Andersons pediatric cardiology 196

Abstract
Magneticresonanceimagingprovidesanewperspectiveonabnormalfetal
cardiovascularphysiologyresultingfromcongenitalheartdisease.Usinga
combinationofcinephasecontrastMRIforvesselflowquantificationand
relaxometryforoximetryandbloodoxygencontentmeasurementithas
beenpossibletoreproducepriorfetalsheepinvasivehemodynamic
measurementsinhumans.Thesehaverevealedthatacombinationof
placentaldysfunction,reducedumbilicalperfusionandinterruptionofthe
normalstreamingofoxygenatedbloodtowardsthecerebralcirculationis
associatedwithdelayedbraingrowthanddevelopment.Thus,fetal
interventionsaimedatimprovingcerebraloxygenationinfetuseswith
congenitalheartdiseasecouldbehelpfulinpreventingtheadverse
neurodevelopmentaloutcomesseeninmanychildrenwithcongenitalheart
disease.

Keywords
fetalcirculation;phasecontrast;magneticresonanceoximetry;inutero
braindevelopment


Introduction
Theabilitytoaccuratelymeasurebloodflowandoxygensaturationsacrossthe
humanlate-gestationfetalcirculationofferedbyadvancesinmagneticresonance
imaging(MRI)hasprovidednewinsightsintotheimpactofcongenitalheart
disease(CHD)onthedevelopingfetus.MRIhasdemonstratedredistributionof
bloodflowandinterruptionofthenormalstreamingofbloodresultingfromthe
obstructionstoflowandabnormalconnectionsthatcharacterizeCHD.Abnormal
fetalhemodynamicshavebeenlinkedtothedysmaturationofthelungsand
brainthataretypicalofnewbornswithmoresevereformsofCHD.Wehavealso
learnedhowdiminishedcardiacoutputinfetuseswithsingle-ventricle
physiologyisassociatedwithreducedfetalperfusionoftheplacentaand

impairedplacentaloxygenexchange,whichmayaccountforthegrowth
restrictiontypicaloflate-gestationfetuseswiththesetypesofcardiac
malformations.Recentdataindicatethatanimpairedmaternal-fetalenvironment
mayhaveanimportantimpactoncongenitalcardiacsurgicalresults.1These
observationsarelikelytobeofrelevanceasweseektounderstandbettertherisk
factorsforadverseoutcomesofneonatalcardiacsurgeryanddevelopnew
therapiestoalterthenaturalhistoryofCHDpriortobirth.


NormalFetalCirculatoryPhysiology
Muchofwhatisknownaboutfetalcardiovascularphysiologyhasbeenlearned
throughinvasiveexperimentsperformedinsheep.Elaboratingonmethodsfor
exteriorizingandcatheterizingthefetallambdevelopedbyJosephBarcroftand
GeoffreyDawesatCambridgeandOxforduniversities,AbrahamRudolphand
hiscoinvestigatorsattheCardiovascularResearchInstituteattheUniversityof
California,SanFrancisco,definedourmodernunderstandingofthedistribution
ofbloodflowandoxygentransportacrossthefetalcirculation.2Rudolph'sflow
measurementswereachievedthroughtheselectiveinjectionofradioactive
microspheresintodifferentvenouscompartmentsthatweresubsequentlytrapped
inthemicrocirculationoftheendorganssuppliedbythefetalheart.By
measuringtherelativeactivityofthedifferenttracersineachofthevariousfetal
organsandapplyingtheFickprincipletomeasureumbilicalflow,bloodflowin
eachofthemajorvesselswascalculated.Theseflowmeasurementswere
combinedwithbloodpressuremeasurementsandoximetryperformedby
conventionalbloodgasanalysisofsamplesobtainedusingcathetersplacedin
thefetalvessels.Basedonobservationsregardingdifferencesbetweenthe
species,includingthesizeofthefetalbrainandhematocrit,Rudolphthen
predictedthedistributionofbloodflowandoxygensaturationsacrossthehuman
fetalcirculation.Tosummarize,thefetalcirculationoperatesinparallel,with
shuntsattheductusvenosus,foramenovale,andductusarteriosus,whichallow

well-oxygenatedbloodreturningfromtheplacentatopassdirectlytothemost
metabolicallyactivefetalorgans,theheartandbrain,whiledeoxygenatedblood
isdirectedbacktotheplacenta.Inthefetus,thebloodwiththehighestoxygen
saturationofapproximately85%isfoundintheumbilicalveinand,downstream,
theoxygensaturationis10%to15%higherintheleftheart(65%)thantheright
(55%)(seeVideo7.1).Infetalsheep,thisisachievedbyaremarkablestreaming
ofbloodemergingfromtheductusvenosusandlefthepaticveintowardthe
foramenovale,whilemoredeoxygenatedbloodpassesfromtheinfrahepatic
inferiorvenacavaandrighthepaticveintowardthetricuspidvalve.
Infetalsheep,therightventricleprovidesthemoredominantcontribution
(approximatelytwo-thirds)towardthecombinedventricularoutput,themajority
ofwhichpassesintotheductusarteriosusanddescendingaorta.Rudolph
suggestedthattherightventriclewasalsodominantinthehuman,withabout
15%ofthecombinedventricularoutputpassingintothepulmonarycirculation,