Andersons pediatric cardiology 685

AtrioventricularJunctions
Aswithheartsinwhichtheatrialappendagesarelateralized,chamberswith
isomericappendagescaneachbeconnectedtotheirownventricles,orthe
atriumscanbeconnectedtoonlyoneventricle.Whentheatrioventricular
connectionsarebiventricular,itisessentialalsotodescribetheventricular
topology.Thisisbecausetherearetwopatternstobefound,irrespectiveof
whethertheisomericappendagesareofrightorleftmorphology.Inthefirst
pattern,theright-sidedatrium,whetherassociatedwithamorphologicallyright
orleftappendage,willbeconnectedtothemorphologicallyrightventricleand
theleft-sidedatriumtothemorphologicallyleftventricle.Thisventricular
topologyisright-handed(Fig.26.17,left).Inthesecondarrangement,therightsidedatrialchamber,whichagainmaypossesseitheramorphologicallyrightor
leftatrialappendage,willbeconnectedtoamorphologicallyleftventricle,and
theleft-sidedatriumwillbeconnectedtoamorphologicallyrightventricle.In
thissecondpattern,theventriculartopologyisleft-handed(seeFig.26.17B,
right).Whenweinitiallydefinedsuchatrioventricularconnections,wedescribed
themasambiguous.Wenowrecognizethatitisbettertodescribethemas
biventricularandmixed,proceedingalwaystodescribetheventriculartopology
presentand,ifnecessary,anyabnormalandunexpectedventricularrelationships.
Biventricularandmixedatrioventricularconnectionsaremuchmorefrequentin
thesettingofisomericleftatrialappendages,withmostofthesepatientshaving
right-handpatternventriculartopology.


FIG.26.17 Conceptofbiventricularandmixedatrioventricular
connections.Theconnectionswouldstillbemixediftheappendageswere
ofleftmorphologybilaterally,ratherthantheisomericrightvariantshownin
Fig.26.5.

Univentricularatrioventricularconnections,typicallywithdoubleinletviaa
commonatrioventricularvalve,aresignificantlymorefrequentinheartswith
rightisomerism.Inaboutone-thirdofsuchinstances,theuniventricular

connectionwillbetoamorphologicallyrightventricle,usuallyleft-sided,butin
someitwillnotbepossibletofindasecondventricle.Themajorityofhearts
willhavedoubleinlettoadominantmorphologicallyleftventricle,butsolitary
andindeterminateventriclesaremorefrequentwithisomerismthaninanyother
setting.Absenceoftheleft-sidedorright-sidedatrioventricularconnectionis
rarebutcanbefoundwithanyventricularmorphology.Thus,whentheatrial
appendagesareisomeric,anytypeofuniventricularatrioventricularconnection
mustbeanticipated,alongwithanypossibleventricularmorphology.Fewer
heartswithleftisomerismhaveuniventricularatrioventricularconnections,but
thesamevariabilitymustbeexpected.
Irrespectiveofthepresenceofbiventricularoruniventricularatrioventricular
connections,mostheartswithrightisomerismhaveacommonatrioventricular
junctionguardedbyacommonvalve(seeFig.26.4).Aswouldbeexpected,
withsuchahighincidenceofatrioventricularseptaldefects,theventricular
septuminthemajorityofheartswithbiventricularatrioventricularconnectionsis
deformedintheanticipatedfashion.Intheveryrarecaseswithrightisomerism
butwithoutanatrioventricularseptaldefect,ventricularseptaldefectsarethe
ruleandcanbeperimembranousormuscular.Inallheartswithuniventricular
atrioventricularconnection,whetherarightorleftisomerism,thosewith
incompleteventricleshaveinterventricularcommunicationsasanticipatedfor
theventricularmorphologypresent.

VentriculoarterialJunctions
Amongtheentiregroupofheartswithisomericatrialappendages,thereisjustas
muchvariabilityintypeandmodeofventriculoarterialconnection,infundibular
morphology,andarterialrelationshipsasencounteredincongenitalcardiac
diseaseasawhole.28Certainpatternsoccurwithsignificantlydifferent
frequenciesinheartswithisomericrightasopposedtoleftappendages.
Pulmonaryobstructionoratresiaissignificantlymorecommoninassociation

withrightthanwithleftisomerism.Whenthereispulmonaryatresia,the
pulmonarysupplyisalmostalwaysductdependent,althoughsupplythrough
systemic-to-pulmonarycollateralarteriescansometimesbefound.Obstruction
oftheleftventricularoutflowtract,withaorticcoarctationoratresia,ismuch
morecommonwithleftisomerism.Analysisoftheventriculoarterialjunctions
asawholeshowssignificantdifferencesbetweenthetwoisomericarrangements.
Ananteriorright-sidedaorta,alongwithsubaorticorbilateralinfundibulum,is
morecommonwithrightisomerism,whileconcordantventriculoarterial
connectionsaremorefrequentinleftisomerism.

ConductionTissues
Themorphologyofthesinusnodereflectsthearrangementoftheatrial
appendages.Inrightisomerism,therearebilateralsinusnodes,eachrelatedto
theterminalcrestandthecavoatrialjunctioninnormalfashion.29Inleft
isomerism,incontrast,therearenoterminalcrestsandnorightatrial
appendages.Thesinusnodethereforecannotoccupyitsnormalposition.Indeed,
inmostheartsstudied,ithasnotprovedpossibletoidentifywithcertaintythe
sinusnode.22Whenacandidateforthesinusnodeisdiscovered,ittendstobe
grosslyhypoplasticandlocatedintheatrialwalladjacenttotheatrioventricular
junctions.
Thedispositionoftheatrioventricularconductionaxisreflectsboththe
atrioventricularconnectionpresentandtheventriculartopology.Whenthereare
biventricularatrioventricularconnections,thedominantfeatureistheventricular
topology.Anatrioventricularnodeinitsregularposition,withaposteroinferior
penetratingatrioventricularbundle,isfoundwithright-handtopology.When
thereisleft-handventriculartopology,thereisusuallyananterior
atrioventricularnode,andtheconductionsystemisfoundasistypicalfor
congenitallycorrectedtransposition.Alternatively,aslingofconductiontissueis
presentrunningalongthecrestoftheventricularseptum,producingaconnection

withtwoatrioventricularnodes.Withuniventricularatrioventricularconnection,
ventricularmorphologybecomesthedominantfeature.Whentheleftventricleis
dominant,theatrioventricularnodeisfoundinanteriorposition,withthebundle
varyingitsrelationshiptotheoutflowtractoftheleftventricleaccordingtothe
positionoftheincompleterightventricle(seeChapter49).Withconnectiontoa
dominantrightventricle,theconductionsystemisnormallysituatedwhenthe
incompleteleftventricleisleft-sided,butananteriornodeorslingmaybefound