Andersons pediatric cardiology 1825

beforeandaftersurgicalinterventions,areshown.Thismultidistribution
circulationisinherentlyinefficientandresultsinprogressivecongestive
heartfailureovertime.Ascanbeseen,thevariabilityinefficiencyis
dependentonanatomy,ventricularfunction,valvarfunction,pulmonary
parenchymalfunction,electrophysiologicstatus,systemicandpulmonary
vascularresistance,ductalpatency(priortosurgery)andthestatusof
surgicalconnections(aftersurgery).Perioperativemanagementistailored
totheassessmentandprovisionofadequateoxygendelivery.When
oxygendeliveryisinsufficient,targetedinterventionstowardthecause(s)
areundertaken,specificallygearedtowardtheindividualanatomyand
physiologypresent.Giventhecomplexityandindividualvariationofthis
multidistributioncirculation,commonlydescribedtreatmentstrategiessuch
as“balancingsystemicandpulmonarybloodflow”and“avoiding
overcirculation”maybetoosimplistic.Increasedpulmonarybloodflowin
isolationdoesnotgenerallyresultininadequatesystemicoxygendelivery
overtheshorttermand,ifpresent,additionalcausesshouldbesought.
Pulmonaryalveolarfunctionmustbeadequateforeffectivegasexchange,
andtotalventricularoutputmustbesufficientinvolumetoprovideenough
systemicvenousreturntomixwithpulmonaryvenousreturn.Seetextfor
details.

Bothbeforeandaftersurgicalmanagementintheneonate,thefUVHis
volume-overloaded,asitsuppliesthepulmonarybloodflow(Qp)plusthe
systemicbloodflow(Qs)plustheregurgitantvolume(Qr)ifpresent.
Additionally,althoughthetermcardiacoutputhasfrequentlybeenused
clinicallytodescribethesystemicbloodflowinchildrenwithabiventricular
circulation,thisisanincorrectterminaneonatewithafUVH.Thetrue
“cardiac”outputisdistributedintwotothreedifferentcompartments.Clinically,
itisimportantintheneonatewithafUVHtobeclearregardingthedifference
betweentheventricularoutput(Qp+Qs+Qr)andthatfractionofthe
ventricularoutputthatdeliversoxygentothetissues—thesystemicbloodflow.

Inaddition,anumberoftermshavecomeintocommonusageoverthepast
decadesbutareneithersensitivenorspecificintheassessmentofphysiologic
stabilityandthemanagementofadequateoxygendeliveryinaneonatewitha
fUVH.AsourunderstandingofthefUVHcirculationhasimproved,wesuggest
thatthefrequentlyusedtermslistedhereareoutdated37–40andshouldbe
modified.
Terminologysuggestedtobemodified/eliminatedinclude:

•Parallelcirculation(discussedearlier)
•Balancingthepulmonaryandsystemicbloodflow


•Pulmonaryovercirculation
•Single-ventriclephysiology
“BalancingthePulmonaryandSystemicBlood
Flow”
Invitrostudies41havesuggestedthattheoptimalratioofQp:Qsis
approximatelyunity(balanced),andinsomeclinicalscenariosthismaybe
true.42,43However,recentinvivostudiessuggestawiderangeinthisratio,
whichdidnotaffectthesystemicdeliveryofoxygenorhospitalsurvival.Inthis
studybyLiandassociates,44systemicdeliveryofoxygenwashighlycorrelated
withtheabsolutevalueofQs,andmuchmoresothantheratioofQp:Qs.In
addition,therearesituationsintheneonatewithafUVHthat,whilethe
circulationis“balanced,”representaprecariousclinicalscenario.Forexample,a
patientwithamixedvenousoxygensaturationof25%,apulmonaryvenous
oxygensaturationof95%,andasystemicoxygensaturationof60%indeedhasa
balancedcirculation—theQptoQsratiois1.0—butnonethelesshasseverely
impairedsystemicoxygendelivery.Theterm“balancedcirculation”isthus
neitherasensitivenorspecifictermtodefinethegoalsofmanagement.

“PulmonaryOvercirculation”
Technically,thistermdescribesincreasedpulmonarybloodflowtothelungs
comparedtothebody,aclinicalscenarioseeninmanychildrenwithseptal
defects,apatentarterialduct,andsoon.However,“pulmonaryovercirculation”
hasunfortunatelybecomesynonymouscolloquiallywithdecreasedsystemic
bloodflow,withanimpliedimbalanceofsystemicandPVRasthecauseof
potentialclinicaldeterioration.
Inthepreoperativeneonate,asstatedearlierandconfirmedwiththeclinical
experienceswaitingfororgantransplantation,slowlyprogressivecongestive
heartfailure—overdaystoweeks—isthemorecommonclinicalscenarioduring
thetransitionalcirculation,ratherthanacutedeterioration.Shouldmoreacute
clinicalchangestakeplace,promptinvestigationofthepatencyofthearterial
ductshouldbeundertaken.Inthepostoperativeneonate,lowsystemicoxygen
deliverymaybeduetopreferentialmaldistributionofflowintothepulmonary
vascularbed(“highQp:Qs”)fromalowPVRcombinedwithananatomically


largeshunt,butasstatedabove,anumberofadditionalcausesmustbe
investigatedbeforeattributingtheclinicalscenariosolelyto“pulmonary
overcirculation”(e.g.,archnarrowing,lowglobaluniventricularoutput,low
oxygencontentormultiplecombinationsofthesefactors,seeFig.70.2).Asin
thepreoperativestate,theconverseistrue:notallbabieswitha“high”oxygen
saturationareatriskofclinicaldeterioration.Inpostoperativepatients,both
hyperventilationandsignificantlyincreasedsupplementaloxygendidnotresult
inclinicaldeterioration.45Duringhyperventilation,therewerenochangesin
systemicormixedvenoussaturation,arteriovenoussaturationdifference,oxygen
excessfactor(Ω),orbloodpressure.Importantly,highlevelssupplemental
oxygenproducedsignificantincreasesfrombaselineinsystemicsaturation(90%
±1%vs.80%±1%;P<.01),mixedvenoussaturation(54%±3%vs.44%±
2%;P<.01),andΩ(2.6%±0.2%vs.2.3±0.2%;P<.01),withnochangein

arteriovenoussaturationdifferenceorbloodpressure.45
Thisstudy,aswellasourclinicalexperience,suggeststhatincreased
pulmonarybloodflowperseinthefaceofnormalsystemicbloodflowand
normalpulmonaryparenchymamayhavenophysiologicconsequences.For
example,apatientwithamixedvenousoxygensaturationof65%,apulmonary
venousoxygensaturationof95%andasystemicoxygensaturation85%hasa
Qp:Qsratioof2:1;however,systemicoxygendeliveryismaintained,as
evidencedbyanormalmixedvenousoxygensaturationandanarrowdifference
betweenthearterialandvenousoxygensaturations.Fundamentallyandmore
accurately,circulatorymaldistributionisamoreinclusiveterm,whichcanbe
usedtodescribesomeofthecausesofastateofdecreasedoxygendelivery,in
particular,redirectionofventricularoutflowtothepulmonaryvascularbed
and/orsignificantatrioventricularvalveregurgitation.However,theclinical
scenariooflowoxygendeliverymayalsobeduetolowuniventricularoutput,
lowoxygencontent,ormultiplecombinationsofthesefactors,asshowninFig.
70.2.Eachofthesecausesofdecreasedsystemicoxygendeliveryhasdifferent
managementstrategies;thereforeanaccurateandcompleteassessmentofthe
multidistributioncirculationmustbeundertaken.
Inconclusion,although“pulmonaryovercirculation”maycauseclinical
instability—particularlyifitcontributestopulmonarycongestion,inadequate
alveolargasexchange,tachypnea,orrespiratorydistress—undercirculationof
eitherthepulmonaryorsystemicvascularbedsisalwaysanunstableclinical
scenario.SimplifyingthecomplexphysiologyshowninFig.70.2—with
terminologysuchashighQp:Qs,pulmonaryovercirculation,andbalancingflow