Andersons pediatric cardiology 166

Indeed,recentstudieshavereporteda10%reductioninoxygensaturationin
theascendingaortainfetuseswithcomplexcardiacdefects,notaccompaniedby
theanticipatedincreasedvolumefloworoxygenextraction.84Thissuggeststhat,
incardiacdisease,cerebraloxygendeliveryandconsumptionisreduced.
Althoughitappearsthatthebrainsoffetuseswithcongenitalheartdefects
developatloweroxygentension,theimpactofthisondeliveryismoredifficult
tomeasure.85Glucosedeliverytothebrainplaysavitalroleinitshomeostasis,86
asdoestheroleofchronicityofthealteredstate.Workinanimalmodelssuggest
responsivenessdiminisheswithchronicityandthehumanfetusmayadaptand
downregulatethebrain'srequirementforoxygenandmetabolicsubstrates.This
metabolicalterationmayaltergeneexpressionandreducemitochondrial
respirationaffectingimportantregulatoryneurohormonalaxes(suchasthe
hypothalamic-pituitaryaxis)andtheusualpatternofbraindevelopmentsuchas
myelinationresultinginitspermanentalteration.87–89

FlowAcrosstheAorticIsthmus
Onlyapproximatelyone-thirdofleftventricularoutput,orone-tenthoftotal
cardiacoutput,flowsthroughtheaorticisthmus.Oneconsequenceofthisisthat
theisthmaldiameterissmallerthanthatofthetransversearchandshowsa
characteristicDopplerpattern(Fig.6.11).Experimentalincreasesinsystemic
impedanceinthelamb,mimickingplacentalinsufficiencyinthehuman,have
beenshowntoreduceorstopisthmalflow.59Inthehuman,whereflowtothe
brainis8to10timesthatofthelamb,thehypoxic-mediatedincreaseallowsa
reductionincerebralimpedance,withreversalofflowabouttheaorticarchthat
canbedemonstratedonpulsedwaveDoppler.90


FIG.6.11 Dopplerflowvelocitypatternsinthefetalaorticisthmus
throughoutgestation.(A)Duringthefirsthalfofpregnancy,forwardflowis
presentbothinsystoleanddiastole.(B)Duringthesecondhalfof
pregnancy,abriefreversalofflowappearsattheendofsystoleas

illustratedinthis32-weekfetus.(C)Inthesamefetusadelayedonsetand
longeraccelerationtimeoftheductalwaveareobservedattheisthmusductusjunction,explainingthelatesystolicreversalofflowintheisthmus.
(FromFouronJC.Theunrecognizedphysiologicalandclinicalsignificance
ofthefetalaorticisthmus.UltrasoundObstetGynecol.2003;22[5]:441–
447.)

ThealterationinDopplerflowprofileintheisthmusinresponsetoalteration
ofcerebralandplacentalimpedanceshasbeencharacterizedbyfiveseparate
profilestermedtheisthmicflowindex(Fig.6.12).31Latterlythesamegrouphas
developedaquantifiableindex,theisthmicsystolicindex,todescribethe
influenceofrightandleftventricularperformanceontheDopplerwaveforms
measuredinthisregion.91Theindexbecomesnegativeduringthefinalweeksof
pregnancy,whereretrogradeisthmicflowiscommonandcorrelateswithright
ventricularoutput.Thisexplainsthecommonfindingofreversedflowinthe
transverseaorticarchinlatergestation,attributabletothefallincerebrovascular
resistance,drivingbloodretrogradelythroughtheisthmusduringsystole.The
metabolicallyactivebrainrespondsbyvasodilationresultinginanincreasein
theproportionofleftventricularstrokevolumegoingtothebrain,thereby
reducingthisretrogradeflow.Aplausiblealternativeexplanationisthatthefall
incerebrovascularresistanceandincreasedcerebralflowtowardtermresultsin


increasedrightheartpreloadwithasubsequentincreasedductalflow,leadingto
retrogradeflowabouttheisthmus.Animalstudiessuggestthatventricular
ejectiontimesofbothventriclesaresimilar,92butthesefindingsmaynotbe
applicableinthehumanfetusbecausetheproportionofcardiacoutputsupplying
thebrainisrelativelygreaterthanmostexperimentalanimalspecies.Itistrue
thattheopeningtimeofthepulmonaryvalveexceedsthatoftheaorticvalvedue
tothelongerelectromechanicaltimeintervaloftherightventricle,butthe
isthmalflowdependsalsoonthecomplianceinthepulmonaryvasculature(see

sectionlater).Thehighertherightventricularoutput,thegreaterthereversalof
branchpulmonaryflowandthereforethemorenegativetheend-systolicvelocity
shouldbeproducinggreaterreversaloftransversearchflow.93

FIG.6.12 Fivepossibletypes(I–V)oftheisthmicflowindex.Dopplerflow
waveformsatthebottomofthefigurearetakenfromfetuseswithplacental
circulatoryinsufficiency.(FromFouronJC.Theunrecognizedphysiological
andclinicalsignificanceofthefetalaorticisthmus.UltrasoundObstet
Gynecol.2003;22[5]:441–447.)

Theabilityoftheisthmustofunctionasatrueshuntexplainswhyreversalof
flowinthetransversearchisoftenseeninthehealthynear-termhumanfetus.It
alsoexplainsthepathophysiologicmechanismsunderlyingthereversedaortic
archflowduetothestealphenomenonincerebralarteriovenousmalformations,
suchasveinofGalenmalformation(Video6.10).However,interpretationofthe
isthmicsystolicindexincardiacdefectsremainsmorecomplexandutilityofthis
indexrequiresdetermination.94

FlowofBloodtotheLungs
Theflowofbloodinthelungsofthenormalhumanfetushasbeencalculated
noninvasivelyfromthedifferenceinestimatedvolumesinthearterialductand
inthepulmonarytrunkusingDopplerultrasound.Inthisway,anincreasewith
ageforpulmonaryflowfrom13%to25%hasbeendescribedinacross-section