Anaesthesia Mc Kh32 C.pdf

7/27/2019 Anaesthesia Mc Kh32 C.pdf

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naesthesia Machine

32.1 N ED FOR N ESTHESISurgical methodsof treatment consists mainly of operations whi ch are normally carried out undersome form of anaes thesia. Anaesthesia serves the following two functions:

1. tensures that the patient does not feel pain and minimizes patient discomfort; and2. tprovides the surgeon with favourable conditions for the work.

When anaesthesia is given so th at the patient loses consciousness, it is calle genera l anaesthesia . In general anaesthesia , the anaes thetic agent is administered to the body so that it reachesthe brain via the blood stream. The usual meth od is inhalation anaesthesia in which gase ou sanaesthetic agents are introduced via the lungs. Exam ples of such agents are diethyl ether,chloroform , hal othane, cyclopropane and nitrous oxide N20 laughing gas). During anaesthesia,not only is the an aesthetic administered in the required amount but also oxygen. Any excesscarbon dioxide is also eliminated . the superficial stages of an aesthesia, the pa tient can breathefor hi mself spontaneous ventilation. At a greater depth of anaesthesia, it may be necessary tosupport the patient with artificial ventilation known as controlled ventilation.32.1.1 Del ivery of Ana esthesiaThe anaes thetic delivery sys tem consists of an anaesth esia ma chine, a patient breathing circuit, aventi lator and airway equipment. .

The m hinecomprises a gas supply--;-delivery unit and an anaesthetic vapourizer.The re th ing circuitconsists of a c losed loop of breathing tubing, containing two uni -directionalbreathing valves and an Adjustable Pres sure Limiting APL)valve, a CO2 absorber, a means forventing excess gases scavenging), a h\lII\idifu; r, and a collapsible reservoir bag.

. A mech nic l ventil tor is us for positive pressure ventilation.. The irw ym n gementequipmen t includ es the mask and endotracheal tube, which interface thepatient with the breathing circuit.


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8 6 ndbook of Biomedical lnstru tation 32.2 A STH SIA MACHI E

An anaesthesia machine is a device which is used to deliver a precisely known but variable g smix ture including anaesthetic nd Iife-sustaining gases to the p e n t ~ s p i r o r y system.Generally. a variable concentration g s mixture of oxygen. nitrous oxide nd anaesthetic vapourlike ether or ha loth ne is obtained from the machine and is made to flow through the breathingcircuit to the pa tient. t iscomposed of two subsystems (Fig.32.1): (i)The gas supply-delivery unit.which cons ists of tu ing nd flowm etres interconnected in parallel; nd (ii) The anaestheticv pourizer, which is used to produce an anaesthetic vapour from a volatile liquid .32.2 .1 Gas Supply SystemGases are provided to the anaesthesia machine from either a pressurized hospi tal central supplyor small storage cylinders attached to the machine.Centralized Supply:Centralised supply systems consist ofulkor cylinder storage for m in ndrese rve supply. control equipment including valves nd pressure regulators. a d istributionpipeline, nd numeroussupply outlets.The system is sodesigned nd operated that the necessarysupply of gases (oxygen and nitrous oxide) is always available. The g s supplied by the hospitalis regulated and maintained aI 275-345 kPa (40-50 psi) at the wa ll outl et.

Gases are supplied to the anaesthesia machine inlet from the central system via a flexible hoseconnected to the operating roomwall outlet. Inorder to prevent interchanging the gas supp lywalloutlet w t the incorrect anaesthesia machine inlet. for exmple, nitrous oxide for oxygen. noninterchangeable connectors reused at each end of the hose.The two types ofnon-interchangeableconnections most commonly used are the Diameter Index Safety System (DISS)and non-interchange ble qu ick couplers. Each type of connection incorporates a male nd female end that isspecially designed for each type of gas.In addition to the connector design, colour-coded hoses foreach specific gas re utilized.Gas Cylinders: A second gas supply source is the cylinders located in yokes attached 10 theanaesthes ia machine. This supply can be utilized as either the main source when a cen tral gassupply does not exist , or a reservewhen central gas supply i s available.Yoke:Eachanaesthesia machine has at least oneyoke for an oxygencylinder but most are providedwith two. In addition to oxygen, mostmachine designs include a nit rou s oxide yoke. In order toprevent incorrect placement of a tnk into thewrong yoke. two pins located in the yoke must fitinto corresponding holes drilled into the tank neck. The placementof these pinsnd correspondingholes is unique for each g s.This identificationsystem, which is referred to as the Pin IndexSafetySystem, has been st nd rdized to prevent the accidental fitting of a wrong cylinder to the yoke.PressureRegula tor Machine pressure regulators red uce cylinder gas pressures to 275kPa (40psi)befo re the g s flows th rou gh the machine. The regulator has one high-pressure inle t. one highpressure outlet nd two-low pressure outlets . The high pressure inlet is connected with thecylinder througha non-return valve.The non-returnvalve prevents the flow into an emptycylinderor back into the central p iping system and also en les its removal and rep lacement when thereserve cylinder is turned on without interrupting the supply of g s.


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; ..[s[Reservlorbag To patient

Expiratoryvalve

CO2absorber115 --..::t--lJ valve

Exhaustvalve

By passvalve

Direct readingvapourizer

RotameterI

Needlevalve

N serviceInlet

Pressureregulator

Cheekvalve

Pressureg ug

N20

2 tt2Service Inlet 2Flush valve

Bubblethroughvapourlzer

>Fig 321 ScIemtl/ic diagram of all allaestlresia machine ami p tint breatlri g circui t


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828 Handbook ofiomedical InstrumentationPressure GaugePressure gauges are attached to the cylinders to indicate the contents of the gasesin the cylinders. For oxygen, the operating range of the gauge isO to 150 kg cm2. Wheneve r thenew oxygencylinder ishooked up nd takenin line, the indicatorshould be above this mark. With the gradual usageof the gas, the reading would drop gradually,when the indicator shows that thepressure has fallen below the minimum level of acceptance, the linder should be refilled. forany reason, the pressure gauge shows a reading above I kg / em during use, the cylindershouldbe disconnected immediately ndreplaced.Fail fe System From the supply, the g s flows into the inlet of the anaesthesia machine nd isdirected through the pressure safety system fail-safesystem) towards the flow delivery unit. Thepressure safety system will not allow nitrous oxide to flow unl ess noxygen supp ly pressureexists in the machine. The fail-safe system consis ts of amaster pressure regulator valve located inthe oxygen supply line. From the master regulator, a reference pressure is provided to the slaveregulator valve controlling the pressure and flow of the nitrousoxide line.When sufficient oxygenpressure of275kPa 40 psi) is present in the master regulator, the reference pressure enables theslave regulator va lve to open nd for nitrous oxide to flow.Unfortunately, pure nitrous oxide canbe delivered with only oxygen supply pressure present; oxygen flow is not requined.

Regulations now require oxygen-nitrous oxide ratio safegu rds , which need a minimumcon tinuous low flow of oxygen varying from 200 to 300 mLmin, as indicated by the low-flowrotameter. newly designed machines, ingenious mechanical devices prevent the delivery ofgasmixtures with an oxygen concentration below a low limit. Oxygen-nitrous oxide rati os vary from25:75 to 30:70, depending on the m nufacturer .Gas Delivery Units From the fail-safe system, the g s is directed to the flow delivery unit. Twomethodsh ve been used to accomplish delivery ndcontrol of the gas mixture: gas proportioningand gas mixing.In a gas proportioning system, the delivered concentration of each gas constituent is thefunction ofa pre-determined, precisely controlled ratio of proportionality which isindependent ofthe total gas flow . For example, for a desired mixture of 70 nitrous oxide and 30 oxygen, themetered ratio ofmass deliverywill always be 7:3,regardless of the total flow rate. Concentration isonly a fun ction of the proportional relationship between constituents. t does not rely on settingindividualgas flows. An oxygen-nitrous oxid e bleeder used in a m nner similar to the oxygenair blenders commonly used with mechanical ventilators performs this function.

Most current anaesthesia machines use gas mixing. n this technique, the flow rate of eachconstituent is ind ependently controlled ndmeasured by a deli very unit consisting of a needlevalve nda rot meter. The needle valve functions as a flow controller and a me ns of turning thegas on and off.The rotameter is a variable orifice flowrnetre ndconsistsof a transparent tubewitha tapered in ternal diameter nda floating bobbin flow indicator.

During the administration of anaesthesia, it may be necessary to i ll the patient bre thingcircuit with oxygen at a rate higher th nwh t the gas delivery unit can supply. For ~ x p sucha situation exists any time the patient is disconnected to the bre thing circuit. This higher flow ofoxygen is supplied via the oxygen flush valve and line .The oxygen flush system provides a highflow ranging from 35 to 75 Lmin at a high pressure 20-45 psi, 270-590 kPa) directly into thepatient bre thingcircuit.


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naesthesiaMachine 829Each gas h sa specific delivery unit. These units are connected in pa rallel and exhaus t into a

commonmanifold prior to leaving the machine.The final concentration and total flow determinedby mixing the component flows are dependent functions and subject to the accuracyof the controland measurement equipment.32.2.2 Vapour DeliveryThe various liquids that possess anaesthetic properties are too poten t (strong) to be used as purevapours. They are thus diluted in a carrier gas such as air and /or oxygen, or ni trous oxide andoxygen. The device that allows vapourization of the liquid ana esthetic agent and its subsequentadmixture with a carrier gas for adm inistration to a patien t is called a vapourizer . Vapourizersthus produce an accurate gaseous concentration from a volatile liquid anaesthetic.The anaestheticvapo ur can then be safely added to the previously metered oxygen and nitrous oxide as themixture leaves the mixing manifold.

Vapourizers are available in one of the two basic designs: the flowmetre controlled or theconcentration-ealibra ted . n either device, the anaesthetic vapours are picked up from thevapo urizer by a carrier gas consis ting either of pure oxygen or an oxygen-nitrous oxide mixturethat bubbles through or passes over the liqui d .The liquid surface area to gas interface is designedto ensure the most efficient vapouriza tion process.

As a result of vapouriza tion, a drop in liquid tempera ture is produced. As the liquid temperatur decreases, a thermal gradient is established between the liquid and the surroundings. hisresults in adecrease in the quantity of the vapour produced.norder to maintain the performanceof the vapourizer, the temperatu re drop is minimized or prevented by the incorporation of athermal source. This is achieved by using a water bath or surrounding the vapo urizing liquidwith a heating element. These devices may also controlthe temperatureo f the carrier gas enteringthe vapourizer.

The ma terials selected for vapourizer construction require both a high specific heat and highthermal conductivity. Materials with high specific heats will change temperature more slowlyand maintain an appropriate thermal inertia.The higher the thermal conductivity, the higher theconduction of heat from the surroundings. Because of its availability and lower cost, copper hasbeen one of the most common materials used . Although not ideal, copperhas a moderate specificheat and a high thermal conductivity. Early vapourizers were accordingly called copp er kettles.

n order to provide a stab le and p red ictable concentra tion of anaes thetic vapour, thevapourizers include a suitable method of ob taining calibrated dilution of vapour to avoidadministration of too powerful volatile anaesthetic agents to the patient. his can be done byseveral means and the vapourizers are accordingly classified into various categories discussedbelow.Variable ypass p o u r i the carrier gas flow from the flowmeter is split into two streamsin a known ratio:one streamwhich is called chamber flow , flows over the liquid agen t while theother stream goes through thebypass path and doesnot enter the vapourizing chamber. The finaconcentration can be controlled by varying the splitting ratio between the vapourizer gas and thebypass gas using an adjustable valve (Fig. 32.2).


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830 Handbook of Biomedical Inst rumentation

,,,t

,, _ c

1 C vpourizerch mber

iqui

> Fig. 32.2 A schem tic diagram ofa variable bypass vapourizer A flo w spli ttingvalve th t can be rotated to alter the relativediameters of the vapourizerand bypass channels

> Fig . 32 3 Schematic diagram of ameasuredflo w vapourizer

-

J---'l 'w lv--{ v.------- -

The splitting ratio of the two flows depends on the ratio of resistances to their flow, which iscontrolled by the concentration control di al and the au tomatic temperature compensation va lve .Usua lly, less than 20 of the gas becomes enriched sa tu rated with vapour and more th an 80 isbypassed, to rejoin at the vapourizer outlet. The outpu t of current variable bypass va pourizers isrelatively constant over the range of fresh gas flows from approximately 250mL/min to 15 L/ min.The outp ut of vapou rizers is lin ear at the ambient temperature (2-35C) due to automatictemperature compensating dev ices that increasecarrier gas flow as the liquid volatile agen t temperature decreases. Also, they are composed ofmetalswith high specific heat and thermal conductivity . tCheck valves are provided to prevent back pressureeffect on the vapourizer from the breathing circuitdu e to pos t v pressure v ntilationMeasuredflow Vapourizers: these devices, theanaesthetic agent is hea ted to a temperature above tthe boiling poin t (so that it behaves as a gas) and isthen met red in to the fresh gas flow (Fig. 32.3).Various anaes thetic agents have Wide ly di ffe

rent potencies and physical properties and hencerequire vapourize rs constructed specifica lly for v2each agent. They are th us agent-specific . They areonly calibra ted for a single gas, usually w ith keyedfilte rs tha t decrease the likelihood of filling th evapourizerwith the wrong agent.


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naesthesia achine 831Vapourizers are provided with various safety related inter-locks which ensure that:- On ly one vapourizer is turned on;- Gas enters only the one which is on;

- Trace vap ou r output is minimized when the vapo urizer is off;- Vapourizers are locked into the gas circuit, th usensuring that they are seated correctly; and- Other impor tant safety featu res are followed includingkeyed filters and secured mountingto minimi ze tipping tilting) which ma y obstruct the working of the valves.

32.2.3 Delivery SystemPatient Breathing System:The function of a patien t breathing system is to deliver anaesthetic andrespiratory gases to and from the patient. describes both the mode of operation an d the apparatusby which inhalation agents are delivered to the pa tient. The breathing system may be

1. Rebreathing Type: This refers to re-breath ing of some or all of the previously exhaledgases, including carbon dioxide and wa ter vapour.2. on-rebrea thingType: In this a fresh gas supply is delivered to the patient and re-breathing of previously exhaled gases is preven ted .Usua lly, non-rebreathing type systems areapp lied in practice. This is achieved by using:

- Non-rebreathing uni-di rectional valve;- Carbondio xide absorption system; involving;

Uni-di rectional circle) system; and Hi-directional to-and-fro) system. .

Figur e 32.4 shows the p rinciple of a non-rebreathing system which uses uni-directionalnon-brea thing va lve. Fresh ga s entering the insp irator y pa rt is eithe r sucked in by the pa tient s

Inspiratorylimb~ Expiratory limb.---_A--_---FG IFresh gasflow) y

Patienta Inspiration

G L

Inspiratorylimb _ _ A-_ _ - Expiratory limb _ _ A- _\atient b) Exhalation> Fig .32.4 Non breathingvalve


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Handbook of Biomedical Instrumentation32 inspiratory effort or blown in during controlled ventilation. The non -rebreathin g valve is sodesigned tha t when it is open to admit inspiratory gas, it does not permit the flow from theexpiratory part to get through it.When the patient exhales, the reverse happens,as the inspiratoryvalve is occluded and the expiratory valve is opened to allow expirato ry.,gases to escape. Theinspi ratory system usually includes a rubberbag of two-litre capacity which acts as a reservoir forfresh gas . The reservoir g is refilled with fresh gas during the expiratory phase. can also becompressed normally to provide assisted or controlled ventilation. The fresh gas supply islinked to a length of corrugated breathing hose minimum length -lDem with an internal volumeof550ml .This rep resents slightly more than the average tidal volume in an anaesthetized adultbreathing spontaneously. This is, in turn, connected to a variable tension, spring-loaded flapvalue for venting off exhaled gases.This valve is located as close to the patient as possible, and iscalled an APL adjustable pressure limiting valve. The APL valve works as a pep-off valve toensure that the pa tient is not subjected to the surges in the gas supply. When the gas encountersresistance from the patient, the excess gas pop. out.The arrngement is shown in Fig.32.5.In thiscase, carbon dioxide elimination is achieved by the flushing action of the fresh gas introducedwith the reathing system, rather than by separation.Obviously, this system retains the potentialfor re-breathing of carbondioxide when the fresh gas flow rates are reduced.

APt

> ig32.5 Mapleson breath ing system usedw ith spontaneous breathing FGFFresh Gas Flow , PL Adjustable Pressure Limiting valve, RReservoirBagCircle Sys tem: The circle is the most popular reathing circui t nd is a closed loopof large-bore,low-pressure tubing divided into an inspiratory and an expiratory limb. Contained within thisloop are two uni-directional valves, a CO2 absorber,circuit gas venting scavenging , an adjustablepressure-limiting valve, reservoir bag, nd airway management equipment including masksand endotracheal tubes. The patient is connected to the absorber by two corrugated hoses, oneinspiratory nd the other expiratory.Fresh gas is introduced proximal to a uni-direction l inspiratory breathing valve. Duringinspiration, gas moves through the absorber from either the reservoir bag or ventilatorbellows ndinspiratory valve into the inspiratory limb of the circuit. The pressure difference between theinspiratory and expiratory limbs keeps the uni-directional expiratory valve closed.During exhalation, the pressure differential reverses.The inspiratory valve closes and the expiratory valve opens,allowing the exhaled gas to flow into the reservoir bag or ventilator bellows nd absorber. TheAPLor the pep off valve enables the anaesthetist to control the circuit volume and pressure by thereguIation ofgas venting from the circuit. Circuit exhaustiseither carried into the room or collectedby a gas scavenging system.


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AnaesthesiaMachine 833Uni-directional brething valves re available in several designs. The disk valve is the most

common in modem systems. This valveconsists ofonly one movable prt , a f lat disk. The disk ism de ofeither plastic or metal and is held against the valve seat by either gravity or a mechanicalspring. The valv es are placed in transparent devices so that the ir action may be observed.The APL is designed to regulate circuit pressures by m nually adjus ting the spring tensionagainst a d isk.When circuit pressure overcomes the valve resistance, the disk is lifted from its seat

nd gas is allowed to exhaust from the circuit. The circuit volume nd pressures throughout thedelivery of anaesthesia re continuously observed so that the APL valve can be ppro priatelyadjusted .The absorbercontains a carbond ioxide absorbent (soda lime) in a c losed contine r. Soda lime

is used in the fo rm of gr nu les so that they have a l rge volume nd large surface area . Thus, theexpired air remains in contact with the soda lime for a relatively long periodof time, increasing theefficiency of absorption. Granules ofn optimum size re selected as, too large a size leads to poorcontact and poor absorption, while too small a sizeclogs the sod limebed nd causes resistanceto the gas flow.Theexhaled g s is made to flow through theabsorberwhere the is removed.The remaining gas is mixed with fresh gas flow ing from the machine and re-breth ed via theinspiratory limb.When the sod a lime gets consumed, its colour changes from pink to yellowish.The reservoir or breathing b g is highly compli nt,with an e sily exp nded volume.The bag

allows the accumulation of gas during exha lation so that a reservoir is available for the nextinspiration. tprovides a me ns for visually monitoring the spontaneous breathing pattern ofthe pa tient nd buffers increases in breathing circuit pressure. The bag also provides a me nsthat can be used to manually ventilate the patient. Either passive or active scavenging sys temsr e utilized in removing the circuit exhaust. In passive scaveng ing, an esthetic waste gases areven ted di rectly into the existing room ventilation systems.The tubing connects either the ventilatoror the APL valve exhaust port of the breathing circuit to the hospital ventilation system . Inactive methods, the anesthetic circuit connects directly to a hig flow vacuum system via anappropriate interface.32 .2.4 umidific tionDry gases supplied by the anaesthesia machine may cause clinica lly significant desiccation ofmucus.This maycontribute to retention of secretion nd the mucus flowm y cease. Lung compliance will consequently fall .Therefore, ir or anaesthetic gases need to be humidified.bsolute Humidity: This isthemaximum mass ofw ter v pourwhich c n becarried by a givenvolume of air (mg / L).This quantity is pre-domin ntly determined by temper ture.Warm air cancarry much moremoisture:Relative Humidity R } This is the percentage of the amountof humidity present in a sample, ascomp red to the absolute humidity possible at the s mple temperture . t is ideal t provide g s s atbody temperaturend 100 RH to the patient sirw y .The humidificationme sures tha t re commonly employed include heated airway humidifiers, nebulizersnd heat nd moisture exchangers.

(i) n the heated humidifiers the air passes over the surface of the hea ted wa ter andvapourization takes place. The temperatu re of the water is thermostatically controlled.


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Handbook of Biomedical Inst rumentation34 -- - . .

1a)

b)Principleof the nebulizerEmp loying the enwulIi ef fecta jet of air m y be used to drawa Iiquid up a small tubefrom areservoir nd to entr in sdropletsb) TIle drop lets may be mde toi inge on n anuil so us-ing them to be brokell up intost ill smallerdroplets

> Fig. 32.6a)

Preferably, two thermostats in series areused, S


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n s th si acltineven tilators prov ide positive pressure venti lation at a controlled min ute volume Tidal vol ume,Rate). They operate either electronically or mechanically with pneu matic or electric power source.

Most of the currently used ventilators cons ist of a bellows contained within another housing.The bellows communicate direc tly with the brea thing circuit and causes a pre-selected volumeofgas to flow into the patient. The flow of gas into the circuit resul ts from collapsing the ventilatorbellows by pressurizin g the surroun ding gas volum e contained within the bellows housing.

The ventilato r is either located wi thin the mainframe of the anaes thesia machine or is attachedas an accessory unit. The outlet of the ventilatorconnects directl y to the patient breathing circuit ofthe anaesthetic delivery sys tem at the loca tion and in place of the brea thing reservoir bag . Theventil ator th us func tions as a controller for both ventilat ion and circuit gas su pply by repl acingthe functions of the reservoir bag and APL valve. 32 2 6 Patient Circu itThe patient circu itconsists of black corrugated an ti-static rubbe r tube, a chrom e plated tube fitting Tjoint), a two litre re-breathing bag, a Heidbrink valve and a face mask with an elbow fitting. Theface mask is designed to fit the patient s face pe rfectly wit hout any leaks and yet to exert theminimum of pressure w hich migh t depress the jaw s and cause respiratory obstruction .

32.3 E O R O NICS I T ANAESTHETIC MACHI NEAny de livery system isexpected to meetaccurately an d safely, the patient s varying requirementsfor respiratory and anaesth etic gases. The sys tem mu st be able to monitor the function of thede livery sys tem itself and the effect of the anaes thesia on the patient. Also, d uring the entireprocedure th e machine pe rformance sh ould not only be mo nitored and controlled, bu t itssta tu s should be continually assessed and recorde d . In order to meet these requ irements, theimpact of electronics on the d esign an d functioning of the anaesthetic m achin e has bee nphenomenal.

The totally pneumatic anaes thetic machine still has man y meri ts, which include its bein g easyto un derstand and easy to maintain, as also its cheaper cost and reliability. How ever, certa inproblems are encountered whi ch d irectly affect the performan ce an d safety of a p n eumatic anaestheti c machine. is is ove rcome thr ough the utilization of newer technologies and automationof instrum en ta tio n and anaesthetic delivery. Microprocessor-base d anaesthetic eq uip mentfacilitates im provements in: Gas supply and p roportioning systems;

Breathing circuits; Gas scavenging an d hu midification devices; and Ventilators.

The use of microprocessor techn ology allows us to fully integrate control and sa fety fun ctionsan d pr otects the patien t fro m: gas suppl y failure, electrical supply failure, hypo xic mixtures,disc onnections, vapourizer function, excessive airway pressure exhaled minute vol ume outsidepre-set limits, oxygep or vo latile agents ou tside the pre-set limit , end-tida l CO2 ou tside presentlim its and technical failur e. All abnormal con ditions ca us e an alarm to appear on the monitorpanel, which also displays the nature of the fault.


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836 Handbook of Biomedical InstrumentationCo mp u ter application with ppr opriate d t processing inpu ts and outputs, automation

and integratio n of machine func tions an d record -keeping are increasing ly becoming possible .Ergonomically designed machines with easy to red and interpret disp lay systems are also beingcommonly used .

Ana esth esia ha s a profound effect upon all physiological syst ems. Most of thes e effects redeleterious, and therefore, it is imp ortnt to kn ow how the human bod y affected by anesthesias.With a view to increasing patient safety and achieving a good degr ee of risk management, allsystems affected by anaesthetic drugs mus t be moni tored . This is d one by us ing monitoringequipment wi th visible and audible alarms as illustrated in Chapter 6.

Fig 32.7 EP Auditory Evoked Potentialrmonitor to measure individual s levelof consciousness during general anaesthesia. Courtesy: A lars MedicalSys tems, U.K

Since individual responses to a pa rticular dose of anaesthetic vary considerably, it is ad visableto measure the effect of the n esthetic on a patient s level of consciousness. One method to do sois to measure ud itory Evok ed Potenti als AEP), which is a neuro-physiological indicator of th echa nges in the level of cons ciousness during anaesthesia . This is an electrical sign al containedwithin the EEG, which is obtained by delivering an aud itory stimulus to th e patient s acousticnerve.The fast extraction of the comp lex AEPsignal, the brain s response to the auditory stimulusof the acoust ic nerve is obtained by m pping the signa l and establishing an index w hichis developed as a gr ap hic curve and a single number on the monitor screen Fig. 32.7). Thisindex which is calculated from a proprietary ma thematical modelling method, qu antifies thelevel of anaest hesia. For exmp le, typ ically, if this ind ex is hig her th an 60, the ptient isawa ke, and decreases in line with decreasing level of consciousness loss of conscious typicallyoccurring below 30). Re-usable o n ap ply stimula tion to the acous tic nerv e to obtainAEP, which is then measured by a set of three disposable electrodes; two electrodes are appliedin the forehead and one behind the ear to estimate th e level of consciousness in a fast ndnon invasive manner

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