Adjusting Ventilator Settings

2/3/2015 AdjustingVentilatorSettings

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AdjustingVentilatorSettings

Ventilatorsettingsareadjustedto(1)normalizebloodgases(ventilation,oxygenation)(2)improvepatientventilatorsynchrony/decreaserespiratorydistressand/or(3)weanapatientfromventilatorsupport.Herewefocusprimarilyonnormalizingbloodgases,withanoverviewofventilatoradjustmentstoimprovesynchronyoralleviaterespiratorydistress.

NormalizingBloodGases

Abloodgasmayindicateeitherabnormalventilation,abnormaloxygenation,orboth.

AdjustingVentilation

AbnormalventilationisindicatedbyanabnormalpHduetoanabnormalPCO2.Withanormalbicarbonate,restoringthePCO2tonormalwillresultinanormalpH.Ifthepatientisinrespiratoryalkalosis(hyperventilationhighpH,lowPCO2),youcanrestoreanormalPCO2bydecreasingtheminutevolume.Ifthepatientisinrespiratoryacidosis(hypoventilationlowpH,highPCO2),youcanrestoreanormalPCO2byincreasingtheminutevolume.Toestimatehowmuchyoushouldincreaseordecreasetheminutevolume,usethefollowingformula:

Forexample,ifamechanicallyventilatedpatientwithaminutevolumeof6L/minhasapHof7.25andaPCO2of60mmHg,youcanrestoreanormalPCO2byincreasingtheminutevolume.Thenewminutevolumewouldbe6.0Lx60/40=9.0L/min,with40mmHgbeingthedesirednormalPCO2.

Exactlyhowyouchangetheminutevolumedependsonthemodeofventilationbeingused.Thetablebelowindicatesthebestwaystoincreaseordecreaseminutevolumeforthecommonmodesofventilatorysupport.

HowtoChangetheMinuteVolumeDependingonVentilatorMode

MODE TOINCREASEVE TODECREASEVEVOLUMETARGETED CMVControl increaseVT decreaserate

CMVAssist/Control increaseVT decreaserateadddeadspace

SIMV increaserateaddpressuresupport

decreaserate

PRESSURETARGETED PCV increase P

increaseratedecrease Pdecreaserate

PSV increase P decrease P



BiPAP increase P(IPAPEPAP)

decrease P(IPAPEPAP)

APRV increase Pincreasereleasefrequency

decrease Pdecreasereleasefrequency

AdjustingOxygenation

WhereasPCO2variesmainlywithoneparameter(minutevolume),bloodoxygenlevelscanbeaffectedbytwotheFIO2andPEEPlevels.Ingeneral,thehighertheFIO2andPEEPlevel,thehigherthePO2/SaO2.

IfthePO2isexcessive(usually>100mmHg),youshouldlowertheparameter(FIO2orPEEP)thatispotentiallymostdangeroustothepatient.Forexample,ifapatienton10cmH2OPEEPand75%oxygenhasaPO2of150mmHg,thehighFIO2isofmostconcern(O2toxicity)andshouldbelowered.Ontheotherhand,ifapatienton18cmH2OPEEPand45%oxygenhasaPO2of150mmHg,thehighPEEPpressureisofmostconcern(barotrauma)andshouldbelowered.

IfthePaO2orSaO2islow(



10 0.5 18 1.0

10 0.6 20 1.0

10 0.7 22 1.0

12 0.7 24 1.0

14 0.7

Usingthistable,iflevelofoxygenationisbelowtheminimumtarget(PaO295%),theyrecommendmoving'down'1step.

Example:AdjustingOxygenationUsingARDSNetwork

Problem:Apatienton10cmH2OofPEEPwithanFIO2of0.60hasaPaO2of50andaSPO2of83%.Ehaydoyuourecommend?

Solution:SincethislevelofoxygenationisbelowtheARDSNetworkrecommendedminimumtarget,youwouldconsulttheabovetableandmovethepatientuptothenexthigherlevelofsupport,i.e.from10cmH2OofPEEPwithanFIO2of0.60to10cmH2OofPEEPwithanFIO2of0.70,andthenrepeatyourassessment.Ifthischangedoesnotbringthepatientabovetheminimumtargetlevels,thenextstepupwouldbetoraisethePEEPfrom10cmH2Oto12cmH2O.

Amorecomplexprocessusedtodetermine'best'PEEPrequiresaccesstocardiacoutputandmixedvenousbloodgasdata(viaapulmonaryarteryorSwanGanzcatheter).Inthismethod(oftencalleda'PEEPstudy'),increasinglevelsofPEEPareappliedwhilesimultaneousmeasuresofoxygenation,pulmonarymechanicsandhemodynamicsaremade(seeexamplebelow).Mostcriteriadefinethe'best'PEEPasthelowestpressurethatyieldssatisfactoryoxygendelivery/tissueoxygenationatasafeFIO2withminimalcardiovascularcompromise.Intheexamplebelow,cardiacoutputandO2deliveryreachtheirmaximums(4.5L/minand869mL/min,respectively)ataPEEPlevelof15cmH2O.However,thislevelofPEEPresultsinwhatmostwouldconsiderpotentiallydangerouspeakandplateaupressures(51and48cmH2Orepectively).Comparablecardiacoutput(4.2L/min),O2delivery(811mL/min)andvenousPO2s(37mmHg)appearpossibleatlowerPEEP,peakandplateaupressures,inthiscase5cmH2OPEEP,whichprobablyrepresentsthebeststartingpointforthispatient.



Abovetablefrom:Pilbeam,SP.MechanicalVentilation:PhysiologicalandClinicalApplications.3ed.St.Louis:Mosby1998.

Morerecently,thebestPEEPlevelhasbeenassociatedwiththelowestpressureneededtoexceedthelowerinflectionpoint(LIP)onthelung'spressurevolumecurve,i.e.,thepointatwhichtheslopeinitiallysteepensandcompliancerises(seefigurebelow).Sincethispoint,alsocalledPflex,conceptuallyrepresentsmaximumalveolarrecruitment,maintainingaslightlyhigherPEEPpressureshouldkeepalveoliopen,whileavoidingoverdistentionassociatedwithhigherpressures.



FromHicks,GHandScanlanCL.Initiatingandadjustingventilatorysupport.InScanlan,CL,Wilkins,RL&Stoller,JK(Eds.).EgansFundamentalsof

RespiratoryCare(7thEd.).St.Louis,MO:Mosby,1999.

Unfortunately,todeterminethebestPEEPlevelusingthismethodisrathercomplex,requiringgenerationofastaticpressurevolumecurveforthepatient.Donemanually,thisnormallyinvolvessedationorparalysisofthepatientandeitherincrementalorslowinflationusingacalibratedsupersyringe,whilemeasuringstaticairwaypressures.Analternativewaytogenerateastaticpressurevolumecurveistherapidendinspiratoryocclusionmaneuver.ThistechniqueinvolvesmanuallyclampingthecircuitbetweentheYpieceandpressure/volumesensoratendinspirationovera10steprangeoftidalvolumes(usuallyrbetween0.1and1.2L).Becausestaticpressurevolumecurvesaresometimesunobtainableusingeitherofthesemethods,theyarenotcommonlyusedinclinicalpractice.However,theyshouldbeunderstoodbyNBRCexamtakers.

AmorereliableandlessriskyalternativetodeterminebestPEEPviapulmonarymechanicsistoassessstaticcomplianceoverarangeofPEEPlevels(withtheVTorPheldconstant).Basedottheresultsofthistest,oneselectsthePEEPlevelatwhichthehighestcomplianceisobserved.Forexample,inthefollowingcase,onewouldselectaPEEPlevelof15cmH2O,correspondingtothethehighestobservedstaticcompliance(38mL/cmH2O)..

PEEP(cmH2O)

StaticCompliance(mL/cmH2O)

6 239 2412 2715 3818 3314 3116 31



Asimilarapproachtakesadvantaqgeofcurrentventilatorgraphicssoftware.Whenusedduringpressurelimitedventilation,itisreferredtoastheequalpressuremethod.Asshowninthefigurebelow,theequalpressuremethodinvolvesmeasuringthevolumeexhaledafterinflatingthelungatthesamedistendingpressureof20cmH2OduringthecourseofthepressuresupportventilationatdifferentlevelsofPEEP(equivalenttoBiPAP).Attheendofeachinflation,thepatientisdisconnectedfromtheventilator,andtheexhaledvolume(VE)isrecorded.TheVEobtainedwiththismaneuverateachlevelofPEEPisthencomparedwiththecorrespondingexhaledvolumeatZEEP,withthedifferencebeingthevolumerecruitedatthatlevelofPEEP.Intheexamplebelow,thevolumerecruitedaboveZEEP(a,b,c)increasesprogressivelyasthePEEPlevelisincreased,withmaximumrecruitment(c)at15cmH2O.

ImprovingPatientVentilatorSynchrony/DecreasingRespiratoryDistress

Inadditiontoadjustingventilatorstoassureadequateventilationandoxygenation,onemayneedtomanipulateventilatorsettingstoimprovepatientventilatorsynchronyand/ordecreaserespiratorydistress.Insomecases,suchaswhenstartingnoninvasiveventilation,alleviatingrespiratorydistressmaybetheprimarygoal.Themostcommonadjustmentshelpfulinimprovingpatientventilatorsynchronyand/oralleviatingrespiratorydistressaresummarizedinthefollowingtable.Intermsofdecreasingrespiratorydistress,oneshouldfirsteliminatedpatientrelatedproblemsasthecausebeforeproceedingwithventilatoradjustments.

Problem/Need ActionInadequateFIO2

TitrateFIO2toSpO2of9092%Check/confirmforadequateO2deliveryHb,cardiacoutput,etc

TriggerproblemsAdjusttriggerleveltominimum(12cmH2O12L/min)



CheckforandeliminateautoPEEP

FlowproblemsIfusingflowlimitedventilation:

IncreaseinspiratoryflowtoeliminateposttriggerpatienteffortUseventilatorthatprovidesflowcompensationSwitchtopressurelimitedventilation

Ifusingpressurelimitedventilation:adjustrisetimetoprovidegoodpressureplateauwithoutspikingIfusingPSV,adjustoffcyclingtoassureeffortfreeandcompleteexhalation

RateproblemsIfusingCMV,setratetoassureadequateexpiratorytimeandproperI:EratioIfusingCMV,considerSIMVIfusingSIMV,increasemandatoryrateuntilspontrateis

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Adjusting Ventilator Settings