Andersons pediatric cardiology 370
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areexcellentgrossbubbletraps.Becausetheyaremicroporousfilters,theyare
susceptibletoobstructionandmanufacturersrecommendplacingaclamped
bypasslinearoundinlinefiltersthatcanbeopenedtopreventtheinterruptionof
CPBifthefilterbecomesobstructed.Integratedarteriallinefiltershaverecently
beendeveloped.Thesefiltersarewrappedaroundthehollowfibermembrane
andareheldwithintheoxygenatorhousing.Advantagesofthisdesigninclude
easeofsetupandlowerprimevolumes;however,theycannotbebypassedif
theybecomeobstructed.
Hemoconcentratorsallowtheperfusionisttoremovewaterandother
electrolytes,suchaspotassium,fromtheblood.Theycontainhollowfibers
similartothosewithinadialysisfilter.Bloodpassesthroughtheinsideofhollow
fibersandvacuummaybeappliedontheoutsidetoencouragewaterremoval.
Everythingsmallerthanthesizeoftheporesofthesemipermeablemembrane
willbeextracted,includingwater,electrolytes,somecytokinesanddrugs.
Everythinglargerthantheporeswillremaininthebloodstream,includingred
cells,plateletsandmostplasmaproteins.Significanthemoconcentrationcanbe
achieved.Someheparinwillberemoved;thusadequacyofheparinizationmust
bemonitoredregularly.186Thehemoconcentratorcanbeusedatanytimeduring
thecase,providedthereissufficientvolumeinthevenousreservoir.Thisisthe
sametypeoffilterusedtoperformmodifiedultrafiltration(MUF),zerobalance
ultrafiltration,ordilutionalultrafiltration(DUF).MUFisutilizedafterCPBto
concentratethehematocritaswellasclottingfactorsandplateletswherezero
balanceultrafiltrationordilutionalultrafiltrationtechniquesareperformed
duringCPBtoreduceinflammatorymediators.187–189
ConductofCardiopulmonaryBypass
RatesofFlow
Althoughthedeterminantsofdeliveryofoxygen,namelyconcentrationof
hemoglobin,saturationofoxyhemoglobin,andratesofflow,aremoreeasily
measuredduringCPBthanatanyothertimeinthelifeofaneonateorinfant,the
adequacyofdeliveryofoxygenshouldalwaysbecontinuouslymonitoredand
adjustedtoavoidovertoroccultinjurytotheorgansthroughoutthe
perioperativeperiod.190,191Ratesofflowhavetypicallybeenguidedby
nomogramsbasedonbodyweightorsurfaceareaandFick'sprinciplesof
deliveryofoxygenandmetabolism.192Theregionaldistributionofbloodduring
CPBisrelatedtohostbiology,anestheticandvasoactivemilieu,andtechnical
factorsduringCPB.Theprobabilityofadequateflowtothewholebodyor
organsisrelatedtothetotalrateofflow.191,193–196Typically,fullflowreferstoa
perfusionindexof2.8to3.6L/m2perminute,whichcorrespondsto150to200
mL/kgperminuteinaneonate.Lowflowisdeliveredatvaryinghypothermic
conditionsaffordingmetabolicprotectionandtypicallyreferstoratesofbetween
one-quarterandhalfoffullflow.196Theratesofflowtothewholebody
necessarytomaintainadequatecerebralperfusionrangefrom30to80mL/kg
perminute.197,198Isolatedperfusionoforgansduringbypassisgovernedbythe
relativedistributionofvascularresistance.Reductionintemperatureto16°Cto
20°Callowsterminationofflowforalimitedamountoftimetopermit
unobscuredaccesstothesurgicalfield,aconditionreferredtoasdeep
hypothermiccirculatoryarrest(DHCA).ThesafedurationofDHCAinany
individualpatientisunknownandhighlyrelatedtothedeterminantsofdelivery
ofoxygen.199Thedeleteriouseffectsofloweredflowmaybemorepronounced
afterhypothermicarrest,200whenahigherperfusionpressureisnecessaryto
reestablishcerebralflow.201Becauseofvariabilitybetweenpatientsand
techniques,itisadvisabletomeasureindicatorsofcerebraloxygenationsuchas
near-infraredspectroscopy(NIRS).202Manipulationoftheindependent
determinantsofdeliveryofoxygen,suchashemoglobinandpartialpressuresof
carbondioxide,canbeusedtorestorecerebraloxygendelivery.203
Hemodilution
HemodilutionhasalmostuniversallyaccompaniedCPBbecauseofthedesireto
primeextracorporealcircuitrywithproductsotherthanblood.Rheologic
considerationsformicrovascularflowduringhypothermiahavesupportedthis
approachandarethoughttooutweighthereductionindeliveryofoxygen
associatedwiththeanemiaproducedbyhemodilution.Theweightofevidence,
however,supportslimitinghemodilutioninneonatesandchildren,targetinga
hematocritofatleast30%evenduringdeephypothermia.204–206While
variationsinprimesolutionsaremainlytargetedatmanipulationofelectrolytes,
oncoticpressureandhemoglobin,theeffectsonprothrombotic,procoagulantand
anticoagulantfactorsshouldalsoberecognizedasimportanteffectsof
hemodilutionandcalculatedbaseduponestimatedbloodvolumeandcircuit
volumeforeachpatient.
TemperatureRegulation
Hypothermiareducesboththecerebralmetabolicrateandtheavailabilityof
oxygenfortransfertothebrain.Theeffectsoncerebralmetabolismarecomplex.
Themetabolismofbrainandothertissueisreducedwithreductioninbody
temperature.207Mostdatasuggestaninverseexponentialrelationship,withas
muchasa3.5-foldreductioninmetabolismforareductionof10°Cin
temperature,referredtoastheQ10.208OthershavefoundaQ10aslowat2.3.209
Thebulkofevidencesuggestsanearlyinverseexponentialreductionin
metabolismisreducedbyanaverageof2.8foldfora10°Cchangein
temperature.210Theresultofmetabolicsuppressionwithhypothermiaisthat
cerebraloxygenextractionisreduced,whileflowofbloodisstillautoregulated,
whethermeasuredbysaturationsinthejugularbulb210,211orNIRS.212,213
Becausetemperatureaffectsthesolubilityofoxygenandcarbondioxidein
solution,andtheirinteractionwithhemoglobin,changesintemperatureare
coupledwithchangesingastensionsandpH.Inabroadrangeofstudies,the
independenteffectofpHissmallcomparedtotheeffectoftemperaturewitha
smallreductioninoxygenconsumptioninmoreacidoticenvironments.209
BecausepHresponsivenessofthecerebralvasculatureremainsineffectat
hypothermia,however,controlandmanipulationofpHisacriticalpartof
temperaturemanagement.
Thecouplingbetweencerebralmetabolismandbloodflowseemstobe
reasonablymaintainedinthetemperaturerangeof32°Cto37°C.214–219Below
30°C,however,uncouplingiscommonlydemonstrated,regardlessofpH,such
thatthemetabolismisreducedmorethanbloodflow.211,220–222Thesolubilityof
oxygeninplasmaandtheaffinityofhemoglobinforoxygenarebothincreased
withhypothermia,suchthatavailabilityofoxygeninthetissuesisreducedat
anygivenrateofflow,theBohreffect.Theincreasedratioofcerebralflowto
metabolismwithhypothermiaiscommonlyviewedascytoprotectivefroman
energeticviewpoint,210butthedecreasedavailabilityofoxygenmaynegatethis
apparentmetabolicprotection.223,224Theresultofincreasedsolubilityand
leftwardoxyhemoglobinshiftisthatthefallincerebraloxygenationwith
ischemiaisnotattenuatedbyhypothermia,eventhoughsaturationsof
hemoglobinarebettermaintained.225Altogetherthecytoprotectiveeffectsof
mildhypothermiaexceedmeasurablemetaboliceffectsandlikelyinvolveother
mechanismsincludingalterationsingeneexpression.226
