Andersons pediatric cardiology 163

FIG.6.7 Volumerenderingofanormal23-weekfetalheartexaminedwith
micro-computedtomography.Cutawayviewshowsrightatrium,left
ventricle,interventricularseptum,andrightventricularoutflowtractwith
opposedpulmonaryvalveleaflets.(FromHutchinsonJC,ArthursOJ,
AshworthMT,etal.Clinicalutilityofpostmortemmicrocomputed
tomographyofthefetalheart:diagnosticimagingvsmacroscopic
dissection.UltrasoundObstetGynecol.2016;47[1]:58–64.)

MagneticResonanceImaging
High-fieldMRIat9.4Twasfounddiagnosticallysuperiortoconventional1.5T
lessthan22gestationalweeks,buttheimagingtimesmaybeaslongas18hours
toobtainresolutionscomparabletothoseachievedbymicro-CTandHREM.49
Thisiscurrentlythemainbarriertoitsuseinpostmortemdiagnosticimagingof
thefetus.


PhysiologyoftheFetalCirculationin
HealthandDisease
Thehumanfetoplacentalcirculationshowsadaptivechangesthatcanbe
measurednoninvasivelyusingDopplerultrasoundandhasallowedcomparison
withthepreviouslyreportedanimalstudies.51,52Initialexperimentalworkin
fetalsheepdemonstratedaredistributionofflowinresponsetohypoxemia.53
WiththeavailabilityofnoninvasiveDopplertechniques,similarinformationon
thealteredDopplerwaveformsassociatedwithabnormalitiesofpregnancyhas
beengathered.Initialstudiesinthehumanusedblindcontinuouswave
ultrasoundoftheumbilicalcordrevealed(e.g.,lowdiastolicflowinthe
umbilicalarteryinassociationwithuteroplacentalinsufficiency).54Whileonthe
fetalsideoftheplacenta,anincreasedresistancetoflowingrowth-restricted
pregnancieswasdescribed.55Technicalimprovements,includingnewercolor
Dopplermodalitiessuchasenergyanddirectionalpower,haveenabledthe
visualizationandinterrogationofsmallervesselsinregionalcirculations,and

indicatorsoffetalwell-beinghavebeenderived.56AcomparisonofDoppler
waveformsinthecarotid,aortic,andumbilicalarteriesandinthemiddle
cerebralarteryhasprovidedevidenceofredistributionofflowinthegrowthrestrictedhumanfetus.57,58Animalworkhassupportedtheconceptthat,inthe
presenceofuteroplacentalinsufficiency,thecerebralcirculationbecomesthe
vascularbedwiththelowestimpedanceinthefetoplacentalcirculation.As
systemicimpedancerises,flowisdirectedretrogradelythroughthearchtoward
thecerebralcirculation.59Increasedflowtothebrainresultsinadecreased
pulsatilityindexrecordedinthemiddlecerebralartery(Fig.6.8).


FIG.6.8 (A)Dopplerpanelillustratingnormalpulsatilityindexofthe
middlecerebralartery(MCAPI).(B)Waveformofredistributionofflow
towardthefetalbraincharacterizedbyincreasedsystolicanddiastolicflow
velocitiesinthisexample,loweringthepulsatilityindex.

UmbilicalCordFlows
Thepulsatilityindexwasderivedinthe1970stoquantifywaveformsinthe
umbilicalcordandassessfetalcompromise.Abnormalitiesofflowinthecord
arecharacterizedfirstbyareduction,andthenareversal,ofdiastolicvelocities,
thusincreasingthepulsatilityindex.Ameta-analysisofsubsequentstudieshas
confirmedumbilicalarteryvelocitymeasurementstobeusefulmeasuresoffetal