Andersons pediatric cardiology 372
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changeinvulnerabledevelopingandpreviouslyinjuredbrains.282,283
PathwaysofCerebralInjuryRelatedtoBypass
Cerebralinjuryisdeterministicallyrelatedtothedeliveryofoxygenwith
irreversiblenecroticcelldeathresultingfromasustainedreductionindelivery
below20%ofnormal.281,284Ratesofdeliveryabovehalfbaselinetypicallydo
notresultininjury,whiledeliveryratesofone-quartertohalfofthenormal
rangeresultincellularinjurywhoseoutcomeismodifiablebyotherfactors,such
astemperatureandfree-radicalscavenging,evenwhenappliedafterthe
insult.285Apoptoticcelldeathensueshourstodaysaftersubnecrotichypoxicischemicinjuryinsusceptiblepopulationsofcellsandthereexistsaspectrumof
ischemicandapoptoticdeathinbothfocalandglobalmodelsof
ischemia.279,284,285Amplificationofinjurythroughexcitatoryaminoacid
neurotransmitter-relatedcalcium-dependentcascadesofuncontrolledneuronal
depolarizationmayplayaroleinbothnecroticandapoptoticcelldeath.284
Modificationofexcitotoxicitycanbedemonstratedwithglutamatereceptor
antagonistssuchasketamineanddextromethorphan,receptoragonistssuchas
anestheticvaporsandbarbiturates,magnesium,andhypothermia.229Although
therapeutictrialshavegenerallybeendisappointinginprofoundischemia,an
incrementaleffectislikelyinmoremoderateinjury.286,287
Althoughhypothermiccirculatoryarrestrepresentsanobviousexampleof
globalischemia,itislikelythatregionalpartialischemiaexistsduringmany
phasesofbypassandtheperioperativeperiod.Inananimalmodeloftissue
oxygenationduringchangingconditionsofbypass,arangeoflevelsofoxygen
inthetissueswasdemonstratedusingthephosphorescentquenchingtechnique,
evenduringhigh-flowbypass.Morehypoxicregionsappearduringlowflow
andhypothermiccirculatoryarrest.288Thecerebralcirculationissusceptibleto
hypoxicinjurythroughouttheperioperativeperiodandpartialischemiais
possibleevenduringhigh-flowbypasswithneuronalfatemodifiableby
postoperativefactors.285,289,290
MyocardialProtectionandCardioplegia
InitiationofCPBoftenhasmyocardialprotectiveeffectsiftheheartisproperly
unloadedbyenhancingtheavailabilityofoxygendeliveredthroughthecoronary
arteriesandreducingconsumptionofoxygen.Hypothermiaisacorecomponent
ofmyocardialprotection.Itfurtherdecreasesconsumptionofoxygenand
preservesstoresofhighenergyphosphates.Myocardialworkcanbefurther
reducedbyinducingahyperkalemicarrestviatheadministrationof
cardioplegia.Thecoronaryarteriesareisolatedfromthedistalaorticcirculation
byplacementofanaorticcross-clampdistaltothecannulausedtodeliver
cardioplegia.Coldsolutionsarethenimmediatelydeliveredat4°C.Aorticvalvar
competenceisnecessarytoensurethecardioplegiaflowstothecoronaryarteries
andnotintotheleftventricle.Iftheaorticvalveisnotcompetent,theaorticroot
canbeopenedandcardioplegiacanbedelivereddirectlyintotheorificesofthe
coronaryarteries.Cardioplegiacanalsobedeliveredinaretrogradefashionvia
acatheterplacedinthecoronarysinus,aslongasthesinusisnotreceivingblood
fromaleftsuperiorcavalvein.Retrogradecardioplegiaisoftenusedasa
supplementarymethodofcardioplegiaevenwhenantegradecardioplegiais
possible.
Localizedmyocardialhypothermiacanbeachievedwithcoolingjacketsand
placementoficeslushinthepericardialspace.Thistechniqueshouldbe
judiciouslyused,however,asitcanresultininjurytothephrenicnerves.184As
anadjuncttohypothermia,thehighconcentrationofpotassiuminthe
cardioplegiaresultsinmyocardialelectromechanicalsilenceanddiastolicarrest.
Theinitialarrestingdoseofcardioplegiaatourinstitutionis20mL/kgofdel
Nidocardioplegiasolutionina1:4,bloodtocardioplegiasolution,ratio.291
Maintenancedosesof10to20mL/kgaregivenevery90to120minutes
thereafteruntiltherepairiscomplete.
Acrucialcomponentofsuccessfulmyocardialprotectionisventricular
decompression.Myocardialconsumptionofoxygenandimpedanceto
subendocardialflowofbloodissignificantlyreducedbyloweringtheventricular
muraltension.292Decompressionisaccomplishedmostcommonlywiththeuse
ofacatheterintroducedthroughtherightsuperiorpulmonaryveinandtheleft
atrium.Thecatheterpassesthroughtheleftatrium,acrossthemitralvalveinto
theleftventricle.Constantsuctionisappliedtothecatheterwiththeuseofa
designatedrollerpumpandaninline,one-way,over-pressurereliefvalveonthe
bypassmachine.Bloodreturningfromtheventingcatheterisreturnedtothe
systemiccirculationviathecardiotomyreservoirasdiscussedpreviously.
Anticoagulation
Althoughotheranticoagulantshavebeenusedinspecialsituations,293,294
heparinisoverwhelminglythemostcommonlyadministered
anticoagulant.295,296Heparinhasarapidonsetofaction,iseasilyreversedwith
protamine,andhasimportantantiinflammatoryeffects.297,298Dosingregimens
includesimpleweight-basedschemes,titrationtoafunctionalendpoint
dependingonactivatedclottingtime,andmeasurementofconcentrationswith
morecomplicatedpredictions.Convincingevidenceforsuperiorityof
approachesislacking.Thegoldstandardofdeterminingadequate
anticoagulationsuitableforbypasshasbeentheactivatedclottingtime.Thistest,
however,doesnottakeintoaccounttheeffectsofvolumeofblood,previous
exposuretoheparin,deficiencyofantithrombinIII,hypothermia,or
hemodilution.TheHepconheparinmanagementsystem(Medtronic)usesa
prebypasstitrationofprotaminetodetermineapatient-specificconcentrationof
heparintobemaintainedthroughoutthebypassrun.Whileonbypass,samples
ofbloodaretakenevery30minutestodeterminebothconcentrationsofheparin
andactivatedclottingtimes.Additionalneedforheparinisbasedonmaintaining
anadequateconcentrationregardlessofanextendedactivatedclottingtime.The
Hepcondevicecalculatesthedoseofprotamineneededtoreverseheparinbased
onthecirculatingconcentrationofheparinattheendofthebypassrun.The
thrombotic,embolic,andinflammatorycomplicationsofCPBareincreasedwith
lowerheparineffect.Adequateanticoagulationiscrucial,otherwiseintravascular
coagulation,thrombosis,oxygenatordysfunction,andconsumptionofclotting
factorsmayoccur.186Heparin-inducedthrombocytopenia,stillanunusual
complicationininfantsandchildren,maybemoredifficulttorecognize.299
DeepHypothermicCirculatoryArrest
DHCAwasfirstdevelopedasaneuroprotectivestrategywhencontinuous
perfusioncouldnotbemaintained.Currently,thereisintensedebateoverthe
degreeofprotectionofferedbyhypothermiccirculatoryarrestcomparedto
hypothermicperfusion.TheBostoncirculatoryarresttrialdemonstratedthata
strategyutilizinghypothermiccirculatoryarrestcomparedtoevenlow-flow
bypassisassociatedwithmoreimmediatecerebralbraininjury.Patients
undergoinghypothermiccirculatoryarresthadmoreseizures,anincreased
tendencytohaveabnormalelectroencephalograms,300andlowerdevelopmental
performanceat1year.192Bothgroupsunderperformedat8years.194Prolonged
hypothermiccirculatoryarrest,greaterthan40minutesat18°C,isassociated
withimpairedneurodevelopmentaloutcome.301,302Therelationshipbetweena
