Andersons pediatric cardiology 982
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Pathophysiology
CirculatoryPhysiology
Thepredominantphysiologicabnormalityintranspositionisthatthesystemic
venousreturnisrecirculatedtothebodythroughtherightventricleandtheaorta,
whilethepulmonaryvenousreturnisrecirculatedtothelungsthroughtheleft
ventricleandpulmonarytrunk.Thus,althoughinthenormalheartthesystemic
andpulmonarycirculationsareinseries,intranspositiontheyaresetupastwo
separateandparallelcircuits.Inthefetus,thisarrangementresultsinmore
desaturationoftheupperbodythanisusualbut,althoughnotassociatedwith
othersignificantcirculatoryinstability,maybepartiallyresponsiblefor
abnormalcentralnervoussystemfindingslaterinlife(seeChapter76).Kept
separated,theoxygenatedpulmonaryvenousbloodcannotreachthesystemic
circuit,andthesystemicvenousreturncannotcirculatetothelungs.Thisresults
inseveresystemicarterialdesaturationfollowingbirthandisincompatiblewith
life.Althoughthisiseasytounderstand,itisthelifesavingcommunications
betweenthesetwocircuitsthataddsignificantcomplexitytothephysiology.
DeterminantsofSystemicArterialOxygenation
andMixing
Systemicflow(QS)isallofthebloodcoursingthroughthesystemiccirculation.
Effectivesystemicflow(QES)isdefinedastheportionofQSthatcontains
saturatedpulmonaryvenousblood.Likewise,pulmonaryflow(QP)isthe
entiretyofthebloodinthepulmonarycirculation,whereaseffectivepulmonary
flow(QEP)isthatdesaturatedportion,havingcompleteditscoursethroughthe
systemiccirculation,nowdirectedtowardthelungs.Inthenormalheart,where
thecardiacconnectionsareconcordantandnoshuntsexist,QP=QS=QEP=
QES.Intransposition,ifnoshuntsexisted,QEPandQESwouldbothequal0and
thepatientwoulddie.Fortunately,twoareasofcommunicationbetweenthe
circuitstypicallyexistinthenewborn.Oneisatthepatentarterialduct.The
otheristhroughtheatrialseptum.Inaddition,aventricularseptaldefect,present
inapproximately40%ofpatients,mayexistaswell.
Inthefirstfewhoursoflife,whenpulmonaryvascularresistanceishigh,
bloodwillshuntpredominatelyfromthepulmonary(saturated)circulationtothe
systemic(desaturated)circulation.Thismayresultintheinterestingfindingof
“reverseddifferentialcyanosis,”wherebythebloodinthelowerextremitiesis
morehighlysaturatedthantheinupperextremities.Thisphenomenonlastsfor
aslongasthereispulmonarytoaorticshuntingattheductallevel.Itmaybe
prolongedinthesettingoftranspositionwithpreductalcoarctation(or
interruption)oftheaorta,wherebythedistalaorticpressureisreducedbythe
obstruction,drivingsaturatedbloodfrompulmonaryarterytoaorta.Inthe
absenceofsuchobstructionasthepulmonaryvascularresistancedrops,
transductalshuntingtakestheoppositecourse,withdesaturatedaorticblood
enteringthepulmonarycirculation,contributingtoQEP.Inturn,pulmonary
venousreturntotheleftatriumisincreased,andtheresultantelevationinleft
atrialpressureservesastheimpetustodrivethesaturatedleftatrialbloodacross
thesecondcommunication,theovalforamenoratrialseptaldefect,tojointhe
systemiccirculation,therebycontributingtoQES.Oncethearterialducthas
closed,itisthebidirectionalflowacrosstheatrialseptumthatmaintainsQEPand
QES.Interventionstoaugmentflowattheductalandatriallevelsallowforthe
successfulstabilizationofthenewbornwithtransposition.
VentricularSeptalDefect
Thepresenceofaventricularseptaldefectcanhaveavariableinfluenceonthe
circulation.Thisinpartdependsonthesizeofthedefect,aswellasonthe
presenceofinteratrialmixing,andthepulmonaryvascularresistance.Alarge
interventricularcommunicationmaycontributetobeneficialcirculatorymixing,
butthepresenceofunrestrictedflowtothelungsmayresultinsymptomatic
heartfailure.Ifsurgeryisnotperformedduringinfancy,thiscanpredisposeto
earlypulmonaryvasculardisease,whichisgenerallyevidentby6to12months
ofage.20–22Thepresenceofadditionalpulmonaryorsubpulmonaryobstruction
furthermodifiestheserelationshipsandmaylimitQP.Patientswhoarewell
balancedduetopulmonaryobstructionaregenerallyasymptomatic,havinga
protectedpulmonarycirculation,adequatemixing,anda“well-trained”left
ventricle.
LeftVentricularPressureandMass
Theessenceoftranspositionissuchthattheleftventriclesupportsthe
pulmonarycirculation.Intheneonatalperiod,whenthepulmonaryvascular
resistanceishigh,leftventricularworkandimpedanceareelevated.Inapatient
withanintactorvirtuallyintactventricularseptum,thenormalpostnatal
reductioninpulmonaryvascularresistanceresultsinaprogressivereductionin
leftventricularpressureandwork,sothatasaresult,leftventricularmyocardial
massprogressivelyfalls.Thisreductioninleftventricularpressureand
myocardialmasswillbeattenuatedinthepresenceofalargeventricularseptal
defect.
Theeffectsofthispostnataltransitionareofcrucialimportancetothe
operativeapproach.Inapatientwithtranspositionwithanintactventricular
septum,whohastraversedthispostnataltransition,anyprocedure(includingthe
arterialswitchoperation),inwhichtheleftventricleiscalledontosupportthe
highresistancesystemiccirculationwillresultinleftventricularfailureinthe
earlypostoperativeperiod.Thus,aswillbediscussed,thearterialswitch
operationisperformedinearlypostnatallife,beforethepostnataltransitionis
complete.Olderpatientswillrequireeitherapreliminaryproceduretoincrease
theloadontheleftventricle,suchasbandingofthepulmonaryarteryto“train”
theleftventricleinpreparationfordefinitivesurgery,oralternativelymay
requireaperiodofleftventricularsupportwithventricularassistance,inthe
earlypostoperativeperiod.Finally,anatrialswitchproceduremaybedesirable
forsomeofthesepatients.
