Andersons pediatric cardiology 159

mechanicalimprovementandmaybeimportantinfuturestrategiesforthe
managementofcardiacfailure.23
However,cautionshouldbeexercisedintheinterpretationofresultsfrom
animalmodels.Areviewofresponsesofthefetalgeneprogramintherodent
modelofcardiomyopathyindiabetesindicatedthatthemostcommonly
measuredgenesinthefetalgeneprogramareconfoundedbythediabetogenic
effects.24
ThemyosinheavychaincarriestheATPasesite.Theenzymickineticsof
ATPasearespecificforeachisoform.Thisisimportantbecauseitistherateat
whichATPasehydrolyzesATPthatprimarilydeterminestheforce-velocity
relationshipduringmyocardialcontraction.Thispartlyexplainsthedifferences
inactiveandpassivemechanicsbetweenfetalandadultmyocardium.25The
transitionfromfetaltoadultisoformsofmyosinheavychainaroundbirthis
similartothecontrolledswitchfromfetaltoadulthemoglobinandrepresents
stage-specificregulationofgeneticexpressionoftheproteinspriortobirth.
Althoughanumberofhormones,andparticularlythyroidhormone,26,27are
knowntomodulatethephenotypicexpressionofthemyosinheavychain,the
factorsresponsiblefortheprecisetimingofthetransitionfromfetaltoadult
isoformsremainunknown,althoughthemolecularmechanismsarethought
likelytobeassociatedwiththemyosinheavychain(MYH)genecluster.28


FetoplacentalCirculation
Thefetalcirculationhastwocirculatorysystemsarrangedinparalleland
characterizedbyfiveuniquefeaturesthatpermitthedeliveryofoxygenated
bloodtotheleftsideofthefetalheartfromtheplacentaanddirectsystemic
venousreturnawayfromthefluid-filledlungsandbacktotheplacentato
becomeoxygenated.Theplacentalcirculationisdesignedtomaximizeexchange
ofoxygenandnutrientsbetweenthemotherandfetus.Theoxygenatedblood
flowsintheumbilicalveinintothefetalliverwhereavariableportionentersthe
venousduct.Thissmallvesselconnectstheintrahepaticportionoftheumbilical

veintotheinferiorcavalvein,andthehighervelocityjetstreamsthroughthe
ovalforamenintotheleftsideoftheheart.Thismechanismisessentialfor
fillingofthefetalleftventricleasthefetallungsarefluid-filledandhavea
relativelylowcirculatoryvolumecomparedwiththeirpostnatalfunction.The
systemicvenousreturnfromthefetalbodyisejectedfromtherightventricle,
withthemajoritydivertedawayfromthepulmonarycirculationthroughthe
arterialduct,intothedescendingaortabelowtheleveloftheisthmus.This
deoxygenatedbloodreturnstotheplacentalcirculationviathetwoumbilical
arteriesforoxygenationandreceiptofnutrients.

Placenta
Theplacentaplaysamajorroleinthefetalcirculation,fulfillingthefunctionsof
thelungforexchangeofgases,andforthekidneyandgastrointestinaltractin
deliveryofnutrientsandexcretionofmetabolites.Thefetalsideoftheplacenta,
whichdevelopsfromthechorion,receivesbloodfrompairedumbilicalarteries,
whichtakeoriginfromtheinternaliliacarteriesofthefetus.Theumbilical
arterieswithinthecordspiralaroundtheumbilicalveinandthendivideinto
branchesatthejunctionofthecordandtheplacenta.Thesebrancheshavea
radialdisposition.Theterminalbranchesperforatethechorionicplateandform
anastomoticplexuseswithinthemainstemofeachchorionicvillus.Eachmain
stempossessesaderivativeoftheumbilicalartery,whichpenetratesthe
thicknessoftheplacenta,dividingtoformahugenetworkofcapillaryplexuses.
Theseprojectintotheintervillousspacesthatcontainmaternalblood.29Asa
result,thereisaveryextensivesurfaceareawithineachchorionicvillus,across
whichexchangeofgasoccursdowngradientsforbothoxygenandcarbon


dioxide.Thereisessentiallynomixingofmaternalandfetalblood.Following
oxygenationwithinthechorionicvilluses,thebloodentersthevenousradicals
withineachmainstem.Theseefferentvenulesbecomeconfluentatthejunction

oftheplacentaandumbilicalcordtoformtheumbilicalvein.Thenormal
umbilicalcordshowsaregularcoilingoftheumbilicalveinandarteries(Video
6.1)thatmaybealteredindiseasestatessuchashypertension(discussedlater)
(Fig.6.2).

FIG.6.2 (A)Grayscaleimageofsegmentofcordshowingarterial
redundancy(UA)overrelativelystraightumbilicalvein(UV).(B)Power
Dopplerhigh-definitionimageof(A)showingarterialloopsdueto
redundancy(arrows).IncaseswherethereismarkedincreaseinUA
length,thearteryistortuousandtherearesegmentswheretheartery
reversesindirectionsimilartoafleur-de-lis.Thiscanbeseenwith(B)and
without(A)colorDoppler.UAtortuosityorredundancyduetoincreased
arteriallengthinrelationisshownwithcolorDoppler(C)andfetoscopically
(D)duringlasersurgery.(FromDonepudiR,MannLK,WohlmuthC,etal.
Recipientumbilicalarteryelongation(redundancy)intwin-twintransfusion
syndrome.AmJObstetGynecol,2017;206.e1–206.e11.)