Andersons pediatric cardiology 1907

FIG.73.7 Theoreticalschematoillustratecirculatorypressurechangesin
normalandFontanpatientsatrest(blue)andduringexercise(red).Inthe
normalcirculation(A),pressureisgeneratedinthesystemicventricle(LV)
toproduceflowintheaorta(Ao)andsystemiccirculation(S).Pressure
dissipatesacrossthesystemicmicrocirculationsuchthatrightatrial(RA)
pressureislow.Theprepulmonarypump(RV)providesthepressureto
generatetheflowinthepulmonaryartery(PA),whichthendissipatesinthe
pulmonarycirculation(P)butissufficienttomaintainpreloadintheleft
atrium(LA).Duringexercise,systemicvascularresistancefallssuchthat
thereislittleincreaseinmeanLVpressurerequirements.However,more
substantialpressureincreasesarerequiredintheRV,andthesepressure
requirementsincreasewithexerciseintensity.IntheFontanpatient(B),the
cavopulmonarybypass(CPB)doesnotprovideanycontractileforce,and
thereforeflowthroughthepulmonarycirculationisdependentonthe
pressuredifferencebetweentheRAandLA.Duringexercise,
transpulmonaryflowcanbeaugmentedonlybyareductioninpulmonary
vascularresistance.Beyondmildtomoderateexercise,pulmonary
vasodilationismaximalandflowincreasesrequireaprepulmonarypump.
Withoutthis,pulmonarypressuredoesnotrise,transpulmonaryflowdoes
notincrease,LApressure(preload)doesnotincrease,andcardiacoutput
cannotsupplythemetabolicdemandsofexercise.(FromLaGercheA,
GewilliqM.Whatlimitscardiacperformanceduringexerciseinnormal
subjectsandinhealthyFontanpatients?IntJPediatr.2010;2010[5]:1–8.)


FIG.73.8 Workversusoxygenuptake(VO2)duringexercise.Inthe
normalcirculation(A)thereisapointabovewhichVO2cannotbe
increaseddespiteanincreaseinworkload.Thisrepresentsthemaximal
VO2(VO2max),whichinthissituationisidenticaltoVO2peak.Inthe
Fontancirculation(B)exercisedurationworkloadandVO2arereduced
comparedwithnormalandfrequentlythereisnoplateauinVO2,suchthat

theVO2maxisnotachieved.

ThereareseveralaspectsoftheFontancirculationthatcontributetoimpaired
exercisecapacity(Fig.73.9).Undernormalconditions,cardiacoutputis
augmentedbyincreasesinpreload,heartrate,andmyocardialcontractilityanda
reductioninafterload.Strokeworkisincreasedsubstantiallymoreinthe
subpulmonary(right)ventriclethanthesystemic(left)ventricle.43Intheabsence
ofasubpulmonaryventriclemanyoftheseadaptiveresponsesareabsentor
compromised.


FIG.73.9 MechanismsofimpairedexercisecapacityintheFontan
patient.Thefourmajorcornerstonestoimpairedexercisetoleranceinthe
Fontancirculationarepreloadinsufficiency,chronotropicincompetence,
restrictivelungdisease,andunderlyingandresiduallesions.Some
featuresofthecornerstonesareinherentinthephysiologyofaFontan
circulation,includingthelackofasubpulmonarypumpandelevated
systemicvenouspressure.Theremaindermakeavariablecontributionto
impairedexercisecapacity,asdootherfactorsincludinganemia,
neurohormonalactivation,arrhythmia,anddeconditioning.

PreloadInsufficiency
IntheFontancirculation,preloadischronicallydepleted,andthiseffectis
magnifiedunderexerciseconditions.Intheabsenceofasubpulmonaryventricle,
systemicventricularfillingisdependentondiastolicfunctionandlow
pulmonaryvascularresistancetopullbloodthroughthepulmonarycirculation.
ThesefactorsaretheprimarydriversofexercisecapacityintheFontan
circulation.Followingvolumeunloadingduringstagedsurgicalreconstruction,
thefunctionallyuniventricularheartreducesinsizeby25%to70%.47Although
remodelingcouldcompensateforthischangebyreducingmyofiberlength,

diastolicdysfunctionpredominatesfromearlyoninthemajorityofFontan