Andersons pediatric cardiology 216
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Pulse-WaveDoppler
Dopplerechocardiographyisapowerfultoolwithwhichtoassesscardiovascular
physiologyandfunctionandisanimportantpartofthecomprehensive
evaluationofthefetalcardiovascularsystem.Indeed,Dopplertechniquesmay
enablethepractitionertosortoutcomplexphysiologiesandtorecognizecardiac
compromise,whichmightnotbeclearby2DandcolorDopplertechniques
alone.TheDopplersignalcanbeanalyzedbythepulsatilityindex,definedas
the(peaksystolicvelocity–end-diastolicvelocity)/time-averagedmeanvelocity,
theresistanceindex,definedasthepeaksystolicvelocity–end-diastolic
velocity/end-diastolicvelocity,orthesystolic/diastolic(S/D)ratio.
UmbilicalArtery
Dopplerevaluationoftheumbilicalarteryprovidesvitalinformationconcerning
thehealthandstateoftheplacentalcirculation.Dopplerinterrogationofthe
umbilicalcordshouldbeperformedinafreeloopofthecordduringfetalapnea
(Fig.8.3).ReferencevaluesfortheumbilicalarterialDopplerflowpatternover
thecourseofgestationhavebeenpublished.48–52Thehealthyplacentashould
haveverylowvascularresistance,andhencetheDopplerspectraldisplaywill
demonstratesubstantialantegradeflowduringbothsystoleanddiastole(Fig.
8.4).However,gestationalagefactorsheavilyintotheinterpretationofthe
umbilicalarterialflowpattern,sincediminishedorevenabsentflowindiastole
isanormalfindinginthefirsttrimester.53Abnormallyelevatedplacental
vascularresistance,asseenincasesofintrauterinegrowthretardation,canbe
identifiedbythepresenceofdiminished,orevenreversed,flowindiastole(see
Fig.8.4).Multiplestudieshavedemonstratedpoorneonataloutcomesor
intrauterinefetaldemiseincasesofabsentorreversedend-diastolicflowinthe
umbilicalartery.54–56Invariousformsofcongenitalheartdisease,theumbilical
arteryresistancemaybeelevated,yetitusuallyremainswithinthe95%
confidenceintervalforgestationalage.57
FIG.8.3 ColorDopplerimageofthenormalumbilicalcord.Twoarteries
(blue)andoneumbilicalvein(red)areseen.
FIG.8.4 Dopplerspectraldisplayofbloodflowintheumbilicalcord.
Umbilicalarteryflowisabovethebaselineandumbilicalveinflowisbelow
thebaseline.(A)NormalDopplerflowpattern.(B)NormalDopplerflow
pattern.Thearrowspointtoreversalofflowintheumbilicalartery,
suggestingmarkedlyelevatedplacentalvascularresistance.
MiddleCerebralArtery
Dopplerinterrogationofthemiddlecerebralarteryprovidesanassessmentof
cerebrovascularresistanceinthefetusandmayalsoofferimportantinformation
regardingfetalwell-being.Fig.8.5demonstratesthenormalDopplerflow
patternforthemiddlecerebralarteryaswellastheDopplerflowpatternsfora
fetuswithlowandhighcerebrovascularresistance.Inthenormalmiddle
cerebralDopplerflowpattern,mostoftheflowoccursinsystole,withonlya
smallportionoccurringindiastole.Referencevaluesforthemiddlecerebral
arteryoverthecourseofgestationhavebeenpublished.50,58,59Anelevatedpeak
systolicvelocityinthemiddlecerebralarteryisasensitivemarkerforfetal
anemia60–65andmayalsobepredictiveofperinatalmortalityinfetuseswith
intrauterinegrowthretardation.66
FIG.8.5 Dopplerspectraldisplayofmiddlecerebralarteryflow.(A)
NormalDopplerflowpattern.(B)Lowcerebrovascularresistancewith
increasedflowindiastole.(C)Highcerebrovascularresistancewith
decreasedflowindiastole.
Changesincerebrovascularresistancemaybeseeninconditionsofaltered
cardiacoutputandwithdifferentformsofcongenitalcardiacdisease.57,67–70
Thesechangesareamanifestationofautoregulatorymechanismsofthefetal
cardiovascularsystem,inwhichthereisanaturaltendencytopreserveflowof
bloodtovitalorgans,suchasthebrain.Whenbloodflowtothebrainis
diminishedduetoanoveralldecreaseincardiacoutputbecauseofmyocardial
dysfunction,orbecauseofanatomicimpediment,vascularresistancewillbe
lowerthannormalinthemiddlecerebralarteryasthebrain“attempts”to
augmentvolumeandflow.Hence,inthepresenceofleft-sidedobstructive
lesions,theresistancemeasuredinthemiddlecerebralarterydecreases.68,69In
thesettingofright-sidedobstructivelesions,incontrast,theresistanceincreases
(seealsoChapter76).69Infetuseswithcardiovascularcompromise,suchas
intrauterinegrowthretardation,theremaybearedistributionofthecardiac
outputawayfromtheplacentaandtowardthebrain,theso-calledbrain-sparing
effect.71,72Typicallytheratioofresistanceinthemiddlecerebralartery
