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Mechanical Ventilation University of Florida Department of Anesthesiology Anesthesiology / Critical Care Clerkship
MechanicalVentilation
UniversityofFloridaDepartmentofAnesthesiology
Anesthesiology/CriticalCareClerkship
BasicPhysiology—Part1
• RespiratoryCycle• Oxygenation• Ventilation• Staticlungvolumes
RespiratoryCycle
• Inhalation&Exhalation• Ingress&Egress• But…• Oxygenation&Ventilation– ThisDivisionhastheGreatestutility• Breakingdownproblems• Implementinginterventions
– Anytimethereisuncertaintystarthere
Oxygenation
• Howcanweascertainsuccess?– ArterialBloodGas(Abg)
• PaO2• Revealsbloodthatenterstracheaanddiffusesintoblood• AlveolarGasEquation
– P(A)O2=FiO2(Patm – Ph2o)– PCO2/RQ» Alveolargas=Ingress(O2)– Egress(CO2)» RespiratoryQuotient(RQ)
• CO2producedperO2metabolized=0.8• Dependsondietandmetabolism
– (A-a)gradienttellsyouoxygenatingcapability
AlveolarGasEquation
Oxygenation
• CanwetellsuccesswithSpO2?– OnlyifpatientisonRoomAir– Forexample• 100%FiO2couldgiveyouanPAO2of760mmHgifperfect• SpO2is100%soalllooksgood
– ActualABGPaO2reveals95-100mmHg– Thus(A-a)Gradient= 660mmHg
• Thisisalimitationofpulseoximetryassessingoxygenation
Ventilation
–MinuteVentilation=TidalVolumexRespiratoryRate• Ve=VtxRR
• SuccessisrevealedbyCO2– HighCO2=Hypoventilation– LowCO2=Hyperventilation– Respiratoryrate?• NO…canbetachypnicandhypoventilating• Onlymovingdeadspacegas• Considerbreathingthroughalongsnorkel:
– Onlysmallamountofeachbreathreachinggasexchange
StaticLungVolumes
FunctionalResidualCapacity
• ResidualVolume+ExpiratoryReserveVolume• Equilibriumofopposingforces– ChestWall(expanding)+LungParenchyma(collapsing)
– Forcesareidiosyncratic• Chestwall:Obesity,pregnancy,paralysis,bodyposition• Lungparenchyma:emphysema,ARDS,
FunctionalResidualCapacity
FunctionalResidualCapacity
• Significance…– ConsiderFRCasaReservoir– Normallyfilledwith21%oxygenand78%nitrogen– CanDe-Nitrogenate(i.e.Pre-oxygenate)• Nowfilledwith100%oxygen• Createstimeforairwayinstrumentation• MaymanipulateFRCtoimprovesafetyprofile
ClosingCapacity
ClosingCapacity(CC)
• CC=ResidualVolume+ClosingVolume
• AlveoliCollapseattheCC– LawofLaplace– Pressure=2xt/r• Surfacetension(t)reducedbysurfactant• Radiusinverselyimpactsairwayclosure
Laplace’sLaw
ClosingCapacity(CC)
• CCnormallylessthenFRCsoalveoliremainopen
• Dependentof:– Age– Bodyposition– pathology
ClosingCapacity(CC)
LungVolumesandCapacities
BasicPhysiology—Part2
• IndicationsforMechanicalVentilation– VentilationFailure– OxygenationFailure
– Asomewhatartificialdivision• Onetypeoffailuremayinvolvingboth• Buthelpstocompartmentalizeandremember
VentilationFailure
• NeurologicDisease
• MuscularDisease
• AnatomicDisease
Neurologicpathology
• CentralPathology
– Sedation• Benzodiazepines• Opiates
– Stroke
– Trauma• TraumaticBrainInjury
NeurologicPathology
• Spinalcordpathology
– Trauma
– Virus
– Stroke
NeurologicPathology
• PeripheralNervepathology
• Neuromuscularrelaxants
• Guillain-Barresyndrome
AnatomicPathology
• ChestWallpathology– Flailchest– CongenitalDiaphragmatichernia– Kyphoscoliosis
• Pleuralinjury– Pneumothorax– Hemothorax– Pleuraleffusions
AnatomicPathology
• AirwayPathology– Burn
– Epiglottis
– Bronchospasm
– Foreignbody
OxygenationFailure
• GasExchangeProblem
– V/QMismatch
– Diffusionabnormality
– VO2/DO2imbalance
– FiO2
Ventilation/PerfusionMismatch
• Matchingisaspectrum
– Deadspace
– V/Q=1:1
– Shunt
DeadSpace
Deadspaceequation
Shunt
Shunt
V/QMatchingSpectrum
V/QMatching:WestZones
OxygenationFailure
• GasExchangeProblem
– V/QMismatch
– Diffusionabnormality
– VO2/DO2imbalance
– FiO2
DiffusionAbnormality
• GasExchangeProblem• EvaluatedwithDLCO• Pathology:– Emphysema– IdiopathicPulmonaryfibrosis– Sarcoidosis– PulmonaryHypertension– InterstitialPneumonitis– PulmonaryVasculitis
OxygenationFailure
• GasExchangeProblem
– V/QMismatch
– Diffusionabnormality
– VO2/DO2imbalance
– FiO2
VO2/DO2Imbalance
• OxygenconsumptionexceedsOxygenDelivery• Causes:– DecreasesinCardiacOutput• Preload• Heartrate• Contractility
– IncreasesinOxygenExtraction• Hypermetabolicstates• Sepsis
OxygenationFailure
• GasExchangeProblem
– V/QMismatch
– Diffusionabnormality
– VO2/DO2imbalance
– FiO2
FiO2
• Inadequateoxygenconcentration
– Requireselevatedconcentration
– ChangeBarometricpressure
– Oxygendisplacedbyothergas
MechanicalVentilation
• VolumeControlled(ACV)
• PressureControlled(PCV)
• SIMV
• PressureSupport(PS)
VolumeControlledVentilation
• 2forms:1. Assist-ControlVentilation2. ControlledVentilation• ControlledVentilation– Providerdialsindesiredtidalvolume&respiratoryrate
– Settingsdeliveredregardlessofeffort(noadditionalbreathspermitted)
– Gasflowstopatienttodeliverpresettidalvolume• MayoccurdespitePeakAirwayPressures
VolumeControlledVentilation
• Assist-ControlledVentilation• Vt andRRdeterminedbyprovider• Anypatientrespiratoryeffort– triggerspresettidalvolume– Defaultstoinitialsettingsifnoeffortattempted– Providerselectstriggerthresholdsensitivity
ControlledVentilation
ControlledVentilation
• Previouscartoon(tobereviewedonnextslide)
– Circlesrepresentareasofinitiatedbreaths(assisted)
– NoEffortfollows2assistedbreaths
– EachBreathexactlysamevolumedelivered
ControlledVentilation
VolumeControlled
• ExcessiveBreathscanleadto“breath-stacking”– AlsocalledAuto-PEEP– Canobstructpreloadorcausepneumothorax– CausesincreaseWorkofBreathing– RiskofsignificantPatient-ventilatorDysynchrony
Breath-StackingNotEnoughtimeallowedforcompleteexhalation.Eachinspiredbreath”Stack”ontopofthepreviousbreath.Significantriskofhemodynamicchangesandbarotrauma(Ptx).
Pressure-ControlledVentilation
• ProviderdialsinRRandfixedPressuretodeliverbreath
• VolumeofeachBreathvariesduetodifferentpulmonarymechanics:– Differentfrompersontoperson– Differentinsamepersonbutdifferentscenarios
• Patientposition,musclerelaxation,abdominalpressures,…– Breathdelivery
• Breathfillingpressuremaintainedatsamelevel• Deceleratingflowwaveallowsforlargetidalvolumes
Pressure-Control
• ComparetoVolume-Controlled:
• 1.Constantpressureleveldelivered
• 2.Volumeaugmentation
• 3.Deceleratinggasflow
Pressure-ControlledVentilation
• Advantages
– Lesslikelihoodofbarotrauma
– GreaterPatient-Ventilatorsynchrony
– LargertidalvolumeswithsmallerDrivingpressure
SIMV
• Synchronized-IntermittentMandatoryVentilation• BreathsaredeliveredinapresetRRandvolume• Patientcanbreathindependentlybetweenbreaths– Noadditionaleffortisprovidedforextrabreaths– Patientmaintainssomeindependenceinventilation– Usedtoweanpatientfromventilator
• Progressivelydialbackmandatorybreaths• Allowprogressivelymorespontaneousventilation
SIMVOvals revealunsupportedadditionalbreaths.Squares showpresetrespiratoryrateandtidalvolumes.ToprevealsControlledventilationwithoutadditionaltriggering.MiddleshowsadditionaltriggeringleadingtorapidRR.
SIMVCanaugmentspontaneousbreathswithpressuresupport.CanaddPEEPtothesystemaswelltoassistoxygenation.
PressureSupportVentilation
• PatientinitiatesEverybreath• Ventilatordeliverssupportwithpresetpressurevalue
• Patientregulates– Ownrespiratoryrate– Owntidalvolume
• ChangesinPulmonaryMechanicsorpatienteffort– AffectstidalvolumeandRR
PressureSupportVentilation
IncreasingeffortorPressuresupportwillincreasedeliveredvolume.DecreasingeffortorPressuresupportwilldecreasedeliveredvolume.
PEEP—OxygenationTool• PositiveEnd-ExpiratoryPressure• Pressureinthelungs(aboveAtm)thatexistsinthelungsafterexpiration
• CoupledwithFiO2fortoolstoimproveoxygenation
• Amountisdialedinbyprovidertomitigateatelectasis– ImprovesV/Qmatching– Canreducetherapidopeningandclosingofalveoli– ARDSnetprotocoltohelpoxygenationinnextslide
FiO2&PEEPtotreatARDS
MonitoringLungMechanics
• ProximalAirwayPressures– PeakInspiratoryPressure– PlateauPressure
• ThoracicCompliance– ChestWallcompliance– Lungcompliance
ProximalAirwayPressures(peak)
• Peakpressure– Pressureinlungsgeneratedduringinspiration
– Combinationof• Resistanceinairway
• Elastanceoflungs
– EndswithEnd-inspiration
PeakAirwayPressure
• Increasedairwayresistance– Airwayobstruction– Bronchospasm– Circuitkinking– Endotrachealtubeobstruction
– anaphylaxis
ProximalAirwayPressures(plateau)
• Plateaupressure• Pressureinthe
airwaywithoutanyairmovement– Duringinspiratory
pause• Determinedby
Complianceoflung– Inversely
proportionaltoelastance
PlateauPressure
• IncreasedPlateaupressure– Pneumothorax– Pneumonia– ARDS– Auto-PEEP– Obesity– Pulmonaryfibrosis
ThoracicCompliance
• =ChangeinVolume/changeinPressure(V/P)• Inverselyproportionaltoelastance• Compliancecaninvolvestaticordynamicmeasurement– Cdyn=Vt/(PIP–PEEP)
• Compliancemeasuredduringgasflow• PIP=peakinspiratorypressure
– Cstat=Vt/(Pplt– PEEP)• Compliancemeasuredwithoutgasflow• Pplt=Plateaupressure
ThoracicCompliance
Lungcompliance• Involveslungparenchyma• Fibrosisvs.Emphysema• Bloodandfluidvolumes• Surfactantdeficiency• pneumothorax
ChestWallCompliance• Involvesmusclesandfat• Morbidlyobese• Largebreast• Musclerelaxation• Positioning
THEEND
