Andersons pediatric cardiology 796
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FIG.31.8 Arrangementoftheleafletsofthecommonatrioventricular
valveasseenwhenthecommonjunctionisviewedfromtheventricular
apex.Thelocationofthemuscularventricularseptumisshownbythe
dashedblacklines,withtheyellowarrowshowingthezoneofapposition
betweenthetwoleafletsthatbridgetheventricularseptum.
Thesefiveleafletsareseentotheirbestadvantagewhenthecommonjunction
isguardedbyacommonvalvarorifice(seeFig.31.7,right).Whenatongueof
valvartissuejoinstogetherthetwoleafletsofthecommonvalvethatbridgethe
ventricularseptum,theeffectistoproduceseparateorificeswithinthecommon
junctionfortheinletstotherightandleftventricles(seeFig.31.7,left).This
tongueisusuallyattacheddirectlytothemusculaturealongthelengthofthe
crestoftheventricularseptum.Theessenceoftheso-calledostiumprimum
defect,therefore,isthatthefusedbridgingleafletsaredepressedintothe
ventricularcavityandalsofusedtothecrestofthescooped-outventricular
septum(seeFig.31.6,left).Thisproducesadouble-orificecommon
atrioventricularvalve.Whenconsideringthebasicarrangement,threeofthe
leafletsareconfinedtooneorotheroftheventricles,withonebeingexclusiveto
theleftventricleandtwototherightventricle.Itistheleftmuralleaflet,which
ismuchlessextensivethanthemuralleafletofthenormalmitralvalve,thatis
tetheredbetweenthesuperiorandinferiorpapillarymusclesoftheleftventricle.
Theanterosuperiorleafletandtherightmuralleafletareconfinedtotheright
ventricle.Thebridgingleafletshavenocounterpartsinthenormalheart.The
spacebetweenthemistheirzoneofapposition(seeFig.31.7).Althoughoften
stillcalleda“cleft”forbrevity,thereisnoquestionthatthespaceistheirzoneof
apposition.Reflectingthezonesofapposition,theleftvalveclosesintrifoliate
fashion(Fig.31.9).
FIG.31.9 Intraoperativeimageshowingthetrifoliateconfiguration
(dashedlines)oftheclosedleftatrioventricularvalveofanatrioventricular
septaldefectwithcommonatrioventricularvalve.
Suchtrifoliateclosureismarkedlydifferentfromthepatternofclosureofthe
mitralvalve,thetwoleafletsofwhichcometogetheralongasolitaryzoneof
apposition(seeFig.31.3,left).Todaythesurgeonseekingtocorrectthelesion
willusuallyclosethespacebetweentheleftventricularcomponentsofthe
bridgingleaflets.Thissurgicalmaneuver,however,neverproducesan
arrangementofleafletsforthenewlyconstructedleftatrioventricularvalvethat
replicatesthearrangementseeninthenormalmitralvalve.Thearrangementof
theleftventricularpapillarymusclesreflectsthelocationoftheleaflets.The
musclesarepaired,asinthenormalheart.Butinsteadofbeinglocatedin
anteroinferiorandposterosuperiorpositions,theyareinmore-or-lessdirect
superoinferiorpositions.Onoccasiontheymaybeevenmoreabnormally
arranged,producingtheso-calledparachutearrangement.18Inreality,the
abnormalvalvelooksmorelikeafunnel,representingthespatialinverseofa
parachute.19Thearrangementoftherightventricularmusclesiscomparable
withthatofthenormalheart,althoughthepositionofthemedialpapillary
muscleisvariable.Thisvariabilityhassignificance,sinceitreflectstheextentto
whichthesuperiorleafletbridgesintotherightventricle,underscoringthe
variabilityseenintheRastelliclassification(seelater).
Asalreadyemphasized,anotherdifferencecharacterizingthemorphologyof
atrioventricularseptaldefectswithacommonatrioventricularjunctionisthe
dimensionsoftheventricularmass.Inthenormalheart,theinletandoutlet
dimensionsoftheleftventricleareapproximatelythesame(seeFig.31.4A).In
atrioventricularseptaldefectswithacommonjunction,thedimensionofthe
outletisconsiderablygreaterthanthatoftheinlet(seeFig.31.4B).Itisoflittle
consequencemorphologicallywhetherthisisbecausetheinletisshorterthan
normalorbecausetheoutletislonger.Probablyitisacombinationofthetwo.
Thedisproportioniswithinthesamerange,bethereacommonvalvarorificeor
separatevalvarorificesattheinletstotherightandleftventricles.Indeed,once
thecurtainofvalvarleafletsisremovedfromtheatrioventricularjunctionsin
anyindividualheart,itisnotpossibletojudgesimplyfromexaminationofthe
ventricularmasswhethertherehadinitiallybeenacommonvalvarorificeor
separaterightandleftatrioventricularorifices.Theextentofdeficiencyofthe
midpointoftheseptum,whichwedescribeasthedegreeof“scooping,”is
usuallygreaterinthoseheartshavingacommonatrioventricularvalvarorifice.20
CategoriesofAtrioventricularSeptalDefect
Allthelesionsdiscussedinthischapterareunifiedbypresenceofacommon
atrioventricularjunction.Despitethecommonalityofthejunctional
arrangements,asalreadydiscussed,itisstillusualforclinicianstorecognizetwo
majorcategories.Theseareconsideredtobe“partial”and“complete,”despite
theiranatomiccomparability.Twoanatomicfeaturesunderscorethisclinical
stratification.Thefirstisthearrangementoftheindividualleafletswithinthe
overallcurtainofvalvartissueguardingthecommonatrioventricularjunction.
Thesecondistherelationshipbetweenthebridgingleafletsofthecommonvalve
andtheatrialandventricularseptalstructures.Problemsareproducedwhen
thesetwofeaturesarecombinedsoastoidentifythepresumedcompleteand
partialvariants,sincethetwoaspectsofanatomyaremutuallyindependent.
