Andersons pediatric cardiology 598
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atleastone,andideallymultiple,leadscontinuously.Thetechnicianobtainsa
complete12-leadrecordingatsettimeintervals,usuallyof1minute,andwhen
neededduringthecourseofanexercisetest.Ideally,allQRScomplexesshould
bestored,eitherasahardcopyordigitally.28Automateddevicesareavailable
onmanyrecorderstodetectarrhythmiasandareusefulbutmaybelimitedwhen
abnormalQRScomplexes,pacemaker-generatedcomplexes,orsignificant
motionartifactarepresent.
Mostcomputerizedrecordershavealgorithmsthatwillaveragetherecorded
signals,thuseliminatingbaselinedriftandbackgroundartifact,permittingeasy
interpretationofchangesintheQRSandSTsegments.Itispossiblethatdata
maybelostwithcertainalgorithmsatveryhighheartrates,soitisimportantto
comparetheaveragedwiththerawdatawheninterpretingthetest.
AnalyzersofRespiratoryGasExchange
Theuseoftheseanalyzershasbecomeroutineinlaboratoriesstudyingpediatric
exercisephysiology.Measurementsofconsumptionofoxygen,productionof
carbondioxide,andpulmonaryfunctions,suchasminuteventilation,tidal
volume,andrespiratoryrate,areeasilyobtainable.Severalsystemsatreasonable
pricesarecommerciallyavailable.Inadditiontomeasuringexpiredgases,these
systemsarefrequentlyequippedtoperformrestingspirometryandtoreceive
inputfromothersources,suchasanelectrocardiographicrecorder.Thisallowsa
singlesystemtogeneratecompleteandfinalreports.
Analyzersusinginertgasescanbeusedtoestimatecardiacoutput.42The
patientbreathesroomairduringmostofthetest.Duringthemeasurementsof
cardiacoutput,ventilationisdivertedviaavalveintoaclosedsystem,usually
througha3-Lrebreathingbag,containingamixtureofoxygen,aninert
diffusiblegas,andaninertnondiffusiblegas.Afteronetothreebreaths,the
nondiffusiblegasequilibratesbetweenthelungsandthebag,andthefinal
concentrationofthisgascanbeusedtocalculatethevolumeofthelungs.The
diffusiblegasisremovedfromthesysteminproportiontotheflowofbloodto
thelungs.Theassumptionwhenassessingcardiacoutputisthatflowsareequal
inthepulmonaryandsystemiccirculations,andthereisnosignificant
intrapulmonaryorintracardiacshunting.Thecalculatedflowofbloodtothe
lungsthenreliablyestimatessystemiccardiacoutput.Unfortunately,residual
pulmonaryand/orintracardiacshuntsfrequentlylimittheusefulnessofthis
measurement.Suchassessmentalsorequirespatientcooperationandtester
expertiseinperformingtherebreathingmaneuverscorrectly.
Acetyleneandheliumarecommonlyusedinertsolubleandinsolublegases,
respectively.Amassspectrometerorsimilardeviceisusuallyrequired(Fig.
23.11).Compact,commerciallyavailablesystemsarecurrentlyavailablebut
mayhavedifficultyinterfacingwithothermetaboliccartsandmeasurement
devices.Onesuchsystemusesnitrousoxideandsulfurhexafluoride.Such
commercialsystemsarelesscostlyandmoreuserfriendlythanindividually
designedsystems.Thisapproachhasbeenvalidatedinchildrenandadultwith
congenitalheartdiseasewhodonothavealeft-to-rightshuntandevenin
mechanicallyventilatedchildren.43–45Alternatively,rebreathingofcarbon
dioxidecanbeusedinsomecommerciallyavailablesystems.Thelattersystem
usestheindirectFickmethodtoestimatecardiacoutput.Anumberof
assumptionsarenecessarytoestimatearterialpartialpressuresofcarbon
dioxide,42,46whichmayresultinsignificantinaccuracies,especiallyinpatients
withcongenitalcardiacdisease.Therebreathingmethodalsohasthe
disadvantageofrequiringsteadystateminuteventilationandthereforeisnot
suitableforprotocolswithfrequentlychangingworkrate.
FIG.23.11 Noninvasivemeasurementofcardiacoutputusingthe
diffusiblegastechnique.Timeisonthex-axis,andthenaturallogofthe
concentrationofthediffusiblegas,inthiscaseacetylene,isonthey-axis.
Thelowercurveshowstheconcentrationofheliumthatequilibrates
betweenthelungsandtherebreathingbagafterseveralbreaths.This
allowscalculationofthelungvolume.Theslopeofthedecayofthe
acetyleneiscalculatedfromthetopcurveandisusedtomeasuretheflow
ofbloodthroughthelungs,inthiscase11.26L/min.Thisshouldequal
systemiccardiacoutputintheabsenceofpulmonarydiseaseor
intracardiacshunting.
Methodsfornoninvasivemeasurementusedlessfrequentlywithexercise
testingincludeelectricalbioimpedanceandDopplermeasurementofaortic
bloodflow.42,46Bioimpedancerequiresverylittlecooperationfromthepatient,
butstudiescomparingthismethodwithdiffusiblegasandothermethodshave
shownpoorcorrelation.46Thesedevicesaresubjecttomotionartifact,which
makesmeasurementduringactualexerciselimited.Measurementsatrestand
immediatelyafterpeakexercise,suchaswitha6-minutewalktest,maybethe
mostusefulforthesetypesofdevices.CorrelationsofDopplerwithdiffusible
gasandothermethodsarealsosuboptimal,andobtaininganadequateDoppler
signalwithexerciseremainsacumbersomeundertaking.
