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Thereforeacommonarterialtrunkisoneformofsingleoutletfromtheheart.
Asamalformationinvolvingtheventriculoarterialjunctions,itmustbe
anticipatedtocoexistwithallpossiblesegmentalcombinations.Inalmostall
instances,nonetheless,therewillbeausualatrialarrangement,withconcordant
atrioventricularconnections.Examplescanbefoundincombinationwith
discordantatrioventricularconnectionsorwithabsenceoftheright
atrioventricularconnection.6Althoughtheatrioventricularjunctionsthemselves
areusuallyseparate,andguardedbymitralandtricuspidvalves,acommontrunk
canrarelybefoundinassociationwithanatrioventricularseptaldefectanda
commonatrioventricularvalve.Inthepresenceofthecommontrunk,thetruncal
valveisalmostalwaysconnectedacrosstheventriculoarterialjunctionswith
bothventricles,thevalvarorificeoverridingtheventricularseptalcrest,and
typicallywithitsleafletsinfibrouscontinuitywiththemitralvalveintheleft
ventricle(Fig.40.3).Suchabiventricularconnectionnecessitatesthepresenceof
ajuxtaarterialinterventricularcommunication.Thedefectisgenerallylarge.Its
flooristhecrestoftheventricularseptum,reinforcedontherightventricular
aspectbythelimbsoftheseptomarginaltrabeculation,orseptalband,andits
roofistheleafletsofthetruncalvalve.Theconeofspacesubtendedbythe
truncalvalvehasrightandleftventricularmargins,butitisusuallytheright
ventricularmarginthatisconsideredtorepresenttheventricularseptaldefect,
anditisthisspacethatisclosedbythesurgeonduringrepair.Inthemajorityof
cases,fusionoftheinferiorlimboftheseptomarginaltrabeculationwiththe
ventriculoinfundibularfoldalongthisrightventricularmarginproduces
musculardiscontinuitybetweentheleafletsofthetricuspidandthetruncal
valves(Fig.40.4,left).Intheabsenceofsuchfusion,thereiscontinuitybetween
theleafletsofthetricuspidandtruncalvalves,makingtheventricularseptal
defectperimembranous(seeFig.40.4,right).Whenpresent,thismuscularbarin
theposteroinferiormarginprotectsthespecializedaxisresponsiblefor
atrioventricularconduction.Inmostinstances,thereisalargedistancebetween
thecoaptingarterialvalvarleafletsandthecrestoftheseptumduringventricular
diastolewhentheleafletsareclosed.However,thisspacemaysometimesbe