Andersons pediatric cardiology 592
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anoverallincreaseinsympathetictone.
SurrogatesofCardiacOutput
Exercisetestingisfrequentlyperformedtoassesstheabilityofthe
cardiovascularsystemtoincreasecardiacoutputinresponsetoanincreased
workload.However,measurementofcardiacoutputduringexerciseisoftennot
practical.Directmeasurementsaretooinvasivetoallowadequatelevelsof
exertion.Noninvasivemethodsareoftentechnicallydifficultandmaybe
inaccurateinthepathophysiologicstatesthatareencounteredinmanytypesof
congenitallymalformedhearts.Thepresenceofobstaclestoaccuratelyand
reliablymeasurecardiacoutputhaveledtotheuseofconsumptionofoxygenas
asurrogateofcardiacoutputinmanyclinicalandresearchsettings.4,18,21Overa
broadrangeofconsumptionofoxygen,thereisanearlylinearrelationship
betweenconsumptionofoxygenandcardiacoutput(Fig.23.6).
FIG.23.6 Relationshipofcardiacoutputtoconsumptionofoxygen
measuredin23subjectsduringsittingcycleergometry.Thesedataare
fromthesamesubjectsshowninFig.23.3.(FromAstrandP,RodahlK.
TheMuscleandItsContraction.TextbookofWorkPhysiology,
PhysiologicalBasesofExercise.3rded.McGraw-Hill;1986:12–53.)
Evenafteraplateauincardiacoutputathigherworkloads,consumptionof
oxygencontinuestoincreaseasaresultofincreasedoxygenextraction,thereby
increasingthearterial–mixedvenouscontentofoxygendifference.Consumption
ofoxygenisdeterminedbytheamountofoxygendeliveredinthebloodthatis
extractedbythemetabolicallyactivetissues.Duringexercise,thisextractionis
determinedbythemyoglobincontentoftheexercisingmuscle,theisoenzyme
characteristicsofthemuscle,andthephysiologicmilieuinwhichthemuscleis
working.4,18Thephysiologicstatewillhaveagreatimpactontheoxygenhemoglobindissociationcurve.Aspreviouslystated,therearemultiplebyproductsofmusclemetabolismthatarereleasedintotheintercellularspace
duringexercise,shiftingtheoxygen-hemoglobindissociationcurvedownandto
theright.4Thelocalincreaseinmuscletemperatureduringexercisealsohasthe
sameeffect.Thenetresultisanincreaseinoxygenunloadingtotheexercising
muscleparticularlyathighworkloads.Thereisthereforeanincreaseinthe
arterial–mixedvenouscontentofoxygendifferenceandacontinuedrisein
consumptionofoxygen,eveninthepresenceofaflatteningorplateauingofthe
riseincardiacoutputnearpeakexercise.4,18,19
Genderdifferencesarepresentintherelationshipbetweencardiacoutputand
consumptionofoxygen(seeFig.23.6).Femalestendtohaveasomewhathigher
cardiacoutputforanygivenoxygenconsumption,aswellasalowermaximal
consumption.19Thesedifferencesarelikelyduetothedifferenceinhemoglobin
concentrationbetweenmalesandfemales.Femaleshavelowerlevelsof
hemoglobincomparedwithmalesandthereforehavelowerarterialoxygen
content,necessitatingahighercardiacoutputtodeliveranequivalentquantityof
oxygen.Lowervaluesinadolescentandadultfemalesmanifestclinicallyin
slightlyhighercardiacoutputsandlowerconsumptionofoxygenatpeak
exercise.Itiseasytocomprehendhowanemiawillresultinreduced
consumptionofoxygenandahigherratioofcardiacoutputtoconsumptionof
oxygencomparedwithnonanemicstates.Thereforetheuseofconsumptionof
oxygenasasurrogateforcardiacoutputisproblematicinthepresenceof
anemia.Mitochondrialdiseasesthatimpairmuscleoxygenutilizationand
hemoglobinopathiesthatalteroxygenaffinitycanhaveanequivalenteffectbut
aremuchlesscommonthananemia.
