Andersons pediatric cardiology 593
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OxygenPulse
Inaclinicalsettingthepractitionerisfrequentlyinterestedinassessingthe
intrinsicinotropicstateofthemyocardiumduringexercise.Aswasstatedearlier,
thisstateisprimarilyreflectedbythestrokevolumeoftheheart.Anoninvasive
measurementofstrokevolumeisausefultooltoassessmyocardialfunction.
Thishasgivenrisetotheuseofoxygenpulseduringexercisetesting.22
Theoxygenpulseisdefinedas:
Because:
Thepulsealsoequals:
Theoxygenpulsecanbemeasuredatanygivenworkrateorconsumptionof
oxygenduringexercisetesting.AslongastheA-Voxygendifferenceremains
stable,ariseorfallinstrokevolumewillcorrelatewithariseorfallinthe
oxygenpulse.Thesechangescanbebluntedsomewhatbyanincreaseor
decreaseinthedifferenceincontentofoxygenbetweenarterialandmixed
venousblood,buttheabilityofmusclestoextractoxygenusuallydoesnot
significantlychangeexceptinresponsetoprolongedtraining.Thisisparticularly
trueatmaximalexercise.Theoxygenpulseisperhapsmostusefulinassessing
changesinmyocardialperformanceovertime,orfollowingtherapeutic
interventionsinanindividualpatient.Assumingnochangeinthehemoglobin
contentorarterialoxygensaturation,thecontentofoxygenofthearterialblood
shouldbeunchanged.Absentlargeweightchanges,theconsumptionofoxygen
requiredtoperformagivenamountofworkonanergometerforanindividual
patientisconstant.Thereforeanyincreaseintheoxygenpulsemeasuredata
givenworkratewouldreflectalowerheartrateneededtoachievethesame
consumptionofoxygenandbeindicativeofahigherstrokevolume.The
conversewouldbetrueforafallingoxygenpulse.Thisdoespresumethe
hemoglobincontentofthebloodhasnotchangedandthatthechronotropicstate
oftheheartisalsounchanged.Asignificantchangeineitheroftheselimitsthe
usefulnessoftheoxygenpulsetoactasamarkerofstrokevolumeduring
exercise.
PulmonaryResponsetoExercise
Consumptionofoxygenandremovalofcarbondioxiderequirethatthe
cardiovascularandpulmonarysystemsworktogetherasanintegratedunit.The
ultimategoalisthatforagivenconsumptionofoxygen,thecarbondioxide
producediseliminatedwhilemaintainingPaCO2andpHwithinanarrow
physiologicrange.Forthisreason,itisnotsurprisingthatthereisaclose
relationshipbetweenminuteventilation,oxygenconsumption,andcarbon
dioxideproduction.Thisrelationshipisexpressedintheventilatoryequivalents
forcarbondioxideandoxygen,representedasVE/VCO2andVE/VO2.The
typicalrelationshipofminuteventilationtoincreasingworkrateandthe
relationshipoftheventilatoryequivalentsforcarbondioxideandoxygento
workratearedepictedinFig.23.7A.Notethesteadyriseinminuteventilation
withincreasingworkrate.Bothventilatoryequivalentsinitiallyfallattheonset
ofexerciseandthenplateau.Duringtheplateauphase,minuteventilationis
increasinginproportiontoboththeincreasesintheproductionofcarbondioxide
andconsumptionofoxygen.Attheonsetoftheventilatoryanaerobicthreshold,
minuteventilationbeginstoincreaseoutofproportiontoconsumptionof
oxygenastherespiratorydriveismoreprominentlystimulatedbyincreased
productionofcarbondioxideoccurringasaconsequenceofthebufferingof
lacticacidtomaintainPaCO2.NoteinFig.23.7Bthat,atthispoint,the
ventilatoryequivalentforoxygenbeginstorisewhiletheventilatoryequivalent
forcarbondioxideremainsstable.Atmaximalexercise,productionoflacticacid
mayrisetoalevelthatcannolongerbeadequatelybufferedbysodium
bicarbonateandfrankmetabolicacidosisensues.Thenormalphysiologic
responsetoacidosisisamarkedhyperventilatoryresponsereferredtoas
terminalhyperventilation(respiratorycompensation).Atterminal
hyperventilation,boththeventilatoryequivalentsforoxygenandcarbondioxide
rise,resultinginasignificantfallinPaCO2.4,10–12
