Andersons pediatric cardiology 594
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FIG.23.7 (A)Relationshipofminuteventilation(VE)torateofworkinthe
samesubjectasshowninFig.23.3.Notethereisasteadyriseinminute
ventilationasrateofworkincreases.(B)Theventilatoryequivalentsof
oxygen(VE/VO2)andcarbondioxide(VE/VCO2)forthesamesubject.The
onsetoftheventilatoryanaerobicthreshold(VAT)andterminal
hyperventilation(TV)aremarked(seetextfordiscussion).
Minuteventilationisdefinedas:
whereVTistidalvolumeandFisrespiratoryrate.Attheonsetofexercise,
minuteventilationinitiallyincreasesprimarilybyanincreaseintidalvolume
ratherthanrespiratoryrate.Tidalvolumeincludesbothalveolarventilationand
thephysiologicdeadspace.Physiologicdeadspaceismadeupofbothanatomic
components,thetracheaandbronchi,andthefunctionaldeadspace,namelythe
ventilationofhypoperfusedornonperfusedpulmonarycomponents.Atrestthe
ratioofdeadspacetotidalvolumeisapproximately30%to35%inadultsand
adolescents.Withtheincreaseintidalvolumeattheonsetofexercise,theratio
falls.Thisisduetoalargertidalvolumerelativetothefixedanatomicdead
space.Inaddition,thereisanimprovementinventilation-to-perfusionmatching
astheincreasednegativethoracicpressurecausesrecruitmentofadditional
capillarybeds,resultinginafallinthephysiologicdeadspace.Atpeakexercise,
theratioofdeadspacetotidalvolumemayfalltoapproximately5%to15%.
Childrentypicallyhavelessefficientventilationthaneitheradolescentsoradults
andtendtohaveahigherrespiratoryrateforanygivenminuteventilation,
resultinginahigherratioatanylevelofexercise.23–27
Athigherlevelsofexercise,minuteventilationincreasesduetoincreasesin
bothtidalvolumeandrespiratoryrate.Muchoftheimprovementintheratio
occursatlowertomoderatelevelsofwork.Therapidfallintheratiois
responsiblefortheinitialsteepfallintheventilatoryequivalentsforoxygenand
carbondioxidethatoccursattheonsetofexercise(seeFig.23.7B).Although
bothconsumptionofoxygenandproductionofcarbondioxideareincreasingin
earlyexercise,thisismorethancompensatedforbytheimprovedefficiencyof
thelungs,resultinginalessthanproportionalriseinminuteventilation.
Inhealthychildrenandadults,ventilationisnottherate-limitingstepin
exerciseperformance.4,22Thisisbecausethecardiovascularsystemusually
reachesitslimitofdeliveryofoxygenbeforethepulmonarysystemis
exhausted.Atthemaximallevelofexercise,mostsubjectshavenotmaximally
stressedtheirpulmonarysystem,andthereispulmonaryreserve,whichis
describedbyexercisephysiologistsasthebreathingreserve.Thisisthe
theoreticalabilitytoincreaseminuteventilationthatremainsuntappedat
maximalminuteconsumptionofoxygen.Thisvalueisintherangeof20%to
50%inhealthychildrenandadults.22
Breathingreserveisdefinedas:
andisexpressedasapercentage,whereVEisthemaximallyachievedVEat
peakexerciseandMVVismaximalvoluntaryventilation.Maximalvoluntary
ventilationisobtainedbyhavingthesubjecthyperventilatefor10secondsas
vigorouslyaspossiblepriortoexercisetesting.Muchlongerperiodsofrapidand
deepbreathingmayresultinfaintingandshouldbeavoided.Thevolume
achievedisthenmultipliedby6,theoreticallytogiveamaximallyachievable
minuteventilation.Alowbreathingreserve,oflessthan20%,mayindicatethat
aprimarypulmonaryabnormalityislimitingexerciseperformance.Because
maximalvoluntaryventilationisahighlyeffort-dependentmeasurement,care
mustbetakenwheninterpretingbreathingreserve.Otherconfirmatorydata,
suchasanabnormalresponseoftheventilatoryequivalentsorabnormalresting
spirometry,shouldbesoughttoconfirmpulmonaryabnormalities.
