Andersons pediatric cardiology 160

VenousDuct
Theumbilicalveincarriesoxygenatedblood,withanoxygensaturationof
between80%and90%,fromtheplacentatotheumbilicalcord(Fig.6.3A).The
umbilicalveinentersthefetalabdomen,whereitdividestoformtheportalsinus
andthevenousduct.Theportalsinusjoinstheportalvein,whilethevenousduct
carriesoxygenatedbloodtotheinferiorcavalvein(Video6.2).Theoriginand
proximalpartofthevenousductactsasaphysiologicsphincter,which,during
hypoxemiaorhemorrhage,resultsinanincreasedproportionofoxygenated
bloodpassingthroughtheducttotheinferiorcavalveinandtotheheart,with
lessexitingtotheportalsinusandtheliver.30Theoxygenatedbloodfromthe
venousductcanbedemonstratedcoursingalongthemedialportionofthe
inferiorcavalveinaftertheirconfluence.Flowintheinferiorcavalvein
continuestowardtheinferioraspectoftherightatrium,whereaproportionof
theoxygenatedbloodisslipstreamedbythelowerborderoftheinfoldedatrial
roof,alsoknownasthedividingcrestor“cristadividens.”Thisstructure
thereforeactsasabaffledivertingbloodintotheatrium,aprocessthatcan
readilybevisualizedduringechocardiography(Video6.3).

FIG.6.3 (A)Simplifiedschemeofthefetalcirculation.Theshading
indicatestheoxygensaturationoftheblood,andthearrowsshowthe


courseofthefetalcirculation.Threeshuntspermitmostofthebloodto
bypasstheliverandthelungs:thevenousduct,theovalforamen,andthe
arterialduct.(B)PowerDopplerinthesagittalviewofthehuman
fetoplacentalcirculationdemonstratestheiliacarteries(IA)arisingfromthe
descendingaortaandcontinuingastheumbilicalarteries(UA)inthe
umbilicalcordandtheumbilicalvein(UV)returningtothefetusand
connectingtothevenousduct(DV).(A,CourtesyKathrynTyler,Pegasus
Lectures,Inc.)

Onlyaproportionoftheoxygenatedbloodisdivertedtotheheart,andthis
proportionvariesindifferentmammalianspeciesandinhealthanddisease
states.Theremainderofthemainlydesaturatedbloodmixeswiththedesaturated
bloodfromthemesenteric,renal,iliacandrighthepaticveinsandwiththatfrom
thecoronarysinusandthebrachiocephalicveins.

OvalForamen
Patencyoftheovalforamenisessentialtoenablefillingoftheleftsideofthe
heartinthefetus,aspulmonaryvenousreturnislow(Video6.4).Theproportion
ofoxygenatedbloodreturningtotheleftsideoftheheartalsovariesbetween
species.Thisoxygenatedbloodmixeswiththedesaturatedbloodreturningtothe
leftatriumviapulmonaryveinssuchthat,aftercompletemixingintheleft
ventricle,theoxygensaturationisapproximately60%,comparedwithlevels
between50%and55%intherightventricle.Bloodfromtheleftventricleis
directedtothebrachiocephaliccirculation,thussupplyingthemostoxygenated
andnutrientrichbloodtothebrain,whichgrowsatadisproportionatelygreater
ratethantherestofthebodyinthehumanfetus.Themajorityofbloodejected
intotheascendingaortabytheleftventricleisdirectedcephaladtotheheadand
upperlimbs,andonlyapproximatelyone-thirdoftheleftventricularstroke
volumecrossestheaorticisthmustoreachthedescendingthoracicaortaand
lowerbody(Video6.5).Althoughthearterialsaturationofoxygenis
comparativelylow,extractionofoxygenbythetissuesisfacilitatedbythe
leftwarddisplacementofthedissociationcurveforoxygenoffetalhemoglobin
comparedwiththatoftheadult.

ArterialDuct
Thesystemicvenousreturnhasanoxygensaturationofapproximately40%.
Thisbloodpassesthroughthetricuspidvalveintotherightventricle,where
mixingwithbloodfromthevenousductoccursbeforeitentersthepulmonary

trunk,wheretheoxygensaturationisbetween50%and55%.Themajorityof
bloodinthepulmonarytrunkpassesthroughthearterialducttothedescending
thoracicaorta,withonlyasmallproportioncontinuingtothelungsviatheright
andleftpulmonaryarteries.Thearterialductentersthedescendingaorta
immediatelydistaltotheisthmusandtheoriginoftheleftsubclavianartery,and
bloodisdirectedtothedescendingthoracicaortabythegeometryofitsinsertion
andalsobyashelflikeprojectionatitsupperinsertion(Video6.6).Thedegree
ofpatencyofthearterialductisregulatedbytheperiductalsmoothmusclecells,
whichproduceprostaglandins.Bloodfromthearterialductmixeswiththat
crossingtheaorticisthmusfromtheaorta.Thisproducesasaturationofoxygen
between50%and55%inthedescendingaorta,fromwhichapproximately30%
returnstotheplacentaviatheumbilicalarteriesforreoxygenation(seeFig.
6.3B).

AorticIsthmus
Theaorticisthmusisawatershedregionoftheaorticarchlyingbetweenthe
aorticarch,justproximaltotheregionwherethearterialductentersthe
descendingaorta.Itistheonlytrueshuntinthefetalcirculation31andallows
communicationoftheleftandrightventricularoutflows(Fig.6.4).Only
approximatelyone-thirdoftheleftventricularstrokevolumecrossestheaortic
isthmusandthusitscaliberisalittlelessthanthatofthetransversearchand
descendingaorta.Thisshuntiscapableofrespondingtothedifferent
impedancesoftheplacentalandcerebralcirculations,aswellasreflecting
differentialventricularperformanceandejectionvolumes.Thusbloodflowis
directedcephaladingrowth-restrictedfetusestoprovideincreaseddeliveryof
oxygenandnutrients(Video6.7)andindiseasestateswherethereisleftheart
obstructionorcerebralvascularsteal.