Andersons pediatric cardiology 798
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dividesthecommonorificeintoseparaterightandleftatrioventricular
components(seeFig.31.7,left).Aswehaveemphasized,theostiumprimum
defectisnomorethanaheartwithacommonatrioventricularvalvewithdual
orifices.Theabnormalitiescanalsoinvolvethepapillarymuscles.Hypoplasiaof
oneorotherpapillarymusclesupportingtheleftatrioventricularvalve,orfusion
ofthemuscles,producesanarrangementreplicatingafunnel,albeitusually,and
misleadingly,describedintermsofa“parachute.”Inthisentity,theorificeofthe
valveisrepresentedbythespacebetweenthebridgingleaflets.Inseverecases
thevalvecantakeonabifoliateconfiguration.Suchaleftvalvewithtwoleaflets
remainsanatomicallydifferentfromthearrangementofthenormalmitralvalve.
RastelliClassification.
Inthepast,itwasconventionaltosubdivideatrioventricularseptaldefectswitha
commonvalvarorificedependingonthemorphologyofthepapillarymuscle
supportingtherightventricularextremityofthesuperiorbridgingleaflet.Such
variabilitywasfirstnotedandhighlightedbyRastelliandcolleagues.22They
describedthreemajortypes.Inthefirst,whichtheydubbed“typeA,”the
bridgingleafletwasmostlycontainedintheleftventricleandwasusuallytightly
tetheredbytendinouscordstothecrestoftheventricularseptum.Inthis
arrangement,thezoneofappositionofthesuperiorbridgingleafletwiththe
anterosuperiorleafletoftherightventricleissupportedbythemedialpapillary
muscle,whicharisesinrelativelynormalfashionfromtherightsideofthe
ventricularseptum.Inthesecondtype,thesuperiorbridgingleafletextends
moreintotherightventricle,usuallybeingunattachedtotheventricularseptum
asitcrossestheseptalcrestbutsupportedbyananomalousrightventricular
papillarymusclearisingfromthebodyoftheseptomarginaltrabeculation.Inthe
thirdtype,thefree-floatingbridgingleaflet—againunattachedtotheseptum—
extendsevenfurtherintotherightventricleandisattachedtoananterior
papillarymuscle.Inthisspectrum,asthesuperiorbridgingleafletbecomes
increasinglycommittedtotherightventricle,thezoneofappositionwiththe
anterosuperiorleafletoftherightventriclealsomovesintotherightventricle,
withcorrespondingdiminutioninsizeoftheanterosuperiorleaflet.The
spectrumcanbeextended,therefore,toincludetheso-calledostiumprimum
defect.Inthislesion,thebridgingleafletsareusuallyfusedtotheventricular
septalcrest,butwithminimalbridgingofthesuperiorleaflet(Fig.31.11).
Variabilityisalsofoundinthearrangementoftheinferiorbridgingleaflet,but
thisisnottakenintoaccountintermsoftheRastelliclassification.Thevariation
intheinferiorbridgingleafletrelatesnotsomuchtotheextentofbridging,since
almostalwaystheleafletextendswellintobothventricles,butmoretoits
tethering.Sometimestheinferiorbridgingleafletisseparatedintorightandleft
ventricularcomponentsbyawell-formedraphe,whichisfirmlyattachedtothe
ventricularseptum.Inotherhearts,thebridgingleafletistetheredbyshort
tendinouscordsasitcrossestheseptum,whereasinstillothersitcanfloatfreely.
Thusfar,noobviousrelationshiphasbeendiscoveredbetweenthedegreeof
tetheringofthetwobridgingleaflets,buttheextenttowhicheitherleafletdoes
bridgeisofobvioussurgicalsignificance.
FIG.31.11 EssenceoftheRastelliclassification,showingtheextentof
thevalvarleafletsasviewedfromtheventricularapex.Thereisincreasing
commitmentofthesuperiorbridgingleaflet(redarrow)totherightventricle
asthespectrummovesfromtypeAtotypeC,withreciprocaldiminutionin
thesizeoftheanterosuperiorleafletoftherightventricle(bluearrow).As
shown,thespectrumcanbeextendedtoincludetheostiumprimumdefect.
LeftVentricularOutflowTract.
Byvirtueofitsanteriorandunwedgedposition,theleftventricularoutflowtract
isparticularlysusceptibletoobstruction.Thisistrueirrespectiveofwhether
thereisacommonvalvarorificeorseparaterightandleftatrioventricularvalves
withinthecommonjunction.Onanatomicexamination,thetractalmostalways
seemsnarrowedascomparedwiththediameteroftheaorticvalve.Itslength,
andtheextentoftheapparentnarrowing,aremoremarkedindefectsinwhich
thesuperiorbridgingleafletisfirmlyfusedtotheseptalcrest—inotherwords,in
ostiumprimumdefects.23Additionallesionscompromisingthealreadynarrowed
channelareresponsibleforhemodynamicallysignificantobstructionor,
alternatively,theeffectsofcorrectivesurgery.Anyofthelesionsthatproduce
subaorticstenosisinthenormalheartcanrapidlyproducesimilarproblemsin
thesettingofatrioventricularseptaldefectswithacommonatrioventricular
junction,particularlywhenthereistetheringofthesuperiorbridgingleafletto
theseptumorseparatevalvarorificesforthetwoventricles.
DominanceofChambers.
Mostcommonlyinatrioventricularseptaldefects,whetherwithseparateright
andleftatrioventricularvalvarorificesoracommonvalvarorifice,therightand
leftcomponentsofthecommonatrioventricularjunctionareofcomparable
circumferenceandtheventriclesareofsimilarsize.Thisproducestheso-called
balancedarrangement.Thecommonatrioventricularjunctioncanbecommitted
initslargerparttotherightventricle,producingrightventriculardominance,or
totheleft,givingadominantleftventricle.Rightventriculardominanceis
usuallyassociatedwithclinicallysignificanthypoplasiaorabnormalityofthe
leftventricularandaorticstructuresbutitismostoftenfoundwithnormal
alignmentbetweentheatrialandventricularseptalstructures.Inthepresenceof
leftventriculardominance,incontrast,itistherightventricularandpulmonary
arterialstructuresthatarehypoplastic,typicallyinassociationwith
malalignmentbetweentheatrialandthemuscularventricularseptalstructures.
Suchheartswithatrioventricularseptalmalalignmentconstitutepartofa
spectrumthatextendstodouble-inletleftventricle,butthroughacommon
