Andersons pediatric cardiology 977
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AnatomyandMorphogenesis
Asemphasized,itisthebasiccombinationsofconcordantatrioventricularand
discordantventriculoarterialconnectionsthatproducetheentitywecall
transposition(Fig.37.1).15Theanatomicsituationcanbecomplicatedbythe
presenceofaventricularseptaldefect,anobstructionwithintheleftventricular
outflowtract,bothofthesemalformations,orbyotherassociatedmalformations.
Caseswithanintactinterventricularseptumandnoobstructionoftheleft
ventricularoutflowtractareusuallyconsideredsimple,eveniftheyare
complicatedbyotherlesionssuchaspersistentpatencyofthearterialduct.
BasicSegmentalCombinations
Thecombinationofsegmentalconnections(Fig.37.2)canbefoundwitheither
usualormirror-imagedatrialarrangements(Fig.37.1).However,thisspecific
combinationcannotexistwhenthereisisomerismoftheatrialappendages.
Heartswithisomericatrialappendagescan,ofcourse,havebiventricular
atrioventricularconnections,right-handtopology,anddiscordant
ventriculoarterialconnections.Suchheartsarecloselyrelatedtotranspositionas
itisdefinedforthepurposesofthischapter,butthepatientswithisomericatrial
appendageshavegrosslyabnormalvenoatrialconnectionsastheirmajorfeature
(seeChapter26).Inthearrangementasdefinedforthepurposesofthischapter,
theatrialanatomyisbasicallynormal,althoughmostfrequentlytheoval
foramenispatent,orthereisadeficiencyoftheflooroftheovalfossa.Evenif
theflapvalveoverlapstherimoftheovalfossa,itisflimsyandcanberuptured
easilybyballoonseptostomy.Inkeepingwiththisnormalatrialanatomy,the
sinusandatrioventricularnodesareintheiranticipatedposition.
FIG.37.2 (A)Morphologicallyrightventricleinthesettingoftransposition.
Itisconnectedbythetricuspidvalvetothemorphologicallyrightatriumand
supportstheaortaaboveamuscularinfundibulum.Theventricularseptum
isintact.(B)Morphologicallyleftventriclefromthesameheart.Itis
connectedtothemorphologicallyleftatriumthroughamitralvalveand
givesrisetothepulmonarytrunk.Notethefibrouscontinuitybetweenthe
leafletsofthepulmonaryandmitralvalves.
Unliketheatrialchambers,ventricularmorphologyissubtlydifferentfrom
normal.Theventricularseptumismuchstraighterthanusual,notshowingthe
multiplecurvessotypicalofthenormalheart.Thepulmonaryvalveisnot
wedgedasdeeplybetweenthemitralandtricuspidvalvesasistheaorticvalvein
thenormalheart.This,inturn,meansthattheareaofoff-settingoftheleafletsof
theatrioventricularvalvesismuchlessmarked,asistheareaoccupiedbythe
membranousseptum.Anotherconsequenceofthisabnormalarrangementisthat
theratioofthedimensionsoftheinletandoutletcomponentsoftheventricular
massareabnormalinfavoroftheoutletdimension,althoughnottotheextent
seeninatrioventricularseptaldefects(seeChapters31and32).Atbirth,the
wallsofthemorphologicallyleftventricularwallaremarginallythickerthan
thoseoftherightventricle.Therightventricularmuralthicknessthenrapidly
increasesinthefirst2yearsoflife,becomingmuchthickerthanthatoftheleft
ventricle.Themostobviousexternalabnormalityistherelationshipoftheaorta
tothepulmonarytrunk.Inthemajorityofpatientswithanintactventricular
septum,theaorticrootistotherightofthepulmonarytrunkinheartsinthe
settingofusualatrialarrangement(Fig.37.2)andtotheleftinthemirror-imaged
variant(Fig.37.3,leftpanel).Patientsarefound,nonetheless,withusualatrial
arrangementandintactventricularseptumwhentheaortaistotheleft(Fig.37.3,
rightpanel).Rarely,theaortamayberightsidedandposterior,evenwhenthe
ventricularseptumisintact.
