Andersons pediatric cardiology 799
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atrioventricularvalve.Inthesevariants,theventricularseptumnolongermeets
theatrioventricularjunctionatthecrux,withimportantconsequencesforthe
dispositionoftheatrioventricularconductionaxis(seelater).24Asimilar
spectrumobviouslyexistswhentherightventricleisdominant,withtheextreme
endofthisspectrumbeingdouble-inletrightventriclewithacommon
atrioventricularvalve.Theconceptofchamberdominancecanalsobeextended
toincludetheatria.Whenoneoftheatriaisdominant,thecommon
atrioventricularjunctionismoreorlessequallysharedbytheventriclesbutis
mostlyconnectedtothedominantatrium.Theonlyexitfortheotheratriumis
acrosstheatrialcomponentoftheatrioventricularseptaldefect.This
arrangementisoftentermeddouble-outletatrium,albeitthisdescriptionis
applicabletoheartshavingabsenceofoneatrioventricularconnectionand
straddlingoftheotheratrioventricularvalve(seeChapter49).
AssociatedMalformations.
Ifnotruledoutbyitsanatomy,anylesionmustbeanticipatedtoexistinhearts
havinganatrioventricularseptaldefectwithacommonatrioventricularjunction.
Wehavealreadymentionedsomeofthemorefrequentmalformations,notably
obstructionswithintheleftventricularoutflowtract,andthoseaffectingtheleft
atrioventricularvalve.Additionaldeficienciesoftheatrialseptumareimportant
andaresometimesdescribedintermsofacommonatrium.Common
atrioventricularvalvescanalsobefoundinheartswithabnormalsegmental
connections,suchasdouble-inletventricleanddiscordantorambiguous
atrioventricularconnections.Inthesesettings,thepatientsfrequentlyalsoexhibit
abnormalventriculoarterialconnections.Double-inletventricle,however,is
usuallynotclassifiedasanatrioventricularseptaldefect.Discordant
ventriculoarterialconnections,forexample,aretheruleinassociationwitheither
double-inletleftventricleordiscordantatrioventricularconnections.Adouble
outletfromtherightventricleisfrequentlyfound,particularlywhenthereis
isomerismoftheatrialappendages(seeChapter27).Oftheotherassociated
lesions,tetralogyofFallotorpulmonarystenosisisparticularlyimportant,
occurringinuptoone-tenthofpatientswithatrioventricularseptaldefectanda
commonatrioventricularjunction.Presenceofasecondmuscularventricular
septaldefectisalsosignificant.Inthoseheartswithobstructionoftheleft
ventricularoutflowtract,rightventriculardominance,alongwithcoarctationor
interruptionoftheaorticarch,shouldbeanticipated.
AtrioventricularConductionTissues.
Inmostinstancestheatrialandventricularseptalstructuresareappropriately
alignedinthesettingofatrioventricularseptaldefectwithacommon
atrioventricularjunction.Thearrangementoftheatrioventricularconduction
axisisdifferentfromnormalbutcomparableinallthephenotypicvariantswith
septalalignment.25–27Thedifferencefromthenormalarrangementreflectsthe
lackoftheatrioventricularseptalstructuresandtheconcomitantlackofanormal
centralfibrousbody.Becauseofthedeficientatrioventricularseptation,the
inferioredgeofthemarginoftheatrialseptumusuallymakescontactwiththe
ventricularseptumonlyatthecrux.Itisatthecrux,therefore,thatthe
atrioventricularconductionaxisusuallypenetratesfromtheatrialtissuestoreach
thecrestofthemuscularventricularseptum.Inconsequenceofthis
arrangement,theentirenodalareaisdisplacedposteriorlyandinferiorly.
Althoughawell-formedtrianglecanbeseenatthislocation,thisnodaltriangle
isnotthesameasthenormaltriangleofKoch(Fig.31.12).
FIG.31.12 Locationoftheatrioventricularconductionaxisinthesettingof
alignedatrialandventricularseptalstructures(A)andrightward
malalignmentofthemuscularventricularseptum(B).Thebestguidetothe
locationoftheatrioventricularnodeisthepointatwhichthemuscular
ventricularseptumjoinstheinferioratrioventricularjunction.Septal
alignmentpermitsrecognitionofaninferiornodaltriangle,whichhasthe
coronarysinusatitsbase.Theinferiortriangle,however,isnolongerthe
sameasthetriangleofKoch.
Theareaofunionbetweentheposteroinferiorextremityoftheventricular
septumandtheatrioventricularjunctionprovidesthemostreliableguidetothe
positionoftheconductionaxis.Thislandmarkistobefoundevenwhenthe
coronarysinusisunroofedoropenstotheleftatrium.Itisalsopresentwhen
thereismalalignmentbetweentheatrialandventricularseptalstructures,butin
thislattersettingtheatrioventricularnodewillbepositionedawayfromthe
cardiaccrux(seeFig.31.12B).Havingtakenoriginfromtheatrioventricular
node,eitherinthenodaltriangleoralongtheinferioratrioventricularjunction,
theelongatednonbranchingbundlerunseitheronthecrestofthemuscular
ventricularseptumortoitsleftside,beingcoveredbytheinferiorbridging
leaflet.Thebundlebranchesarefoundmoreposteriorlythaninthenormalheart.
Onlytherightbundlebranchextendsalongthecrestinthebareareafoundinthe
presenceofacommonorifice.Inheartswithseparatevalvarorificesfortheright
andleftventricles,thispartoftheaxisiscoveredoverbytheconnectingtongue
ofleaflettissue.Thisfeatureisofmajorsurgicalimportance,sinceitpermits
suturestobesecuredontheleftsideofthefibrousrapheorwithinthebridging
leafletsthemselveswithoutcourtingdamagetotheunderlyingconduction
tissues.Theleftventricularoutflowtract,byvirtueofitsunwedgedlocation,is
unrelatedtotheconductionaxis.Thisfeatureeliminatestheriskofsurgical
damagecomparedwiththenormalheartshouldremovalbeattemptedofthe
obstructinglesions.
RelatedLesions
Knowledgeoftheanatomichallmarksofatrioventricularseptaldefectwitha
commonatrioventricularjunctionpermitsrecognitionofotherlesionswith
similarfeaturesthatdonotfitintothegroupasdefinedbecausetheydonot
possessacommonatrioventricularjunction.Themostobviouscandidateinthis
regardistheheartwithanormallylocatedsubaorticoutletbutadeficient
atrioventricularcomponentofthemembranousseptum.Suchlesions,knownas
directGerbodedefects,permitshuntingfromtheleftventricletotheright
atrium.Thisispossibleonlybecausethereareseparaterightandleft
atrioventricularjunctions(seeChapter32).Patientswiththesedefectshavea
normallywedgedsubaorticoutflowtractandareveryrare.28Closelyrelatedare
theheartswithseparaterightandleftatrioventricularjunctionsandanormally
wedgedaortabutwithdirectshuntingacrossaperimembranousinletventricular
septaldefectintotherightatriumbecauseofanomalousattachmentofthe
