Page 5/1/2009 Corrado P. Marini, MD Director of Surgical Education Geisinger Health System Danville, Pennsylvania AtlantiCare Regional Medical Center May 5, 2009 MECHANICAL VENTILATION Modes of Ventilatory Support HISTORY OF MECHANICAL VENTILATION “But that life may be restored… an opening must be attempted in…the trachea, in which a tube or reed should be put; you will then blow into this, so that the lung may rise again…And as I do this, and take care that the lung is inflated in intervals, the motion of the heart and arteries does not stop…” Andreas Wesele Vesalius, 1543 HISTORY OF MECHANICAL VENTILATION • The 1947-1948 polio epidemic resulted in breakthroughs in the treatment of patients with respiratory paralysis • Endotracheal intubation and mechanical ventilation was pioneered in Denmark HISTORY OF MECHANICAL VENTILATION • Stephen Hales – Used a manual bellows to inflate the lungs (1743) 9first mechanical ventilator – Treatise on Ventilators (1751) – Also identified 9Blood pressure 9Treatment for bladder stones 9Carbon dioxide
Transcript
Page 5/1/2009
Corrado P. Marini, MDDirector of Surgical Education
Geisinger Health SystemDanville, Pennsylvania
AtlantiCare Regional Medical Center May 5, 2009
MECHANICAL VENTILATIONModes of Ventilatory Support
HISTORY OF MECHANICAL VENTILATION
“But that life may be restored… an opening must be attempted in…the trachea, in which a tube or reed should be put; you will then blow into this, so that the lung may rise again…And as I do this, and take care that the lung is inflated in intervals, the motion of the heart and arteries does not stop…”
Andreas Wesele Vesalius, 1543
HISTORY OF MECHANICAL VENTILATION
• The 1947-1948 polio epidemic resulted in breakthroughs in the treatment of patients with respiratory paralysis
• Endotracheal intubation and mechanical ventilation was pioneered in Denmark
HISTORY OF MECHANICAL VENTILATION
• Stephen Hales– Used a manual bellows to
inflate the lungs (1743)first mechanical ventilator
– Treatise on Ventilators (1751)– Also identified
Blood pressureTreatment for bladder stonesCarbon dioxide
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MODERN VENTILATORS
• Mechanical ventilation evolved significantly in the 1970’s and 80’s with the introduction of microprocessors
• Our understanding of acute lung injury also evolved greatly
The goal of mechanical ventilation is to optimize pulmonary gas exchange
Existing Controversies– Controlled vs. spontaneous ventilation– Large vs. small tidal volume– PEEP vs. no PEEP– Recruitment vs. no recruitment maneuvers– Wet vs. dry lungs– Invasive vs. noninvasive ventilation
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Goals of Ventilator Modes
1. Maintain adequate oxygenation
2. Maintain adequate ventilation
3. Reduce work of breathing
4. Improve patient comfort
MECHANICAL VENTILATIONSupport of Adequate Oxygenation
• Allow a normal, spontaneous breathing pattern whenever possible
• Maintain a PaCO2 between 35 - 45 mmHg
• Maintain a PaO2 sufficient to meet cellular oxygen demands but avoid oxygen toxicity
• Avoid respiratory muscle fatigue and atrophy
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MECHANICAL VENTILATIONBreath Types
• There are two basic breath types– Spontaneous or demand
Initiated by the patient– Ventilator or mandatory
Initiated by the ventilator (time triggered)
• Breaths are defined by three variables– Trigger: initiates the inspiratory phase – Limit: maximal set inspiratory pressure or flow – Cycling (the factor that terminates the I cycle)
MECHANICAL VENTILATIONBreath Types
• Mandatory– Ventilator does the work– Ventilator controls start and stop
• Spontaneous– Patient takes on work– Patient controls start and stop
The Control VariableInspiratory Breath Delivery
• Flow (volume) controlled- pressure may vary
• Pressure controlled- flow and volume may vary
• Time controlled (HFOV)- pressure, flow, volume may vary
Trigger VariableStart of a Breath
• Time - control ventilation
• Pressure - patient assisted
• Flow - patient assisted
• Volume - patient assisted
• Manual - operator control
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Inspiratory - delivery limits
Maximum value that can be reached but will not end the breath-–Volume–Flow–Pressure
End of Inspiratory PhaseCycling mechanisms
The phase variable used to terminate inspiration–Volume–Pressure–Flow–Time
TRIGGER VARIABLE MODES OF VENTILATION
• Volume controlled– Controlled by inspiratory flow– Limited by a preset volume or maximal inspiratory
pressure– Cycled by volume or time
• Pressure controlled– Controlled by pressure (inspiratory + PEEP)– Limited by pressure (inspiratory + PEEP)– Cycled by time or flow
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Inspiration Expiration
120
1 2
3
-3
0
20
0 21
20
0 1 2
3
-3
0
20
0 2
Inspiration Expiration
Volume/Flow Control Volume/Flow Control Pressure ControlPressure Control
Time (s) Time (s)
PawPaw
Pressure
Volume
Flow
0 00
0
If compliance decreases the pressure increases to maintain the same Vt
Volume Control Breath TypesVolume Control Breath Types
11 22 33 44 55 66
SECSEC
11 22 33 44 55 66
PPawawcmHcmH2200
6060
--2020
120120
120120
SECSEC
INSPINSP
EXHEXH
FlowFlowL/minL/min
BASIC MODES OF SUPPORT
• Demand breaths– Spontaneous breathing– Pressure Support Ventilation (PSV)
• Created as a technique to reduce ventilator imposed work of breathing
• Patient determines rate, volume, and flow– widely used to improve patient comfort
• Advantages over traditional modes– improves patient - ventilator synchrony– reduces work of breathing– decreases dead-space to tidal volume ratio– prevents respiratory muscle fatigue
• Commonly used in conjunction with SIMV
• Uses high gas flow (up to 250 L/min)
• Reduces work of breathing by overcoming the resistance of the ventilator and endotrachealtubes
• Useful in patient weaning
• Does not have a “back-up” rate should apnea develop
PRESSURE SUPPORT VENTILATION
PATIENT COMFORT LEVEL
10 0
SpontaneousBreathing
PressureSupport
Ventilation
CONTROLLED MECHANICAL VENTILATION
• Inspiration initiated by – Time (ventilator)
• Expiration initiated by– Volume (ventilator)– Pressure (ventilator)– Time (ventilator)
Volume limited - Time cycled - Ventilator triggered
Note absence of patient effort
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CONTROLLED MECHANICAL VENTILATION
• Most common mode of mechanical ventilation
• Allows no interaction between patient and ventilator
• Very uncomfortable if patient is awake– almost always requires pharmacologic paralysis
• Allows no patient work of breathing
• Can result in high peak inspiratory pressures
PATIENT COMFORT LEVEL
10 0
SpontaneousBreathing
ControlledMechanicalVentilation
PressureSupport
Ventilation
ASSIST CONTROL VENTILATION
• Patient determines respiratory rate, but not volume• Attempts to improve patient comfort by allowing
patient - ventilator interaction• Patient receives full preset tidal volume each breath
– results in hyperventilation, hypocarbia, and respiratory alkalosis if patient is tachypneic
– can result in high peak inspiratory pressures• Requires minimal patient work of breathing
– leads to respiratory muscle atrophy and weakness
Volume limited - Time cycled - Patient / ventilator triggered
PatientPatient
Different tidal volumes
• Originally intended as a method of weaning
• Most common mode of ventilation in the SICU setting
• Allows patient to choose rate and tidal volume– more natural, physiologic breathing pattern– more comfortable for patient– requires less patient sedation
• Allows spontaneous breathing while still providing larger tidal volume breaths to prevent atelectasis
• Can result in high peak inspiratory pressures on mechanical breaths
Volume limited - Time cycled – Patient / ventilator triggered
PatientVentilator
Same tidal volume
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PRESSURE CONTROL VENTILATION
• Inspiration initiated by – Negative pressure
change (patient)– Time (ventilator)
• Expiration initiated by– Pressure (ventilator)– Time (ventilator)
Pressure limited - Time cycled - Patient / ventilator triggered
Same pressure
PatientVentilator
• Limits peak inspiratory pressures– used as part of a “lung protective” strategy”
smaller tidal volumesincreased respiratory rates
– can lead to hypercapnia, inadequate ventilation
• May require pharmacologic paralysis to prevent patient-ventilator disynchrony
• May be used with prolonged/reversed inspiratory:expiratory times as “inverse ratio ventilation”
– Inspiration occurs before complete exhalation leading to “air-trapping” or “auto-PEEP”
PRESSURE CONTROL VENTILATION
INVERSE RATIO VENTILATION
Paw
Flow
TI
TE
time
TI / TE < 1 TI / TE > 1
Incomplete lung emptying before next breath results in air trapping and intrinsic PEEP
PATIENT COMFORT LEVEL
10 0
SpontaneousBreathing
ControlledMechanicalVentilation
AssistControl
Ventilation
SynchronizedIntermittentMechanicalVentilation
PressureSupport
Ventilation
PressureControl
Ventilation
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New Modes of VentilationDual-Controlled Modes
Adaptive support ventilationHamilton; GalileoDual control breath to breath:
SIMV
Pressure-regulated volume control
Adaptive pressure ventilation
Autoflow
Variable pressure control
Siemens; servo 300
Hamilton; Galileo
Drager; Evita 4
Cardiopulmonary corporation; Venturi
Dual control breath to breath:
Pressure-limited time-cycled ventilation
Volume support
Variable pressure support
Siemens; servo 300
Cardiopulmonary corporation; Venturi
Dual control breath to breath:
Pressure-limited flow-cycled ventilation
Volume-assured pressure support
Pressure augmentation
VIASYS Healthcare; Bird 8400Sti and Tbird
VIASYS Healthcare; Bear 1000
Dual control within a breath
NameManufacturer; ventilatorType The Respiratory Therapist sets :– pressure limit = plateau seen during VC– respiratory rate– peak flow rate (the flow if TV < target)– PEEP– FiO2– trigger sensitivity– minimum tidal volume
Dual Control within a BreathVolume-assured pressure support
Dual Control Breath-to-BreathPressure-limited time-cycled ventilation
Pressure Regulated Volume Control
Servo 300 Maquet Servo-i
Dual Control Breath-to-BreathPressure-limited time-cycled ventilation
Pressure Regulated Volume Control
• Delivers patient or timed triggered, pressure-targeted (controlled) and time-cycled breaths
• Ventilator measures VT delivered with VT set on the controls. If delivered VT is less or more, ventilator increases or decreases pressure delivered until set VT and delivered VT are equal
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Dual Control Breath-to-BreathPressure-limited time-cycled ventilation
Pressure Regulated Volume Control
The ventilator will not allow delivered pressure to rise higher than 5 cm H2O below set upper pressure limit
Example: If upper pressure limit is set to 35 cm H2O and the ventilator requires more than 30 cm H2O to deliver a targeted VT of 500 mL, an alarm will sound alerting the clinician that too much pressure is being required to deliver set volume
Pressure Regulated Volume Control
PRVC. (1), Test breath (5 cm H2O); (2) pressure is increased to deliver set volume; (3), maximum available pressure; (4), breath delivered at preset E, at preset f, and during preset TI; (5), when VT
corresponds to set value, pressure remains constant; (6), if preset volume increases, pressure decreases; the ventilator continually monitors and adapts to the patient’s needs
Volume fromVentilator=
Set tidal volume
time= setInspiratory time
Pressure limitBased on VT/Ctrigger cycle off
calculatecompliance
Calculate newPressure limit
no
yes
yes
no
Control logic for pressure-regulated volume control and autoflow
Pressure Regulated Volume Control
Disadvantages and RisksVarying mean airway pressureMay cause or worsen auto-PEEPWhen patient demand is increased, pressure level may diminish when support is neededMay be tolerated poorly in awake non-sedated patientsA sudden increase in respiratory rate and demand may result in a decrease in ventilator support
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Pressure Regulated Volume Control
Indications1. Patient who require the lowest possible
pressure and a guaranteed consistent VT
2. ALI/ARDS3. Patient with the possibility of CL or Raw
changes
Pressure Regulated Volume Control
AdvantagesMaintains a minimum PIPGuaranteed VT and VE
Patient has very little WOB requirementAllows patient control of respiratory rate and VE
Variable VE to meet patient demandDecelerating flow waveform for improved gas distributionBreath by breath analysis
Pressure then rises to assure that the set tidal volume is delivered
New Volume Targeted Breath New Volume Targeted Breath Pressure Variability is ControlledPressure Variability is Controlled
1 2 3 4 5 6
SEC
1 2 3 4 5 6
PawcmH20
60
-20
120
120
SEC
INSP
EXH
FlowL/min
PPawawcmHcmH2200
6060
--2020
6060
FlowFlowL/minL/min
VolumeVolume
Set flow limit
Set tidal volume cycle threshold
Set pressure limit
Tidal volume met
Tidal volume not met
Switch from Pressure control toVolume/flow control
Inspiratory flowgreater than set flow
Flow cycleInspiratory flowequals set flow
Pressure limitoverridden
LL
0
0.6
4040
Pressure at Pressure support
delivered VT≥ set VT
flow= 25% peak
Cycle offinspiration Insp flow
> Set flow
PAW <PSVsetting
delivered VT= set VT
Switch to flow controlat peak flow setting
trigger
yes
no
no
no
no
no
yes
yes
yes
Control logic for volume-assured pressure-support mode
yes
Dual Control within a BreathVolume-assured pressure support
• This mode allows a feedback loop based on the volume
• Switches even within a single breath from pressure control to volume control if minimum tidal volume has not been achieved
Bear 1000Tbird
Bird 8400Sti
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Dual Control within a BreathVolume-assured pressure support
• If pressure is too high, all breaths are pressure-limited.
• If the peak flow setting is too high , all breaths will be volume-controlled
• If the pressure is set too high or the minimum tidal volume is set too low; the volume guarantee is negated
• If peak flow is set too low, the switch from pressure to volume is late in the breath, inspiratory time is too long.
Dual Control Breath-to-BreathPressure-limited flow-cycled ventilation
Volume Support
• Tidal volume is used as feedback control to adjust the pressure support level
• All breaths are patient triggered, pressure limited, and flow-cycled.
• Automatic weaning of pressure support as long as tidal volume matches minimum required VT (VT set in a feedback loop to adjust pressure).
Dual Control Breath-to-BreathPressure-limited flow-cycled ventilation
Volume Support
Servo 300 Maquet Servo-i
Volume Support versusVolume Assured Pressure Support
How does volume support differ from VAPS? – In volume support, we are trying to adjust
pressure so that, within a few breaths, desired VT is reached.
– In VAPS, we are aiming for desired VTtacked on to the end of a breath if a pressure-limited breath is going to fail to achieve VT
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Volume Support• Entirely a spontaneous mode
• Delivers a patient triggered (pressure or flow), pressure targeted, flow cycled breath
Can also be timed cycled (if Ti is extended for some reason) or pressure cycled (if pressure rises too high).
• Similar to pressure support except VS also targets set VT. It adjusts pressure (up or down) to achieve the set volume (the maximum pressure change is < 3 cm H2O and ranges from 0 cm H2O to 5 cm H2O below the high pressure alarm setting
• Used for patients ready to be “weaned” from the ventilator and for patients who cannot do all the WOB but who are breathing spontaneously
Volume Support
(1), VS test breath (5 cm H2O); (2), pressure is increased slowly until target volume is achieved; (3), maximum available pressure is 5 cm H2O below upper pressure limit; (4), VT higher than set VT delivered results in lower pressure; (5), patient can trigger breath; (6) if apnea alarm is detected,
ventilator switches to PRVC
Volume fromVentilator=
Set tidal volume
Flow= 5% ofPeak flow
Pressure limitBased on VT/Ctrigger cycle off
calculatecompliance
Calculate newPressure limit
no
yes
yes
no
Control logic for volume support mode of the servo 300
Dual Control Breath-to-BreathPressure-limited flow-cycled ventilation
Volume Support
• Little data to show it actually works.
• If pressure support level increases to maintain TV in pt with increased airways resistance, PEEPi may increase.
• If minimum TV set too high, weaning may be delayed.
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Volume Support
IndicationsSpontaneous breathing patient who require minimum VE
Patients who have inspiratory effort who need adaptive supportPatients who are asynchronous with the ventilatorUsed for patient who are ready to wean
Volume Support
AdvantagesGuaranteed VT and VEPressure supported breaths using the lowest required pressureDecreases the patient’s spontaneous respiratory rateDecreases patient WOBAllows patient control of I:E timeBreath by breath analysisVariable VI to meet the patient’s demand
Volume SupportDisadvantages
Spontaneous ventilation requiredVT selected may be too large or small for patientVarying mean airway pressureAuto-PEEP may affect proper functioningA sudden increase in respiratory rate and demand may result in a decrease in ventilator support
Dual Control Breath-to-BreathAdaptive Support Ventilation
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Adaptive Support Ventilation
A dual control mode that uses pressure ventilation (both PC and PSV) to maintain a set minimum VE (volume target) using the least required settings for minimal WOB depending on the patient’s condition and effort
– It automatically adapts to patient demand by increasing or decreasing support, depending on the patient’s elastic and resistive loads
Adaptive Support Ventilation• The clinician sets patient’s IBW, % desired VE . The ventilator
then delivers 100 mL/min/kg.
• A series of test breaths measures the system C, resistance and auto-PEEP
• If no spontaneous effort occurs, the ventilator determines the appropriate respiratory rate, VT, and pressure limit delivered for the mandatory breaths
• I:E ratio and TI of the mandatory breaths are continually being“optimized” by the ventilator to prevent auto-PEEP
• If the patient begins having spontaneous breaths, the number of mandatory breaths decrease and the ventilator switches to PS at the same pressure level
• Pressure limits for both mandatory and spontaneous breaths are always being automatically adjusted to meet the E target
Adaptive Support Ventilation
IndicationsFull or partial ventilatory supportPatients requiring a lowest possible PIP and a guaranteed VT
ALI/ARDSPatient requiring high and/or variable Patients not breathing spontaneously and not triggering the ventilatorPatient with the possibility of work land changes (CL and Raw)Facilitates weaning
Adaptive Support Ventilation
AdvantagesGuaranteed VT and VE
Minimal patient WOBVentilator adapts to the patientWeaning is done automatically and continuouslyVariable to meet patient demandDecelerating flow waveform for improved gas distributionBreath by breath analysis
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Adaptive Support Ventilation
Disadvantages and RisksInability to recognize and adjust to changes in alveolar VD
Possible respiratory muscle atrophyVarying mean airway pressureIn patients with COPD, a longer TE may be required A sudden increase in respiratory rate and demand may result in a decrease in ventilator support
Automode• The ventilator switch between mandatory and
spontaneous breathing modes
• Combines volume support (VS) and pressure-regulated volume control (PRVC)
• If patient is paralyzed; the ventilator will provide PRVC. All breaths are mandatory that are ventilator triggered, pressure controlled and time cycled; the pressure is adjusted to maintain the set tidal volume.
• If the patient breathes spontaneously for two consecutive breaths, the ventilator switches to VS. All breaths are patient triggered, pressure limited, and flow cycled.
• If the patient becomes apneic for 12 seconds; the ventilator switches back to PRVC
BILEVEL VENTILATIONWhat is BiLevel Ventilation?
• Is a spontaneous breathing mode in which two levels of pressure and hi/low are set
• Enabled utilizing an active exhalation valve
• Substantial improvements for spontaneous breathing– better synchronization, more options
for supporting spontaneous breathing, and potential for improved monitoring
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BiLevel Ventilation
Synchronized TransitionsSpontaneous Breaths
Spontaneous Breaths
PPawawcmHcmH2200
6060
--20201 2 3 4 5 6 7
What is BiLevel Ventilation?
At either pressure level the patient can breathe spontaneously– spontaneous breaths may be supported by PS – if PS is set higher than PEEPH, PS supports
spontaneous breath at upper pressure
BiLevel Ventilation
PEEPPEEPHH
PEEPPEEPLL
Pressure SupportPressure SupportPEEPPEEPHigh High + PS + PS
PPawawcmHcmH2200
6060
--20201 2 3 4 5 6 7
Then What Is APRV?• Is a Bi-level form of ventilation with sudden short
releases in pressure to rapidly reduce FRC and allow for ventilation
• Can work in spontaneous or apneic patients
• APRV is similar but utilizes a very short expiratory time for pressure release and a prolonged time on Phigh
This short time at low pressure allows for ventilation
• APRV always implies an inverse I:E ratio
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Airway Pressure Release Ventilation• Provides two levels of CPAP and allows spontaneous breathing at
both levels when spontaneous effort is present
• Both pressure levels are time triggered and time cycled
Airway Pressure Release Ventilation• Allows spontaneously breathing patients to breathe at a
high CPAP level, but drops briefly (approximately 1 second) and periodically to allow CPAP level for extra CO2 elimination (airway pressure release)
• Mandatory breaths occur when the pressure limit rises from the lower CPAP to the higher CPAP level
Airway Pressure Release Ventilation
Indications1. Partial to full ventilatory support2. Patients with ALI/ARDS3. Patients with refractory hypoxemia due to
collapsed alveoli4. Patients with massive atelectasis5. May use with mild or no lung disease
Airway Pressure Release VentilationAdvantages
1. Allows inverse ratio ventilation with or without spontaneous breathing (less need for sedation or paralysis)
2. Improves patient-ventilator synchrony if spontaneous breathing is present
3. Increases mean airway pressure4. Improves oxygenation by stabilizing collapsed alveoli5. Allows patients to breath spontaneously while
continuing lung recruitment6. Lowers PIP7. May decrease physiologic deadspace
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Airway Pressure Release Ventilation
Disadvantages and Risks1. Variable VT2. Could be harmful to patients with high
expiratory resistance (i.e., COPD or asthma)3. Auto-PEEP is usually present4. Caution should be used with
hemodynamically unstable patients5. Asynchrony can occur if spontaneous
breaths are out of sync with release time6. Requires the presence of an “active
exhalation valve”
Airway Pressure Release Ventilation
Comparison of three different modes of ventilation HFOV – HFJV
What is different?
Modified ET tubeStandardET tube
Vaporizer, nebulizer, humidity entrainment
Standard humidifierHumidity
Gas trapping by increased f and set PEEP
Direct setting; No gas trapping
CPD Control
PassiveActiveExhalation
1-10 Hz (60-600)3-15 Hz (180-900)Frequency
JetOscillatorMechanisms
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High Frequency Oscillatory VentilationPrinciples of gas exchange
1. Convection (Bulk Flow) Ventilation
2. Asymetrical Velocity Profile
3. Taylor Dispersion
4. Molecular Diffusion
5. Pendelluft
6. Cardiogenic Mixing
Oscillator
HFOV
Decrease TV’s to physiological dead space and increase frequency
CDPAdjust Valve
Oscillator
BIAS Flow
ET Tube
Patient
Taylor Dispersion
Low flow
High flow
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Gas Profile Pendelluft Effect
CO2 Elimination Variables of Oscillator Breaths
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HFOV Principle:HFOV Principle:Pressure curves Pressure curves CMVCMV / HFOV/ HFOV Control Variables of HFVO
CO2 Elimination
CO2 Elimination = VT2 x fVT = oscillatory volumeF = oscillatory frequency
Oscillatory volume versus frequency and amplitude
AmplitudeFrequency
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WHAT WE DO KNOW…
• Lungs are heterogeneous; alveoli are not all alike
• Injured alveoli are non-compliant; “normal” distention of injured alveoli results in overdistention of the truly normal alveoli
• Cyclical inflation and deflation of alveoli using large tidal volumes and low PEEP injures lung parenchyma and can cause both “atelectrauma” and “volutrauma”
• Optimal mechanical ventilation ensures adequate oxygenation while minimizing the detrimental effects of alveolar overdistention
THE COMPLIANT LUNG
0
200
400
600
800
1000
0 10 20 30 40 50 60
Pressure (cm H2O)
Volu
me
(mL)
Inspiration
ExpirationA small change in pressure results in a large change in volume
