Andersons pediatric cardiology 364
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Tycron)haveminimalmemory,holdknotswell,andareusedfrequentlyto
securepatchesusedtocloseventricularseptaldefects,ortosecureprosthetic
valves.Polypropylene(Prolene)isamonofilamentsuturethatmaylastfrom2to
6years.Itslongdurability,easymaneuverability,andtendencytoglideeasily
thoughtissue,makeitthesutureofchoiceforcardiacreconstructionand
vascularanastomoses.TheGore-Texsuture,madeofPTFE,isamonofilament
nonabsorbablesuturealsousedforvascularanastomoses.Thesutureis
frequentlyusedwithGore-Texgraftstoimprovehemostasisofthesutureline.
PatchMaterialandValvelessConduits
Oneoftheeasiestmaterialstouseisfreshautologouspericardium.Thematerial
iseasilyharvestedatthetimeofaprimaryoperation,thoughitislesseasily
harvestedduringreoperation.Thematerialispliable,sterile,andhasno
likelihoodofincitinganimmunologicreaction.Somepreferforthepericardium
tohavefirmertexturetoincreaseeaseofhandling,inwhichcaseitisfixedin
glutaraldehyde.Glutaraldehydefixationresultsincross-linkingofcollagen,
makingthepericardiumstiffer.Suchfixationmayreduceaneurysmalformation
whenpericardiumisusedforvascularreconstruction.72Whennative
pericardiumisnotavailable,glutaraldehyde-treatedbovinepericardiumcanbe
used.Thoughthistissueisreadilyavailable,andhasminimaltonoriskof
transmissionofdisease,itdoesundergocalcificdegeneration.Photo-oxidized
bovinepericardiumisanewerpreparationthatusesdye-mediatedphotooxidationtocrosslinkpericardialcollagenratherthanthechemicalfixation
associatedwithglutaraldehyde.Thehandlingcharacteristicsofthistypeof
bovinepericardiumissimilartothatofnativepericardiumandthereappearsto
beareducedriskofcalcification.73
Homograftaortasorpulmonaryarteries—inotherwords,cadaverictissues—
canalsobeusedforvascularreconstruction.Suchmaterialiscommonlyused
duringreconstructionofthepulmonaryarterialtreeandconfluence,ortorelieve
isolatedpulmonarystenoses.Theyarealsocommonlyusedforreconstructionof
theaorticarchintheneonate.74Thematerialishemostaticandeasytohandle.It
is,however,expensive,anditrequirestimeforthawing,carriesariskof
calcification,andcomesinvaryingthicknessesthatareoftendifficulttopredict.
Inaddition,homograftmaterialcaninciteanimmuneresponseresultingina
nonspecificincreaseinantihumanlymphocyteantibodies,potentially
complicatingfuturetransplantation.75
Dacron,asyntheticpolyestermaterial,hasbeenamainstayinvascular
reconstructionsinceitsintroductioninthe1950s.Itisusedinitstubularformto
reconstructvascularsegments.Itcanalsobeusedasapatch,andiscommonly
usedforclosureofventricularseptaldefects.TheDacronmaterialcanincitea
fibrousreaction.Thisisanadvantageforclosureofventricularseptaldefects,in
thatsuchfibrousingrowthmayallowclosureofthetinyresidualdefectsatthe
edgesofthepatch.FibrousingrowthisevenfurtherenhancedwhentheDacron
iscoveredwithvelour.76Caution,however,shouldbeusedwhenDacronis
placedclosetoanarterialvalve,asfibrousingrowthcanimpairmotionofthe
valvarleaflets.
Whenusedasaconduit,theporosityofDacronposesaproblemfor
hemostasis.Inordertoimprovehemostasis,graftscanbepreclottedbysoaking
themineitherautologousbloodoralbumin,andthenheatingthegraftsinan
autoclaveorapplyingfibringlue(fibrinogenplusthrombin)tothegraftsonthe
operatingroomtable.Inordertomakethegraftseasiertouse,theycanbe
manufacturedwithsealantsalreadyappliedtothegrafts.Oneexampleisthe
HemashieldDacrongraft(MedoxMedical),inwhichformaldehydecross-linked
collagenisusedtodecreaseporosity.
PTFE,withthetradenameTeflon,canbeexpandedorstretchedsuchthatit
carriesaporesizeof20to30µm,asizethathasbeendeterminedtobethe
optimalporesizeforhealing.77,78TheresultisexpandedPTFE,commonly
knownasGore-Tex.Itisusedextensivelyforsystemic-to-pulmonaryartery
shuntsandforarterialreconstruction.Gore-Texisthrombogenicanda
pseudointimallayerwilldevelopovertimethatistheresultofacombinationof
thrombusandcellularingrowth.79–83Thispseudointimallayerwilldecreasethe
caliberofsmallgraftsandcanresultincriticalreductionofflow.Aspirinhas
beenshowntoimprovepatencyofsmallcaliberGore-Texgrafts.84Gore-Tex
graftscanbeheparinbondedtoreducetheriskofthrombosisandlossofluminal
patency.Whenusedforperipheralarteriovenousprostheticaccessinadult
patients,thereisimprovedpatencyofthegraftwithasignificantlylowerriskof
earlythrombosisduringthefirst5monthsoffollowup.85
ValvesandValvedConduits
Thedecisionregardingvalvereplacementinchildreniscomplex.Unlikeadult
patients,thechoiceofaprosthesisisinfluencedbygrowthofthepatient,the
needfordurability,andthepotentialforavoidanceofanticoagulationinactive
childrenwheretheinjuryprofileisnotinsignificant.86Fourbroadcategoriesof
prosthesisexist:mechanical,xenobioprosthetic,homograft,andautograft.
MechanicalValves
Thecurrentlymanufacturedmechanicalvalveshaveabileafletdesign(Fig.
16.8).Allmechanicalvalvesarethrombogenicandrequireanticoagulationand
carrytheriskforthromboembolicandbleedingcomplications.Anticoagulation
isaccomplishedwithvitaminKantagonistssuchaswarfarin,acenocoumarol,or
phenprocoumoncommonlycombinedwithantiplateletagents(typicallyaspirin),
anddipyridamole.Theriskofthromboembolicandbleedingcomplicationsis
present,eveninpatientswithoptimallymanagedanticoagulation,andis
influencedbyfactorsspecifictothepatient,alongwiththepositionofthevalve.
Theannualrateofthrombosiswithamechanicalprosthesisrangesfrom0.1%to
5.7%.87–90Alowerthromboembolicriskisobservedinpatientswithaorticvalve
replacementcomparedtomitralvalvereplacementandpatientswithbothaortic
andmitralvalvereplacementhavethegreatestrisk.Preservedventricular
functionisassociatedwithalowerriskofthromboemboliccomplications.Ina
single-centerreport,childrenundergoingaorticvalvereplacementhad93%
freedomfromthromboemboliccomplicationsat20-yearfollow-up91andlittle
anticoagulation-relatedhemorrhage.92Theriskofcomplicationsrelatedto
anticoagulationisgreatestformechanicalvalvesplacedinmitralorleft-sided
atrioventricularvalveposition.Threeseparatereportssummarizingindividual
institutionalexperiencesinchildrentotaling115patientsreported10-year
freedomfromthromboemboliccomplicationsabove92%,andtheriskof
complicationsrelatedtobleedingatbetween76%and97%.93–97Thetrue
incidenceofthromboemboliccomplicationsinchildrenundergoingreplacement
ofthemitralvalvewithamechanicalprosthesisisprobablyunderestimatedby
single-centerreports.Amulticenterstudyfoundthat4of102survivorsofmitral
valvereplacementrequiredre-replacementforthrombosisatameanfollow-up
of6.0years.98Prospectivestudiesofadults,withcombinedenrollmentofover
1000patients,showedthatbileafletmechanicalvalvesinthemitralposition
wereassociatedwithanincidenceofboththromboemboliceventsand
complicationsduetobleedingatafrequencybetween1%and3%perpatientper
year.99,100Freedomfromthromboemboliccomplicationsafter10yearswas
85.5%,andfreedomfrombleedingwas81.7%.99Themostwidelyused
