Basic Pacing Concepts Part III. Sensing Sensing zSensing is the ability of the pacemaker to...

Basic Pacing ConceptsBasic Pacing ConceptsPart IIIPart III

SensingSensing

SensingSensing

Sensing is the ability of the pacemaker to “see” when a natural (intrinsic) depolarization is occurring

– Pacemakers sense cardiac depolarization by measuring changes in electrical potential of myocardial cells between the anode and cathode

An Electrogram (EGM) is the Recording of Cardiac An Electrogram (EGM) is the Recording of Cardiac Waveforms Taken From Within the HeartWaveforms Taken From Within the Heart

Intrinsic deflection on an EGM occurs when a depolarization wave passes directly under the electrodes

Two characteristics of the EGM are:

– Signal amplitude

– Slew rate

Slew Rate of the EGM Signal Measures the Change in Slew Rate of the EGM Signal Measures the Change in Voltage with Respect to the Change in TimeVoltage with Respect to the Change in Time

The longer the signal takes to move from peak to peak:

– The lower the slew rate

– The flatter the signal

Higher slew rates (number in mV) translate to greater sensing

– Measured in volts per second

Vo

lta

ge

Time

Slope

Slew rate=Change in voltage

Time duration ofvoltage change

A Pacemaker Must Be Able to Sense A Pacemaker Must Be Able to Sense and Respond to Cardiac Rhythmsand Respond to Cardiac Rhythms

Accurate sensing enables the pacemaker to determine whether or not the heart has created a beat on its own

The pacemaker is usually programmed to respond with a pacing impulse only when the heart fails to produce an intrinsic beat

Accurate Sensing...Accurate Sensing...

Ensures that undersensing will not occur –the pacemaker will not miss P or R waves that should have been sensed

Ensures that oversensing will not occur – the pacemaker will not mistake extra-cardiac activity for intrinsic cardiac events

Provides for proper timing of the pacing pulse – an appropriately sensed event resets the timing sequence of the pacemaker

Undersensing . . . Undersensing . . .

Pacemaker does not “see” the intrinsic beat, and therefore does not respond appropriately

Intrinsic beat not sensed

Scheduled pace delivered

VVI / 60

OversensingOversensing

An electrical signal other than the intended P or R wave is detected

Marker channel shows intrinsic

activity...

...though no activity is present

VVI / 60

Sensitivity – The Greater the Number, the Sensitivity – The Greater the Number, the LessLess Sensitive the Device to Intracardiac EventsSensitive the Device to Intracardiac Events

SensitivitySensitivityA

mp

litu

de

(mV

)

Time

5.0

2.5

1.25

Accurate Sensing Requires That Accurate Sensing Requires That Extraneous Signals Be Filtered OutExtraneous Signals Be Filtered Out

Sensing amplifiers use filters that allow appropriate sensing of P waves and R waves and reject inappropriate signals

Unwanted signals most commonly sensed are:

– T waves

– Far-field events (R waves sensed by the atrial channel)

– Skeletal myopotentials (e.g., pectoral muscle myopotentials)

Accurate Sensing is Dependent on . . .Accurate Sensing is Dependent on . . .

The electrophysiological properties of the myocardium

The characteristics of the electrode and its placement within the heart

The sensing amplifiers of the pacemaker

Factors That May Affect Sensing Are:Factors That May Affect Sensing Are:

Lead polarity (unipolar vs. bipolar)

Lead integrity

– Insulation break

– Wire fracture

EMI – Electromagnetic Interference

Unipolar SensingUnipolar Sensing

Produces a large potential difference due to:

– A cathode and anode that are farther apart than in a bipolar system

_

Bipolar SensingBipolar Sensing

Produces a smaller potential difference due to the short interelectrode distance

– Electrical signals from outside the heart such as myopotentials are less likely to be sensed

An Insulation Break May Cause Both An Insulation Break May Cause Both Undersensing or OversensingUndersensing or Oversensing

Undersensing occurs when inner and outer conductor coils are in continuos contact

– Signals from intrinsic beats are reduced at the sense amplifier and amplitude no longer meets the programmed sensing value

Oversensing occurs when inner and outer conductor coils make intermittent contact

– Signals are incorrectly interpreted as P or R waves

A Wire Fracture Can Cause Both A Wire Fracture Can Cause Both Undersensing and OversensingUndersensing and Oversensing

Undersensing occurs when the cardiac signal is unable to get back to the pacemaker – intrinsic signals cannot cross the wire fracture

Oversensing occurs when the severed ends of the wire intermittently make contact, which creates potentials interpreted by the pacemaker as P or R waves

Electromagnetic InterferenceElectromagnetic Interference

Electromagnetic Interference (EMI)Electromagnetic Interference (EMI)

Interference is caused by electromagnetic energy with a source that is outside the body

Electromagnetic fields that may affect pacemakers are radio-frequency waves

– 50-60 Hz are most frequently associated with pacemaker interference

Few sources of EMI are found in the home or office but several exist in hospitals

EMI May Result in the Following Problems:EMI May Result in the Following Problems:

Oversensing

Transient mode change (noise reversion)

Reprogramming (Power on Reset or “POR”)

Oversensing May Occur When EMI Signals Are Oversensing May Occur When EMI Signals Are Incorrectly Interpreted as P Waves or R WavesIncorrectly Interpreted as P Waves or R Waves

Pacing rates will vary as a result of EMI:

– Rates will accelerate if sensed as P waves in dual-chamber systems (P waves are “tracked”)

– Rates will be low or inhibited if sensed in single-chamber systems, or on ventricular lead in dual-chamber systems

EMIEMI

“Noise” sensed by the pacemaker

Should have paced

Noise ReversionNoise Reversion

VPVPSRSR SR SR

Noise Sensed

Lower Rate Interval

VVI/60

Continuous refractory sensing will cause pacing at the lower or sensor driven rate

EMI May Lead to Inadvertent Reprogramming EMI May Lead to Inadvertent Reprogramming of the Pacing Parametersof the Pacing Parameters

Device will revert to Power on Reset (POR or “backup” mode)

– Power on Reset may exhibit a mode and rate change which are often the same as ERI

– In some cases, reprogrammed parameters may be permanent

New technologies will continue to create new, unanticipated sources of EMI:

Cellular phones (digital)

Sources of EMI Are Found Most Sources of EMI Are Found Most Commonly in Hospital EnvironmentsCommonly in Hospital Environments

Sources of EMI that interfere with pacemaker operation include surgical/therapeutic equipment such as:

– Electrocautery

– Transthoracic defibrillation

– Extracorporeal shock-wave lithotripsy

– Therapeutic radiation

– RF ablation

– TENS units

– MRI

Sources of EMI Are Found More Rarely in:Sources of EMI Are Found More Rarely in:

Home, office, and shopping environments

Industrial environments with very high electrical outputs

Transportation systems with high electrical energy exposure or with high-powered radar and radio transmission

– Engines or subway braking systems

– Airport radar

– Airplane engines

TV and radio transmission sites

Electrocautery is the Most Common Electrocautery is the Most Common Hospital Source of Pacemaker EMIHospital Source of Pacemaker EMI

Outcomes

– Oversensing–inhibition

– Undersensing (noise reversion)

– Power on Reset

– Permanent loss of pacemaker output (if battery voltage is low)

Precautions

– Reprogram mode to VOO/DOO, or place a magnet over device

– Strategically place the grounding plate

– Limit electrocautery bursts to 1-second burst every 10 seconds

– Use bipolar electrocautery forceps

Transthoracic Defibrillation Transthoracic Defibrillation

Outcome

– Inappropriate reprogramming of the pulse generator (POR)

– Damage to pacemaker circuitry

Precautions

– Position defibrillation paddles apex-posterior (AP) and as far from the pacemaker and leads as possible

Magnetic Resonance Imaging (MRI) is Generally Magnetic Resonance Imaging (MRI) is Generally Contraindicated in Patients with PacemakersContraindicated in Patients with Pacemakers

Outcomes

– Extremely high pacing rate

– Reversion to asynchronous pacing

Precautions

– Program pacemaker output low enough to create persistentnon-capture, ODO or OVO mode

Lithotripsy Shock Waves May Have an Lithotripsy Shock Waves May Have an Effect on Pacemaker SystemsEffect on Pacemaker Systems

Outcomes indual-chamber modes:

– Inhibition of ventricular pacing

Outcomes in rate adaptive pacemakers

– High pacing rates

– Piezoelectric crystal damage

Precautions:

– Program pacemaker to VVI or VOO mode

– Lithotriptor focal point should be greater than 6 inches from the pacemaker

– Carefully monitor heart function throughout procedure

Radiation Energy May Cause Radiation Energy May Cause Permanent Damage Permanent Damage

Certain kinds of radiation energy may cause damage to the semi-conductor circuitry

– Ionizing radiation used for breast or lung cancer therapy

Damage can be permanent and requires replacement of the pacemaker

Therapeutic Radiation May Therapeutic Radiation May Cause Severe DamageCause Severe Damage

Outcomes:

– Pacemaker circuit damage

– Loss of output

– “Runaway”

Precautions:

– Keep cumulative radiation absorbed by the pacemaker to less than 500 rads; shielding may be required

– Check pacemaker after radiation sessions for changes in pacemaker function (can be done transtelephonically)

Pacemaker Features That Pacemaker Features That Address Interference Address Interference

InputBandpass

filterAbsolute

valueReversion

circuitLevel

detector

Sensitivity adjustment

Pacemakerlogic

Pacemaker sensing circuits amplify, filter and either process or reject incoming signals

Rate Responsive PacingRate Responsive Pacing

Rate ResponseRate Response

Rate responsive (also called rate modulated) pacemakers provide patients with the ability to vary heart rate when the sinus node cannot provide the appropriate rate

Rate responsive pacing is indicated for:

– Patients who are chronotropically incompetent (heart rate cannot reach appropriate levels during exercise or to meet other metabolic demands)

– Patients in chronic atrial fibrillation with slow ventricular response


Cardiac output (CO) is determined by the combination of stroke volume (SV) and heart rate (HR)

SV X HR = CO

Changes in cardiac output depend on the ability of the HR and SV to respond to metabolic requirements


SV reserves can account for increases in cardiac output of up to 50%

HR reserves can nearly triple total cardiac output in response to metabolic demands


When the need for oxygenated blood increases, the pacemaker ensures that the heart rate increases to provide additional cardiac output

Adjusting Heart Rate to Activity

Normal Heart Rate

Rate Responsive PacingFixed-Rate Pacing

Daily Activities

A Variety of Rate Response Sensors ExistA Variety of Rate Response Sensors Exist

Those most accepted in the market place are:

– Activity sensors that detect physical movement and increase the rate according to the level of activity

– Minute ventilation sensors that measure the change in respiration rate and tidal volume via transthoracic impedance readings


Activity sensors employ a piezoelectric crystal that detects mechanical signals produced by movement

The crystal translates the mechanical signals into electrical signals that in turn increase the rate of the pacemaker

Piezoelectric crystal


Minute Ventilation (MV) is the volume of air introduced into the lungs per unit of time

MV has two components:

– Tidal volume–the volume of air introduced into the lungs in a single respiration cycle

– Respiration rate–the number of respiration cycles per minute


Minute ventilation can be measured by measuring the changes in electrical impedance across the chest cavity to calculate changes in lung volume over time

Pacing systems

Electrical concepts

Stimulation thresholds

Sensing

Electromagnetic Interference (EMI)

Rate response

Summary of Basic Pacing Concepts Summary of Basic Pacing Concepts ModuleModule

General Medtronic Pacemaker DisclaimerINDICATIONS

Medtronic pacemakers are indicated for rate adaptive pacing in patients who may benefit from increased pacing rates concurrent with increases in activity (Thera, Thera-i, Prodigy, Preva and Medtronic.Kappa 700 Series) or increases in activity and/or minute ventilation (Medtronic.Kappa 400 Series).

Medtronic pacemakers are also indicated for dual chamber and atrial tracking modes in patients who may benefit from maintenance of AV synchrony. Dual chamber modes are specifically indicated for treatment of conduction disorders that require restoration of both rate and AV synchrony, which include various degrees of AV block to maintain the atrial contribution to cardiac output and VVI intolerance (e.g., pacemaker syndrome) in the presence of persistent sinus rhythm.

9790 Programmer

The Medtronic 9790 Programmers are portable, microprocessor based instruments used to program Medtronic implantable devices.

9462

The Model 9462 Remote Assistant™ is intended for use in combination with a Medtronic implantable pacemaker with Remote Assistant diagnostic capabilities.

CONTRAINDICATIONS

Medtronic pacemakers are contraindicated for the following applications:

Dual chamber atrial pacing in patients with chronic refractory atrial tachyarrhythmias.

Asynchronous pacing in the presence (or likelihood) of competitive paced and intrinsic rhythms.

Unipolar pacing for patients with an implanted cardioverter-defibrillator because it may cause unwanted delivery or inhibition of ICD therapy.

Medtronic.Kappa 400 Series pacemakers are contraindicated for use with epicardial leads and with abdominal implantation.

WARNINGS/PRECAUTIONS

Pacemaker patients should avoid sources of magnetic resonance imaging, diathermy, high sources of radiation, electrosurgical cautery, external defibrillation, lithotripsy, and radiofrequency ablation to avoid electrical reset of the device, inappropriate sensing and/or therapy.

9462

Operation of the Model 9462 Remote Assistant™ Cardiac Monitor near sources of electromagnetic interference, such as cellular phones, computer monitors, etc. may adversely affect the performance of this device.

See the appropriate technical manual for detailed information regarding indications, contraindications, warnings, and precautions.

Caution: Federal law (U.S.A.) restricts this device to sale by or on the order of a physician.

Medtronic Leads

For Indications, Contraindications, Warnings, and Precautions for Medtronic Leads, please refer to the appropriate Leads Technical Manual or call your local Medtronic Representative.

Caution: Federal law restricts this device to sale by or on the order of a Physician.

Note:

This presentation is provided for general educational purposes only and should not be considered the exclusive source for this type of information. At all times, it is the professional responsibility of the practitioner to exercise independent clinical judgment in a particular situation.

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