Andersons pediatric cardiology 161

FIG.6.4 (A)Diagramofthefetalcirculationillustratingtheuniqueposition
oftheaorticisthmus,betweentheaorticandpulmonaryarches.(B)During
systole,theleftandrightventricular(LV,RV)strokevolumeshaveopposite
effectsonthedirectionofflowthroughtheisthmus.(C)Duringdiastole,the
twodownstreamvascularimpedancesaretheonlydeterminantsofthe
directionoftheisthmicflow.(FromFouronJC.Theunrecognized
physiologicalandclinicalsignificanceofthefetalaorticisthmus.Ultrasound
ObstetGynecol.2003;22[5]:441–447.)


MaturationalChangesintheEarlyFetal
Heart
InvivoStudies
Diagnosticimagingofthefetalheartispossiblefrom12gestationalweeks,even
inmultiplepregnancies.32,33Modernultrasoundtransducerswithlimitsof
resolutionofapproximately50µmintheaxialplaneat6MHzandlessthan100
µminthelateralplanepermitdiagnosticviewsatnormalobstetricscanning
depths.Asaresult,morphologicandphysiologicdatahavebecomeeasierto
recordandmorereliable.Improvedresolutionshouldincreasetherobustnessof
calculatedmeasuressuchasindexedcombinedcardiacoutputthatrelieson
measurementsofbiometricvariablesandvalvediameters.z-Scoreshavebeen
derivedtotakeaccountoftheeffectsoffetalgestationandgrowthonthesizeof
vessels,valves,andchambers.34,35z-Scoresareparticularlyusefulfor
quantitativecomparisonwhencardiacstructuresarehypoplastic,andvaluesare
availableonlineorviaasmartphoneapp.36–38
Newindexesreflectingthedevelopmentalabnormalityofcardiacshapein
variousdiseasestateshavebeendevelopedincludingthesphericityindexofthe
heart(mostoftenabnormalitiesoffetalgrowth),anda24-segmentapproachto
itsassessmenthasbeenproposedusingspecializedofflinesoftware(Fig.
6.5).39,40

FIG.6.5 Computationofthesphericityindex(SI).(A)Mostcommon
previouslypublishedmethodstocomputethefetalSIinwhichtheenddiastolicbasal-apicallength(greenarrows)isdividedbythebasal
transverselength(bluearrows),orthemid-transverseend-diastoliclength


(redarrows)isdividedbythemidbasal-apicallength(greenarrows).(B)
Techniqueusedinwhichthemidend-diastoliclengthisdividedbyeachof
the24transversesegments.LA,Leftatrium;LV,leftventricle;RA,right
atrium;RV,rightventricle.(FromDeVoreGR,KlasB,SatouG,SklanskyM.
The24-segmentsphericityindex:anewtechniquetoevaluatefetalcardiac
diastolicshape.UltrasoundObstetGynecol.2018;51:650–658.)

Moreadvancedultrasoundtechniques,usingthree-dimensional(3D)andfourdimensional(4D)technologywithshortenedacquisitiontimeslessthan3
secondsandmagneticresonanceimaging(MRI),permitphysiologicassessment
ofventricularvolumesnoninvasivelybutonlywhereimagingisoptimal.41,42
LackofresolutionofbothmodalitiesandexpenseofMRIhavepreventedtheir
introductionintoroutineclinicalpractice.
However,anewautomatedprogram,combiningcolororbidirectionalpower
Dopplerultrasoundwithfetalintelligentnavigationechocardiographyshows
promiseinprocessing3Dvolumesetstogeneratestandardviews.43,44

PostmortemStudies
Thefirstsystematicstudyofcardiacgrowthinthehumanfetuswasmadeusing
alargeseriesofnormalheartsobtainedatpostmortem.Thisestablishedthe
relationshipbetweentotalbodyweight,totalheartweight,andthechangeof
heartweightwithgestationalage.45However,impressive,nearhistologicdetail
hasbeenmadepossiblebynewertechnologies.Thereisaclinicalimperativeto
developtechniquestoachievea“noninvasivepostmortem,”andthesehavebeen
investigated,bothforwholebodyautopsyandtoexaminesingle-organ
specimensobtainedfollowingpregnancylossortermination.Thesetechniques

includehigh-resolutionepiscopicmicroscopy(HREM),micro-computed
tomography,andhigh-fieldMRI.

High-ResolutionEpiscopicMicroscopy
HREMisanexvivotechnique.Usedmorefrequentlytoexaminesmallanimal
hearts,ithasbeenappliedinthefirstandearlysecondtrimesterhumanfetal
heartwhenthesmallsizeandfragilityofstructuresmakeothertechniques
difficult.46Theheartspecimensareprocessed,embeddedinplasticresin(JB4,
Polyscience),andseriallysectionedtoproducemorethan1000sectionsfrom
eachheart.HREMautomaticallycapturesanimageofhighresolution(minimum
1µm)fromeachsectionandcompilestheserialimagesinperfectalignmentto