Andersons pediatric cardiology 201
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FetalHemodynamicAssessment
InasimilarapproachtotheuseoftheFickequationformeasuringcardiac
outputbasedonoxygenconsumptionandarterialandvenousbloodoxygen
content,thecombinationofvesselflowmeasuredbyphasecontrastMRIand
vesseloxygencontentbyMRoximetrycanbeusedtocalculatefetaloxygen
delivery,oxygenconsumption,andoxygenextractionfraction.Ifweignorethe
contributionofoxygendissolvedinplasma(whichisnegligibleinthefetus)then
fetaloxygendeliveryistheproductofumbilicalveinoxygencontentand
umbilicalveinflow,whilefetaloxygenconsumptionisgivenbythedifference
betweenumbilicalveinandarteryoxygencontentbyumbilicalflow.Fetal
oxygenextractionistheratioofoxygenconsumptiontooxygendelivery.A
similarapproachcanbeappliedtoindividualorgans,inparticularwehavebeen
interestedtoestimatefetalcerebraloxygendeliveryandconsumption.
Unfortunately,thecarotidarteriesandjugularveinsaretoosmallforoximetry
andflowmeasurementsinthefetususingourcurrentapproach.However,the
flowintheSVCislikelytoconsistprimarilyofcerebralvenousreturn,witha
smallandrelativelyconstantcontributionfromtheupperlimbsandotherparts
oftheheadandneck.Thereforewehaveapproximatedfetalcerebraloxygen
deliveryastheproductofSVCflow,aorticarchoxygencontent,andfetal
cerebraloxygenconsumptionasthedifferencebetweenSVCandaorticarch
oxygencontentbySVCflow.
NormalLate-GestationHumanFetal
CirculationbyMagneticResonance
Imaging
Table7.1detailsreferencerangesforthedistributionofbloodflowacrossthe
normallate-gestationhumanfetalcirculationbycinephasecontrastMRIwith
metricoptimizedgating.TheseMRIfindingsareremarkablyclosetothe
estimatesprovidedbyRudolph,withtheexceptionofumbilicalflow,whichwas
lowerthanpredictedwhenmeasuredbyMRI.However,theumbilicalflowswe
observedbyMRIareinkeepingwithsomeofthereportedultrasound
measurements.4InkeepingwithRudolph'sobservations,wefoundcerebral
bloodflowandpulmonarybloodflowtobehigherinthehumanthanthesheep,
whileumbilicalflowislower.Rudolphhassuggestedthatthelowerumbilical
flowseeninthehumanreflectsthehigherhematocritinthehuman,which
providessimilaroxygendeliverydespitethelowerflow.InTables7.2and7.3
MRImeasurementsoftheoxygensaturationsinsomeofthemajorhumanfetal
vesselshavebeenprovided,aswellasfetaloxygendeliveryandconsumption.
Again,thesefindingsareverysimilartothefetalsheep,indicatingthatthesame
streamingmechanismsareoperatingtopreferentiallyoxygenatethemost
metabolicallyactiveorgans:thebrainandtheheart.Adiagramshowingboth
oxygensaturationsandflowdistributioninasmallergroupofnormallategestationhumanfetalsubjectsisshowninFig.7.11.15
Table7.1
DistributionofBloodFlowin40Late-GestationHumanFetusesby
PhaseContrastMagneticResonanceImaging
Meanflow(mL/min/kg)
SD
95%CI
Meanflow(%CVO2)
SD
95%CI
Modeledmeanflow(%
CVO2)
CVO
477
79
378–
664)
MPA
274
64
177–
435
57
7
42–67
57
AAo
189
35
130–
255
40
7
30–55
40
SVC
146
44
83–
259
31
9
18–50
29
DA
189
43
120–
279
40
9
25–57
41
PBF
82
52
14–
190
17
9
3–29
16
DAo
255
56
172–
385
54
10
37–75
52
UV
136
33
97–
202
29
8
20–46
29
FO
127
56
34–
226
28
13
7–54
27
Flows(withSDand95%CI)areprovidedinmL/min/kgandaspercentagesofthecombined
ventricularoutput.ModelledmeanpercentageofCVO2wereextrapolatedfrommeasuredflows
usinganonlinearoptimizationconstrainedbyconservationofflow(bottomline).
AAo,Ascendingaorta;CI,confidenceinterval;CVO,combinedventricularoutput;DA,ductus
arteriosus;DAo,descendingaorta;FO,foramenovale;MPA,mainpulmonaryartery;PBF,
pulmonarybloodflow;SD,standarddeviation;SVC,superiorvenacava;UV,umbilicalvein.
Table7.2
Late-GestationHumanFetalVesselOxygenSaturationsbyT2
MappingComparedWithFetalSheepReferenceData
Vessel
MRI(mean±SD)
Lambreference
UV
84±10
80
AAo
69±13
65
MPA
55±12
55
SVC
38±11
40
DAo
56±12
60
AAo,Ascendingaorta;DAo,descendingaorta;MPA,mainpulmonaryartery;MRI,magnetic
resonanceimaging;SVC,superiorvenacava;UV,umbilicalvein.
Table7.3
Late-GestationHumanFetalOxygenDelivery(DO2)andConsumption(VO2)byMagnetic
ResonanceImagingComparedWithFetalSheepReferenceData
DO2
VO2
FetalSheep
~20
7–8
Late-GestationHuman
20.4
6.9
