Andersons pediatric cardiology 768
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consideronlythepartsthatcanberemovedwithoutcreatinga
communicationwiththeextracardiacspacetobetrueseptalstructures.
Thus,whenoneviewstheseptalsurfaceoftherightatrium,itseemsatfirst
sightthatalargeexpanseoftheatrialwallsbetweentheorificesofthesuperior
andinferiorcavalveinsandtheattachmentoftheseptalleafletofthetricuspid
valvearepotentiallyinterposedbetweenthecavitiesoftherightandleftatria
(Fig.29.2,left).
FIG.29.2 Septalsurfaceoftherightatrium.Theyellowlineshowsthe
planeofsectiontakentoproducetheimageatright,whichshowsthatthe
anteriorandposteriorrimsoftheovalfossaareinfoldingsoftheatrial
walls.(AnorthogonalsectionofthesameheartisshowninFig.29.3.)Note
thatthetendonofTodaroisthecranialcontinuationoftheeustachian
valve.Itextendsthroughtheanteroinferiorbuttressofthefossatoinsert
intothecentralfibrousbody.ICV,Inferiorcavalvein;SCV,superiorcaval
vein.
Whentheheartissectionedacrosstheovalfossa,itsanteriorandposterior
rimsareseenasfoldsbetweentheatrialwalls(seeFig.29.2,right).Sections
takenthroughtheatrialchambersinthefrontalplaneshowthatthesuperiorrim
ofthefossa,oftenconsideredtorepresenttheseptumsecundum,issimilarlya
fold(seeFig.29.1).ThisfoldistheareaknowntosurgeonsastheWaterstonor
Sondergaardgroove.Inferiorly,therimseparatesthefossafromtheorificeofthe
coronarysinus,withthemouthofthecoronarysinusitselfseparatedfromthe
mouthoftheinferiorcavalveinbyanadditionalfold,thesinusseptum.The
tendonofTodaro,oneoftheimportantlandmarksofthetriangleofKoch,runs
throughthisareatoinsertintothecentralfibrousbody(seeFig.29.2,left).This
anteroinferiormuscularbuttress,traversedbythetendon,isanothertrueseptal
structure(Fig.29.3).
FIG.29.3 TheheartshowninFig.29.2hasbeensectionedagaintoshow
thatthesuperiorrimoftheovalfossaisalsoafoldbetweentheatrialwalls.
Notethewallsofthecoronarysinusintheleftatrioventriculargroove.The
starshowsitsmouthintherightatrium.Thereisanadditionalfoldbetween
thewallsofthesinusandtheinferiorcavalvein,whichisknownasthe
sinusseptum.ICV,Inferiorcavalvein;SCV,superiorcavalvein.
Thusonlytheflooroftheovalfossaandtheanteroinferiormuscularbuttress
constitutetrueinteratrialseptalstructures,whichcanberemovedwithoutexiting
thecavitiesoftheheart(seeFig.29.1).
TypesofInteratrialCommunication
Recognitionoftheextentofthenormalseptumunderscoresthefactthatnotall
interatrialcommunicationsareseptaldeficiencies.5Ofthechannelsthatpermit
interatrialshunting,thosewithintheconfinesoftheovalfossa,alongwiththe
muchrarerdeficienciesoftheanteroinferiormuscularbuttress,aretrueseptal
defects.Theostiumprimumdefects,alongwiththesinusvenosusdefectsand
thosefoundatthesiteofthemouthofthecoronarysinus,areinteratrial
communications,buttheylieoutsidetheconfinesoftheatrialseptum(Fig.
29.4).
FIG.29.4 Locationofthevariouslesionsthatpermitinteratrialshunting.
Theovalfossaandvestibulardefectsaretrueatrialseptaldeficiencies.
Theremaininglesionsareproperlydescribedasinteratrialcommunications
ratherthanatrialseptaldefects.
DefectsWithintheOvalFossa
Theselesions,whicharetrueseptaldefects,arebyfarthecommonesttypeof
interatrialcommunication.Althoughmostfrequentlytermedsecundumdefects,
thistermisjustifiedonlybecausethedefectrepresentspersistenceofthesecond
interatrialcommunicationtobeformedduringcardiacdevelopment.Shouldthey
befoundpostnatally,itisbecauseofdeficienciesofthefloorofthefossa,which
