Andersons pediatric cardiology 365
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prostheticvalveofthelasttwodecadesistheSt.Judevalve.Itisalowprofile,
bileafletvalveconstructedofpyrolyticcarbon.Initsmostrecentiteration,ithas
arotatablevalvarmechanism.CompetingbileafletdesignsincludetheATS,OnX,andCarbomedicsvalves.Therehasbeenconsiderableinterestoflateinthe
potentialofutilizingtheOn-Xvalvewithaspirinastheonlyantithrombotic
agent.Themanufacturerclaimsthatthelackofsiliconcoatingonthecarbon
usedtoconstructthevalvemakesitlessthrombogenic.Controlledtrials
(PROACT)areunderwaytoevaluatetheriskofthromboemboliccomplications
usingalessaggressiveanticoagulationgoal(internationalnormalizedratio
[INR]1.5to2.0).Theinterimresultsfromthetrialshowedamodest1.8%per
yearreductionintheriskofmajorbleedingwiththelowerINRtargetandno
significantincreaseinthromboembolism.101Therearealsoisolatedreportsof
aspirin-onlyanticoagulationforothermechanicalvalvessuchastheSt.Jude
prosthesis.102
FIG.16.8 (A)Typicalbileafletmechanicalvalve.Suchvalvesareatrisk
forthromboemboliccomplicationsandanticoagulationisnecessary.(B)
Adherentthrombusinanexplantedvalve.(CourtesySt.JudeMedical,Inc.,
St.Paul,MN.)
Mechanicalvalveshavebeenproducedforadultsbutrecently,smaller
mechanicalprosthesesinthe15-mmand17-mmsizerange,suitableformany
smallchildrenandeveninfantsandnewborns,havebecomeavailable.The
HALOIDEtrialsponsoredbytheFoodandDrugAdministrationinconjunction
withSt.JudeMedicalwillinvestigatetheefficacyofthe15-mmprosthesisin
patients5yearsofageorless.Whilethestudyisongoing,itisnotcurrently
recruitingparticipants.Despitetheavailabilityofsmallersizesofprostheses,
annularenlargementsuchastheanteriorenlargementofKonno,103orthe
posteriorenlargementofNicks104orManouguian105,106mayberequiredwhen
thediameterofthenativeaorticrootdoesnotpermitplacementofavalve
sufficientlylargetomeetthehemodynamicneedsofthepatientandallowfor
growth.Inthemitralposition,theanatomydoesnotlenditselfaswelltoannular
expansion.Othertechniquesmustbeutilized.Valvescanbeplacedinthesupraannularposition,oratangles.Thetechniquescanpermitplacementofa
modestlylargerprosthesis.Significantoversizingofmitralprosthesesshouldbe
avoided,asthiscanresultinsubaorticobstructionandheartblock.
XenobioprostheticValves
Xenobioprostheticvalvesincludeporcineaorticvalvesandvalvesmanufactured
frombovineorequinepericardium.107,108Xenobioprostheticmaterialhas
typicallybeentreatedwithglutaraldehydeinordertodecreaseimmunogenicity
andimprovedurability(Fig.16.9A).Xenobioprostheticvalveshavelow
thrombogenicityandgenerallydonotrequirechronicanticoagulation.In
children,xenobioprostheticvalvesplacedinthesystemiccirculationundergo
rapidandunpredictablecalcificdegeneration,limitingtheiruse(Fig.
16.9B).109,110Recenteffortshavebeendirectedatnewertechniquesfor
preservationthatlimitthisprocessofcalcificationwithmixedresults.111
Xenobioprostheticvalvedconduitshavebeenusedinrarecircumstanceswhere
anticoagulationmustbeavoided,butingeneral,xenobioprostheticvalvesare
rarelyusedinchildreninthesystemiccirculation.Xenobioprostheticvalvesare
commonlyusedinchildreninthepulmonaryandtricuspidposition.Inthelower
pressureright-sidedcirculationthevalvesaremoredurableandchronic
anticoagulationisnotrequired.
FIG.16.9 (A)Typicalxenobioprostheticstentedporcinevalve.(B)Such
valves,wheninsertedinchildren,undergorapidcalcificationand
degeneration.(A,CourtesyEdwardsLifesciences,Irvine,CA.)
HomograftValves
Valvedhomograftmaterialcanalsobeusedforvalvarreplacementand
reconstruction(Fig.16.10).Thehomografthascommonlybeenusedfor
replacementoftheaorticvalveandaorticrootinthefaceofendocarditis.112–117
Smallhomograftvalvedconduitsareidealforcomplexreconstructionin
neonatesandsmallinfants,suchasrepairofcommonarterialtrunk,and
tetralogyofFallotwithpulmonaryatresia.InNorthAmerica,however,
homograftsinsizessuitableforneonatesandsmallinfantsarebecominglessand
lessavailable.Theuseofhomograftstojointherightventricletothepulmonary
arteriesisdiscussedmorefullywhenconsideringselectionofvalves.
