Andersons pediatric cardiology 369
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FIG.16.14 Cannulationforanarterialswitch.Venousdrainageis
accomplishedwithseparatevenouscannulaforthesuperiorcavalvein
(SVC)andinferiorcavalvein(IVC).Oxygenatedbloodisinfusedviathe
arterialcannulapositionedatthejunctionoftheascendingaortaandaortic
arch.Thecross-clamphasbeenappliedisolatingthecoronaryarteriesfor
deliveryofcardioplegiaviaacannulaintheproximalascendingaorta.Left
ventriculardistensionisavoidedbyplacingaleftventricular(LV)vent
throughthejunctionoftherightsuperiorpulmonaryveinandleftatrium,
advancingitacrossthemitralvalveintotheleftventricle.(FromTweddell
JS,LitwinSB.Transpositionofthegreatarteries.OperTechThorac
CardiovascSurg.2002;7:49–63.)
Additionalrollerpumpsprovideactivesuctionforuseasfieldsuckers,aleft
ventricularvent,oraventintheaorticroot.Avarietyofpressuretransducers,
leveldetectors,bubbledetectors,andinlinebloodgasanalyzersareusedfor
enhancedprecisionandsafety.
Oxygenators
Hollowfibermembranesaremanufacturedtomimicthepulmonarycapillary
bedbypackingtogethermicroporousfibersinaspiral184orcrosshatched
fashion.Theflowrateandcompositionofgasesdeliveredtotheoxygenatorcan
bemanipulatedtocontrolthetransferofoxygen,carbondioxide,andanesthetic
vaporsintotheperfusate,takingintoaccountthesurfaceareaofthemembrane,
andthediffusionandblood-gassolubilitycoefficientsofdifferentgases.
Surfacesexposedtothebloodcanbecoatedwithsomeformofabiomimetic
treatment.Severaltypesoftreatmentsareavailable,allhavingthegoalof
increasingthebiocompatibilityofthecircuit,reducingdamagetobloodand
minimizingtheimpactofbypassontheinflammatoryresponse.183
Reservoirs
Twotypesofreservoirsareutilized,venousandcardiotomy.Theformerisa
collectionchamberonlyforthevenousbloodandmaybeintheformofabagor
hard-shellreservoir.Thelattercollectsallshedbloodreturningfromthe
operativefieldviacardiotomysuctionandvents.Bothreservoirsarefiltered.
Thecardiotomyfiltersaredesignedtoremovedebrissuchastissue,fat,
macrothrombiandsuturematerial.184Manyreservoirsareavailablethatcombine
thevenousandcardiotomysuctionasasingleunit.Inthisconfiguration,the
separationofthechambersismadeinternally.Afterthefiltrationprocess,their
volumescombineintoasingleoutlet,whichsimplifiesconnectionsandpermits
visualizationofairandtheleveloffluidinthereservoir.Accordingtothelatest
publishedsurvey,97%ofpediatricinstitutionrespondentsinNorthAmericaand
90%ofinternationalrespondentsusehard-shellvenousreservoirs.
Pumps
Althoughmanytypesofpumpshavebeendescribedintheliterature,thereare
primarilytwotypesofarterialpumpsinusetoday,namelyrollerandcentrifugal.
Rollerpumpscontainalengthoftubinglocatedinsideacurvedracewayand
havegonelargelyunchangedintheirfunctionsincetheiroriginalusebyGibbon
intheearly1950s.Thisracewayisplacedatthetravelperimeterofrollers
mountedontheendsofrotatingarmspositionedoppositeeachother.These
rollersaremountedinsuchawaythatonerolleriscompressingthetubingatall
times.Bycompressingasegmentoftheblood-filledresilienttubing,thepump
pushesbloodaheadofthemovingroller,producingcontinuousflow.Theoutput
oftherollerpumpisdeterminedbytherevolutionsperminuteofthepumpand
thevolumedisplacedwitheachrevolution.Thisstrokevolumedependsonthe
internaldiameteroftubingandthecircumferentiallengthoftheraceway.185The
rollerpumpheadisreusable.Theflowrateissimpletodeterminebymultiplying
strokevolumebytherevolutionseachminute.Multiplesizesoftubingcanbe
usedinthesamepump,makingitapplicabletopatientsofallsizes.These
pumps,however,dohavedisadvantages.Thepumpdisplacesanytubing
contentssoitcanpumpbothairandblood,necessitatingtheneedforasystemto
detectbubbles.Becauseitisanocclusivepump,pressuretransducersmustbe
connectedtothesystemtodetectexcessivepressures,reducingtheriskof
particulatemicroembolizationfromtubingspallation,shear-inducedblood
damage,orcircuitrupture.Adjustmentofocclusion,orspacingbetweenthe
rollerandbackwalloftheraceway,isimperativefortheaccuratecalculationof
flowrateandtominimizebloodtrauma.
Centrifugalpumpsconsistofanimpellerarrangedwitheithervanesoranest
ofsmoothplasticconesinsideaplastichousing.Thesterile,disposableimpeller
iscoupledmagneticallywithanelectricmotorspinninginthedriveconsole.
Whentheimpellerrotatesrapidly,itgeneratesapressuredifferential,causing
bloodtoflow.186Centrifugalpumpsarenonocclusiveandpressuredependent.
Theygenerateincreasedflowwheneitherthepreloadincreasesortheafterload
decreases.Thenonocclusivenatureofthepumpeliminatestubingwear,
spallation,andexcessivelinepressuresassociatedwithrollerpumps.However,
sincetheyarepressuredependent,aflowtransducerisnecessarytodetermine
accurateratesofflow.Duetotheirnonocclusivedesign,ifthepumpslowsor
stops,reversalofflowcanoccur.Centrifugalpumpsaremoreexpensive
comparedtotherollerpumpandarenotreusable.Asaresultofthedescribed
limitationsofcentrifugalpumps,arecentsurveyfoundthatrollerpumpswere
thepredominantpumpdeviceinpediatriccenters.185
FiltersandHemoconcentrators
Inadditiontothecardiotomyandvenousreservoirfiltersmentionedpreviously,
allbutapproximately8%ofdomesticandinternationalpediatricinstitutionsuse
arteriallinefiltersasalastlineofdefenseagainstgaseous(GME)and
particulatemicroembolization.185Theycomeinavarietyofsizes,with
preestablishedlimitstoratesofflowandareavailableineitherinlineor
integrateddesigns.Inlinearterialfiltersrequireadditionalvolumetoprime,but
