Andersons pediatric cardiology 69
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andthoseoftheheart,weshouldstilldescribethecardiacstructuresrelativeto
thebodilycoordinates.Suchattitudinallyappropriatedescription1isabasicrule
ofhumananatomy,buthasnotalwaysbeenfollowedbythosedescribingthe
heart.Thecardiacsilhouette,whenprojectedtothefrontalsurface,ismoreor
lesstrapezoidalwithhorizontalupperandlowerborders,arelativelyvertical
rightborderjustoutsidetherightedgeofthesternum,andaslopingleftborder
extendingouttotheapexinthefifthintercostalspace(seeFig.2.1).
Thetruebaseoftheheartistheposterioraspectoftheatrialchambers,where
theorganisanchoredwithinthemediastinum.Morefrequently,itisthe
ventricularmassthatisdescribedashavingabase.Allfourcardiacvalvesare
seenwhenthisareaisviewedfromtheatrialaspect(Fig.2.2,left).
FIG.2.2 Locationofthecardiacvalvesasrevealedbyreconstructionfrom
amultidetector-rowcomputedtomogramobtainedinanadultindividual.
Left,Arrangementwithinthebaseoftheventricularmassasviewedfrom
aboveandfromtheleft.Inadditiontothelocationofthevalves,the
reconstructionalsoshowsthelocationofthemembranousseptum,andthe
interleaflettrianglebetweentheleftcoronaryandthenonadjacentsinuses
oftheaorticroot.Right,Locationofthehingesofthevalvarleafletsas
viewedinthefrontalprojection.Notethecentrallocationoftheaorticroot
inbothprojections.
Whenviewedfromthefrontinattitudinallyappropriatefashion,the
pulmonaryvalveislocatedsuperiorlyandtotheleft,withtheaortic,mitral,and
tricuspidvalvesoverlappingwhentracedinrightwardandinferiordirection(see
Fig.2.2,right).Thekeyfeaturefromthestanceofunderstandingnormalcardiac
anatomyisthecentrallocationoftheaorticrootwiththeaorticvalvarleaflets
andtheirsupportingaorticsinuses,beingrelatedtoallthecardiacchambers(see
Fig.2.2,left).
Theheartitself,occupyingthemiddlepartofthemediastinum,isencased
withinthepericardialsac.Thefibrousouterlayerofthepericardiumfunctionsas
thecardiacseatbelt,withitsattachmentstothediaphragm,alongwiththe
reflectionsattheentrancesandexitsofthegreatveinsandarterialtrunks,
anchoringtheheartwithinthemediastinum(Fig.2.3A).Thetoughfibrous
componentislinedwithaserouslayer,theparietalpericardium,whichisitself
reflectedontothesurfaceoftheheartastheepicardium.Withinthecavitythus
formedaretworecesses,thetransverseandobliquesinuses(seeFig.2.3B).
FIG.2.3 (A)Segmentedmultidetector-rowcomputedtomographicdataset
obtainedfromanadultindividualshowingthereflectionsoftheserous
pericardiumwithinthesacofthefibrouspericardium.(B)Locationsofthe
transverseandobliquesinuses.
Embeddedwithinthefibrouspericardiumarethevagusandphrenicnerves.
Bothsetsofnervestraversethelengthoftheheartoneachside,withthephrenic
nervesanterior(Fig.2.4)andthevagusnervesposteriortothehilumsofthe
lungs.
FIG.2.4 Computedtomographicdataset,obtainedfromanadult
individual,reconstructedtoshowthelocationoftheright(left)andleft
(right)phrenicnerves.Thecavitiesofthecardiacchambersandgreat
vesselshavealsobeenreconstructed.
Therecurrentlaryngealnervesbranchfromthevagusnerveswithinthe
mediastinum,recurringaroundthebrachiocephalictrunkontherightside,and
aroundthearterialduct,oritsligamentousremnant,ontheleft.Thethymus
glandisalsoaprominentstructurerelatedtotheanteriorandlateralaspectsof
thepericardialsacintheregionofexitofthegreatarterialtrunks.Itis
particularlyprominentinneonatesandinfants.Theesophagus,tracheal
bifurcation,anddescendingaortaareadditionalimportantmediastinalstructures
relatedposteriorlytotheheart,withtheirlocationsnowwelldemonstratedusing
computedtomography(Fig.2.5).
