Andersons pediatric cardiology 78
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entitiesthatsupportsthesemilunarattachmentsoftheleaflets.Atleastthree
zoneswithinthecomplexcanjustifiablybedescribedasdiscreterings(seeFig.
2.18).Thefirstisthesinutubularjunction.Thesecondistheanatomic
ventriculoarterialjunction,whichistheareaoverwhichthefibroelasticwallof
thearterialtrunkjoinswiththesupportingventricularwalls.Thisanatomic
junctionshouldbedistinguishedfromthehemodynamicboundarybetween
ventricleandarterialtrunk,whichismarkedbythesemilunarattachmentofthe
valvarleaflets.14Thesemilunararrangementresultsincrescentsofventricular
wallbeingincorporatedwithinthetruncalsinuses,andtrianglesofarterialwall
beingincorporatedwithintheventricularoutflowtracts(seeFig.2.17).Thefinal
ringwithinthevalvarcomplex,identifiedbyechocardiographersastheannulus,
hasnoanatomiccounterpart.Itisformedbyjoiningthebasalattachmentsofthe
leafletswithintheventricles.Manysurgeonsdescribeasemilunar“annulus.”13
Thisisbecausehavingremovedtheleafletsofadiseasedvalve,thesemilunar
remnantsareseenaspointsofanchorageforthesuturesplacedtosecurethe
prosthesesusedasvalvarreplacements.
SeptalStructures
Althougheachoftheseptalstructureshasalreadybeendescribed,itisworth
reemphasizingtheirstructure,themoresosinceonlycursorymentionhasbeen
madeoftheimportantmembranouspartoftheventricularseptum.Inthis
respect,onlythosepartsofthecardiacwallsthatseparateadjacentchambers
shouldbedescribedasseptal.15Wallsthatseparatethecavityofachamberfrom
theoutsideoftheheart,evenwhenfoldedonthemselves,arenotpartofthe
septalstructures.
Theatrialseptum,whendefinedinthisfashion,iscomposedmostlyofthe
flooroftheovalfossa,thisbeingtheflapvalve,whichisderivedfromthe
developmentalprimaryseptum.Inadditiontothefloor,onlytheanteroinferior
rimofthefossaisalsoatrueseptuminitsownright.Theothermarginsarethe
infoldedatrialwalls,orelsetheatrialwalloverlappingthebaseofthe
ventricularmass(seeFig.2.9).Theso-calledsinusseptumisnomorethanthe
branchingpointofthecoronarysinusandtheinferiorcavalvein.Contiguous
withbothsinusseptumandtheinferiormarginoftheovalfossaisthearea
knownasthetriangleofKoch.Thisimportantzoneistheatrialaspectofthe
atrioventricularmuscularsandwich(seeFig.2.10).Inthisarea,theleafletsofthe
tricuspidvalveareattachedmoreapicallythanarethoseofthemitralvalve.The
apexofthetriangleofKochismadeupofthefibroustissueintheposterior
componentoftheaorticroot.Withinthistissueisincorporatedthefibrouspartof
theatrioventricularseptum,itselfapartofthemembranousseptumoftheheart.
Thisistheonlytrueatrioventricularseptum.Takenoverall,themembranous
septumiscontiguouswiththefibroustrianglebeneaththezoneofapposition
betweenthenonadjacentandrightcoronaryleafletsoftheaorticvalve(seeFig.
2.23).Itisthelineofattachmentofthetricuspidvalveonitsrightaspectthat
dividesthemembranousseptumintoitsatrioventricularandinterventricular
components,althoughtheproportionsofthetwocomponentsvarymarkedly
fromhearttoheart.
Theinterventricularcomponentofthemembranousseptum,whenconsidered
relativetothebulkofthemuscularventricularseptum,isinconspicuous,but
formsthekeystoneoftheseptumwithintheaorticroot.Wethoughtinthepast
thatwewereabletodividethemuscularventricularseptumintoinlet,apical
trabecular,andoutletcomponents,thinkingthattheseseptalcomponents
matchedthecorrespondingpartsoftheventricularcavities.16Inreality,theparts
oftheseptumseparatingtheinletandoutletoftherightventriclefromtheir
comparablecomponentsintheleftventriclearenotnearlyaslargeasinitially
thought.Becauseofthedeeplywedgedlocationofthesubaorticoutlet
component,muchoftheseptumsupportingtheseptalleafletofthetricuspid
valveseparatestheinletoftherightventriclefromtheleftventricularoutlet(see
Fig.2.21).Becauseofthefree-standingnatureofthesubpulmonary
infundibulum,themuscularseptumofthenormalheartlacksanytrueoutlet
septum.Inthenormalheart,therefore,therearenoobviousboundariesthat
dividethemuscularventricularseptumintocomponentparts.Theoutletseptum
achievesaseparateexistenceonlywhentheventricularseptumitselfisdeficient,
particularlywhenitsoutletcomponentismalalignedrelativetotherestofthe
muscularseptum.
FibrousSkeleton
Manytextbooksillustratetheshort-axisbasalsectionoftheheartascontaininga
fibrousskeletonthatembracestheoriginsof,andprovidestheattachmentsfor,
theleafletsofallfourcardiacvalves.Thereisnofoundationinanatomicfactto
supportthisnotion,themoresosinceunlikethebonyskeleton,thefibrous
componentsoftheheartdonotprovideoriginsandinsertionsforthemyocardial
components.Asalreadyshown,theleafletsofthepulmonaryvalveare
supportedonanextensivesleeveoffree-standingrightventricularmusculature.
Theleafletsoftheaorticvalvealsoariseinpartfromtheseptalmusculature,and
inpartfromazoneoffibrouscontinuitywiththeaorticleafletofthemitral
valve.Thetwoendsofthezonearethickenedtoformtherightandleftfibrous
trigones(Fig.2.27).
FIG.2.27 Leftventricularoutflowtractopenedanteriorlyandspreadto
revealtheareaoffibrouscontinuitybetweentheleafletsoftheaorticand
mitralvalves.Theendsoftheareaofcontinuityarethickenedtoformthe
fibroustrigones,withthecontinuitybetweentherighttrigoneandthe
membranousseptumformingthecentralfibrousbody.
