the result of theaction potential produced by aIl these motor units;the resulting integral being greater than the integralobtained for a single motor unit.

183

By using a slower sweep speed in conjunction withthe system shawn in Fig. 12-6, the EMG and absoluteintegra1 for a series of contractions may bedisp1ayed as shown in the bottom photograph.

(Al

SINGLE MOTORUNIT ACTIONPOTENTIAL

MILD VOLUNTARY CONTRACTIONSHOWING ACTION POTENTIALFROM A SINGLE MOTOR UNIT.

.6 DIVISIONS = 11 X 10-6 VOLT SECONDS

(Bl

FORCEFUL VOLUNTARY CONTRACTIONSHOWING SUMMATION OF ACTIVITYFROM MANY MOTOR UNITS.

1.2 DIVISIONS = 43 X 10-6 VOLT SECONDS

(C)

A SERIES OF MILD VOLUNTARYCONTRACTIONS - MAY BE USEDTO SHOW FATIGUE.

EACH CONTRACTION 0.2 DIVISIONS= 18 X 10-6 VOLT SECONDS/CONTRACTION

UPPER TRACE EMG lmV/DIV

LOWER TRACE JEMG (Al O. lV/DIV = 18 X 10-6 VOLT SECONDS/DIV(Sl 0.2V/DIV = 36 X 10-6 VOLT SECONDS/DIV(C) 0.5V/DIV = 90 X 10-6 VOLT SECONDS/DIV

SWEEP SPEED (Al AND (8) = 20ms/DIV(C) = 100ms/DIV

Fig. 12-7. Results obtained with the system shown in Fig. 12-6.

184

el ectri ca 1quantity

voltseconds

ca 1ibrat ion

The quantity of electrical activity produced bya muscular contraction of a montomic function ofthe strength of the contraction. Since it isdifficu1t to estimate this quantity from anobservation of the EMG waveform, the absoluteintegral of the EMG is used as a measure of thisquantity. The integrator output is calibrated inunits of electrica1 quantity, that is, volt seconds.Since one volt second is a substantial quantity ofelectricity, it is preferable to refer to EMGrs insmaller units, one "unit" being equal to 10-6 voltseconds. It can be seen from Fig. 12-7 that for aparticular subject and a particu1ar muscle Il unitsare produced by a single motor unit, 18 units by amild muscular contraction and 43 units by a forcefuIcontraction.

Referring to Fig. 12-6, the absolute integral isobtained by full-wave rectification of theelectromyograph and then integrating this rectifiedsignal with an integrator gated "on" for theduration of the oscilloscope sweep. The wholesystem consisting of the differential amplifier, thefull-wave rectifier, the integrator and the displayoscilloscope can be calibrated using an oscilloscopecalibrator waveform as shown in Fig. 12-6. Thesystem shown in Fig. 12-6 may be duplicated by usingone Type 56lB Oscilloscope in conjunction with aType 3A9 DifferentiaI Amplifier and a 3A8Operational Amplifier and one Type 564B StorageOscilloscope in conjunction with a Type 3A72 DualTrace Amplifier and a 3B4 Time Base unit. Detailsof the absolute value adapter and gating adapterfor the 3A8 are given in Chapter 29.

audiooutput

12.7

185

The EMG is often presented audibly in clinicalapplications and the trained listener can judge thecondition of the muscle by the volume andcharacteristic tones produced by the audio systemduring a muscular contraction.

ELECTROMYOGRAPHYDURING ELECTRlCALST~TION

vo 1untaryversuss+ imuIlatedEMG

The EMG produced during a voluntary twitch is spreadout over a period of 100 mi11iseconds or more as thenerve impulses to various motor units are not timecoincident as the propagation delay from the spinalcord to the muscle concerned is different for aIlnerve fibers. Also, since the contraction isvoluntary, any one motor unit may produce severalaction potentials, the frequency of discharge beingdetermined within the spinal cord. Such is not thecase when recording EMG produced by electricalstimulation. AlI neurons with thresholds above thestimulating intensity are simultaneously stimulatedby the electrical impulse, thus aIl muscle fibersdischarge simultaneously, producing substantialactivity for a brief period of time, typically lessthan 10 milliseconds. Although the response obtainedwhen stimulating is referred to as an EMG,. it is anunnatural occurrence and should perhaps be morecorrectly referred to as a "myographie response" ora "muscle action potentiaL. Il The stimulus pulse usedto initiate this response usually has an amplitudeof >100 volts and is either 0.1 millisecond, 0.3millisecond, or, occasionally, 0.5 millisecond wide.

186

---..--~..STIMULATOR TO SCOPE~TRIGGER

TO STORAGEOSCILLOSCOPE

(Al USING AN ISOLATED STIMULATOR

TO SCOPETRIGGERSTIMULATOR

'PICKUPELECTRODES(SURFACEELECTRODES)

TO STORAGEOSCILLOSCOPE

(8) US 1NG A GROI:JNDEDSTiMULATOR

TO SCOPETRIGGER

LATENCY ACTION POTENTIAL4ms 7ms

OIPHASIC RESPONSE

1'" ,..1AMPLIFIER RECOVERYFROM OVERLOAD


MONOPHASIC RESPONSE


TRIPHASIC RESPONSE

(C) US 1NG A.COI\CENTRIC NEEDLE ELECTRODE

Fig, 12-8.. EMG produced by electrical stimulation.

latency

diphasicresponse

monophasicresponse

triphasicresponse

187

A typical EMG produced by electrical stimulation isshown in Fig. l2-8A. A 0.3 millisecond stimuluspulse was used to initiate the response. A delayoccurred between the stimulating pulse and theresponse; othis delay is referred to as latency. Theaction potential shown in Fig. l2-8A has a latencyof 4 milliseconds and produces an action potentialcovering the following 7 milliseconds. This actionpotential is referred to as diphasic as it shows asingle positive deflection followed by a singlenegative deflection.

The action potential shown in Fig. l2-8B is referredto as monophasic as the action potential appears tobe comprised of only one positive deflection. Thisaction potential is partially camouflaged by theamplifier recovery characteristics due to the use ofa nonisolated stimulator. Such amplifiercharacteristics were covered earlier in this chapter.If a grounded stimulator is used on tissue, theground electrode should aZways be placed between the"hot" stimulating electrode and the recordingelectrodes, to minimize stimulus artifacts.

The EMG shown in Fig. l2-8C is referred to astriphasic as two positive deflections and onenegative deflection are exhibited. This EMG wasrecorded using a concentric needle pickup electrodewhich effectively locates two electrodes less than1 millimeter apart in the muscle in an attempt torecord the action potential produced by a singlemotor unit rather than by the complete muscle. AlIthree EMG's shown in Fig. 12-8 are consideredacceptable and basically serve to show that the nerveand muscle relationship is functioning correctly asthe latency is not excessive and an action potentialis, in fact, generated.

12.8 THE H REFLEX

The muscular reflex response generated within thespinal cord was covered in Chapter 3. When recordingEMG's produced by electrical stimulation, thestimulating current excites the motor nerve which inturn initiates a response in the muscle concerned.

188

muscleresponseviaref 1 exstimulation

This stimu1ating current a1so excites the sensorynerve. It is possible to decrease the stimulus leve1to a point where the stimulus intensity isinsufficient to excite the motor nerve but issufficient to excite the more sensitive sensorynerve. The depolarization p~lse propagated in thesensory nerve as a resu1t of-this stimulation trave1sto the spinal cord where a reflex response is in turnpropagated in the motor nerve. This reflex responsepropagates a10ng the motor nerve to the muscleconcerned, initiating a muscular response.

STIMULATINGr: ELECTRODE

V MOTOR/1\ NERVE

SENSORYNERVE

INALCORD

[CORDINGELtCTRODES

MUSCLE ACTION PRODUCED BY STIMULUS CURRENTDEPOLARIZING MOTOR NERVE (DIRECT) OR BY STIMULUSCURRENT DEPOLARIZING SENSORY NERVE WHICH lN TURNDEPOLARIZES MOTOR NERVE DUE TO REFLEX RESPONSEGENERATED lN SPINAL CORD (INDIRECT).

5ms/DIV....j~

LOW STIMULUS CURRENT PRODUCES ARESPONSE GENERATED BY REFLEX ACTIONlN THE SPINAL CORD. NO DIRECT MOTOR

~--- NERVE REACT 1ON TO ST1MULUS.

INCREASING STIMULUS CURRENT

•...--MOTOR NERVE DIRECTLY STIMULATED BYHIGH STIMULUS CURRENT; RECOVERYBLOCKS "H" RESPONSE.

NORMAL "M" RESPONSE

~---~ LATENCY OF "H" REFLEX RESPONSE - 20ms

"H" REFLEX RESPONSE

~STIMULUSlms

Fig. 12-9. The H-Reflex response.

H reflex

M ref 1ex

189

The physical relationship between the muscle, thenerve and the spinal cord is shown in Fig. 12-9,together with the results obtained when recordingthe EMG with progressively increasing stimulusintensity. At low stimulus intensity a responsehaving a latency of approximately 20 milliseconds isdetected, known as the "H reflex" response. Thislatency is due to the conduction time from thestimulating point along the sensory nerve to thespinal cord and thence from the spinal cord alongthe motor nerve to the muscle concerned. As thestimulating current is progressively increased thisH reflex response decreases and a normal or "Mreflex" response appears with a normal latency of7 milliseconds, representing the conduction timefrom the stimulus site, via the motor nerve, to themuscle concerned. The "H reflex" response can beused to determine the condition of the reflex system.

12.9 NERVE CONDUCTION

motor nervepropagationvelocity

The propagation velocity of the nerve impulse alongthe motor nerve from the stimulus site to the musclecan be determined as shown in Fig. 12-10. ln theexample shown, the peroneal nerve of the left leg isstimulated behind the knee and a muscular responseis detected in the foot, using either surfaceelectrodes or needle electrodes. The response shownhas a latency of Il.5 milliseconds. The stimuluselectrodes are then moved to a point behind theankle and a response obtained in the foot having alatency of 4 milliseconds. The difference betweenthese two latencies is attributed to the conductiontime required for the nerve impulse to propagatealong the motor nerve from the knee to the ankle.

PERONEAL NERVE OF THE LEFT LEG

MeTOR NERVE STIMULATED AND ACTIONPOTENTIAL DETECTED lN APPROPRIATE MUSCLE.

tST1MULUS SITE 11----.,..

KJIIEE

KJIIEETOANKlE38cm

STIMULUS SITEANKLE

-L..._ __ TO OSC 1LLOSCOPE /

TIME DIFFERENCEBETWEEN ACTION

POTENTIAL - 7.5msCONOUCTION VElOCITY - 38cm lN 7.5ms

- 51m/s

Fig. 12-10. Motor nerve conduction velocity determination.

190

sensorynervepropagationvelocity

The propagation velocity can be determined bymeasuring the distance from the knee stimulationpoint to the ankle stimulation point and dividingit by the difference in latencies. A 100 voltstimulus pulse, 0.3 millisecond wide, is usually used.Occasionally, pulse widths of either 0.1 millisecondor 0.5 millisecond are used. The stimulus should berepeated several times to ensure that the responseobtained is consistent.

A similar technique can be used to measure sensorynerve conduction velocity as shown in Fig. 12-11.ln Fig. 12-10 we moved the stimulus site withrespect to a fixed recording site, however, whenattempting to record sensory nerve conductionvelocity, it is necessary to stimulate at a fixedsense receptor site and to record the propagation ofthis stimulus pulse along the sensory nerve bydetecting the nerve impulse or "traveling wave ofdepolarization" at various sites along the nerve.Fig. 12-11 shows the results obtained whenstimulating the hand and recording the propagationof this pulse along the ulnar nerve at four pointsalong the length of the nerve using a four-channeloscilloscope. If the vertical position of the fourchannels on this oscilloscope are adjusted torepresent distance in centimeters from the stimulus~ite, the various latencies obtained should bedirectly proportional to this distance, resulting inthe straight line shown in Fig. 12-11. Anydeviation in straightness in this line wouldrepresent a change in conduction velocity. Injuryto the nerve will normally result in decreasedconduction velocity in the injured part of the nerve.

74116cm

58 1t17cm

41 11

28cm

,,/t

13cm

o t

191

ULNAR NERVE OF THE RIGHT'ARM

SENSORY NERVE STIMULATED AND "TRAVELING WAVE OF DEPOLARIZATIONAND REPOLARIZATION" DETECTEO WITH SURFACE ELECTRODES

\\_RECORDING SITE #1 "~ NECK 80

e+--RECORD 1NG SITE #2ARMPIT

70

60

#1

t 50

40

.;-- RECORD 1NG SITE 113ELBOW

cm

RECORDING SITE #4WRIST

30

20

10

o

/#4

1l,NEAR STRAIGHT LINE SHOWS CONSTANTCONDUCTION VELOCITY ALONG NERVE

THE FOUR RECORDINGS ARE STACKEDSO THAT THEIR DISTANCES FROM THEBASELINE REPRESENT THE DISTANCEFROM STIMULUS SITE TO RECORDING SITE

STIMULUS SITE

'" - AT LITTLE FINGER

DISTANCE (lN cm) ALONGULNAR NERVE FROMSTIMULUS SITE

CONDUCTION VELOCITY - #1 TO #4 - (16 + 17 + 28)cm lN 12.5ms49m/s

Fig. 12-11, Sensory nerve conduction velocity determination,

192

1imb nervesinvestigated

40 to 60mis normal

electrodiagnosis

When measuring conduction velocity four nerves areprincipally investigated: the ulnar and mediannerves of the arm and the peroneal and tibial nervesof the legs. The ulnar, peroneal and tibial nervesperform both general sensory and motor functions,however the median nerve primarily performs asensory function. Responses from the ulnar nerveare normally detected on the back of the hand or onthe fingers, the median nerve on the thumb or on thethick fleshy part of the hand near the thumb, theperoneal nerve on the four smaller toes or the topof the foot and the tibial nerve at the side of thefoot near the larger toe or on the larger toe.

Normally, conduction velocity is measured, or EMG'sare recorded, in an attempt to diagnose anabnormality in the subject and, since thisabnormality usually manifests itself in only one sideof the subject at a time, the limb on the other sideof the subject may be regarded as normal andc~nduction velocities or EMG's compared between thetwo limbs. Although conduction velocity depends onthe nerve under investigation, conduction velocitiesin most healthy nerves fall in the range from 40 to60 meters per second; injury or abnormality beingindicated by lower conduction velocities extending tobelow 10 meters per second. The termelectrodiagnosis is applied to the general study ofnormal motor unit behavior. Completeelectrodiagnosis should include most of thetechniques covered in this chapter.

12.10 REPETITIVEST~TION

recoverycharacteristics

criticalfrequency

So far this chapter has dealt with the effects of asingle stimulating pulse on muscle and nerve fiber.ln attempting to determine the recoverycharacteristics of a motor unit it is necessary tostimulate with a double pulse and to determine thedelay required between the two pulses for thestimulus pulse to be "seen" by the muscle as twoseparate stimuli rather than as only one stimulus.This minimum pulse separation, when translated tofrequency, is known as the critical frequency. Thecritical frequency varies for different muscles inthe same subject and for the same muscle at adifferent temperature and in a different state offatigue. The critical frequency for most of themajor muscles in the human body lies between 5 and15 Hz corresponding to minimum pulse separations of200 milliseconds and 66 milliseconds.

193

Multiple pulse stimulation, rather than single pulseor double pulse stimulation, is used to determinethe fatigue characteristics of nerve and muscle fiber.Under normal conditions, the response obtained duringprolonged stimulation should show little change fromthe response obtained from a single stimulus as longas a brief relaxation period is allowed between eachpulse.

12.11 EMOOTI-IMUSCLE POTENTIALS

electrogastrogram

Previously in this chapter we have dealt with theelectrical activity produced in muscles known asskeletal muscle. Skeletal muscle producescontractions in a series of muscle fibers, thecombined effect producing continuous motion. Othermuscles, particularly the muscles surrounding themajor organs in the torso, produce overall continuouscontraction and are known as "smooth" muscles."Smooth" refers to the microscopic appearance of themuscle, in contrast to "striated" skeletal muscle,with its characteristic cross stripes. Electrodesplaced on, in, or over smooth muscle can be employedto detect contractions in these muscles. Electrodesappropriately inserted into the stomach, bladder,etc. or placed on the surface of the body over theseorgans can serve to monitor the slowly varyingpotentials generated by the muscles in these organs.These potentials are known as smooth musclepotentials. Although these potentials are rarelymonitored, the recording of the electrical activityproduced by the stomach has been termed theelectrogastrogram. The electrogastrogram ischaracterized by a slowly changing "DC" potential(below 1 Hz) which would normally be recorded on aDC-coupled instrument at a sensitivity of 10millivolts per division and at a sweep speed ofperhaps 1 second per division.

194

AREA OF SUBJECT WITHA HIGH CONCENTRATIONOF SWEAT GLANDS(PALMS AND SOLES)

TO 1. IMPEDANCE MEASURING CIRCUITTO DETECT ôR

OR

2. DC AMPLIFIER TO DETECT E

GSR DESIGNATED BYEITHER CHANGE lN RESISTANCE (ôR)OR GENERATION OF POTENTIAL (E)BETWEEN ELECTRODES

AREA OF SUBJECT DEVOIDOF SWEAT GLANDS(TRUNK, EARS, AND LIMBS)

Fig. 13-1. GSR measurement.

195

GALVANIC SKIN REFLEX - GSR

13.1 THE AUTONOMIe NERVOUS SYSTEM

sweatglandactivity

Fereeffect

Tarchanoffeffect

The human autonomic nervous system is the systemwithin the body that regulates body functions suchas temperature, respiration and glandular activity.When a subject is psychologically excited or is insome other elevated state of psychological activity,the subject perspires or "sweats." This is due toan emotional stimulus initiating a response in theautonomic nervous system which in turn produces aresponse in the subject's sweat glands. Detectionof this sweat gland activity is, thus, an indicationof the subject's psychological state or state ofarousal.

Activity of the sweat glands is referred to by oneor more of the following terms: electrical skinresistance (ESR), galvanic skin reflex (GSR),electrodermal response (EDR) and psychogalvanicreflex (PGR). Occasionally, the term GSR is referredto as the galvanic skin reaction or galvanic skinresistance. These terms aIl relate to one or bothof the following physiological changes associatedwith sweat gland activity: a change in resistanceand the generation of a potential between areascontaining many sweat glands and areas almost devoidof them. The change in resistance is referred toas the Fere effect on the exosomatic response of theGSR. A decrease in the subject's resistanceindicates arousal. Relaxation is indicated by anincrease in resistance. The generation of a potentialdifference is referred to as the Tarchanoff effector the endosomatic response of the GSR. Thisresistance change and potential generation isrepresented by ôR and E in Fig. 13-1.

196

O. 1Hz -

LEAD ELECTRODES PLACED ON HAND

REFER TO CHAPTER 29FOR DETAILS OF THECONSTANT CURRENTPULSE GENERATOR hd400ms

AC COUP LED3dB LOWER FREQUENCY

~

CONSTANTCURRENTPULSE GEN

50kn

25kn~ ~

5s/DIV

AMPLIFIER GAIN AND POSITION ADJUSTEDWITH 25kn AND 50kn CAL 1BRAT IONRESISTORS FOR SCALE CALIBRATION.ALLOWS FOR EFFECT OF AC COUPLINGAND 3A9'S lMQ INPUT RESISTANCE.

Fig. 13-2. GSR measurement.

197

13.2 GSR MEASUREMENT BY RESISTANCE CHANGE

DCtechniques

ACtechniques

GSR measurement by this technique involves thedetection of an impedance change between twoelectrodes on the subject. The GSR primarily changesthe resistive component of this impedance, it is thusimportant that the measurernent technique used beinsensitive to reactive component changes. Thesimplest technique would appear to be the passageof a DC current via the electrodes and the detectionof the voltage drop produced between the electrodesdue to this current flow. Since, however, the-GSRis usually recorded for prolonged periods, theelectrodes used cannot be relied upon as theseelectrodes will undoubted1y produce a DC offsetpotential of several hundred millivolts due to thepassage of current for a prolonged period. Thisoffset potentia1 could not be differentiated from apotential produced by a change in the subject'sresistance. For this reason, DC techniques have beenfound to be unsatisfactory.

Since it is desirable to emp10y a measurernenttechnique sensitive primari1y to resistance change,and since it is undesirab1e to use DC measurementtechniques, very low frequency AC techniques areinvariably used. These measurement techniquesinvolve the passage of an AC current of perhaps10 microamperes peak at a frequency of 2 or 3 Hz.The resulting voltage drop between the electrodescan then be detected as an AC signal which will beindependent of any DC offset potentials generatedat the electiodes; such a technique is shown inFig. 13-2. Referring to Fig. 13-2, a constantcurrent pulse generator adapter, used in conjunctionwith a Type 561B or 564B oscil1oscope's calibrator,provides pulses of 10 microamperes, with a durationof 40 milliseconds. Further details on this adapterare given in Chapter 29. Since this pulse waveformhas only a 10% duty cycle, a change in subjectresistance, when using an AC-coupled amplifier, willprimarily alter the displayed pulse amplitude andwill cause almost no shift in the oscilloscope zerolevel or base line.

198

electrodes

typicalresistances

Since, when recording the GSR, we are recordingresistance changes due to action of the sweat glandsat the surface of the skin, electrodes should beused that make direct contact with the skin as theuse of any conductive electrode paste would interferewith the action of the sweat glands. The electrodesused should also have no chemical effect on theaction of the sweat glands. Thin lead plates arepreferred for GSR electrodes as they meet the aboverequirements and they are also malleable and can bemolded to suit the subject's contour. Lead plateelectrodes are used in the photograph shown inFig. 13-2; one electrode is placed on the palm ofthe hand in a region of high sweat gland concentrationand the other electrode is placed on the back of thehand in a region almost completely devoid of sweatglands.

The range of resistances encountered in normalsubjects when recording between two electrodes onthe hand is from 20,000 ohms to perhaps 0.2 megohms.ln sorneinstances, particularly if the autonomicnervous system is malfunctioning and the sweat glandsare effectively denervated, the resistance betweenthe electrodes will exceed 1 megohm.

13.3 GSRMPASURFMENTBY POTENTIAL DETECfrON

offsetpotential

GSR measurement by this technique involves thedetection of a DG potential between two electrodeson the subject. This DG potential will normally beless than one millivolt. An electrode offsetpotential in excess of one millivolt may be producedat the electrode/subject interface and any unbalancein this offset potential between the two electrodescannot be differentiated from the GSR potential.Although solid silver electrodes, or perhaps silverplate electrodes, are used, this offset potentialunbalance is difficult to control and willinvariably contribute a considerable De potential tothe GSR potential. For this reason, GSR measurementby this technique is rarely attempted.

199

13.4 ELECTRICAL SKIN RESISTANCE

sweatglanddistribution

dermometer

The electrical skin resistance (ESR) is basicallythe same as the GSR. The term ESR is, however,usually reserved for measurement of the distributionof sweat glands on the human body rather than theactual change in the activity of these sweat glands.The ESR is measured in the same way as the GSR,however the ground electrode is applied by meansof a silver clip attached to the ear and the activeor exploring electrode consists of a noncorrodingsilver or lead disc or a small roller wheel. Sweatgland distribution is detected by moving the positionof this exploring electrode. The change inresistance between a sweating and a nonsweating areais distinct and a sma11 movement of the exploringelectrode can result in a resistance change inexcess of 100 percent. Instruments specifica1lydesigned for measurement of the electrical skinresistance are referred to as dermometers.

acousticalreflections

advantages

201

ULTRASONOGRAPHY

Ultrasonography is a technique by which ultrasonicenergy is used to detect internaI body organs. Burstsof ultrasonic energy are transmitted from atransducer through the skin and into the internaIanatomy. When this energy strikes an interfacebetween two tissues of different acoustical impedance,reflections are returned to the transducer. Thetransducer converts these reflections to anelectrical signal. This electrical signal 1samplified and displayed on an oscilloscope, eachtissue interface appearing as a vertical deflectionalong the baseline of the oscilloscope at a distanceproportional to the depth of the interface. Thisultrasonic technique is similar to the time-domainreflectometry technique used to rneasureelectricalcable length and the sonar technique used to detectobjects under water.

While the use of pulse-echo ultrasonic energy issomewhat similar to the use of X-ray, the resultsobtained differ from an X-ray picture, being across-sectional projection or simply a linearprojection rather than a profile of the areaexamined. AIso, in contrast to X-ray,ultrasonography uses mechanical energy at a levelwhich is not harmful to human tissue, thus, it maybe used with safety on pregnant subjects and forfrequent examination. Ultrasonography can detectmaterials that are not radiopaque, thus angiographicdyes are unnecessary. As commercial ultrasonicdiagnostic instruments are easy to operate,ultrasonography is rapidly becoming a valuablediagnostic technique.

202

TRANSMITTER AND RECEIVER(COMMON TO ALL SYSTEMS)

r---ff TRANSDUCER

500HzTIMI G CLOCK

IMPULSE GENERA TOR<TRANSM ITTERl

TRANSMIT/RECEIVESWITCH<DIODES)

NONLINEAR(LOGARITHMIC)AMPLIFIER AND

ENYELOPE DETECTOR

ECHOSIGNAL

lOOY/DI

'-----l~TR 1GGERPULSE

IMPULSE RECE 1VED ECHO

EXCITES TRANSDUCER OSCILUATION ATITS RESONANT FREQUENCY (2.5MHz)

ATTENUATED REPLICA OF TRANSMITTEDWAVESHAPE PROOUCED AT TRANSDUCER

"A" SCAN BLOCK DIAGRAM "T-M" MODE BLOCK DIAGRAM

XECHO

SIGNAL ---1_------ YZ

SWEEPGENERATOR

(FASTl

MONITOROSCILLOSCOPE

SWEEPTR 1GGER ___....._GENERATOR

{FASTlrRIGGER

MQNITOROSCILLOSCOPE

r---X'-------' y

Z

ECHOSIGNAL

SWEEPGENERATOR

(SLOW)

"B" SCAN BLOCK 0 1AGRAM

TRA SDUCER

" X

YMQNITOR

OSCILLOSCOPE

MECHANICALSCANN 1NG SYSTEM(MANUAL OR SERVO)

ECHO SIGNAL~Z

Fig. 14-1. Ultrasonic diagnostic sy tems -block diagrams.

displaymodes

203

While the basic function of ultrasonic diagnosticequipment is to measure distances between interfacesthat separate body structures by timing the echosproduced by these interfaces, this timed-echoinformation may be processed in various ways toproduce different forms of display. The threecommonly used display modes are the "A" scan mode,the "T-M" mode and the "B" mode; further details onthese three operating modes are given later in thischapter.

14.1 ULTRASONIC SYSTEMS

transducerfrequencies

operationaltheory

soundvelocity

Ultrasonic energy for use in ultrasonography isproduced by exciting a piezoelectric crystalreferred to as the ultrasonic transducer. Thesepiezoelectric crystals normally have a self-resonantfrequency of between 1 MHz and 10 MHz; the mostcommon type used for ultrasonography having aself-resonant frequency of 2.5 MHz. When thiscrystal transducer is excited by an electricalimpulse, it will ring at its self-resonantfrequency and produce a train of damped oscillations.Fig. 14-1 shows the input pulse used to excite a2.5 MHz transducer and a reflected pulse detectedby the same transducer. The input pulse has anamplitude of almost 300 volts and a duration of 1microseconde The transducer oscillates at 2.5 MHz,the da~ping factor being dependent on the transducerdesign and on the type of tissue in the transducer'spath. As this transmitted damped oscillation reachesan interface between materials having differentacoustical impedances, a reflection or echo isproduced. By the time this echo has returned to thetransducer, the transducer is passive and is thenagain excited by vibrations produced by thisreturning echo. This echo signal is then amplifiedand processed in a logarithmic amplifier and anenvelope detector. This process is repeated at anapproximate 500 Hz rate. The time between thegeneration of the transducer exciting pulse and thedetection of the received echo represents the timetaken for the ultrasonic energy to travel from thetransducer, to the interface, and thence from theinterface back to the transducer. The velocity ofsound waves, and thus of ultrasonic energy, in bodytissue is about 0.125 centimeters per microseconde

204

depthversusresolution

transducer/tissuecoup 1ing

It is rarely necessary to know this velocityaccurately as relative interface distances are moreimportant than actual interface distances. If theseactual distances are required, the system can becalibrated against a known tissue/interface distance.

Depth and resolution of the system depends on thecrystal's resonant frequency, a 1 MHz crystal provideslow resolution, however, reflections can be detectedfor tissue 50 centimeters from the transducer. Thecommonly used 2.5 MHz crystal can be used to atleast 20 centimeters; whereas a 10 MHz crystalprovides excellent resolution, however it cannot beused above about 5 centimeters. Relating distanceto velocity, a tissue interface 5 centimeters fromthe transducer will produce a reflection 80microseconds after the impulse.

It is important that the transducer be firmly heldagainst the tissue (skin) as any air in thetransducer's path will severely attenuate ~"~,ultrasonic energy. A satisfactory transducèr/tissuecoupling can be assured by using a liquid couplersuch as water. ln certain ultrasonic scanningsystems it is impossible to locate the transduceragainst the tissue and a substantial depth of wateris used as a coupling medium.

14.2 "A" SCAN ULTRASONOGRAPHY

Y-T mode

ca 1ibratedsweepgenerator

"A" scan ultrasonography displays the amplifiedecho signal on the vertical channel of anoscilloscope with the horizontal channel beingdeflected by a conventional sweep generater. Thissweep generator is triggered from the impulse signaland the time delay between the beginning of the sweepand the echo appearing on the CRT screen isproportional te tissue depth. The sweep generatormay be a conventional sweep generator or may bespecifically calibrated in tissue depth (centimeters)rather than in time per division. A sweep speed of100 microseconds per division would correspond toapproximately 6.25 centimeters of tissue perdivision. Faster sweep speeds would givecorrespondingly greater resolution. The fastestusable sweep speed would probably be 2 microsecondsper division which would correspond to 1.25millimeters of tissue per division. This fastersweep speed would only be usable with 10 MHztransducers.

echoencephalography

205

The most common usage of "A" scan ultrasonography isin echoencephalography. Echoencephalography detectsbrain midline position and possible displacement ofthis midline due to an abnormal space occupying masswithin one side of the skull, such as a tumor. Whenperforming an "A" scan with the transducer heldagainst the side of the subject's head, echos arereceived from the two halves of the brain as shownin Fig. 14-2. The echo produced by the midline istermed the "M" echo and should be symmetricallyplaced between the echos received from each side ofthe subject's skull.

1 1 Il IlI~ Il:,~k :: /~--~

1 I:f :: 1II~""""'" Il ~~

1 (.{I ,.'1 1 Il 1 1

Il

A TYPICAL INSTRUMENT CONFIGURATIONFOR "A" SCAN ULTRASONIC DIAGNOSIS

(PICKER X-RAY)

1 11 1 Il Il1 fooIl••l----I~~II.. ~ 1 1

BRAIN 1 BRAIN

BRAINMIDLINE

---1 1-+-50lJs/DIV

::3.lem TISSUEPER DIV

Fig. 14-2. "A" scan echoencephalography.

206

fore ignobjectdetection

TRANSDUCERWITH

WIDE ANGLEOF

DISPERSION

FOREIGN~ OBJECT

1 11 1

1 11 11 1

"A" SCAN OFTHE EYE

~ r--lO)Js/D 1V

Fig. 14-3. "A" scan detection of a foreign object.

"A" scan ultrasonography may also be used to detectforeign objects imbedded in a subject. Fig. 14-3shows a typical "A" scan obtained when scanning aneye containing a foreign object with a 10 MHztransducer. The response attributed to the foreignobject can be verified by scanning the other eye andnoting the lack of any response at this timeposition.

Many manufacturers produce equipment suitable for"A" scan ultrasonography. A typical instrumentconfiguration is shown in Fig. 14-2; a TektronixType 56lB or 564B Oscilloscope is used with aTektronix Type 2B67 time-base unit and a specialvertical plug-in unit, designed for ultrasonography,produced by the Picker X-Ray Corporation. Suitableprobes are also produced by the Picker X-RayCorporation. The complete instrument may be used inconjunction with a Tektronix Scope-Mobile® Cart andTektronix Trace-Recording Camera.

207

14 . 3 lIT IME- MOTION" MODE ULTRASONOGRAPHY

Z-T mode

Considering "A" scan ultrasonography as discussedpreviously; if an interface producing an echo wasmoving in relation to the transducer, the responseobtained on the CRT would also move horizontallyrelative to the beginning of the sweep. If this "A"scan system was to be modified slightly by removingthe echo signal from the vertical channel (y) andconnecting it to the intensifying channel (Z), anecho response would then appear as an intensifiedspot on the screen. Any "motion" in this echoresponse would appear as a horizontal motion of thisspot between successive sweeps of the horizontalsweep generator. No vertical signal would beinvolved.

ln time-motion or "T-M" mode ultrasonography, theabove hypothetically modified "A" scan system isused with a slow sweep generator moving the displayvertically. Thus, any motion in the echo responseis displayed in real time by the slow vertical sweep.This system is shown diagrammatically in Fig. 14-1,and a typical "T-M" mode ultrasonogram from motionof the mitral valve of the heart is shown inFig. 14-4.

VERTICAL SWtEP 1DISPLAYS VALVEMOTION lN REALTIME.

LOPE ex VELOCITY

TRANSVERSE SECTIONTHROUGH SUBJECT

Fig. 14-4. 'T - M" mode cardiography.

208

TRANSDUCER SCANNED ACROSSSUBJECT. 6 INSTANTANEOUSPOSITIONS SHOWN.

INTERNALORGAN

A TYPICAL MANUAL SCANNING SYSTEM PRODUCEDBY THE PICKER X-PAY CORP., LONGMONT, COLO.

<, _# 1<,

X-YOUTPUT. EACH OF THE 6 INSTANTANEOUSTRANSDUCER POSITIONS PRODUCES 3INTENSIFIED AREAS. THESE INTENSIFIEDAREAS ARE RECORDED ON A STORAGE SCOPEAND LINKAGE OF THE "DOTS" PRODUCESAN OUTLINE OF THE INTERNAL ORGAN.

TYPICAL XY9 AUTOMATIC SCANNING SYSTEM.THE POSITION OF THE TRANSDUCER IS PRESENTED AS FOUR VOLTAGES, REPRESENTINGTHE DISTANCES ALONG THE X AND Y AXESAND THE SINE AND COSINE OF THE ANGLEBETWEEN THE LINE OF THE ULTRASONICBEAM AND THE X-AXIS.

Fig. 14-5. U1trasonic scanning - "B" mode.

209

Referring to Fig. 14-4, the motion of the mitralvalve is clearly shown on the vertically-swept traceon the face of the CRT. Fig. 14-4 also shows themotion of the walls of the heart, however echoes fromother interfaces in the transducers path have beengated out in this instance. This particular

cardiography application, known as "T-M" mode cardiography isparticularly valuable in the detection of mitralstenosis. Since the actual movement of the mitralvalve is shown in centirneters in real time (verticalsweep), the velocity of movement of this valve canbe determined from the slope of the trace obtained.ln the example shown in Fig. 14-4, the maximumvelocity of the mitral valve is 16 centimeters persecond, the overall movement of the valve is 2.2centimeters and the heart rate is 86 beats perminute.

14.4 ULTRASONIC SCANNING

mechanicalX-Y scan

manualscanning

Ultrasonography using the "B" mode technique isreferred to as ultrasonic scanning. ln this mode,the echo signal is connected to the Z axis of amonitor to provide intensification as in the "T-M"mode, however the X and Y monitor input signals arederived from a mechanical scanning system andprovide signals proportiona1 to the position anddirection of the probe to form part of a physicalpicture of the organs being examined. The resultobtained is a two-dimensiona1, cross-sectionalpresentation of part of the subject.

ln its simplest forrothe "B" mode ultrasonic scanneris manua1ly scanned by an operator across the partof the subject of interest. Referring to Fig. 14-5,with the transducer p1aced in each of the sixpositions shown, a series of intensified areasappears on the CRT face. Their position bearing anexact physica1 relationship to the positions oftissue interface within the subject. With only sixtransducer positions as shown in Fig. 14-5, only sixrows of intensified dots would appear on the CRT,however, in practice, the transducer is slowlyscanned over the subject and is producing echopatterns at a 500 Hz rate. The display thus appearscontinuous rather than composed of several dots.The manual scanning system shown in Fig. 14-5, whi1enot offering the sophistication of an automaticscanning system, is economical and may also be usedfor both "A" mode scanning and "T-M" mode scanningrather than being restricted to "B" mode use.

210

automaticX-Y-8scanning

The mechanism associated with an automatic X-Y-8cornpoundscanning system is also shown in Fig. 14-5.Automatic compound scanning systems can be dividedinto two distinct classes, in one class the scanningmovements of the probe are predeterrninedand relatedto the geometry of the machine itse1f, in the otherthe scanning pattern is dictated by the contours ofthe particular part of the individual patient beingexamined. The first type is referred to as animmersion scanner since a liquid-filled space isrequired between the probe and the subject and thesecond type is referred to as a dry scanner sincethe transducer is in direct contact with the subject.The mechanism associated with immersion scanners issornewhatless complicated than the mechanism requiredfor direct contact dry scanners and the requirementof having a 1iquid filled space is satisfied byhaving a plastic membrane separating the subjectfrom the liquid in which he or she is immersed.Immersion scanners are particularly suited to areassuch as the breasts and the eyes where direct contactscanning is impractical.

POSITION OF FETUS IN UTERO CORNEAL THICKNESS DETERMINATIW

NORMAL BREAST TISSUENIPPLE"

/NIPPLE

ART 1FACTS ARTIFACTS

DETECTION OF CYST lN BREAST TISSUE

Fig. 14-6. Typical "B" mode displays from a compound scanning system.

ultrasonographicapp 1 icat ions

211

Fig. 14-6 shows typica1 "B" mode displays obtainedwith a compound scanning system. These displays areself-explanatory and indicate sorneof the potentialadvantages of ultrasonography. Neither of the threedisplays shown in Fig. 14-6 could have been obtainedby conventional X-ray techniques as; for the fetusin utero, fetal development is impaired by X-rayenergy; for the breast cyst, such cyst tissue isundetectab1e by conventional X-ray techniques; and,for the scan of the eye, resolution with X-raytechniques cannot be obtained if the X-ray platecannot be located directly behind the regionconcerned.

14.5 DOPPLER ULTRASOUND

moveme[ltcausesfrequencysh i ft

Continuous u1trasonic energy, rather than bursts ofultrasonic energy as used in u1trasonography, isused to detect motion within a subject by the Dopplerprinciple. When ultrasonic energy is reflected froma moving object it is shifted slightly in frequency,the frequency shift being proportional to the speedof the objecte ln the living body there arenumerous movements which reflect ultrasonic energy:b100d flowing through arteries, the action of theheart, intestinal movements and passage of urine andgastric juices. The most common application ofDoppler ultrasonics is in obstetrics to detectmovement of the fetal heart and fetal blood flow;such fetal activity can be detected as early as thetenth week of gestation. Another major applicationfor Doppler ultrasonics is in the detection of bloodflow in the peripheral circulation of the body.

213

SECTION III

INSTRUMENTATION

The following chapters (]5-]8) describe the'characteristics of the instrumentation required toimplement the measurement techniques covered inSection II as weIl as the characteristics ofinstrumentation in current use for biophysicalmeasurements. A working knowledge of electronicinstrumentation principles is assumed; this materialprimarily discusses the instrumentationcharacteristics that are unique to the biophysicalsciences.

Since the following material is generally limited toa discussion of those instrumentation characteristicsthat are unique to biophysical measurement, it shouldnot be regarded as a general reference source on anyone type of instrumentation. An attempt is made ineach of the following chapters to direct a readerwho is somewhat unfamiliar with general electronicinstrumentation to a general reference source. Thisgeneral reference source should first be studied bysuch readers before preceding with a study of thematerial presented in the following chapters.

STIMULATION

STIMULATQR(CHAPTER 22)

SUBJECT. ,.

SAFETY(CHAPTER 17l -=-

SENSING PROCESSING

SHIELDING(CHAPTER 17)

ELECTRODES(CHAPTER 16)

AMPLIFIER(CHAPTER 19)

TRANSDUCER(CHAPTER 18)

SIGNALPROCESSOR

(CHAPTER 20)

INFORMATION REPLAY

DISPLAY RECORD DISTRI8UTIO'J•• ,. •• ~

OSCILLOSCOPE(CHAPTER 21)

CAMERA(CHAPTER 24)

DISPLAY DEVICETV SIGNAL(CHAPTER 23)

GRAPHICRECORDER

(CHAPTER 25)

INTENSIVEFR~ 1 CARE UNIT

OTHER SUBJECTS I-------~ (CHAPTER 28)

MAG"'ETICL_ ~ TAPE RECORDER

(CHAPTER 26)

Fig. 15-1. Biomedical instrumentation configuration(Showing references to appropriate chapters in this book).

biomedicalsignalsource

electrodesandtransducers

215

INSTRUMENTATION SUMMARY

A b10ck diagram of the instrumentation used forbiomedica1 measurements is shown in Fig. 15-1. Whilethe majority of measurement systems do not utilizeaIl of the components shown in this diagram, theultimate measurement system may contain aIlcomponents. The fo1lowing 13 chapters (Chapters16-28) describe each of these instrumentation groupsin more detail.

The biomedical signal to be measured is derived fromthe subject. This signal may be continuous1yproduced by the subject (such as the ECG) , it maybe vo1untarily initiated by the subject (such as amuscular contraction) or it may be produced byartificial stimulation of the subject (such as anevoked response). This artificial stimulation isnorma1ly produced by an electric current; however,other forms of stimulators, such as mechanica1 oroptical stimulators, are occasiona11y used.Stimu1ators are covered in more detail in Chapter 22.

The biomedica1 signal is either direct1y avai1ablefrom the subject as an e1ectrical potentia1 detectedby using e1ectrodes or it is availab1e in sorneotherform, in which case an electrical potentia1corresponding to the biomedical signal is producedwith the aid of a transducer. Biomedicalmeasurements such as body impedance are regardedas transducer-type measurements (although e1ectrodesare used to perform this measurement) as themeasurement involves the detection of an impedancerather than the detection of a bioe1ectric potential.Electrode systems are covered in more detail inChapter 16 and transducer systems are covered in moredetail in Chapter 18. Electrode shie1ding andsubject safety is covered in Chapter 17.

216

ampl ifyingandprocessing

displaying

permanentrecord

Whether the biomedical signal is obtained directlyvia electrodes or produced via transducers, itinvariably requires amplification and perhaps signalprocessing. Amplifiers are discussed in Chapter 19and signal processes are discussed in Chapter 20.Many transducer systems incorporate the necessaryamplification into the system as discussed inChapter 18.

Once the biomedical signal has been amplified andprocessed, then biomedical measurement techniquescan be forgotten as electronic measurement and.display techniques are now employed. The amplifiedand processed signal may be displayed on aconventional oscilloscope (Chapter 21), on sorneotherform of display device such as a television receivervia a scan converter (Chapter 23), on a paper chartrecorder (Chapter 25) or it may be recorded onmagnetic tape (Chapter 26). The signal may bedirectly routed to further data processing and datatransmission equipment (Chapter 27), or it may becombined with signals from other patients in anintensive care unit (Chapter 28).

If the signal is recorded on magnetic tape or on achart recorder, then a permanent record of the signalis obtained. When monitoring the signal on aconventional oscilloscope or a display device, apermanent record must be obtained by photographiemethods as covered in Chapter 24. Use of a magnetictape recorder or data processing and transmissionequipment allows information replay through theinstrumentation system for a detailed analysis ofbiomedical signals and, in many cases, frees theoperator to concentrate on recording the biomedicalinformation.

15.1 BIQ~DlCAL SIGNALS

Fig. 15-2 lists the biomedical measurements coveredin Section II of the book. This summary isnecessarily very brief and references to theappropriate sections in Section II are given todirect the reader to more detailed information.Fig. 15-3, produced by courtesy of BeckmanInstruments, Inc., is a more complete summary ofbiophysical measurements and gives the ranges andcharacteristics of the various biomedical signals.(Note: Not all of the biomedical measurementscovered in Fig. 15-3 are included in this book.)

217

PHYS 1OLOG1CAL REFERENCE EASUREMfP'1T SENSI""; DEVICES PARAMETER PARAI4CTER

,>ARAMETER (THI S BOOK) REQUIRED USEO TYPICAL FREQUfNCYP-P VALUE CHARACTER1ST ICS

FUNDAMENTAL SPECT~Hz FI",) Hl

CIRCUlATORY SYSTEM

HEART POTENTIALS OiAPTERS ELECTROCARDIOGRAH SURFACE ELECTRODES ZrttV 1.3 .05 - 802 AND 5

CHAPTER 17 H[ART ElECTRODES 5001V 1.3 ,05 - saCHAPTER 6 VECTORCARDIOGRAM SURFACF ELEr.TRODES 2mV 1.3 ,05 - 80

CHAPTER 7 FETAL ELECTROCARDIOGRA'4 SURFACE ELECTRODES lMOTHER) 10~V 2.5 2 - 100

BLOOO PRESSURE SECT 8,1 DIRECT ARTERIAl PRESSURE PRESSLflE TRANSrucER 120mmHq 1.3 oc- 20AT BRACHI,t,L ARTERYOR FEMORAL ARTERY MERCURYMANOMETER 170nvrtig " "

SECT 8,1 DIRECT VENOUS PRESSURE PRESSURE TRANSDOCER 9mmHg 1.3 DC - 20

WA TER MANOMETER 12C11lH2O 1.3 -SECT 8.2 INDIRECT ARTERIAL PRESSURE SPHYGHOMA~ETER WITH 120/8DnrnHg - -

KOROTKOFF MICROPHONE 15001V 1.3 30- 500

SECT 8,3 RELAT IVE ARTER IAL PRESSURE PLETHYSMOGRAPH RELATIVE 1.3 .05 - la

IMPEDANCE PLETHYSMOGRAPH 0.1% CHANGE 1.3 .05 - la

BlOOO FLOW SECT 6.4 PERIPHERAL FLOW ELECTRCf-lAGNET1C FLO~ETER IODOcc/mln 1.3 DC-5OlAND BLOOO VELOCITY) 1SOTHERMAL FLOft'olETER " " ..

ElECTROTURB 1NOMETER " " "ULTRASONIC FLOW'IETER " " "

SECT 6,5 CARO1AC OUTPUT FLOIoiMETERAT AORTA 5000cc/ml n 1.3 OC- 50

DYE DilUTION 50ODcc/mln - -BLOOO VOLUME SECT 6.6 BLOOO VOLUME MOD1FIED OYE Dl LUT ION 5500cc - -MITRAL VALVE FU~T ION SECT 14.3 ULTRASONIC ECHO T-M MODE ULTRASONICS 16cm/s 1.3 DC- 50

RESPIRATORY SY~TEM CHAPTER 9

BREATHING SECT 9.2 PNEUMOGRAM THERMISTOR PNEUMOGRAPH 5ODcc/BREATli 0.25 .05 - 2

1MPEDANCE PNEUMOGRAPH " " DC- 1ELASTIC FORCE GAGE " " ..

RESPIRATORY FLOW SECT 9.3 PNEUMOTACHOGRAM PNEUHOTACt.IOORAPHIII TH 20,000cc/mln " ..PRESSURE TRANSDOCER

RESPIRATORY VOLUME SECT 9.4 SPIROGRAM SP1R().lETER 4000cc " DC- .5

BRA1N FUNCT1ONS

ELECTRICAL ACTIVITY CHAPTER 10 ELECTROENCEPHALOGRAM SCALP ELECTROOES 5O~V 10 .5 - 100

INTRACRANIAl ELECTRODES 500~V " "EVOKED RESPONSES CHAPTER Il INTRACElLULAR POTENTIALS MICROElECTRODES lOO1nv 1 - 10,000

EXTRACElLULAR POTENTIALS NEEDlE ELECTRODES 50~V 1 - 1000

EYE RESPONSES SECT 11.6 ELECTRORET1NOGRAM CONTACT LENS ELECTRODE 100~V .05 - 20

BRAIN MIDL INE POSITION SECT 14.2 ULTRASONIC ECHO "A" SCAN ULTRASONICS

MUSCULAR FUNCTIONS CHAPTER 12

IolISCLE EXCITABI L1TY SECT 12.3 S-D CURVE ST lMULATE WITH SURFACEELECTRODES

MUSCLE STRENGTH secr 12.4 MYOGRAM NEEDLE OR SURFACE ElECTRODES 300uSTRAIN .5 DC-5O

MUSCLE POTENTIAlS SECT 12.6 ELECTR().lYOGRAM NEEDLE OR SURFACE ElECTRODES ImV 10 - 5000

SECT 12.7 ELECTROHYOGRAHW1TH NEEOLE OR SURFACE ELECTRODESST IMULAT ION STI"-'LATE WITH SURFACE .. " "-ELECTRODES

NERVE CONQUCTION SECT 12.8 H REFLEX RESPONSE AS ElECTROMYOGRAPHW1TIlREOUCED ST 1MULAT ION

SECT 12.9 CONOUCTION VELOCITY AS ELECTROHYOGRAPH

SMJOTH MUSCLE ACT 1V!TY SECT 12.11 ELECTROGASTRQGRAH SURFACE ELECTRODES 20mV .25 .05 - 2

AUTONOHIC NERVOUS SYSTEM

SWEAT GLAND ACTIVITY CHAPTER 13 GALVANIC SI(IN REFLEX LEAD SURFACE ELECTRODES 50kll

ELECTRICAL SKIN .. .. " "RES1STANCE

BOOY TEMPERATURE SECT 9.5 TEMPERATURE THERM1STOR PROBE 96°FTHE~ETER

ANATQMY

1NTERNAl ORGAN POS1T ION CHAPTER 14 ULTRASON1C ECHO ULTRASON1C SCAI'.N1NG SYSTEM

Fig. 15-2. Biophysical measurements covered in this book.

218

CARO10VASCULAR SYSTEM

BLOOO PRESSURE, DIRECT METHOO

BLOOO PRESSURE, 1NO1RECT "'ETHOO, 1NTERM1TTENTSYSTOLIC AND OIASTOLIC

PULSE WAVES, DIRECT '1ETt-«lD, ARTERIAL

PULSE WAVES, 1~ID1RECT METHOO, PER1PHERAL ARTERY

PHONOCAROIOGRA'1

PLETHYSMOGRAM(VOLUME MEASUREMENTS)

SALL 1STOCARDIOGRAM

HEART RATE

OXIMETRY

CARO1AC OUTPUT

BLOOO FLOW

HEART SHUNT DETECTION

ELECTROCARDIOGRA'"

PR1MARY SIGNAL RANGES AND CHARACTERIST1CS

FREQUENCYRA'lGE: CC TO 200Hz; CC TO 60Hz USUALLY ADEQUATE. PRESSURE RANGE,ARTERIAL: 40 TO 300mnHg; VENOUS0 TO l5mmHg.

AUSCULTATQRYCRITERION (KOROTKOFF SOUNOS): 30 TO 150Hz USUALLY ADEQUATE. PALPATORYCRITERION: 0.1 TO 60Hz. BOTH REQUIRE ADDITIONAL SIGNAL SHOWINGOCCLUDING PRESSURE.

(SEE BLOOO PRESSURE, DIRECT 1

FREQUENCYRANGE: 0.1 TO 60Hz USUALLY ADEQUATE. PULSE TRACE SIMILAR TO BLOOOPRESSURE, DIRECT, BUT WITHOUT BASELINE ZERO.

FREQUENCYRANGE: 5 TO 2000Hz; MAJOR DIAGNOSTIC COMPONENTSLIE lN 20 TO 200Hz RANGE.

FREQUENCYRANGE: De TO 30Hz.

FREQUENCYRANGE: CC TO 40Hz.

AVERAGE RATE, HUMAN: 45 TO 200 BEATS/min; LAS ANIMAL; 50 TO 600 BEATS/min.

FREQUENCYRANGE: 0 TO 60Hz; 0 TO 5Hz USUALLY ADEQUATE.

FREQUENCYRANGE: 0 TO 60Hz; 0 TO 5Hz USUALLY AOEQUATE.

FLOW RANGE, HUMAN: 5cc/mln TO 10 LlTERS/mln. FREQUENCYRANGE: 0 TO 60Hz;o TO 20 USUALLY AOEQUATE.

ELECTROOE SIGNAL RANGE, Po2: 0 TO 16OmmHg. HYDROGENAND SOOIUM ASCORBATE: QUALITATIVE.

FREQUENCYRANGE: 0.05 TO 80Hz. SIGNAL RANGE: 1000VTO 5mV INCLlXlES FETAL RANGE.

RESP1RATORY SYSTEM

FLOW RATE (PNEUMOTACHQGRAM)

EflEATHING RATE CALCULATED FROMRECORD (WITHAPPRQXIMATE RELATIVE RESPIRATQRY VOLIJ.4E)

T IDAL VOLUME (MEASURED PER EREATH ORItlTEGRATED TO PROVIDE VOLUME/min)

CO2, N20, OR HALOTHANE CONCENTRATION .1N RESP1REO AIR

DIFFUSION OF INSPIRED GAS (USING NITROGENI

PUI.IOlARY DIFFUSIIlG CAPACITY WSING CARBON MONQXIDEI

FREQUENCYCOIoIPONENTSTO 40Hz. NORIoIALFLOW RANGE: 250 TO 500ml/s; MAXIMUM 8 LlTERS/s.

AVERAGE RATE: HUMAN, 12 TD 20 BREATHS/min; LAB ANIMAL, 8 TO 60 BREATHS/mln.

TYPICAL VOLU"'E, AOUn tfJMAN: 6OOmI/EflEATH: 6 TO 8 LlTERS/mln.

NORMALRANGE, CO2: 0 TO 10~; END-TiOAL CO2, HUMAN: 4 TO 6~. N20: 0 TO 100~. HALO-THANE: 0 TO 3~.

NORMALRANGL OF NITROGEN CONCENTRATIONDIFFERENTIAL: 0 TO 10~.

NORMAL RANGE, HUMAN: 16 TO 35m1CO/mmHg/m1n.

DI SSOLVED GASES ANO pH

PARTIAL PRESSURE OF OISSOLVED O2, lN VIVO OR lN VITRO

pH, lN VITRO

PARTIAL PRESSURE OF DI SSOLVEO CO2, lN VITRO

FREQUENCYRANGE: CC TO 1Hz USUALLY ADEQUATE. NORMALMEASURE"'ENTRANGE: 0 TOBOOmmHgPo2• HYPERSARIC P02 RANGE: 800 TO 3000.

SIGNAL RANGE: 0 TO HOOmV CQVERS pH RANGE.NORMAL SIGNAL RANGE: 0 TO .15OmV COVERS RANGE FROIoI 1 TO 1000mmHg Pco2•

BI OELECTRIC POTENT IALS

ELECTROENCEPHALOGRAM

CEREBRAL POTENTIALS, INTRACRANIALLY RECORDED

ELECTROIoIYOGRAM(PRIMARY SIGNAL)

ELECTROMYOGRAM(AVERAGED)

SMOOTHMUSCLE POTENTIAl (e.g., ELECTROGASTROGRAM)

ELECTRORET1NOGRAM

ELECTROCARDIOGRAM

ELECTRONYSTAGMOGRAM

FREQUENCYRANGE: CC TO 100Hz; "'AJOR OIAGNOSTIC COMPONENTSLIE lN 0.5 TO 60Hz RANGE.NORMAL SIGNAL RANGE: 15 TO lOO~V

NORMAL SIGNAL RANGE: 10~V TO 10OmV. PULSE DURATION: O.6ms TO 20ms.

FREQUENCYRANGE: 10 TC 2000Hz. PULSE DURATION: O.6ms TO 2Oms.AN AVERAGE OF THE PRIMARY SIGNAL, AFTER FULL WAVE RECTIFICATION.

FREQUENCYRANGE: CC TO O. 6Hz. NORMAL SIGNAL RANGE: 0.5 TO BOmV.

FREQUENCYRANGE: OC TD 20Hz ADEQUATE. NORMAL SIGNAL STRENGTH: i~V TO ImV.

(SEE LIST ING UNOERCARDIOVASCULAR SYSTEM)

DIRECT: FREQUENCYRANGE, 0 TO 20Hz. TYPICAL SIGNAL STRENGTH, 100.V/10· EVE HOVEMENT.DERIVATIVE OR VELOCITY: FREQUENCYRANGE, 0 TO 20Hz. SIGNAL OERIVED FROIoIDIRECT READING.

PHYSICAL QUANT1T 1ES

TEMPERATURE

VDICE

SKIN RESISTANCE (GSR)

ISOMETRIC FORCE, DIMENSIONAL CHANGE, BODY FLUIDANO BODY CAV1TY PRESSURES

Fig, 15-3.

FULL RANGE OF SIGNALS.

FREQUENCYRANGE: 20 TO 20 .oooue.RES1STANCE RANGE: 1k TO 5OOk.

FULL RANGE OF'SIGNALS.

Biophysical signal ranges

tissueelectrodeinterface

ionicelectroniccurrents

219

ELECTRODES

The electronics ~ngineer, accustomed to measuringpotentials between various points on an electroniccircuit, may tend to regard two physiologicaielectrodes simply as two probes applied to a subjectin order to measure a potentiai difference. Such anoversimplification cannot be made as the conductionof current in tissue, as in any other liquid system,is ionic; that is to say, by the migration ofpositive and negative ions from point to point. Tomeasure eiectricai effects in tissue it is necessaryto make a transfer from this ionic conduction to theelectronic conduction which occurs in the measuringcircuit. This transfer is accomplished at thetissue-electrode interface.

16.1 ELECTRODE OFFSET POTENTIAL

ha 1f ce 11potentials

electrodepotentials

If two ~issimilar metais are inserted into acontainer of eiectrolyte, a potential difference canbe measured between these metals. If one of thesemetais were silver and the other copper, thispotential would be referred to as the silver/coppercell potential and would be in the order of 0.4 volts.This potential is comprised of two separate componentsadded aigebraically together: a "half cell"potential due to the silver electrode and a "halfcell" potential due to the copper electrode. Thesehalf cell potentials, although difficult to measure,are the potentials produced across the metalelectrolyte interfaces. To a first orderapproximation, neglecting many chemical factors thatwould be important if the electrodes were used in achemical measuring application rather than abiophysical measuring application, the half cellpotential is approximately equal to the electrodepotential of the metal concerned. Thus, the

220

CORRODED END(~NODIC ORLEAST NOBLE)

PROTECTED END(CATHODIC ORMOST NOBLE)

METAL 10NIC ELECTRODESYMBOL POTE TIAL (EP)*

ALUMI IUM AI+++ -1.66VOLTS

ZINC Zn++ - .76

IRON Fe++ - .44

LEAD Pb++ - .12

HYDROGEN H+ 0

COPPER Cu++ + .34

+ + .80SILVER Ag

PLATINUM Pt+ + .86

GOLO Au+ +1.50

*APPROXIMATELY EQUAL TO HALF-CELL POTENTIALlN PHYSIOLOGICAL ENVIRONMENTS.

Fig. 16-1. Electrode poten tials of metals, the electrochemical series.

+.70V

"'.46V OF OFFSETINTO AMPLIFIER

.---------1 +.----------1 +

Cu

EQUIVALENTTO

Ag

SILVER ELECTRODE

COPPER ELECTRODE

SILVER LAELECTRODEON SUBJECT

CONTAINER OFELECTROLYTE

SUBJECT ACTSAS ELECTROLYTE

OFFSET POTENTIAL PRODUCED BY DISSIMILAR METALSAPPROXIMATELY EQUALS DIFFERENCE lN ELECTRODE POTENTIALS.

OFFSET ~ EP Ag - EP Cu'"O.80V - O.34V'"O.46V

Fig. 16-2. Electrode offset potential.

electrodeoffsetpotential

differentialamp offsetpotential

221

potential produced between the silver electrode andthe copper electrode will be approximately equal tothe difference in the electrode potentials of silverand copper, that is, approximately equal to 0.80 minu~0.34 or 0.46 volts. The exact value of thispotential will depend on many chemical factors, themore important of which are the electrolyte used andthe concentration of this electrolyte; however thevalue given is probably within about 100 millivoltsof the actual value that may be measured in aphysiological environment. A list of the electrodepotentials for the more common metals is shown inFig. 16-1. This listing is referred to as theelectrochemical series. The example given of asilver and copper electrode is illustrated inFig. 16-2, which shows the similarity betweenelectrodes in a container of electrolyte andelectrodes on a subject. ln electrophysiology thedifference in half cell potentials th~t can bedetected between two electrodes is referred to as"offset potential."

It is important not to confuse the electrode offsetpotential produced between two electrodes connectedto a subject and the DC-differential offset potentialavailable in many differential amplifiers. TheDC-differential-offset-potential capability indifferential amplifiers is intended to be used tocancel the electrode offset potential producedbetween electrodes.

ln the biomedical situation, the electrode offsetpotential produced between electrodes may be unstableand unpredictable. Thus, it is desirable that thispotential be as low as possible. ln order to reduceelectrode offset potential it is first necessary tounderstand its origine At any electrode/electrolyteinterface there is a tendency for the electrode todischarge ions into solution and for ions in theelectrolyte to combine with the electrode. Thesechemical reactions may be represented as follows:

Metal ~ electrons + metal ions(Oxidization reaction)

Electrons + metal ions ~ metal(Reduction reaction)

222

'.".'.....' ..' "

G

;;::'.f:"'~\':::·'8 Gt<i;tll~t~~G 8

ELECTRODE /?',::,,~,; G";if1(~~~~1~~:;;~{G

',:,:,,:,':~'ELECTROLYTE.,': :: ::: "

------------ELECTRODE DOUBLE

LAYER FORMS(COMPLEX)

POTENTIAL EXISTS BETWEEN THE ELECTRODE ANDELECTROLYTE DUE TO THE FORMATION OF THEELECTRODE DOUBLE LAYER.

(A) METALLIC ELECTRODE

,:,:>'i:', ELECTROLYTE. :::' ..'.":.' ,', ;....:~:.•.'::.:;:': :.~.:;. :.::::' ., .'.. . "'..', ":..,.:.).::~.<::.....:

~

SILVER CHLORIDE FORMS FREE SILVER IONS (Ag+)AND CHLORIDE IONS (CI-) WHICH PREVENT THEFORMATION OF THE ELECTRODE DOUBLE LAYER.

(B) SILVER/SILVER CHLORIDE ELECTRODE

Fig. 16-3. Electrode/electrolyte interface.

nonpolarizableversuspolarizable

reversibleversus nonreversible

part iail Yreversibleelectrodes

223

The net result of these reactions is the creation ofa charge gradient, the spatial arrangement of whichis called the electrode double layer. This doublelayer may be represented, as shown in Fig. l6-3A, inits simplest form as two parallel areas of chargeof opposite signs. Electrodes in which no nettransfer of charge occurs across the metalelectrolyte interface are referred to as perfectlypolarized or perfectly nonreversible electrodes,that is, only one of the two chemical reactionsshown above can occur. Electrodes in whichunhindered transfer of charge is possible arereferred to as perfectly nonpolarizable or perfectlyreversible electrodes, that is, both of the equationsreferred to above occur with equal ease. Practicalelectrode systems have properties that lie betweenthese ideal limits. The electrode double layerexisting between a practical electrode andelectrolyte may be regarded as a battery in parallelwith a reasonably high impedance as shown inFig. 16-4. This impedance may typically be 10,000ohms and the battery may be equivalent to a 10 ~Fcapacitor. The potential of the battery, or thecharge on the capacitor, will be the half cellpotential of the electrode.

1krl 10kl"l-,

~~i~t,TO

TISSUE i AMPL 1FIERu.:',ELECTRODESON TISSU[

2MnEQUIVALENT

TO 100n

Ikn 10kn<TYP ICAL VALUESFOR 1cm2 PLAT[)

10\JF

~SKIN OOURLE

RESISTANCE LAYERIMPEDANCE

AMPLIFIERINPUTRESISTANCE

BULKTISSUE

RESISTANCE

~....._-v--_./IONIC

CURRENTFLOW

ELECTRONICCURRENTFLOW

Fig. 16-4. Electrode equivalent circuit for partially reversible electrodes.

224

si Iver!si Iverchlorideelectrodes

toxicity

chloriding

The silver/silver-chloride electrode shown inFig. 16-3B consists of either a solid silver surfacecoated with a thin layer of solid silver chlorideor, as is the case with Tektronix silver/silverchloride electrodes, consist of silver powder andsilver chloride powder compressed into a solidpellet. The presence of the silver chloride allowsthe electrode to behave as a near perfectnonpolarizable or reversible electrode as itprohibits the formation of the electrode doublelayer, the silver chloride dissociating to silverions and chloride ions which are free to migratebetween the electrode and the electrolyte, thusopposing the formation of the double layer. The netresult is a low impedance, low offset potential,interface between the silver and the electrolyte.Both silver/silver-chloride and zinc/zinc-sulfateelectrodes exhibited this characteristic.

Most electrodes form soluble rnetallic salts and thusare highly toxic and can be used only as surfaceelectrodes on intact skin. Silver electrodes are,however, nontoxic, as silver chloride is almostinsoluble in a chloride-containing solution, thusvery few free silver ions exist and tissue damagefrornthem is negligible. Thus, silver or silver/silver-chloride electrodes are definitely preferredfor use on exposed tissue. Although, as statedpreviously, zinc/zinc-sulfate electrodes produce lowoffset potential characteristics similar to silver/silver-chloride electrodes, they are highly toxic toexposed tissue due to the passage into the tissue offree zinc and/or sulfide ions.

A layer of silver chloride may be deposited onsilver electrodes, converting thernto silver/silverchloride electrodes, by a process known aschloriding. This is achieved by making the silverelectrodes positive to a solution containing sodiumchloride or saline (0.9% concentration) and passinga current through the electrode at the rate of1 mA/cm2 of surface for several minutes. The silverelectrode should be cleaned to rernovesurfacecontaminents before chloriding.

225

jell y(paste)contact

Mechanical disturbance of the electrode double layercauses electrode noise because the double layer actsas a region of charge gradient, disturbance of whichgives rise to a change in capacitance and thus achange in voltage. The electrical stability of anelectrode is considerably enhanced by mechanicalstabilization of the electrode-electrolyte interface.This is achieved by the use of indirect-contactfloating electrodes which interposes an electrolyte"jelly" or "paste" between the electrode and thetissue.

The preceding discussion of electrode offsetpotential has been highly simplified, as inelectrophysiology it is unnecessary to know the exactvalue of the electrode offset potential produced andit is therefore unnecessary for users of electrodesto understand aIl of the chemistry involved.Interested readers wishing to more fu11y understandthe chemica1 process invo1ved shou1d refer to Geddesand Baker, Principles of Applied BiomedicalInstrumentation, Wi1ey, 1968, Sections 9.2, 9.3, Il.1,Il.2 and Il.3 and a1so Dewhurst, PhysicalInstrumentation in Medicine and Biology, Pergamon,1966, Section 21.2 (specifica11y Chapters 21, 22, 26and 28).

16.2 ELECTRODE OFFSET POTENTIAL CHARACTERISTICS

drift andnoise

The previous section discusses the electrode offsetpotential produced at an electrode-electro1yteinterface and states that this potentia1 is unstableand unpredictab1e. When e1ectrodes are connected toa subject in order to record a bioe1ectric event ona DC-coupled oscilloscope, long-term changes ine1ectrode offset potential appear as baseline driftand short-term changes appear as noise on the trace.

226

dischargingoffset

If a pair of new electrodes are applied to a subject,an electrode offset potential will exist as theelectrodes can essentially be regarded as twobatteries. When these electrodes are then connectedto a DC-coupled oscilloscope amplifier, the inputimpedance of this amplifier will act as a load on thebatteries and tend to discharge them. Since theseelectrodes form a very poor battery, a typicalTektronix oscilloscope differentiaI-input impedanceof two megohms will discharge the batteries over aperiod of several hours. It is recommended that newelectrode pairs be shorted together in 0.9% salinefor a few hours before use in an effort to acceleratethis process. As these batteries are dischargedtheir effective potential is reduced, this decreasingpotential appearing as electrode offset potentialdrift as shown in Fig. 16-5. Although the source isrepresented as a battery in preceding discussions,it may equally weIl be represented as a capacitorwith the amplifier differentiaI-input impedancetending to discharge the capacitor as depicted in theequivalent circuit shown in Fig. 16-4. Eitheranalogy is only a first order of approximation to thetrue situation.

SILVER/SILVER CHLORIDE

ELECTRODES

ZERO. (INPUT SHORTED)

-DRI FT FROM 2min TO 4min

DRIFT FROM 10min TO 12min

COPPERELECTRODES -DRIFT FROM 2min TO 4min

-DRIFT FROM 10min TO 12min- ZERO. (1NPUT SHORTED)

STAINLESS STEELELECTRODES

1FT FROM 2min TO 4min

ZERO. (INPUT SHORTEO)

ALL 5mV/DIV

DRIFT FROM 10min TO 12min--------ERRAT 1C CHARACTER 1ST 1CS

ALL O.2min/DIV

Fig. 16-5. Electrode offset potentiallong term stability-drift.

dri ftcompar ison

noisecomparison

227


ELECTRODES-NOl SE FREE

COPPERELECTRODES -NOISY


-NOISY

iALL 50~V/DIV1Hz - 100HZl ~ r-

ALL ls/DIV

Fig. 16-6. Electrode offset potential short term stabili ty - noise.

Referring to Fig. 16-5, the electrode offsetpotential drift for silver/silver-chloride electrodes,copper electrodes and stainless steel hypodermicneedle electrodes are shown. ln each case theelectrode offset potential drift is shown over atwo-minute segment after the amplifier load had beenapplied to the electrodes for two minutes and for .ten minutes. A "zero" trace is also included on eachphotograph. The electrode offset potential producedby the silver/silver-chloride electrodes and thecopper electrodes appears to decrease exponentially;however, the stainless steel electrodes are clearlyerratic and, for the particular pair of electrodesused for this investigation, show an abrupt changein potential after about ten minutes. The electrodeoffset potential drift for the silver/silver-chlorideelectrodes and the copper electrodes would beconsidered acceptable, however the erraticperformance of the stainless steel electrodes maymake them undesirable for physiological recording.

Short term changes in electrode offset potential arereferred to as electrode noise. If electrodes areused in conjunction with an AC-coupled highsensitivity amplifier as shown in Fig. 16-6, thelong-term drift characteristics will be rejected bythe low frequency characteristic of the amplifier.

228

storedcharge

storedchargecomparison

However, short-term instabi1ities are c1ear1ydisp1ayed. From Fig. 16-6 it is c1ear that thesi1ver/silver-chloride e1ectrodes are essential1ynoise free, whereas copper e1ectrodes and stain1esssteel electrodes exhibit noisy characteristics thatmay be intolerab1e for high-sensitivity physio1ogicalrecording.

If current is passed through electrodes, this currentwill tend to charge the "battery" or "capacitor"formed at the electrode-e1ectro1yte junction. Ifelectrodes are used for tissue stimulation, thiscurrent flow is deliberate. However, any electricalactivity associated with a subject, such asstimulation or defibrillation, will cause sornedegreeof current flow in the recording e1ectrodes. Theability of the electrodes to be charged like abattery or a capacitor by current flow is termed thee1ectrodes' "offset-potential stored-chargecharacteristic." It is desirab1e that physiologicalelectrodes store as Iowa charge as possible, therebyallowing an amp1ifier's differentiaI-input impedanceto discharge this energy in as short a time aspossible. Referring to Fig. 16-7, silver/silverchloride electrodes, copper electrodes and stainlesssteel e1ectrodes were applied to a subject and usedin conjunction with an amplifier having a 2-megohmdifferentiaI-input impedance. A charge of 0.003coulombs was then applied between these electrodesby discharging a capacitor through them. It can beseen from Fig. 16-7 that the silver/silver-chlorideelectrodes recovered to within 50 mV in 0.2 secondsand had almost completely recovered within 5 seconds.The copper e1ectrodes exhibited inferior storedcharge characteristics, taking 3.5 seconds to recoverto 50 mV and several minutes to completely discharge.The stored-charge characteristics of these copperelectrodes may be acceptable if no deliberate currentwere applied through the subject.

LL

The stainless steel electrodes exhibit particularlypoor stored-charge characteristics, taking 120seconds to recover to within 50 mV and in excess ofsix minutes to be sufficiently stable for ECG use.The stainless steel electrodes rnaycause erraticrecording even if no current is deliberately passedthrough the subject since electrostatically andelectromagnetically induced currents are alwayspresent.


ELECTRODES

COPPERELECTRODES


fAU SOmV/DIv

t

IRECOVERY TO sOmvOFFSET lN 0.25

RECOVERY TO SOmV

10FFSET 1N 3.S5(18 TIMES AS LONG ASFOR Ag/AgCI ELECTRODES)

RECOVERY TO sOmv10FFSET lN 12051<600 TIMES AS LONG ASFOR Ag/AgCI ELECTRODES)

UPPER 2 WAVEFORMS15/DIVLOWER WAVEFORM125/DIV

Fig. 16-7. Electrode offset potential stored charge after .003 coulombsurge - recovery.

230

EFFECT ON MEDIUM SENSITIVITY MEASUREMENT AT VARIOUS LOW FREQUENCY CUTOFFFREQUENCIES.

f-4-O.2s/DIV

ELECTRODE DC OFFSETDRIFTING AT lmV/s.

EFFECT ON HIGH SENSITIVITY MEASUREMENT AT VARIOUS LOW FREQUENCY CUTOFFFREQUENCIES.

50~V/DIV

ELECTRODE DC OFFSETDRIFTING AT lmV/s.

Fig. -8. Electrode offset drift effect on AC coupled amplifier.

drift andAC-coup 1ing

electrodeoffsetversusampl ifiermaximuminput specifications

check'unknown'electrodes

231

Long term electrode offset potential changes (drift)cannot simply be ignored when using AC-coupledinstrumentation as the rate of change of thispotential must be related to the AC-coupling timeconstant in the recording amplifier. Fig. 16-8shows the baseline changes exhibited when usingelectrodes whose offset potential is drifting at arate cf 1 mV/s. At a sensitivity of 0.5 mV/div,as would be used to record the ECG, a O.Ol-Hz lowfrequency response offers little improvement overa DC-coupled instrument. When recording at highsensitivities, say 50 wV/div as is the case with theEEG, this l-mV/s electrode offset potential drift,used in conjunction with an amplifier having aO.l-Hz low-frequency response, produces intolerabledeflection and al-Hz low-frequency response must beused for satisfactory results. The drift rate of1 mV/s chosen is typical of sorneof the poorerquality electrodes; however, silver/silver-chlorideelectrodes and many commercial electrodes can beexpected to exhibit drift rates of between 10 wV/sand 100 wV/s after having been applied for more thantwo minutes.

The absolute value of the electrode offset potentialis rarely significant unless this absolute valueexceeds the maximum differentiaI-DG-input voltagecharacteristics of the amplifier concerned. Whenusing DC-coupled . strumentation this electrodeoffset potential may exceed the maximum DCdifferential offset available within theinstrumentation and it would therefore be impossibleto bring the trace onto the CRT screen.

If the characteristics of an electrode are unknown,it is recommended that tests be performed on theelectrode before the electrode is used forphysiological recording. Electrodes may be testedin situ or, alternatively, the subject may bereplaced by a 0.9% concentration saline solution aswere most of the electrode characteristics presentedin this chapter.

232

16.3 OTHER ELECTRODE CHARACTERISTICS

inertness

mechanicalcharacteristics

photosensitivity

5 ilverpurity

The most important electrode characteristics are thecharacteristics associated with electrode offsetpotential, that is, drift, noise and recovery. Otherfactors must also be considered when selectingelectrodes. The electrode must, of course, be a goodconductor and must be chemically inert. Inertness,or the relative inactivity of the material inchemical reaction, is approximately proportional tothe metal's elettrode potential as shown in Fig. 16-1.Metals with a high negative electrode potential suchas aluminum tend to be more chemically active thanmetals with a high positive electrode potential suchas silver. If an electrode is not relatively inert,the formation of ions by the electrode material andthe reactions between these ions and the acidity ofthe subject's skin produce poisons which canirritate the subject.

Mechanical characteristics of electrodes areimportant in many applications. Although it is ofparamount importance that the electrode besatisfactory electrically, it must also bemechanically rugged, easy to clean and/or sterilizeand easy to apply to the subject.

Silver/silver-chloride electrodes exhibitphotosensitive characteristics. It is not fuIIyunderstood whether the effect of light on thesilver/silver-chloride electrode actually results ina potential being generated, or whether it resultssimply in a change in the electrode's offsetpotential. Nevertheless, in sorneinstances, it maybe desirable to shield the electrode from Iightinterference to insure satisfactory operation. Sofar we have been unable to detect any photosensitivecharacteristics in the Tektronix silver/silverchloride electrodes.

It should be noted that if silver is used forelectrodes, either in its raw state or as a silver/silver-chloride electrode, it should be ofspectroscopic grade (99.999% pure) to ensure noisefree operation. "Fine" jeweller's silver is nowherenear pure enough.

233

16.4 REUSABLE SURFACE ELECTRODES

reusableversusdisposable

indirectcontact

directcontact

indirectcontactbestperformance

There are two principal categories of electrodes inuse -- reusable electrodes and disposable electrodes.Reusable electrodes are intended to be used on manysubjects after having been cleaned and perhapssterilized after each use. Disposable electrodesare intended to be discarded after use on a singlesubject. Price is rarely a paramount factor inselecting a reusable electrode type; however,disposable electrodes must be inexpensive to bepractical thereby limiting their performance.Reusable electrode types usually offer improvedperformance over disposable electrode types and maybe expected to cost between one dollar and tendollars for each electrode.

Each of the two principal electrode types (reusableand disposable electrodes) can be further categorizedinto indirect contact (floating) and direct contacttypes. Indirect contact electrodes are built insuch a way as to be spaced sornedistance from theskin and rely on an electrolytic bridge between theelectrode and the skin. This electrolytic bridge isformed by conductive electrode paste or Itjelly,"suchas the electrode paste manufactured by Day-Baldwinlnc. that is currently supplied as a standardaccessory with the Tektronix Type 410 Monitor.Indirect contact electrodes typically produce lessmotion artifact than the direct contact types andtheir performance is somewhat more predictable asthe electrode is always used in conjunction withonly one type of electrolyte -- the electrode paste.

Direct contact electrodes, as the terminology implies,are designed to make direct contact witb the subjectand they may, if necessary, be applied withoutelectrode paste (dry). Since most direct contactelectrodes are used with a smali amount of electrodepaste to improve electrode-skin contact impedance,they should probably more correctly be categorizedbetween the indirect contact and the direct contacttypes. The electrodes supplied by Tektronix for usewith the Type 410 Physiological Monitor and alsothe Beckman electrodes shown in Fig. 16-9 areindirect-contact silver/silver-chloride electrodes.It is generally agreed that this type electrodeprovides the best available electrode performancefor surface electrode applications.

234

Recommended appl ication configuration

ELECTRODES SUPPLIED BY TEKTRONIX FOR USE WITH THE TYPE 410 MONITOR

Type currently supplied

Earlier types suppl ied

Alternate appl ication configuration Alternate application configuration

SILVER PLATE ELECTRODES

235

SILVER EEG ELECTRODES

SELF RETAINING ELECTRODES

LEAD PLATE GSR ELECTRODES BECKMAN ELECTRODES

--•• ••

Fig. 16-9. Reusable electrode types.

THE 2 PIN PLUG USED ON THE ADAPTERCABLES MAY BE REMOVED AND REPLACEDWITH A BNC CONNECTOR, TEKTRONIXPART NUMBER 131-0297-00, TO ALLOWUSE WITH INSTRUMENTATION HAVINGBNC INPUT SOCKETS.

~

4-40 U.N.F.THREAD USED

ELECTRODE --------lf...---- •••..PIN 119-0197-00

BARE W 1RE ADAPTER -----+---- ...•..PIN 103-0080-00

NEEDLE ADAPTER ------4___!_-~PIN 103-0108-00

PLATE ADAPTER -------11---- ...•...••••••PIN 103-0079-00

ADAPTER CABLES CUSTOM ADAPTERS FOR USELENGTH COLOR MARKING TEKTRONIX PIN WITH OTHER ELECTRODE TYPES

MAY BE FABRICATED USING6' GREEN RL 012-0172-00 THE PLATE ADAPTER

6' RED LL 012-0170-00

4' WHITE RA 012-0171-00

4' BLACK LA 012-0169-00

4' BROWN C 012-0173-00

2.5' YELLOW EEG+ 012-0174-00

2.5' YELLOW EEG- 012-0175-00

Fig. 16-10. Tektronix electrode cables and adapters.

reusableelectrodetypes

adaptercables

237

Fig. 16-9 shows various types of reusab1e surfaceelectrodes. Earlier e1ectrode types supp1ied byTektronix for use with the Type 410 Physio1ogica1Monitor were permanently affixed to an electrodecable; however, the types currently supplied aredetachable from the electrode cable. Fig. 16-9shows the current Tektronix silver/silver-chlorideelectrode applied to a subject using the adhesiverings and electrode paste supplied by Tektronix withthe Type 410 Monitor. Alternative applicationconfigurations, using surgical tape over theelectrode or using an adhesive-backed foam pad overthe e1ectrode, are also shown. The large, silverplated, copper electrodes historically used to recordthe ECÇ, the self-retaining electrodes currentlypreferred as a chest electrode for ECG recording,silver EEG electrodes, lead plate electrodes intendedfor GSR recording, and silver/si1ver-chloridee1ectrodes produced by Beckman are shown. AlI of thee1ectrodes in Fig. 16-9 are photographed to the samesca1e. These electrodes can be adapted for use withthe Tektronix Type 410 Physiological Monitor usingeither a plate adapter, a need1e adapter or a barewire adapter. These adapters are shown in use inFig. 16-9 and further details on adapter cables andadapters avai1able from Tektronix are shown in thenext figure.

Referring to Fig. 16-10, Tektronix supplies 2t-, 4-and 6- foot adapter cables to al10w signaIs to beconnected from e1ectrodes to the Type 410 Monitor.These adapter cables can be directly connected tothe Tektronix silver/si1ver-ch1oride electrodes orcan be connected to other e1ectrode types via eitherplate adapters, need1e adapters or bare-wire adapters.These are the only adapter types avai1ab1e fromTektronix; however, enterprising users havesuccessfu11y adapted other e1ectrode types for usewith the Tektronix Type 410 Monitor. The electrodeand adapter system shown in Fig. 16-10 is primari1yintended for use with the Tektronix Type 410 monitor;however, if the two-pin plug supplied with thesecab1es is replaced by a BNC connector, these cablesand adapters can be used with other instrumentationhaving BNC input sockets.

238

UNITED A 1RCRAFT CORPORATION; HAMILTON STANDARDDIVISION; ADHESIVE "TELECTRODE" DISPOSABLE ELECTRODE

ELECTRONICS FOR MEDICINE INC. DISPOSABLE PURE SILVER ELECTRODES

239

LEXINGTON INSTRUMENTS INC. DISPOSABLE SILVER PLATED ELECTRODE

DEVICES INC.DISPOSABLE

ADHESIVE ELECTRODE

PHYSIO-CONTROL CORP. ADHESIVE "DERMA-CIRC"DISPOSABLE ELECTRODE

BECTON, DICKINSON AND COMPANY; ELECTRODYNEDIVISION; "DISPOS-EL" DISPOSABLE ELECTRODE

Fig. 16-11. Disposable electrode types.

240

16.5 DISPOSABLE SURFACE ELECTRODES

disposableelectrodetypes

spray-onelectrode

Tektronix does not produce a disposable electrodefor use with the Type 410 Physiological Monitor.Disposab1e e1ectrodes produced by six differentmanufacturers are shown in Fig. 16-11. As statedearlier, disposable e1ectrodes rnustbe inexpensiveand one would expect to pay less than a dollar foreach electrode. While disposable electrodes areindeed convenient and offer adequate performance forrnanyapplications, they appear to be unsuited toapplications where the subject may receivestimulation and/or defibrillation as their electrodeoffset potential stored-charge characteristics appearto be substantially worse than for si1ver/si1verchloride electrodes. While Tektronix would not wishto recommend any particular brand of disposableelectrode, aIl are usable with the Type 410 Monitorfor ECG recording if the patient is not expected torequire defibrillation, stimulation or cauterization.

A relatively new and unique disposable electrode isthe spray-on e1ectrode developed by NASA for ECGrecording in space flight. This electrode system iscommercia1ly available from the Hauser Research andEngineering Company in Boulder, Colorado. Theseelectrodes basically consist of a film of conductiveadhesive that is sprayed onto the skin with a contactwire imbedded into this adhesive during its dryingcycle. These spray-on electrodes are particularlysuited for long-term monitoring on subjects who arephysically active.

16.6 NEEDLE ELECTRODES

hypodermicelectrodes

Needle electrodes fall in the disposable electrodecategory and are primarily used for ECG monitoringduring surgery or where extremely fast electrodeapplication is desirable, as in an emergencysituation. Needle electrodes may simply be stainlesssteel hypodermic needles. Stainless steel is asomewhat unsatisfactory rnaterial for electrodes(covered elsewhere in this chapter) as its

241

characteristics are.inferior to most other electrodematerials. Stainless steel must however, be used asit is one of the few materials that can offersufficient mechanical strength. Typical long-needleelectrodes are shown in Fig. 16-12. These needlesare regular hypodermic needles but short hypodermicneedles are preferable as it is only necessary toinsert the needle just under the subiect's skin.

(A) HYPODERMIC NEEDLE ELECTRODES

INSULATED HYPODERMIC NEEDLE(OR SURGICAL NEEDLE)

CONCENTRIC NEEDLE ELECTRODE(HYPODERMIC NEEDLE WITH ONECONDUCTOR THREADED THROUGH IT)

DIFFERENTIAL CONCENTRIC NEEDLEELECTRODE (HYPOOERMIC NEEDLEWITH TWO CONDUCTORS)

(S) CONCENTRIC NEEDLE ELECTRODES

Fig. 16-12. Needle electrodes (disposable).

242

concentricelectrode

differentialconcentricelectrodes

The hypodermic needle electrode referred topreviously is intended to perform similar functionsto those of the surface electrodes. Special typesof needle electrodes, such as concentric needleelectrodes or insulated needle e1ectrodes, may beused in applications where it is necessary to insertthe need1e to a bioe1ectric site within the bodyrather than to record the electrical activityproduced near the surface of the body. Concentricneed1e e1ectrodes may be two or three inches longfor use in e1ectromyography of the ske1eta1 musclesand may be up to six inches long for certainapplications in neurosurgery. Various arrangementsof concentric needle e1ectrodes are also shown inFig. l6-12B. The simplest form of "concentric"needle e1ectrode is an insulated hypodermic needle.The insulated need1e effectively only recordse1ectrical activity generated in the region of thetip of the need1e. A simple concentric need1eelectrode consists of a hypodermic needle with alength of 36-gauge enamel-covered silver wirethreaded through it. The enamel covering on thesilver wire insulates it from the stainless steelneedle and is removed at the electrode tip area toprovide a need1e electrode similar to a miniaturecoaxial cable. A bioelectric signal may be recordedby connecting the recording amplifier between thecenter conductor and the outer concentric needle or,preferably, two concentric needle electrodes may beused with a recording amplifier connected betweenthe center conductor of each and the outer needle ofeach acting as a grounded shield. Two pieces ofenamel-covered platinum wire may be used in ahypodermic needle to produce a differential concentricneedle electrode. This form of electrode is primarilyused with the outer needle grounded and with therecording equipment connected between the two innerconductors. This effectively records the electricalactivity produced in the region of electrodes spacedless than l millimeter apart. This type ofdifferential concentric needle electrode may be usedfor recording the EMG and evoked responses.

243

16.7 MICROELECTRODES

glassmicroelectrodes

metalmicroelectrodes

mlcroelectrodecharacteristics

Microelectrodes are used to record the electricalactivity within individual cells, thus, the activeelectrode tips of these microelectrodes must besmall enough so as not to interfere with the cell'snormal function. Microelectrodes may be expectedto have tip diameters within the range from 0.1micron to 10 microns (one micron = 10-6 meters).Microelectrodes can be divided into two broadclassifications -- glass microelectrodes and metalmicroelectrodes. Glass microelectrodes consist ofan electrolyte-filled glass tube that has been drawnto a capillary with a silver wire making connectionto the electrolyte within the tube. The transferfrom ionic conduction to electronic conduction ismade at the relatively large surface area interfacebetween the platinum wire and the electrolyte. Metalmicroelectrodes consist of a finely tapered metallicrod covered with a layer of insulation. With metalmicroelectrodes, the transfer from ionic conductionto electronic conduction occurs at the electrode tip,that is, within the cell itself.

The characteristics of glass microelectrodes arecovered in considerable detail in Chapter Il. Theequivalent electrical circuit of the glassmicroelectrode is shown in Chapter Il and is alsoshown in Fig. 16-13 together with the equivalentcircuit for the metal microelectrode. Referring tothe equivalent circuit for the metal microelectrode,most of the microelectrode resistance (Rm) is locatedwithin the first few microns from the electrode tip.With the metal microelectrode inserted into the cell,this resistive area will be located within the cell.The capacity between the electrolyte within the celland the metal microelectrode (wich themicroelectrode insulating material acting as thedielectric) is represented by C

mc.

244

GLASS MICROELECTROOE WITH SILVER WIRE

RmTYPICAL:

C :: O.lpFme

Cmf :: O.2pF

R :: 108nm

A GLASS"MICROELECTROOE PULLER"

RESIN INSULATED METAL MICROELECTRODE

RESIN INSULATION

GOLO PLATEDCONNECTOR

Rm

TUNGSTEN CARBIDEPOL 1SHED ROD

METALL ICTHIN FILM

TYPICAL:

C = 15pFme

Fig. 16-13. Microelectrodes.

METALMICROELECTRODE

Rand Cm meLOCATED lNTHIS REGION

mlcroelectrodeconstruction

245

Metal microelectrodes are formed by etching a metalrod while the metal rod is being slowly withdrawnfrom the etching solution, thus forming a tapered tipon the rode The etched metal rod is then dipped intoan insulating solution bath to form a layer ofinsulation over the rode This insulating layer isprevented from covering the tip by the surfacetension action. Glass microelectrodes are formedby heating a special quality glass tubing and thendrawing the soft glass tubing apart with controlledtension to form a capillary. This is usuallyachieved with a mechanical device known as a"microelectrode puller" consisting of a heatingelement and tensioning system. A microelectrodepuller is shown in Fig. 16-13.

16.8 ELECTRODE APPLICATION

Electrodes may be affixed to a subject in a varietyof ways and application recommendations are usuallyincluded with electrodes at the time of purchase.Fig. 16-9 shows various techniques for applyingmany electrode types and, in the case of theTektronix electrodes, shows application by eitheran adhesive ring, adhesive tape or an adhesive foampad. The silver EEG electrode shown in Fig. 16-9 isglued to the subject's forehead with a commercialadhesive known as collodion. AlI electrode typesoffer better performance if applied after cleaningand abrading the skin with a small amount of slightlyabrasive electrode paste. This tends to remove deadskin cells from the surface and thus decreases theelectrode contact impedance.

246

lYr----------.-----------r----------,ELECTRODE PAIR OFFSET POTENTIALCHANGES WITH TIME FROM APPLICATIONTO SUBJECT

ELECTRODE PAIR OFFSET POTENTIALCHANGES WITH TIME AFTER PASSINGA CURRENT IMPULSE OF .003 COULOMBS

O. 1mV O. 1mY L----L---L....L..I..I..I~---L---'-....L..Jl...J..J..UJ---....L....].-J.-L.J..J..l.u1 15

TIME (MINUTES) (SECONDS)

REUSABLE ELECTRODES - SEE FIG. 16-9RESISTANCE

AFTER 40 ml n

CURRENT TEKTRONIX ELECTRODES 170n

2 EARL!ER TYPE TEKTRON 1X ELECTRODES - SMALL 320n

3 EARLIER TYPE TEKTRON 1X ELECTRODES - LARGE 100n

4 SILYER PLATE ELECTRODES 260n

5 SELF RETAINING ELECTRODES 250n

6 "SOLDER" EEG ELECTRODES 400n

7 LEAD PLATE GSR ELECTRODES 260n

DISPOSABLE ELECTRODES - SEE FIG 16-11

8 UN 1 TED AIRCRAFT 380n

9 ELECTRONICS FOR MEDICINE 360n

10 LEXINGTON INSTRUMENTS 280n

11 PHYSIO-CONTROL CORP 500n

12 NEEDLE ELECTRODES - SEE FIG. 16-12 350n

OISE AFTER 30 min50IlY/DIV

Fig. 16-14. Characteristics of electrode types shown in Figs. 16-9, 16-11 and 16-12.

247

16.9 ST~TING ELECTRODES

stimulatingversusrecordingelectrodes

Almost aIl of the electrode types discussedpreviously may be used as stimulating electrodes.When using electrodes for stimulation, current isdeliberately passed between the electrodes. Thiscurrent, unless it is truly biphasic, will increasethe electrode offset potential, thus making theelectrodes increasingly unsuitable for future useas recording electrodes, even if silver/silverchloride electrodes are used as stimulatingelectrodes. It is advisable that, once any electrodehas been used as a stimulating electrode, it beidentified in sornemanner and not used thereafter asa recording electrode. Silver should be used forstimulating electrodes as it is nontoxic and doesnot tend to produce iontophoresis due to the passageof electric current. Ionotophoresis is the drivingof external ions in through the skin which canproduce considerable irritation if unsuitableelectrodes are used.

16.10 COMPARISON OF ELECTRODE TYPES

30-minutedriftcomparison

There is undisputed evidence to suggest thatsilver/silver-chloride electrodes are far betterthan other types for recording physiological signaIs;however, they are not weIl suited to sorneapplications and may offer unnecessary performancein other applications. Many non-silver/silverchloride electrodes are used for physiologicalmonitoring and are discussed in previous sectionsin this chapter. A comparison between 12 of theelectrode types discussed in this chapter is shownin Fig. 16-14. Several pairs of each of the 12electrode types were tested and, in each case, thepair exhibiting the poorest performancecharacteristics is shown in this characteristicsummary. The first graph respresents the electrodeoffset potential drift of each of these 12 electrodetypes over a 30-minute period after the electrodeswere applied to a subject and coupled to an amplifierhaving a 2-megohm differentiaI-input impedance. lnaIl cases, the reusable electrodes had been usedseveral times previously.

248

applyingcurrentpulse

recoverycomparison

noisecomparison

conclusion

After observing the drift characteristics of theseelectrodes over a 30-minute period, a current impulseof .003 coulombs was applied through the electrodesby discharging a capacitor across them. The secondgraph shows the change in electrode offset potentialfor each of these 12 electrode types during the firstfive minutes after application of the current impulse.ln aIl cases the electrodes were tested in anelectrolyte consisting of 0.9 percent sodiumchloride. The insertion of the electrodes into thesodium chloride bath was adjusted to match theelectrode surface area in the bath to the electrodesurface area normally in contact with a subject.The separation of the electrodes within the bath wasmaintained at approximately three centimeters.Fig. 16-14 shows the resistance of the electrode pairto a 10 mA De current after having been in theelectrolyte and connected to the amplifier for aperiod of 40 minutes. Fig. 16-14 also shows thenoise produced by these electrodes over a 10-secondperiod after having been in the electrolyte bath andconnected to the amplifier for a 30-minute period.

While we would not wish to make a detailed analysisof the results shown in Fig. 16-14, it is apparentthat reusable electrode types generally exhibit lowerelectrode offset potential and lower noisecharacteristics than the disposable electrode types.It should also be noted that, particularly fordisposable electrodes, manufacturing techniques areconstantly changing and the performance typical ofan electrode type may change as the manufacturingprocess is changed.

249

GROUNDING - SAFETY

artifact

The term artifact refers to the presence of anunwanted signal such as power line frequencyinterference or noise. When recording, for example,the EEG, the presence of ECG or EMG information onthe recording also constitutes artifact. By far themost common source of artifact is power lineinterference; however, power line leakageconsiderations are of paramount importance from asafety viewpoint and it is not uncommon for safetyconsiderations to seemingly contradict artifactelimination considerations.

17.1 GROUNDING

If a subject were deliberately connected across aone-volt AC source, an AC current of perhaps 100 ~Awould flow through the subject due to the finiteimpedance (~lO,OOO ohms) of the subject and skinconnections. Although such a connection should neverbe deliberate, many instrumentation systemsinadvertently produce a similar situation in the formof a ground loop.

250

GROU!"JOED'IL

MAGNETIC FIELD FROM UTILITYTRANSFORMERS AND WIRING INOUCESA CURRENT INTO THE GROUNO LOOP.

UOTHER~.--e MONITORI G

EQUIPIIIIENT

GROUNOEORECEPTACLES

ECGMO"JITOR

"GROUNO" CURRENTS RESULT lN A POTENTIALDROP ACROSS THE SUBJECT'S RESISTANCE.THIS APPEARS 0 THE ECG MON 1TOR AS60Hz NOISE.

POWERCOROGROUND

IF THE GROUND LOOP CANNOT BE ELIMINATED BY"OPEN CIRCUITING" THE LOOP BY REMOVING AGROUNO CONNECTOR; EITHER

1 CREASE IMPEDANCE OF GROUND LOOP SOSUBJECT IMPEDANCE IS NEGLIGIBLE (ADDlOOkQ lN SERIES WITH RL)

100kn

ORREWIRE GROUNDS SO THAT GROUNO CURRENTSDO NOT PASS THROUGH SUBJECT ( USE SINGLEPOINT GROUNDING).

:?GHZGROU 0LOOP

Fig. 17-1. Induced ground current from a "ground loop".

induced60-Hz

groundloop

el iminati nggroundloop

251

If a 10-foot 1ength of wire is connected between apatient's left arm and his right 1eg, a 60-Hz ACcurrent will be induced through the patient as thiswire acts as a transformer secondary, with currentmagnetica1ly induced from adjacent power wiring andtransformers. Although the magnetic couplingmateria1 involved is air and the amount of couplingtherefore is extreme1y small, it is finite and anEMF will be induced into the loop which will in turncause current to flow through the subject. ThislO-foot length of wire may, in practice, be twoseparate grounding wires attached to a subject(Fig. 17-1).

Referring to Fig. 17-1, if either the ECG monitoror the "other monitoring equipment" is connected tothe subject, a ground loop is not produced. However,if both instruments are simultaneously connected tothe subject, the ground connections for eachinstrument form a ground loop. The ground loop shownproduces a potentia1 between the subject's right legand left arm causing a current to flow through thepatient's bulk body resistance. This current flowproduces a voltage drop across different parts ofthe body which would appear as a 60-Hz artifact onthe ECG monitor.

The ground loop produced by the configuration shownin Fig. 17-1 may simply be eliminated by removingone of the ground connections to the subjectwhenever both items of monitoring equipment are usedsimultaneously. If this is particularlyinconvenient, the ground loop may be eliminated byadding a high impedance in series with one of theseground leads to reduce current flow to a negligiblevalue or the subject may be grounded at a singlereference point so any induced current flow does notflow through the subject's bulk body impedance. Bothof these ground loop changes are shown in Fig. 17-1and both changes effectively eliminate the currentflow through the subject, thus eliminating 60-Hzpotential differences between various points on thesubject.

252

17 • 2 INDUCED GROUNDCURRENTS

transformerinducedcurrent

buttockplateqroundloop

old styleelectrocauteryunits

A ground loop is an example of an induced groundcurrent, the value of which can be reduced oreliminated by modifying the grounding configuration.Another form of induced ground current is illustratedin Fig. 17-2 where a current is induced into a groundwire via an instrument's power transformer. ln thisspecific case, an induced 60-Hz ground current isinduced into the grounding circuit of anelectrocautery unit. This ground current flows viathe electrocautery ground plate, through the subjectand to ground via a perspiration-dampened bedsheet,a conductive rubber mat and the grounded operatingtable.

The above type of induced ground current isparticularly difficult to eliminate as safetyconsiderations demand that the subject be groundedand that the electrocautery-unit buttock plate beweIl grounded to the electrocautery unit and to thesubject to eliminate burns at electrode sites duringelectrocautery. This particular problem is oftenencountered when trying to monitor a subject's ECGin an operating room using an older styleelectrocautery unit. When these.older units weredesigned it was not expected that the ECG may bemonitored while the electrocautery unit was connectedto the line ready for use. Modern electrocauteryunits have more carefully designed internal-groundingconfigurations and incorporate shielding to preventany magnetically" induced or capacitively inducedcurrent from entering the subject via the buttockplate. Perhaps the only solution that can be offeredfor an electrocautery unit of the older design is todiscard the unit. Although this "solution" may seemharsh, the laws of physics offer no alternative and,more importantly as will be seen later in thischapter, the induced current produced by theelectrocautery unit may be sufficient to cause deathif grounded catheters are used on the subject.

253

SUSJECT LYING ON MOIST (PERSPIRATION)

/

SEO SHEET AND CONDUCTIVE RUBBER MAT.ELECTROCAUTERY BUTTOCK PLATE UNDER SUBJECT.

GROUND CURRE~',-Nr

~~

JGROUNDED

OPERATING TABLE

MONITOR WILL RECORD60Hz VOLTAGE DROPACROSS SUBJECT

ELECTROCAUTERYUNIT

1NDUCE7(;({CURRENT

1115V 60Hz

LINE

EQUIVALENT 1

TO tINDUCED

60Hz CURRENT

SUBJ ECTRESISTANCE1000

115V 60HzLINE

AN INDUCEO 60Hz CURRENT OF 350nA RMS WILL PRODUCE2mm OF 60Hz NOISE ON A STANDARD ECG MONITOR

Fig. 17-2. Induced ground current from electrocautery unit.

254

115 VOLTUT 1LlTYWIRING

11

_.L-C-r-I11

11

_J._ C-,-11

11

_1- C-T-I1

1

115V 60HzLINE

CAPACITIVE COUPLING FROM 1POWER LINE TO SUBJECTCAUSES CURRENT I, CTHROUGH SUBJECT.

l

TO EEGRECORDER SUBJECT TO EEG

1Don RECORDER~

IF C = 3pF

X 900MncIRMS = 130nA

ERMS l3IJV

Ep_p = 36IJV (O.6cm DEFLECTIONON EEG RECORDER)

TO ELIMI ATE CURRENT FLOW

T7:R~ EITH::1ELoTHESUruECT.SHIELD UTILITY WIRING

GROUNDEDSHIELD

i

1_J._ C-,-

t

Fig. 17-3. Electrostatic shielding.

i

f

2SS

17.3 ELECTROSTATlCALLY INDUCED CURRENTS

uti 1 itywiringcapacitance

uti 1 itywiringshielding

recordingleadshielding

Capacitive coupling between the lIS-volt w1r1ng andthe subject constitutes an impedance which will allowcurrent to flow through the subject. The situationis illustrated in Fig. 17-3 which shows that a straycapacitance of 3 pF between the utility wiring andthe subject can produce 6 centimeters of 60-Hzartifact on an EEG recording.

To eliminate electrostatically induced currents, itis necessary to provide an alternative path for thiscurrent to flow. This may be achieved by eithershielding the lIS-volt utility wiring, in which casethe current flows from this wiring to the shield, orby placing the subject inside an electrostaticshield (Faraday Cage), in which case the currentflows to ground via the shield rather than via thesubject. ln either case, shielding is expensive andit may be preferable to locate a recording site inan alternative part of the hospital or laboratorywhere electrostatically induced current is lessbothersome. ln the research environment, wherehigh sensitivity amplifiers are invariably used, thesubject and parts of the recording equipment shouldbe enclosed in a screened room to effectively isolatethe subject from capacitively induced currents fromany source.

Currents may be capacitively coupled to the leadsconnecting the subject to the recording equipment,thus these leads must be screened to effectivelyshield them. Care should be taken to ensure thatthese shields are, however, only grounded at onepoint otherwise, in attempting to eliminateinterference by electrostatic shielding, one mayinadvertently create 60-Hz interference byintroducing a ground loop. This interference may beproduced by 60-Hz power wiring or it may be highfrequency interference produced by radio and/ortelevision transmitters.

17.4 ELECTRIC SOOCK CURRENT 1HRESHOLDS

When electric current is passed through the body,part of this current will pass through the heart andinterfere with the normal function of the heart,perhaps causing death. ln cases of death byelectrocution, almost aIl subjects are killed by thepassage of electric current through the heart ratherthan by sornerelated occurrence such as burns or

256

voltageversuscurrent"5hoc k"

shockcurrentthresholds

ventricularfibrillation

vulnerableperiod incardiaccycle

muscular paralysis of the muscles controllingbreathing action. If a person receives a l15-Vshock, it is not the voltage of this shock that isimportant but rather the current through the body.Thus, if a person receives a l15-V "shock" standingin a dry environment insulated by normal clothing,the "shock" may hardly be felt as the current may beweIl below 1 mA. If, however, a person receives al15-V shock while standing barefooted on moistground, the person would receive a severe shock andprobably be killed as the current may exceed 100 mA.

Referring to Fig. 17-4; experimental investigationon numerous subjects (both male and female) hasindicated that up to 300 ~A applied to the surfaceof the body, such as from one arm to the other, maybe suggested as being reasonably safe for mostsubjects. It can be seen that 99.5 percent of thepopulation require 400 ~A or more of 60-Hz currentto perceive the current and that the threshold ofperception or sensation increases as frequencyincreases above 100 Hz and below 10 Hz. At 10,000 Hzand at De, the threshold of sensation isapproximately five times greater than at 60 Hz.While current above the threshold of sensation maynot be detrimental to a healthy person, it may causecomplications in a hospitalized patient and willcertainly create anxiety which would be detrimentalto the patient's general weIl being. From Fig. 17-4it can also be seen that "cannot-let-go" currentthresholds are an order of magnitude above sensationcurrent thresholds and that they also increase forfrequencies below 10 Hz and above 100 Hz.

Fig. 17-5 shows the physiological effects of 60-Hzarm-to-arm current and shows a threshold ofsensation at 300 ~A, a threshold of pain at 1 mA, a"cannot-let-go" threshold at 10 mA and a ventricularfibrillation-induction threshold at 100 mA. 99.5%of the population will have thresholds above thesevalues. Ventricular fibrillation refers .tomalfunctioning of the ventricular musculature whichwill interfere with the normal blood-pumping actionof the heart and eventually cause death. Ventricularfibrillation is produced by current directly throughthe heart during a specifie portion of the cardiaccycle known as the "vulnerable period." Thevulnerable period for ventricular muscle occursduring the upstroke of the T-wave and a single shockimpulse lasting for less than 0.1 second could causeventricular fibrillation if received during thisvulnerable periode

257

"CANNOT LET GO"LIMIT FOR 99.5%POPULATION

RMSCURRENT

mA

THRESHOLO OFSENSATION FOR99.5% POPULATION

300uA SUGGESTEDSAFE LlMIT

10 100 1000 10k lOOk

FREQUENCY OF ARM-TO-ARM CURRENT (HERTZ)

Fig. 17-4. Physiological effects versus current frequency.

THRESHOLD OF THRESHOLD OF CANNOT VENTRICULAR FIBRILLATION SEVERE BURNS

SENSATI~J1III Ir J~ll1IIIIr ~~I~B~:I~~IATHtL~~~,S~K100uA 1mA 10mA 100mA lA 10A

60Hz SHOCK CURRENT ARM-TO-ARM

CATHETER 1NTOCIRCULATORY

SYSTEM

MAXIMUMACCEPTABLE.::15lJA

MAXIMUMACCEPTABLE.::300lJA

BODY SURFACE TO HEART BODY SURFACE TO BODY SURFACEARM-TO-ARM -- PART OF THECURRENT FLOWS THROUGH THEHEART.

Fig. 17-5. Physiological effects of 60Hz electric current.

258

currentdensityversusthresholds

internaicurrentthresholds

internaiprobes andelectrodes

highaccidentrate

The 300 ~A threshold refers to 60-Hz current appliedfrom one arm to the other arm and is intended as aguide to acceptable leakage current levels. Theacceptable level for any one subject may be somewhatabove this value and will also depend on the size ofthe current contact. A very small point contactcan undoubtedly be felt at 0.3 mA, but a current inexcess of perhaps l mA may not produce sensationif the contacts are somewhat larger. Depending onthe size of contact, the threshold of pain may alsobe considerably above 1 mA, probably 10 mA if thecontacts are large enough. It is thus evident thatcurrent density is also important in determining thephysiological effects of electric current.

The threshold of sensation of electric currentsdiffers greatly between currents applied arm-to-armand currents applied internally to the body. If acurrent is applied internally, a far greaterpercentage of the current rnayflow via the arterialsystem directly through the heart, thus less over-allcurrent is required to produce ventricularfibrillation. Experiments with dogs have indicatedthat in sorneanimaIs ventricular fibrillation can beproduced with currents as low as 17 ~A applieddirectly to the dog's heart. ln general, as thethreshold of.sensation in man may not be much higherthan for the dog, 30 ~A of 60-Hz current through theheart may produce ventricular fibrillation and 15 ~Acould perhaps be considered as a safe upper limit.5 ~A has been postulated as a safe upper 1imit undercertain adverse conditions.

A probe or e1ectrode within the body such as acardiac catheter or pacemaker e1ectrode will providea direct electrical pathway to the heart and thusthe 15 ~A current limit proposed in the precedingparagraph should be considered as an upper limit forcurrent flow through these conductors. The impedancefrom these conductors to the subject's skin surfacemay typically be 1,000 ohms, thus a potential of30 mV between these conductors and a point on thesurface of the subject's body is sufficient to causeelectric shock, ventricular fibrillation and possibledeath! It has been proposed that the introductionof the internaZ eZectrode has been the leadingfactor in the present high incidence of accidents inhospital patient care areas and operating rooms.

259

Cardiac catheterization for diagnostic or therapeuticpurposes has become common in recent years. Sensingcatheters may have electrodes or transducers attheir tips for ECG recording, blood pressuremeasurements and other diagnostic procedures. Fluidfilled catheters are also in common use and, althoughthey are not normally considered to be a probe orelectrode within the body, the fluid in thesecatheters may be conductive, thus they also mayprovide an electrical pathway to the heart.

17.5 INSTRUMENTATION SAFETY CONSIDERATIONS

unprecedentedshock hazarddemandssystemapproach

powertransformerleakagecurrents

Twenty years ago the electrocautery unit was the onlyitem of electronic equipment routinely used in theoperating room. Any electrical shock hazardassociated with this instrument was far outshadowedby the risk of explosion·from flammable anestheticgases. Nowadays the operating room is packed withelectronic equipment, aIl of which is necessary forthe fulfillment of today's modern surgicalprocedures, and the risk of explosion from flammableanesthetic agents is considered of secondaryimportance. AlI of the instruments used in theoperating room may be used alone with relativeimpunity however, in combination, they produce anunprecedented electrical shock hazard of giantproportions. The interaction of various unitsconnected to the subject demand a system approachwhich may be foreign to the training of medical staffand may also not be fully understood by themanufacturers of the various items ofinstrumentation.manufacturers tendand tend to regarditself rather than

Sornemedical instrumentto be oblivious to this problemtheir product as an entity withinas a part of an over-all system.

The major source of potentially lethal currents inany instrument is leakage current from the 115-V(or 230-V) power-transformer primary. This leakagecurrent is largely due to capacitive coupling fromthe power transformer primary to other parts of thetransformer and/or other parts of the instrument.Instruments are usually designed so this leakagecurrent flows to the instrument case and then toground via the three-wire power cord provided withthe instrument. Most modern general-purposeinstrumentation may be expected to produce up to500 ~A of leakage current, however instrumentation

260

shockcurrentsfromungroundedinstruments

shockcurrents toisolatedsubjects

designed for use in the medical environment shouldpreferably have a leakage below 100 ~A. By specialdesign techniques it is possible to decrease thisleakage to weIl below 10 ~A. Although this may notalways be practical, as long as'the instrumentis adequately grounded, the leakage current willnot flow through the subject. A problem arises wheninstrumentation is used in an ungroundedconfiguration, as would be the case if the three-pinpower plug provided with the instrument was replacedby a two-pin plug to allow the instrument to be usedwith the older style two-pin power receptacles.Under these conditions the ground leakage currentmay flow through the subJect as shown in Fig. 17-6and if one of the conductors is an internaIelectrode, such as a catheter, death will almostcertainly result.

It is apparent that if the subject were "floating"or isolated, then a path to ground would not beprovided for the Ieakage current mentioned in theprevious paragraph and no current would flow throughthe subject. ln practice, however, this does notreduce the risk as fault currents could still flowbetween various items of equipment without involving"ground" in the current circuit; also, if a subjectinadvertently touched a grounded item, such as a bedframe or water pipe, then the isolation would benullified and electrocution may result. It is,therefore, apparent that protection systems must beorganized on a grounded system concept.

INSTRUMENT CASE

115V 60HzLINE

mALEAKAGE CURRENT OF 0.5mA TO GROUNO OR,IF FAULTY GROUNO, TO SUBJECT

CAPACITANCE FROM PRIMARY TO EITHER SHIELO, CORE OR GROUNOEO SECONOARYRESULTS lN CURRENT FLOW FROM PRIMARY TO INSTRUMENT GROUNO (CASE).

ASSUMING CAPACITANCE OF .01~F:X AT 60Hz = 250knc 115

RMS CURRENT = 250mA = O.SmA

Fig, 17·6. Transformer capacitive leakage currents.

secondordershockprotection

current1imiting

current1imit ingwith fieldeffectdiodes

261

The various hazards mentioned so far in this chapterprimarily refer to current flowing through thesubject if a fault occurs or if something abnormalhappens. As in any situation, faults do occur andabnorma~ities do happen, thus it is desirable tooffer sornesecond order protection to the subject.

Under normal operation of, for example, an ECG unit,only a small signal current should flow in any ofthe active electrodes or the right-leg groundelectrode. Under abnormal or fault conditions,however, current may flow through any one of thesepaths and second order protection can be offered byincorporating sorneform of current limiting. Thesimplest form of current limiting, and a form adequatefor use with the right-leg ground electrode, is touse a large value resistor in series with theelectrode. Series resistance current limiting isnot, however, always possible. ln any amplificationequipment, such as an ECG monitor, series resistancewill degrade the common-mode rejection-ratiospecification of the instrument to a point where60-Hz interference may be intolerable. It issometimes necessary to use a more sophisticated formof current limiting.

The Tektronix Type 410 Physiological Monitor isspecifically designed for operating room use andincorporates series-resistance current limiting inthe right-leg ground electrode and more sophisticatedcurrent limiting in series with the active electrodes.This current limiting utilizes field effect diodes,the impedance of which is approximately 1,000 ohmswhen the current is less than 1 ~A. However, thediodes limit fault currents to a maximum of 300 ~Aas their impedance increases as the current attemptsto exceed the 300 ~A level. It is thus apparentthat current limiting within the Tektronix Type 410Physiological Monitor provides adequate second ordersubject protection for "arm-to-arm" fault currentsdelivered by surface electrodes. It does not provideadequate protection to allow the Type 410 to be usedwith intracardiac-catheter electrodes. It should benoted that the instruction manual provided with theTektronix Type 410 Physiological Monitor specificallystates that the unit is not intended for use withinternaI electrodes of any type.

262

current1imitingadapters

Many oscilloscopes produced by Tektronix andelectronic instrumentation produced by othermanufacturers are used in operating rooms and inintensive-care rooms on live human subjects. Itshould be noted that this type of instrumentationwas designed for use in the electronics laboratoryand does not incorporate any secondary subjectprotection. If a Tektronix product, other than theType 410 Physiological Monitor, is to be used inconjunction with a human subject, we suggest thatsorneform of current limiting be included in serieswith any electrode in contact with the subject.Spacelabs, Inc., Chatsworth, California, market anelectrical shock protector which can be used inseries with electrodes if the nominal line voltageis below 120-V AC or, alternatively, a currentlimiting adapter is described in Chapter 29 whichis suited for use in areas with either 120-V or240-V power. It provides secondary protection bylimiting current to 300 ~A. Secondary protectiondoes not guarantee safety, it simply reduces theprobability of an accident. Each protection featureadded to instrumentation should reduce thisprobability.

17.6 ELECTRICAL SERVICE GROUNDING

potentialsbetweendifferentgroundingsystems

Section 17.4 pointed out that a potential of only30 mV between two supposedly grounded conductorsmay allow enough current to flow through a subjectto cause death by electrocution. If multiple groundpoints are available in an operating or intensivecare room there is no guarantee that aIl of these areat the same potential and a voltage may exist between,for example, a grounded wall outlet and a nearbywater pipe. These subtle ground potentialdifferences are difficult to detect, but they may befatal if left undetected. It is not uncommon tofind two electrical power services available in olderhospitals, the second service having been providedto handle increased power demand since the hospitalwas constructed. This second service may come froma di~ferent part of the hospital and the potentialat its ground conductor may differ from the potentialof the ground conductor of the older service by a

remotefaultcurrents

263

volt or more. If subtle differences in groundpotential are detected, they can usually be almostcompletely eliminated by interconnecting each groundpoint with a heavy-gauge copper grounding bus. Ascurrents of several amperes may flow in thisinterconnecting grounding bus, the copper wire usedshould be at least a 1/4 inch in diameter. Suchsubtle ground differences can be detected with adifferential AC voltmeter or a differentialoscilloscope.

Referring to Fig. 17-7, it is apparent that twoground points from the same power service may showa potential gradient due to a fault current inequipment connected elsewhere in the power service.Although the fault current of 1 A shown in Fig. 17-7is insufficient to trip a circuit breaker in theactive line of the power service, it is sufficientto produce a voltage drop of 50 mV across 20 feetof standard service ground wire. Once again, this50 mV may be enough to cause a fatality. To avoidthis difficulty the service outlets should be groupedtogether to provide a single point ground.

OPERATING ROOM REMOTE ROOM

CATHETER 1 SUBJECT'SVEIN OR ARTERY

FAULTYINSTRUMENT~-- •••• 1 NSTRUMENT

lAGROUNDFAULT

POWER UTILITY THIRD WIREGROUNDING SYSTEM #14 GAUGECOPPER WIRE

20ft; 50mV20ft OF GROUND W 1RE ,..05n

lA FAULT CURRENTVOLTAGE DROP ; 50mv

lA GROUND FAULT CURRENT

THIS 50mv APPEARS BETWEEN THE SUBJECT'S CIRCULATORYSYSTEM (HEART) AND THE OPERATING TABLF CAUSING ACURRENT FLOW OF 5G~A FOR A lOOOn IMPEDANCE -- ENOUGHCURRENT FOR VENTRICULAR FIBRILLATION.

Fig. 17-7. Subject injury caused by a "remote" electrical fault.

264

responsetime ofprotectiondevices

A catastrophic remote electrical fault may producea fault current of perhaps 50 amperes which willcause the primary circuit breaker to trip. Thiscircuit breaker may, however, take several hundredmilliseconds to disconnect the service after thefault has occurred, thus the fault current may flowfor more than 100 milliseconds. As discussed inSection 17.4, if this fault current occurred duringthe "vulnerable period" of a subject's cardiac cycle,any potential produced between different groundpoints attached to the subject may be sufficient tocause death.

17.7 OPERATING ROOM ISOLATION

staticchargeprevention

faultcurrentmonitors

ln an effort to reduce the risk of flammableanesthetic ignition in an operating theater,conductive and grounded flooring is used, the subjectis connected to a grounded operating table by aconductive rubber mat and moist bedsheet and theoperating personnel wear conductive clothing. Theseprecautions prevent the build-up of staticelectricity and the subsequent spark caused by thedischarge of this static electricity.

Due to the presence of this extensive grounding inthe operating room, the electrical service tooperating rooms normally has both conductors isolatedfrom ground so if a fault develops between oneconductor and ground, no fault current can flow. Theintegrity of this isolation is monitored continuouslyby ground fault detectors installed between eitherside of the service and ground. These detectorsnormally signal any ground fault causing a currentto flow in excess of about l mA. Unfortunately,there is no more maligned item of electricalequipment in.the operating room than the groundfault monitor; if the monitor alarm sounds, theoperating room personnel often as not suspect themonitor rather than heed its warning. It should alsobe noted that the 1 mA level at which the groundfault monitor alarm sounds may be low enough toprotect the subject against currents flowing fromsurface contacts to his body but is not low enoughto protect the subject from shock from internalelectrodes or catheters.

265

17.8 ELECTROCAUTERY AND DEFIBRILLATION

electrocautery

The electrocautery unit is extensively used inoperating rooms and is used to provide a source ofhigh-frequency RF current for either cutting tissueor welding tissue together. The electrocauteryprobe provides a high frequency source of 2 MHz andup to 15 kV with respect to ground. When this probeis applied to a subject during surgery, currentflows between this probe and ground. Theelectrocautery unit is always used in conjunctionwith a large buttock plate underneath the subjectin such a way as to provide a large surface area,good electrical contact, ground to the subject. Anelectrocautery current of several amperes will thusflow between the electrocautery probe and the buttockplate, however the energy dissipated will causeappreciable heating and burning only in the regionof the probe as the current density flowing in theregion of the large surface area buttock plate willbe quite low.

ECGMJNITOR

ELECTROCAUTERYUNIT GROUNDI GBUTTOCK OR "BACK"PLATE

ELECTROCAUTERYCURRENT FLOWS VIAECG ELECTRODE

ELECTROCAUTERYUNIT

"'8kV2MHz

FAULT lN ELECTROCAUTERYUNIT GROUNDING CONFIGURATION

PART OF THE 8kV APPEARSACRO~S THE ELECTRODE -SUBJECT INTERFACE (100000)

POWER = RE2 (2000)2 = 400 WATTS = BURN10000

Fig. 17-8. Electrocautery burns at ECG electrode sites.

266

electrodeburns duringelectrocautery

defibrillation

If the electrocautery unit does not provide anexcellent ground to the subject, either due to poorapplication or due to a fault within the buttockplate grounding circuit, then the electrocauterycurrent will be diverted to other grounded points onthe subject. It is not uncommon for ECG electrodesto provide this alternative grounding point as aIlECG monitoring equipment essentially provides a lowimpedance path to this high-voltage, high-frequencycurrent. Unfortunately, however, the ECG electrodedoes not behave in the same manner as the buttockplate as the contact area involved is smaller by afactor of at least 100, perhaps 1,000. The currentdensity is thus increased by a corresponding factorand will almost certainly cause heating and burns atECG electrode sites. The high-frequency currentused in electrocautery is at too high a frequencyto cause ventricular fibrillation. ln practice, ifa surgeon suspects that his electrocautery unit "isnot cutting too weIl," he should be suspicious ofthe electrocautery unit ground circuitry and shouldhave the electrocautery unit adequately checkedbefore continuing to use it.

The defibrillator is another high voltage devicecommonly used in patient-care areas. Thedefibrillator provides a single short-duration pulseof up to 10 kV between two large electrodes. Theseelectrodes are normally applied to a subject's chestin cases where the heart is in ventricularfibrillation or has stopped completely. The highenergy defibrillation pulse is transmitted betweenthe electrodes and a great portion of this energyis dissipated in the heart. Defibrillation hopefullyeither re-starts the heart or reverts it fromventricular fibrillation to a normal beating action.Defibrillators should have both electrodes isolatedfrom ground.when the high voltage pulse is appliedand can normally be expected to provide safeoperation if other personnel are not in contact withthe subject during the defibrillation process.

Although a defibrillator's output is isolated, thisoutput will be unbalanced capacitively to ground.Thus, unless monitoring devices, such as an ECGinstrument, incorporate input circuit protection theymay be damaged by the defibrillator 'pulse.

267

cardioversion

If a subject's heart has not stopped beatingaltogether but is beating erratically, adefibrillator may be operated in a synchronized modeto revert the heart to a normal rhythm. This isreferred to as cardioversion. The defibrillatorsynchronizing pulse is obtained from the subject'sECG R-wave, the defibrillation pulse occurringsometime after the ECG R-wave but avoiding thevulnerable period during the upswing of the ECGT-wave.

defined

biophysicalmeasurementsonly

269

TRANSDUCERS - TRANSDUCER SYSTEMS

A transducer may be generally defined as a devicewhich converts one form of energy into another. Asfar as this text is concerned, a transducer isdefined as a device which converts any form of energyother than electrical energy into electrical energy.Transducers are used to meas'ure physical quantitiesand phenomena by producing an electrical outputproportional to this quantity or phenomena. Sometransducers require electrical excitation, othersproduce an electrical output without the aid ofexcitation. A complete transducer system consistsof the transducer as weIl as the necessary excitationvoltage source if excitation is required. Transducersystems may also incorporate some output signalamplification.

This chapter should not be regarded as a generalreference on transducers, it specifically discussestransducers from a biophysical measurementstandpoint and makes no attempt to review some of themore sophisticated transducer systems used for highaccuracy physical measurements. A companion volumeentitled Transducer Measurement Concepts provides amore general purpose reference source for transducersand transducer measurements.

270

18.1 RESISTrvE TRANSDUCER CONCEPTS

A resistive transducer is a device whose resistancechanges in proportion to sornephysical quantity.This resistance change is thén converted to anelectrical output signal by utilizing additionalcircuitry. Various types of resistive transducersare discussed later in this chapter. For the purposeof this text, resistive transducers may be divided

two into two broad classifications: Those where theclasses

type andappl ication

voltageexcitation

change in resistance expected with a changingphysical quantity will be greater than five percentof the transducer's nominal resistance and thosewhere the change in resistance will be less thanfive percent. This ratio of change in resistance to

~nominal resistance is normally shown as~. This

classification is necessary as the circuits used inconjunction with each of these transducer types arequite different.

Any one particular transducer may encorupassboth ofthe above broad classifications. For example, athermistor used to measure body temperature must beparticularly sensitive and is only expected to beused over a narrow temperature range. Thistemperature range would not normally change thethermistor's resistance by more than five percent.A thermistor used, however, to record outside airtemperature would be expected to operate over a broadtemperature range and the thermistor's resistance maybe expected.to change by 50 percent or more. Thus,the transducer type as weIl as the transducer'sapplication determines the classification.

Resistive transducers are normally used in one ofthe three circuit configurations shown in Fig. 18-1.The circuit shown in Fig. 18-lA essentially powersthe resistive transducer from a constant voltagesource and monitors the current through thetransducer. This circuit is generally the easiestto implement as the excitation source may simply bea low-voltage power supply or battery; however, thecircuit is nonlinear unless the value of the currentsensing resistor R2 is negligible in proportion tothe value of the transducer's resistance RI. The

271

output voltage eo is thus very sma1l with respect tothe excitation voltage E. Since this circuit issomewhat insensitive it can normally only be used inapplications where the change in resistance expectedwith a change in physical quantity will be greaterthan five percent of the transducer's nominal

!illresistance, i.e. ~ > 0.05.

+

E

e IS PROPORTIONAL TO Rl if R2 « Rla THU$ e «E. NOT RECOMMENDEDa

(A) CONSTANT VOLTAGE

l UR)

e IS PROPORTIONAL TO Ra RECOMMENDED IF CHANGE lN R 1$ GREAT

(i .e., THERMISTOR)

(B) CONSTANT CURRENT

R3

E R2 R4e e = E Rl + R2a a R3 + R4NOMINALLY: Rl = R2 AND R3 = R4R4

UNBALANCED WHEATSTONE BRIDGERECOMMENDED IF CHANGE lN RIS $MALL(i .e., STRAIN GAGE)

(C) CONSTANT VOLTAGE BRIDGE

NOTE: R = TRANSDUCERR = FIXED RESISTOR

Fig. 18-1. Resistive transducer circuits.

272

currentexcitation

voltageexcitedWheatstoneBridge

gagefactor

The constant current excitation circuit shown inFig. 18-IB should also be used in applicationswhere the change in resistance expected with achanging physical quantity is greater than fivepercent of the transducer's nominal resistance. Itdoes not, however, suffer the nonlinearity due tothe presence of a current sampling resistor as doesthe previous circuit. This circuit requires a highinput impedance-recording circuit but this is notnormally an important factor with modern electronics.It derives its excitation from a constant currentsource which may not be as readily available as aconstant voltage source.

The Wheatstone Bridge powered from a constant voltagesource shown in Fig. l8-le is recommended inapplications where the change in resistance expectedwith a changing physical quantity will be less thanfive percent of the transducer's nominal resistance,

i.e. ~ < 0.05. This circuit has the advantage over

the previous two circuits discussed in that theoutput voltage is zero when the transducer'sresistance is at its nominal value. The bridge maybe, however, somewhat nonlinear as discussed in thefollowing section, thus it should be limited toapplications where ~ is small compared to R.

The feature of providing zero output when thetransducer's resistance is at its nominal value isperhaps the Wheatstone Bridge's most desirablecharacteristic. "Standing" output in the circuitsshown in Fig. 18-IA and l8-IB must often be "buckedout" by an offset potential within the amplifier orrecording device for the circuits to be useful.

The gage factor of a resistive transducer is theratio of the relative change in resistance of thetransducer to the relative change in the physicalquantity being measured. ln the particular case ofa resistive displacement transducer which, in itssimplest forro,may consist of a length of resistancewire, displacement can be coupled to the wire tochange its length and the gage factor G is computedfrom the following formula:

~-G R= =~l

i

~R lR ~l

determiningstrain andgage factor

273

~Z and Z refers to the change in length of theresistance wire and the original length of this wire

respectively. The term ~~ is referred ta as strain,

symbol E.

As an example of the above, consider a 1 meter lengthof resistance wire having a resistance of 100 ohms.If a displacement is mechanically coupled ta thiswire, the length of the wire may be stretched byone cm to 101 cm. The strain would then be equal

1to 100 or 0.01. ln physical measurement, this

strain would be considered large and would bereferred to as 10,000 ~strain or 10,000 ~E. If instretching the wire, its resistance changes from

100 ohms to, say 102 ohms, then the ratio ~ is equal

to 0.02 and the gage factor G is equal ta 2, beingthe ratio of the resistance change, 0.02, to thestrain, 0.01.

18. 2 THE UNBALANCEDWHEATSlDNEBRIDGE

balanced -unbalancedbridge

The Wheatstone Bridge circuit was discussed brieflyin the previous section, however further discussionof the characteristics of this circuit is desirabledue to its extensive use in transducer measurementsystems. The Wheatstone Bridge can be used ineither of two distinct modes: as a baZanced bridgewhere an unknown resistance is measured by adjustingthe value of one of the other resistors in the bridgefor zero output, or as an unbaZanced bridge wherean unknown resistance is measured by measuring theoutput voltage produced by bridge imbalance. Thebalanced bridge is only used in static measurementsituations, however the unbalanced bridge is used inboth static and dynamic measurement situations. Mosttransducer applications use the Wheatstone Bridge inits unbalanced condition.

The following paragraph discusses the output voltageobtained from an unbalanced Wheatstone Bridge whenone of the resistors comprising the bridge changesits value by~. If only one resistor changes invalue, the bridge is said to contain only one activeelement. This change in value would normally becaused by a change in a physical quantity acting ona transducer.

274

calculatingeo

+.2 ONLY ONE RESISTOR lN THEBRIDGE CHANGES VALUE

+.1

-.1

o

IDEAL RELATIONSHIP ..","UNBALANCEDeo = O.25E~ t,....................... BRIDGE ERROR

•..•.....

,......ACTUAL RELATIONSHIP

eo = O.25~ [1 - ~ + (~r_...]eoE

••"L1NEAR" RELAT 10NSH1P

ACCURACY ±5%

-.2+---~--~--~--~--r---~--~--~--~~.5 .6 .7 .8 .91.01.11.21.31.41.5

R + AR RELATIVE CHANGE lN-R- = ANY ONE RES 1STOR

-.5 -.4 -.3 -.2 -.1 0 +.1 .2 .3 .4 .5AR = RATIO OF CHANGE lN ANY ONE RESISTORR TO ITS NOMINAL VALUE

Fig. 18-2. Output voltage from unbalanced Wheatstone Bridge.

The Wheatstone Bridge circuit is shown in Fig. 18-1together with a formula for determining the outputvoltage from the circuit. If Rl and R2 are equal,and R3 and R4 are equal, eo will be zero. If theresistance of one of the resistors, say R4, ischanged in value by ~, the output voltage may becomputed from the equation given in Fig. 18-1. Thealgebraic solution of this equation reverts to acomplex binomial function; however in most instances,second-orde~ terms can be neglected and the equationfor the output voltage can be simplified to:

~eo = 0.25 E~. The percentage error is approximately

±5% for ±10% changes in R, i.e., the unbalancedWheatstone Bridge with one active element is accurate

~to ±5% for values ofR from -0.1 to +0.1. Fig. 18-2

shows both the actual output voltage obtained froman unbalanced Wheatstone Bridge with one activeelement and the "ideal" output voltage computed fromthe above equation neglecting second-order binomialterms. The graph presented in Fig. 18-2 confirmsthe accuracy statement above and also shows that for

~large ratios of ~ the unbalanced bridge with one

more thanone activeelement

275

active element is substantially in error if thesecond-order binomial terms are neglected. It shouldbe nrited that, if the changing resistive element isa thermistor, the curvature of its characteristiclargely corrects the nonlinearity of the bridge overthe temperature range of clinical interest.

Many transducer systems are arranged so that achanging physical quantity causes a resistancechange in two of the resistors in the unbalancedWheatstone Bridge circuit or possibly aIl four ofthe resistors in the unbalanced bridge circuit.These bridges are then referred to as having twoactive elements and four active elements,respectively. Re-analysis of the unbalancedWheatstone Bridge equations will show that, if thephysical quantity to be measured has an oppositeeffect on both active elements in a bridge containingtwo active elements, then the output is twice theoutput of a bridge containing only one activeelement. Similarly, the output for a bridgecontaining four active elements is four times theoutput of a bridge containing only one activeelement. Re-analysis for two or four active elementswill also show that the solution of the equationgiven in Fig. 18-1 is simple and no second orderbinomial terrnsare derived; thus no errors areproduced by neglecting such terms. The outputvoltage formulas for the unbalanced Wheatstone Bridgecircuit are summarized below:

If the bridge contains one active element:

0.25 Iill to ±5% fore = E Raccurate0

-0.1Iill 0.1< -- <R

If the bridge contains two equally active elements:

e = 0.5 Elill exactly.0 R

If the bridge contains four equally active elements:

e = Elill exactly.0 R

276

III TRANSDUCED QUANTITY Q1,11 (DISPLACEMENT,PRESSURE etc.)

eo

l '

~( \, 11~1 ~, 1

f, /l '" ,1 ~ t1l -, ,5, ,

L...__ ---"_ -L-.'::- =-" __ r - - -------I~

OUTPUTVOLTAGE

EXCITATIONVOLTAGE

'TRANSDUCER SENSITIVITY FACTOR F

EXAMPLE: PRESSURE TRANSDUCER WITH SENSITIVITY FACTORSUPPLIED WITH TRANSDUCER OF 118~V/V/cmHq PRESSURE.

SAY, CONVENIENT EXCITATION VOLTAGE OF 6V DC

e FOR ONE cmHg PRESSURE = FEQ = 118 x 10-6 x 6 x 1 = .708rnVoTHUS, WITH 6.0V OF EXCITATION, SYSTEM GIVES

.708rnVOF eo/cmHg PRESSURE.

ALTERNATIVELY: ADJUST EXCITATION VOLTAGE FOR A MORE CONVENIENTCALIBRATION FACTOR.

SAY WE WISH lrnVOF eo/cmHg PRESSURE;

THEN, e = FEQo

1 x 10-3 = 118 x 10-6 x Ex 1

E = 8.49V

THUS, WITH 8.49V OF EXCITATION, THE SYSTEM GIVES

1.0rnVOF e /crnHg PRESSURE.oTHE EXCITATION VOLTAGE MUST BE WITHIN THE TRANSDUCER'S RATING.FOR MOST TRANSDUCERS THE EXCITATION RANGE IS 5V TO 8V.

Fig. 18-3. Transducer system calibration using sensitivity factor Fsupplied with transducer.

18.3 PRACTlCAL TRANSDUCER SYSTEMS USING lliE UNBALANCED WHEATS'IDNEBRIŒE

sensitivityfactor F

calculatingF

Practical transducers are self-contained units thatproduce an electrical output proportional to theexcitation voltage and the physica1 quantity to be -measured. Although the transducer may internallyconsist of a Wheatstone Bridge having one or moreactive e1ements, it is unnecessary to know thecharacteristics of the elements comprising thisbridge if a calibration factor or transducersensitivity factor P is supplied with the transducerat the time of purchase. The transducer sensitivityfactor F is norma11y given as volts output per voltof excitation per unit of physica1 quantity to bemeasured. The actual output voltage is determinedas shown in Fig. 18-3.

If the transducer sensitivity factor for a particu1artransducer is unknown, it may be determined bysubjecting the transducer to a known amount of thephysica1 quantity that the transducer is designed tomeasure. If the transducer is then excited from a

cal ibrationresistor

277

known source, and the output voltage measuredaccurately with a calibrated oscilloscope orvoltmeter, the sensitivity factor may be calculatedby rearranging the equation shown in Fig. 18-3.

With the above system, an accurate measurement ofthe physical quantity under investigation dependson the accuracy of the transducer sensitivity factor,the accuracy of the excitation voltage, and theaccuracy of the output voltage indication device.Many transducer systems incorporate a calibrationresistor as shown in Fig. 18-4. This calibrationresistor is chosen to have the same effect on thetransducer as would a certain amount of the physicalquantity to be measured. It is thus only necessaryto connect this calibration resistor as shown inFig. 18-4 and to adjust the output voltage indicationdevice for a known deflection. This resistor fully"calibrates" the transducer system and it isunnecessary to know the exact value of the excitationvoltage or the exact sensitivity of the outputvoltage indication device. Many transducers aresupplied by manufacturers at the time of purchasecomplete with a calibration resistor. If thetransducer sensitivity factor F is known, anappropriate calibration resistor value can be selectedas shown in Fig. 18-4.

CAllBRATION RESISTOR<TEMPORARY)

ReRe = (OQ;5 - 0.5) R

OR

Q 0.25= F (:e + 0.5)

WHEREQ = SIMULATED PHYSICAL

QUANTITY

F = TRANSDUCER SENSITIVITY FACTOR,V t/V. /UNIT Qou ln

EXCITATIONVOLTAGE

OUTPUTVOLTAGE eo

---_...

\ TRANSDUCERBRIDGE RESISTANCE OF RSENSITIVITY FACTOR OF F

BRIDGE RESISTANCE R IS THE NOMINAL RESISTANCE OF EACH OF THE FOUR ARMS OF THE BRIDGE.IT MAY BE OBTAINED FROM THE MANUFACTURER'S DATA SHEET FOR THE TRANSDUCER OR MEASUREDWITH AN ACCURATE RESISTANCE BRIDGE BY SIMPLY MEASURING THE RESISTANCE BETWEEN THEEXCITATION TERMINAlS WITH THE OUTPUT TERMINALS OPEN CIRCUITED OR VICE VERSA.

EXAMPLE: PRESSURE TRANSDUCER SENSITIVITY FACTOR SUPPLIED WITHTRANSDUCER OF 118~V/V/emHg PRESSURE -- BRIDGE RESISTANCE 350nWE WISH TO SIMULATE A PRESSURE OF 10emHg:

Re = (0(.>;5- 0.5)R = (10 x ~i~\10-6 - 0.5)350 = 74.2kfl

Fig. 18-4. Simulation of transduced quantity with a calibration resistor.

278

18.4 AC- AND DC-BRIDGE SYSTFMS

DC-bridgesystems

simple ACbr idgesystems

The previous discussion applies to transducersystems having either DC or AC excitation sources.ln practice, both DC-bridge and AC-bridge systemsare used. The DC-bridge systems shown in Fig. 18-5simply consist of a battery or DC power supply asthe excitation source and incorporate a DC-coupledamplifier in the output voltage indication system.Since the excitation is DC, this system cannot beused with reactive transducers; that is, transducerswhere the active elements consist of inductors orcapacitors rather than resistors. Until recentlythiE DC-bridge system was not preferred for highaccuracy or high-sensitivity measurements due to therelative instability of DC-coupled amplifiers.Stable DC-coupled amplifiers, such as the TektronixType 3A9 DifferentiaI Amplifier, are now availableand thus DC-excited bridge systems are regainingusage.

Perhaps the most economical bridge system is thesimple AC-bridge system shown in Fig. 18-5. Withthis system, the bridge is excited by a sinewaveoscillator, normally within the audio-frequency range,and the output is amplified via either a wide-bandAC-coupled amplifier or, preferably, a narrow-bandAC-coupled amplifier. Since the amplifier is ACcoupled, DC stability is unimportant. The amplifiedbridge output signal is detected with a simpledetector, however, as this detector is not phasesensitive, this simple AC-bridge system will provideessentially "positive" output for both "positive"and "negative" changes away from the balancecondition in the physical quantity to be measured.Often, however, the nature of this physical quantityovercomes this apparent ambiguity.

279

Rl R3

- r--'- DCAMPLI FIER

INDICATOR-

R2 R41

OSCILLATOR

(A) DC SYSTEM

DETECTORINDICATOR

NARROW BANDAC

AMPLIFIER

(B) SIMPLE AC SYSTEM

NARROW BAND,I----1AC

AMPLIFIER1 ND 1 CATOR

PHASESENSITIVEDETECTOR

(Cl PHASE SENSITIVE AC SYSTEM

Fig. 18-5. DC and AC bridge systems.

280

carrieramp 1 if iersystems

3C66

The detector used in the simple AC system discussedin the previous paragraph may be replaced with aphase-sensitive detector that solves the apparentambiguity by producing both positive and negativeoutputs. Such a system is referred to as a phasesensitive AC system or a carrier amplifier system asshown in Fig. 18-5. The Tèktronix ·Type 3C66 plug-inunit for the Type 56lB Oscilloscope or 564B StorageOscilloscope employs this phase-sensitive AC-bridgesystem.

The Tektronix Type 3C66 Carrier Amplifier wasprimarily intended for use with strain gages and isthus calibrated in microstrain or ~E. Thisinstrument does, however, incorporate switchingcircuitry to allow a built-in l50-kD shuntcalibration resistor to be used. Thus the ~ STRAIN/DIV switch may be regarded as a variable gain controlon the transducer's output voltage detectioncircuitry and a calibration resistor may be used tocalibrate the system as discussed in Section 18.3.The Tektronix Type 3C66 Carrier Amplifier system isshown in Fig. 18-6. The built-in l50-kQ shuntcalibration resistor in the Type 3C66 may be replacedwith any desired value of calibration impedance. Thecalibration impedance must be of the same form as theimpedance that is changing in the transducer, thusresistive transducers require calibration resLst.ors,inductive transducers require calibration inductorsand capacitive transducers require calibrationcapacitors. .

281

HTYPE 3C66 CARRIER AMPLIFIER

C

5VRMS25kHzEXCITATION

CALIBRATION RESISTORMAY BE SWITCHEDINTO CIRCUIT

D

TOOSCILLOSCOPE

----"l '

SI \, 1,\l '"', 1

{, /l '- 1l '"':., t1" ,~l ,1 1

'----.,.- - - - -\j Î - - -+-------<J~----.L _

~ LETTERS REFER TO 3C66 ,NPUTSOCKET PIN DESIGNATION.

TRANSDUCERSENSITIVITY FACTOR OF FBRIDGE RESISTANCE OF R

NARROW BANDAC AMPLIFIER

&PHASE SENSITIVE

DETECTOR

SELECT A CALIBRATION RESISTOR AS DESCRIBED lN FIG. 18-4. PLACE THIS CALIBRATIONRESISTOR INSIDE THE TYPE 3C66 ON THE TERMINALS AS DESCRIBED lN THE TYPE 3C66MANUAL. ADJUST TYPE 3C66 ~STRAIN/DIV SWITCH AND GAIN CONTROL FOR A CONVENIENTDEFLECTION. --_

EXAMPLE: PRESSURE TRANSDUCER - SENSITIVITY FACTOR SUPPLIED WITHTRANSDUCER OF 118~V/V/cmHg PRESSURE - BRIDGE RESISTANCE OF 350n-

- SELECT A CALIBRATION RESISTOR TO SIMULATE A PRESSURE OF, SAY,10cmHg. FROM FIG. 18-4, R = 74.2kn.c

- INSTALL THIS 74.2kn RESISTOR INTO THE 3C66.

- ADJUST TYPE 3C66 CONTROLS FOR CONVENIENT DEFLECTION, SAY,2 DIVISIONS, WITH ~STRAIN/DIV AT 500.

NOTE CALIBRATIO: 10cmHg 2 DIVISIONS AT 500~STRAIN/DIV:.10cmHg = 1OOO~STRA 1N

~ SENSITIVITY = 100~STRAIN/cmHg PRESSURE

Fig. 18-6. Tektronix Type 3C66 Carrier Amplifier.

282

13rnAQC FRor~564B CAL 1BRATOR 564B STORAGE OSCILLOSCOPE

CALIBRATION ICAL1BRATORI CALRESISTOR AND 40VPUSH BUTTON _l_ DC~ 0-- ~

VEPT HORIZ

-0+(- - - -~", r \i( ':. 1\ ••.• 1 3A9 2867

11 •...•, 1 DIFFERENTIAL TI""E BASE

(, '} A"1PLIFIER1

\ l '- t!1•.... )•... 'i1 " ,,

_! - - -\,11

_(")-r- -L ____

-=- "-TRA~SDUCEO

Fig. 18-7. Transducer system using conventional oscilloscopeand plug-in units.

TYPE 410 MONITORCABLE

D WITH

0 IAUX 1 Po MONITOR"'1 P

150mm/~ D-----,...

l '( '- 1

/ ~Il '"',1{, "}

l '" 1•...• t{l '7,. ,Il '1

'----..,.- - -- -....r- - ..•..--'L _

TRANSDUCER

TRANSDUCER CONNECTED TO 410 PATIENT CABLE USING7 PIN PLUG. TEKTRONIX PIN 134-0090-00.

POWER TO TRANSDUCER = 13.5V FROM 1200 SOURCE IMPEDANCE.PROVIDES = 10V FOR 3500 TRANSDUCER.

Fig. 18-8. AC coupled transducer system with Type 410 Monitor.

283

5648system

A DC-transducer system may be fabricated using aTektronix Type 564B Storage Oscilloscope with a3A9 Vertical Amplifier and 2B67 Time Base unit asshown in Fig. 18-7. This system excites thetransducer from an approximate 13 mA current source,however since the transducer's impedance isessentially constant, the voltage between thetransducer's excitation terminaIs will be constantand the system will operate as if it were excitedfrom a constant voltage source. This system issuitable for use with transducers having a nominalbridge resistance of between 100 ohms and 2,000 ohms.

410system

The transducer system shown in Fig. 18-8 may be usedin sornelimited applications where the physicalquantity to be measured is essentially an AC signal.This system is thus suitable for recording thedifference between diastolie and systolic pressuresusing a pressure transducer, however sinee theType 410 is insensitive to DC signaIs, it will notshow the absolute values of these pressures.

18.5 DISPLACEMENT TRANSDUCERS

A displaeement transducer converts linear orrotationa1 displacement into an electrieal output.Various forms of resistive displacement transducersare shown in Fig. 18-9. Each of these transducerschanges resistance due to displaeement; thisresistance change is then converted to an electrica1output by using any one of the circuits shown inFig. 18-1.

potentJometer

Perhaps the simplest resistive disp1aeementtransducer is the eommon potentiometer. Theresistance of the potentiometer changes withrotational disp1acement which may be obtained from alinear displacement via either a lever system or apulley. This type of resistive displacementtransducer is commonly used in spirometers asdiseussed in Section 9.4.

284

f 1u id-f i liedrubber tube

bondedstraingage

unbondedstraingage

semiconductorstra ingage

displacementinertia

The fluid-filled rubber tube shown in Fig. 18-9changes its cross-sectional area and length due toa linear displacement or force acting on the rubbertube. This changing cross-sectional area and lengthcauses a change in the resis~ance of a conductivefluid within the tube as measured between each endof the tube. Mercury has historically been used asthe conductive fluid in this type of transducer,however the resistance of mercury is extremely lowand special techniques are required to detect anychange in this resistance. Fluids with much higherresistivities are preferred and are indeed used inmany commercial fluid-filled rubber-tube transducers.

The three strain-gage resistive-displacementtransducers shown in Fig. 18-9 represent the basisfor most modern transducers. The bonded straingage consists of a length of fine resistance wirebonded to a backing of paper or epoxy. As lineardisplacement is coupled to this backing material, itstretches and the resistance of the wire bonded tothis material therefore increases. The unbondedstrain gage is somewhat similar to the bonded straingage with the exception that the resistance wire isself supporting as shown in Fig. 18-9. The bondedsemiconductor strain gage is also somewhat similarto the bonded strain gage with the exception thatthe resistance wire element is replaced with afilament of semiconductor mater{al, usually silicon.The principal advantage of the bonded semiconductorstrain gage is its high gage factor, in the order of120 compared to 2 or perhaps 3 for the bonded orunbonded resistance-wire strain gages.

With any of the displacement transducers shown inFig. 18-9, the mechanical source providing thedisplacement must also provide sorneforce to overcomeinertia in the transducer. The fluid-filled tube mayrequire considerable force in conjunction with thelinear displacement to be able to stretch the rubbertube. The unbonded strain gage requires sorneforceto stretch the resistance wires comprising the gage.Such devices should more correctly be classed asforce transducers that may be used for displacementmeasurements if the force driving the system islarge compared with that required to drive thetransducer.

POTENTIOMETER:

ROTATIONALDISPLACEMENT

(rr1 ~LINEAR ORROTATIONAL

DISPLACEMENT

\J_-------~Ir--- ...•. LINEAR DISPLACEMENTRUBBER TUBE OR FORCEFILLED WITH

CONOUCTIVE FLUID

BONDED STRAIN GAGE: FINE RESISTANCEWIRE

LINEARDISPLACEME T

FINE RESISTANCEWIRE

UNBQNDED STRAIN GAGE:(FOUR ACTIVE ELEMENTS)

LINEARDISPLACEMENT

BONDED SEMICONDUCTOR STRAIN GAGE: LINEARDISPLACEMENT

FILAMENT OF SEMICONDUCTOR(SILICON MATERIAL)

Fig. 18-9. Resistive displacement transducers.

285

286

PIEZOELECTRIC:

(

LINEAR DISPLACEMENT ) t FORCEOF A BEAM MAY PRQDUCE

THIS FORCE PIEZOELECTRICCRYSTAL

OIFFERENTIAL TRANSFORMER:

CRYSTALSUPPORT

ELECTRICAL OUTPUT

LINEAROISPLACEMENT

OIFFERENTIAL CAPACITOR:

LIGHT TRANSMISSION:

~SEC 1) 1

O~TPUT

COMMON(SEC 2)

(PRI)

PRIMARYCOlL

TWQSECO'mARY

'-------------- COILS

--------;- FIXEOPLATES

":'!I---_ L1NEAR.............._~ 01SPLACEMENT

(MQVES CENTER PLATE)

LIGHTSOURCE

APERTURE

LINEAR01SPLACEMENT

Fig. 18-10. Nonresistive displacement transducers.

plezoelectric

differentialtransformer

287

Various forms of nonresistive displacementtransducers are shown in Fig. 18-10. Thepiezoelectric force transducer produces an electriccharge between the faces of a solid crystal when thiscrystal is subjected to bending which may be causedby displacement of a beam, etc. A principaladvantage of the piezoelectric crystal transducer isthat no excitation source is required, howeverdisplacement does produce a charge rather than avoltage and this charge is normally quicklydissipated by the input impedance of the amplifierused in conjunction with this transducer. It is thusprimarily suited to the measurement of AC quantities,however the transducer can be used in conjunctionwith a charge amplifier to provide good low-frequencyresponse characteristics. A piezoelectricdisplacement transducer is used in the Korotkoffsound microphone shown in Chapter 8, Fig. 8-10.

A differentiaI-transformer displacement transduceris shown in Fig. 18-10. The differential transformerconsists of three inductors coupled magnetically bya common armature. One of these three inductors isexcited from an AC source (primary winding) andthis excited winding induces voltages into the othertwo windings (secondary windings), which areconnected in opposition. The induced voltage isdependent on the amount of coupling and thus on therelative position of the armature. With the armaturecoupling energy equally from the primary winding toeach of the secondary windings, voltages of equalamplitude appear across each winding. When the .coreis moved off center, however, each secondary is notlinked identically. One secondary produces a largervoltage than the other and thus the potentialsproduced in the secoRdaries do not cancel and anoutput voltage is obtained. The two secondarywindings of a differential transformer may beconsidered as two elements of an unbalancedWheatstone Bridge with the other two elementsconsisting of fixed resistors. The differentialtransformer is particularly suited for use witheither of the AC-bridge systems shown in Fig. 18-5.They are also available in De-to-DC versions withaIl the necessary electronics built into the actualtransducer.

288

ditferential The differential-capacitor displacement transducercapacitor shown in Fig. 18-10 essentially consists of two

capacitors arranged so that a common plate for bothof these capacitors is coupled to a lineardisplacement. Any movement of this cornmonplateincreases the capacitance in one capacitor whi1esimultaneously decreasing the capacitance in theother. These two capacitors may be used to forrotwo elements in an AC Wheatstone Bridge system withthe other two elements consisting of fixed resistors.

The light-sensitive displacement transducer shown inFig. 18-10 employs a variable-width slot coupled tothe disp1acement. Light is passed through this slot,the width of the slot determining the amount oflight that can pasSe The varying 1ight intensity isnormally detected with a 1ight-sensitive transduceras discussed in Section 18.7.

18.6 FORCE, PRESSURE AND ACCELERATION TRANSDUCERS

Force, pressure and acceleration transducers arerelated to displacement transducers as shown inFig. 18-11. A force transducer converts force tolinear displacement using a spring or sorneother

force resi1ient material. Force on the transducer producesstress in the sensing element. This stress causesstrain and the strain is then detected with a straingage disp1acement transducer.

The pressure transducer shown in Fig. 18-11 ispressure basica1ly a force transducer with fluid pressure

being converted to force via a diaphragme Sincethe force produced is proportional to the fluidpressure, the transducer's resistance change isproportional to the fluid pressure.

The acceleration transducer shown in Fig. 18-11simply consists of a force transducer coup1ed to a

acceleration known masse This mass converts an acce1eration toa force. Although the acceleration transducer shownin Fig. 18-11 is sensitive to acce1eration in onedirection only, modifications to this principalallow acceleration transducers sensitive toacce1eration in any direction to be constructed.The weight transducer shown in Fig. 18-11 i8essentially a force transducer with the force beingproportional to the weight of a body app1ied to thetransducer and to the gravity constant g.

velocitypickups

289

DISPLACEMENT: VARIABLE RESISTANCE ELEMENT

L L==~=]~-i~DISPLACEMENT~ ~ CHANGES RESISTANCE

FORCE: PRESSURE:

FORCE

SPRING

ACCELERATION: DIAPHRAGM

ACCELERATION = FORCEMASS

WEIGHT OF MASS mCOUPLED TO ACCELERATION

WEIGHT: 1 MASS m EXERTS A FORCET ON THE TRANSDUCER OF mg

(g = ACCELERATION DUE TO GRAVITY)

Fig. 18-11. Transducer evolu tion.

Velocity pickups, consisting simply of a coil locatedin a magnetic field, are commonly used for vibrationpickups and in dynamic microphones. Displacementand acceleration may be obtained from the output ofa velocity transducer by integrating ordifferentiating this output voltage. Conversely,displacement can be measured with an accelerationtransducer by double integration of the outputvoltage and acceleration can be measured with adisplacement transducer by double differentiation.

290

pressuretransducerpredominant

vo 1umedisplacement

The mathematical relations involved are shown here.

Differentiation processing:

dxV =-dt x = Displacement

dv d2x V = Velocitya =- =--dt dt2 a = Acceleration

t = TimeIntegration processing:

C = Constant

V = CI f adt

x = C2 f vdt = ClC2 f adt

For sinusoidal motion, the above may be simplifiedto:

Vpeak = 2Trfx kpea

a = (2Trf) 2xpeak peak

f = Frequency ofsinusoidal motion

Calibration of systems using this indirect methodcan be difficult, thus these methods are primarilyused when uncalibrated or relative measurements aresatisfactory. Double differentiation can oftenresult in excessive noise and thus should be avoided.

Although force, pressure, acceleration and weighttransducers are aIl used in biophysical measurements,the pressure transducer is of particular interestdue to its extensive use for monitoring the activityof the circulatory system and respiratory system asdiscussed in Chapters 8 and 9. An importantcharacteristic of a pressure transducer used forbiophysical measurements is its volume displacement,that is, the amount of fluid that must flow into thepressure transducer to produce movement of thediaphragm within the transducer. Volume displacementin pressure transducers is normally measured in cubicmillimeters per 100 mm Hg pressure change. Volumedisplacements range from below .01 cubic mm per 100 mmHg pressure to above 20 cubic mm/lOO mm Hg pressure.Transducers with high volume displacements normallyhave poor high-frequency response characteristicsdue to the finite time required to move the largevolume of fluid involved. Transducers for bloodpressure measurement should have a volumedisplacement of less than .04 cubic mm/lOO mm Hgpressure.

291

18.7 TRANSDUCERS FOR NONMECHANlCAL QUANTITIES

The range of physical quantities that can bemeasured with the aid of a transducer is almostlimitless, as is the range of transducers that maybe useful in the biophysical measurement sciences.Transducers are used extensively in the biophysicalsciences to measure the following nonmechanicalquantities: temperature, light, radiation, liquidchemical composition and gaseous chemicalcomposition. These transducer types are discussedbriefly below, however more detailed information isavailable from the principal transducermanufacturers; perhaps sorneof the more prominentare Statham Instruments, Inc., Oxnard, California;Beckman Instruments, Inc., Palo Alto, California;E & M Instrument Company, Inc., Houston, Texas andmany others. An excellent reference for theapplication of transducers to biomedicalinstrumentation is Chapters 1 through 9 of Principlesof Applied Biomedical Instrumentation by L. A. Geddesand L. E. Baker, published by John Wiley and Sons,Inc., New York, N.Y.

Temperature transducers can be divided into twocategories; temperature dependent resistors and

thermistor thermoelectric generators. The thermistor is atemperature dependent resistor whose resistancedecreases as temperature is increased. A typicalthermistor may be expected to produce a one percentchange in resistance for a temperature change of oneor two degrees within normal environmentaltemperature ranges. For the resistance change tobe detected, sorneform of resistance detectioncircuitry is necessary. It is important that thepower level involved in this circuitry does notcause self-heating within the thermistor.

thermocouple A thermocouple consisting of a junction between twodissimilar metals is a thermoelectric generator,however, this form of temperature transducer israrely used for biophysical measurements due to itsrelative insensitivity compared to thermistors.

292

1ightsensitiveres istor

radiationdetection

chemicaldetection

Optical transducers sensitive to light are used inthe mechanical displacement transducer shown inFig. 18-10 and in the plethysmographs shown in Chapter8. Although many different forms of opticaltransducers are available, the most common type usedin biophysical measurements is the light-sensitiveresistor. Light-sensitive resistors, or photoresistors, consist of a thin film of resistivematerial deposited on the inside of an evacuatedglass or plastic chamber. The most commonly usedmaterial is cadmium sulphide, however, cadmiumselenide and semiconductor materials are used incertain applications. The response of these photoresistors is reasonably linear; the resistancedecreases proportionally with an increase in thelevel of illumination. A typical "dark" resistancewould be above 10 megohrns, this resistance woulddecrease to a few kilohms with normal roomillumination of about 100 foot-candles.

Radiation transducers are used in biophysicalrneasurernentsto check for the possibility of aradiation hazard. Radiation transducers may detectone or more of the three principal forms ofradiation: alpha particles, beta particles and gammarays. Gamma radiation is usually considered thernostdangerous because of its higher energy content.Radiation transducers either detect radiationdirectly or detect light emitted by certain crystalswhen exposed to radiation. Perhaps the most cornrnonform of radiation detector is the Geiger-Mullercounter which, when subject to radiation producesionization in a sealed gas-filled tube which allowscurrent to flow between electrodes placed in thetube.

Liquid chemical transducers are used in biophysicalmeasurements to provide a continuous chemicalanalysis of body matter, particularly blood. Liquidchemical transducers are used to measure the pH orrelative acidity of body fluids and are also usedfor routine blood analysis to determine the amountof dissolved oxygen or carbon dioxide in the blood.ln addition, liquid chemical transducers areavailable to determine the relative amounts ofpotassium and sodium ions in body fluide Gaseouschemical transducers are used to analyze the gaseouscontent of expired air, particularly the amounts ofoxygen and carbon dioxide, and also are used asoxygen detectors in atmospheric warning systems.

293

AMPLIFIERS

Physiological signaIs acquired by either electrodesor transducers are typically below 10 mV in amplitudeand must therefore be amplified to be compatiblewith display devices and recorders. This chapterdiscusses the characteristics of amplificationsystems as seen by electrodes. The discussion willbe limited to the process of amplification, additionalsignal processing is discussed in Chapter 20. lnpractice, most physiological amplifiers include bothamplification and signal processing within the onepackage. For the purpose of this discussion,therefore, an amplifier is defined as a device havinga high input impedance and a low output impedanceand providing either a fixed or perhaps a variablevoltage gain. The amplifier may or may not containDC-offset capabilities to offset the DC level of theoutput with respect to the input DC level.

294

••••••

LINE FREQUENCYINTERFERENCEELECTROSTATICALLYCOUPLED ORMAGNETICALLYINDUCED TO SUBJECT

Z AND ZI = ELECTRODE CONTACT IMPEDANCEE = DESIRED PHYSIOLOGICAL SIGNAL

El = INTERFERENCE SIGNAL

EQUIVALENT TO

••Z

ZI

.Fig. 19-1. Typical physiological measurement configuration.

295

19.1 THE DIFFERENTIAL AMPLIFIER USED WITH ELECTRODES

desiredandinterferencesignal

electrodecontactimpedances

differentialand commonmode signal

A typical physiological measurement configuration isshown in Fig. 19-1. ln almost all physiologicalmeasurement situations, two signaIs are produced bythe subject: a desired physiological signal suchas the ECG or EEG and an interference signal,typically 60 Hz, due to electrostatically coupledor magnetically induced interference. The purposeof the differential amplifier is to reject thisinterference signal and to amplify the desiredsignal. Two impedances, Z and Zl, representingelectrode contact impedance are shown in Fig. 19-1.The source of these impedances is discussed inChapter 16 when referring to the characteristics ofvarious electrode systems. Z and Zl will normallylie in the range from 1,000 ohms to 50,000 ohms andit is possible for the difference between Z and Zlin any one system to be as great as 10,000 ohms.Although aIl modern differential amplifiers haveinput impedances above 1 megohm, these finiteimpedances Z and Zl cannot be ignored as will beseen later in this chapter. The desiredphysiological signal shown in Fig. 19-1 is connectedso that it appears between the two input terminaIsof the differential amplifier and is referred to asthe differential signal. The interference signalshown appears between both inputs of the differentialamplifier and ground and is referred to as the commonmode signal.

19.2 COMMON MODE REJECTION

As stated in the previous paragraph, the purpose ofa differential amplifier is to reject common mode

CMRR signaIs. The "common mode rejection ratio" or CMRRof an amplifier is its relative ability to rejectcommon mode signaIs. The common mode signal isdefined as the signal that is applied to both inputsin the same phase, that is:

commonmodesignal

(si~nal tO)+ (Si~nal tO)= + lnput - lnput

2

296

CMRR versussourceimpedance

Common mode rejection ratio is defined as the ratiobetween the amplitude of the common mode signal tothe amplitude of an equivalent differential signalthat would produce the same output from theamplifier. As an example, consider a differentialamplifier with a single l-V source connected fromground to both inputs 01 the amplifier, that is, incommon mode. If the output signal is equivalent tothat produced by a 10 ~V differential signal, thecommon mode rejection ratio would then be the ratioof this apparent differential signàl to the truecommon mode signal; that is, IV:IO~V or 100,000:1.Common mode rejection is often specified in decibels;a CMRR of 100,000:1 being referred to as a commonmode rejection of -100 dB. The relationship is

Common Mode Rejection in dB = 20 10glO CMRR

The common mode rejection ratio calculated previouslywas obtained by applying the same one volt commonmode signal to both inputs of the differentialamplifier. Let us now consider the case where animpedance of 150 ohms is inserted between the onevolt source and the positive input of thedifferential amplifier with the negative input ofthe differential amplifier connected directly to thesource as previously. Considering the positive inputcircuit and assuming an input impedance to groundfrom this positive input of 10 megohms, the l50-ohmresistor in conjunction with the 10-megohm inputresistance of the differential amplifier will forroa small but nevertheless highly significant voltagedivider. Now, instead of the full one volt of commonmode signal reaching the differential amplifier,15 ~V appear across the 150-ohm resistor and onlyone volt minps 15 ~V or .999985 volts reach thepositive input of the amplifier. Since no resistoris inserted in series with the negative input, thefull one volt common mode signal reaches thisnegative input. A common mode signal is therefore1 + .999985

2 = .9999925 volts. The signal at the

+input is 7.5 ~V less than this and the signal atthe -input is 7.5 ~V greater than this. Theadditional 15 ~V on the negative input with respectto the positive input is a differential signal andwill be amplified as a differential signal. Assuminga gain of 20,000, this 15 ~V will appear as 300 mVat the output.

sourceimpedancereducesCMRR

sourceimpedanceunbalance

output hasthreemajorcomponents

297

It is thus apparent that the amplifier described inthe preceding paragraph with a CMRR of 100,000:1appears to produce an additional output due to theISO-ohm "source impedance unbalance." If 200 mV ofcommon mode signal appear at the output due to theCMRR of the amplifier and 300 mV of common modesignal appear at the output due to source-impedanceunbalance, a maximum of 500 mV of common mode signalmay appear at the output. These two signaIs wouldnormally not add directly, but being AC signaIs,they must be added algebraically and it isconceivable that they may in fact tend to canceleach other. An equivalent differential signal that

500 mVwould produce this same output would be 20 000 or,25 ~V. The apparent CMRR is then the ratio of thisequivalent differential signal to the true commonmode signal; that is .999992SV:25~V or z 40,000:1.It,is thus apparent that the ISO-ohm source-impedanceunbalance has reduced the CMRR of the amplifier from100,000:1 to an apparent CMRR of 40,000:1. As thesesignaIs add algebraically and may tend to canceleach other, 40,000:1 will be a worst-case CMRR. lnpractice the apparent CMRR may be increased by thesource impedance unbalance.

Extension of the previous discussion will show thatit is not the absolute value of the source impedancethat determines this apparent reduction in CMRR butrather the difference between the source impedances(source impedance unbalance) between the positive andnegative inputs of the differential amplifier. Asstated in the preceding section, it is possible fora source-impedance unbalance of up to 10,000 ohms tooccur when recording physiological signaIs. It isthus desirable either to attempt to minimize thisunbalance, or to increase the input impedance of thedifferential amplifier to the common mode signal inan effort to increase the ratio between the sourceimpedance unbalance and the common mode inputimpedance of the differential amplifier.

From the preceding discussion it is apparent thatthe output of any differential amplifier will consistof three components --

1. A desired output component due toamplification of a differential signal bythe differential amplifier,

298

fourthcomponent

calculationof outputcomponents

2. An undesired component due to incompleterejection of common mode interferencesignaIs by the differential amplifier and,

3. An undesired component due to sourceimpedance unbalance allowing a smallproportion of a common mode signal toappear as a differential signal to theamplifier.

These three output components are shown in Fig. 19-2together with the equations used for determining thevalues of each of these three output-signalcomponents.

When a differential amplifier is used on a subjectthere will also be a fourth output; an undesiredoutput component due to amplification of an undesireddifferential input from the subject, due to thesubject's finite resistance between the inputterminaIs and the presence of AC interferringcurrents through this resistance. This fourth outputlimits the useful rejection ratio of an ECG amplifierto about 100,000:1.

Referring back to the amplification system discussedearlier in this section, this system is illustratedin Fig. 19-3 with the one volt common mode signalconsisting of a one volt P-P, 60-Hz sinewave and adesired differential signal consisting of a 100 ~VP-P, 2-Hz sinewave. Applying the formula shown inFig. 19-2 to this amplification system gives a desiredoutput of 2 volts of 2-Hz sinewave and an undesiredoutput of 0.5 volts of 60-Hz sinewave. Thisundesired output consists of 0.2 volts due to theinherent CMRR of the amplifier and 0.3 volts due tothe ISO-ohm source-impedance unbalance of the system.

When measur~ng the CMRR of modern arnplifiers,it isnecessary to use about one volt of common modesignal so that the output due to this signal can bediscerned from noise. (1 V cornmonmode with100,000:1 CMRR is equivalent to a 10 ~V differentialinput signal.) Typical common mode signaIs inbiophysical measurements are from 1 mV to 100 mV.The CMRR to these signaIs will be slightly greaterthan the CMRR to a one volt signal due to smallnonlinearities in the amplifier's input devices.

299

l' ...... DI FFERENTIAL-,AMPLIFIER GAIN - A

"- "- INPUT IMPEDANCE - Z.Z "- "- ln...... CMRR-X:l"- "-

Zln<,

<, ....OUTPUT) •• THREE COMPONENTS;'

Z. ,/

Tln ,/

,/,/

,/

,/

,/

". DESIRED COMPQNENT UNDESIRED COMPONE NT UNDESIRED CQMPONENT DUE TODUE TO PHYSIOLOGICAL DUE TO INTERFERENCE INTERFERENCE SIGNAL El ANDSIGNAL E SIGNAL El AND CMRR ELECTRODE CONTACT RESISTANCES

OF AMPLIFIER Z, Zl-

1-

IF Z. » Z AND Zl AND El » Eln

AEIAEl (1

Z.

- Z1 )OUTPUT AE + + ln

X Z. + Zln

AEl (~+ Z.

ZI )OUTPUT AE + 1 - lnZ. + Z -ln

THE COMPONENT OF THE OUTPUTIS THE UNWANTED OR COMMON MODESIGNAL REFERRED TO THE INPUTOF THE DIFFERENTIAL AMPLIFIER

Fig. 19-2. Common mode rejection.

DIFFERENTIAL INPUT100~V OF 2Hz SINEWAVE

~1'.1~F"ir~·.11~-'E_ 1~~PU""IY O~ uOH~ SI\LWAVl

"".",,,u"lu'f.,.u ••'t'r"t'"1II"lfl'lt1,,,·'\tttttl"11"I'"I~'lIj"'lIlIllilllhllllillll;ltllll,'1i11 111111"

A - 20,000CMRR = 100,000:1Z. = 10Mnln

A - 1DEALB - ACTUAL WITH NO SOURCE Z UNBALANCEC - ACTUAL WITH 150n SOURCE Z

UNBALANCE

OUTPUT = AE + AE 1 (~ + 1 - Z in ~ i~ _ Z1)

( 1 10 x 106 )20,000 x 100~V + 20,000 x lV 100,000 + 1 - 10 x 106 + 150 _ 0

= 2V OF 2HzSINEWAVE

+ 0.2V OF 60HzSINEWAVE DUE TOCMR OF AMPLIFIER

+ 0.3 OF 60Hz SINEWAVEDUE TO 150n INPUT ZUNBALANCE

Fig. 19-3. Example of common mode rejection calculation.

300

coup 1ingcapacitorunbalance

The principal common mode signal encountered inbiophysical recording is line frequency interferenceat either 50 Hz or 60 Hz. Let us now examine thevalidity of inserting coupling capacitors in serieswith the differential amplifier inputs to provide anAC-coupled amplifier. ln physiological situationsthese capacitors may be intended to block theDC-offset potentials encountered in the electrodesystem. Assuming a low frequency response to 0.1 Hzwith 10-megohm input resistors, then 0.16 ~F couplingcapacitors are required. The capacitive reactanceof 0.16 ~F at 60 Hz is 16,700 ohms. When consideringthis impedance in the light of the earlier CMRRcalculations, it becomes obvious that ordinary 10%tolerance capacitors will have a substantialimpedance unbalance to this 60 Hz and thus willprevent a high common mode rejection ratio from beingobtained. Even if the coupling capacitors arematched to ±l%, the reduction in CMRR due to theirimbalance will be significant. It is thus desirablenot to use coupling capacitors at the inputs of adifferential amplifier but to use them in later .amplification stages after common mode signaIs havebeen rejected.

19 •3 INPUT RESISTANCE

inputresistanceto commonmode anddifferentialsignais

For any differential amplifier there are twosignificant input resistance values: the inputresistance of the amplifier to a differential signaland the input resistance to a common mode signal.From the preceding discussion it is clear that theinput resistance to common mode signaIs (common modeinput resistance) should be as high as possible,definitely above 10 megohms and preferably above100 megohms. Relating back to the discussion onelectrodes in Chapter 16, electrode offset potentialsare stabilized over a period of time due to theloading effect of the amplifier; that is, the inputresistance of the amplifier to differential signaIswill tend to discharge the "batteries" formed at theelectrode-tissue interfaces. It is thus desirablethat a differential amplifier's input resistance todifferential signaIs (differential input resistance)be of a value that will al10w e1ectrodestabilization within two or three minutes afterapplication. It is undesirab1e however, to use tooIowa differentia1 input resistance as too lowresistance produces a distorted pulse response to aphysio10gica1 signal as discussed in Section 16.1.

301

In practice, a differential input resistance of2 megohms has proved satisfactory and couId perhapsbe regarded as an optimum value for most physiologicalrecording or from surface electrodes.

Referring to Fig. 19-4, the input resistanceconfiguration of differential amplifiers determinesthe ratio between the differential input resistanceand the common mode input resistance. The inputresistance configuration shown in Fig. 19-4A usesseparate input resistors to ground for each input.

DIFFERENTIAL AMPLIFIER WITH SEPARATEINPUT RESISTANCE ON EACH INPUT

o------~ G /' '"","'~ DIFFERENTIAL INPUT RESISTANCE = 28

'"'" TO GATE OF COMMON MODE 1 NPUT RES 1 STANCE = 0.5Rv '"/' 1NPUT DEV ICE (FETl

l' .....1 .•.•..•.••

.•.•. ......+O-----:--...-~~G "

.•.•. .•.•.<,<, ...

+0-----'---..... G "' <,

/''"

(Al

DIFFERENTIAL AMPLIFIER WITH SEPARATEINPUT RESISTANCE ON EACH INPUT AND ANADDITIONAL RESISTANCE TO REDUCE THEDIFFERENTIAL INPUT RESISTANCE

<,<,>

DIFFERENT 1AL 1NPUT RES 1STANCE;:; .•x 812R + Rl

COMMON MODE INPUT RESISTANCE;:; 0.5R

(6)

DIFFERENTIAL AMPLIFIER WITH AN INPUTRESISTANCE ON EACH INPUT SHARING ACOMMON RESISTANCE TO GROUND

DIFFERENTIAL INPUT RESISTANCE;:; 2RCOMMQN MODE INPUT RESISTANCE = 0.5R + R2

«»

Fig. 19·4. Common mode and differential input resistance.

302

configurationdeterminesratio

This configuration gives a high differential inputresistance with a lower commpn mode input resistance,a somewhat undesirable situation. The inputresistance configuration shown in Fig. 19-4B issimilar to that shown in Fig. 19-4A with the additionof resistance RI to reduce the differential inputresistance to an acceptable value. This configurationis satisfactory for use in most physiologicalsituations and can be achieved from the circuit shownin Fig. 19-4A with the simple addition of oneresistor. The input resistance configuration shownin Fig. 19-4C provides acceptable values for boththe differential input resistance and the common modeinput resistance and is particularly recommended asit has the added advantage of having only onephysical resistor forming most of the common modeinput resistance. Typical values for this circuitmay be 1 megohm for Rand 100 megohms for R2. Thiscircuit also lends itself to input guarding asdiscussed in the following section.

19.4 INPUT GUARDING

singleendedinput

The common mode input impedance of an amplifiercan be greatly increased by "guarding" or"bootstrapping" the input circuit of the amplifier.Input guarding can best be understood by viewing theamplifier from the input, by considering DC conditionsand then expanding the analysis to include practicalAC conditions. A conventional single-ended inputcircuit is shown in Fig. 19-5A. The input impedanceof this circuit consists of the input resistance Rin parallel with the reactance of the capacitance Cof the shielded cable used for interconnection betweenthe input voltage and the amplifier. Assume an inputvoltage of one volt and an input resistance R of onemegohm. With the input voltage connected to theamplifier, a current of one ~A will flow. As far asthis input voltage is concerned, it is one voltseeing a load that requires one ~A of current; theload thus appears as one megohm to the input voltage.

r---------------------1 + AMPLI FIER: 1NPUT CKT

GAIN A

11 -:-1L _

(A)CONVENTIONAL INPUT CIRCUIT

r---------------------1 + AWLIFIER1 INPUT CKT: GAIN A

SHIELDED CABLE :CAPACITY C 1-----t--

f R

'-..•...---+---_~OUTPUT

L _

(B) GUARDED INPUT CIRCUIT

+ AWLIFIERINPUT CKT

CQIIM)N MODEGAIN A

SHIELDED CABLECAPACITY C

\R

COMMON MODE+--+---+---.-~SIGNAL OUTPUT

R

+

(C) GUARDED DIFFERENTIAL INPUT CIRCUIT

+

10~

EMlnERFOLLOWER 1----t-----,

Ql10Qll1

+(0)GUARDED DIFFERENTIAL INPUT CIRCUIT

AS USED lN THE TEKTRQNIX TYPE 410PHYS 10LOGlCAL MON 1TOR

303

INPUT IMPEDANCEZj n = RN Xc

INPUT IWEDANCEz. (RI Xc),n

1 - A

TYPICAL VALUE FORAMPLIFIER STAGE GAINA = 0.99

COMMON MODEINPUT IWEDANCEZjn(cm) = 0.5 (~

1 - A

DIFFERENTI ALINPUT IMPEDANCEZjn(dj f) = 2R

AT OC -C(M.I()NMOOE 1NPUT 1MPEDANCE 36MnDIFFERENTIAL INPUT IMPEDANCE 2~

AT 60Hz -

COMMON MOOE INPUT IMPEDANCE = 75~DIFFERENTIAL INPUT IMPEDANCE 2MO

CONSTANTCURRENTSOI.RCE

Fig. 19-5. Input guardingor "bootstrapping".

304

singleendedguardedinput

laap gainamp 1if iesR.ln

Now consider the single-ended guarded input circuitshown in Fig. 19-5B. Assume the same conditions asbefore with the input FET having a gain of 0.90between gate and source. With the amplifier inputresistance connected to the FET source rather thanto ground, a one volt input will produce a one voltpotential at the FET gate and therefore a 0.90 voltpotential at the FET source. The potentialtherefore appearing across the input resistance Ris the difference between the potentials, that is,0.1 volt. The current through the one megohm inputresistor is now 0.1 ~A. Now, as far as the inputvoltage is concerned, it is one volt seeing a loadthat requires 0.1 ~A of current; the load thusappears as 10 megohm to the input voltage eventhough the actual value of the input resistance isonly one megohm. The value of the input resistanceis thus amplified by a factor of 10 by connectingthe input resistance in a guarded configurationrather than by simply connecting it to ground.

ln actual practice, a more typical value for the FETgain between the gate and the source would be 0.990,resulting in an input resistance "amplification" of100X. Thus, a physical l-megohm resistor in aguarded circuit would appear as a 100-megohmresistor to the input voltage. The closer theamplifier bootstrap loop gain is to unity , the higherthe input resistance as seen from the input. Theabove discussion applies equally weIl to AC signaIs,however, both the input resistance R and the inputcapacitance C formed by the shielded cableconnecting the input voltage to the amplifier mustbe considered as the input components of the circuit.As with the DC conditions discussed previously, theinput reactance of the circuit can also be"amplified" by connecting the shield of the inputcable to the FET source rather than simply to ground.A formula for the input impedance of this circuit isgiven in Fig. 19-5B.

commonmodeguardedinput

410 guardedinput

guardingused withinampl ifier

305

The above discussion can be further extended to adifferential amplifier input circuit as shown inFig. 19-5C. With a differential amplifier thecommon mode input impedance is increased by guarding.As discussed in the previous section, it is desirableto have as high a common mode input impedance aspossible to minimize the effect of source impedanceunbalance.

Tektronix Type 410 Physiological Monitor uses aguarded differentiaI-input circuit as shown inFig. 19-5D. Since high common mode input impedanceis primarily required to reject 60-Hz signaIs, theguarding circuit can be AC coupled to the inputresistors and cable shields as it is unnecessary toprovide high common mode impedance to DC signaIs.The 410 Physiological Monitor has a 38-megohm commonmode input resistance to DC signaIs and a commonmode input impedance to AC signaIs of considerablyabove 750 megohms.

Guarding can also be used within an amplifier toprovide constant operating conditions for the inputFET's. ln the 410 Physiological Monitor, as the gatesof both input FET's go positive, the output of theinput-guarding emitter follower, QllO and Qlll, alsogoes positive. This emitter-follower output is usedto guard the various components of the inputimpedance and is also used to determine the drainpotentials on each of the input FET's. The circuitis so arranged that a constant voltage appearsbetween the source and drain no matter what thecommon-mode input-signal level.

306

l IG FETAMPLIFIER

E IRIOMO

- -- -

+200pA

lAMPLIFIERINPUT CURRENTl = IR - IG

CURRENT FLOW"INTO" AMPLIFIER

!~ 1NPUT RES 1STANCE~ IS SLOPE OF THIS

LOAD LINE t>ER = tlI = 10MO

o IL-------:::~---+----_+___I~ Ett

CURRENT FLOW"OUT OF" AMPLIFIER

2 3INPUT VOLTAGE (mV)

-200pA --- LOAD LINE FOR AMPLIFIER WITH 10MflINPUT RESISTANCE AND ZERO INPUTGATE CURRENT

-- LOAD LINE FOR AMPLIFIER WITH 10MnINPUT RESISTANCE AND -100pA INPUTGATE CURRENT

Fig. 19-6. Amplifier input current.

19.5 INPUT CURRENT

input gatecurrent

Consider the simplified amplifier configurationshown in Fig. 19-6. The input impedance of such anamplifier is defined as the ratio of an incrementalchange in the input voltage to the resultingincremental change in the input current. Thus, ifthe input voltage shown in Fig. 19-6 changed by 1 mVand the resulting input-current change was 100 pA,

h . . Id b 1 mV 10 ht e lnput reslstance wou e 100 pA = mego m.

If the only component of input current was due tothe input resistance, then Ohm's law dictates thatthe input current would be zero when the input voltageis zero. ln aIl physiological amplifiers anadditional input current, due to the amplificationdevice, is always present. ln the case of a modernFET amplifier, this input current would normallybe below 1 nA and would possess a negative signindicating that the FET is acting as a "source" ofcurrent rather than a "sink" of current. This currentis referred to as "input gate current."

input gridcurrent

inputcurrentversusinputresistance

input currentproducesoffsetpotential

inputcurrentcompensation

307

ln older style amplifiers using vacuum tubes ornuvistors for the input element, the input currentmay be several nanoamps and may be of eitherpolarity, indicating that the input device is eithera "sink" or "source" of current. Input current inthese amplifiers is referred to as "input gridcurrent."

The two load lines shown on Fig. 19-6 defining therelationship between input voltage and input currentboth represent an input resistance of 10 megohm (theslope of the line), the upper line representing atheoreticaZZy ideaZ amplifier having zero input gatecurrent and the lower line representing a practicaZFET amplifier having an input gate current of -100 pA.Since the component of the input current associatedwith the input amplification device is constant anddoes not vary with the input voltage, it must beshown separately when specifying an amplifier andcannot in any way be included with the amplifier'sinput resistance specifications.

Input gate current can be a source of undesirableoffset potential if an amplifier is used inconjunction with a high impedance source. Considerthe amplifier shown in Fig. 19-6 used in conjunctionwith a source having a l-megohm output impedance.Assume also that this source is zero volts. The-100 pA input grid current will flow through theoutput impedance of the source creating a potentialdrop of 100 ~V. The output of the amplifier wouldthen indicate a 100-~V input voltage even though theinput voltage is in fact zero. This situation isonly troublesome when making DC measurements as thegate current is a DC current.

Many amplifiers provide sornedegree of input gatecurrent compensation by adding a constant-currentdrain between the gates of the field effecttransistors and ground. Thus, an amplifier such asthe Tektronix Type 3A9 Differential Amplifier, hasan input gate-current specification of ±20 pA at25°C even though the actual internal FET-gate currentmay be in excess of -100 pA.

308

19.6 DYNAMIC RANGE, ne OFFSET AND RECOVERY

overloadingamp 1if iers

transientover 1oad ingversusrecovery

As either the common mode or differential inputvoltage to a differential amplifier is increased,a point is reached when the amplifier will overloadand the'output voltage will not be representativeof the input voltage. This overload may occur withinthe amplifier input stages or elsewhere within theamplifier. If, howeyer, the amplifier's dynamicrange specifications are not exceeded, overload willnot occur. Dynamic range characteristics for adifferential amplifier are specified as follows:

1. Input differential dynamic range -- Themaximum peak differential voltage that canbe applied between the differentialamplifier input terminaIs,

2. Input common mode dynamic range -- Themaximum peak common mode voltage that can beapplied between the inputs of thedifferential amplifier and ground,

3. Output dynamic range -- The maximum peakoutput voltage that can be expected fromthe amplifier.

The amplifier is referred to as "overloaded" if anyone of these three limits is exceeded.

ln physiological applications it is not uncommon fortransients to be produced that are several orders ofmagnitude greater in potential than the physiologicalsignal under observation. An important characteristicof a physiological amplifier is the time taken for itto recover from such overloads. Amplifier recoverycharacteristics can be defined by the time taken forthe amplifier to recover from a transient within thedynamic range specifications of the amplifier and bythe time taken for the amplifier to recover fromtransients far greater than the dynamic rangecharacteristics. DC-coupled amplifiers normallyrecover almost instantaneously from transients withinthe amplifier's dynamic range specifications as thesetransients are within the common mode rejectioncapabilities of the amplifier. Recovery time fortransients that exceed the amplifier's dynamicrange specifications are difficult to specify as

AC coup 1ingandtransientsversusrecovery

410 recovery

DCdifferentialoffsetversuselectrodeoffset

309

this recovery time depends on the precisecharacteristics of the overload transient. With suchtransients, thermal effects and the inherent unbalanceproduced by such thermal effects must be considered.Typical amplifiers may take several seconds to recoverfrom severe overload.

AC-coupled differential amplifiers may haveparticularly bad recovery characteristics due tothe inherent time constant involved in the ACcoupling network. Recovery time may be ten timesas long as this time constant. Although AC couplingat the amplifier input is not recommended, ACcoupling at a later stage in the amplifier isdesirable as it eliminates electrode offsetpotentials, etc. Amplifiers that are AC coupledafter several stages of DC-coupled amplification arereferred to as "AC-stabilized" amplifiers. Manyphysiological amplifiers, such as the amplifierincorporated into the Tektronix Type 410Physiological Monitor, incorporate AC coupling afterseveral stages of DC amplification and internaIlimiting to improve the amplifier's recoverycharacteristics. The amplifier in the Type 410Physiological Monitor has a DC-coupled input stageand is AC coupled after this input stage at a timeconstant of two seconds. Its recovery time is lessthan four seconds for any overload signal. Thiscbaracteristic is particularly important in anamplifier intended for ECG monitoring in intensivecare and surgery as the amplifier will certainly beoverloaded if the subject is defibrillated and it ishighly important that the medical personnel be ableto use the amplifier immediately after thedefibrillation process is complete.

Physiological signaIs obtained from electrodesinclude a DC component due to the electrode-offsetpotential. A differential amplifier capable ofrecording these physiological signaIs in a DC-coupledmode must therefore, include the capability to rejectthis electrode-offset potential. This is achievedby incorporating DC-differential offset within thedifferential amplifier which effectively producesa DC-differential voltage at the differentialamplifier input which cancels the electrode-offsetpotential.

310

electrodeoffsetexceedingdynamicrang'e

dynamicwindow

If DC-differential offset is not available within anamplifier, AC coupling must be used to reject theelectrode-offset potential. As it is undesirableto incorporate AC coupling at the amplifier's inputthe amplifier must be AC coupled after one or moreamplification stages and care must be taken to ensurethat the electrode-offset potential is not exceedingthe dynamic-range characteristics of the amplifier.Often the user may not be aware that the amplifier'sdynamic range is being exceeded and erroneous resultsare obtained.

The term "dynamic window" refers to the use of adifferential amplifier with DC-differential offset.Dynamic window can best be understood by consideringthe following example. Consider a Tektronix Type3A9 DifferentiaI Amplifier operating at its maximumgain of 100,000 with no DC-differential offset. Asthe output dynamic range for this amplifier is ±5volts, the maximum differential input voltage thatwill not exceed this output dynamic range is

±5 volts +50 V Th l'f' , bl to "see"100 000 or - ~. e amp ~ ~er ~s a e,a dynamic window of ±50 ~V. Now assume that the Type3A9 is used in the DC OFFSET position. The DC OFFSETrange available is the same as the differentialdynamic range, that is ±l volt. Assume that thecourse and fine DC ·OFFSET controls are set to provide+0.3 volts of DC-differential offset. Now themaximum differential input voltage that can be appliedto the amplifier is ±50 ~V +0.3 volts. The amplifieris now able to "see" a dynamic window of ±50 ~V from0.3 volts, that is from 0.299950 volts to 0.300050volts.

19.7 NOISE AND DRIFT

noiseversusdri ft

stabi 1ity

hum

Noise and drift are both unwanted signaIs that occurwithin an amplifier. The term "noise" normallyrefers to unwanted signaIs generated at a frequencyabove 0.1 Hz and DC drift normally refers to slowchanges in the output level at frequencies below0.1 Hz. The term "stability" is also used toindicate relative DC-drift characteristics. Thecomponent of noise occurring at the power-linefrequency and harmonies of the power-line frequencyis often referred to as "hum."

quai ifyingnoise

equivalentrelated toRMS

RMS noise

311

The noise generated within an amplifier is oftenspecified in microvolts and refers to this noise asif it were a differential input voltage. The noiseproduced within an amplifier is either specified asmicrovolts P-P, microvolts RMS, microvoltstangential or by specifying an equivalent noiseresistance. The equivalent noise resistanceconsiders aIl the noise in an amplifier to beproduced by an equivalent resistance producingthermal noise. The RMS value of this thermal noiseis related to the resistance value by the followingexpression:

ERMS = 7.4 x 10-12 x~ R bw et + 273)

or(E ) 2 x 1024

RMSR = --~~---------55 bw (t + 273)

R = Equivalent NoiseResistance -- ohms

bw = Bandwidth -- Hz

t = Temperature -- °c

Thus, an amplifier specified as having an equivalentnoise resistance of 100,000 ohms would produce 7 ~vRMS of noise at 30 kHz bandwidth at 20°C. Equivalentnoise resistance relates, therefore, to RMS noise.

The RMS noise, and thus the equivalent noiseresistance, of an amplifier is measured by shortingthe inputs to ground ànd measuring the output voltageat maximum sensitivity with an RMS-indicatingvoltmeter. The RMS noise generated within theamplifier will then be this RMS output voltagedivided by the gain of the amplifier. For aTektronix Type 3A9 DifferentiaI Amplifier operatingat a bandwidth of 30 kHz and at its maximum gain of100,000, the RMS noise output was 0.32 volts. Thus,the RMS noise produced within this amplifier is3.2 microvolts RMS and the equivalent noiseresistance for the amplifier is 20,000 ohms.

Amplifier noise is normally specified in microvoltsRMS if the amplifier is to form part of aninstrumentation system. This noise specificationgives no information as to the peak component ofnoise or to the relative appearance of a CRT displayincorporating this noise, thus either P-P noise ortangential-noise measurements may be desirable insorneinstances.

312

RMS NOISE

INPUT CQUPLING SWITCHESTO GROUNO POS 1T ION.

EQUIYALENT NOISE RESISTOR

INPUT CQUPLING SWITCHESTO GROUND POSITION.

P-P NOISE

INPUT CQUPLING SWITCHESTo GROUND POSITION.

TANGENTIAL NOISESQUAREWAVE GENERATOR OUTPUT REDUCEDUNT 1L THE NO1SE BANDS SHOWN ON THEOUTPUT-MON 1TOR 1NG ose ILLOSCOPE MERGEINTO ONE WloE NOlSE BAND.

501lATTENUATOR ••.••----"

SYSTEM

SQUAREWAVEGE ERATOR

=lkHz

RMS NOISE

RIolSYOLTMETER

RMS VOLTMETERREADS O.32Y

3A9 AMPLIFIERbw DG - 30kHzGAIN 100,000

TEMPERATURE WITHIN AMPLIFIER 34·C

SPACING BETWEEN TWO NOISE BANDS REDUCEDUNTIL THE BANDS MERGE. THIS OCCURS WITHTHE INPUT GENERATOR PROVIDING 6~V OFSQUAREWAVE TO THE AMPLIFIER.

0.32 Q...J2RMS NOISE = GAIN Z lOS = 3.2~V(RMS)

EQUIVALENT NOISE RESISTORCOMPUTED FROM RMS NOISE AND TEMPERATURE.

R = (8RMS 2 x 102~ = b.2 x 10-6)2 X 102~

55 x bw t + 273) 55 X 30 X 103 X 30720kll

P-P NOISE

TOseOPEIv/oiV

P-P NOISE : ~ = 9~V(P-P FOR 99S1

TANGENTIAL NOISE - 6~V

TRIGGERED DISPLAYTO

SCOPE

FREE RUN DISPLAY

Fig. 19-7. Amplifier noise measurement.

peak-topeak noise

tangentialnoise

313

If an amplifier is to be used in conjuction withpeak detector or trigger circuitry, it is importantto know the peak value of noise produced by anamplifier rather than simply the RMS value of thisnoise. Peak-to-peak noise for an amplifier can bemeasured in the same way as RMS noise by using anoscilloscope to display the noise output. Thedisplayed noise for the amplifier system discussedpreviously is shown in Fig. 19-7. From observationof the oscilloscope display at a sensitivity of 1volt per division, the P-P deflection shown appearsto be approximately 0.9 divisions referring to anoutput of 0.9 volts. The P-P noise generated withinthe amplifier is thus 9 ~V P-P referred to the input.Closer examination of the noise shows that, while 99percent of the information appears to be within 0.9divisions, random spikes are produced over an apparentoverall range of 1.2 divisions. (Theoretically,random spikes can extend out any distance.) The P-Pnoise generated by this amplifier could, therefore,be more correctly stated as 12 ~V subjective P-Pwith 99 percent of the noise below 9 ~V P-P.

ln the majority of physiological measurements anoscilloscope is used to display the output from anamplifier. It is thus desirable to know theamplifier's noise characteristics with respect tothis oscilloscope display. A "tangential"measurement of displayed noise effectively measuresthe trace width produced by noise and therefore givesan idea of the resolution obtainable from theoscilloscope screen. The following discussion ontangential noise measurement technique will furtherclarify this resolution concept. Consider a lowamplitude squarewave being applied to the input ofthe differential amplifier as shown in Fig. 19-7.As it is necessary to know the amplitude of thissquarewave, it is desirable to obtain a relativelyhigh-amplitude squarewave for ease of measurementand then to accurately attenuate this via a 50-ohmattenuation system. Although the output from this50-ohm attenuation system is single ended, that is,only one output source is produced with respect toground, this output should be coupled differentiallyinto an amplifier by connecting the positive inputof the amplifier to the output source and thenegative input of the amplifier to the referenceground for this output source. For the configuration

314

tangentialx 2 equalsRMS

=->derivationfrom RMS

shown in Fig. 19-7, the oscilloscope will clearlyshow the squarewave together with the noise generatedfrom within the differential amplifier; this displayis shown in the upper photograph of the tangentialnoise section of Fig. 19-7. If the oscilloscope'ssweep generator is then operated in a free-runningmode, the squarewave information will be lost andthe CRT display will appear as two bands of noise.As the output from the squarewave generator isincreased in amplitude, these bands will tend toseparate; as the output is decreased in amplitude,these bands will tend to merge into one. The inputsquarewave level at which these two bands just mergeinto a single band is referred to as the tangentialnoise specification for the amplifier. ln theparticular configuration shown in Fig. 19-7, thetangential noise specification for the amplifier is6 ~V and a free-running display of this 6-~Vsquarewave together with inherent noise from withinthe amplifier is shown in the bottom photograph inFig. 19-7.

If an amplifier's frequency response is perfectlyGaussian, then a mathematical analysis of RMS-noisemeasuring techniques and tangential-noise measuringtechniques will show tangential noise to be twiceRMS noise. Since most physiological amplifiers areessentially Gaussian, RMS noise can be simplydetermined once tangential noise has been measured.This indirect method of determining RMS noisesidesteps the need for a true RMS-reading voltmeter.

If the distribution of noise within an amplifieris Gaussian, and the noise distribution in mostwideband amp·lifiers is approximately Gaussian, thenthe noise distribution will follow a "normaldistribution" curve. From the curve we can deducethat:

RMS noise x 2 = P-P noise for 68% of the noise



RMS noise x 6 = P-P noise for 99.9% of the noise

RMS noise x 8 = P-P noise for 99.99% of the noise

excessnoise

short- andlong-termdri ft

temperatu redri ft

315

Examination of the equation relating RMS noise toequivalent noise resistance indicates that RMSnoise is proportional to the square root of theamplifier's bandwidth. This noise component isknown as "thermal noise" or "Johnson noise." Theabove proportionality is essentially true forwideband amplifiers (100 kHz and above) but, however,does not hold true for narrowband amplifiers or whenwideband amplifiers are subject to bandwidthlimiting. Under these conditions an additional lowfrequency noise component generated within the inputfield-effect transistors becomes an appreciableproportion of the amplifier's total noise and cannotbe neglected. This extra noise component is referredto as "excess noise" (also occasionally referred to

as If} nO:Î:-se"or "flicker noise") and is discussed in

more detail in Chapter 20 when discussing bandwidthlimiting considerations.

As previously stated, drift can be regarded as verylow-frequency noise generated within an amplifier.Drift specifications, as with noise specifications,are in microvolts and refer to the drift as if itwere a differential input voltage. Because of itsinherent low frequency nature, drift is normallyspecified in P-P units. Drift specifications formost amplifiers are self-explanatory, however, afull evaluation of drift should include both longterm drift and short-term drift measurements. Shortterm drift measurements are measured in microvoltsper minute and indicate to the user the drift inmicrovolts expected over a one-minute period afterthe amplifier has had sufficient time to warm up.Long-term drift specifications are in microvolts perhour and indicate the total drift that may beexpected over a one-hour periode Long-term driftspecifications will always exceed short-term driftspecifications.

Both long-term drift and short-term drift assume aconstant operating temperature within thedifferential amplifier and an additionalspecification must be generated to determine thecharacteristics of this drift with a change intemperature. Temperature drift is specified inmicrovolts per degree Celsius and indicates thedrift that may be expected from the amplifier fora IOC change in ambient temperature.

316

PRINCIPAL CHARACTERISTICS ONLY SHOWN WITH THE 3A9 USED AS AN INPUT AMPLIFIER TO PROVIDE GAINFROM A DIFFERENTIAL INPUT TO THE SIGNAL OUTPUT CONNECTOR AT A BANDWIDTH OF FROM DC TO 30KHz.80TH WIDER AND NARROWER BANDWIDTHS ARE AVAILABLE AS DISCUSSED lN CHAPTER 20 - SIGNAL PROCESSORS.GAIN SETTINGS BELOW xl00 ARE NOT CONSIDERED.

CHARACTERISTICS * MOD 1FIED PERFORMANCEPERFORMANCE

GAIN - CALIBRATED

- UNCAL 1BRATED

CMRRDY AMIC RANGE

RECOVERY

INPUT IMPEDANCE

1NPUT CURRE T

NOISE

DC DRIFTAFTER 1 HR WARMUP

OUTPUT DYNAMIC RANGE

DC OFFSET

xl00 TO xl00,000i 10 STEPSlN A 1-2-5 SEQUENCEVARIABLE CONTROL PROVIDESCONTINUOUSLY VARIABLE GAINBETWEEN STEPS100,000:1± 1 VOLT DIFFERENTIAL± 10 VOLTS COMMON MODETO WITHIN 0.5% lN LESS THAN10~s IF OVERLOAD VOLTAGEWITHIN DYNAMIC RANGES SHOWNABOVE2MQ 24pF DIFFERENTIAL0.5MQ 94pF COMMON MODEMAXIMUM ±20pA AT EACHINPUT AT 25°C

6~V MEASURED TANGENTIALLY(TYPICAL)

5~V/MIN (PEAK TO PEAK)lO~V/HOUR (PEAK TO PEAK)50~V;oC±5 VOLTS FROM 100n SOURCE±1 VOLT

SAME AS STANDARD PERFORMANCE

"

""""

"5.0Mn 94pF CQMMON MODE

TYPICALLY < ±100pA AT EACHINPUT AT 25°CSAME AS STANDARD PERFORMANCE

"

""

*3A9 MODIFIED TO INCREASE INPUT IMPEDANCE- REMOVE LINKS ON 3A9 CIRCUIT BOARD AS

SHOWN ON PHOTOGRAPH.- ADD 2.2Mn RESISTOR BETWEEN + INPUT

AND - INPUT IF DESIRED TO MAINTAIN2MQ DIFFERENTIAL INPUT RESISTANCE. REfvlOVE

LINKS

ADD 2.2MnBETWEENTHESEPOINTS

Fig. 19-8. Characteristics of the Tektronix Type 3A9 Differential Amplifierwhen used as a physiological differential amplifier.

powersupplydrift

317

If high-stability power supplies are not used withinan amplifier, drift may be caused by variations insupply voltage. This would be specified asmicrovolts drift for a supply voltage change of±lO% from nominal. Drift with line-voltage changeis negligible in modern FET amplifiers.

19.8 SPEClALIZED AMPLIFIERS

optimumcharacteristics

Only the characteristics of general-purposephysiological amplifiers have been discussed in thischapter. A typical general-purpose physiologicalamplifier is the Tektronix Type 3A9 DifferentiaIAmplifier, abbreviated specifications for which areshown in Fig. 19-8. Many physiological-monitoringsituations are particularly dependent on one ormore characteristics of an amplifier and it is oftennecessary to use amplifiers with characteristicsoptimized for a particular measurement requirement.An ECG amplifier intended for use duringdefibrillation must possess particularly goodrecovery characteristics. An EEG amplifier must beparticularly free of noise and an amplifier intendedfor use with microelectrodes for recording evokedresponses must have an extremely high input Impedanceand low input current. Amplifiers specifica11yintended for use with microelectrodes are discussedin more detai1 in Chapter Il. A relatively newconcept is the use of plug-in probes that allow aphysiological amplifier to be optimized for aparticular measurement requirement. Thus, the oneamplifier may be used for general purpose recording,for high sensitivity recording, or for use inconjunction with microelectrodes simply by changingprobes.

319

SIGNAL PROCESSORS - OPERATIONAL AMPLIFIERS

The signal available as an output from aphysiological amplifier may not be in a formcompatible with other instrumentation. Signalprocessing is often required to either achievecompatibility or to improve the presentation of aphysiological signal. Signal processing is alsoused when more than one category of information isto be obtained from the same physiological signal.

20.1 OPT~ BANDWrDTH FOR PHYSIOLOGlCALSIGNALS

DC-l0,OOO Hzbandwidth

Spectrum analysis of the action potentialgenerated when a single cell depolarizes indicatesthat essentially aIl of the signal is produced withinthe frequency domain from De to 10,000 Hz. Since aIlphysiological signaIs are the sum of one or moreaction potentials, the frequency domain of thesephysiological signaIs will be less than or equal tothe frequency domain of the single-cell actionpotential. Thus, the optimum bandwidth for aphysiological amplifier capable of monitoring anyphysiological signal is from De to 10,000 Hz. Thisbandwidth is adequate for use in almost aIlphysiological monitoring situations; however, in theresearch environment, sorneworkers postulate theexistence of higher frequency components and abandwidth from De to 30,000 Hz should be adequateto allow these postulated components to be observed.

320

INPUT SIGNAL

PRF = 100Hztr AND tf = O.lms

WIDTH = O.6ms

FREQUENCY SPECTRUMOF INPUT SIGNAL

1kHz

HIGH FREQUENCY RESPONSE LIMITING

-3dB

100kHz

30kHz

10kHz

3kHz

1 kHz ïiiiiiïiiiiiiiii

DISPLAYED RISE- ~TIME OR FALLTIME

=

(

~~~~U~R FALLTIME)2 ( 0.35 )2COMPONENT lN THE + HIGH FREQUENCYINPUT SIGNAL -3dB POINT

HIGH FREQUENCY -3dB CUTOFF REQUIREDFOR EGLIGIBLE OISTORTION, THAT IS,NO VISUAL WAVEFORM CHA GE

1.5MAXIMUM RISE- ORFALLTIME COMPONENTlN THE INPUT SIGNAL

HIGH FREQUENCY -3dB CUTOFF REQUIRED FORACCEPTABLE DISTORTION, THAT IS, NO VISUALRISETIME DEGRADATION BUT SOME ROUNDINGOF ABRUPT CHANGES

1.0MAXIMUM RISE- ORFALLTIME COMPONENTlN THE INPUT SIGNAL

LOW FREQUENCY RESPONSE LIMITING

-3dB

DC

10Hz

100Hz

DISPLAYED "TILT" OR "SAG" lN % =~x 100%yDISPLAYED TI LT

_ % TILT ON THE t LOW FREQUENCY)- INPUT SIGNAL +,30 x W x -3dB POINT

LOW FREQUENCY -3dB CUTOFF REQUIREDFOR NEGLIGIBLE DISTORTION, THAT IS,< 1% TILT ADDED

.0016= MAXIMUM w lN THEINPUT SIGNAL

LOW FREQUENCY -3dB CUTOFF REQUIREDFOR ACCEPTABLE DISTORTION, THAT IS,< 5% TILT ADDED

.008= MAXIMUM w lN THEINPUT SIG AL

Fig.20-1. Input signal degradation due to frequency response limiting.

distortionversusbandwidthrisetimeproduct

5 imu 1atedactionpotential

321

The optimum bandwidth requirement of 10,000 Hzproposed in the previous paragraph is somewhattheoretical as it supposes that a single-cell actionpotential could be coupled to an amplifier withoutdistortion. ln practice, these potentials aredistorted by the monitoring techniques used and abandwidth of substantially less than 10,000 Hz willnot produce additional distortion. No visualdistortion of most waveforms can be observed on anoscilloscope if the product of the oscilloscope'samplifier bandwidth and the fastest risetimecomponent in the signal exceeds 1.5. Only veryslight distortion can be observed on an oscilloscopeif this bandwidth-risetime product is between 1and 1.5. These relationships should thus be observedwhen considering the optimum bandwidth required formonitoring a particular physiological signal.

Referring to Fig. 20-1, a single-cell actionpotential can be experimentally simulated with apulse generator having a rise- and falltime of 0.1 msand a pulse width of 0.6 ms. Such a pulse is shownin Fig. 20-1 together with a frequency domainpresentation showing the predominant energy contentof this pulse to be below 10,000 Hz. The effects onthis pulse of both high-frequency response limitingand low-frequency response limiting are also shownin Fig. 20-1.

20.2 AMPLIFIER NOISE REDUCTION BY BANDWIDTH LTIMITING

The noise generated within a physiological amplifieris discussed in Chapter 19, Section 19.7. Thisdiscussion refers to two sources of noise within anamplifier, "Johnson noise" and "excess noise."

322

Johnsonnoiseversusbandwidth

excessnoiseversusbandw idth

0.5

.05

EexEXCESS NOISE

COWONENT

Fig. 20-2. Amplifier noise/bandwidth relationship(Tektronix Type 3A9 Amplifier).

Examination of the equation relating the Johnsonnoise produced within an amplifier to the amplifier'sequivalent noise resistance indicates that Johnsonnoise is proportional to the square root of theamplifier's bandwidth. It is thus apparent that ifthe amplifier's bandwidth is limited, the RMS noiseproduced within the amplifier will also be decreased.This Johnson noise-bandwidth relationship is shownin Fig. 20-2.

The excess noise component of the total RMS noiseproduced within an amplifier is not mathematicallyrelated to the amplifier's bandwidth. There islittle change in this excess noise component whenthe amplifier's bandwidth is reduced from 300,000 Hzto 3,000 Hz, thus the frequency content of thisnoise for modern FET amplifiers is predominantlybelow 3,000 Hz. The relationship between the excessnoise produced within a Tektronix Type 3A9 Amplifierand the amplifier's bandwidth is also shown inFig. 20-2. Excess noise has historically been

referred to as } noise.

tota 1 no iseversusbandwidth

3A9 signalprocessingfeatures

323

Since both Johnson noise and excess noise areessentially random, the total of these two noisecomponents will be the square root of the sum of thesquares of each component. The total RMS noiseproduced within a Type 3A9 Amplifier is shown inFig. 20-2. It is apparent from this figure that anamplifier bandwidth reduction from 300,000 Hz to10,000 Hz results in a 280% reduction in the noiseproduced within the amplifier. It is also apparentthat a further reduction in the amplifier'sbandwidth from 10,000 Hz to 1,000 Hz is of lessvalue in reducing noise as the resulting decrease inthe amplifier's noise is only about 33%. Manyphysiological signaIs can be recorded with bandwidthssubstantially less than 1,000 Hz and in such cases,limiting the amplifier's frequency response can alsosubstantially reduce the noise generated within theamplifier.

An amplifier such as the Tektronix Type 3A9 Amplifierincorporates bandwidth limiting as shown onFig. 20-3. The high frequency response of theamplifier can be adjusted in nine steps between1 MHz and 100 Hz and the low frequency response ofthe amplifier can be adjusted in seven steps betweenDC and 10,000 Hz. DC differentiaI-offsetcapabilities are also provided to offset the effectof electrode-tissue interface potentials asdiscussed in Chapters 16 and 19.

LOW FREQUENCYRESPONSELIMITING

HIGH FREQUENCYRESPONSE -----!-.•.•LIMITING

±1V OF-- DC OFFSET

Fig. 20-3. Signal processing con trols on the Type 3A9 Amplifier.

324

\

INPUT SIGNAL BEFORE ATTENUATION 5~V SIGNAL AFTER ATTENUATION

AMPLI FIERUPPER -3dB

lMHz

lms/DIVALL WAVEFORMS O.lMHz

ALL lOfjV/DIV

Fig. 20-4. Noise reduction by high frequency response limiting.

INPUT SIGNAL ANDBASELINE DRIFT DUETO ELECTRODE OFFSETPOTENTIAL DRIFT.AMPLIFIER DC COUPLED

AMPLIFIER LOWER -3d8CUTOFF AT 10Hz

Fig. 20-5. Drift reduction by low frequency response limiting.

1imiti ngupper 3-dBpoint

325

The effect of bandwidth limiting within anoscilloscope vertical amplifier on the oscilloscope'sdisplay is shown in Fig. 20-4. With the oscilloscopedisplaying a 5-~V pulse with an amplifier bandwidthof 1 MHz, the pulse is completely swamped by theamplifier's internally generated noise. If theamplifier's upper 3-dB cutoff is limited to 10,000 Hzthe noise is substantially reduced and the inputsignal predominates. A further reduction in theamplifier's bandwidth has little effect on thedisplayed noise; however, it does produce undesirabledistortion.

20.3 AMPLIFIER LOW FREQUENCY RESPONSE L~ITING

1 imit i nglower 3-dBpo int

While limiting an amplifier's low frequency responsehas a negligible effect on the noise produced withinthe amplifier, it can be useful in eliminatingunwanted low frequency signaIs such as drift withinthe amplifier or drift in the electrode-offsetpotential produced between two physiologicalelectrodes. Fig. 20-5 depicts a typicalphysiological response obtained with a DC-coupledamplifier and one obtained using low frequencybandwidth limiting.

20.4 LINE FREQUENCY REJECTION

high CMRRminimizesinterference

Sorneyears ago line-frequency rejection filterswere incorporated into physiological monitoringsystems to eliminate 50- or 60-Hz line frequencyinterference from a physiological signal. Ofcourse, if the physiological signal also containedfrequency components at 50 or 60 Hz, thesecomponents were also eliminated and thephysiological signal was distorted.

With the advent of the modern high CMRR differentialamplifier it was found that the increased commonmode rejection eliminated most line-frequencyinterference, indicating that this interference wasfrom a common mode source rather than from adifferential source. When using an amplifier withhigh common mode rejection ratio, it is rarelynecessary to include any form of line frequencyfilter and, as such filters may cause signaldistortion, line-frequency rejection filters shouldbe considered only as a last resort.

326

20.5 NOISE REDUCTION BY SIGNAL AVERAGING

signalaveraglngminimizesnoise

Signal averaging is a signal processing techniquefor extracting a wanted signal from a background ofunwanted noise. Signal averaging can only be usedif the desired signal can be generated a number oftimes, either periodically or aperiodically, and canbe used to effectively reduce the noise accompanyingthe signal by several orders of magnitude. Thesubject of signal av~raging is adequately covered inChapter 11, Section 11.4, when discussing thetechniques used to record evoked cortical responses.Because of its inherent expense, signal averagingis normally used only when absolutely necessary andis primarily used for recording cortical responsesand in research applications. It is, however, animportant physiological signal processing techniqueand should not be overlooked. This subject isdiscussed elsewhere in this book.

20.6 OPERATIONAL AMPLIFIERS

characteristics

referencesources

Electronically, an operational amplifier is simplya high gain inverting amplifier designed to remainstable with large amounts of negative feedback fromoutput to input. An operational amplifier, by meansof this negative feedback, is capable of processinga signal in many different ways, dependent only onthe passive components selected as the feedbacknetwork and possibly as a series input network. Theideal operational amplifier has infinite gain,infinite input impedance and zero output impedance.

This chapter should not be regarded as a generalreference on operational amplifiers, it specificallydiscusses operational amplifiers from a biophysicalmeasurement standpoint and makes no attempt to reviewsorneof the more sophisticated operational amplifiersystems used within the general electronics industry.A companion volume entitled Operational AmplifierCircuits published by Tektronix, Inc. provides a moregeneral reference source on the use of operationalamplifiers. The instruction manual provided byTektronix for use with the Type 3A8 OperationalAmplifier plug-in also provides an extensivereference on operational-amplifier signal-processingcircuits. The major manufacturers of modular orIC-packaged operational amplifiers also produce

negativefeedback tocurrentnu Il atinput

327

application handbooks containing many circuits thatwould be useful for biophysical measurements. lnparticular, we would recommend Applications Manualfor Operational Amplifiers published by Philbrick/Nixus Research, Dedman, Massachusetts.

Referring to Fig. 20-6, an operational amplifierwith negative feedback operates in a self-balancingconfiguration providing, through the feedbackelement, whatever current is necessary to hold theinput at ground potential. The output signal is afunction of this current and of the impedance ofthe feedback element. If current is applied to theoperational amplifier input, it would tend to developa voltage across the feedback impedance and thus movethe operational ampll.fier input away from groundpotential. The output, however, changes in theopposite direction, providing current to balance theinput current and thus maintains the operationalamplifier input at ground potential. If the feedbackimpedance is high, the output voltage must becomequite high to provide enough current to balance evena small input current.

FEEOBACKCURRENT

If Z FEEOBACKf IMPEDANCE

INPUTCURRENTI.ln

;-------l~ OUTPUTeo/ VOLTAGE OUTPUT

FROM A LOW SOURCEIMPEDANCE

CURRENT NULL POINTAT GROUNO POTENTIAL(ASSIJME ZERO CURRENTTO AMPLIFIER)

e.. I. +_2. = 0

ln Zf

OR e =o

- I.lnz;-

Fig.20-6. Operational amplifier principle.

328

(Al MODULAR OPERATIONAL AMPLIFIERS US1NG DISCRETE COMPONENTS OR IC'SGENERALLY LOW POWER OUTPUTS

<100mW

(8) TEKTRONIX TYPE 3A8 OPERATIONAL AMPLI FIER PLUG-IN UNIT

MEDIUM POWER OUTPUT±25V ±7.5mA 190mW

(C) KEPCO POWER OPERATIONAL AMPLIFIER

HIGH POWER OUTPUT±36V ±5A 180W

Fig, 20-7. Typical operational ampliflers,

voltageinput

non invert inginput

classifications

low power

329

ln physiological measurements we are more oftenrequired to deal with an input voltage rather thanan input current. We thus use an additional element,referred to as an input impedance (Zi), whichconverts an input voltage signal to an input currentsignal.

The input to an inverting high-gain amplifier iscommonly referred to as the inverting input or"- input." Many operational amplifiers also provideaccess to a noninverting or "+ input." If thisnoninverting input is used in an operationalamplifier circuit, negative feedback must still bemaintained between the output and inverting inputand the operating principles remain as discussedpreviously with the exception that the current nullpoint will be maintained at the potential of thenoninverting input rather than at ground potential.

Many types of operational amplifiers are commerciallyavailable, sorneof which are shown in Fig. 20-7.Operational amplifiers can generally be classifiedas either general purpose or specialized, thisdiscussion will refer only to general purposeoperational amplifiers. Operational amplifiers canbe further classified as to their output voltage andcurrent capabilities.

Low power operational amplifiers are available inmodular or integrated circuit packages as shown inFig. 20-7A and generally provide an output power ofless than 100 milliwatts. A typical IC-packagedlow-power operational amplifier may provide a maximumoutput of ±10 volts at ±5 milliamperes. Modularoperational amplifiers are particularly suited foruse in custom signal processing equipment as theyneed only be supplied with the necessary DC power,feedback impedance and input impedance to performa signal processing function. The Fairchild ~A74loperational amplifier is particularly suitable.

330

mediumpower

The operational amplifier shown in Fig. 20-7B isone of the two operational amplifiers incorporatedinto the Tektronix Type 3A8 Operational Amplifierplug-in unit. The Type 3A8 can be used withTektronix 560-Series Oscilloscopes or with the Type129 Plug-in Unit Power Supply. It contains twoseparate but identical operational amplifiers plusa display amplifier. The display amplifier monitorsthe output of either operational amplifier or can beused as an independent oscilloscope amplifier. Theinput impedance and feedback impedance for eachoperational amplifier can be internally selectedfrom a range of resistors and capacitors, thuseliminating the need for external components if theoperational amplifier is to be used for simple signalprocessing applications. If more sophisticatedapplications are required the necessary inputimpedance and feedback impedance networks can bepermanently constructed on an adapter assembly(Tektronix PN 013-0048-01) which can be plugged intothe operational amplifier when desired. Thistechnique is used extensively in the variousoperational amplifier circuits used throughout thisbook and discussed in greater detail in Chapter 29.The operational amplifiers incorporated into theType 3A8 are classified as medium-power operationalamplifiers providing an output power below 1 watt.The Type 3A8 can provide a maxium output of ±25 Vat ±7.5 mA.

high power

The high-power operational amplifier shown inFig. 20-7C has an output power capability of 180watts. Such high power capability operationalamplifiers are rarely used in biophysicalmeasurements; however, high-power operationalamplifiers having output powers of several watts maybe required to drive chart recorders and other lowimpedance devices.

20.7 OPERATIONAL AMPLIFIER APPLICATIONS

The references mentioned at the beginning of theprevious section (20.6) should be consulted whenconsidering the use of an operational amplifier fora particular signal processing application.Operational amplifier applications can, however, beclassified into several broad groups as shown inFigs. 20-8 and 20-9. The following will discuss /

noninvertingamp 1 if ier

ose il 1 ator

331

these configurations as relating to one of theoperational amplifiers in the Tektronix Type 3A8Operational Amplifier plug-in unit. Sornepracticaloperational amplifier circuits using the Type 3A8are discussed in Chapter 29.

The noninverting amplifier shown in Fig. 20-8 can beused to provide gain in any physiological measuringsystem. It also provides a high input impedanceand a low output impedance and thus can providecurrent gain as weIl as voltage gain in a system.If the Zi and Zf components of this noninvertingamplifier are infinity and zero respectively, thenthe configuration becomes a unit voltage gainamplifier providing only current gain between theinput and output.

If positive feedback is added to an operationalamplifier, an oscillator is formed as shown inFig. 20-8. One or more frequency dependent elementsin the positive-feedback circuit determines thefrequency of oscillation, however, sornenegativefeedback is also required to stabilize theoscillation amplitude. This negative feedback maysimply consist of resistive feedback elements oroften nonlinear devices are used to provide sornedegree of output voltage regulation.

e = Eo 1e = Eo 1

eorE.1

j_

NONINVERTING AMPLIFIER UNITY GAIN AMPLIFIER

..........__NEGATIVE FEEDBACK"'STABILIZES OSCILLATION

AMPLITUDE

eo~ POSITIVE FEEDBACK

~PRODUCES OSCILLATION

FREQUENCY DEPENDENTELEMENTS

OSC 1LLATOR

Fig. 20-8.Operational amplifier applications using thenoninverting (+) input to the amplifier.

332

eo = -IjZf

E. = 01

Zfe = -E-o j Z.

1

CURRENT TO VOLTAGE CONVERTER VERTING AMPLIFIER

IF ZfZj ARE RESISTIVE, THE AMPLIFIER IS WIDEBAND WITHIN THE LIMITATION OF THE

OPERATIONAL AMPLIFIERS GAIN BANDWIDTH PRODUCT.

IF ZfZj ARE RC COMBINATIONS THE AMPLIFIER FREQUENCY RESPONSE WILL DEPEND ON THE

RC RATIO.

IF ZfZj ARE NONLINEAR WITH VOLTAGE OR CURRENT (DIODE) THEN THE AMPLIFIER IS

NONLINEAR, EITHER COMPRESSION OR EXPANSION.

C

SWITCH OPENEDDURING INTERVAL

t, - t2R

R

-M.1t;t RC

_L VOLT/s

E.1

OUTPUT PROPORTIONAL TOINPUT VOLTAGE AND TIME

OUTPUT PROPORTIONALTO RATE OF CHANGEOF INPUT VOLTAGE

1 TEGRATOR DIFFERENTIATOR

Fig. 20-9 Operational amplifier applications using theinverting (-) input to the amplifier.

current tovoltageconverter

invertingamp 1if ier

compensatingadapter

bandwidthmodification

333

The previous circuits use the noninverting + inputto an operational amplifier as weIl as the inverting- input. The following circuits use only the - inputto the operational amplifier with the + inputgrounded. The basic operational amplifier andfeedback impedance discussed in Section 20.6 isessentially a current to voltage converter as shownin Fig. 20-9 which provides an output voltageproportional to the input current and feedbackimpedance.

The input current for a current to voltage convertercircuit may be obtained from an input voltage by theuse of an input impedance resulting in the invertingamplifier shown in Fig. 20-9. The output voltage isthen proportional to the input voltage and ta theratio between the feedback impedance and inputimpedance. By selecting appropriate impedance valuesthis inverting amplifier may also be used as aninverting attenuator.

ln most applications the input impedance and feedbackimpedance used for an inverting amplifier areresistors and the amplifier is essentially wideband.

ZfFor largeZ. ratios the gain of the circuit will be

1.

high and the use of resistors for Zi and Zf mayproduce sornehigh frequency losses due to straycapacitance. These losses can be minimized by theuse of the Tektronix Compensating Adapter (PartNumber 013-0081-00). This adapter compensates forthe stray capacitance associated with the internaIZi and Zf resistors.

If the Zi and Zf components of an inverting amplifierare combinations of resistors and capacitors, thenthese impedances will have nonlinear frequencyresponse characteristics and the amplifier maytherefore be tailored to suit particular frequencyresponse requirements. Various values of resistorsand capacitors for the input impedance and feedbackimpedances can be chosen to provide high-passfiltering, low-pass filtering or other bandwidthmodification in a biophysical measurement system.

334

logarithmicdisplay

If the input impedance and feedback impedanceelements in an inverting amplifier are nonlinearwith respect to voltage or current then theamplifier's gain will be nonlinear with respect tovoltage or current. If the ;ype 3A8 is used with aTektronix Log Adapter (PN 013-0067-00) therelationship between the input voltage and the outputvoltage of the operational amplifier will belogarithmic and thus allows the measurement of highamplitude signaIs mixed with low amplitude signaIs.Thus, biophysical signaIs spanning three voltagedecades can be displayed logarithmically on anoscilloscope.

20.8 INTEGRATION AND DIFFERENTLATION WITH OPERATIONAL AMPLIFIERS

integration

Operational amplifier circuits are particularlysuited to integration and differentiationapplications, that is, where the output voltage isproportional to either the integral or thedifferential of the input voltage.

An operational amplifier used as an integrator isshown in Fig. 20-9. The output voltage isproportional to the integral of the input voltagein volt-seconds during the time interval tl-t2' lnintegration it is necessary to specify this timeinterval, for the output voltage will continue toincrease as long as a positive input voltage ispresent and, when the input voltage is removed, theoutput voltage will remain at a fixed level. lnorder to return this integrator output to zero,allowing the integrator to once more perform usefulintegration, the feedback capacitor of theintegrator must be discharged either by a manualswitch, a reed switch controlled by other circuitryor by the use of the Tektronix Gating Adapter(PN 013-0068-00). This gating adapter is designedfor use with one of the operational amplifiers inthe Type 3A8 and allows repetitive signaIs with anet integral other than zero to be integrated anddisplayed. The use of an integrator in biophysicalmeasurements is further discussed in Chapter 12,Section 12.6, when discussing the electromyogramproduced by a voluntary muscular action. Detailsof a simple gating adapter for use with anintegrator are given in Chapter 29.

differentiation

335

An operational amplifier may be used fordifferentiation as shown in Fig. 20-8B. The outputvoltage is then proportional to the differential ofthe input voltage, i.e., it is proportional to therate of change of the input voltage in volts persecond. Differentiation is rarely used directly inbiophysical measurement systems due to the excessivenoise it produces, however, indirectly it may beused to intensity compensate an oscilloscope displayas discussed in Chapter 21.

The values of Rand C used in integration anddifferentiation circuits depend upon the amplitudeand frequency of the input signal and are usuallyselected experimentally to produce an adequate outputvoltage without attempting to exceed the operationalamplifier's ±25-V output capability.

referencematerial

337

OSCILLOSCOPES

A physiological signal, after having been amplifiedand processed, can be displayed and/or recorded onmany different devices as shown in Chapter 15,Fig. 15-1. The oscilloscope is the most popular ofthese devices and is incorporated within almostevery biophysical instrumentation system. ln manycases, the oscilloscope is the final component inan instrumentation system. However, as anoscilloscope does not provide a permanent record,sorneform of recorder, such as a magnetic taperecorder, a graphic recorder or an oscilloscopecamera, may also be necessary.

This chapter should not be regarded as a generalreference on oscilloscopes. It specifical1ydiscusses the unique characteristics of oscilloscopesas they relate to biophysical measurements. Manycompanion volumes relating to either oscilloscopemeasurement concepts or oscilloscope circuit conceptsare available from Tektronix; a complete list ofthese volumes is given at the end of this book.We would also recommend Typical OscilloscopeCircuitry, published by Tektronix, as a good singlereference on oscilloscope principles. Also, theTektronix catalog of oscilloscopes and associatedinstruments provides a useful reference while listingthe characteristics of aIl of Tektronix' products.This catalog includes a list of US field engineeringoffices and international field offices anddistributors where additional information onoscilloscopes and their applications are available.Tektronix also produces an applications orientedperiodical entitled TEKSCOPE. For regular receiptof this periodical, contact the before-mentionedfield office or distributor in your area.

338

classifications

conventionalscope

slavescope

displaydevice

The three principal components of a simpleoscilloscope are the vertical amplifier, thehorizontal amplifier/sweep generator and the cathoderay tube. These components are discussed in detailin the before-mentioned references and thecharacteristics of amplifiers suited for use withphysiological signaIs are further discussed inChapter 19. An oscilloscope may contain anamplifier suitable for displaying a physiologicalsignal directly from electrodes or transducers(such as the Type 5031) or it may incorporate asimpler amplifier which can be used in conjunctionwith a preamplifier for physiological applications.

It is important to differentiate between threeessentially similar devices that present informationon a cathode-ray tube: the conventionaloscilloscope, the slave oscilloscope and the displaydevice. A conventional oscilloscope is a versatileinstrument incorporating a vertical amplifier anda horizontal amplifier/sweep generator inconjunction with a CRT display. Both the verticalamplifier and the sweep generator's characteristicscan be varied over wide ranges by controls on thefront of the oscilloscope. A slave oscilloscope'sfunction is to provide a conventional oscilloscopewith an additional CRT display. The slaveoscilloscope is coupled to a conventionaloscilloscope in such a way as to present the samedisplay on its CRT as presented on the CRT of theconventional oscilloscope. The coupling between aslave oscilloscope and "master" conventionaloscilloscope is usually achieved with high impedanceprobes limiting the physical distance between twooscilloscopes to 12 feet. A display device issomewhat similar to a slave oscilloscope; however,it is intended to be used in a remote location forthe display of any signaIs on x-y-z coordinates.These signaIs may be derived via low impedanceoutputs from an oscilloscope or they may be derivedfrom other instrumentation systems.

21.1 OSCILLOSCOPE VERTICAL AMPLIFIERS

To preserve linearity and focus on a CRT display,it is necessary to drive both the CRT verticaldeflection plates and horizontal deflection plateswith differential signaIs. The oscilloscopeamplifier in its simplest form may simply amplify aninput signal and present this amplified signal tothe CRT deflection plates as a differential signal.

multitracescopes

339

To obtain various deflection sensitivities the gainof this amplifier must be variable both in discretesteps and over a continuous range. A simpleoscilloscope vertical amplifier, such as theTektronix Type 3A75 Amplifier, accepts an inputsignal into a l-MQ impedance and attenuates thissignal by discrete steps to 50 mV/cm of deflectionon the CRT display. The 3A75 then amplifies thesignal and provides a differential output of +10 V/cmand -10 V/cm to the CRT deflection plates. The CRTthus receives a total deflection voltage of 20 V/cm.

Multitrace amplifiers allow the presentation ofseveral channels of information by switching a CRTdisplay between several amplifiers in such a mannerthat the display appears to a viewer as beingproduced by several separate CRT deflection systemswithin the one CRT. The Tektronix Type 3A3 DualTrace DifferentiaI Amplifier switches or "chops"between two amplifiers at a 0.2 MHz rate. The CRTdisplay thus consists of a sequence of 2.5 ~ssegments from each channel in turne At the maximumsweep speed commonly used for physiological signaIsof 0.1 ms/cm, the two traces on the CRT appear ascontinuous lines since 20 chopped segments occurduring each centimeter of sweep and therefore theseparate segments cannot be discerned. Four-traceamplifiers such as the Type 3A74 incorporateswitching circuitry to switch between four separateamplifiers, presenting a four-channel display on aCRT.

~1.2 OSCILLOSCOPE HORIZONTAL AMPLIFIERS/SWEEP GENERATORS

The oscilloscope horizontal system, as with thevertical system, requires a differential voltage todeflect the CRT beam and is similar to the verticalamplifier with the exception that the variable andstep gain controls are often omitted Ca constantlevel of sweep signal is usually used in conjunctionwith the horizontal amplifier). A sweep generatorprovides a ramp to the horizontal amplifier, thuscausing a linear horizontal sweep across the CRTface. For physiological applications the sweepgenerator ramp duration should be variable from50 seconds to less than 1 ms, providing sweep speedsfrom 5 s/div to 0.1 ms/div. Sorneapplications mayrequire faster sweep speeds; most Tektronix sweepgenerators are capable of sweeping to at least1 ~s/div.

340

A SIMULATED ECG DISPLAYED AT O.5s/DIV

USABLE PERSISTENCE FOR 1.55 MAXIMUM

(A) OSCILLOSCOPE WITH P31 PHOSPHOR

USABLE PERSISTENCE FOR 45 MAXIMUM

(B) OSCILLOSCOPE WITH P7 PHOSPHOR

EFFECTIVELY HAS INFINITE PERS 1 STENCE

(C) STORAGE OSCILLOSCOPE SUCH ASTYPE 564B

NONSTOREDTRIGGERED DISPLAYSHOWING NOISE

SAME S IGNAL---i~AS ABOVESTOREO FROMA SINGLE SWEEP

(0) STORAGE DISPLAYS MAY HELP TO REDUCE APPARENTNOISE ON A TRACE AS SHOWN BY THE NONSTOREDAND STOREO DISPLAYS PRESENTED ABOVE

Fig. 21-1. Persistence and storage.

341

21.3 OSCILLOSCOPE CRT DISPLAYS

persistence

storage

Most conventional oscilloscope CRT displays areintended to be viewed at a distance from 2 feet to6 feet and are presented in a format approximately10 centimeters wide and 8 centimeters high. Speciallarge screen CRT displays are available for viewingat greater distances or for viewing by a number ofobservers.

The persistence of a CRT display refers to the lengthof time that the CRT phosphor continues to emit lightafter the CRT beam has excited the phosphor.Referring to Fig. 2l-lA, the standard phosphorsupplied with many Tektronix oscilloscopes is a P3lphosphor which provides usable persistence for up to1.5 seconds; however, for most practical purposesits persistence is considered to be under 0.5 seconds.Most Tektronix oscilloscopes are available on specialorder with a Type P7 phosphor (see Fig. 2l-lB) whichprovides considerably greater persistence than theP3l phosphor. A storage oscilloscope such as theTektronix Type 564B or Type 5031 effectively providesinfinite persistence (see Fig. 2l-lC) and should beconsidered for aIl oscilloscope applicationsrequiring sweep speeds below 10 ms/div or where theinformation displayed is transient in nature. Thestorage oscilloscope stores a trace on the CRTphosphor until such time as an erase control ismanually activated or activated by a remoteelectrical signal. As shown in Fig. 2l-lD, thestorage oscilloscope may also be useful in place ofa conventional oscilloscope in reducing the apparentnoise on a CRT display.

342

21.4 TYPICAL OSCILLOSCOPES

Type 5031

560 series

Oscilloscopes representative of Tektronix productssuited to biophysical measurements are shown inFig. 21-2, 21-3 and 21-4. The Tektronix Type 5031Dual BearnStorage Oscilloscope shown in Fig. 21-2 isa non-plug-in storage oscilloscope providing ageneral purpose instrument particularly suited tobiophysical measurements. This oscilloscope isreferred to as a dual-beam oscilloscope, having twoseparate vertical deflection systems and two separatepairs of vertical deflection plates within the CRT.This contrasts to the dual-trace amplifier discussedearlier that presents two channels of informationvia a beam switching circuit. Each vertical amplifierhas a maximum sensitivity of 10 ~V/div with adifferential input impedance of 2 Mn and a commonmode input impedance of 0.5 MD (modifiable to 5 Mn).No electrodes or other input coupling items areprovided as standard accessories with this product.

The Tektronix 560 Oscilloscope series shown inFig. 21-3 is representative of the plug-inoscilloscope concept. Both the vertical amplifierand the horizontal amplifier/sweep generatorcharacteristics in the 564B Storage Oscilloscopeand 561B conventional Oscilloscope can be variedover broad ranges by the use of various verticaland time-base plug-in units. Fig. 21-3 shows onlythose plug-in units that are suited to biophysicalmeasurements. The 560 series of oscilloscopesincludes many more vertical and time-base plug-inunits for specialized, non-physiological applications.

The Type 565 Oscilloscope shown in Fig. 21-3 doesnot utilize a time-base plug-in unit; however, itis a dual-beam oscilloscope (having two separateand independent deflection systems) utilizing twovertical plug-in units. A Type 565, when used inconjunction with, for example, two Type 3A74 fourtrace plug-in units, can present a total of eighttraces on its CRT.

SISTABLE SPLIT-SCREEN STORAGE AND CONVENTIONAL DISPLAYSWITH VARIABLE VIEWING TIME AND AUTOMATIC ERASE

SCALE FACTOR READOUT ADJACENT TO THE CRTDUAL BEAM DIFFERENTIAL. EACH BEAM TO 10j.lV!DIVAT lMHzOR 5kHz WITH 100~OOO:1 CMRR

SELECTABLE X-Y MODE AT 10j.lV!DIV

343

ALSO AVAILABLE lN A RACKMOUNTING OR LOW PROFILECABINET CONFIGURATION.

TYPE RS031

ALSO AVAILABLE AS ANONSTORAGE OSCILLOSCOPElN A RACK MOUNTING ORLOW PROFILE CABINETCONFIGURATION

TYPE RS030

Fig.21-2. The Tektronix Type 5031 Dual Bearn Storage Oscilloscope.

344

VERTICAL PLUG-IN UNITS

3A72SINGLE-ENDED INPUTS

3A74 3A75TVPE 3A7~AMI'LlFIER

SINGLE TRACE50rnV/DIV

4MHz

DUAL TRACE10rnV/DIV650kHz

FOUR TRACE20rnV/DIV

2MHz

3A9

. -t • 1-.-

3A3DIFFERENTIAL INPUTS

SINGLE TRACElO~V/D 1V

IMHz100,000:ICMRR

TYPE lA3 ".=_~..~.

"~-:T9..-t.,-- .

3L5

SPECTRUM ANALYZER10~V/DIV10Hz-1MHz

DUAL TRACE100~V/01 V500kHz

50,000: ICMRR

3A8SPECIAL PURPOSE

3C66

DUAL OPERATIONALAMPLI FIERS A 0 ADISPLAY AMPLIFIER

CARRIER AMPLIFIERFOR USE WITHTRANSDUCERS

Fig. 21-3. Part of the Tektronix 560 Series of oscilloscopes.

PLUG-IN OSCILLOSCOPES

564B

VERTICALPLUG-IN

UNIT

TIME-BASEPLUG-IN

UNIT

BISTABLE SPLlTSCREEN STORAGEAND CONVENTIONALDISPLAYS

AVAILABLE WITHVARIABLE VIEWING TIME ANDAUTO ERASE

USES INTERCHANGEABLE VERTICAL ANDTIME-BASE PLUG-INUNITS

4; USES 1 NTERCHANGEABLE VERTICAL ANDTIME-BASE PLUG-INUNITS

561B

VERTICALPLUG-IN

UNIT

TIME-BASEPLUG-IN

UNIT

CONVENTIONALNONSTORAGEDISPLAYS ONLY

TIME-BASE PLUG-IN UNITS

2B67

1~s/DIV-5s/DIV5X MAGNIFIERSINGLE SWEEPLIMITED USE ASAN AMPLIFIER

3B4TYPE 38<1 TlME BAS!'

..._ -_-===O.2~s/DIV-5s/DIV50X MAGNIFIERSINGLE SWEEPAS AN AMPLIFIERO.2V/DIV-5V/DIV

345

DUAL-BEAM OSCILLOSCOPE HAVING TWOVERTICAL AND TWO HORIZONTAL SYSTEMS.

CONVENTIONAL NONSTORAGE DISPLAY ONLY

USES INTERCHANGEABLE VERTICAL PLUG-INUNITS

HORIZONTAL SYSTEMS - 1~s/DIV-5s/DIV10X MAGNIFIERSWEEP DELAYL lMITED USE ASAN AMPLIFIER

565_-

6

MONITORS ECG, EEG OR RELATIVE BLOOD PRESSURE

ALARM ON LOSS OF ECG OR PRESSURE SIGNALBATTERY OPERATED PORTABLE - 12.5 POUNDS

THE TYPE 410 WITH STANDARD ACCESSORIES

OPTIONAL ACCESSORIES FOR USE WITH THETYPE 410 MONITOR

THE TYPE 410 MONITOR POSITIONED AT THE 5ftLEVEL ON AN ANETHESIOLOGIST'S GASMACHINE DEMONSTRATING USE OF THE MOUNTINGSTAND SHOWN AS AN OPTIONAL ACCESSORY

REF OTY DESCRIPTION PART NUMBER

1 2 INSTRUCTION MANUALS 070-0658-002 1 POWER CABLE ASSEMBLY - 7ft 161-0058-003 1 PATIENT CABLE ASSEMBLY - 10ft 012-0120-003A INCLUDES SHEET CLAMP 344-0146-004 1 ELECTRODE RL GREEN - 6ft 012-0121-255 1 " LL RED - 6ft 012-0121-226 1 " RA WHITE - 4ft 012-0121-297 1 " LA BLACK - 4ft 012-0121-208 1 " C BROWN - 4ft 012-0121-219 1 TUBE ELECTRODE PASTE 006-1098-0010 1 PACKAGE 100 ADHESIVE RINGS 006-1099-0011 5 PLATE ADAPTER 103-0079-0012 5 NEEDLE ADAPTERS 103-0108-00

REF DESCRIPTION PART NUMBER

2

3

44A48567891011121314

012-0184-00LIMB LEAD CABLE PACKAGELIMB LEAD CABLE WITH FOURELECTRODES INSTALLED,ELECTRODE PASTE AND ADHESIVERINGS (FOR ECG MONITORINGlN SURGERY)

CHEST LEAD CABLE PACKAGECHEST LEAD CABLE WITH THREEELECTRODES INSTALLED,ELECTRODE PASTE AND ADHESIVERINGS (FOR ECG MONITORINGlN INTENSIVE CARE)

PATIENT CABLE PACKAGESAME AS STANDARD ACCESSORYPACKAGE (ABOVE) lESS POWERCORD AND MANUALS.

PULSE SENSOR ASSEMBLYCONSISTING OF PULSE SENSOR

AND FINGER HOLDERELECTRODE +EEG YELLOW -2Ht

" -EEG Il 2HtBATTERY PACKMOUNTING CUPl'JOUNTINGSTAND8ARE WIRE ADAPTERPLUG - MINI-PHONEPLUG 2 PIN ELECTRODEPlUG 7 PIN AUXILIARYPLUG OUTPUT

012-0183-00

012-0185-00

015-0104-00015-0102-00015-0103-00012-0121-24012-0121-23016-0107-02407-0393-01016-0110-00103-0080-00134-0079-00134-0089-00134-0090-00131-0551-01

Fig. 21-4. The Tektronix Type 410 Physiological Monitor.

Type 410

347

The Tektronix Type 410 Physiological Monitor shownin Fig. 21-4 is a specialized oscilloscope intendedfor monitoring during surgery or in intensive care.The Type 410 provides a minimum of operator controlsto allow its use by nontechnical personnel withinthe hospital environment. A full complement ofelectrodes and a patient cable are provided asstandard accessories with many optional accessoriesavailable as shown in Fig. 21-4.

Full specifications on the oscilloscopes discussedhere a~e given in the current Tektronix catalog ofoscilloscopes and associated instruments.

348

A STANDARD OSCILLOSCOPE DISPLAY OFA O.5ms WIDE PULSE AT A SWEEP SPEEDOF 5ms/cm (NO RATE INTENSIFICATION)

OSCILLOSCOPE DISPLAY OF THE SAMEWAVEFORM AS SHOWN AT LEFT WITH RATEINTENSIFICATION

ANY OSCILLOSCOPE

lNSID Z-AXIS OR INTENSITY-MODULATING SIGNAL

PUTGNAL

[HORil 1DIFFERENT1 ATOR ABSOLUTE-VALUE

VERT vv AMPL 1FIERT <. ~>1 IL l~ f-

VERTICAL Il -:SIGNAL ~+i r1~IJlJJLJlE JlJLIL I1E j 1 J

uÎ 1 ,

IF NEGATIVE INTENSITY MODULATINGSIGNAL IS REQUIRED, REVERSE DIODESlN ABSOLUTE-VALUE AMPLIFIER

(A) PRINCIPLE

LOWER VERTSIG AL OUT~~~----------~

TYPE 3ABOP AMP

OPERATIONAL AMPLIFIER 20PERATESAS AN ABSOLUTE-VALUE AMPLIFIERUSING THE ABSOLUTE-VALUE ADAPTERDESCRIBED lN CHAPTER 29

TYPE 565 OSC OPERATIONAL AMPLIFIER 1 OPERATESAS DIFFERENTIATOR WITH Z. ; .OOl~FAND Zf ; 200kn 1

VERT AMPLPLUG-I NSUCH ASTYPE 3A9

LOWER BEAMCRTG~

(B) A TYPICAL RATE INTENSIFIED SYSTEM

Fig.21-5. Rate intensification.

349

21.5 RATE INTENSIFICATION

differentiation andabsolutevalueampl ification

practicalsystem

The left-hand oscilloscope waveform shown in Fig. 21-5is typical of a pulse display on an oscilloscope.Only the baseline and top of the pulse are displayedon the CRT because during the transitions, the CRTbeam is moving 100 times faster than during thebaseline or pulse top periods. This difference inrate results in a difference in intensity on theCRT display.

ln order for the baseline, the pulse top and thetransitions to be aIl presented on the CRT at thesame intensity, the intensity of the CRT displaymust be made proportional to the rate of change ofthe vertical information. Fig. 2l-5A shows theprinciple of rate intensification. A differentiatorprovides a signal proportional to the rate of changeof the vertical signal. The output on thedifferentiator is, however, direction sensitive andmust be "full-wave rectified" in an absolute-valueamplifier to produce an output proportional to theabsolute rate of change of the vertical signal. Thisoutput is then used in conjunction with the z-axisinput of an oscilloscope to intensity modulate theCRT beam.

The above principle can be applied in practice asshown in Fig. 2l-5B. The CRT photographs shown inFig. 21-5 were taken with this system. Thedifferentiator used consists of one of theoperational amplifiers in a Type 3A8 OperationalAmplifier plug-in unit operating as a differentiator.The differentiator's input impedance and feedbackimpedance are selected by the controls on the frontof the 3A8 at .001 ~F and 0.2 Mn, respectively.The output from this operational amplifier is thenfed to the second operational amplifier on the Type3A8 which is used as an absolute-value amplifier inconjunction with the absolute-value adapter describedin Chapter 29. The output from this absolute-valueamplifier is then fed into the lower-beam CRT gridto modulate the lower beam intensity. Rateintensification is particularly useful whenattempting to photograph a CRT display or when manychannels of information are presented on a singleCRT display.

350

MASTER OSCILLOSCOPE SLAVE OSCILLOSCOPE

y~ULT 1-TRACE

VERT AMPO-----i (1N CHOPPED MODE) 1-.--+ -1

CHOPPED.;IBLANKING

SWEEP('~K1~SWEEP GEN

ANDTRIGGER

HORIZONTALAMPLI FIER x

CHANNEL BANDWIDTH MUST BE SUFFICIENTy TO OISPLAY CHOPPING INFORMATION WITH

NEGLIGIBLE DISTORTION

% CHANNEL BANDWIDTH AND DELAY MUST BE ADEQUATETO PRODUCE EFFECTIVE CHOPPED BLANKING

IF MAXIMUM SWEEP SPEED IS < O. lms/cmX BANDWIDTH OF 100kHz IS ADEQUATE

MASTER OSCILLOSCOPE DISPLAY SLAVE OSCILLOSCOPE DISPLAY

FOUR CHANNELS DISPLAYED lNCHOPPED ~DE WITH SINEWAVEINPUT TO CHANNEL 1

Fig. 21-6. Oscilloscope slaving considerations.

351

21.6 SLAVE OSCILLOSCOPES

additionalCRT display

nondistortingcoupl ing

high cost

The purpose of the slave oscilloscope, as mentionedpreviously, is to provide a conventional oscilloscopewith an additional CRT display. A slave oscilloscopeshould, thus, contain only those controls necessaryto alter the characteristics of the slaveoscilloscope display, that is, intensity, focus andperhaps astigmatism and scale illumination. Althougha slave oscilloscope will contain a verticalamplifier, a horizontal amplifier and a z-axisamplifier, it is unnecessary to vary thecharacteristics of these amplifiers after the slaveoscilloscope has been coupled to the masteroscilloscope; thus, these amplifiers need no externalcontrols. A block diagram of a slave oscilloscopesystem is shown in Fig. 21-6 and the idealrelationship between a master oscilloscope displayand the slave oscilloscope display is also shown inFig. 21-6. Any change in the master oscilloscopedisplay should be perfectly reproduced by the slaveoscilloscope display.

When considering slave oscilloscopes, two basicrules should generally be adhered to. First, therequired signal from the master oscilloscope shouldbe coupled to the slave oscilloscope via highimpedance, low-capacity probes so as not to interferewith the characteristics of the master oscilloscope.Tektronix XlO or XlOO probes are recommended as,even with l2-foot lengths, they provide a load onthe master oscilloscope of 10 Mn and less than 13 pF.Invariably, the master oscilloscope is not designedto allow low-impedance probes or directinterconnection and the use of either will degradethe performance of the master oscilloscope. Thesecond basic rule of slave oscilloscopes relates toeconomy. ln general, the cost of a masteroscilloscope/slave oscilloscope system will be equalto or will exceed the cost of two separateoscilloscopes. Thus, the criteria for using a slaveoscilloscope system should not be based on economybut rather on the desirability of having one set ofoperator controls controlling two CRT displays.

352

vertical andhorizontalslaving

sweep andchoppedblankingslaving

The slave oscilloscope block diagram shown inFig. 21-6 shows both the vertical (y) and horizontal(x) signaIs coupled differentially from the mas teroscilloscope deflection plates to the slaveoscilloscope deflection amplifiers. It is desirableto use differential inputs to eliminate unwantedsignaIs that may appear on the master oscilloscopeamplifier outputs but may not appear on the masteroscilloscope display due to the inherent differentialrejection capability of the deflection plates. Ifa multitrace vertical amplifier is used in themaster oscilloscope vertical, the slave oscilloscopevertical amplifier must have sufficient bandpass toallow the chopped vertical signal to be coupled intothe slave oscilloscope without risetime reduction.For most Tektronix multitrace vertical amplifiers,this necessitates a slave oscilloscope verticalamplifier bandpass considerably in excess of 1 MHz.The bandwidth of the slave oscilloscope horizontalamplifier is noncritical in biophysical measurementsapplications and 0.1 MHz is usually sufficient.

A master oscilloscope display uses intensitymodulation via the CRT grid and/or cathode to turnthe master oscilloscope CRT beam off during sweepretrace and during the interval that the multitracevertical amplifier is switching from one channel toanother. These signaIs are referred to as sweepblanking and chopped blanking, respectively. Thez-axis amplifier must be capable of accepting bothof these blanking signaIs to allow the slaveoscilloscope CRT beam to be turned off insynchronism with the master oscilloscope CRT beam.The chopped blanking signal consists of a narrowpulse occurring during the chopping interval; thisnarrow pulse has a risetime considerably less than1 ~s, thus, a z-axis amplifier bandwidth of severalmegahertz is necessary to allow faithful chopped

differentialversussingle-probecoupl ing

353

blanking of the slave oscilloscope display. If asingle trace vertical amplifier is used, then nochopped blanking signal is involved and only thesweep blanking need be coupled to the z-axisamplifier. Under these conditions the bandwidth ofthe z-axis amplifier would be noncritical forbiophysical measurement applications and, as withthe horizontal amplifier, 0.1 MHz should besufficient.

Various compromises may be made to the basic slaveoscilloscope block diagram shown in Fig. 21-6;however, it is important to fully understand anyperformance degradation associated with thesecompromises. AlI Tektronix oscilloscopes utilizeregulated power supplies and high quality verticaland horizontal amplifiers. Thus, it is notabsolutely necessary to couple the vertical andhorizontal signaIs from the master oscilloscope tothe slave oscilloscope in a differential mode.Negligible distortion will be observed on the slaveoscilloscope display if the signaIs are coupledbetween master and slave with single probes. Itshould be noted, however, that the output from themas ter oscilloscope vertical and horizontalamplifiers will contain a standing DC level and itis necessary for the slave vertical and horizontalamplifiers to be able to cancel out this standingDC level if only single-probe coupling is used.Most Tektronix vertical and horizontal amplifierssuitable for use with slave oscilloscopes incorporatesufficient range in the DC BALANCE control andPOSITION control to cancel out this DC voltage. Itis, however, necessary to fully consider this andto ensure that sufficient position range is .available in the slave oscilloscope to center thedisplay on the slave CRT.

354

BLANKING DIRECTLYCONNECTED TO MASTEROSCILLOSCOPE CRT GRID

"FUZZY" TRACES DUETO LACK OF CHOPPED BLANKING

(A) DIRECT SWEEP-BLANKING INTERCONNECTION. SINGLEPROBES USED TO PICK OFF X AND Y INFORMATION.

INFORMATION DISPLAYEDDURING SWEEP RETRACE DUETO LACK OF SWEEP BLANKING Y

"FUZZY" TRACES DUETO LACK OF CHOPPED BLANKING

(Bl NO Z AXIS INFORMATION. SINGLE PROBES USED TOPICK OFF X AND Y INFORMATION

y

SWEEPBLANKING

X

LOSS OF GAIN AND REGISTRATION ON SLAVE DISPLAYDUE TO DIFFERENCES lN SENSITIVITY AND GEOMETRYBETWEEN SLAVE CRT AND MASTER CRT. SLAVE CRT HIGHVOLTAGE ADJUSTMENT WILL OFFER SOME COMPENSATION.

MASTER OSCILLOSCOPE DISPLAY IS ALSO DEGRADED.•..---- DUE TO CAPACITIVE LOADING REDUCING THE VERTICAL

AMPLIFIER'S BANDWIDTH

(Cl DIRECT INTERCONNECTION OF X, Y AND SWEEP BLANKING BETWEENMASTER AND SLAVE - 12ft LONG UNSHIELDED WIRES.

Fig. 21-7. Oscilloscope slaving compromises.

fuzziness

no sweepblanking

pa ra Ile 1 i ng

355

Many slave oscilloscopes are used with multitracevertical amplifiers in the master oscilloscope butwith no chopped blanking signal coupled between themaster oscilloscope and the slave oscilloscope. Nochopped blanking is, therefore, presented to theslave CRT resulting in a fuzziness on the traces asshown in Fig. 21-7A and B. Due to thecharacteristics of the film used to record thewaveform shown in Fig. 21-7A and B, this fuzzinessdoes not appear to be excessive; however, a viewerwould find the lack of chopped blanking quiteannoying when viewing the slave oscilloscope display.

If a slave oscilloscope system does not incorporatesweep blanking, as shown in Fig. 21-7B, sweep retracecan clearly be observed on the slave oscilloscopeCRT and under certain input signal conditions canbe misleading as weIl as annoying.

Many attempts have been made to create a slaveoscilloscope by simply paralleling an additional CRTwith the master oscilloscope CRT; the results, asshown in Fig. 21-7C, are far from ideal. This systemalso degrades the performance of the masteroscilloscopè as weIl as presenting a poor display onthe slave oscilloscope and is, therefore, notgenerally recommended. Also, as the additional CRTwill not have exactly the same characteristics asthe master oscilloscope CRT, the slave oscilloscopedisplay will never be a direct replica of the masteroscilloscope display.

356

slavesystems

probecoupl ing

Two practical slave oscilloscope systems are shownin Figs. 21-8 and 21-9. The single-channel slaveoscilloscope system shown in Fig. 21-8 uses eithera Type 601 Storage Display Unit or a Type 602(conventional) Display Unit as a slave oscilloscope.The master oscilloscope consists of either a Type56lB (Conventional) ~scilloscope or a Type 564BStorage Oscilloscope with a 2B67 Time Base plug-inunit and any single-channel vertical amplifierplug-in unit. The x and y bandwidth of the slaveoscilloscope is 1 MHz, sufficient for a singlechannel operation. The y input to the slaveoscilloscope is derived via a single probe from theoutput of the vertical amplifier. The positioncontrol within the slave oscilloscope is sufficientto cancel out the standing DC level at the outputof the vertical amplifier. The x input to the slaveoscilloscope is derived in the same manner as for they input. As the system is only intended for singlechannel operation, no chopped blanking is requiredand the z input to the slave oscilloscope must simplyprovide sweep retrace blanking. The z input of theslave oscilloscope is derived via a probe from theblanking pulse output of the time base plug-in unit.ln order to provide sufficient gain range in the 601or 602 display unit, R25 and R75 in these units mustbe shorted out with a wire link.

The Type 601 and 602 Storage Display Units have100,000-ohm input impedance arnplifiers. TektronixP6006 (XIO) and P6007 (XlOO) probes are intended foruse with amplifiers having a l-megohm imputimpedance. These probes can, however, besuccessfully used with the 601 or 602 display unit;the XlO probe actually providing an attenuation ofXIOO and the XIOO probe actually providing anattenuation of X200. It should be noted that thecompensation on aIl probes used in any slaveoscilloscope system must be correctly adjusted.Correct probe compensation adjustment for probesused with the z-axis amplifier may require anadditional oscilloscope to view the z-axis waveformduring adjustment.

357

MASTER OSCILLOSCOPE SLAVE OSCILLOSCOPE

3A9

DIFFERE TIALAMPLIFIER

2B67

TIMEBASE

y

21

564B STORAGE OSCILLOSCOPE

%

21

R25 AND R75 lN THE 601 OR 602 DISPLAYUNIT DEFLECTION AMPLIFIERS TO BEREPLACED BY A SHORT CIRCUIT.

PIN NUMBER ON PLUG-IN

(INTERCONNECTION SOCKETlN 564B OSCILLOSCOPE

21•• TEKTRONIX Xl00 PROBE

12ft LONG - TYPE P6007TEKTRONIX PN 010-0154-00

~ TEKTRONIX Xl0 PROBE12ft LONG - TYPE P6006TEKTRONIX PN 010-0148-00

5648 MASTER OSCILLOSCOPE DISPLAY TYPE 602 SLAVE OSCILLOSCOPE DISPLAY

Fig. 21- 8. A single channel slave oscilloscope system.

358

NONSTORAGE MASTER OSCILLOSCOPE STORAGE SLAVE OSCILLOSCOPE

561B OSCILLOSCOPEt

213A74

4 TRACEVERT ICAL

AMPLIFIER*

2B67

TIMEBASE

13

21

+x -x

17

*ANY PLUG-IN TYPES DESIGNED FOR USEWITH TEKTRONIX 560 SERIES OSCILLOSCOPES~1AYBE USED

tMAY ALSO USE TYPE 564BSTORAGE OSCILLOSCOPE

PIN NUMBER ON PLUG-IN

(

1NTERCONNECT 1ON SOCKETlN 561B OSCILLOSCOPE

21 TEKTRONIX Xl00 PROBE- •••. - 12ft LONG - TYPE P6007

TEKTRONIX PN 010-0154-00

TEKTRONIX Xl0 PROBE"ClCl- 12ft LONG - TYPE P6006

TEKTRONIX PN 010-0148-00

TEKTRONIX ~2 ATTENUATOR"INPUT NORMALIZER"TEKTRONIX PN 067-0541-00

4501 SCAN CONVERTER DISPLAY

NOTE: THE TYPE 4501 SCAN CONVERTER PROVIDES EITHER A NONSTORAGE OR A STORAGE DISPLAY.

WHEN USING THE TYPE 4501 TOPROVIDE A STORED DISPLAY, THE4501 PROVIDES THIS DISPLAY lNVIDEO FORMAT FOR DISPLAY ON ATELEVISION RECEIVER OR MONITOR.REFER TO CHAPTER 23.

Fig, 21·9. A multichannel slave oscilloscope system.

Type 4501slavesystem

359

A multichannel slave oscilloscope system is shownin Fig. 21-9. The Type 4501 Scan Converter Unit isbasically a five-inch storage display unit havinghigh input impedance (1 MD, 47 pF) differential inputinto the x, y, and z axes. The bandwidth of the xand y amplifiers is 10 MHz and the bandwidth of thez-axis amplifier is 5 MHz. The x, y, and z amplifiercharacteristics of the Type 4501 make it particularlysuited as a slave oscilloscope as these amplifiercharacteristics exceed aIl of the desirable criteriadiscussed earlier in this section. This system shownin Fig. 21-9 is self-explanatory; either XIO or XlOOprobes are used to couple the necessary signaIs fromthe mas ter oscilloscope to the slave oscilloscopeand, where necessary, Tektronix "input normalizers"are used to provide an additional 2X attenuation.The x and y signaIs are coupled differentially betweenthe master oscilloscope and the slave oscilloscopeand the differential z-axis amplifier is actuallyutilized as a summing amplifier to add together themaster oscilloscope chopped blanking waveform andsweep retrace blanking wave f orm,

360

The Type 4501 Scan Converter Unit, as weIl as beingbasically a five-inch storage display unit, caninternally scan information stored on the storageCRT and present this information on bright, largescreen, TV monitors or receivers. The Type 4501 ScanConverter Unit is discussed in greater detail inChapter 23.

Fig. 21-10. A complete four-channel slave oscilloscope.

stimulation

361

PULSE GENERATORS AND STIMULATORS

As covered in various chapters in Section II ofthis book, the passage of electric current throughvarious types of cells may cause these cells todepolarize and to generate an action potential. Thispassage of electric current is normally achievedbiologically by a nerve impulse propagated from withinthe central nervous system; however, it may beartifically produced by a stimulator. The stimulatormay be a self-contained instrument or it may becontrolled by a separate pulse generator. For thepurpose of this discussion of stimulus systems thepulse generator portion will be defined as that partthat generates the pulse waveshape required forstimulation and the stimulator will be defined asthat part that amplifies the signal to a levelsufficient for artificial biological stimulation.Typically, a pulse generator may generate a 10 voltpulse; this pulse is then amplified within thestimulator to 100 volts or more for stimulation.

362

1 PUTTRIGGER

OR\1A'JUA

TRIGGER

PULSE GENERATOR

'- ~------------STIMULATOR

~-

~ P,-~7,~A'IEFO~·1 PU SE PULSE V ~GE GE GE .J -

--t----- ~ f--L @ OUT 1 OUT 2 OUT 3 ST 1 ~/ULUS

PUS'"! ISOLATIONUNI r

OUr S

ItPUT TRISGER_k. ___jl...___

.•.. RAr~P__ ~~RATE

TIMING RAMP CONTROLS~SUBSEOUENT PULSE

GENERATING cUNCTIONS

OUTPUT 2

n nlpuLsE_j IL.. ..J AMPL 1TUDE

----J••I4I..•.~ - ~ l''_ZERO PULSE

PULSE DELAY DURATION

OUTPUT 3 LJ"""-----.-j r+-

PULSEDURATION

LJ,.._PULSEDELAY

OUTPUT 4 & 5

(COMBINED OUTPUTS2 A'.JD3l

lTOTAL OUTPUTAMPLITUDE

Fig. 22-1. A typical physiological stimulus system.

+ OR - UP TO SOYOUT AT UP TO SOmALOADING WITH RESPECTTO GROU ND

tGROUNDEDOR ISOLATEDSTIMULATOR USED

~+ OR - UP TO SOmAOUT AT 100Y COMPLIA CE(ISOLATED FROM GROU Dl

363

22.1 STTIMULUSSYSTEMS

typicalsystem

A typical physiological stimulus system is shown inFig. 22-1. The pulse generator section of thisstimulus system uses a ramp generator to control twopulse generators. These pulse generators aretriggered at discrete ramp levels, thus providingdelay between the initiation of the ramp and theinitiation of the pulse. With the system shown inFig. 22-1, the ramp can either be generatedrepetitively, generated via an external triggersource or can be initiated manually from a pushbutton on the waveform generator. The output pulsesfrom the pulse generators are combined to produce 2positive pulses or a positive/negative pulse pair.

A stimulus system suited to most biophysicalmeasurement requirements may have the fo1lowingcharacteristics:

PULSE GENERATION

Double pulse capability with each pulseindependently controlled.

Pulse widths from .01 ms to 300 ms.

Pulse rise- and falltimes <1 ~s.

Pulse repetition rate, controlled by ramprate, from 1,000 Hz to 0.1 Hz with singlepulse capability.

Delay between pulses in a double-pulseformat from ~O to 300 ms.

Output pulses from constant current sourcesto allow mixing. Current amplitude andvoltage compliance must be compatible withpower operational amplifiers and stimulusisolation units. Usually la mA withcompliance to 10 volts is adequate.

STIMULATION VIA POWER OPERATIONAL AMPLIFIER

Input configuration to allow summation ofeither voltage source or current sourceoutputs.

Slew rate of >50 V/~s with feedback elementsadjusted for maximum output from a 1 voltor 10 mA input pulse.

364

brainstimulation

Up to ±50 V/±50 mA capability as either avoltage source or as a current source.

STIMULATION VIA STIMULUS ISOLATION UNIT

Tristable output capability for stimulationwith pulses having positive and negativecomponents.

Isolation between output and source of <5 pF,>lolOn and between output and ground of<30 pF, >lolOn.

Up to ±100 V/±50 mA capability as either avoltage source or as a current source.

Open cortical stimulation (stimulation of the exposedbrain) requires considerably less stimulus energythan peripheral stimulation (stimulation of the armsand legs). Cortical stimulation requires from 0.1to 10 V/O.l-IO mA while peripheral stimulationrequires from 10 to 200 V/1-50 mA. Cortical stimulusenergy should be maintained below 1 watt peak and0.3 ~coulombs to avoid thermal and/or electrolyticinjury to brain cells. Peripheral stimulationenergy should be maintained below pain thresholdswhenever possible.

22.2 TEKTRONIX 160 SERIES PULSE GENERATORS

Type 162operation

The Tektronix Type 162 Waveform Generator shown inFig. 22-2 may be operated in one of three separatemodes as shown in Fig. 22-3. When operating in therecurrent mode, the ramp will immediately resetitself to zero after completing a ramp cycle andbegin a second rampe This process will continueindefinitely. When the Type 162 is operated in thegated mode, the ramp generator will free-run forthe duration of the input gate pulse, a rampbeginning when the input gate pulse goes positiveand the last ramp being completed after the gatepulse again returns to zero. With the Type 162operating in the triggered mode, the ramp generatormay be triggered from external positive triggerpulses or by internaI trigger pulses generated bymanually depressing the trigger push-button on thefront of the generator. One ramp is generated foreach input trigger, thus, if only one input triggeris received, or if the front panel manual triggerpush-button is depressed only once, a single rampwill be generated.

l'YH '"_'"'flot. tA/~UAIO.

•GENERATES A TIM-1NG RAMP FROM100115TO 105DURATION

"'1 !io'l'VI Sf $(I'IIUi'O*

----- .

•GENERATES + OR -OUTPUT PULSE OFUP TO 50V WITHRISE- AND FALLTIMES '"1115.DURATION VARIABLElOllS TO 0.15

tTH ,.$I"UlSfGfHlt.MOr

..._--..._- ...•

GENERATES + OR -OUTPUT PULSE OFUP TO 25V WITHRISE- AND FALLTIMES", 0.2115.OURATION VARIABLElllS TO 10ms

•OISPLAY UNITY = 0.05-50V/DIVX = 11-15V/01 VZ = SOV

365

•POWER SUPPLYFOR 160 SERIESGENERATORS

Fig. 22-2. Tektronix "160" Series Pulse Generator system.

(Al RAMR GENERATOR FREE RUNS

__jEXTERNAL GATE INPUT

~PULSE OR "MANUAL"PUSH-BUTTON DEPRESSED

(S) RAMP GENERATOR GATED

_l._ ________,~___,l EXTERNAL TRIGGER INPUT~ PULSES OR TR1GGER 1NTERNALLY

GENERATED EACH TIME "MANUAL"PUSH-SUTTON IS DEPRESSEO

RAMP RATE

(el RAMP GENERATOR TRIGGERED

Fig, 22-3. Timing ramp control.

366

Type 161operation

____~l~ ~l~ ~l _INPUT TRIGGER

___ ---'n._ __ __.nL-__ ~n!_____,l.__~~p•.••L~~TUDE~ ~ t OUTPUT PULSE CAN BEPULSE POSITIVE OR NEGATIVE

DURATION

(A) PULSE GENERATOR TRIGGERED FROM INPUT TRIGGER

INPUT RAMP

DELAYPULSE SHOWN DELAYED BY= 50% OF RAMP DURAT ION

(B) PULSE GENERATOR CONTROLLED BY INPUT RAMP

____~l~ ~l ~l _INPUT TRIGGER

INPUT RAMP

PULSE INITIATED BY TRIGGER'--_ INPUT AND BY RAMP INPUT

AFTER DELAY(C) DOUBLE-PULSE GENERATION

Fig. 22-4. Output pulse control.

The Type 161 Pulse Generator is shown in Fig. 22-2and its various operating modes are shown inFig. 22-4. The pulse generator may be triggeredfrom an external positive trigger source, in whichcase the pulse output will begin when the triggerpulse goes positive. The pulse generation cycle maybe initiated by an input Tamp. Level-sensitivetrigger circuitry within the pulse generatortriggers at a precise level on this ramp and therebyprovides a delay corresponding to the time taken forthe ramp to reach the preset triggering level. Thepulse generator may be triggered by both an inputtrigger and an input ramp, thus providing doublepulse generation.

Types 163,360 and160A

367

The Type 163 Pulse Generator shown in Fig. 22-2 issimilar to the Type 161 but provides a loweramplitude output pulse with faster rise- andfalltimes. The Type 360 Indicator is basically adisplay device; its primary purpose being to displayoutputs from the 160 Series to simplify output pulsewaveform adjustment. The Type l60A Power Supplyprovides regulated power to other 160 Series modules.

22.3 HIGH E OR I OUTPUT VIA A POWER OPERATIONAL AMPLIFIER

constantcurrentandconstantvoltage

The output level from most pulse generators isinsufficient for tissue stimulation, therefore, someform of pulse amplification is required between thepulse generator and the stimulating electrodes. Apower operational amplifier, as discussed inChapter 20, is ideally suited to this application.

Chapter 12, Section 12.1, discusses thecharacteristics of stimulators required for EMG useand points out that two different forms ofstimulators are currently in use -- constant voltagestimulators and constant current stimulators.Although tissue can be stimulated with either aconstant voltage source or a constant current source,it is difficult to correlate results obtained whenusing these different stimulating sources. Thus,while both systems are entirely adequate, theclinical electromyographer or physiologist willnormally have a preference for either constantvoltage or constant current stimulation, thispreference probably being influenced by his earliermedical training.

368

Jl n Rf

RIEl

R2----u--u- E2

R3~ E3

INPUT PULSES FROM VOLTAGE 1_ejç_. __ JSOURCES

IF INPUT PULSES ARE FROM CURRE TSOURCES, OMIT RI, R2 AND R3 (EQUAL ZERO)

eO=Rf(II+I2+I3)Rf CQNTROLS TOTAL OUTPUT VOLTAGEAND RI, R2 AND R3 CONTROL PROPORTIONOF EACH INPUT SIGNAL lN THE OUTPUT

(Al VOLTAGE SOURCE OUTPUT

1 PUT PULSES FROM VOLTAGESOURCES

_Il ru.,IF 1 PUT PULSES ARE FROM CURRENTSOURCES, GROUND E2'NPUT TERMINAL,

SUM ALL CURRENTS INTO El INPUT TERMINAL

AND ADD Re FROM El INPUT TO GROUNDVALUE OF R = R

C 5

10 = -(:r)(II + 12 + ..• )

R

(9)CURRENT SOURCE OUTPUT

Fig. 22-5. Pulse combination and amplification with a power operation al amplifier.

Il - -'---=-r---~-=--~---="""""ll~PLAST IC CASE":' BATTERY POWERED":' oc/oc CONVERTER

1 1

+ OR - 1INPUT PULSES ;+JFROM CURRENT 1

SOURCE1

":' L __

OUTPUTCIRCU 1TRY 1--'---...•.

+ OR - CURRENTJ----,:---l~ OR

VOLTAGE SOURCEOUTPUT

_ _j

~COUPLI G FROM

OUTPUT TO 1NPUT" 5pF > lOi 00

COUPL 1NG FROM ~OUTPUT TO GROUND'" 30pF > 101°0

Fig 22-6. Equivalent circuit of a stimulus isolation unit.

constantvoltage

constantcurrent

369

A power operational amplifier used in a voltagesource configuration for constant voltagestimulation is shown in Fig. 22-5A. The input tothe operational amplifier can be derived from eithervoltage sources or current sources and the outputfrom the operational amplifier exhibits essentiallyconstant voltage characteristics with an outputimpedance of considerably less than 10 ohms in mostpower operational amplifiers. Power operationalamplifiers suitable for tissue stimulation shouldhave an output capability of at least 50 V at 50 mA.It should be noted that the operational amplifierconfiguration shown in Fig. 22-5A inverts the outputsignal with respect to the input; in most instances,reversaI of the stimulating electrodes effectivelyinverts the stimulating signal back to its originalpolarity.

A power operational amplifier used in a currentsource configuration for constant currentstimulation is shown in Fig. 22-5B. Since anoperational amplifier inherently provides a voltagesource output, a current sensing resistor (Rs) isused to detect the output current and to modifyfeedback around the operational amplifier to keepthis current constant. Inputs from either voltagesources or current sources may be used. The outputexhibiting constant current characteristics shouldhave an output impedance typically in excess of0.1 M~. An operational amplifier suitable forconstant current stimulation should have an outputcapability of up to 50 mA with a voltage complianceto at least 50 V.

22.4 HIGH E OR I OUTPUT VIA A STIMULUS ISOLATION UNIT

characteristics

A stimulus isolation unit provides amplification ofan input pulse or pulses and isolates the outputpulse from both ground and the input pulse source.The reasons for output pulse isolation are coveredin detail in Chapter 12, Section 12.2. An equivalentcircuit for a typical stimulus isolation unit isshown in Fig. 22-6. Since stimulus isolation may beused in applications requiring high amplitudestimulation into a relatively high impedance (suchas in stimulation of the extremities using electrodeson the surface of the skin), the output from astimulus isolation unit should have a greater voltagecapability with a somewhat lower current capabilitythan the output from a power operational amplifier.

370

bistableandtristable

A typical stimulus isolation unit operating in aconstant voltage stimulation mode may produce avoltage output of up to 100 V with a currentcapability of 50 mA. Similarly, when operating in aconstant current stimulation mode, it may provide acurrent output of up to 50 mA with compliance up to100 V.

Stimulus isolation units are typically bistable ortristable devices. A bistable stimulus isolationunit offers either positive or negative outputs,having only two stable states -- output on and outputoff. A tristable stimulus isolation unit offerssimultaneous positive and negative outputs, havingthree stable states -- output on positive, outputoff and output on negative. With a tristable stimulusisolation unit, the current or voltage provided bythe "output-on-positive" pulse can be controlledindependently from the current or voltage providedby the "output-on-negative" pulse.

22 • 5 CARnlAC PACHv1AKERS

As discussed in Chapter 2 when dealing with theheart and the circulatory system, the steady rhythmof the heart is maintained by a biological"pacemaker" within the sinoàtrial node of the heart.Failure of this pacemaker will cause the pumpingaction of the heart to be interfered with, causinga seizure and possible death.

Emergency resuscitation from a seizure can beaccomplished by external electrical stimulation or

heartstimulation

battery1ife

371

cardiac massage of the heart by trained medica1personnel. Since in many subjects seizures areunpredictable, continuous stimulation is requiredto ensure continuous reliable operation of the heart.Long term stimulation from an external electricalsource requires a considerable leve1 of stimulatingcurrent to ensure that a sufficient 1eve1 of currentpasses direct1y through the heart, thus causingconsiderable pain to the subject. This pain may beavoided by using external electrical stimulation viainternaI electrodes placed directly on the heart;however, tissue irritation and rejection present aproblem that is difficu1t to overcome.

The modern cardiac pacemaker concept avoids tissueirritation and rejection by surgica1ly implantinga pacemaker within the subject's abdomen and .connecting it to the heart via internaI electrodes.Modern internaI pacemakers use mercury batterieswhich can operate the pacemaker continuously for ayear or more. Since the pacemaker is placed in thesubject's abdomen rather than in the heart, arelatively unsophisticated surgical procedure isnecessary on a routine basis to replace the pacemakerwith one having a fresh battery. The problem ofdetermining just when an internaI pacemaker's batteryis nearly discharged, and thus, when the pacemakermust be replaced, has not yet been comp1etely solved;however, many researchers have proposed varioustechniques for determining pacemaker end-of-life byanalyzing the pacemaker output pulse as recorded onthe surface of the body. At this stage there doesnot appear to be sufficient evidence in favor of anyone method for it to be universally accepted.

372

pacemakeroutput

The internaI cardiac pacemaker consists of atransistorized blocking oscillator producing pulsesof approximately 10 V in amplitude, a fewmilliseconds in width and at an approximate 60-perminute rate. The equivalent circuit for a pacemakerand subject, together with a photograph of acommercial pacemaker, is shown in Fig. 22-7.

PACEMAKER SUBJECT~~

",,70011

~---=,_W 1RES CONNECT PACEMAKEROUTPUT TO ELECTRODESATTACHED TO THE HEART

EQUIVALENTCIRCUIT

PACEMAKER SURGICALLY1MPLANTED 1N SUBJ ECT 1 SABDOMINAL CAVITY

ELECTRODE WIRES CAN BEDISCONNECTED FROM PACEMAKERTO ALLOW PACEMAKER TO BEREPLACED BY SIMPLE SURGERY

TYPICAL ECG FROM A SUBJECT WITHAN IMPLANTED CARDIAC PACEMAKER

MYOCAROIALELECTRODE

WIRES SELF-LOCKINGSEALEOCONNEC TORS

PACEMAKER6.4 x 7.2 x 2.0cWEIGHT = 1809

A COMMERCIAL PACEMAKER PRODUCEO BYELECTRODYNE COMPANY INC., MASS., USA

Fig.22-7. The implantable cardiac pacemaker.

373

ECG SIGNAL 1NFOR SV CHRONIZED USE

ENERGV

Ô •PUSH TOCHARGE

SV CHRO- ce 1NSTANNIZED TANEOUS

~LARGE LLE ELECTROOES ALLOI< ",GH CUR"'"TSTO BE PASSED THROUGH SUBJECT WITH LOW CURRENTDENSITY AT THE ELECTRODE SITES. ALSO PROVIDESLOW ELECTRODE-SUBJECT RESISTANCE.

POWERLINE 1-+--.•Ims -~

ELECTRODESON SUBJECT

~~)VACUUM RELAY CONTROLLED C ~FROM DISCHARGE BUTTON ORSVNCHRONIZING CIRCUIT

Fig. 22-8. The De defibrillator.

22.6 CARDLAC DEFIBRILLATORS

ventricularf i b ri 11at ion

watt-secondshocks

Resuscitation from either a heart seizure or fromventricular fibrillation can be accomplished byexternal electrical stimulation. Ventricularfibrillation is produced within the human heart dueto a variety of causes, including accidentalelectrocution. When the heart is in ventricularfibrillation, individual portions of the ventricularmuscle contract independently instead ofsynchronously and effective output of blood ceases.External stimulation can be achieved with a cardiacdefibrillator which essentially consists of acapacitor charged to several thousand volts which isthen discharged through the subject via largesurface-area "paddle" electrodes as shown inFig. 22-8. The energy produced within a cardiacdefibrillator is normally measured in watt-seconds

374

or joules and is equal to 0.5 CE2, (C is the valueof the storage capacitor and E is the voltage levelto which it 1s charged). Defibrillation shocksfrom 200 to 400 watt-seconds are normally requiredto achieve satisfactory cardiac defibrillation. NotaIl of this energy will be available for dissipationat the subject as the efficiency of discharge willbe considerably less than 100%; typically 20% to 70%.

The cardiac defibrillator may be operated in one oftwo modes: instantaneous or synchronized. When

instantaneous operated in the instantaneous mode, the energy frommode the charged capacitor is discharged through the

subject when the discharge button (usually locatedon one of the hand-held paddle electrodes) isdepressed. It has been found that cardiacdefibrillation is most efficient if thedefibrillating pulse occurs during the falling partof the ECG R-wave and that defibrillation can bedetrimental to sornesubjects if it occurs during theT-wave.

synchronizedmode

cardioversion

With a cardiac defibrillator operating in thesynchronized mode, the discharge pulse is notimmediately applied to the subject after the dischargebutton has been depressed but is delayed to occurduring the falling part of the following R-wave. Useof a defibrillator in the synchronized modenecessitates an ECG signal being applied to thedefibrillator for synchronizing purposes. Thus, adefibrillator can only be used in the synchronizedmode if the ECG signal generated by the subject isof sufficient quality to allow the synchronizingcircuit within the defibrillator to detect theR-wave. Many cardiac disorders produce abnormalECG signaIs having detectable R-waves. Thesedisorders may often be remedied by using adefibrillator in the synchronized mode, thedefiprillator pulse forcing the heart to revert toa normal operating rhythm. This process is known ascardioversion.

displaydevicesversusosc i 1loscapes

375

DISPLA y DEVI CES AND INDICATORS

Bath display devices and indicators are used tavisually display the output from a biophysicalinstrumentation system. The display device utilizesa cathode-ray tube as the display medium, theindicator uses sorneother visual device such as apanel meter or a numerical readout device such asNixie tubes. The following material discussesdisplay devices from a biophysical measurementsviewpoint only; therefore, we would recommend acompanion volume entitled Information DisplayConcepts published by Tektronix, Inc. as a generalreference source for display devices.

A display device may appear ta be simply anoscilloscope, and indeed, an oscilloscope is oftenused as a display device. Display devices, however,generally differ from oscilloscopes as the CRTdisplay within a display device is optimized fordisplay characteristics (resolution, contrast,brightness and screen size) whereas, in contras~, theCRT display within an oscilloscope is optimized formeasurement capability (vertical "amplifier bandwidthand writing rate). A display device is intended tabe used within a specific instrumentation system andthus has no external contraIs ta change thecharacteristics of the display. An oscilloscope may,however, contain numerous external contraIs ta changethe characteristics of the display ta suit thedesired measurement requirement.

376

5in CRT UNITS 8cm x 10cm

TYPE 601 STORAGE DISPLAY UNIT

x AND Y - 1V FULL-SCREEN SENSITIVITY100kHz BANDWIDTH

- 100kn 50pF INPUT RCz-AXIS - BISTABLE: ON > 1V OFF < 0.5V

- > 100kHz EFFECTIVE BANDWIDTH- 100kn 50pF INPUT RC

CRT - BISTABLE STORAGE- 12.5 STOREO LINE-PAIRS/cm

RESOLUTION- 5cm/ms WRITING SPEED

TYPE 602 DISPLAY UNIT

x AND Y - 1V FULL-SCREEN SENSITIVITY- 1MHz BANDWIDTH- 100kn 30pF INPUT RC

z-AXIS - LINEAR: ON 1V RANGING TO OFF OV- 1MHz BANOWIOTH- 100kn 70pF INPUT RC

CRT - NONSTORAGE. P31 OR P7 PHOSPHOR- < .014 INCH TRACE WIDTH

(A)

1lin CRT UNIT 16.2 x 21cm

TYPE 611 STORAGE OISPLAY UNIT

x AND Y - 1V FULL-SCREEN SENSITIVITY100kO 60pF INPUT RC

z-AXIS - BISTABLE: ON > 1V. OFF < 0.5V> 100kHz EFFECTIVE BANDWIOTH

- 100kn 50pF INPUT RCCRT - BISTABLE STORAGE

- 19 STORED LINE-PAIRS/cmRESOLUTION

- 25cm/ms WRITING SPEED

TYPE 611 MOO 162C - HORIZONTAL FORMAT

SCAN CONVERTER - 5in CRT TO 14in OR 17in TV MONITOR

377

TYPE 4501 SCAN CONVERTER UNIT

x AND Y - 1V FULL-SCREENSENSITIVITY

- 10MHz BANDWIDTH- lMQ 47pF INPUT RC

z-AXIS - LINEAR OR LIMITING:ON lV. OFF OV

- 5MHz BANOWIDTH- 1MQ 47pF INPUT RC

CRT - BISTABLE STORAGE- 12.5 STOREO LINE

PAIRS/cm RESOLUTION- 5cm/ms WRITING

SPEED

TYPE 4501 WITH 17in CONRAC MONITOR

(B)

Fig. 23-1. Tektronix display units.

23.1 TEKTRONIX DISPLAY UNITS

displayunitcharacteristics

Tektronix currently produces four display units -the Type 602 Disp1ay Unit, the Types 601 and 611Storage Display Units and the Type 4501 ScanConverter Unit. These four products are shown inFig. 23-1 together with a brief summary of theirprinciple characteristics. AlI Tektronix displayunits require one volt of x and y input for fullscreen deflection horizontally and vertically andrequire one volt of z-axis input to turn the CRTbeam on and off. As these x, y and z sensitivitiesare on1y adjustable over a relative1y narrow range,

378

Types 601and 602

Type 611

it is necessary to externally amplify or attenuatesignaIs to a level consistent with the sensitivityof the display unit before coupling such signaIs tothe display unit. Tektronix display units may beused to provide an additional CRT display for aconventional oscilloscope, that is, in a slaveoscilloscope configuration as discussed in Chapter21.

The Type 602 Display Unit utilizes a nonstorage,high resolution, CRT providing a display area of8 cm vertically by 10 cm horizontally. The Type601 Storage Display Unit has similar characteristicsto Type 602 Display Unit and incorporates a storageCRT to allow displayed information to be stored asdiscussed in Chapter 21. Erasure of storedinformation can either be accomplished manually witha push-button on the front of the Type 601 or may beaccomplished by an electrical signal.

The Type 611 Storage Display Unit provides four timesthe display area of the Type 601 by utilizing anIl-inch storage CRT. The display area of the Type611 is 16.2 cm horizontally by 21 cm vertically; theinstrument is also available in a horizontal formatproviding 16.2 cm vertically and 21 cm horizontally.A stored dis~lay on the Type 611 may be eithererased by a push-button or remotely by an electricalsignal.

Type 4501

379

The Type 4501 Scan Converter Unit (see Fig. 23-lB)is a unique instrument providing a large screenCRT display via a television system. The Type 4501Scan Converter may basically be considered as a5-inch storage display unit (similar to the Type601) coupled with a television system to displayinformation stored on the scan converter's CRT on alarge screen television monitor or a commercialtelevision receiver. The bright displays achievedvia scan conversion are ideal for individual orgroup viewing under high ambient light conditions.

Readout from the storage CRT is accomplished byscanning the CRT storage target with a TV raster.AlI necessary scanning, sync, and video circuitsare contained in the scan converter, providing acomposite TV signal of the information stored on theCRT.

The TV signal is available as either ElA 525-line,60-field format or in CClR 625-line, 50-field formatat 1 volt P-P. This signal can be used inconjunction with a studio-quality TV monitor forlarge screen display. Modulated RF at 55.25 MHzthrough 67.25 MHz is also produced to allow the useof commercial TV receivers for a display.

380

STORED DISPLAYON SCAN CONVERTER CRT

OISPLAY MAY BE REPROOUCED TOANY SIZE VIA TV MONITORPROJECTION SYSTEM OR TV MULTI-CHANNEL OSCILLOSCOPE DISPLAYS

COMPUTER GENERATEO DISPLAYS

SUBJECT ECG OVER ONE MINUTE(REFER TO FIG. 23-4)

Fig. 23-2. Type 4501 Scan Converter capabilities.

variety ofinformation

381

As shown in Fig. 23-2, the Type 4501 Scan Converteror any other display unit can be used to presentmany different types of information. The Type 4501Scan Converter's application as a slave oscilloscopeis discussed in Chapter 21. A typical four-channelslave oscilloscope display on a television monitoris shown in Fig. 23-2 together with a computergenerated display of a subject's variation intemperature over a 36-hour period and a steppedtrace display of a subject's ECG measured over a oneminut.e period using the system discussed later inthis chapter.

382

THE RESOLUTION OF AN OSCILLOSCOPE DISPLAY IS MEASURED lNLINE-PAIRS AS SHOWN

OSCILLOSCOPE DISPLAY1 LINE-PAIR l ----

(1 LINE + 1 SPACE~ _ _ _

SCAN ON A TYPE 602 DISPLAY UNIT CRT SHOWING LINE-PAIRRESOLUTION AND EFFECT OF CLOSER SCANNINS

25-LI NE/cm SCAN

RESOLUTION IS 25LINE-PAIRS/cm ASLINE AND SPACE ARETHE SAME WIDTH

36-LINE/cm SCAN 50-LINE/cm SCAN

SCAN AT TWICE LINEPAIR RESOLUTION RATEJUST MERGES INTO NONRESOLVABLE SCAN

TOP LEFT-HAND QUADRANT OF HIGH RESOLUTION DISPLAYS ONTEKTRONIX DISPLAY UNITS SHOWING RESOLUTION UNIFORMITY

TYPE 601 STORAGE TYPE 602 TYPE 611 STORAGE

Fig.23-3. Resolution of Tektronix display units.

383

23.2 RESOLUTION

1ine-pcirsversusscan 1ines

determiningresolution

spot size

The principal difference between the CRT display onan oscilloscope and a display unit is the increasedbrightness and resolution obtainable with a displayunit as shown in Fig. 23-3. The resolution of adisplay unit or oscilloscope CRT display isspecified in line-pairs, one line-pair being equalto one bright written line and one space of equalwidth. This specification should not be confusedwith scanning resolution specified in Zines asassociated with television systems; in such systemstwo scan lines are required to produce one line andone space.

A scan of 25 lines/cm on a Type 602 Display Unit isshown in Fig. 23-3. From an observation of thisdisplay it appears that the width of the dark spacebetween these Zines is equaZ to the width of theZines, thus it can be said that this display has aresolution of 25 line-pairs/cm. If the scanningdensity is now increased to 36 lines/cm as shown onFig. 23-3, the individual scanning lines can stillbe clearly discerned; however, it is apparent thatthe width of the scanning lines is somewhat greaterthan the width of the dark space between the scanninglines. At a scanning rate of 50 lines/cm the darkspace between the scanning lines begins to disappearand the individual scanning lines begin to mergeinto a nonresolvable scan. The resolution of thisparticular CRT could be quoted as 25 line-pairs/cmor, when dealing with a television type display, ashaving a resolution capability of 50 lines/cm.

The spot size of the before referenced display iscalculated to be .008 inches as the display is capableof 25 line-pairs/cm, that is,·50 spots/cm. It shouldbe noted that the spot size specified for theTektronix Type 602 Display Unit is .014 inches,almost twice the spot size calculated above. Spotsize will vary from one CRT to another; it will alsovary slightly over the total display area of the CRTand varies considerably with the intensity of thedisplay. A1though the typica1 spot size for a Type602 Display Unit may be .008 inches, thespecification of .014 inches represents the maximumspot size that would be encountered at any point onany Type 602 CRT when displayed at a relatively highintensity level.

384

+IV~ SWEEPOV

+ 1 V .-----,......,OV_j U L

:r

BLANKING

MONITOR

y

TYPE 611

ERASE

SELF-RESETIINGSTA 1RSTEP GENERA TOR 1-- ----'

TRIGGERED FROMll-AXIS INPUT

(Al USING THE TYPE 611 lN A SYSTEM

564B STORAGE OSCILLOSCOPE REMOTE-ERAS

D SIGNALS FOREACH CRT HA

PHYSIO-LOGICALSIGNAL 3A6 3B4

INTO CHIDUAL-TRACE TIME BASEVERTICAL (TRIG ON(ADD MODEl CHI SIG) SWEEP

STAIRSTEP GATEINTO CH2 OUT

'--- SELF-RESETIING16-STEP GE ERATOR

TRIGGERED FROMSWEEP GATE

DISPLAY JUST BEFOREE THE END OF THE

EIGHTH SWEEP

REFER TO CHAPTER 27 FOR DETAILSOF THE STEP GENERA TORS

LF

LOWER HALF OF THE {CRT ERASES JUSTPRIOR TO THE STARTOF THE FIRST SWEEP

UPPER HALF OF THECRT ERASES JUST {PRIOR TO THE STARTOF THE NINTH SWEEP

1

DISPLAY JUST AFTERTHE START OF THENINTH SWEEP

ECG DISPLAY OVER A 3-MINUTE PERIOO

(B) SELF-CONTAINED SYSTEM USING A TYPE 564B SPLIT-SCREEN STORAGE OSCILLOSCOPE

Fig. 23-4. Expanded x -axis storage-CRT displays.

385

The uniformity of resolution between the center ofa CRT display and the corners of the display isparticularly good with Tektronix display units asshown in the lower series of photographs in Fig. 23-3.An observation of these photographs will show thatlittle loss of resolution can be detected whencomparing the resolution at the center of the CRTdisplay to the resolution at the top left-hand edgeof the CRT display.

23.3 LARGE SCREEN EXPANDED-SWEEP DISPLAYS

sweepstepping

expandedx axis

signaldensity

The normal 5-inch CRT of most oscilloscopes limitsthe amount of information that may be presented onthe CRT without confusion. The large screencapabilities of the Type 611 Storage Display Unitand the Type 4501 Scan Converter Unit allow agreater quantity of information to be displayed.This greater quantity of information may be derivedfrom many different channels of information or may bederived from a single channel of information over aprolonged period of time using vertical sweep steppingas shown in Fig. 23-4.

The displays shown in Fig. 23-4 require the use ofa step generator capable of being updated with eachsuccessive sweep. The step generator may alsocontain an adjustment to allow the number of sweepsto be varied to suit the display requirements.Details of two self-resetting stair-step generatorsare given in Chapter 29.

The expanded x-axis display shown in Fig. 23-4Auses the Type 611 to store a large volume of analoginformation on its Il-inch CRT. Although the displayshown represents a subject's ECG history over a threeminute period, the resolution of the Type 611 is suchas to allow similar displays to be presented over fargreater periods by using slower sweep speeds, a lowerlevel of ECG signal and less spacing between sweeps.An erase command from the step generator will erasethe CRT when the step generator resets to zero toonce again begin its stepping cycle or, alternatively,the information may be held on the CRT for aprolonged period or until the step generator ismanually reset to erase the information and beginthe cycle once again. While this system has theadvantage of a large screen display, aIl of theinformation stored on the screen must be erased

386

programmederase

before beginning a new series of sweeps, thus, theamount of "history" presented on the CRT varies fromzero just after erasure to three minutes, as withthe display shown in Fig. 23-4A when the stepgenerator has almost completed its cycle and thescreen is full of information. This disadvantage isovercome in the system shown in Fig. 23-4B.

The expanded x-axis storage display shown inFig. 23-4B uses a Type 564B Storage Oscilloscopewith a Type 3A6 vertical plug-in unit and a Type 3B4horizontal plug-in unit in conjunction with a selfresetting l6-step generator. Although this systemonly uses aS-inch CRT display, it has the advantageof being a self-contained system and, as describedbelow, uses the split-screen storage feature of theType 564B to alternately erase segments of the CRT.

The expanded x-axis display system shown inFig. 23-4B provides 16 sweeps, the first sweepbeginning at the bottom left-hand corner of the CRTand the last sweep finishing at the top right-handcorner of the CRT. Just prior to the completion ofthe last sweep the CRT will present the previous 16sweeps of stored information. At the end of thelast sweep the step generator will reset to zeroand the lower half of the CRT will be erased, thusleaving only the previous 8 sweeps presented on theupper half of the CRT. The system will then commenceto cycle through its first 8 sweeps, storing themon the lower half of the CRT. At the end of theeighth sweep the step generator will cause the upperhalf of the CRT to be erased, leaving only theinformation generated by the previous 8 sweeps onthe lower half of the CRT. The step generator willthen commence to cycle through the remaining 8 sweeps,storing information on the upper half of the CRT.The cycle is then repeated continuously. This systemhas the advantage that at least 8 sweeps are alwayspresented on the storage CRT at any one time, thusthe amount of "history" presented at any one timevaries from 8 sweeps to 16 sweeps rather than, aswith a system using a nonsplit-screen storageoscilloscope, from zero to 16 sweeps.

387

23.4 INDlCATORS

movingcoi 1 meter

digitalmeters andindicators

As stated previously, indicators may consist of amoving-coil panel meter or sorneform of digitaldisplay device. The moving-coil panel meter, whilebeing considerably less expensive than a digitalindicating device, is somewhat difficult to read andthus should only be used for the indication ofnoncritical parameters or where digital displaydevices would be uneconomical. Moving-coil metersmay be directly coupled to other components in abiophysical monitoring system or may be coupled tothese components via amplification circuits.

Digital meters and indicators accept voltage levelsfrom other components in a biophysical monitoringsystem and convert them to a format suitable fordisplay in numeric form. They offer the advantageof showing the measured quantity in a form that canbe read at a glance or at a considerable distance.The digital indicator is also free of any readingerror that may occur when reading a moving-coilpointer against a fixed scale. Digital displayindicators are generally custom built for specifiesystems. Electronics for Medicine, New York,produces a digital display unit for use in anoperating room providing 6 channels of numericdisplay to indicate the subject's temperature, pulserate, venous pressure, arterial diastolic pressure,arterial systolic pressure and mean arterialpressure. A numeric indicator device may consist ofa CRT display unit and a character generator forcharacters on the CRT. The character generator maybe a separate instrument or may utilize a digitalcomputer to generate characters with the aid ofdigital to analog converters.

Digital displays are not the whole answer to datapresentation. Particularly if the user is harassedor tired, they are easily misinterpreted. Further,they give no indication of rate-of-change of thevariable shown, and the human brain seems to be ableto accept rate-of-change information separately fromabsolute levels. It is far easier for the humanbrain to accept a pattern than a digital value.

388

The best way of looking at the matter is to ask howmany bits of information, or information levels, areaetually required. Is it just "Too mueh" or "Toolittle," or is it "Mueh too mueh," "Too mueh,""O.K.," "Too little" or "Mueh too little?" A leveldisplay whieh may inelude absolute values forreeording or eloser examination if time permits isfar more valuable than just numbers.

continuousmotionphotography

389

OSCILLOSCOPE CAMERAS

Biophysical information displayed on an oscilloscopeor display unit must be photographically recordedif a permanent record of this information is to bemaintained. While a storage oscilloscope doesmaintain a record for an hour or more, it too mustbe photographed if a more permanent record isrequired.

Information displayed on a CRT may be photographedusing either conventional photographie techniques ormoving-film photographie techniques. Conventionalphotography of information displayed on a CRTinvolves photographing the CRT display with anoscilloscope camera. While this technique isentirely satisfactory for most applications, it doesnot provide a continuous record of data versus time,but only a record of data over a discrete period oftime (normally the duration of one CRT horizontalsweep). Continuous data may be recorded byphotographing a CRT display with a movie camera;however, the movie film can only be displayed byprojection and does not provide a convenient recordthat can be studied with ease. A modification ofmovie photography, known as continuous-motionphotography, provides a permanent record of dataversus time on photographie film by eliminating thehorizontal sweep from an oscilloscope and byproviding the horizontal "sweep" by moving the filmpast the face of the CRT as discussed in Section 24.3.

390

TYPE C-12 CAMERA

TYPE C-27 CAMERA

DIRECT VIEWING WITHOUT THE USE OFA MIRROR, THUS TRANSMITTING MAXIMUM LIGHT TO THE FILM. ACCEPTS AWIDE RANGE OF LENSES AND FILM BACKS.

PROJECTED GRATICULE ACCESSORYFOR TYPE C-12 CAMERA

PROJECTED GRATICULE ALLOWS SPECIALGRATICULES OR HAND WRITTEN DATA TOBE SUPERIMPOSED ONTO THE FILM TOGETHER W 1TH DATA FROM THE CRI.

BEAM -SPLITT 1NG MIRROR MIN 1M 1ZESVIEWING PARf\LLAXWITH CRTS HAVINGEXTERNAL GRATICULES. ACCEPTS A WIDERANGE OF LENSES AND FILM BACKS.

TYPE C-50 CAMERA

Fl.9 LENS 1:0.7 MAGNIFICATIONEASE OF OPERATION AFFOROEO SY SUILTlN TRACE-SRIGHTNESS PHOTOMETER,RANGE-FINDER FOCUSING AND ACCURATEEXPOSURE CONTROL.

(A) FOR SIMULTANEOUS PHOTOGRAPHY AND VIEWING - FOR USE WITH MOSTTEKTRONIX OSCILLOSCOPES WITH 5in CRTS

TYPE C-30A CAMERA

FOR USE WITH TEKTRONIX OSCILLOSCOPESHAVING 3, 4, OR 5in CRTS. Fl.9 LENSMAGNIFICATION VARIABLE lN INDEXEO STEPSFROM 1:0.7 TO 1:1.5. ACCEPTS A WIDERANGE OF OPTIONA~ FILM SACKS.

TYPE C-IO CAMERA

FOR USE WITH TEKTRONIX OSCILLOSCOPESOR MON 1TORS HAVING 11in CRTSF8 LENS - FIXED FOCUS - 1:0.5 MAGNIFICATION

(B) FOR PHOTOGRAPHY OR VIEWING WITH CAMERA REMOVED OR "SWUNG AWAY"

Fig. 24-1. Tektronix oscilloscope cameras suited to biophysical measurements,

391

24.1 CONVENTIONAL CRT PHOTŒRAPHY

Type C-12with beamsp 1 itt ingmi rror

projectedgraticule

Conventional oscilloscope photography involvestaking a still photograph of the CRT. Specializedcameras are, however, required to eliminateextraneous light from the photographie record andto allow the CRT display to be photographed withoutdistortion by a lens with a relatively short focallength. Since it is usually undesirable to waitfor a film to be processed before being able toanalyze data recorded on it, CRT cameras normallyutilize Polaroid* film, providing a permanent recordwithin 10 seconds. Polaroid film is produced by thePolaroid Corporation, Massachusetts. It is oftendesirable for an oscilloscope camera to incorporatea viewing tunnel or sorneother form of viewingmechanism to allow the CRT display to be viewedduring photography.

The Tektronix Type C-12 Camera shown in Fig. 24-1uses a beam-splitting mirror between the lens andthe CRT. This mirror permits part of the light fromthe CRT to pass through the mirror to the lens andpart of the light from the CRT to be reflected bythe mirror into the viewing tunnel. This permitsviewing from an effective viewing position directlyin front of the CRT, thus minimizing parallax errorbetween an external graticule and the informationdisplayed on the CRT. The beam-splitting mirrorwithin the Type C-12 Camera also allows lightprojected from beneath the camera to be reflectedinto the lens and to pass through the mirror intothe viewing system. A projected graticule accessoryfor the Type C-12 Camera provides a light sourcebeneath the camera and allows various specialgraticules and masks to be inserted between thislight source and the camera. The projected graticuleaccessory eliminates any parallax from an externalgraticule CRT by providing a supplementary graticulein the same plane as the CRT phosphore Manyspecialized graticule formats are available fromTektronix for use with the projected graticule; mostof these special graticules include a clear area toallow the user to write information on the graticulewhich will then appear on the final photographierecord.

*Trademark Polaroid Corporation

392

Type C-27

Type C-50

Type C-30A

Type C-l0

fil m backs

Although the Tektronix Type C-27 Camera (Fig. 24-1)is similar to the Type C-12 Camera, it incorporatesdirect angular viewing of the CRT rather than abeam-splitting mirror. This permits the maximumtransmission of light from the CRT to the lens.As it does not include a beam-splitting mirror, itdoes not allow the use of the projected graticuleaccessory. The Type C-27 Camera shown includes aspeed computer and an electric shutter; eithermanual shutters or electric shutters are availableon many Tektronix oscilloscope cameras.

The Type C-50 Camera (Fig. 24-1) is particularlyeasy to operate due to the inclusion of a built-intrace brightness photometer, a range finder focusingmechanism and an accurate exposure control system.The Type C-50 provides angular viewing of the CRTand does not include a mirror between the CRT andthe lens.

The Type C-30A Camera (Fig. 24-1) does not includeprovision for simultaneous photography and viewing;it must be either removed from the oscilloscope forviewing or it may be swung away to one side via itshinged mounting bezel. The Type C-30A Camera isparticularly versatile, being suited for use withmost Tektronix oscilloscopes and including bellowsbetween the lens, CRT plane and film plane to allowvariable magnification from 1:0.7 to 1:1.5.

The Type C-lO Camera (Fig. 24-1) is intended for usewith Tektronix oscilloscopes or monitors havingIl-inch CRT's, such as the Type 611 Display Unit orthe Type T4002 Graphic Computer Terminal.

AlI of the T~ktronix oscilloscope cameras shown inFig. 24-1, with the exception of the Type C-lO, areshown with film backs for use with Polaroid Type 107Pack Film, providing "instant" photographie recordson 3 1/4-incn X 4 1/4-inch Polaroid film. AlI ofthese cameras can be used with Graflok* film holdersand accessories to allow the camera to be used witheut-film holders, film-pack adapters, roll-film (120)holders, etc. Graflok backs and accessories areavailable from local camera shops. The Type C-lOTrace-Recording Camera incorporates a Graflok backto allow its use with Polaroid Type 57 4 by 5-inchcut film.

*Trademark Graflex, Inc.

393

PLAN VIEW OF CURVED FACEPLATE CRT AS USED!N THE TYPE 410 PHYSIOLOGICAL MO ITOR

MARKERS 0 CRT PHOSPHOR LINEUP WITH GRATICULE LINES WHENVIEWED FROM INFINITE DISTANCE

~ ! ! ! !.~~. .. .. :/~r LI NES : -, -, / ~ARKERS ON CRT

GRATICULE lN FRON0:::-..... / PHOSPHOR 00 NOT LINEOF CRT HAVE 1cm SPAC 1NG , / UP W 1TH GRAT 1CULE LINES

'.... 1 / WHEN VIEWED FROM LENS

----~~C1J~-----OSC 1LLOSCOPE CAMERA 1// . l ' <,

LENS PLANE ~/ l ""/ .....••.....

/ l "OSCILLOSCOPE CAMERA/ l -, FILM PJLANE

/ l "/ "

.82cm

/____,r-

.94cm

/CE TER SCREEN

CAMERA MAGNIFICATION1S 1 :0.82

RIGHT EDGE OF SCREENCAMERA MAGNIFICATION

1S 1:0,94

NO LINEARITY OF MARKER SPACI G = .94 - .82 ~ 15%DUE TO CURVED CRT FACEPLATE .82

Fig.24-2. Photography of a curved faceplate CRT.

24.2 PHOTOGRAPHY OF CURVED FACEPLATE CRT'S

curvedfaceplates

AlI Tektronix oscilloscope cameras are intended foruse with CRT's having flat faceplates, the lensesare thus compensated for good linearity when usedwith flat-faceplate CRT's. SorneCRT's, such as theCRT incorporated within the Tektronix Type 410Physiological Monitor, have curved faceplates. Acurved-faceplate CRT such as the Type 410 CRT willproduce an approximate 15% nonlinearity in thephotographie record when photographed with aconventional oscilloscope camera as shown inFig. 24-2.

394

THE NIHON KOHDEN CAMERA MOUNTS DIRECTLY ON MOST TEKTRONIXOSCILLOSCOPES, THUS ELIMINATING THE NEED FOR CUSTOM ADAPTERS

OPENS FOR SIMULTTANEOUS PHOTOGRAPHYAND CRT VIEWING BYOPERATOR

CONTROL UNIT SELECTS CONTINUOUS MOTIONOR SINGLE FRAME OPERATION AND FILMSPEED FROM 100cm!s TO .05mm!s

ILLUMINATES ADATA CARO FORRECOROING PERMANENT DATA ONTHE FILM

Fig.24-3. The Lehigh Valley Electronics, Inc., Nihon Kohden PC-2Acontinuous-rnotion camera.

395

24.3 CONTINUOUS MOTION CAMERAS

A continuous-motion camera provides continuous motionof film or sensitized paper (usually 35 mm) past anoscilloscope CRT via a lens system. Since horizontalinformation is provided by the moving film, thehorizontal sweep of the oscilloscope display isturned off leaving only vertical informationdeflecting the CRT beam. This recording system canbe likened to a conventional chart recorder withvertical movement of the CRT beam acting in a similarmanner to vertical movement of the stylus on thechart recorder. Two commercial continuous motion

Nihon cameras are shown in Figs. 24-3 and 24-4. The Nihoncamera Kohden Model PC-2A Continuous Motion Camera shown in

Fig. 24-3 is marketed in the USA by Lehigh ValleyElectronics, lnc. The camera is produced in Japanand is available with a full range of adapters toallow the camera to be conveniently mounted on mostTektronix oscilloscopes. A broad range of filmtransport speeds is provided from 1000 millimetersper second to 0.05 millimeters per second.

396

HORIZONTAL MOUNTING lN FRONTOF CONVENTIONAL OSCILLOSCOPE

RACK ASSEMBLY WITH CAMERA CUSTOM MOUNTED

CRT

BEAM-SPLITTING MIRROR(1NCLUDED W 1THPN 122-0635-00)

TEKTRONIX OSCILLOSCOPE/CAMERA ADAPTERPN 016-0226-01 FOR MOST 5-INCHROUND CRT'SPN 016-0217-01 FOR MOST 5-INCHRECTANGULAR CRT'SREFER TO CURRENT TEKTRONIXCATALOG FOR PN VERIFICATION

OSCILLOSCOPE CRT MAY BEVIEWED NORMALLY BY ENLARGING THE OPENING lNTHE C-12 CAMERA FRAMEAND COVERING IT WITH ANAMBER FILTER

PART OF TYPE C-I2 CAMERA(PN 122-0635-00) REMOVEVIEWING TUNNEL AND REARFRAME MOUNTING STUDS.LENS MOUNTING HOLE MUSTBE BLOCKED IF USED UNDERHIGH AMBIENT ILLUMINATION.

Fig.24-4. The Grass Instrument Company, Kymograph continuous-motion camera. .

Kymographcamera

397

The Kymograph* Continuous Motion Camera shown inFig. 24-4 is manufactured by the Grass InstrumentCompany in the United States. Film transport speedsfrom 1000 millimeters per second to 0.25 millimetersper second are provided. The Grass InstrumentCompany does not produce adapters to allow thiscontinuous motion camera to be easily adaptable tomost commercial oscilloscopes, thus sorneform ofcustom mounting, either in a horizontal or verticalformat, must be developed as shown in Fig. 24-4.This custom mounting may be achieved using part ofthe Tektronix Type C-12 Camera to provide a beamsplitting mirror to allow the CRT to be viewedduring continuous motion photography. An amberfilter must be incorporated into the viewing pathto block out aIl but amber extraneous light fromentering the camera lens via the viewing path. Mostrecording films are blue sensitive, thus the amberlight will not expose the film. If, however,panchromatic film 1s used, then the amber extraneouslight can be rejected with a blue filter in front ofthe camera lens. The Nihon Kohden cameraincorporates a beam-splitting mirror and filters forsimultaneous viewing and photography. Both theNihon Kohden and Grass continuous motion cameras canalso be used for conventional, single-pictureoscilloscope photography with the oscilloscope timebase operating.

If either camera (1ncorporating a 110 volt motor) isto be used on a higher voltage supply by using astep-down transformer, the transformer should be ofadequate rating (500 volt-amp or more) to deliverthe starting surge current of the motor.

*Trademark Grass Instrument Company

basiccomponents

399

GRAPHIC RECORDERS

A continuous visual record of physiological dataversus time may either be obtained with a graphicrecorder or with an oscilloscope and a continuousmotion camera as covered in the previous chapter.Continuous motion CRT photography, while offeringalmost unlimited bandwidth and extremely fastrecording rates, does not provide an immediate recordof the recorded data. A graphic recorder, whileinherently possessing limited high-frequency responsecharacteristics, does provide an immediate record ofthe recorded data for observation. Graphic recordersare often referred to as oscillographs,oscillographic recorders, strip chart recorders orchart recorders. Graphic recorders utilize sorneform of stylus to traverse a strip of chart paperwhile the chart paper is in motion, leaving a visibletracing which is a record of the time variations ofthe input voltage or current.

Most graphic recorders consist of the following fivebasic components:

An electromechanical device to convert anelectrical input signal to mechanicalmovemerit ,

A stylus arm to transmit the mechanicalmovement from the electromechanical deviceto the stylus,

A stylus to leave a written record on chartpaper as the stylus moves across the chartpaper,

A chart paper assembly consisting of a chartpaper supply roll, a chart paper writingtable and a chart paper takeup roll and

A paper drive mechanism to move the chartpaper across the writing table from thesupply roll to the takeup roll at a constantspeed.

WRITINGPRINCIPLEREFER SECTION 25.3 INK THERMAL PRESSURE INK INK

1 1 1 1

COMMERCIALLY GALVANOMETRIC GALVANOMETRIC GALVANOMETRIC GALVANOMETRIC POTENTIOMETRIC SPECIALAVAILABLE RECTILINEAR RECTILINEAR RECTILINEAR CURVILINEAR RECTILINEAR PURPOSERECORD ERS INK WRITING THERMAL WRITING PRESSURE WRITING INK WRITING INK WRITING

GRAPHIC RECORDERS

BASICMECHANISMREFER SECTION 25.1 GALVANOMETRIC POTENTIOMETRIC

RECORDINGFORMATREFER SECTION 25.2 CURVILINEAR RECTILINEARRECTILINEAR

Fig. 25-1. Graphie recorder classifications.

OTHERS

.t:"aa

characteristics

401

Graphic recorders may be broad1y c1assified as tothe fo11owing three characteristics:

Basic Mechanism

The basic mechanism used to convert an inputcurrent or voltage into mechanica1 movernentof the recording stylus may use either aga1vanometric or a potentiornetric princip1eas discussed in Section 25.1.

Recording Format

The geornetrica1 re1ationship between sty1usmovement and the chart paper will eitherresult in a curved line or a straight linebeing transcribed on the chart paper whenthe sty1us is abrupt1y moved to a newposition (referred to as curvi1inear orrecti1inear recording respectively asdiscussed in Section 25.2).

Writing Princip1e

A written record is transcribed on the chartpaper with either an ink-pen sty1us, a heatedsty1us or a rounded-point sty1us (referredto as ink writing, thermal writing andpressure writing respective1y as discussedin Section 25.3).

402

INK WRITING INK PENSTYLUS

CONVENTIONALCHART PAPER WITHCURVILINEAR GRID

CHART PAPER PULLEDPAST STYLUS BYDRIVE MECHANISM

(Al CURVILINEAR RECORDER

THERMAL WRITING STYLUS ARM

HEAT SENSITIVECHART PAPER WITHRECTILINEAR GRID

INK WRITING

INK WRITING PSEUDORECTILINEAR

LONG PEN lN CONJUNCTIONWITH LIMITED ARC PRODUCESCURVILINEAR RECORDINGSTHAT ARE DIFFICULT TODIFFERENTIATE FROMRECTILINEAR RECORDINGS

(Bl RECTILINEAR RECORDERS

MECHANICAL LINKAGETRANSFORMS PIVOTALMOVEMENT OF THE COlLINTO LATERAL MOTIONOF THE PEN TO PRODUCERECTILINEAR RECORDING

Fig. 25-2. Curvilinear and rectilinear galvanometric recordermechanisms.

403

25.1 BASIC RECORDER MECHANISMS

galvanometricmechanisms

damping

An input current or voltage is converted tomechanical movement of the stylus by either agalvanometric or a potentiometric principle.Galvanometric recorders utilize a moving coil andmagnet assembly with a stylus arm attached to themoving coil. This assembly is somewhat similar tothe common d'Arsonval galvanometer assembly used inmost conventional panel meters; however, theassembly used in a graphic recorder must developconsiderably more torque than the assembly used ina panel meter to overcome forces associated withstylus pressure and to provide good transientperformance (damping). The various recorder typesshown in Fig. 25-2 aIl use galvanometric recordermechanisms. These mechanisms are normallyconstructed so that the angular deflection of thestylus arrnis proportional to the magnitude ofinput current, that is, the relationship betweeninput current and angular deflection is linear.

Damping is particularly important in a galvanometricrecorder. If the moving-coil mechanism is at restand a current is suddenly applied to the coil, thecoil, the stylus arm and stylus will commence tomove to a new position. The stylus and arm willgain momentum during this movement and when the newequilibrium position is reached this momentum willcause them to overshoot this position. The processis then reversed with the stylus and arm oscillatingabout the equilibrium position for sornetime untilfinally coming to rest. Damping must be added to agalvanometric recorder mechanism to overcome theseoscillations, the mechanism being referred to ascritically damped when the stylus and arm assume anequilibrium position as quickly as possible withoutovershooting the position. It is most importantthat pen-to-paper friction constitute only a verysmall part of the pen system damping, which shouldbe supplied electrically, otherwise the damping willbe unreliable over a period of time, and will tendto vary from one side of the chart to the other.

404

mechanismfrequencyresponse

potentiometricmechanism

galvanometricversuspotentiometric

Due to finite force developed by a galvanometricrecorder mechanism and the finite mass associatedwith the stylus and arm, the maximum angularacceleration of the system is defined and thus thehigh frequency response of the system is defined.Most galvanometric recorder mechanisms offer amaximum high frequency response of less than 200 Hz,the high frequency response being directlyproportional to the force developed by thegalvanometric mechanism and inversely proportionalto the moment of inertia of the coil, stylus arm andstylus assembly.

Potentiometric recorders operate on a servo principlewith the position of the stylus arm being detectedvia a contact mechanism attached to the arm and incontact with a fixed slide-wire as shown inFig. 25-3. The servo system will move the stylus armand stylus until the potential between the slide-wirecontact and the input voltage is zero. The linearityand accuracy of a potentiometric recorder mechanismis dependent only on the characteristics of the slidewire and associated electronics, that is, therelationship between input voltage and the positionof the contact on the slide wire is linear.Potentiometric recorder mechanisms in general providehigher accuracy and better linearity thangalvanometric mechanisms.

Damping in a potentiometric recorder mechanism isnormally achieved eiectronically within the servoamplifier. Due to the inherent speed limitations ofthe servo system and the finite mass and frictionalforces associated with the stylus arm, stylus andslide wire, potentiometric recorder mechanisms areinherently low frequency devices and rarely offer ahigh frequency response in excess of 20 Hz uniessstylus deflection is limited to a centimeter or so.

Since high accuracy and linearity is rarely ofparamount importance in biophysical measurements andsince a high frequency response of 20 Hz is notadequate for most biophysical recording applications,galvanometric recorders are generally preferred topotentiometric recorders for biophysical applications.Galvanometric recorders are also, in general, lessexpensive than potentiometric recorders, particularlyif muiti-channei instruments are considered.

405

ARM fvlQVES ASSERVO MOTORROTATéS SHAFT

+

INKPEN~ SLIDE-WIRE

SIGNALINPUT

VOLTAGE

r----____;~- - - - - ,1111

1 SERVO MOTOR COUPLED :1 TO SLIDE-WIRE AND PEN 1•.- - - - - - - - - - - - - - - - - ---1

11

Fig. 25-3. Rectilinear, potentiometric (servo) recorder mechanisms.

25.2 RECORDING FORMATS

curv i 1 inear

The geometrical relationship between stylus movementand the chart paper is either referred to ascurvilinear or rectilinear. A pivoted stylus armwill transcribe an arc at its tip if the arm iscaused to rotate about its pivot point. A stylus atthe tip of the arm will, therefore, transcribe acurved line on stationary chart paper located beneaththe stylus. Since the line is curved, the recordingformat is referred to as curvilinear recording.The basic galvanometric mechanism shown in Fig. 25-2Aproduces a curvilinear recording. A typical ECG andsinewave recorded with a curvilinear recorder isshown in Fig. 25-2A. Although the chart paper usedwith curvilinear recorders is marked with acurvilinear grid, the distortion produced bycurvilinear recording is, at least, an inconvenienceand thus rectilinear recorders are preferred.Curvilinear recorders are, however, considerably lessexpensive than rectilinear recorders and areextensively used, particularly for less criticalapplications such as may be encountered in a studentlaboratory.

406

rect i 1 inear

geometrycauseserror

Movement of a stylus in a straight line perpendicularto the direction of movement of the chart paper wouldtranscribe a straight line on stationary chart paperlocated beneath the stylus. Since the line isstraight, the recording format is referred to asrectilinear recording. The potentiometric mechanismshown in Fig. 25-3 produces a rectilinear recording.Rectilinear recording may also be achieved with thegalvanometric recorder mechanisms shown in Fig. 25-2B.

The thermal writing rectilinear recorder shown inFig. 25-2B transcribes a curvilinear arc at the endof the stylus arm; however, the contact between thearrnand the chart paper effectively moves up anddown the stylus arm as the arm rotates about itspivot point, a straight line is, therefore,transcribed onto stationary chart paper. lnanalyzing the geometry of this technique it isapparent that the movement of the stylus on the chartpaper is not directly proportional to the angulardeflection of the stylus arrn,thus producing alinearity error in the recorder. This linearityerror is less than 1% if the total stylus deflectionin both directions from its central resting point islimited to half the perpendicular distance betweenthe "knife edge" and the pivot point.

Various recorder manufacturers have designedingenious mechanical linkages between the stylus armand chart paper to produce true rectilinear recordingfrom a galvanometric rnechanisrnas shown in the inkwriting recorder in Fig. 25-2B. If stylus deflectionsare srnallwhen compared with the length of the stylusarm then the recording obtained, while being a truecurvilinear recording, is difficult to differentiatefrorna rectilinear recording. This technique isextensively used in EEG recorders and is showndiagrarnatically in the ink-writing, pseudorectilinear recorder in Fig. 25-2B.

407

25.3 WRITING PRINCIPLES

ink writing

thermalwriting

pressurewriting

The stylus in a graphic recorder must cause a writtenrecord to appear on the chart paper as the stylusmoves across the surface of the paper. This writtenrecord may be achieved with a pen attached to thestylus arm, a heated stylus attached to the stylusarm, or with a rounded point attached to the stylusarm. The writing techniques are referred to as inkwriting, thermal writing and pressure writing,respectively.

Ink-writing graphic recorders use ink pens operatingon a capillary and siphon principle to record onuntreated chart paper. The ink pens need constantattention to prevent blockage; however, the recordingpaper used is conventional paper, being considerablyless expensive than the treated papers used foreither thermal writing or pressure writing recorders.Thus, the ink-writing recorder is preferred inapplications where a large volume of recording paperis required such as in electroencelphalography.

The thermal writing recorder uses a heated stylus inconjunction with specially coated chart paper. Thestylus is heated by electric current flowing througha resistive element at the end of the stylus arm, asthe heated stylus travels across the chart paper itmelts off a white coating from the chart paper toexpose a dark undersurface. While the thermalwriting recorder requires little maintenance, thespecially treated paper is somewhat expensivemaking this technique unsuited to applicationsrequiring long term recording.

Pressure writing recorders also utilize special chartpaper. Carbon treated paper is transcribed by arounded stylus at the end of the stylus arm withthe pressure of the stylus leaving a black trace onthe paper. Pressure writing recorders in generalexhibit poor high speed characteristics due to thefinite pressure required and, thus, the finitefrictional forces involved at the stylus-paperinterface.

408

TECHNI-RITE ELECTRONICS INC.MODEL TR711

3dB FREQUENCY RESPONSE FROM DG TO 35HzFOR 4em P-P DEFLECTIONS AND FROM DG TO125Hz FOR O.Sem P-P DEFlECTIONS.

RECTILINEAR THERMAL WRITINGlmm/s TO 50mm/sAC POWEREOWEIGHT - 10 POUNDS

A SINGLE-CHANNEL, PORTABLE, HIGH-SPEED RECORDER

HONEYWELL MODEL 2506 SHOWN WITHMODEL 125 ATTENUATOR

FLAT FREQUENCY RESPONSE FROM DC TO 30HzFOR 4em P-P DEFLECTIONS AND FROM De TO80Hz FOR lem P-P.

RECTILINEAR INK WRITINGlmm/mln TO 250mm/sAC POWEREDWEIGHT - 65 POUNDS

A SIX-CHANNEL HIGH-SPEED RECORDER

BECKMAN INSTRUMENTS INC.TYPE S11 DYNOGRAPH 11

FLAT FREQUENCY RESPONSE FROM DG TO 42HzFOR 4em P-P DEFLECTION AND FLAT WITHIN±30% FOR 0.8em P-P DEFLECTION.

RECTILINEAR PRESSURIZED lNKWR1T1NG

O.lmm/min TO 250mm/s

AC POWEREDRACK MOUNTED

*TRAOEMARK BECKMAN INSTRUMENTS, INC.

AN EIGHT-CHANNEL HIGH-SPEED RECORDER

Fig. 25-4. Typical commercial recorders,

409

25 . 4 CQ\MERCIAL GALVANa.1ETRI C RECORDERS

Type 410recorderoutput

Three commercial high speed recorders are shown inFig. 25-4. AlI of these recorders are suited tobiophysical measurements, providing chart speedsconsistent with biophysical signals containingfrequencies to 100 Hz or so. The single channelrecorder shown in Fig. 25-4 is ideally suited foruse in conjunction with the Tektronix Type 410Physiological Monitor. The Type 410 provides a highlevel signal output via a rear panel connector whichcan be directly connected to the single channelrecorder. Adequate output is provided by the Type410 for use with almost any recorder and isdirectly compatible for use with any of the recordersshown in Fig. 25-4 as aIl of these recorders includeinput signal attenuation.

410

25.5 SPECIAL PURPOSE RECORDERS

low castrecorder

The preceding discussion has covered the recordertypes in common use for biophysical measurements;however, many different types of graphic recorders,such as optical recorders and x-y recorders, are incommon use and are occasionally used for biophysicalmeasurements.

A low cost, single-channel, low speed recorder isshown in Fig. 25-5. This recorder is limited to lowspeed applications providing a maximum chart paperspeed of 0.5 inches per second. This recorder usesa galvanometric movement that is almost identical tothe movement used in most panel meters. The stylusarm is completely free to move and, thus, nofrictional forces are involved. Once every second amechanical lever forces this stylus arm and stylusagainst pressure-sensitive chart paper, leaving asmall impression. Since the chart speed is limited,the written information on the chart paper appearsas a series of closely spaced dots. This form ofrecorder is particularly suited for recording verylow frequency physiological signaIs such as bodytemperature, heart rate variation, etc. Very slowchart speeds are available to allow 24 hours ofinformation to be recorded on less than 2 inches ofchart paper.

RUSTRAK INSTRUMENT CO.MODEL 88

FOR DC OR LOW FREQUENCY USE ONLYRESPONSE TIME IS ~O.8s

RECTILINEAR PRESSURE-SENSITIVEPAPER WRITING SYSTEMO.06in/h TO O.5in/s

AC OR LOW VOLTAGE DC POWERED

WEIGHT - 5 POUNDS

Fig.25-5. A single-channellow speed, low cost recorder.

Fig.25-6. An x -y recorder.

opticalrecorder

411

HONEYWEll MODEl 560

llln x 16in PAGE SIZEDe TO 5Hz (3dB)INK WRITINGAC POWEREDTIME BASE

0.02 TO 5.0em/s

Many graphic recorders use optical coupling betweenthe basic recorder mechanism and the chart paper,the "chart paperlt consisting of photographic film.High frequency performance to several kHz isobtainable with these optical recorders; however, inmost instances, if high frequency performance inexcess of that obtainable with conventional graphicrecorders is desired, an oscilloscope and continuousmotion camera is preferable to an optical recorderfor they are far less susceptible to mechanicalshock and vibration and, in general, far moreversatile.

x-y recorder An x-y recorder is a device used to plot twovariables against one another on chart paper ratherthan, as with a conventional graphic recorder,plotting one variable against time on chart paper.A single sheet of graph is commonly used as the chartpaper, the stylus moving in both x and y directionsacross the paper. A typical x-y recorder is shownin Fig. 25-6. The transient response performanceof x-y recorders limits them to low frequencyapplications. Except where extreme accuracy isrequired, an oscilloscope and conventionaloscilloscope camera is normally preferred to thex-y recorder as the combination offers superiortransient performance and greater versatility.

"holding"device

soft/ha rdcopy

413

MAGNETIC TAPE RECORDERS

A magnetic tape recorder is an analog storagedevice. EZectricaZ signaIs fed into a magnetictape recorder are stored on the magnetic tape withthe same eZectricaZ signaIs available from themagnetic tape recorder when required. Although theoscilloscope camera and the graphic recorder areinformation storage devices, once information hasbeen recorded by these devices it is extremelydifficult to regenerate the information in itsoriginal electrical format. The magnetic taperecorder is the only convenient device that permitsrecording of data in such a manner as to allow itto be reproduced at sornelater time in its originalelectrical format, thus, making it a "holding" deviceas weIl as a recording device. A magnetic taperecording may be referred to as a "soft copy" as itis capable of being reconverted to the originalelectrical variable. The oscilloscope camera andgraphic recorder produce "hard copy" which is notcapable of convenient reconversion.

The magnetic tape recorder allows biophysicalsignaIs to be recorded in real tirneduring aphysiological experiment and then allows theexperimental results to be reproduced at sornelatertime. This allows those performing the experimentto concentrate on obtaining data during theexperirnentand then to concentrate on analyzingthe data after the experiment has been completed.Sornemonitoring of the recorded signal must beused, however, as it is frequently too late tocorrect sorneobvious fault in the recording systemunless signaIs are rnonitoredwhile they are beingstored on rnagnetic tape.

MODULATORS

SIGNALS SIGNALSlN OUT@---;..._------+--<>1 0-D"""'1R"'""E,..."..CT=------_._~@

11 1L JBIAS

OSCILLATOR

ERASEOSC 1LLATOR1----------.....,(1F USED)

TENSION IDLERFOR CONSTANTTAPE TENSION

MAGNETICERASE HEAD(IF USED)

STACKEDMAGNETIC

RECORDINGHEADS

REPRODUCTIONAMPLIFIERS

STACKEDMAGNETIC

REPRODUCTIONHEADS

DEMODULATORS

MAGNETIC TAPE

",","",

"

CAPSTAN AND IDLERASSEMBLY FOR CONSTANTSPEED TAPE TRANSPORT

CAPSTAN MOTORSPEED CONTROL

CIRCUITRY

Fig.26-1. Basic components of an instrumentation tape recorder.

record slow,playbackfast

record fast,playbackslow

415

Since most magnetic tape recorders have thecapability of recording information at one speedand then reproducing it at a different speed, timeexpansion or compression of the information can beaccomplished. By recording at a slow speed andreproducing at a faster speed, many hours ofexperimental data can be reproduced in just a fewminutes, thus taking full advantage of thecapabilities of other instruments in the measuringsystem such as the high speed capability of dataprocessing equipment or the high frequencycapability of a spectrum analyzer. By recordinginformation at a high speed and'reproducing it at aslower speed, high frequency information isreproduced as low frequency information, thusallowing more time for detailed analysis of theinformation and allowing the subsequent use of lowfrequency instrumentation such as a graphic recorder.

Two basic recording techniques are used ininstrumentation magnetic tape recorders suitable forbiophysical applications -- direct recording andindirect frequency-modulation (FM) recording. Thesetwo techniques are covered separately in Sections26.1 and 26.2.

26.1 DIRECT MAGNETIC TAPE RECORDING

recordingcircuitdescription

The following discussion assumes that the reader isfamiliar with magnetic tape recorder principles asthey relate to the unsophisticated audio taperecorder commonly used for home entertainment. Theserecorders aIl use the direct recording technique.

Fig. 26-1 shows the basic components of aninstrumentation tape recorder and shows switchingin both the recording circuitry and the reproductioncircuitry for switching between DIRECT or FM. Thetape recorder is operating in the direct recordingmode with the switch in DIRECT. Referring toFig. 26-1, in the direct mode the input signal isalgebraically added to a high frequency bias signalof about 100 kHz and then fed directly into arecording amplifier which provides the recordingcurrent necessary to drive the magnetic recordinghead. The recording head converts the current intoa varying magnetic flux which changes the residualmagnetism on the magnetic tape as it moves past therecording head. This magnetism on the tape, in turn,

416

"direct"mode1imitations

produces a change in the output voltage from themagnetic reproduction head. The voltage producedat the reproduction head is then amplified in areproduction amplifier to provide a high leveloutput signal.

Direct magnetic tape recording has two severedisadvantages -- poor low frequency response andpoor output level stability. The poor low frequencyresponse is due to the magnetic characteristics ofthe recording and reproduction heads as theefficiency of the heads decreases as frequency dropsbelow a few hundred hertz. For biophysicalapplications it can be stated that direct magnetictape recording is unsatisfactory for signaIs below100 Hz.

If a constant-level, audio-frequency signal isrecorded on a home entertainment audio tape recorder,the level of the output signal will fluctuate by30% or more primarily due to variations in themagnetic coating on the tape and, also, to variationsencountered in the contact between the tape and therecording and reproduction heads. This 30%variation, while being quite acceptable for homeentertainment recorders, is unsatisfactory forbiophysical measurements. By the use of high qualitymagnetic tape and careful design of the tapetransport system, this amplitude stability can beheld within 10% or so. This 10% output levelvariation exists on most instrumentation taperecorders operating in the direct mode and is asevere limitation in many applications. Thislimitation is completely overcome by recording viathe indirect FM technique and this technique alsoextends low frequency response to De.

417

26.2 INDIRECT FM MAGNETIC TAPE RECORDING

"FM" mode1imitations

Referring to Fig. 26-1 with the recorder operatingin the FM mode as shown, the input signal is appliedto a modu1ator where it is used to frequency modulatea carrier frequency, this modulated carrier is thenfed to the recording amplifier which provides therecording current necessary to drive the magneticrecording head. The subsequent voltage from thereproduction head is then amplified and demodulatedto provide an output signal. Indirect FM taperecording permits frequencies down to and includingDe to be recorded.

The high frequency response of any magnetic tapesystem is limited by the characteristics of therecording and reproduction heads, by the signal-tonoise ratio that is acceptable, and to sorneextentby the characteristics of the magnetic recording tapeused. Since the high frequency response of themagnetic tape system is limited and thereby limitingthe FM carrier frequency, the high frequency responseof the indirect FM system is further limited due tothe maximum deviation limits of the FM modulatorand demodulator (normally about 20% of the FM carrierfrequency).

418

sidebands1 imitbandwidth

DIRECT OR FM RECORD/REPRODUCE SYSTEMSSIX TAPE TRANSPORT SPEEDS

FROM 15/16in/s TO 30in/sUSES EITHER 1/4in WIDE OR 1/21n WIDE

MAGNETIC TAPESEVEN DATA CHANNELS AND AN ADDITIONALVOICE CHANNEL FOR VOICE ANNOTATIONS

DIRECT RECORD/REPRODUCE SYSTEM:100Hz TO 150kHz AT 301n/s50Hz TO 5kHz AT 15/16In/s

FM RECORD/REPRODUCE SYSTEM:DG TO 10kHz AT 30in/sDG TO 312Hz AT 15/16in/s

PORTABLE CASE OR RACK MOUNTEDWEIGHT - 80 POUNDS

Fig. 26-2. The Model PR - 500 seven-channel instrumentationrecorder (Ampex Corporation).

Referring to the instrumentation recorder shown inFig. 26-2, the specifications for the highfrequency response of the direct record/reproducesystem is only 5 kHz at a tape speed of 15/16 inchesper second. The maximum frequency component in themodulated FM carrier frequency must, therefore, bebelow 5 kHz. Since 8 sidebands are required on eachside of an FM carrier frequency for almost distortionfree FM reproduction, the high frequency response ofthe recorder in the FM record/reproduce mode is,thus, limited to one-sixteenth of the high frequencyresponse of the direct record/reproduce system oronly 312 Hz at a tape speed of 15/16 inches persecond. As the tape speed of the system isincreased, the high frequency performance of therecording and reproduction heads is increased, theFM modulation frequency can be increased, and,therefore, the high frequency performance in the FMrecord/reproduce mode is proportionally increased.

ln the indirect FM recording mode, amplitudestability between the signal applied to the recordinghead and the signal received from the reproductionhead need not be considered because aIl instabilitywill either be removed in a limiting stage in thereproduction amplifier or will be of no consequencedue to the insensitivity of the demodulator taamplitude modulated signaIs.

419

26.3 TAPE TRANSPORTrvœCHANISM

tape speeds

tape speedstab i 1ity

The basic components of the tape transportmechanism used in an instrumentation tape recorderis shown in Fig. 26-1. The tape supply reel maysupply either 1/4- or 1/2-inch wide tape from a10 1/2-inch diameter reel which supplies 3600 feetof normal thickness tape. A rotating capstan andidler assembly pull this tape past the magneticheads at a constant speed; the tape is thencollected on a takeup reel. Tension idler assemblieson both sides of the capstan are used to provideconstant tension on the tape.

The speed at which the magnetic tape is drawn pastthe magnetic heads is controlled by the speed of thecapstan motor which drives the capstan and idlerassembly. Most instrumentation tape recorders offera speed range from 15/16 in/s to 30 in/s with fourintermediate speeds. Sorneinstrumentation recordersoperate up to speeds of 240 in/s.

Tape speed stability is an important considerationin instrumentation recorders. Relative tape speedinstability is referred to as "wow" if occurring atless than about 2 Hz and "flutter" if occurring atgreater than about 2 Hz. With the recorder operatingin the direct mode, any instability in the tape speedwill appear as a change in frequency of the outputsignal. With the recorder operating in the FM mode,any instability in the tape speed will appear as anamplitude variation in the output signal. Controlcircuitry within instrumentation tape recordersmaintains a constant tape speed by using sorneformof servo mechanism; the feedback to the servo isderived via a tachometer attached to the capstan orvia the output from a constant frequency source thathas been recorded onto one channel of a multi-channelrecorder.

420

26.4 OTHER MAGNETIC TAPE RECORDER CONSIDERATIONS

erasureIf a magnetic tape has been used previously torecord information, this information must be erasedfrom the tape before the tape can be reused forrecording. Erasure can be achieved with a bulktape eraser that exposes a complete reel of tape toan intense low frequency magnetic field. Erasuremay also be accomplished via a magnetic erase headon the tape recorder; this head exposes the tape toan intense high frequency magnetic field before thetape reaches the magnetic recording head. Manyinstrumentation tape recorders, particularly multichannel recorders, do not incorporate erasure as itis generally more convenient and less expensive touse a bulk eraser.

track width

The previous discussion assumes only one channel ofinformation being recorded on the tape recorder.Most commercial instrumentation tape recorders offermore than one channel of information; the recordershown in Fig. 26-2 offers seven channels forinformation plus an additional channel for voiceannotations. Obviously, if seven tracks plus a voicetrack are to be stacked onto a 1/4-inch wide tape,the width of each track must be extremely narrow.The instrumentation recorder shown in Fig. 26-2 usestrack widths of 0.024 inch.

The tape transport used in an instrumentation taperecorder must be capable of quickly winding tape inthe forward direction to allow a desired segment oftape to be reached quickly. Similarly, the transportmust be capable of quickly rewinding the tape backonto the supply reel. The braking systemincorporated into the tape transport mechanism mustbe capable of quickly stopping tape movement when astop command is received.

421

noise The maximum achievab1e signal to noise ratio in thebest qua1ity tape recorders, in either the director FM modes, is about 44 dB, it Ls thus necessary torecord at as high a signal leve1 as the system willpermit and to use sornefi1tering during p1ayback toreduce the noise as much as possible. The signalto noise ratio in the FM mode may be expected toapproach 44 dB, however in the direct mode the signalto noise ratio will rare1y exceed 30 dB.Instrumentation grade magnetic tape should be usedfor physio1ogica1 recording, in contrast to lowerqua1ity entertainment grade tape. The use ofinstrumentation grade tape wilL insure that themaximum signal to noise ratio of the system isobtained, it will insure that the maximum amplitudestabi1ity in the direct mode is obtained and it willinsure that there is no "information dropoutll due toIIdropoutsll on the tape. Carefu1 maintenance isparticu1arly important with magnetic tape recordersas the signal to noise ratio of the recorder will bedrastically reduced shou1d the record and/or p1aybackheads become worn, dirty and/or magnetized.Manufacturers maintenance instructions usuallyinclude head cleaning and demagnetizing procedures.

423

DATA TRANSMISSION AND PROCESSING

Transmission of data between various elements in abiophysical measuring system is normally accomplisheddirectly with shielded cable. Shielded cable is,however, unsuited for many data transmissionrequirements, particularly for transmission over longdistances or if it is desired for one item ofinstrumentation to be completely isolated bothphysically and electrically from other items in thesystem. Data transmission via a radio link, referredto as telemetry, is used in these applications. Datatransmission becomes particularly important if thephysiological signal is to undergo data processingin a computer as, in most instances, the computer islocated remotely from the data source.

27.1 DATA TRAN9v1ISSION VIA SHIELDED CABLE

Data transmission via shielded cable is the simplestform of data transmission; the signal is transmittedin its original analog voltage forme For faithfultransmission, the transmission medium must becapable of passing the physiological signal over itsmaximum conceivable amplitude and bandwidth range.ln a direct-wire unterminated transmission link

cable losses using shielded cable, losses in signal amplitude arecaused by cable reactance and are in directproportion to cable length. For most physiologicaldata transmission applications, cable reactanceimposes a practical maximum cable length of about1,000 feet when the cable is driven by normalinstrumentation providing signaIs from an outputimpedance of perhaps 100 ohms.

424

determiningcable systembandwidth

Within the bandwidth of interest in biophysica1measurements (De to 30,000 Hz) the capacitance fromthe shielded cable inner conductor to the shie1d isthe most important single factor 1imiting the highfrequency performance of the transmission system.The bandwidth of a shie1ded cable transmissionsystem, when driven from a voltage source, isgiven by:

.16 x 1012f3dB = RCl

R is the output resistance of the driving source inohms, C is the nominal capacitance of the shieldedcable in picofarads per foot and l is the length ofthe cable in feet. Instrumentation providing anoutput signal via a vacuum-tube cathode-fol1owercircuit provides a typical driving source impedanceof ~1,000 ohms; instrumentation providing an outputsignal via a transistor emitter-fol1ower circuitprovides a typical driving source impedance of~100 ohms. Most shie1ded cable has a capacitanceof between 20 and 40 pF/ft; the common 1/4-inchdiameter shielded cable used in many physiological1aboratories exhibits a typica1 capacitance of 30pF/ft. Relating this information to the previousformula, a 1,000-foot 1ength of 30 pF/ft shie1dedcable, when driven from a source impedance of 100ohms, provides a transmission system bandwidth of50 kHz. Practically, the system bandwidth will besomewhat 1ess than this as the driving amplifierdoes not "see" on1y the capacitance of the line,but its inductance and resistance as weIl. At50 kHz, a 1,QOO-foot line, a110wing for propagationve10city in the cable, is a significant fraction ofa wavelength long.

Although unterminated direct wire transmissionlinks using shielded cable can be used to about1,000 feet it is recommended that terminatedsystems be used for ana10g data transmission inexcess of 50 feet. Terminated systems require the

425

shielded cable to be driven from a source of itsown characteristic impedance and be terminated inits characteristic impedance. To avoid ground loops,it is customary, and usually essential, to transmitfrom a single-ended driver having the linecharacteristic impedance, and to receive by adifferential amplifier between line and shield havinga high common mode impedance, and a differentialinput impedance equal to the characteristic impedanceof the line.

The overall bandwidth of a biophysical measurementsystem is given by:

f =1

_Il 1 1Vfl2 + f22 + f32 + ....etc.

where fl, f2, f3, etc. are the bandwidths (upper -3 dBfrequency limits) of the individual components in thesystem including the transmission components betweenindividual instruments.

Wire transmission can be used over very longAM techniques distances by employing an amplitude modulated carrier

frequency with the information to be transmittedbeing used to amplitude modulate a carrier frequency.This carrier frequency must be higher than thehighest frequency component in the modulating signalby a factor of at least five to allow low-distortiondemodulation at the receiver. Any amplitudemodulated transmission system is only as good as theability of the system to faithfully reproduceamplitude variations. System attenuation, systemnoise, system nonlinearity and/or interference fromother sources produce errors in the informationtransmitted. For this reason, amplitude modulateddata transmission techniques are limited to wiretransmission rather than to radio link transmission.

426

27.2 DATA TRANSMISSION VIA A TELEMETRY LINK

shortrangeappl ications

singlechanneltelemetry

Telemetry links used in biophysical measurementsrange from the short range systems used in behavioralstudies laboratories to the extremely long rangesystems used in the aerospace industry. Theprincipal difference between the short range andlong range systems is the power output capability ofthe transmitter and the sensitivity of the receiver.

Short range telemetry systems are used in behavioralstudies to completely isolate a subject or laboratoryanimal from a recording system. A small telemetrytransmitter may be attached to the subject totransmit, for example, the subject's ECG; a telemetryreceiver may be located only a few feet away to allowthis ECG signal to be processed by a physiologicalmeasurement system. This allows complete subjectfreedom of movement and thus allows for more naturalsubject behavior. As a telemetry transmitter isnormally a completely isolated battery operateddevice, telemetry offers a degree of safetyunattainable with conventional instrumentation dueto the complete elimination of direct electricalconnections between the subject and theinstrumentation.

Short range telemetry systems usually use FMtechniques and may transmit in the FM broadcastband (88 to 108 MHz). They may be simple systemsproviding only one data channel or more complexsystems providing multichannel capability by usingsubcarrier modulation. A typical single-channeltelemetry lin~ is shown diagramatically in Fig. 27-1.The single-channel battery-powered FM transmittershown would normally be one cubic inch or so involume providing several hundred hours of operationfrom one or two miniature mercury batteries. Thereceiver must provide DC coupling from thediscriminator to give the system a DC signaltransmission capability, and, to avoid DC drift,both transmitter and receiver should be crystalcontrolled.

ELECTRODES FORMONITORING ECG

BATTERY-POWEREDFM TRANSMITTER

FM TRANSMITTER~-----..----~----..-----~FM

OSCI LLATOR

IOOMHz CENTER FREQUENCY±O-20kHz DEVIATIO

427

FMRECEIVER ECG SIGNAL OUT

FM RECEIVER~-----..----~----..-----~SIGNAL

OUT

Fig. 27 -1. A typical single-channel telemetry link.

FM MIXER AND2 OSCILLATOR FM

OSCILLATOR

100MHz CENTER FREQUENCY±O-20kHz DURATION

3 FM IRIGPROPORTIDNALSUBCARRIERBANDS (Hz!OSCILLATOR NOMINAL

LOWER CENTER UPPER IIAND INTELL.IIAND UMIT fREQ LlMIT WIDTH FREQ

1 370 400 430 60 6---- 2 518 560 602 84 8

SUBCARRIER OSCILLATORS J 675 730 785 110 II4 888 960 1.032 144 145 1.202 1.300 1.398 196 20

3kHz CENTER FREQUENCY 6 1.572 1,700 1.828 256 25

±O-225Hz DEVIATION 7 2.127 2,300 2,473 346 358 2.775 3.000 3.225 450 459 3.607 3.900 4,193 586 60

2 3.9kHz CENTER FREQUENCY 10 4.995 5.400 5.805 810 80

±O-293Hz DEVIATION 11 6.799 7.350 7.901 1.102 11012 9.712 10,500 11.288 1.576 16013 13.412 14.500 15.588 2.176 220

3 5.4kHz COMPUTER FREQUENCY 14 20.350 22.000 23.650 3.300 330±0-40SHz DEVIATION 15 27.750 30.000 32.250 4,500 450

16 37.000 40.000 43.000 6.000 60017 48.560 52.500 56.440 7.880 79018 64.750 70.000 75.250 10.500 1.05019 86.025 93.000 99.975 13.950 1.40020 114.700 124.000 133.300 18.600 1.900

FM/FM RECElVER 21 152.625 165.000 177.375 24.750 2.500

A 18.700 22.000 25.300 6.600 660B 25.500 30.000 34.500 9.000 900C 34.000 40.000 46.000 12.000 1.2000 44.620 52.500 60.380 15.760 1.600E 59.500 70.000 80.500 21.000 2.100F 79.050 93,000 106.950 27.900 2.800G 105.400 124.000 142.600 37.200 3.700H 140.250 165.000 189.750 49.500 5.000

428

FM/FM TRANSMITTER

FMOSCILLATOR

BANDPASSFILTER

2.7-3.3kHz

BANDPASSFILTER

3.6-4.2kHz

BANDPASSFILTER

4.9-S:9kHz

DISCRIMINATOR DISCRIMINATOR DISCRIMINATOR

SIGNAL OUT

2 3

Fig. 27 ·2. A typical multichannel telemetry link - frequency division multiplexed.

PHYS 10-LOGICALSIGNALSlNA--,---<B_..:....I __

c-..:....I--0-..:....1--

L.__

0 1 1 -- -,LPF A

0 DATA TRANSMISSION LINK~BCARRIES ALTERNATE SEG-0 MENTS OF A, B, C & 0 ~C

0 _j L o ___J 1 LPFr D

ENCODER DECODER

ENCODER AND DECODER KEPT lN LPF - LOW PASS FILTERS TOSYNC BY THE ADDITION OF A REMOVE MULTIPLEXING-SYNCHRONIZING SIGNAL lN THE FREQUENCY COMPONENTENCODED INFORMATION lN THE OUTPUT SIGNALS

Fig. 27·3. Principle of time division multiplexing.

429

A typical multichannel telemetry link is showndiagramatically in Fig. 27-2. The input signal isused to frequency modulate subcarrier oscillatorsoperating below 100 kHz; the frequency modulatedoutputs from these three subcarrier oscillators aremixed and used to frequency modulate a high frequency

FM/FM system oscillator. This system is referred to as an FM/FMsystem as both the subcarrier oscillators and themain oscillator are frequency modulated. A list ofIRIG FM/FM subcarrier frequency bands is shown inFig. 27-2. The maximum signal that can betransmitted via the telemetry link is known as thenominal intelligence frequency and is directlyproportional to the bandwidth of the subcarrierchannel used. It can be seen from Fig. 27-2 that,at a subchannel bandwidth of 24.75 kHz, the nominalintelligence frequency is only 2.5 kHz. Manymultichannel telemetry systems use wider channelbandwidths and thus require correspondingly greaterchannel separations. Subcarrier bands 1 through 21shown in Fig. 27-2 have relatively narrowbandwidths; however, subcarrier bands A through Hhave wider bandwidths to allow higher frequencyinformation to be transmitted.

27.3 TTIMEDIVISION MULTIPLEXING

frequencydivisionversustimedivision

Multiplexing refers to the combining of severalchannels of information to allow this information tobe transmitted via one data transmission link. TheFM/FM telemetry system discussed previously is anexample of frequency division multiplexing as thesignaIs are aIl transmitted simultaneously usingvarious frequency bands to separate the signaIs.Whereas frequency division multiplexing is truesimuZtaneous transmission of separate channels ofdata, time division multiplexing consists ofsequentiaZ transmission of separate channels of data.This multiplexing technique is best illustrated bythe operation of a rotating commutator in which theseveral input signaIs are switched sequentially ontoa common output channel. The multiplexed outputmay be transmitted directly over wire or used tomodulate a high frequency carrier. At thereceiving end the signal is separated back intoindividual channels by a decoder that is synchronizedwith the transmitting encoder. This technique isillustrated in Fig. 27-3. The frequency at whichthe encoder switches between channels should besubstantially higher than the maximum informationfrequency content of either of the channels.

430

other datatransmissionsystems

Although FM/FM multiplexing and time divisionmultiplexing are commonly used for biophysicalapplications, many other information transmissionsystems are used for special applications. Thesetransmission systems are not unique to thebiophysical sciences and any text on radiocommunications should provide a good reference asto other possible data transmission systems.

27.4 DATA PROCESSING WITH AN ANALOGCOMPlITER

principle

The analog computer is a highly versatile signalprocessor. Analog computers consist of precisionelectronic modules such as operational amplifiers,voltage sources, voltage dividers, multipliers andother special devices. The analog computer is ableto analyze nonelectronic systems by representingthese systems in electronic units. Basic systemquantities are normally represented by varyingvoltage or current; system constants are representedby either discrete resistors or by voltage dividers.Once a physical quantity has been converted to anelectronic analog, the analog computer processesthese electronic quantities by using variousoperational amplifier configurations foramplification, attenuation, integration, summationand numerous other functions.

A sample problem and its solution via an analogcomputer are shown in Fig. 27-4. The equation shown(Fig. 27-4) represents the behavior of a blood flowsystem when subjected to a step change in pressure.If this step change in pressure (p) is representedby a voltage step and the desired pressure in someother part of the flow system (y) is representedas an output voltage from the analog computer, thenthe differential equation relating p to y can besolved by the circuit shown. This circuit only usesfive operational amplifiers and two precisionattenuators; however, typical problems solved by ananalog computer may involve the use of many morecomponents. Commercial analog computers may vary insize from units offering only six operationalamplifiers to extremely large installations offeringseveral hundred operational amplifiers. Themechanical arrangement of operational amplifiers andother modules in analog computers is normallyarranged so that interconnection between variousmodules can be achieved by a patching network.

431

PROBLEM - A PHYSIOLOGICAL BLOOD-FLOW ANALYSIS

A STEP PRESSURE CHANGE TO A FLOW SYSTEM (p) CAUSES A DAMPED SINUSOIOAL PRESSURE CHANGE (y) TO BE PROPAGATEDTHROUGHOUT THE SYSTEM. SYSTEM ANALYSIS YIELDS THE FOLLOWING SECOND OROER DIFFERENTIAL EQUATION WITHSYSTEM CONSTANTS REPRESENTED BY CI, C2 AND C3.

C2 ~ + CI* + C3 y = P

SOLUTION - SOLVE THE ABOVE EQUATION WITH OPERATIONAL AMPLIFIERS AND CALIBRATED DIVIDERS.

SINCE THE EQUATION REPRESENTS A DYNAMIC SYSTEM, SOLUTION OF THE ABOVE WOULD YIELD A COMPLEX SERIES OF TERMSTO REPRESENT THE OUTPUT OF THE SYSTEM. THE ANALYSIS CAN BEST BE ACCOMPLI SHED WITH AN ANALOG COMPUTER.THE ABOVE EQUATION CAN BE REWRITTEN:

...e_ _ 0 È1L _ C3 = d2yC2 C2 dt C2 y 1 dt2

SOLUTION POINTLEFT SIDE = RIGHT SIDE

~dt OUTPUT = y

C2 - CI C2- C3

ÇJ_~C2 dt

- ÇJ_ ~C2 dt CI C3

C3C2 y

-_ ------

f1 \.' '\./"'- -~- -t-,- ,/\,

1 -

INPUT P1 - ATTENUATION & INVERSION

2 - $UMMATION & INVERSION

3 - INTEGRATION

4 - INTEGRATION

5 - INVERSION

OUTPUT Y

Fig.27-4. Physiological system analysis with an analog computer.

432

27.5 DATA PROCESSING WITH A DIGITAL COMPUTER

timesharing

digitalana logconversion

real timeprocessing

With the advent of digital computer time sharingtechniques, the use of the digital computer is noweconomically feasible for many biophysicalmeasurement applications. While the centralprocessing unit of a digital computer can normallyonly process information from one source at any onetime, it can often process this information in afraction of a second and then begin processinginformation from another source. This is referredto as time sharing. ln a time shared system, dueto the speed of the computer, the user is oftenunaware that the central processing unit is sharingits time between him and many other users.

Data must be presented to, and is received from, adigital computer in a digital format. If analogdata is to be processed with a digital computer, itmust, therefore, be converted via an analog todigital converter. Conversely, if the data receivedfrom the digital computer is required in an analogformat, it must be converted to this format in adigital to analog converter. If a computer islocated remotely from the analog data gathering site,it is preferable to transmit and receive data inanalog form. If the data is digitized on site, itmust be either transmitted in parallel to thecomputer (one cable per bit), which is veryexpensive, or serially, which is also expensive andslow as weIl. It is quite feasible for ECG or EEGdata, but impractical for direct brain recording oraction potentials.

Since the time scale of analog data is usuallyimportant, this time scale must be maintained whenprocessing analog data via a digital computer. Itis thus necessary that the computer process thisdata in real time as shown in Fig. 27-SA. Thisprecludes the direct use of a time shared computingsystem unless the priorities in the time sharedsystem are arranged so that processing of thisanalog information takes absolute priority over anyother processing. ln this case, the time sharedcomputer system would process this data in real time.For this reason, many self contained small computersare used in this application.

preservingtime scale

433

DATATRANSMISSION

LINK

DIGITAL DATAFROM COMPUTER

ANALOG TODIGITAL

CONVERTER

ANALOG DATAFOR ---+1

PROCESS ING

DIGITAL DATA1NTO COMPUTER

ANALOG DATAAFTER

PROCESSING

DIGITAL TOANA LOG

CONVERTER

(A) REAL-TIME COMPUTING SYSTEM

ANALOG DAT"

D"TA TRANSFER FROMDIGIT"L STORAGE TOCOMPUTER IS UNDER

TER CONTROL

PROCFOR ~ANALOG TO DIGITALESSING DIGITAL r--- STORAGE DIGI

CONVERTER DEVICE INTOf--'"

DATAINTERCHANGEABLE TRANSMISSIONlN SOME SYSTEMS LlNK

OG DAT" ~ DIGIFTER r-- FROMESSING DIGITAL TO- ANALOG f4- STORAGE

CONVERTER

TAL DATACOMPUTER

ANALA

PROC

TAL DATAC().IPUTER

(B) TIME-SHARED COMPUTING SYSTEM

Fig.27-5. Analogdata to and from a digital computer.

Although the time scale of analog data is usuallyimportant and must be maintained during a dataprocessing procedure, it is possible for a timeshared computer system to indirectly process thisdata in other than real time and then to reconstructthe data in real time after a finite delay throughthe use of storage devices. Referring to Fig. 27-SB,analog data is converted to digital data via ananalog to digital converter; this digital data is.then stored in a digital storage device (normallylocated at the computer site). The time sharedcomputing system then accepts data from the digitalstorage device on a time shared basis with transferbeing controlled by the computer. Digital data fromthe computer will then be produced in segments asthe computing system finds time to process storeddata. These segments of digital data can then beconverted to analog data and stored or they may bestored in a digital storage device before converting.AlI of the analog data can then be withdrawn fromthe storage device to preserve the time scale of thedata.

434

TEKTRONIX TYPE T4002 GRAPHIC COMPUTERTERMINAL WITH ANALOG INPUT CAPABILITY

PHYSIOLOGICAL SIGNALTO AUXILIARYINPUT INTERFACE --- ...•.

KEYBOARD ALLOWS COMMANDSTO BE SENT TO THE COMPUTERALONG WITH PHYSIOLOGICALSIGNAL lN DIGITAL FORMAT

Fig.27-6. A computer terminal used for physiological signal analysisand processing via a remote computer.

Type T4002

435

The Tektronix Type T4002 Graphie Computer Terminalshown in Fig. 27-6, when interfaced with a customanalog input module, is ideally suited to analoginput data processing with a time shared computing.system. The terminal, when used in conjunction withan acoustic coupler or "modem" (MOdulatorDEModulator) and data communications service,provides the necessary electronics to implement thetime shared computing system discussed previouslyand shown diagramatically in Fig. 27-5. The storagefunction is achieved by using the storage cathode-raytube of the terminal. The communication line providedby the data communication service should havesufficient bandwidth (baud rate capability) to allowdata transmission in real time. The ECG signaltypically requires a 400 baud line.

27.6 DATA PROCESSING APPLICATIONS AND SOFTWARE

signalaveraging

Data processing applications in biophysicalmeasurements require application software (computerprograms) to control the computer in performingspecifie tasks. This software is continuously beingdeveloped by computer manufacturers, computer usersand consulting firms and most computer manufacturershave a "library" of software programs available totheir customers.

The digital computer is particularly suited to signalaveraging. Digital Equipment Corporation has asignal averaging program for their PDP-8/1 computerthat also permits prestimulus averaging andcomputation of statistical information such asstandard deviation and trends. The Digital EquipmentCorporation's PDP-8/1 computer may be used inconjunction with the Tektronix Type 611 StorageDisplay Unit and an analog to digital converter asa complete signal averaging instrument.

436

ECGanalysis

Computers are now extensively used for analysis ofECG waveforms. The information storage and waveformcomparison capability of the computer allow it tocompare an ECG signal with other ECG signaIspreviously analyzed to determine if the signal iswithin acceptable statistical limits. The ControlData Corporation provides software forelectrocardiogram analysis to use with their Model1704 Computer; the complete package of computersoftware and peripheral equipment is designated the"1700 Computer Electrocardiogram Analysis System."This system identifies, measures and analyzeswaveforms of l2-lead electrocardiograms by performinga pattern recognition analysis on theelectrocardiogram and provides a printout of theresults for diagnostic use.

Computers are ideally suited to simulation ofphysiological systems, thus, in many cases, allowingan analysis of physiological systems within acomputer rather than by performing actualphysiological measurements.

intensivecare un its

437

INTENSIVE CARE CONCEPTS

Most diseases of the heart and of the circulatorysystem, referred to as cardiovascular diseases,strike without warning and prompt treatment isrequired if death is to be averted. Such treatmentis best provided in a specialized area of a hospitalreferred to as an "intensive care unit." Thesespecialized hospital units provide constantobservation of the subject, constant monitoring ofthe subject's physiological condition and provideimmediate emergency treatment should it be required.Coronary intensive care units are used for treatmentof diseases of the heart such as the myocardialinfarction or "heart attack." Stroke intensive careunits are used for treatment of diseases of thecirculatory system such as the stroke. Pulmonaryintensive care units are used for treatment ofrespiratory diseases.

An intensive care unit may consist of one or moresubject-monitoring sites, referred to as "beds" aseach site is, in fact, a bed. Electronicinstrumentation at each subject-monitoring sitemonitors various physiological signaIs from onesubject and may activate alarms should thesephysiological signals be above or belowpredetermined limits. The complete intensive careunit consists of not only the necessary monitoringequipment but also the necessary trained personneland emergency equipment to allow immediate treatmentof cardiac malfunctions.

438

Intensive care units are usually constructed tosuit a particular hospital's requirements. Widelyvarying approaches are taken as to the physiologicalfunctions to be monitored and the type of monitoringequipment required and, in general, no two intensivecare units are alike. Intensive careinstrumentation is continuously being developed toaccomplish more advanced physiological monitoringtechniques. The following discussion on intensivecare concepts is intended to present some of thephysiological functions that may be monitored andsorneof the typical instrumentation that may beused to monitor these functions. The discussion isconceptual in nature and in no way attempts tosurvey aIl current intensive care applications andinstrumentation.

20 1 PHYSIOLOGICAL FUNCTIONS TO BE MONITORED DURING INTENSIVECARE

Since subjects in coronary intensive care units aresuffering from cardiovascular diseases, aIlphysio1ogical functions associated with the heartand the circulatory system should be monitored. lnmonitoring these physiological functions, it shouldbe borne in mind that the subject is in a recoverysituation, thus, the physiological monitoring devicesshould not hinder the recovery process and should beas unobjectionable as possible to the subject.

ECGmonitoring

The principal physiologica1 signal monitored in anintensive care unit is often the electrocardiogram.The electrocardiogram is usually monitored in thelead II configuration with two active electrodesplaced approximately 12 inches apart along themaximum potential axis of the subject's heart. Athird electrode (ground) should be located elsewhereon the chest. This electrocardiogram monitoringconfiguration is referred to as a three-lead chestcluster. Tektronix produces a patient cable foruse with their Type 410 Physiological Monitor thatis specifically intended for monitoring duringintensive care. The electrodes used for ECGmonitoring during intensive care must be suited forlong term monitoring applications. The Tektronixsilver/silver-chloride electrode system provided withthe Type 410 Physiological Monitor is ideally suited

bloodpressure

Korotkoffsystem

plethysmograph

439

to this application as the electrode paste suppliedproduces no subject discomfort or skin irritationand the relatively large amount of paste requiredbetween the subject and the electrode prevents thepaste from drying out due to evaporation and skinabsorption.

The second physiological parameter often of primeimportance in intensive care monitoring is bloodpressure. Blood pressure can be and often ismonitored using an intra-arterial catheter andtransducer; however, the catheter results inconsiderable subject discomfort and many intensivecare units prefer to monitor blood pressure by sornealternate method.

Blood pressure can be monitored using the automaticcuff pump and Korotkoff microphone blood-pressurerneasurement system shown in Fig. 8-11. Althoughthis system is occasionally used in intensive careunits, it also possesses the disadvantage of beingsomewhat uncomfortable to the subject (bruises),and more importantly, being a sarnplingtechnique, itdoes not provide a continuous record of the subject'sblood pressure. Thus, if for sornereason thesubject's blood pressure were to suddenly drop, thissystem rnay take a minute or so to detect this pressuredrop.

Blood pressure monitoring with a plethysmographoffers the least discomfort to the subject; however,it provides only a relative indication of the weIlbeing of the circulatory system rather thanproviding absolute values for diastolic and systolicpressure.

Although considerable controversy still exists asto the best method for routine blood pressuremeasurement in an intensive care unit, each of thesetechniques possesses attributes and a singleintensive care unit may employ one or more of thesetechniques and, indeed, aIl three may be availableif required. Although diastolic and systolicarterial pressure are cornmonlyrnonitored,meanarterial pressure and venous pressure are alsornonitored in sorneinstances.

440

respiration

bodytemperature

pacemaker

It is often desirable to monitor the subject'srespiratory activity during intensive care; thismay be accomplished with a thermistor pneumographplaced in the subject's nostril. It is often alsodesirable to monitor body temperature in intensivecare subjects via a rectal or armpit thermistorprobe.

Monitoring of the physiological signaIs referred topreviously necessitates numerous electrodes, etc.being placed on the subject. ln addition, it isoften desirable to have cardiac pacemaker electrodesapplied to the subject's chest. Although theseelectrodes are not used during routine intensivecare, they should be connected to a cardiacpacemaker for immediate emergency use if required.

28.2 INTENSIVE CARE INSTRUMENTATION

A conceivable intensive care instrumentation systemis shown in Figs. 28-1, 28-2 and 28-3. Fig. 28-1depicts the total instrumentation layout in a fourbed intensive care unit. Each of the four bedsincludes separate subject-monitoring instrumentationproviding an indication of the subject'sphysiological condition as indicated in Fig. 28-2.SignaIs from each of these four instrumentationmodules are also connected to a central nurse'sstation to permit monitoring by the nurse on duty,to allow selective recording of the ECG, and toallow the ECG signal and/or audio-visual informationto be transmitted throughout the hospital via aclosed-circuit television link. The televisioncamera and closed circuit link may be regarded as"luxury items," most other features shown areessential.

441

VIACLOSED-CIRCUIT

TV SYSTEMCENTRAL NURSE'S STATION

NURSE OVERSEES THE SYSTEM: OBSERVES INFORMATIONFROM SUBJECT MONITORS AND TRANSMITS INFORMATIONRELATING TO POSSIBLE PROBLEMS TO THE TELEVISIONRECE IVERS.

ELECTROCARDIOGRAM WAVEFORM AND/OR ALPHANUMERICSDISPLAYED ON LOW-COST COMMERCIAL TELEVISION RECEIVERSLOCATED THROUGHOUT THE HOSPITAL. ALLOWS AUOIO-VISUALCOMMUNICATION FROM THE NURSE'S STATION TO MEDICALSTAFF THROUGHOUT THE HOSPITAL.

VIACLOSEO-CIRCUITTV SYSTEM

INSTRUMENTATION CONSOLE

VIA SHIELDED CABLES

INTENSIVE CARE WARD -- FOUR BEO

~ REMOTE-GONTROl TV CAMERAMOUNTED ABOVE SUBJECTS

Fig. 28-1. Instrumentation in an intensive care unit.

(~ RELATIVE ARTERIAL BLOOD.\\l\tM"'~)lr---PRESSURE VIA PLETHYSMO- --4'r--l

~ Il GRAPH ON FOREHEAD 1

RESPIRATORY ACTIVITY"'---+--- V 1A THERM 1STOR PNEUMO

GRAPH lN NOSTRIL

ELECTROCARDIOGRAM VIA~-- 3-LEAD CHEST CLUSTER

ELECTRODE CABLE

\CHEST ELECTRODES FOR

\...._-----....y"-'-- USE W 1TH CARO 1ACPACEMAKER IF REQUIRED

t BODY TEMPERATUREL VIA A RECTAL

THERMISTOR PROBE

HIGH/LOWRATE/AMPLITUDE

ALARM

PULSERATEf----

ALARM ---.~

11 11

RESPIRATORYHIGH/LOW r--- RATE ---+-~

RATE/AMPLITUDEALARM ALARM

1 PHYSIOLOGICAL MONITOR C1

L..._

•.... -_

ÔIECG 1

~ s

ARDIACRATE --+--+-~ECG

AUDIO

CARO lACPACEMAKER

ALARMJ lN

AUDIO COMMUNICATIONAND ALARM VOICE

1N/OUT --+--~

l0lI~

HIGH/LOWTEMPERATURE

ALARM

_ BODYTEMPERATURE

1---- ALARM ---+--~

Fig. 28-2. Subject monitoring instrumentation for one intensive care "bed."

CABLING TOCENTRAL NURSES'STATION

plethysmograph

pneumograph

Type 410Monitor

thermi storprobe

televisionmon itori ng

443

Referring to the subject-monitoring instrumentationlocated beside each intensive care bed as shown inFig. 28-2, relative arterial blood pressure,respiratory activity, the electrocardiogram and bodytemperature are monitored. Relative arterial bloodpressure is monitored via a plethysmograph on eitherthe subject's forehead, his nasal septum or the lobeof his ear. Finger plethysmographs are rarely usedin intensive care due to their susceptibility tosubject movement. The instrumentation associatedwith the plethysmograph may provide an indication ofcardiac rate; it may also provide an alarm signalshould the cardiac rate go above or below presetlimits or should the amplitude of signal producedby the plethysmograph fall below a predeterminedlevel (indicating a loss in blood pressure or flow).

Respiratory activity is monitored via a thermistorpneumograph in the nostril with the associatedinstrumentation providing an indication ofrespiratory rate as well as providing an alarm ifthis rate falls outside predetermined limits or ifthe signal level produced by the thermistorpneumograph is reduced below some predeterminedamplitude (indicating a loss of respiratory activity).

The electrocardiogram is monitored by a TektronixType 410 Physiological Monitor using a Tektronixthree-lead chest-cluster electrode cable. Thephysiological monitor provides an ECG output froma low impedance source as well as an audio outputto audibly indicate the cardiac rate or to indicatea loss of cardiac activity.

Body temperature is monitored via a rectal thermistorprobe. The associated instrumentation indicatingbody temperature may also contain an alarm systemwhich will be activated if the body temperatureshould fall outside predetermined limits.

Instrumentation located beside each intensive carebed should, preferably, be away from the subject'srange of vision as its presence can be somewhatdisconcerting to some subjects. The intensive careward may also contain a closed-circuit televisionsystem to allow one or more subjects to be viewedvia a television camera. This television camera maycontinuously scan the subjects in the intensivecare ward or its position may be controlled from thecentral nurse's station.

444

additionalequipment

centralnurse'sstation

ln addition to physiological and visual monitoringof the subject, a cardiac pacemaker module and audiocommunication and alarm module are also includedwith the instrumentation at_.eachintensive care bed.The cardiac pacemaker provides variable-amplitude,variable-rate pulses for cardiac pacemaking shouldit be required. The audio communication and alarmpanel provides audio-visual alarm indication ofabnormalities in blood pressure, cardiac rate,respiratory activity or body temperature and providesaudio communication between the intensive care bedand the nurse's station.

An intensive care unit central nurse's station isshown in Fig. 28-3. Multiconnector cable connectsthe output from the four subject-monitoring siteslocated beside each intensive care bed to the centralnurse's station. Each subject's ECG is continuouslydisplayed via a four channel oscilloscope and iscontinuously recorded on a memory loop tape recorder.This tape recorder contains the previous one-minuteECG history for each subject by recording the ECG ona tape loop "one minute" in length. While sornecentral stations duplicate all physiologicalindicators at the central station, this is normallyunnecessary and, in the interest of simplicity,it is preferable to only provide one set ofindicators for relative blood p~essure, respiratoryactivity and body temperature. These indicators canbe manually switched between the four beds or theswitching may be activated by the alarm system withthe monitors being automatically switched to the bedproviding the alarm signal. When an alarm isreceived at the central nurse's station, it may alsobe used to connect the appropriate ECG signal to ascan converter and ECG chart recorder and to startthe chart recorder. ln this way a permanent recordis achieved on the chart recorder beginning oneminute prior to the alarm being sounded andinformation is displayed on the scan converter fortransmission via the hospital closed-circuit TVsystem to other medical personnel involved. Thescan converter and closed-circuit TV system may alsoincorporate alphanumeric input to allow alphanumericdata relating to the intensive care subject to bedisplayed on television receivers located throughoutthe hospital.

445

CHART RECORDER STARTED BY ALARM LOGIC

3

ALARM ELECTRONICS [TI RELATIVEAND BLOOD

OM SUBJECT ALARM LOGIC PRESSURENITORING RATEIPMENT Il

11 ~RESPIRATORY1 ACTIVITY1 RATE11111lm 4-CHANNEL 1 [îJ BODYOSCILLOSCOPE 1

(ECG) MANUAL SELECTION TEMPERATUREOR CONTROLLED TEMPBY ALARM LOGIC

111

4-CHANNEL TAPE-LOOP 1 M ECGMEMORY (1 MINUTE 1

1] CHART ~1MEMORY OF ECGl 1 RECORDER11

1'>IL x

;:, - L..'::-~ SCAN

1

CONVERTERr--

AUDIO-VISUAL lCONTROL...••.

TO CLOSED-CIRCUIT TV SYFOR INFORMATION DIS

THROUGHOUT HOSPALPHANUMERICINPUT FOR TV

SYSTEM

M TV CAMERAVE SUBJECTS

STEMPLAYITAL

FRMOEOU

2

4

FROABO

Fig. 28-3. Central nurses' station instrumentation for 4 intensive care beds.

446

computersinintensivecare

Increasing use is being made of digital computers inintensive care units. These computers provide storageof the subject's physiological data and cancontinuously interrogate this data to determine ifit is within predetermined limits. The computer canperform multivariant analysis of the subject's ECGand can compare his ECG with previous records toindicate changes in the ECG which may be clinicallysignificant. The advent of the time shared computersystem allows one computer to simultaneously performmany tasks throughout a hospital, thus making thecomputer economically feasible for many hospitals.

447

SECTION IV

APPENDIX

Chapter 29 describes custom instrumentation requiredto perform the measurements discussed in Section II.While most of these custom items are simple andshould more correctly be referred to as adapters,they are necessary for the correct operation of themeasurement systems and are not generally availablecommercially.

Chapter 30 gives formaI definitions for the biomedicalterminology used throughout this book. It is hopedthat this chapter contains the definitions of anyterms used that would not normally be regarded ascommon terminology by the electronics engineer.

449

CUSTOM INSTRUMENTATION

This chapter provides circuits and constructiondetails for various items of custom instrumentationthat are required to implement sorneof thebiophysical measurements discussed in Sections IIand III. Details of these items are presented heresince similar items do not appear to be availablecommercially. These items have been constructed andused satisfactorily for the measurement techniquefor which they were intended. Normal tolerancevariations in components may, however, necessitateminor circuit changes in sorneinstances. It isanticipated that the items presented will beconstructed by competent electronic technicians orelectronic engineers who are able to evaluate theperformance of the device and modify this performanceif necessary.

Although the Tektronix, Inc. part number is providedfor ail the parts necessary to construct these items,most of the parts are common electronic componentsand are available through local suppliers. We wishto encourage purchase from these local supplierswhenever possible. If necessary, however, parts maybe purchased in small quantities through Tektronixfield offices located throughout the United Statesor through Tektronix subsidiaries or distributors inother countries. These field offices, subsidiariesand distributors are listed in the current Tektronixcatalog. When purchasing these parts from Tektronix,please quote the Tektronix part numbers for theindividual components shown with the circuits. WhileaIl parts were available from Tektronix at the timeof preparation of this text, Tektronix makes noguarantee as to the continutd availability of theseparts.

450

The items described in Sections 29.1 through 29.5are intended for use in conjunction with theTektronix Type 410 Physiological Monitor.

The items described in Sections 29.6 through 29.9 areintended for use with the Tektronix Type 3A8Operational Amplifier plug-in unit. These items mayalso be used in conjunction with the Tektronix Type 0Operational Amplifier plug-in unit. It is alsoprobable that these circuits may be suited for usewith other operational amplifier modules.

The items described in Sections 29.10 through 29.15are independent items of instrumentation and are notintended for use with any specifie instrument.

29.1 TYPE 410 MODIFICATION FOR FETAL ECG USE

Monitoring of the fetal ECG requires the use of aphysiological monitor with an amplifier having a lowfrequency 3 dB point of >1 Hz. The standard 410Physiological Monitor has a low frequency responseof <0.1 Hz and is thus unsuited for monitoring thefetal ECG. The EEG function of the Type 410Physiological Monitor may be modified to allow it tobe used to record the fetal ECG. This modificationconsists of modifying the low frequency response ofthe Vertical Amplifier in the EEG position from<0.1 Hz to >1 Hz. See Fig. 29-1.

The low frequency response of the amplifier ischanged by decreasing the time constant of the ACcoupling network from 1 microfarad/2.2 megohm to 1microfarad/75 kilohm by adding 75 kilohm resistorswhen the 410 is operated in the EEG mode. This alsoreduces the amplifier's gain by a factor of 2. Sinceno spare switch positions are available on the 410INPUT SELECTOR switch, switching of the 75 kilohmresistors must be accomplished using transistors.The 410 horizontal and audio circuit produces apositive logic level when the 410 is operated in theEEG mode. This level is used to freerun the sweepand to disable the audio circuit, however, it mayalso be used in this modification to turn on theswitching transistors QlA and QlB.

TO JUNCTION0134, 0135

RI75kn

QIA

+ CI.047]..lF

451

Q1B

TO JUNCTION0235, 0236

CIRCUIT BOARDJUNCTION OF

0236, 0235, R234 ,C234, 0260, 0137B

CIRCUIT BOARDJUNCTION OF

0135, 0134, R134,C134, 0160, Q137A

R4360kn Dl ADOITIONAL COMPONENTS

LOCATEO WITHIN THE 410.TO HORIZONTAL

R5 AND AUDIO CIRCUIT360kfl BOARD. PIN H.

CKT NO TEKTRONIX PNDESCRIPTION

CI .047]..lF35V CAPACITOR 290-0282-00C2 .047]..lF35V CAPACITOR 290-0282-00Dl C06538 DIODE 152-0185-0001 DUAL PNP TRANSISTOR 151-0261-00RI 75kn 1/8W RESISTOR 317-0753-00R2 75kn 1/8W RESISTOR 317-0753-00R3 360kn 1/8W RESISTOR 317-0364-00R4 360kn 1/8W RESISTOR 317-0364-00R5 360kn 1/8W RESISTOR 317-0364-00

OUAL TRANSISTOR SOCKET 136-0235-008 INCH INSULATEO WIRE

TO HORIZONTAL ANDAUOIO CIRCUIT BOARD

PIN H

OUALTRANSISTOR

SOCKET

ALL COMPONENTS ARE MOUNTEOONTO THE TRANSISTOR SOCKET.THE ASSEMBLY IS THEN WIREOONTO THE VERTICAL AMPLIFIERCIRCUIT BOARD WITHIN THE 41(.

Fig.29-1. Type 410 modification for fetal ECG use.EEG position now 5mm/SO/-LV, 1 Hz - 100 Hz.

452

)-------:-: P 1 TO 410PATIENT CABLE

'-'-----oIJt:==:::: P2

TO Rl ON 410'-L...__ .r:-_-_.•P3 PAT1ENT CABLE

QTY DESCRIPTION TEKTRON 1X PN

2 BNC CONNECTORS 131-0352-013 2-PIN PlUGS (Pl,P2,P3l 134-0089-003 (ftl SHIElDED CABLE 175-0284-001 BOX w/HARDWARE * 013-0073-00

*CONSTRUCTION NOTE:

- REMOVE 3 BANANA PlUGS FROM BOX- ENlARGE THE 2 OUTER VACATED HOlES TO 5/15 INCH- TAP ENlARGED HOlES 3/8 INCH x 32 TAP

FOR 501JV/cm FOR 5001JV/cm FOR 5mV/cmSENSITIVITY SENSITIVITY SENSITIVITY

EEG + ECG lA AUX +

EEG - ECG RA AUX -

THIS PIN TO~ / CABLE CENTER

~.... CONDUCTOR

'- THIS PIN TOWIRING FOR CABLE SHIElDPl, P2, P3

Fig.29-2. A BNC input adapter for the Type 410 Monitor.

111111\\

\111111

11,111\

,--_____:'-~- - - - - -PlPlUGS INTO P50ON REAR OF 410

CKT NORIR2R3R42 ea6 ea (ftl1 ea

eaeaea

DESCRIPTION TEKTRONIX PN1.47kn RESISTOR 321-0209-005kn POTENTIOMETER 311-0171-003kO RESISTOR 318-0099-003kO RES 1 STOR 318-0099-00BNC CONNECTORS 131-0352-012 CONDUCTOR SHIElDED CABLE 175-0152-01BOX w/HARDWARE * 013-0073-007-PIN CONNECTOR (Pll 131-0551-01WASHER, lOCKING 210-0046-001/4 1NCH NUT '210-0583-00

*CONSTRUCTION NOTE:- REMOVE 3 BANANA PlUGS FROM BOX- ENlARGE THE 2 OUTER VACATED HOlES TO 5/16 INCH- TAP ENlARGED HOlES 3/8 INCH x 32 TAP- FilE SlOT lN BOX FOR CABLE CLEARANCE- ENlARGE THE CENTER VACATED HOlE TO 1/4 INCH

+OUTPUT

-OUTPUT

Fig.29-3. A signal output adapter for the Type 410 Monitor thatprovides a differential signal output from the Type 410Monitor at 100 mV for each cm deflection on the 410 CRT.

453

29.2 A BNC INPUT ADAPTER FOR THE TYPE 410 MONITOR

Refer to Fig. 29-2. This adapter requires littleexplanation, being entirely passive. The adaptersimply provides BNC to "410 System" adaption toallow conventional BNC accessories, etc., to be usedto connect physiological signaIs to the Type 410Monitor.

Referring to the markings located on the Type 410patient cable, the adapter may be connected to theAUX+ and AUX- positions to allow the 410 to be usedin the auxiliary mode, providing a sensitivity of5 mV per centimeter. With the adapter connected tothe ECG LA and ECG RA positions, the 410 must beoperated in the ECG lead l mode and will provide asensitivity of 500 ~V per centimeter. With theadapter connected to the EEG+ and EEG- positions,the Type 410 must be operated in the EEG mode andwill provide a sensitivity of 50 ~V per centimeter.

29.3 A SIGNAL OUTPUT ADAPTER FOR THE TYPE 410 MONITOR

Refer to Fig. 29-3. This adapter provides avertical signal output from the Type 410 VerticalAmplifier via conventional BNC connectors. Adifferential output of 100 mV for each centimeter ofvertical deflection on the Type 410 CRT is provided.The adapter should, preferably, be used withauxiliary equipment having a differential input,however satisfactory results may be obtained whenusing only one output from the adapter inconjunction with equipment having only a "singleended" input. The sensitivity in the single endedmode will be 50 mV for each centimeter of verticaldeflection and care should be taken to avoidexcessive common mode signaIs at the 410 input.

The Type 410 provides a differential vertical signalout at the rear of the instrument of approximately65 ~A for each centimeter of CRT deflection. Adifferential output voltage is produced across aload on this current source consisting of RI, R2,R3 and R4. R2 varies this load to allow the outputvoltage to be varied. If alternative output voltagesare required the values of R3 and R4 should bechanged accordingly.

454

29.4 A RESISTIVE TRANSDUCER ADAPTER FOR THE TYPE 410 MONITOR

Refer to Fig. 29-4. This adapter allows resistivetransducers to be used with the Type 410 Monitoroperating in the auxiliary mode. A change in theresistive transducer's nominal resistance of lessthan 0.1% produces 1 centimeter of verticaldeflection on the Type 410 CRT. The frequencyresponse of the system is from 0.1 Hz to 250 Hz.

RI, R2, QI and Q2 provide a constant current sourcefor the resistive transducer, thus the voltageacross the transducer is proportional to thetransducer's resistance. Capacitors CI and C2 couplechanges in this voltage to the vertical amplifier ofthe Type 410 Monitor.

Almost any resistive transducer having a nominalresistance below 10 kn may be used, however theadapter is not intended for use with bridge typeresistive transducers. The adapter was specificallyconstructed for use with the thermistor pneumographdescribed in Section 29.5.

455

TRANSDUCER VALUE OF TEKTRONIX PN OPTIMUM TRANSDUCER SENSITIVITY CHANGE lN RESISTANCE (ôR) FORSUPPLY CURRENT R2 FOR R2 NOMINAL RESISTANCE 1 DIV VERTICAL DEFLECTION ON 410 CAT

I e lmA 680n 317-0681-00 10kn - 5kn ôR " 5 OHMS/DIV2mA 330n 317-0331-00 5kn - 2kn 2.5 OHMS/DIV5mA non 321-0108-00 2kn - 1kn 1 OHM/DIV

10mA 62n 317-0620-00 LESS THAN lkn 0.50HM/DIV\

Cl11lF+

0 0

0 0 +Pl C2 11lF+0

0 Rl-0 100kn I

01

R3~RESISTIVE~TRANSDUCER

B

~cE 01, 02

CKT NO DESCRIPTION TEKTRON 1X PN

Cl luF 35V CAPACITOR 290-0308-00C2 IIlF35V CAPACITOR 290-0308-00Pl 7-PIN PLUG 134-0090-0001 SILICON NPN 151-0206-0002 SILICON NPN 151-0206-00Rl 100kn 1/8W RESISTOR 317-0104-00R2 * 1/8W RESISTOR *R3 RESISTIVE TRANSDUCER * --

OTY - 4 ft SHIELDED CABLE 175-0284-00

*SEE CHART FOR VALUE AND PN

ALL COMPONENTS ARE MOUNTED ONTO THEPINS OF THE 7-PIN PLUG. THE PLUGCOVER IS THEN FITTED LEAVING THESHIELDED CABLE PROJECTING FOR CONNECTION TO A RESISTIVE TRANSDUCER.

COVER, PART OF Pl,SNAPS OVER PLUGAND COMPONENTS

PlPl

Fig. 29-4. A resistive transducer adapter for the Type 410 Monitor.

456

- SOLDER THERMISTOR TO CABLE- SLIDE HOLDER OVER JUNCTION- FIX HOLDER lN PLACE WITH CEMENT- SOLDER CABLE TO TRANSDUCER ADAPTER

2 3 4

ITEM DESCRIPTION QTY TEKTRONIX PN

RESISTIVE TRANSDUCER INTERFACE REFER FIG 29-4ADAPTER - 5mA. FOR DETAILS

2 PVC-COVERED COAXIAL CABLE 4 (ft) 175-0284-003 HOLDER - LARGE 7/16 INCH DIA 1 352-0077-00

OR SMALL 1/4 INCH DIA 352-0078-004 lkn THERMISTOR 307-0127-01

THERMISTOR PNEUMOGRAPHw/LARGE HOLDER(FOR ADULTS)

OPTIONAL SMALLHOLDER THERMISTOR PNEUMOGRAPH

(FOR CHILDREN) FITTED TO SUBJECT

Fig.29-5. A therrnistor pneumograph for the Type 410 Monitor.

457

29.5 A THERMISTOR PNEUMOGRAPH FOR THE TYPE 410 MONITOR

Refer to Fig. 29-5. This pneumograph allows asubject's respiration to be monitored by monitoringthe temperature difference between inhaled andexhaled air with a thermistor.

A thermistor having a nominal resistance of 1 kn isused in conjunction with suitable mechanical holdersand the resistive transducer adapter described inSection 29.4. Either a large holder (for adults)or a small holder (for children) may be used to holdthe pneumograph in place at the subject's nostril.

458

29.6 AN INPUT NEUTRALIZING ADAPTER FOR THE 1YPE 3A8 OPERATIONALAMPLIFIER

Refer to Fig. 29-6. This adapter provides theextremely high input impedance necessary to allowamplification of signaIs derived frommicroelectrodes. The input resistance into thisadapter is greater than 109 ohms, the inputcapacitance is less than 1 picofarad and the adapterhas a gain of 2X and a low output impedance to allowit to be used 1n conjunction with conventionalinstrumentation. The adapter uses both operationalamplifier units available in the Tektronix Type 3A8Operational Amplifier plug-in unit.

Operational amplifier #1 is operating as a noninverting amplifier having a gain of 2 determinedby R2, R3 and R4. RI, CI and C2 provide frequencyresponse compensation for these gain settingresistors. Positive feedback is produced via R7,C3 and C4 to a current summing point at the junctionof R5 and R6. This positive feedback current isadjusted so as to equal the sum of the input currentto the operational amplifier via R5 plus the current(through R6. Thus the current from the input must bezero. The output from operational amplifier #1provides the outpu~ from the adapter and alsoprovides an input to operational amplifier #2operating as a unity gain follower. The componentsassociated with the positive input of operationalamplifier #2 form a 2X attenuator, thus the outputfrom operatianal amplifier #2 is half of the outputof operational amplifier #1, that is, equal ta theinput to operational amplifier #1. The output fromoperational a~plifier #2 is used to drive anungrounded shield surrounding the input to theadapter. This provides input "guarding" toeffectively reduce the capacitance of the inputcable.

459

REFER ALSO TO 3A8 INSTRUCTIONMANUAL.

r-I RI

47011

1 R2lOOk

1

1-R

--,1

1

1

1

1

1

1

1

1

1

C227pF R. > 10911

1

Cr < lpFGAIN 2X

SET 3A8 OPERATIONALAMPLIFIERS Zj AND ZfSWITCHES TO EXT,GRID SELECTION TO + GRID.

R3 R4950kn lMI1

R522011

INPUT

R7lMI1

C3R6 100pF C4lMO

-c

L

r1

1

1

1

1

1

1

1

L

--,OUTPUT 1

I--~'__--e

1

1

1

1

1

1

1

_j

R810kO SET CI FOR OPTIMUM TRANSIENT

RESPONSE WITH INPUT CONNECTEDDIRECTLY TO 40mV CALIBRATOR.

R910kO

SET C6 AND RIO FOR OPTIMUM TRANSIENT RESPONSE WITH INPUT CONNECTED TO 40mV CALIBRATOR VIA 10MOSERIES RESISTANCE.RIO

lkl1 R2 AND C4 ARE FRONT PANELADJUSTMENTS.

CKT NO DESCRIPTIONCI 9-35pF CAPACITORC2 27pF CAPACITORC3 100pF CAPACITORC4 VAR CAPACITOR (-C)C5 27pF CAPACITORC6 9-35pF CAPACITORRI 4700 1/4W 5% RESISTORR2 100kO POTENTIOMETER (-R)R3 950kO 1/8W 1% RESISTORR4 lMn 1/8W 1% RESISTORR5 2200 1/4W 5% RESISTORR6 lMO 1/8W 1% RESISTORR7 IMO 1/8W 1% RESISTORR8 10kO 1/8W 1% RESISTORR9 10kl11/8W 1% RESISTORRIO lkl1POTENTIOMETERWl 1 ft 2-CONDUCTOR SHIELDED CABLEW2 1 ft l-CONDUCTOR SHIELDED CABLE

1 ft SHIELDING BRAID WIRE

TEKTRON1X PN MECHANICALQTY TEKTRONIX PNDESCRIPTION281-0092-00

283-0076-00283-0060-00

1 ADAPTER-TERMINALASSY 013-0048-011 COMPENSATION ADAPTER * 013-0081-001 1/4 INCH GROMMET 348-0020-003 5/16 INCH GROMMETS 348-0003-001 CONNECTOR BNC 131-0106-01

KNOB FOR R2 366-0153-00NUT FOR R2 210-0583-00WASHER-LOCKING FOR R2 210-0046-00LUG-SOLDER FOR R2 210-0223-00

-- *283-0076-00281-0092-00315-0471-00311-0467-00318-0095-00321-0481-00315-0221-00321-0481-00321-0481-00321-0289-00321-0289-00311-0635-00175-0072-00175-0284-00176-0047-00

*C4 AND ASSOCIATED HARDWARE ARE SUPPLIEDAS PART OF THE COMPENSATING ADAPTER. THETEKTRONIX PN FOR C4 IS 281-0090-00.

Fig. 29-6. An input neutralizing adapter for the Type 3AS Operational Amplifier.

460

29.7 AN ABSOLUfE VALUE ADAPTER FOR THE TYPE 3A8 OPERATIONALAMPLIFIER

Refer to Fig. 29-7. This adapter provides full waverectification of input signaIs producing positiveoutput signaIs for both positive and negative inputsignaIs. The adapter has an absolute gain of 1.

With a negative going input, diode Dl conducts andthe operational amplifier operates as an invertingamplifier with unity gain determined by RI, R2, R3and the output impedance of the input source. Witha positive going input, diode D2 conducts and theoperational amplifier operates as a unity gain,

~ noninverting amplifier, as "2i" at the operationalamplifier negative input is effectively infinite.R4 ensures that the adapter has the same inputresistance for both positive and negative signaIs,thus maintaining a constant potential on CI. DiodesDl and D2 effectively reduce the input voltage levelby approximately 0.3 volts.

INPUTVIA 3A8

@ C215pF

.---------i I!t---"""'il

DlRl

100kn R2200kn

OUTPUTVIA 3A8

@

ADJUST R4 FOR 50% DUTY FACTOR WITH SINEWAVEADJUST R2 FOR SYMMETRY WITH SINEWAVE

CKT NO

CIC2

Dl l'0.2R1R2R3R4

MECHANICALQTY

12

DESCRIPTION

11lF 100V CAPAC ITOR15pF CAPACITOR

MATCHED PAIR DIODES

100kn 1/4W RESISTOR200kn POTENT '1 OMETER'2.2Mn 114W RES 1 STOR200kn POTENT10METER

TEKTRONIX PN

285-0815-00 !

281-0509-00

152-01 ]:0-00

315-0104-00311-0660-00316-0225-00311-0660-00

ADAPTER TERMINAL ASSY 013-0048-0151 l' 6 1NCH GROMMETS 348-0003-00

461

INPUT

~.

OUTPUT

R4

1

R2

PERFORMANCE AT 500Hz

r, \,(\ /~(\ (\~ t r i"

\ \1 \ ~ { \{ I{ : 1/ ., i . 1 1 \ l ',i

1\ VVV\)10V P-P INPUT OUTPUT = Il NPUT 1

IV P-P INPUT OUTPUT = 0.7511NPUTI

0.1V P-P INPUTOUTPUT = 0.25 IINPUTI + 5mV

Fig, 29'-7. An absolute value adapter for the Type 3A8 Operational Amplifier.

462

REED RELAY (51) OPENS AND CLOSESlN LESS THAN lms

POSITIVE GATING PULSE OPENS S1

(FROM TEKTRONIX 5618OR 5648 OSCILLOSCOPECAL 18RATOR)

13mA@=-----------.--,Rl33kQ

CAL

DlGATE

L1

ADAPTER TERMINAL ASSY 013-0048-01

CKT NO DESCRIPTION

Dl DIODE - SILICONL1 REED DRIVE COlLQ1 2N1893 TRANSISTORR1 33kQ 1/4W RESISTOR51 REED SWITCH

MECHANICALQTY

TE KTRON 1X PN

152-0185-00108-0442-00151-0096-00315-0333-00260-0877-00

PERFORMANCE AS GATED INTEGRATOR - .5ms/div

1NTEGRATOR - Z 1 = 0.1M!l. Zf = O.lIJFINTEGRATOR GATED "ON" 8Y 384 FOR 2.3msINPUT TO INTEGRATOR IV DC

384GATING PULSE

INTEGRATOROUTPUT

"ON" DELAY0.3ms

i10V/cm

f

iO. lV/cm

f

HOFFtI DELAY0.3ms

Fig. 29-8. A low-speed gating adapter for the Type 3A8 Operational Amplifier.

463

29.8 A LOWSPEED GATING ADAPTER FOR THE TYPE 3A8 OPERATIONALAMPLIFIER

Refer to Fig. 29-8. This adapter provides a reedswitch between the negative input and output of anoperational amplifier for various gatingapplications, particularly for gated integrationapplications. The reed drive coil, LI, is poweredfrom the 13 mA available when the calibrator in a

- Tektronix Type 56lB or 564B Oscilloscope is operatedin the 40 volt De position. With no input signal,this 13 mA flows in LI, causing SI to close. Whena positive level is applied to the base of QI, QIconducts, diverting the 13 mA to QI and thus causingSI to open. RI is chosen as 33 kD to allow theadapter to be powered from the 20 volt plus gatesignal available from the Type 3B4 Time Base unit.This adapter may be used in conjunction with lowervoltage gating signaIs by correspondingly reducingthe value of RI. A delay of 0.3 ms is producedbetween the application of the gating pulse and theactivation of SI.

464

INPUT(VIA 3A8l@

04

1 1

RI5kfl

NUI.18EROF STEPSVARIES FROM 4TO >500 WITHCf = O.IIlFANDINPUT PULSESOF 25 VOLTS

OUTPUT(VIA 3ABl@

-+itf.--1 1

+15V_~ __I__I_--1 11

OV- l '

R510kfl ERASE PULSE

'--"IN\.--I_ 1F REQU1REDR48.2k

082.2mA

C3.01u

SELF RESET'_..()-o~J----tl>--__ "'_---l ~

SPRING RETURNACTION FROMRESET TO HOLO

DESCRIPTIONCKT NO TEKTRONIX PN

CI .03UF 100V CAPACITOR 285-0702-00C2 .001UF 200V CAPACITOR 283-0067-00C3 .01UF 200V CAPACITOR 283-0005-00Ct SELECTED BY TYPE 3A8 Zf CONTROL (O.luF)Dl DIODE T12G 152-0008-00

02-06 5 EACH SI DIODE CD61165 5 EACH 152-0245-0007 DIODE T12G 152-0008-0008 2.2mA TUNNEL DIODE TD-2 152-0402-00RI 5kflMIN POT 311-0310-00R2 lMfl1/4W RESISTOR 315-0105-00R3 10kn 1/4W RESISTOR 315-0103-00R4 8.2kfl1/4W RESISTOR 315-0822-00R5 10kn 1/4W RESISTOR 315-0103-00QI 2N130B TRANSISTOR 151-0072-00

MECHANICALQTY1 ADAPTER TERMINAL ASSY 013-0048-011 3-POSITION SWITCH 260-0614-001 KNOB FOR RI 366-0189-002 WASHERS FOR RI AND SWITCH 210-0905-002 NUTS FOR RI AND SWITCH '210-0562-00

STEP OUTPUT WAVEFORM~

OUTPUT VOLTAGE ISPROPORTIONAL TO R4

ERASE PULSE OUT CAN BE USED TOERASE A STORAGE OSCILLOSCOPE WHENTHE STEP RESETS TO ZERO

EXCELLENT STEP LINEARITYFOR SWEEP STEPPING ORSCANNING APPLICATIONS

Fig. 29-9. A resetting step generator adapter for the Type 3A8Operational Amplifier.

-------------- ------- --

465

29.9 A RESETTING STEP GENERATOR ADAPTER FOR THE TYPE 3A8OPERATIONAL AMPLIFIER

Refer to Fig. 29-9. This adapter produces astairstep waveform, the stepping commands beingreceived from an external input. This external inputmay typically be a gating pulse derived from the timebase of an oscilloscope. The output stairstep mayalso be used to erase a storage oscilloscope duringstep reset.

The circuit shown provides a step ramp of 15 voltspeak, with the number of steps continuously variablefrom 4 to 500 or more. The required step commandinput is a 20 to 25 volt gate waveform from a sourceimpedance of 5,000 ohms or less. Componentselection allows for the use of nearly any stepcommand waveform greater than about 1.5 volts.Linearity appears to be better than ±3%.

A negative charge is applied to D2 from the input,the charge is then transferred to Cf by theoperational amplifier to provide a voltage step atthe output. The charge is the charge in CIdeveloped during the "on" time of the input pulse.The charge is transferred to Cf during the off timeof the input pulse. The amount of voltage developedacross Cf for each step is dependent on the inputpulse amplitude, the setting of Rl and the ratioCI/Cf·

The positive going stairstep at the output providesboth collector voltage for QI and current for thetunnel diode D8 which is normally in the Iow voltagestate. When the stairstep reaches the amplitudeat which the current through R4 and D8 reaches2.2 mA, the tunnel diode switches to its highvoltage state, turning on QI. The collector of QIsnaps sharply negative. This negative going stepis coupled to the +grid of the operational amplifiervia C2 and D5, causing the output to fall negative.The negative transition at the output is alsocoupled via C2 and D5 to the positive·grid,completing a regenerative loop and forcing theoutput to continue falling even when D8 has revertedto its low voltage state and QI is turned off. D8and QI, then, only trigger the reset; the "work" isdone by the operational amplifier.

466

D3 prevents the falling output from driving the-grid negative and cancelling the regenerative action;D4 catches the output when it reaches groundpotential. D6 prevents the collector of QI frombeing driven negative.

The negative transition drives the junction of C2and R3 to approximately -15 volts, it then recoversto ground potential via the C2-R2 time constant.Dntil the +grid recovers to 0 volts and D5disconnects, further input pulses to the -grid areshunted to ground via D3 and do not charge thestairstep. This provides a natural holdoff action.The circuit comes out of holdoff with a sharpregenerated step to 0 v.

29.10 A SELF-CONTAlNED RESETTING STAIRSTEP GENERATOR

Refer to Fig. 29-10. This stairstep generatorprovides a 16 step ramp of 1 volt peak, the stepcommand input requiring a negative going transitionof greater than 2 volts in amplitude to initiate thestepping action. This input may typically be thegating pulse derived from the time base of anoscilloscope. The output stairstep may also beused to erase a storage oscilloscope during stepreset or to erase one half of the screen of a splitscreen oscilloscope after 8 steps have beencompleted.

Dl consists of 4 toggling flip-flops with the 111"output of each flip-flop driving the clock input ofthe adjacent flip-flop. Rl, R2, R3, R4 and R5 forma digital to analog converter to allow the digitallevels produced by the 4 flip-flops to be transformedto analog steps. R5 may be reduced in value toprovide less ramp output. A decrease in R5 wf.I.Lalso improve the linearity of the circuit. Thelinearity of the circuit may also be improved byselecting Dl for optimum linearity.

467

t/l;_OV

~

_-IV

16 STEP OUTPUTUl RI R3 R4 RS

SN7493N l')kfl 4kO 2kO 2700 -=-FOUR-BIT BINARYCOUNTER IC --\r- --,,r- -, ,

",

"-,__,

"16" ERASE OUTPUTERASE AFTER 16 STEPS

R8100kO

+ --,___JI "8" ERASE OUTPUT

ERASE AFTER 8 STEPS

CIINPUT .001\lFL:r

Si

STEP GENERATOR WILL STEP ONCE FOR EACHNEGATIVE TRANSITION. MINIMUM AMPLITUDE 2V.

CKT NO DESCRIPTION

C1 .001\lFSOOV CAPACITORDl SILICON DIODED2 SILICON DIODED3 lN751A ZENER DIODEQI 2N3904 TRANSISTORRI 16.2kO ±1% RESISTORR2 7.96kfl±1% RESISTORR3 4.02kO ±1% RESISTORR4 2.0kfl±1% RESISTORRS 2700 ±S% RESISTORR6 20kO ±5% RESISTORR7 1500 1/2W RESISTORR8 100kO ±5% RESISTORUl INTEGRATED CKT TI

SN7493NMECHAN1CAL

QTY

TEKTRONIX PN

283-0078-00152-0185-00152-018S-00152-0279-00IS1-0190-00321-0309-00321-0638-00321-0251-00321-0222-0031S-0271-0031S-0203-00301-0151-00315-0104-00

156-0032-00

11131122

BOX - TERMINAL 202-0054-00CONNECTOR - 8NC MALE 131-0428-00CONNECTOR - BNC FEMALE 131-0106-00CONNECTOR - TERMINAL JACK 131-02S1-00SOCKET - INTEGRATED CKT 136-0269-00COYER - BOTTOM 200-0252-00SETSCREW 4-40 213-0048-00SCREW THREAD-FORMING 213-0141-00

R7lS00

-1 2V 1N (SOmA)

STEPLINEARITY

WITHO. IV OUTR5 = 270

WITH RS AT 2700 THE OUTPUTIS ~ IV BUT IS SLIGHTLYNONLINEAR.WITH R5 AT 270 (TEKTRONIXPN 315-0270-00) THE OUTPUTIS REDUCED TO ~ O.lV, HOWEVER, LINEARITY IS IMPROVED.R8, Q1 AND "8" ERASE OUTPUTONLY REQUIRED W1TH SPLITSCREEN STORAGE OSCILLOSCOPES SUCH AS TYPE 564B.

STEPLINEARITY

WITH1V OUT

R5 = 2700

IF ONLY 8 STEPS REQUIRED,D 1SCONNECT [jJFROM [Œ ANDCONNECT 1T TO Ii}! .LINEARITY MAY BE IMPROVEDBY SELECTING U1.

Fig. 29-10. A self-contained resetting stairstep genera tor.

468

r R1 10kn

C

_1\U' '. ,'"R2

R3 12kn • 75kO '_~ 'y -.:R4 47kn

1 R5 36kn~y 'v

R6 30kn

,... R7 ~1kO /"_- - - ,y

J R8

-:R9 68kll

'> 120kO

0RIO 12kn /"_

1 v ,-Rl1 30kll

, y

r

...• R12 15kn /"_~ -R13 10kn '_v- _ v ':"R14 100kO

'-' - - y

TRODES AND

+

A +

E

M

LL

H

RL

8 ElECELECTRODE CAB LES

+

-

CKT NO DESCRIPTION

RI 10kn 1/8W RESISTORR2 75kO 1/8W RESISTORR3 12k1l1/8W RESISTORR4 47kn 1/8W RESISTORR5 36kn 1/8W RESISTORR6 30kll 1/8W RESISTORR7 24k1l1/8W RESISTORR8 120kll1/8W RES 1STORR9 68kn 1/8W RESISTORRIO 12kn 1/8W RESISTORR11 30kn 1/8W RESISTORRI2 15kn 1/8W RESISTORR13 10kn 1/8W RESISTORR14 100kn 1/8W RESISTOR

MECHANICALQTY

1ACEMLLHRL

6 ea1 ea1 ea

ELECTRODE WITH 4ft CABLEELECTRODE WITH 4ft CABLEELECTRODE WITH 4ft CABLEELECTRODE WITH 4ft CABLEELECTRODE WITH 4ft CABLEELECTRODE WITH 6ft CABLEELECTRODE WITH 4ft CABLEELECTRODE WITH 6ft CABLEBNC CONNECTORSADAPTER-TERMINAL ASSY1/2 INCH GROMMET

TEKTRONIX PN

317-0103-00317-0753-00317-0123-00317-0473-00317-0363-00317-0303-00317-0243-00317-0124-00317-0683-00317-0123-00317-0303-00317-0153-00317-0103-00317-0104-00

012-0121-21012-0121-21012-0121-21012-0121-21012-0121-21012-0121-22012-0121-21012-0121-25131-0352-01013-0048-01*348-0005-00 1

*CONSTRUCTION NOTE:

- REMOVE 6 BANANA PLUGS FROM TERMINAL ASSY- ENlARGE VACATEO HOLES TO 3/8 INCH- EPOXY 6 BNC CONNECTORS INTO HOlES- REMOVE 2 BANANA SOCKETS FROM TERMINAL ASSY- ENLARGE AND J01N VACATED HalES FOR 1/2 INCH GROMMET- REMOVE PLUGS FROM THE ENDS OF THE 8 ELECTRODES

Fig. 29-11. A "Frank" network for vectorcardiographicuse.

469

29.11 A FRANK NETWORK FOR VECTORCARDIOGRAPHIC USE

Refer to Fig. 29-11. This network provides thenecessary electrode signal interconnections andattenuations for vectorcardiographic use usingthe Frank system. Eight silver/silver-chlorideelectrodes are applied to the subject and the threeorthogonal vectorcardiographic signals, x, y, and z,are available as differential signals via three pairsof BNC connectors.

470

29.12 A CURRENT LIMITING ADAPTER FOR PROTECTION FROM ELECTRICSHOCK

Refer to Fig. 29-12. This adapter limits currentbetween the input and output or between the outputand input to less than 300 ~A peak, thus allowingconventional electronic instrumentation to be usedin conjunction with human subjects.

Current limiting is achieved by field effect diodesDl and D2 which limit current in either direction to300 ~A within the voltage rating of the diodes(100 volts). Neons Bl and B2 ensure that thesediodes are not subjected to voltages that exceedtheir voltage rating. Rl and R2 provide sornecurrent limiting to protect Bl and B2 should highamplitude, high-power-capability signals be appliedto either the input or to the output. PotentiometerR3 allows the dynamic impedance of two such adaptersto be balanced to achieve optimum common moderejection ratio in a differential amplifier.

INPUT(FROM

SUBJECT> ~~

NOTE: TWO OF THESE ADAPTERS AREREQUIRED FOR USE WITHDIFFERENTIAL AMPLIFIERS.

BANDWIDTH - DC to 500kHzMAXIMUM SIGNAL INPUT - 50V PEAKADJUST R3 FOR COMMON MODE BALANCE

WHEN USING 2 ADAPTERS WITH ADIFFERENTIAL AMPLIFIER. CMRROF 3A9 WITH 2 ADAPTERS IS100,000:1.

CKT NO DESCRIPTION TEKTRONIX PN

BI NEON BULB 5AH-B 150-0067-00B2 NEON BULB 5AH-B 150-0067-00Dl FEl DIODE 152-0328-00D2 FET DIODE 152-0328-00RI RESISTOR 2kn 1/2W 323-0222-00R2 RESISTOR 2kO 1/2W 323-0222-00R3 POTENTIOMETER 500n 311-0634-00-- GROM'-1ElFOR R3 348-0003-00-- ACCESSORY HOUSING 011-0081-00

TWO ADAPTERS lN USE WITH A TEKTRONIXTYPE 3A9 DIFFERENTIAL AMPLIFIER

LIMITS CURRENT FLOW TO THE SUBJECTFROM THE AMPLIFIER TO LESS THEN 300uA

Fig. 29-12. A current-limiting adapter for protection from electric shock.

471

ClOCKWISE..._1NPUT O-:--.JW"I,.-----'VW"l,.--

RI = lMnMAX INPUT 50V

RI R2330n 5kn CI

.Olj.1F

...--...------:-.• OUTPUT

TO OSCILLOSCOPEOR AMPLIFIERWITH R. = lMn

1

Ci < .001j.1FSIOPENS AT FULL

CLOCKWISE ROTATIONOF R2

R2 VARIES 3dB CUTOFF FREQUENCY BETWEEN2.3kHz AND 35kHz. WITH R2 lN FULLClOCKWISE POSITION, SI IS OPEN AND 3dBCUTOFF IS ABOVE 100kHz.

ACCESSORY HOUSINGKNOB - FOR R2NUT - FOR R2WASHER-FLAT - FOR R2WASHER-lOCKING - FOR R2

011-0081-00366-0189-00210-0583-00210-0905-00210-0046-00

CKT NO DESCRIPTION TEKTRONIX PNCI .Oij.lF50V ±10% CAPACITOR 283-0155-00C2 .0033j.1F100V ±5% CAPACITOR 283-0051-00RI 330n 1/4W ±5% RESISTOR 315-0331-00R2 5kn POTENTIOMETER 311-0656-00SI SWITCH ON R2

MECHANICALQTY

Fig.29-13. A low-pass filter for physiological signal processing.

29.13 A LO-PASS FILTER FOR PHYSIOLŒICAL SIGNAL PROCESSING

Refer to Fig. 29-13. This filter consists of asingle Re network to allow the frequency responseof instrumentation to be limited in order tocorrespondingly lirnitnoise. When used inconjunction with a conventional oscilloscope verticalamplifier, and with R2 fully clockwise, the inputirnpeaanceis 1 MD in parallel with .0033 ~F providinga frequency response in excess of 100 kHz. As R2 isrotated counterclockwise, the frequency response islimited between 35 kHz and 2.3 kHz depending on theposition of R2.

472

01INPUT

40V, 1kHzSQUAREWAVE

RIFROM TEKTRONIX561B OR 564B 33kfl

OSCILLOSCOPE

02CI +

5.6'1103

R2lkn

20mV/DIV

t

C2 04.056'11

CKT NO

CIC2C301020304QIRIR2R3R4R5

MECHANICALQTY

111122

DESCRIPTION TEKTRONIX PN

5.6'11FTANTALUM CAPACITOR 290-0247-00.056'11F±5% CAPACITOR 285-0684-00''11F±10% TANTALUM CAPACITOR 290-0183-00SILICON DIODE 152-0185-00SILICON DIODE 152-0185-00SILICON DIODE 152-0185-0010V ±5% ZENER DIODE 152-0149-002N3904 TRANSISTOR 151-0190-0133kfl5% 1/4W RESISTOR 315-0333-00lkfl5% 1/4W RESISTOR 315-0102-00220fl5% 1/4W RESISTOR 315-0221-003.3kf15% 1/4W RESISTOR 315-0332-0018n 5% 1/4W RESISTOR 315-0180-00

BOX - TERMINALCONNECTOR - BNC MALECONNECTOR - BNC FEMALECOYER - BOTTOMSETSCREW, 4-40SCREW, THREAD-FORMING

202-0054-00131-0428-00131-0106-00200-0252-00213-0048-00213-0141-00

~ ~0.2s/DIV

OUTPUT

E = 50mVotr = O.lms

tf = 0.2ms

Z = 18n/l'11Fo

Fig. 29-14. A pulse-shaping circuit simulating the "action potential.'

473

29.14 A PULSE SHAPING CIRCUIT STIMULATING THE ACTION POTENTIAL

Refer to Fig. 29-14. This device provides pulseshaping on the 1 kHz calibrating squarewaveproduced by Tektronix 56lB or 564B Oscilloscopes.The output amplitude is 50 mV, the output risetime0.1 ms and the output falltime 0.2 ms. The energyof the output pulse is almost all contained in thefrequency spectrum from DC to 10 kHz.

When the signal produced by the oscilloscopecalibrator is at 40 volts, e2 is charged in a pseudoconstant current mode via R2, providing a risetimeof 0.1 ms. When the charge on e2 reaches 10 volts,D4 conducts limiting further voltage increase. This10 volt level is attenuated by R4 and R5 to providea 50 mV output. When the signal produced by theoscilloscope calibrator is at zero volts, a chargeis maintained on CI, allowing Ql to conduct in apseudo-constant current mode to discharge e2 at aconstant rate, providing a falltime of 0.2 ms.

474

29.15 A CONSTANT CURRENT PULSE SOURCE FOR GSR AND OTHER USES

Refer to Fig. 29-15. This pulse generator requiresa 40 volt De power supply which may be obtained froma Tektronix 56lB or 564B Oscilloscope calibrator.Shockley diode D2 is functioning as an oscillator,the frequency of oscillation being determined by Rland Cl. The oscillations produced at the anode ofD2 turn QIan and off via Dl and R2, thus providinga squarewave at the collector of QI which isattenuated via R3 and R4. A constant current 1sproduced by this attenuated squarewave via R5.

1NPUT 0;---.----------,40V DC FROM

561B OR 564BOSCILLOSCOPE

RI100kO

CI4.71lF

2]JA/DIV(ZV, lMO>

R222kO R3

100kO

R4470kO

CKT NO DESCRIPTION TEKTRONIX PN

CI 4.71lFTANTALUM CAPACITOR Z90-0187-0001 33V ZENER DIODE 152-0Z41-0002 SHOCKLEY DIODE 152-0136-0001 ZN4250 TRANS 1STOR 151-0Z19-00RI 100kO 1/4W RESISTOR 315-0104-00RZ ZZkn 1/4W RESISTOR 315-0Z23-00R3 100kO 1/4W RESISTOR 315-0104-00R4 470kn 1/4W RESISTOR 315-0474-00R5 Z.ZMO 1/4W RESISTOR 315-0225-00

ACCESSORY HOUSING 011-0081-00

==10]J5PULSE'"2.5Hz

., T'.'''-OfllIJlOI1-lXII1.00

Fig. 29-15. A constant-current, pulse source for GSR and ether uses.

475

DEFINITIONS

This chapter provides definitions for thebiomedical terminology used throughout this book.These definitions have been taken from eitherWebster's International Dictionary (Second EditionUnabridged 1959), Webster's Seventh New CollegiateDictionary (1967) or Dorland's Illustrated MedicalDictionary (24th Edition, 1965). The appropriatereference is given at the end of each definition.The Websters' are published by G. & C. MerriamCompany, Springfield, Massachusetts, USA and theDorland's is pub1ished by W.B. Saunders Company,Philadelphia, Pennsy1vania, USA and London, U.K.

A accretion - The process of growth or enlargement.(Webster)

alveolus An air cell of the lungs. (Webster)

amnion - A thin membrane forming a closed sacsurrounding the embryos of reptiles, birds andmammals. It contains a thin, watery fluid, theamniotic fluid, in which the embryo is immersed.(Webster)

amniotic - See amnion.

oangstrom - Abbreviated A.

meter. 1 Â = 10-8 cm.One ten-billionth of a(Webster)

anterior - Situated before or toward the front.(Webster)

aorta - The great trunk artery that carries bloodfrom the heart to be distributed by branch arteriesthrough the body. (Webster)

aortic - Pertaining to the aorta. (Webster)

476

arborizations - Formation of or into a formresembling a tree in properties, growth, structure,or appearance, or such a form and arrangement.(Webster)

arrhythmia - An alteration in rhythm of theheartbeat either in time or force. (Webster)

arterioZe - One of the small terminal twigs of anartery that ends in capillaries. (Webster)

artifacts - Any artificial product; any structure orfeature that is not natural. (Dorland)

atria - Plural of atrium.

atrio-ventricuZar - Located between an atrium andventricle of the heart. (Webster)

atrium - An anatomical cavity or passage; especiallythe main chamber of an auricle of the heart or theentire auricle. (Webster)

auricZe - The chamber or either of the chambers ofthe heart that receives blood from the veins andforces it into the ventricle or ventricles.(Webster)

autonomic - Acting independently of volition;relating to, affecting, or controlled by theautonomic nervous system. (Webster)

axon - A usually long and single nerve-cell processthat, as a rule, conducts impulses away from thecell body. (Webster)

B bioeZectric - See bioelectricity.

bioeZectricity - The electrical phenomena whichappear in living tissues. (Dorland)

biophysicaZ - Pertaining to the branch of knowledgeconcerned with the application of physicalprinciples and methods to biological problems.(Webster)

brachiaZ - Relating to the arm or a comparableprocess. (Webster)

477

bradycardia - A slow heart rate. (Webster)

bronchi - Plural of bronchus.

bronchus - Either of two primary divisions of thetrachea that lead respectively into the right andthe left lung; broadly - bronchial tube. (Webster)

bundZe of His - A small band of cardiac musclefibers transmitting the wave of depolarizationfrom the auricles to the ventricles during cardiaccontraction.

c capiZZaries - Any of the smallest vessels of theblood-vascular system connecting arterioles withvenules and forming networks throughout the body.(Webster)

cardiac - Pertaining to the heart. (Dorland)

cardioZogy - The study of the heart and its actionand diseases. (Webster)

cardiovascuZar - Relating to the heart and bloodvessels. (Webster)

catheter - A tubular medical device for insertinginto canals, vessels, passageways, or bodycavities usually to permit injection or withdrawalof fluids or to keep a passage open. (Webster)

ceZZ - A small usually microscopie mass ofprotoplasm bounded externally by a semipermeablemembrane, usually including one or more nueleiand various nonliving products, capable alone orinteracting with other cells of performing aIlthe fundamental functions of life, and forming theleast structural aggregate of living matter capableof functioning as an independent unit. (Webster)

cephaZic - Directed towards or situated on or in ornear the head. (Webster)

cerebeZZum - A large dorsally projecting part of thebrain, especially concerned with the coordinationof muscles and the maintenance of bodilyequilibrium. (Webster)

478

cerebrum - An enlarged anterior or upper part ofthe brain. (Webster)

chronaxy - The time required for the excitation ofa nervous element by a definite stimulus; theminimum time at which a current just double therheobase will excite contraction. (Dorland)

cochlea - A division of the labyrinth of the ear ofhigher vertebrates that is usually coiled like asnail shell and is the seat of the hearing organ.(Webster)

cornea - The transparent part of the coat of theeyeball that covers the iris and pupil and admitslight to the interior. (Webster)

cortex - The outer or superficial part of an organor body structure; especially the outer layer ofgray matter of the cerebrum and cerebellum.(Webster)

cortical - Of, relating to, or consisting of thecortex. (Webster)

cranium - The part that encloses the brain.(Webster)

curare - A dried aqueous extract especially of avine used in medicine to produce muscularrelaxation. (Webster)

cytoplasm - The protoplasm of a cell exclusive ofthat of the nucleus. (Dorland)

D defibrillation - The stoppage of fibrillation ofthe heart. (Dorland)

defibrillator - An apparatus used to counteractfibrillation (very rapid irregular contractionsof the muscle fibers of the heart) by applicationof electric impulses to the heart. (Dorland)

dendrite - Any of the usual branching protoplasmicprocesses that conduct impulses toward the bodyof a nerve celle (Webster)

depolarize - To cause to become partially or whollyunpolarized. (Webster)

479

diastole - A rhythmically recurrent expansionespecially the dilatation of the cavities of theheart during which they fill with blood. (Webster)

diastolic - See diastole.

dicrotic - Having a double beat; being or relatingto the second expansion of the artery that occursduring the diastole of the heart. (Webster)

dorsal - Relating to, situated near or on the back.Especially of an animal or of one of its parts.(Webster)

E ECG - Abbreviation for electrocardiogram. (Dorland)

ectopic - Located away from normal position.(Dorland)

EEG - Abbreviation for electroencephalogram.(Dorland)

electrocardiogram - The tracing made by anelectrocardiograph. (Webster)

electrocardiograph - See electrocardiography.

electrocardiography - The making of graphie recordsof the electric currents emanating from heartmuscle, as a method for studying the action of theheart muscle. (Dorland)

electrode - A conductor used to establish electricalcurrent with a nonmetallic part of a circuit.(Webster)

electroder,mal- See electrodermography.

electrodermography - The recording of the eleetricalresistance of the skin, whieh varies with theamount of sweating, and constitutes a sensitiveindex to the activity of the autonomie nervoussystem. (Dorland)

electroencephalogram - The traeing of brain wavesmade by an eleetroeneephalograph. (Webster)

electroencephalograph - See electroeneephalography.

480

eZectroencephaZography - Recording of electriccurrents developed in the brain by means ofelectrodes applied to the scalp, to the surfaceof the brain, or placed within the substance ofthe brain. (Dorland)

eZectrogastrogram - The graphic record obtained bythe synchronous recording of the electrical andmechanical activity of the stomach. (Dorland)

eZectroZyte - A nonmetallic electric conductor inwhich current is carried by the movement of ions.(Webster)

eZectromyogram - The tracing of muscular actionpotentials by an electromyograph.

eZectromyograph - See electromyography.

eZectromyography - The recording of the changes inelectric potential of muscle. (Dorland)

eZectrophysioZogy - The science of physiology in itsrelations to electricity; the study of theelectric reactions of the body in health.(Dorland)

emboZus - An abnormal particle (as an air bubble)circulating in the blood. (Webster)

embryo - A human or animal offspring prior toemergence from the womb or egg; hence, a beginningor undeveloped stage of anything. (Webster)

EMG - Abbreviation for electromyography. (Dorland)

epiZepsy - Any of various disorders marked bydisturbed electrical rhythms of the central nervoussystem and typically manifested by convulsiveattacks usually with clouding or consciousness.(Webster)

extraceZZuZar - Situated or occurring outside a cellor the cells of the body. (Webster)

extracorporeaZ - Situated or occurring outside thebody. (Dorland)

F fibuZa - The outer and smaller of the two bonesof the leg. (Dorland)

481

fibular - Pertaining to the fibula. (Dorland)

fluoroscopy - Process of using an instrument toobserve the internaI structure of an opaque object(as the living body) by means of X-rays. (Webster)

frontal plane - Section drawing, etc. para1lel to themain axis of the body, and at right angles to thesagittal plane. (Webster)

G galvanic - Of, relating to, or producing a directcurrent of electricity. (Webster)

H hemisphere - Half of any spherical or roughlyspherical structure or organ, as demarcated bydividing it into approximately equal portions.(Dorland)

homogeneity - The quality or state of beinghomogeneous. (Webster)

homogeneous - Of uniform structure or compositionthroughout. (Webster)

infarct - An area of necrosis in a tissue or crganresulting from obstruction of the localcirculation by a thrombus or embolus. (Webster)

infarction - See infarct.

inhomogeneities - See inhomogeneity.

inhomogeneity - Something which is not homogeneous.(Webster)

intracellular - Being or oceurring within aprotoplasmic cell. (Webster)

ion - An atom or group of atoms that carries apositive or negative electric charge as a resultof having lost or gained one or more electrons.(Webster)

iris - The circular pigmented membrane hehind thecornea of the eye. (Dorland)

iso-electric - Uniformly electric throughout, orhaving the same electric potential, and heneegiving off no current. (Dorland)

482

isothermaZ - See isothermic.

isothermie - Having the same temperature. (Dorland)

isotropie - Exhibiting properties with the samevalues when measured along axes in all directions.(Webster)

isotropy - See isotropic.

L ' Zateney - A state of seeming inactivity, such asthat occurring between the instant of stimulationand the beginning of response. (Dorland)

Zobe - A somewhat rounded projection of division ofa bodily organ or part. (Webster)

Zumen - (1) the cavity of a tubular organ (thelumen of a blood vessel). (2) the bore of atube (as of an organ). (Webster)

M manometer - An instrument for measuring the pressureof gases and vapors: pressure gauge. (Webster)

membrane - A thin layer of tissue which covers asurface or divides a space or organ. (Dorland)

metaboZism - The sum of all the physical and chemicalprocesses by which living organized substance isproduced and maintained. (Dorland)

micron - A unit of length to one thousandth of amillimeter. l~ = 10-3 mm = 10-6 meters. (Webster)

mitochondria - Small granules or rod-shapedstructures found in differential staining in thecytoplasm of cells. (Dorland)

mitraZ stenosis - A narrowing of the left atrioventricular orifice. (Dorland)

myocardiaZ - See myocardium.

myocardium - The middle muscular layer of the heartwall. (Webster)

myograph - An apparatus for recording the effects ofa muscular contraction. (Dorland)

myographie - See myograph.

483

N necrosis - Death of tissue, usually as individualcells, groups of cells, or in small localizedareas. (Dorland)

neuron - A nerve cell with its processes,collaterals, and terminations regarded as astructural unit of the nervous system. (Dorland)

neuronal - See neuron.

nomography - A graphie method by which the relationbetween any number of variables may be representedgraphically on a plane surface, such as a piece ofpaper. (Dorland)

nuclei - Plural of nucleus. (Dorland)

nucleus. - A central point, group, or mass aboutwhich gathering, concentration or accretion takesplace -- essential portion of a cell -- groupof nerve cells in the central nervous system.(Webster)

o occipital - See occiput.

occiput - Of or relating to the back part of thehead or skull. (Dorland)

organ - A differentiated structure consisting ofcells and tissues and performing sornespecifiefunction. (Webster)

orthogonal - At right angles to.

orthogonality - See orthogonal.

p parietal - Of, relating to, or forming the upperposterior wall of the head. (Webster)

permeable - See permeate.

permeate - To pass through the pores or interstices.(Webster)

peroneal - Pertaining to the fibula or to theouter side of the leg. (Dorland)

piezoelectric - See piezoelectricity.

484

piezoelectricity - Electricity or electric polaritydue to pressure especially in a crystallinesubstance. (Webster)

plethysmography - The recording of the changes inthe size of a part as modified by the circulationof the blood in it. (Dorland)

plastid - Any specialized organ of the cell otherthan the nucleus. CDorland)

pneumatic - Relating to, or using air, wind, orother gas (a) moved or worked by air pressureCb) adapted for holding or inflated withcompressed air. (Webster)

pneumograph - An instrument for recording thethoracic movements or volume change duringrespiration. (Webster)

pneumotachygraph - An instrument for recording thevelocity of the respired air. (Dorland)

posterior - The hinder parts of the body. (Webster)

protoplasm - The colloidal complex of protein, otherorganic and inorganic substances, and water thatconstitutes the living nucleus, cytoplasm, plastids,and mitochondria of the cell and is regarded asthe only form of matter in which the vitalphenomena are manifested. (Webster)

protoplasmic - See protoplasme

psychogalvanic - See psychogalvanometer.

psychogalvanometer - A galvanometer for recordingthe electrical agitations produced by emotionalstresses. (Dorland)

pulmonary - Relating to, functioning like, orassociated with the lungs. (Webster)

pupil - The contractile aperture in the iris ofthe eye. (Webster)

485

R radical - A group of atoms that is replaceable by asingle atom, that is capable of remaining unchangedduring a series of reactions, or that may show adefinite transitory existence in the course of areaction. (Webster)

radioisotope - An isotope which is radioactive,produced artificially from the element or from astable isotope of the element by the action ofneutrons, protons, deuterons, or alpha particlesin the chain-reacting pile or in the cyclotron.Radioisotopes are used as tracers or indicatorsby being added to the stable compound underobservation, so that the course of the latter inthe body (human or animal) can be detected andfollowed by the radioactivity thus added to it.The stable element so treated is said to be"labeled" or "tagged." (Dorland)

real-time spectrum analyzer - A spectrum analyzerthat performs a continuous analysis of theincoming signal with the time sequence of eventspreserved between input and output.

retina - The sensory membrane of the eye thatreceives the image forrnedby the lens, is theimmediate instrument of vision and is connectedwith the brain by the optic nerve. (Webster)

rheobase - The minimum potential of electric currentnecessary to produce stimulation. (Dorland)

s sagittal - Of, relating to, or situated in themedian plane of the body or any plane parallelthereto. (Webster)

scalp - That part of the integument of the headwhich normally is covered with hair. (Dorland)

selenide - A compound of selenium with an elementor radical. (Webster)

selenium - A nonmetallic element relating to sulphurand tellurium and resembling them chemically.(Webster)

486

semipermeable - Partially but not freely or whollypermeable (Webster)

sinoatrial node - A microscopic collection ofatypical cardiac muscle fibers which isresponsible for initiating each cycle of cardiaccontraction.

sinus - A cavity in the substance of the bone ofthe skull that usually communicates with thenostrils and contains air. (Webster) Also, anyirregular cavity, particularly in the circulatorysystem.

spatial - Pertaining to space. (Dorland)

sphygmomanometer - An instrument for measuringblood pressure and especially arterial bloodpressure. (Webster)

spirometer An instrument for measuring the airentering and leaving the lungs. (Webster)

stereotaxie - Pertaining to or characterized byprecise positioning in space. (Dorland)

stimulate - To excite to functional activity. (Dorland)

stroboscope - An instrument for determining speeds ofrotation or frequencies of vibration made in theform of a rapidly flashing light that illuminatesan object intermittently. (Webster)

stroboscopie - Of, utilizing, or relating to astroboscope. (Webster)

synapse - The point at which a nervous impulsepasses from one neuron to another. (Webster)

synaptic - See synapse.

systemic - Pertaining to or affecting the body as awhole. (Dorland)

systole - The contraction, or period of contraction,of the heart, especially that of the ventricules.It coincides with the interval between the firstand second heart sound, during which blood isforced into the aorta and the pulmonary trunk.(Dorland)

487

systoZic - See systole.

T tachycardia - Relatively rapid heart action.(Webster)

tetrahedron - A solid body having four faces.(Webster)

thermistor - An electrical resistor made of amaterial whose resistance varies sharply in aknown manner with the temperature. (Webster)

thermocoupZe - A thermoelectric couple (a union oftwo conductors -- as bars of dissimilar metalsjoined at their extremities -- for producing athermoelectric current) used to measuretemperature differences. (Webster)

thermoeZectric - Of or relating to phenomenainvolving relations between the temperature andthe electrical condition in a metal or incontacting metals. (Webster)

thoracic - See thorax.

thorax - The part of the body of man and othermammals between the neck and the abdomen.,(Webster)

thrombus - A clot of blood formed within a bloodvessel and rema~n~ng attached to its place oforigin. (Webster)

tibia - The inner and usually larger of the twobones of the vertebrate hind limb between theknee and ankle. (Webster)

tibiaZ - See tibia.

tissue - An aggregation of similarly specializedcells united in the performance of a particularfunction. (Dorland)

torso - The human trunk. (Webster)

trachea - The main trunk of the system of tubes bywhich air passes to and from the lungs.(Webster)

488

transducer - A device that is actuated by power fromone system and supplies power in any other formto a second system. (Webster)

u ulnar - Pertaining to the inner and larger bone ofthe forearm, on the side opposite that of thethumb. (Dorland)

utero - (1) A combining form from uterus, (Webster),as utero-vaginal.

(2) The Latin dative of uterus, as in utero,in the uterus.

uterus - The hollow muscular organ in female animaIswhich is the abode and the place of nourishmentof the embryo and fetus. (Dorland)

v vasoconstriction - Narrowing of the lumen of bloodvessels especially as a result of vasomotoraction. (Webster)

vasomotor - Any element or agent that affects thecaliber of a blood vessel. (Dorland)

vector - A quantity that has magnitude, direction,and sense and that is commonly represented by adirected line segment whose length represents themagnitude and whose orientation in space representsthe direction. (Webster)

ventricle - A chamber of the heart which receivesblood from a corresponding atrium and from whichblood is forced into the arteries. (Webster)

ventricular - See ventricule.

venule - A small vein; especially one of the minuteveins connecting the capillary bed with the largersystemic veins. (Webster)

vertex - The top of the head. (Webster)

viable - Capable of living; especially born alive,with such form and development of organs as to benormally capable of living. (Webster)

viability - See viable.

Acceleration transducer,288-290

Acoustica1 techniques, 201-211Acoustic coupler, 434-435Action potentia1 (defined),

14-15evoked, 148-170simu1ated, 321

Adioventricu1ar rhythm(defined), 66

Alpha rhythm, 44, 45, 131,133, 147

Amp1ifiers, 63-65, 156-158,175-176, 293-317

differentia1, 63-65, 103,156-158, 175-176, 181,196, 226, 294-303,305-335

operationa1, 124, 156-158,326-335

sing1e-ended, 304Ana10g to digital conversion,

432, 435Anions (defined), 8Aorta, 25-27, 95, 112, 475Aortic valve, 26-27Arrhythmia (defined), 66, 476Arteria1 catheterization, 95,

439Arterioles (defined, 25, 476Artifact (defined), 133, 476Atrioventricu1ar bund1e, see

Bund1e of HisAtrioventricu1ar conduction

time, 30Atrioventricu1ar node, 27, 29Atrioventricu1ar valves,

26-27, 207-209, 217Atrium, 25-30, 95, 476Augmented vector (defined),

54Auric1e, 66, 476

489

INDEX

Autonomic nervous system,195-199, 217

Aveo1i (defined), 115Average reference recording

(EEG), 139-140aVF, aVL, aVR (defined), 54AV node, see Atrioventricu1ar

nodeAxial vectorcardiogram, 74-75Axon (defined), 39,476Ba11istocardiogram, 218Bandwidth (amp1ifiers), 319,

321-325, 333Bandwidth 1imiting, 321-325,

333Base1ine drift, see DriftBasi1ar membrane, 37Beta rhythm, 133Bioe1ectric potentia1 (defined),

7Biphasic stimulation, 172-173Bipolar ECG, 53-54Bipo1ar recording (EEG),

139-140B10ndheim configuration, 83B100d f10w measurements, 93,

108-112, 217-218, 430-431dye-dilution techniques,

112-113, 217e1ectroturbinometer, 112

217f1owrneters, 108-111, 217isotherma1, 112, 217thermistors, 112, 217u1trasonic, 112, 211, 217

B100d pressure measurements,93-107, 217-218, 259, 290,387, 439, 442-445

brachial artery, 95catheter use, 94-97, 439direct, 93-99femora1 artery, 97

490

B100d pressure measurements(continued)

indirect, 93, 101-107instrumentation, 102-107,

290, 387Korotkoff sounds, 101-102,

439need1e use, 97p1ethysmograph, 104-107,

217, 439pulse sensor, 104-107relative, 93, 104-107, 217,

439·,442-445sphygmomanometer, 100, 217syringe use, 97termino1ogy, 98-99dye-dilution techniques,

112-113, 217e1ectroturbinometer, 112,

217f1owmeters, 108-111, 217isotherma1, 112, 217thermistors, 112, 217u1trasonic, 112, 211, 217

B100d volume measurements,113, 217-218

Body temperature, seeTemperature

Bonded strain gage, 284-285Bootstrapping, see

Input guardingBrachial artery, 95Brain, 38-45Bronchi, 114-115, 477Bund1e of His, 27, 29-30, 66,

477Burger triangle, 52Cameras, 78, 87, 88, 389-397Cardiac catheters, see

CathetersCardiac output (defined), 112Cardiac vector (defined), 50Cardiography, 209Cardiovascu1ar system, 23-30,

49-112, 207-208,437-446

monitoring in intensivecare, 437-446

Cardioversion, 267Catheters, 63, 94-97, 112,

258-261, 439, 477cardiac, 94-97, 258-261e1ectrode, 63

Cations (defined), 8Cautery, 240-252-253, 259,

265-266Ce11 action potentia1, 7,

14-21Ce11 e1ectrica1 activity, 7-21,

42-43Ce11 size, 7Ce11 stimulation, 16-18Central su1cus, 41Cerebe11um, 41, 477Cerebral circulation, 25Cerebral hemisphere, 41-42, 133Cerebrum, 41, 478Character generator, 387Chart recorders, see

Graphic recordersChemica1 ana1ysis transducers,

292Chemica1 gradient (defined), 8Chopped b1anking, 352, 359Chronaxy (defined), 178,478C1osed-circuit te1evision,

440-441, 443-445CMRR, see Common-mode

rejection ratioCommon-mode rejection ratio

(defined), 295-296Common-mode signal (defined),

295Computers, 159-163, 326, 381,

387, 430-436, 446ana1og, 430-431digital, 432-433, 446signal averaging, 159-163,

376, 435software, 435termina1s for, 434-435time sharing, 432-435

Contact 1ens e1ectrode, 169,217

Continuous-motion photography,389, 394-397, 399

Corpus ca11osum, 41-42Cortex, 40-42, 45, 131, 134,

145, 164-166, 478Cortical stimulation, 364Cranium, 41, 478Cube vectorcardiogram, 74-75Current 1imiting, 261-262, 470Curvi1inear graphic

recordings, 405Cyst detection, 210-211Damping (graphic recorders),

403Data processing, 423, 430-437Data transmission, 423-430Defibrillation, 240, 266-267,

309, 373-374, 478Delta rhythm, 45, 133, 146Dendrite (defined), 39,478Densitometer, 112Depo1arization (defined), 12

asynchronous, 21synchronous, 18thresho1d, 15, 43travelling wave of, 21

Dermometer, 199Diaphasic response (defined),

187Diastole (defined), 26, 479Diasto1ic pressure (defined),

98Dicrotic notch (defined), 99DifferentiaI amplifier, 63,

65, 103, 156-158,175-176, 181, 196, 226,294-303, 305-310

DifferentiaI capacitortransducer, 288

DifferentiaI transformertransducer, 287

Digital indicators, 387Digita1-to-ana1og conversion,

432Disp1acement transducers,

120-121, 283-289Disp1ay devices, 77-78, 338,

356-357, 375-386Doppler u1trasound, 211

491

Drift (amplifier), 225-231,246-247, 310, 315-317

Drift catheters, 97Dua1 beam (defined), 342Dua1 trace (defined), 342Dye dilution techniques,

112-113Dynamic window (defined), 310Ear, 36-37ECG, Bee E1ectrocardiographyEchoencepha1ography, 205Ectopic beats (defined), 66EDR, Bee E1ectroderma1 responseEEG, Bee E1ectroencepha1ographyEinthoven's triangle, 50-59EKG, Bee E1ectrocardiographyE1astic force gage, 121, 217E1ectrica1 skin resistance,

195, 199E1ectric shock, 255-267E1ectric shutter, 392E1ectrocardiography, 28-30,

49-91, 217-218, 231, 237,240, 249-253, 259-261,266-267, 295, 298, 317,385, 405, 426, 432~ 436,438, 440, 442-446,450-451, 479

Einthoven's triangle, 50-59feta1, 80-91, 450-451frontal plane, 48-59, 66interpretation of, 65-66sagittal plane, 48-49, 63transverse plane, 48-49,

60-62vectorcardiography, 67-79

E1ectrocautery, 240, 252-253,259, 265-266

Electrode adapters, 237Electrode application, 245Electrode cab1es, 237Electrode double 1ayer

(defined),223Electrode materia1s,

copper, 219, 226-227, 229,237

1ead, 198-199, 235

492

Electrode materia1s (continued)si1ver, 135, 198-199,

234-235, 237, 242-243,245, 247

si1ver/si1ver ch1oride,135, 169, 173, 224,226-229, 231-232, 237,240, 247

stain1ess steel, 226-227,229, 240, 242

zinc/zinc sulfate, 224Electrode offset potentia1

(defined), 221Electrode paste, 225, 233Electrode placement, 52-63,

69-74, 82-87, 135-136,143, 169

ECG, 52-63EEG, 135-136ERG, 169feta1 ECG, 82-87intracrania1, 143VCG, 69-74

Electrode potentia1s, 220Electrode types, 15, 34-35,

63, 98, 135, 143,150-156, 164, 166,169, 173, 182, 187,189, 198-199, 217,219-248, 258, 261

catheter, 63, 258, 261contact 1ens, 169, 217micro, 15, 34-35, 98,

150-156, 164, 166,217, 243-245

glass, 151-153, 243-245meta1, 151, 243-245pressure, 98

need1e, 34-35, 143, 166,182, 189, 217, 240-242

concentric, 35, 143,166, 187, 242

insu1ated, 35, 143, 242spray on, 240surface, 135, 169, 173,

189, 198-199, 217,219-240, 369, 439

direct contact, 233

disposab1e, 240indirect contact (f1oating),

233reusab1e, 233-239

E1ectroencepha1ography,44-45, 131-147, 162-163,217-218, 231, 235, 237,245, 249, 254-255, 295,317, 406, 432, 480

e1ectrodes, 134-139, 231, 235,237, 245

externa1 stimu1ii, 147line interference, 135,

254-255recording modes, 139-140signal averaging, 162-163spectrum ana1ysis, 141

E1ectrogastrogram, 193,217-218, 480

E1ectromagnetic f1owmeter,108-111, 217

E1ectromyography, 35, 166,181-193, 217-218, 242,249, 367, 480

E1ectronystagmogram, 218E1ectroretinogram, 37, 169-170,

217-218E1ectroturbinometer, 112, 217Emergency gate, 34EMG, Bee E1ectromyographyEndosomatic response, 195Epi1epsy, 145-146, 480Equivalent noise resistance,

311ERG, Bee E1ectroretinogramESR, see E1ectrica1 skin

resistanceEvoked action potentia1s, 45,

149-170Excess noise (amplifier), 315,

321-322Expiratory reserve volume

(Lung), 116Eye, 36-37, 206, 210-211Fau1t current detector, 264F-EGG, Bee

Feta1 e1ectrocardiographyFemoral artery, 97

Fere effect, 195Fetal electrocardiography,

80-91, 217conflict with maternaI ECG,

82, 90electrode placement, 82-87multiple pregnancy, 90subject preparation, 82-83

Fetal ultrasonography, 210-211Flicker noise, 315Flowmeter, 108-111, 217Force transducers, 284-288,

290Frank electrode system, 70-73Frequency response, 319, 404,

416-418, 471amplifiers, 319graphic recorders, 404tape recorders, 416-418

Frontal lobe, 41Frontal plane ECG, 50-59,

66-67bipolar limb 1eads, 53-54Burger's triangle, 52cardiac vector, 50-57chest c1uster, 58Einthoven's triangle, 50-59e1ectrode position, 53-58noise, 58unipo1ar 1imb 1eads, 53-59waveform po1arity, 54

Frontal VGG, 68-77Functiona1 residua1 capacity

(lung), 116Gage factor (defined),

272-273Ga1vanic skin reflex,

194-198, 217-218, 235,237, 474

Galvanometic graphicrecorders, 403-404

Gamma rhythm, 133Geiger-Muller counter, 292Graphic recorders, 63-64,

129, 133, 136, 140,399-411, 444-445

Grounding, 249-255, 259-260,262-266

493

Ground loop, 249-251, 425GSR, Bee Ga1vanic skin reflexHair ce11s (ear), 37Half-ce11 potentia1 (defined),

219Heart potentia1s, 23-30, 49-91,

217, 249-267 Bee alBOE1ectrocardiogram

feta1 e1ectrocardiogram,80-91

safety, 249-267vectorcardiogram, 67-79

Hepatic portal circulation, 25Hodgkin-Huxley theory, 7-8Horizontal amp1ifiers

(oscilloscope), 339H reflex, 187-189, 217Hum (defined), 310Impedance pneumograph,

121-122, 217Indicators, 375, 387-388Indifferent e1ectrode

(defined), 54Inferior vena cava, Bee

Vena cavaInput guarding (defined),

302-303Input neutralization, 157-158,

458-459Input norma1izers, 359Inspiratory capacity (lung),

116Inspiratory reserve volume

(lung), 116Intensive care, 437-446Iontophoresis, 247Jitter, 162Johnson noise, 315, 321-322Korotkoff sounds, 101-102, 287,

439Latency (defined), 164, 482Light-sensitive resistor

transducer, 292Line interference, 135, 137,

254-255, 294-295, 325Line-pair (defined), 383Lowpass fi1ter, 158Lumen (defined), 109,482

494

Lung, 115capacity of, 116-117physio1ogy of, 114-115

Magnetic tape, BeeTape recording

Manometer, 98, 217, 482Median nerve, 192Medu1la ob1ongata, 41Microe1ectrode, Bee

Electrode typesMicromanipulator, 166Middle ear, 36-37Minute respiratory volume

(defined), 125Mitral valve, Bee

Atrioventricu1ar valveModem, 434-435Monophasic response (defined),

187Motoneuron, 31Motor cortex, 41-42Motor end-plate, 31Motor nerve propagation

ve1ocity, 189-190Motor unit (defined), 31Moving-coi1 meter, 375, 387M reflex, 188-189Mu1tiplexing (defined), 429Muscle action, 32, 180-193Mye1in sheath, 38Myocardia1 infarction, 23,

437Myocardium, 30, 482Myography, 180, 217, also Bee

E1ectromyographyNeed1e e1ectrodes, see

Electrode typesNernst relation, 8, IlNerve conduction, 189-192Nerve propagation ve1ocity,

189-192Nerve tissue thresho1d

(defined),177Net gradient (defined), IlNeurog1ia (defined), 40Node of Ranvier, 38Noise, 135, 158-163, 225,

227, 246-247, 310-315,421, 471

Nucleus (defined), 39Occipital lobe, 41, 131, 483Offset potentia1 (defined), 221One ov~r f noise, 315, 322Optica1 graphic recorders, 411Optica1 transducers, 286, 292Osci11ographs, Bee

Graphic recordersOscilloscope cameras, 78, 82,

88, 389-397Oximetry, 218Pacemaker, 258, 370-372, 440,

444Parietal lobe, 41, 483Peak-to-peak noise, 313Peronea1 nerve, 192, 483Persistance (phosphor), 341PGR, Bee

Psychoga1vanic reflexPhonocardiogram, 218Photographic film, 389,

391-392, 397Photoresistor, 292Physio1ogica1 monitors, 64-65Piezoe1ectric transducer,

286-287P1ethysmograph, 104-105,

217-218, 439, 442-443,484

Pneumograph, 117-122, 217,442-445, 483

conductive-f1uid in tube,121, 284

strain gage, 121thermistor, 118, 442-443torso impedance, 122

PneumotachQgraph, seePneumotach

Pneumotach, 122-125, 217-218,484

Po1arized ce11 (defined), IlPotentiometic graphie

recorders, 404Power supp1y noise, 317Pressure transducers, 93, 95,

97-98, 288-290PR interva1, 28Probes (oscilloscope),

356-359

495

Projected Graticu1e, 390-391Protection (subject), see

SafetyPsychoga1vanic reflex, 195Pu1monary arteries, 25-27,

112, 114Pu1monary circulation

(defined), 25Pu1monary valve, 26-27Pu1monary veins, 27, 114Pulse generators, 361-374Pulse sensor, see

P1ethysmographPurkinje system, 27, 29P wave, 28, 30, 57, 66, 74QRS segment, 28, 30, 55, 57,

66, 72, 74, 77, 91QT interva1, 28Radiation transducer, 292Radioisotope, 112Rate intensification, 348Recti1inear graphic

recordings, 406Reflex response, 33-34Refractory period (defined),

16Regenerative breakdown, 15Renal circulation, 25Residua1 volume (lung), 116Resistance pneumograph,

121-122Resistive transducers,

270-285, 454-455Resolution (CRT), 382-385Respiration, 114-127, 217,

442-445air f1ow, 122-125instrumentation, 118-1271ung volume, 116-117physio1ogy of, 115-117pneumograph, 117-122, 217,

442-445, 454-457pneumotach, 122, 125

Resting potentia1, 7Retina, 36-37Rheobase (defined), 177, 485Risetime, 321RMS noise, 311

R wave, 28, 50, 55, 82, 88,91, 267

Safety, facing l, 65, 79, 169,224, 255-267, 470

Sagittal plane ECG, 48-49, 63,67

Sagittal VCG, 68-77SA node, see

Sinoatria1 nodeSchwann ce11, 38S-D curve, see

Strength-duration curveSemiconductor strain gage,

284-285Sensitivity factor (defined),

276Sensory cortex, 41-42Sensory nerve propagation

ve1ocity, 190-192Sensory system, 36-37Shie1ded cable, 423-425

bandwidth, 423-425losses, 423

Shock (e1ectric), 255-267,470Signal averaging, 159-163, 326Simu1ated action potentia1, 321Sinoatria1 node, 27, 29-30, 486Ske1eta1 muscle (defined), 193Skin resistance, see Ga1vanic

skin reflexSlave oscilloscope, 338,

351-360, 378Smooth muscle (defined), 193Sodium-potassium pump

(defined), 15Soma (defined), 39Spectrum ana1ysis (EEG), 141Speed Computer, 392Sphygmomanometer, 100, 217, 486Spinal nerves, 38-45Spirogram, 117, 124, 217Spirometer, 117, 126, 217, 486Spot size, 383Spray-on e1ectrode, 240Sterotaxic instruments, 166-167Stewart-Hamilton dye dilution

technique, 112Stimu1ating e1ectrodes, 247

496

Stimulations, 166, 169-179,361-374

biphasic, 172-173constant current, 172-173,

367-369constant voltage, 172,

367-369cortical, 364grounded, 175, 187iso1ated, 175-176, 364,

368-3701ight, 169

Stimulus artifact (defined),176

Stimulus isolation, 175-176,364, 368-370

Stimulus thresho1d (defined),16

Strain (defined), 273Strain gage, 284-285Strength/duration curves,

176-179, 217Striated muscle (defined),

193Strip chart recorder, see

Graphic recordersStroke, 437ST segment, 28Subject protection, see

SafetySuperior vena cava, see

Vena cavaSurface e1ectrodes, see

Electrode typesS wave, 28, 55Sweat gland activity, 184-191Sweep b1anking, 352, 359Sweep generators

(osci11oscope),339Sweep stepping, 385-386Synapse (defined), 39Systemic circulation

(defined),25Systole (defined), 26, 486Systo1ic pressure (defined),

98Tachycardia (defined), 66, 487Tangentia1 noise, 158, 313

Tape recording, 63, 136,413-421, 444

direct recording, 415-416frequency response, 416-418indirect FM recording,

417-418noise, 421tape speeds, 415, 419transport mechanism, 419-420

Tarchanoff effect, 195TA wave, 28, 30Te1emetry, 423, 426Television, 360, 379-381Temperature, 128-129,

217-218, 387, 410, 440,442-445

Temporal lobe, 41Terminal arborization

(defined), 39Tetrahedron vectorcardiogram,

74-75Thermal noise, 315Thermistors, 112, 118-121,

128-129, 217, 270, 275,291, 440, 442-433,454-457, 487

b100d f1ow, 112body temperature, 128-129pneumograph, 117-121, 217,

442-433, 454-457respiration, 118-121temperature, 128-129

Thermocoup1e, 291, 487Thermometer, 129, 217Theta rhythm, 133Tibia1 nerve, 192Tida1 volume (lung), 116, 218Time-division mu1tip1exing,

428-430Time jitter, 162Time-motion u1trasonography,

207-209Trachea, 114, 487Transducer systems, 269-292,

439Transverse plane ECG, 48-49,

60-62, 67Transverse VCG, 68-77

Travelling wave ofdepolarization, 21,34-35

Tricuspid valve, seeAtrioventricular valve

Triphasic response (defined))187

Tumors, 146T wave, 28, 30, 57, 66, 74,

256, 267Ulnar nerve, 192Ultrasonic scanning, 209-211,

217Ultrasonography, 201-211,

217Unbonded strain gage, 284-285Unipolar ECG, 53-54Unipolar esophageal lead ECG

(defined),63Unipolar recording (EEG),

139-140U wave, 28, 30VCG, see VectorcardiographyVectorcardiography, 67-79,

217axial electrodes, 74-75cube electrodes, 74-75electrode placement, 69-74Frank electrode system,

70-73, 468-469frontal, 68instrumentation, 70-79normal response, 72, 74sagittal, 68spatial, 68tetrahedron electrodes, 75transverse, 68

Velocity transducer, 289Vena cava, 25, 95, 112Ventricle, 25-30, 95Ventricular fibrillation, 66,

256-258, 267Vertical amplifiers

(oscilloscope),338-339VF, VL, VR (defined), 54Vibration transducer, 289Video ~isplay, 360, 379-381Visua1 cortex, 41-42, 164-165

497

Vital capacity (lung), 116V lead measurements (defined),

60Voltage differentiation and

integration, 334-335Waveform generators, 363-364Weight transducer, 288, 290Wheatstone bridge, 271-282, 287X-y chart recorders, 411

499

REFERENCES TO TEKTRONIX PRODUCTS

C-10 Trace-Recording Camera, 78,390, 392

C-12 Trace-Recording Camera,390-391, 397

C-27 Trace-Recording Camera, 390,392

C-30A Trace-Recording Camera, 82,88, 390, 392

C-50 Trace-Recording Camera, 390,392

o Operationa1 Amplifier, 4502A63 DifferentiaI Amplifier, 1032B67 Time Base, 106, 108, 120,

124, 156, 180, 205-206,282-283, 345, 356-358, 360

3A3 Dua1-Trace DifferentiaIAmplifier, 344

3A6 Dua1-Trace Amplifier, 384,386

3A72 Dua1-Trace Amplifier, 181,344

3A74 Four-Trace Amplifier, 79,344, 358, 360

3A75 Amplifier, 3443A8 Operationa1 Amplifier, 124,

156-158, 181, 326, 330-335,344, 348-349, 458-466

3A9 DifferentiaI Amplifier, 181,196, 278, 282-283, 307, 311,317, 322-323, 344, 348, 357

3B4 Time Base, 181, 345, 384, 3863C66 Carrier Amplifier, 98, 103,

106-107, 120-123, 180,280-281, 344

3L5 Spectrum Ana1yzer, 142-143,344

129 P1ug-in Unit Power Supp1y,330

160A, 161, 162, 163, 360 PulseGenerator Series, 364-367

410 Physio1ogica1 Monitor, 64-65,77-79, 85-88, 104-105,118-119, 144, 169-170,233-237, 261-262, 282-283,305, 309, 346-347, 393, 409,438, 442-443, 450-457

504 Oscilloscope, 160561B Oscilloscope, 79, 197,

205-206, 280-281, 342, 345,356, 360, 473-474

564B Storage Oscilloscope, 79,103, 106, 120, 122, 124,127, 142-143, 156-157, 180,181, 197, 205-206, 280-283,342, 345, 356-357, 384,386, 473-474

565 Dual-Beam Oscilloscope, 342,345, 348

601 Storage Display Unit,356-357, 376-378, 382

602 Display Unit, 356-357,376-378, 382-383

611 Storage Disp1ay Unit, 77-78,376-378, 382, 384-385, 435

T4002 Graphie Computer Terminal,434-435

4501 Scan Converter Unit,358-360, 377, 379-381

5031 Dual-Beam StorageOscilloscope, 78, 338,342-343

BOOKS lN THIS SERIES:

CIRCUIT CONCEPTS

tille

Digital Concepts

Horizontal Amplifier Circuits*

Oscilloscope Cathode-Ray Tubes*

Oscilloscope Probe Circuits*

Oscilloscope Trigger Circuits*

Power Supply Circuits*

Sampling Oscilloscope Circuits

Spectrum Analyzer Circuits

Storage Cathode-Ray Tubes and Circuits

Sweep Generator Circuits*

Television Waveform Processing Circuits

Vertical Amplifier Circuits"

*7-book set covering Real-Time Oscilloscopes

MEASUREMENT CONCEPTS

Automated Testing Systems

Engine Analysis

Information Display Concepts

Probe Measurements

Semiconduetor Deviees

Spectrum Analyzer Measurements

Television System Measurements

Time-Domain Refleetometry Measurements

Transdueers Measurements

Biophysieal Measurements

part number

062-1030-00

062-1144-00

062-0852-01

062-1146-00

062-1056-00

062-0888-01

062-1172-00

062-1055-00

062-0861-01

062-1098-01

062-0955-00

062-1145-00

062-1180-00

062-1106-00

062-1074-00

062-1005-00·

062-1120-00

062-1009-00

062-1070-00

062-1064-00

062-1244-00

062-1246-00

062-1247-00

Tektronix - Biophysical Measurements (1970)

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Transcript of Tektronix - Biophysical Measurements (1970)