Andersons pediatric cardiology 168

FIG.6.14 Dopplerrecordings.(A)Progradecoronaryflowinafetuswith
twin-twintransfusionsyndromesuggestinga“heart-sparing”response
secondarytofetalhypoxemiaordistress.(B)Bidirectionalcoronaryblood
flowinafetuswithpulmonaryatresiawithintactventricularseptumanda
rightcoronaryarterytorightventriclefistula.Thereisnormalvelocity
Dopplersuggestingnoelevationofrightventricularpressure.(C)In
contrastto(B),thisrecordingshowshigh-velocityreversalofflowalongthe
rightcoronaryarteryat2.9m/sandnormalvelocityforwardflow.

Animalstudiesofmyocardialflowshowthatcoronaryreserveismediatedby
nitricoxideandchangesduringhypoxemia.108Thisfindinghasbeentermed
fetalcardiacsparing.109Accordingly,visibleflowinthecoronaryarteriesis
attributedtoanincreasedvolumeofflowsecondarytolowfetalarterialcontent
ofoxygen.
Visualizationofcoronaryflowcoincideswithimportantincreasesinthe


Dopplervelocityz-scoresintheumbilicalartery,inferiorcavalveins,andvenous
duct.Thegreatestchangewasobservedinthevenousductz-scoreoccurring24
hoursbeforevisiblecoronaryarterialflowwasidentified.Thesechangeswere
associatedwithadverseperinataloutcomes.110

DevelopmentalChangesinCardiacFunction
Theabilitytoassessfetalcardiacfunctionhasimprovedasimagingtechniques
haveadvanced.Moreover,thepotentialforfetaltherapyforcertainconditions,
suchassemilunarvalvestenosis,TTTS,anddiaphragmaticherniahasstimulated
amorecomprehensiveassessmentoffetalcardiovascularfunctiontoguidetheir
timingandmonitortheirsuccess.111
Intheearlyembryo,gradientsacrosstheatrioventricularorificesactasa
resistanceto,andregulate,theflowofblood,thusinfluencingventricular
development.112Astheventricularmassbecomestrabeculated,soitsmass

increasesandstiffnessdecreases,thusoptimizingventricularfillingandejection.
Increasingcardiacefficiencyisassociatedwithincreasingmyocardialmassand
competenceoftheatrioventricularandarterialvalves.113

DiastolicMaturation
Atrialpressureexceedsventricularpressurethroughoutfilling,andfromearly
gestationthereisacleardistinctionbetweentheso-calledpassiveandactive
fillingphases,referredtoastheEandAwaves,respectively.5Thetermsare
misleadingbecausewehavelongrecognizedthatearlydiastoleisan“active”
relaxation,butthetermspersistincommonusage.114Activevelocitiesarehigher
thanpassivevelocitiesinthefetusandinthenewbornperiod,resultinginaratio
betweentheEandAwavesthatislessthan1.
TransvaginalDopplerultrasoundofthehumanfetalhearthasconfirmedthat
ventricularinflowwaveformsaremonophasicbefore9weeksofgestation,
becomingbiphasicby10weeks.115Thepatternsofventricularfillingchange
withage,witharelativeincreaseinearlydiastolicfilling,representedbytheE
wave,comparedwiththelatediastoliccomponent,orAwave,suggesting
improvedventricularrelaxation.116–118Referencerangesbetween8and20
weeksofgestationshowagreatervolumeofflowpassingthroughthetricuspid
thanmitralvalveatallgestationalages.TheE/Aratioisalsohighlydependent
onpreload.Itcannotprovideaload-independentassessmentofventricular


function.Itisthereforeaparticularlyunsuitablemeasureinfetallife,when
directpressurescannoteasilybemeasured,andthereisdebateaboutthe
interpretationoflongitudinalstudies.Althoughpatternsofventricularfilling—
monophasiccomparedwithbiphasic—havebeenproposedasmoresimple
barometersofdiastolicfunction,thesehavefailedtocorrelatewithchangesin
downstreamimpedance.119
Maturationalchangesinventricularpropertiesinhumanfetusesaccelerate

aftermid-gestationasdiastolicfillingincreasesmainlyafter25weeks.Theyare
associatedwithadecreaseintheratiooftheareaofthemyocardialwalltothe
end-diastolicdiameteroftheleftventricle.Thusthedecreaseinleftventricular
wallmassrelatedtogestationalagemaybeoneimportantmechanism
responsibleforthealterationsindiastolicpropertiesnotedinthefetalheart.
Thesearecoincidentwiththereductioninplacentalimpedanceassociatedwith
normaladaptationofthespiralarteries.29,120
Atrioventricularvalvarregurgitationisacommonfindingfrom9weeks
onwards.115Tricuspidregurgitationiscommonlyfoundinthefirsttrimesterand
isthoughttobemorecommoninfetusessufferingfromaneuploidy,particularly
trisomy21.Ithasbeenincorporatedintoearlyscreeningprogramsandisusedto
adjusttheage-relatedriskfortrisomy21.121Itisnotclearwhytricuspid
regurgitationismorecommoninthesefetusesbutmaybeassociatedwith
delayeddevelopmentoftheatrioventricularcushionsbecauseitresolves
spontaneouslyinmostcasesandhasnophysiologicconsequenceslaterin
pregnancyorafterbirth.Mostcasesoftricuspidregurgitationreportedinlater
gestationarealsotransientandtrivial.Theyareassociatedwithanormal
outcome.122

SystolicMaturation
Thesecondhalfofpregnancyisassociatedwithanincreaseinmeanvelocities
throughtheoutflowtractsandadecreaseinisovolumicrelaxationand
contractiontimes.Theventricularstrokevolumerises,andtheafterloadreduces,
whichismorepronouncedontheleftsideoftheheart.Thepeakvelocitiesinthe
ascendingaortaaregenerallyhigherthaninthepulmonarytrunk,andthe
gestationalincreaseislinearinlongitudinalstudies.Cardiacoutputis
traditionallycalculatedfromtherightandleftventriclesusingthevelocitytime
integralofthemaximumvelocityenvelopesthroughthevalves,andastatic
assessmentofvalvardiametermeasuredatthehingepoints.Themeantotal