Andersons pediatric cardiology 1077

NormalFetalCirculation
Approximatelytwo-thirdsofthefetalcardiacoutputoriginatesfromtheright
ventricle,withonly5%to10%passingthroughthelungs.15,16Assuch,the
majorityofrightventricularoutputpassesthroughthearterialductintothe
descendingaorta,anditspresenceisessentialfornormalfetaldevelopment,
permittingrightventricularoutputtobedivertedawayfromthehigh-resistance
pulmonarycirculation.Prematureconstructionorclosuremayleadtorightheart
failure,resultinginfetalhydrops.17


EmbryologyandPathogenesis
Duringearlyfetaldevelopment,fivearterialarcheslinktheaorticsacwiththe
paireddorsalaortas,althoughallarchesareneverpresentsimultaneously.The
archesarenumbered1to6.However,tothebestofourknowledge,thereisbuta
singleexampleidentifiedofafiftharcharterythatoccupiesadiscretesegment
ofpharyngealmesenchyme.Ithasbecomeconventionaltointerpretseveral
congenitalmalformationsonthebasisofpersistenceofthisenigmaticentity.
TheseexamplesarediscussedinChapter48.Theinitialsymmetricarrangement
ofthepharyngealarcharteriesdoesprovidethebasisfortheunderstandingthe
patternofthehypotheticaldoublearchproposedbyEdwards18toexplain
vascularringsandslings.Thisbilaterallysymmetricarrangement,withpaired
brachiocephalicarteriesandarterialducts(Fig.41.1,left),isusuallytransformed
totheconfigurationseeninpostnatallifebythedisappearanceofsomearterial
segmentsandrealignmentofothers(seeFig.41.1,right).19

FIG.41.1 Left,PrimitivesystemofdoublearchesproposedbyEdwardsto
explainvascularringsandslings.Thearchespersistonboththeright(R)
andleft(L)sides,unitingposteriorlytoformaneutraldescending(desc.)
aorta.Eacharchgivesrisetosubclavianandcommoncarotid(CA)
arteries,alongwithanarterialduct,whichjoinstheappropriatepulmonary

artery.Theaorticsacandoutflowtracthavedividedtoformtheaortaand
thepulmonarytrunk(PT).Thedoublearchsurroundsthe
tracheoesophagealpedicle(OE,T).Right,Withnormaldevelopment,the
rightarchandrightductregress,leavingaleftarchfeedingthedescending
aorta,anarterialductontheleftside,withthebrachiocephalicartery(BCA)
givingrisetotherightcommoncarotidandsubclavianarteries.LCA,Left
carotidartery;LPA,leftpulmonaryartery;LSA,leftsubclavianartery;RCA,
rightcarotidartery;RPA,rightpulmonaryartery;RSA,rightsubclavian
artery.

Thereforethenormalarterialductdevelopsfromthedorsalportionoftheleft
sixtharch.Fromtheirinception,thesixtharchesareassociatedwiththe
developinglungs,withthearteriesfeedingthedevelopinglungsdeveloping
withintheanteriorwallofthemediastinum.Thearteriestaketheiroriginofthe
flooroftheaorticsac,whichinitiallyfeedsalsothebilateralarteriesofthesixth
arches(Fig.41.2,left).

FIG.41.2 Reconstructionofthedevelopingarteriesofthepharyngeal
archesinembryonicmicesacrificedduringthe11th(left)and12th(right)
daysofdevelopment.Thereconstructionsareviewedfromthefront,with
right-sidedstructuresseentotheleft.Attheearlystage,thearteriesofthe
secondarcharebeginningtoregress,withthearteriesofthethird,fourth,
andsixtharchesformedinsymmetricfashion.Thecavityofthedistal
outflowtractisacommonstructureatthisstage.Thepulmonaryarteries
arebeginningtodevelopwithintheventralcomponentofthepharyngeal
mesenchyme.Bythe12thday,therightsixtharcharteryisbeginningto
regress.Thedistaloutflowtracthasnowseparatedintotheintrapericardial
componentsoftheaortaandpulmonarytrunk.Thepulmonaryarteriesnow
arisefromthepulmonarytrunk,withtheleftsixtharcharteryformingthe