Andersons pediatric cardiology 164
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well-being,andarterialandvenousDopplerwaveformshavebecome
incorporatedasstandardmeasurementsinsurveillanceofthehigh-risk
pregnancy,60,61withthelattershowntoprovideamorespecificpredictoroffetal
compromise(Fig.6.9).62OftheDopplermeasurementsusedinevaluation,
absenceorreversalofend-diastolicflowinthedescendingaortaorumbilical
arteryofthefetusisseenfirstandmaybetoleratedforaperiodofweeksinthe
compromisedgrowth-restrictedfetus,butoncechangesareseeninthesystemic
veins,emergentdeliveryisoftenthemostappropriatemanagement.63
FIG.6.9 Seriesofumbilicalartery(UA)andumbilicalvein(UV)Doppler
recordingsillustratingworseningplacentalfunction.(A)Dopplerofnormal
flowintheUAshowingprogradeflowindiastole.(B)NormalflowintheUV
showingabsenceofvenouspulsation.(C)Absenceofend-diastolicflowin
theumbilicalartery.(D)Abnormalvenouspulsationsassociatedwithfetal
hypoxemia.(E)Reversedend-diastolicflowintheumbilicalartery
signifyingincreasingplacentalresistance.(F)Triphasicflowpatterninthe
umbilicalvein.
Whenplacentalfunctionisseverelyreduced,asinfetuseswithrestricted
growth,increasedplacentalresistanceleadstoareductionintotaldeliveryof
arterialoxygentothefetusbecauseofthereductioninmeanplacentalreturn,
eventhoughthecontentofoxygenoftheumbilicalvenousbloodisoftennear
normal.Itisvitaltoexamineothervascularbedsinthehumanfetusasthefetal
brain,heart,andadrenalglandsrespondtothispathologicstatebydrawing
increasedflow,thusrequiringanincreaseincombinedventricularoutputto
provideit.Inthehumanfetus,thebrainisthelargestorgan,andthehealthy
responsivefetusisabletoreducecerebralresistancebyarteriolardilation.This
hasledtotheadoptionoftheratioofthecerebraltoplacentalresistancesasboth
anindicatorofpoorfetalgrowthandanindependentpredictorofintrapartum
fetalcompromise.64
FlowintheVenousDuct
Thevenousductoccupiesauniquephysiologicpositionasaregulatorofoxygen
inthefetoplacentalcirculation.Animalstudiesfirstdemonstratedstreamingof
oxygenatedbloodfromtheumbilicalveinthroughtheovalforamenintotheleft
