basic-mechanical-ventilation-1203112540131385-3.ppt

BasicsofMechanicalVentilation,AlainBroccard,MDJohnMarini,MDUniversityofMinnesotaRegionsHospitalStPaul,MN,Objectives,Tounderstand:HowpositivepressureventilationhelpsReducetheworkofbreathingRestoreadequategasexchangeThebasicsofInvasivepositivepressureventilation(IPPV)Noninvasivepositivepressureventilation(NIPPV)TheprinciplesofbedsidemonitoringPressureandvolumealarmsFlowandpressuretimecurves,PhysiopathologyofRespiratoryFailure30,ResistanceAW,ComplianceR,DVA/Q,WorkofBreathing,Fatigue,Hypercapnia,Neuromusculardisorders,VE,VO2VCO2pH,Hypoxemia,AW=Airrway;R=respiratroysystem;VE=minuteventilation,VO2=Oxygenconsumption,VCO2=carbondioxideproduction,IndicationsandRationaleforInitiatingIPPV,Unprotectedandunstableairways(e.g,coma)IntubationandIPPVallowsto-Securetheairways-Reducetheriskofaspiration-MaintainadequatealveolarventilationHypercapnicrespiratoryacidosisIPPVandNIPPV-Reducetheworkofbreathingandthuspreventsrespiratorymusclefatigueorspeedsrecoverywhenfatigueisalreadypresent-Maintainadequatealveolarventilation(preventorlimitrespiratoryacidosisasneeded)HypoxicrespiratoryfailureIPPVandNIPPVhelpcorrecthypoxemiaasitallowsto-DeliverahighFiO2(100%ifneededduringIPPV)-ReduceshuntbymaintainingfloodedorcollapsedalveoliopenOthersIntubationtofacilitateprocedure(bronchoscopy),bronchialsuctioning,ImportantPitfallsandProblemsAssociatedwithPPV,PotentialdetrimentaleffectsassociatedwithPPVHeartandcirculation-Reducedvenousreturnandafterload-HypotensionandreducedcardiacoutputLungs-Barotrauma-Ventilator-inducedlunginjury-AirtrappingGasexchange-Mayincreasedeadspace(compressionofcapillaries)-Shunt(e.g.,unilaterallungdisease-theincreaseinvascularresistanceinthenormallungassociatedwithPPVtendstoredirectbloodflowintheabnormallung),DecreasedpreloadPositivealveolarpressurelungvolumecompressionoftheheartbytheinflatedlungstheintramuralpressureoftheheartcavitiesrises(e.g.,RAP)venousreturndecreasespreloadisreducedstrokevolumedecreasescardiacoutputandbloodpressuremaydrop.Thiscanbeminimizedwithi.v.fluid,whichhelpsrestoreadequatevenousreturnandpreload.Patientswhoareverysensitivetochangeinpreloadconditions(e.g.,presenceofhypovolemia,tamponade,PE,severeairtrapping)areparticularlypronetohypotensionwhenPPVisinitiated.ReducedafterloadLungexpansionincreasesextramuralpressure(whichhelpspumpbloodoutofthethorax)andtherebyreducesLVafterload.Whenthecardiacperformanceismainlydeterminedbychangesinafterloadthaninpreloadconditions(e.g.,hypervolemicpatientwithsystolicheartfailure),PPVmaybeassociatedwithanimprovedstrokevolume.PPVisveryhelpfulinpatientswithcardiogenicpulmonaryedema,asithelpstoreducepreload(lungcongestion)andafterload.Asaresultstrokevolumetendstoincrease.,ImportantEffectsofPPVonHemodynamics,Generallyspeaking,theeffectsofPPVonthecardiacchambertransmuralpressuresvaryinparallelwith:Airwaypressure(e.g.,airwaypressurevenousreturn)Lungcompliance(e.g.,compliancevenousreturn)Chestwallstiffness(e.g.,intheobesepatients,agivenchangeinairwaypressureandlungvolumewillhavemoreimpactonthehemodynamics,giventhatthepressurerisearoundtheheartisgoingtobehigherthaninpatientswithcompliantchestwall,everythingelsebeingequal),EffectsofPPVonHemodynamics,Marini,Wheeler.CritCareMed.TheEssentials.1997.,AlveolarPressureandGasExchange,Adaptedfrom:Marini,etal.CritCareMed.1992.,Notethatasairwaypressureincreasesaboveacertainlevel(e.g.,highPEEPpositiveend-expiratorypressure):OxygentransportstarttodeclinedespitetherisingPaO2ascardiacoutputstartsfalling.Deadspacealsotendstoincreaseduetocompressionofalveolarcapillariesbyhighalveolarpressure,creatingventilatedbutpoorlyperfusedalveolarunits.,OtherPotentiallyAdverseEffectsofMechanicalVentilation,Excessiveairwaypressureandtidalvolumecanleadtolunginjury(ventilatorinducedlunginjury)andcontributetoincreasedmortality.,TheAcuteRespiratoryDistressSyndromeNetwork.NEnglJMed.2000;342:1301-1308.,Lungsofdogsventilatedforafewhourswithlargetidalvolumedemonstrateextensivehemorrhagicinjury.,OtherPotentiallyAdverseEffectsofMechanicalVentilation,Inthesettingofobstructivephysiology(e.g.,asthmaandCOPD),adjustmentofthetidalvolumeandrateminuteventilationtorestoreanormalpHandPaCO2canleadtoairtrapping,pneumothoraces,andseverehypotension.,Tuxenetal.AmRevRespDis1987;136:872.,UpperPanel:Whenairwayresistancesarehigh,thereisforafewbreathmoreairgoinginthancomingoutofthelungs(dynamichyperinflation).Subsequently,anewequilibriumisreached.Theamountofairtrappedcanbeestimatedinapassivepatientbydiscontinuingventilationandcollectingtheexpiredvolume(lowerpanel).Thevolumeoftrappedgasislargelydeterminedby:TheseverityofairwayobstructionTheventilatorsettings(seeadvancecoursefordetails).Ofallthesettings,theimposedminutesventilation(setratexVT)andthemostimportantone.ThetimeleftbetweentidalbreathforexhalationislessimportantifalowVTandVEaretargeted.,PositivePressureVentilation:TheEquationofMotion,Inapassivesubject,airwaypressurerepresentstheentirepressure(P)appliedacrosstherespiratorysystem.Theworkrequiredtodeliveratidalbreath(Wb)=tidalvolume(VT)xairwaypressureThepressure(P)associatedwiththedeliveryofatidalbreathisdefinedbythesimplifiedequationofmotionoftherespiratorysystem(lungsoneneedstoovercometheresistanceofthestrawandtheelasticityoftheballoon.,IntrinsicPEEPandWorkofBreathing,Volume,VT,VT,FRC,Pressure,PEEPi,DynamicHyperinflation,PEEPi=intrinsicorautoPEEP;greentriangle=tidalelasticwork;redloop=flowresistivework;bluerectangle=workexpendedinoffsettingintrinsicPEEP(anexpiratorydriver)duringinflation,Whenpresent,intrinsicPEEPcontributestotheworkofbreakingandcanbeoffsetbyapplyingexternalPEEP.,+,+,+,+,ThePressureandWorkofBreathingcanbeEntirelyProvidedbytheVentilator(PassivePatient),Ventilator,WorkofBreathingUnderPassiveConditions,Whenthelungisinflatedbyconstantflow,timeandvolumearelinearlyrelated.Therefore,themonitoredairwaypressuretracing(Paw)reflectsthepressure-volumeworkareaduringinspiration.Apressure-sensingesophagealballoonreflectstheaveragepressurechangeinthepleuralspaceandthereforetheworkofchestwallexpansion.,TheWorkofBreathingcanbeSharedBetweentheVentilatorandthePatient,PAW,PES,patient,machine,time,ACmode,Theventilatorgeneratespositivepressurewithintheairwayandthepatientsinspiratorymusclesgeneratenegativepressureinthepleuralspace.,Paw=Airwaypressure,Pes=esophagealpressure,RelationshipBetweentheSetPressureSupportLevelandthePatientsBreathingEffort,Carreyetal.Chest.1990;97:150.,ThechangesinPes(esophagealpressure)andinthediaphragmaticactivity(EMG)associatedwiththeincreaseinthelevelofmaskpressure(Pmask=pressuresupport)indicatetransferoftheworkofbreathingfromthepatienttotheventilator.,PartitioningoftheWorkloadBetweentheVentilatorandthePatient,Howtheworkofbreathingpartitionsbetweenthepatientandtheventilatordependson:Modeofventilation(e.g.,inassistcontrolmostoftheworkisusuallydonebytheventilator)PatienteffortandsynchronywiththemodeofventilationSpecificsettingsofagivenmode(e.g.,levelofpressureinPSandsetrateinSIMV),CommonModesofVentilation,Volumetargetedventilation(flowcontrolled,volumecycled)ACPressuretargetedventilationPCV(pressurecontrolled,timecycled)PSCombinationmodesSIMVwithPSandeithervolumeorpressure-targetedmandatorycycles,PressureandVolumeTargetedVentilation,Pressureandvolumetargetedventilationobeythesameprinciplessetbytheequationofmotion.Pressureandvolumetargetedventilationobeythesameprinciplessetbytheequationofmotion.Inpressure-targetedventilation:anairwaypressuretargetandinspiratorytimeareset,whileflowandtidalvolumebecomethedependentvariables.Involumetargetedventilation(flow-controlled,volumecycled),atargetvolumeandflow(orinspiratorytimeincertainventilator)arepresetandpressureandinspiratorytime(orflowintheventilatorwhereinspiratorytimeispreset)becomethedependentvariables.Thetidalvolumeistheintegraloftheflowduringinspiration=areaunderthecurveoftheflowtimecurveduringinspiration(seenextslide).,PressureandVolumeTargetedVentilation,Marini,Wheeler.CritCareMed.TheEssentials.1997.,VT,Assist-control,SetvariablesVolume,TIorflowrate,frequency,flowprofile(constantordecel)PEEPandFIO2MandatorybreathsVentilatordeliverspresetvolumeandpresetflowrateatasetback-uprateSpontaneousbreathsAdditionalcyclescanbetriggeredbythepatientbutotherwiseareidenticaltothemandatorybreath.,SIMV,KeysetvariablesTargetedvolume(orpressuretarget),flowrate(orinspiratorytime,Ti),mandatedfrequencyPEEP,FIO2,pressuresupportMandatorybreathsVentilatordeliversafixednumberofcycleswithapresetvolumeatpresetflowrate.Alternatively,apresetpressureisappliedforaspecifiedTiSpontaneousbreathsUnrestrictednumber,aidedbytheselectedlevelofpressuresupport,PeakAlveolarandTranspulmonaryPressures,P(t)=VT/CR+FlowxRR+PEEPtot,meanPaw,ExternalPEEP,IntrinsicPEEP,Plateaupressure,PeakAirwayPressure,Palveolar,Ppleural,Ptranspulmonary=Palveolar-Ppleural,Pplat=MaximumPalveolar,AlveolarPressure,Transpulmonarypressureisakeydeterminantofalveolardistension.,MonitoringPressureinVolumeTargetedVentilation,Plateaupressuretracksthehighesttidalalveolarpressure,akeydeterminantofalveolardistension.Plateaupressure(Pplat)is,however,onlyasurrogateofpeakalveolardistendingpressure(transpulmonarypressure=Pplatpleuralpressure).e.g.,inapatientwithalowchestwallcompliance,agivenPplatistypicallyassociatedwithahigherpleuralpressurebutlessalveolardistension(smallertranspulmonarypressure)thaninapatientwithacompliantchestwall.ThedifferencebetweenthePpeakandPplattrackstheresistivepressure,asdictatedbytheequationofmotion.Duringaninspiratorypause,flowbecomeszero,theresistivepressureiseliminatedandtheairwaypressuredropsfromitspeaktotheplateaupressure.,AirwayResistanceandRespiratorySystemCompliance,Underconditionsofconstantflow,thedifferencebetweenpeakandplateauairwaypressuresdrivesend-inspiratoryflow.Thequotientofthisdifferenceandtheflowsettinggivesameasureofairwayresistanceatendinspiration.Whenairflowisstoppedinapassivelyventilatedpatientbyocclusionoftheexpiratorycircuitvalveatendinspiration(plateaupressure)andendexpiration(totalPEEP),thepressureneededtoovercometheelasticrecoilofthelungsandchestwallduringdeliveryofthetidalvolumeisgivenasthedifferenceinthesevalues.Dividingthedeliveredtidalvolumebythisdifferencequantifiestherespiratorysystemcompliance.,MeanAirwayPressure,Althoughmeasuredintheconnectingcircuit,meanairwaypressureisavalidmeasureofthepressureappliedacrossthelungandchestwall,averagedacrossbothphasesoftheventilatorycycle-butonlyunderpassiveconditions.Changesinmeanairwaypressureareproducedbychangesinminuteventilation,PEEP,andI:Eratio.Meanairwaypressuresaffectpleuralpressureandlungdistention.Therefore,changesinmeanairwaypressureduringpassiveinflationmayinfluence:ArterialoxygenationCardiacoutput,PressureControlledVentilation,Keysetvariables:Pressure,TI,andfrequencyPEEPandFIO2MandatorybreathsVentilatorgeneratesapredeterminedpressureforapresettimeSpontaneousbreathsPCV-ACmode:sameasmandatorybreathsPCV-SIMVmode:unsupportedorPSImportantcaveatItisimportanttounderstandthatinpressure-controlledventilationtherelationbetweenthesetrateandminuteventilationiscomplex.Aboveacertainfrequency(e.g.,whenintrinsicPEEPiscreatedduetoareducedexpiratorytime),thedrivingpressure(setPCpressurePEEPtotal)startstodrop-andsodoesthedeliveredtidalvolume.Apneumothoraxorotheradversechangeinthemechanicsoftherespiratorysystemwillnottriggerahighalarmpressurebutalowtidalvolumealarminstead.,PressureSupport,Pressure=setvariable.Mandatorybreaths:none.SpontaneousbreathsVentilatorprovidesapresetpressureassist,whichterminateswhenflowdropstoaspecifiedfraction(typically25%)ofitsmaximum.Patienteffortdeterminessizeofbreathandflowrate.,PCV:KeyParametertoMonitorisVT,Changeinmechanicsairwayresistance:.e.g.,bronchospasmrespiratorysystemcompliance.e.g,pulmonaryedema,pneumothoraxAutoPEEPexpiratoryresistanceexpiratorytimee.g.,rateInspiratorytimee.g.,rateifI:Eratioconstant,WhatCausesaDecreasedVTDuringPCV?,Auto-PEEP(IntrinsicPEEP,PEEPi),Marini,Wheeler.CritCareMed.TheEssentials.1997.,NotethatAutoPEEPisnotequivalenttoairtrapping.Activeexpiratorymusclecontractionisanoftenunderappreciatedcontributor(leftpanel)topositivepressureattheendofexpiration,SuspectingandMeasuringAutoPEEP,SuspectAutoPEEPifflowattheendofexpirationdoesnotreturntothezerobaseline.,AutoPEEPiscommonlymeasuredbyperformingapauseattheendofexpiration.Inapassivepatient,flowinterruptionisassociatedwithpressureequilibrationthroughtheentiresystem.Insuchconditions,proximalairwaypressuretracksthemeanalveolarpressurecausedbydynamichyperinflation.,Endexpiratorypause,InterimSummaryandKeyPoints,Mechanicalventilationhelpstoimproverespiratorygasexchangeandcanprovidecompleteorpartialworkofbreathingassistance.Safeandeffectiveimplementationofmechanicalventilationrequiresunderstandingtheequationofmotionfortherespiratorysystem.Monitoringdynamicandstaticairwaypressuresandflowsyieldsvitalinformationforinterpretingthemechanicsoftherespiratorysystemandforadjustingmachinesettingsforoptimalperformance.,MechanicalVentilationandGasExchange,RespiratoryacidosisHypoxemia,HypercapnicAcidosis,DeterminantsofPaCO2PACO2=0.863xVCO2/VAVA=VE(1-VD/VT)CausesofhypercapniaInadequateminuteventilation(VE)Deadspaceventilation(VD/VT)CO2production(VCO2)CorrectivemeasuresforrespiratoryacidosisWhenappropriate,increasetheminuteventilation(e.g.,therateorthetidalvolume),VD=deadspaceVA=alveolarventilationVE=minuteventilationVT=tidalvolumeVCO2=CO2production,MechanismforArterialHypoxemia,ReducedFiO2(e.g.,toxicfumes,altitude)HypoventilationImpaireddiffusionVentilation/perfusion(VA/Q)mismatchingHighVA/QLowVA/QShuntingIfsignificantshuntingispresent,theFIO2requirementistypically60%,MinuteVentilationandGasExchange,TherelationshipbetweenPaCO2andminuteventilationisnotlinear.Inpatientswithhypoventilation,smallchangesinminuteventilationmayhavelargeeffectonthePaCO2.,Shunting:EffectsofFiO2onPaO2,Notethatasthe%ofshuntrises,increasingtheFIO2haslessandlessimpactonPaO2.Undersuchconditions,reducingtheshuntfractioniskeytotheabilitytoimprovegasexchange,andthistypicallyrequiresPPVandPEEP.,KeyFactorsDeterminingtheEffectsofShuntingonArterialO2Saturation,SvO2Shuntfraction,NotethatinpatientswithshuntphysiologyareductioninarterialO2saturationmaybeduetoachangein:theintraparenchymalshuntfraction(e.g.,atelectasis)ortheSvO2(e.g.,dropincardiacoutput).AsuddendropinO2saturationinpatientswithARDSwarrantsathoroughassessmentoftherespiratoryandcardiovascularsystem.,50%,50%,MeanAirwayandAlveolarPressures,Adaptedfrom:Ravenscraftetal.CritCareMed.1992.,Innormallungs,theinspiratoryresistivepressureissimilartotheexpiratoryresistivepressure(lightshadedareaundertheairwaypressure-timecurve)sothatmeanairwaypressure(Paw)canbeusedtotrackmeanalveolarpressure(Palv).,PEEP,RegionalLungVolume,andShunting,Lungregionswithshunttendtodistributepreferentiallyinthedependentregions.Tidalventilationhelpsopencollapsedregions,andPEEPhelpstomaintainthoseregionsopenthroughoutexpirationandtoreduceshunt.NotethatlevelofPEEPrequiredtoachievesuchvariesalongthegravitationalaxis.,EffectofPEEP-inducedAlveolarRecruitmentonPaO2,Malbuissonetal.AJRCCM.2001:163:1444-1450.,ApproachtoMV,IsMVindicated?,Conservativetreatmentandperiodicreassessment,NO,YES,ContraindicationtoNIPPV?,NIPPV,NO,Success?,InvasiveMV,NO,NO,YES,NoninvasiveVentilation,VentilatorysupportprovidedwithoutinvasiveairwaycontrolNotracheostomyNoETT,KeyDifferencesBetweenNIPPVandIPPV,AllowsthepatientstomaintainnormalfunctionsSpeechEatingHelpsavoidtherisksandcomplicationsrelatedto:IntubationSedationLessventilator-associatedpneumonia,LessairwaypressureistoleratedDoesnotprotectagainstaspirationNoaccesstoairwayforsuctioning,AdvantagesofNIPPV,DisadvantagesofNIPPV,ClinicalUseofNIPPVinIntensiveCare,DecompensatedCOPD(HypercapnicRespiratoryFailure)CardiogenicpulmonaryedemaHypoxicrespiratoryfailureOtherpossibleindicationsWeaning(post-extubation)ObesityhypoventilationsyndromePatientsdeemednottobeintubatedPost-surgeryAsthma,Adaptedfrom:AmJRespirCritCareMed.2001;163:283-291.,ContraindicationstoNIPPV,CardiacorrespiratoryarrestNonrespiratoryorganfailureSevereencephalopathy(e.g.,GCS90%IncreaseIPAPgraduallyupto20cmH2O(astolerated)to:-alleviatedyspnea-decreaserespiratoryrate-increasetidalvolume-establishpatient-ventilatorsynchrony,SuccessandFailureCriteriaforNPPV,ImprovementsinpHandPCO2occurringwithin2hourspredicttheeventualsuccessofNPPV.Ifstabilizationorimprovementhasnotbeenachievedduringthistimeperiod,thepatientshouldbeconsideredanNPPVfailureandintubationmustbestronglyconsidered.OthercriteriaforafailedNPPVtrialinclude:worsenedencephalopathyoragitation,inabilitytoclearsecretions,inabilitytotolerateanyavailablemask,hemodynamicinstability,worsenedoxygenation.,Conclusions,Agoodunderstandingofrespiratoryphysiologyisrequiredforthejudiciousmechanicalventilation.Theequationofmotionisthesinglemostusefulguidetounderstandingmechanicalventilation.Unlesscontraindicated,NIPPVisbecomingthefirstmodalitytotryinmanysettings.MonitoringkeyvariablessuchasPplateauandauto-PEEPismandatorytosafeandeffectivepractice.,PostModuleTesting:Case1,29-year-oldpatient(weight120kg,height170cm)ARDSsecondarytobilateralpneumoniaVentilatorsettings:ACwithVT800mlandback-uprate10/min,PEEP5cmH2O,FIO280%Measuredvariables:rate25,VE=20l/min,Ppeak40cmH2O,Pplat35cmH2OABG:pH7.40,PaO255mmHg,PaCO238mmHg,O2saturation85%,Question1,Whatmechanismbestexplainsthepatientshypoxemia?V/QmismatchShuntAbnormaldiffusionInadequateoxygendeliveryandhightissueextraction,Answer1,Ifinaventilatedpatient,FIO260%isneeded,shuntiscertainlythemaincauseforthehypoxemia(correctresponse:2).Asarule,increasingtheFIO2willcompensateforVA/Qmismatchingbutnotforshunt.WhenVA/Qmismatchingispresent,hypoxemiatypicallycorrectswithanFIO230cmH2O.Remember,however,thattheactualdistendingalveolarpressureisthetranspulmonarypressure(Pplat-Ppleural).HigherPplatcanbeacceptedinapatientwithlowchestwallcompliance,aslessalveolardistensionwillbepresentforthesamePplat,everythingelsebeingequal.,Case2,67-year-oldfemale(weight50kg)withsevereemphysemaisadmittedforCOPDdecompensation.ShefailedNIPPVandrequiredsedation,paralysis,andintubation.S