Mechanical Ventilation
Overview
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Intro
NIV
Basic Modes
Settings
Specific Conditions
Ventilators
Other modes
Acute respiratory failure
• Hypoxia (PO2 < 60mmHg)
– Low inspired O2
– Hypoventilation – CNS, peripheral neuro, muscles, chest wall
– V/Q mismatch
• Shunt – pneumonia, APO, collapse, contusions
– Alveoli perfused but not ventilated
– Venous admixture
• Anatomical shunt – cardiac anomaly
• Increased dead space (hypercapnia) – hypovolaemia, PE, poor cardiac
function
– Diffusion abnormality – severe destructive disease of the lung – fibrosis,
severe APO, ARDS
• Hypercapnia (PCO2 >50mmHg)
– Hypoventilation
– Dead space ventilation
– Increased CO2 production
Shunt
0
mmHg
450
mmHg
100%
70%
85%
Mechanical Ventilation
• Pump gas in and letting it flow out
• Function
– Gas exchange
– Manage work of breathing
– Avoid lung injury
• Physics
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Flow needs a pressure gradient
Pressure to overcome airway resistance and inflate lung
Pressure (to overcome resistance) = Flow x Resistance
Alveolar pressure = (Volume/Compliance) + PEEP
Airway pressure = (Flow x Resistance) + (V/C) + PEEP
Gas Exchange
• Oxygenation – get O2 in
– FiO2
– Ventilation (minor effect) – alveolar gas equation, CO2 effect
– Mean alveolar pressure
• Mean airway pressure – surrogate marker, affected by airway
resistance
• Pressure over inspiration + expiration
• Set Vt or inspiratory pressure
• Inspiratory time
• PEEP
– Reduce shunt
• Re-open alveoli – PEEP
• Prolonging inspiration – improve ventilation of less compliant alveoli
• Ventilation – get CO2 out
– Alveolar ventilation = RR x (Tidal volume – Dead space)
Adverse Effects
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Barotrauma
– High alveolar pressure
– High tidal volume
– Shear injury –
• Repetitive collapse + re-expansion of alveoli
• Tension at interface between open + collapsed alveoli
– Pneumothorax, pneumomediastinum, surgical emphysema, acute lung injury
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Gas trapping
– Insufficient time for alveoli to empty
– Increase risk
• Airflow obstruction – asthma, COPD
• Long inspiratory time
• High respiratory rate
– Progressive
• Hyperinflation
• Rise in end-expiratory pressure – intrinsic-PEEP, auto-PEEP
– Result – Barotrauma, Cardiovascular compromise (high intrathoracic pressure)
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Oxygen toxicity
– Acute lung injury due to high O2 concentrations
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Cardiovascular effects
– Preload – positive intrathoracic pressure reduces venous return
– Afterload - positive intrathoracic pressure reduces afterload
– Cardiac Output – depends on LV contractility
• Normal – IPPV decreases CO
• Reduced – IPPV increases CO
– Myocardial O2 consumption - reduced
Gas Trapping
NIV
• CPAP
– Similar to PEEP
– Splint alveoli open – reduce shunt
– Spontaneous breathing at elevated baseline
pressure
• BiPAP
– Ventilatory assistance without invasive
artificial airway
– Fitted face/nasal mask
– Initial settings 10/5
NIV
NIV
• Indicator of success
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Known benefits
Younger age
Lower APACHE score
Cooperative
Intact dentition
Moderate hypercarbia
(pH<7.35, >7.10)
– Improvement within first 2 hrs
• Contraindications
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Cardiac/Resp arrest
Non-respiratory organ failure
Encephalopathy GCS <10
GIH
Haemodynamically unstable
Facial or neurological surgery,
trauma or deformity
– High aspiration risk
– Prolonged ventilation
anticipated
– Recent oesophageal
anastamosis
NIV Benefits
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General
COPD
Cardiogenic pulmonary oedema
Hypoxaemic respiratory failure
Asthma
Post-extubation
Immunocompromised
Other diseases
What is a Mode?
• 3 components
• Control variable
– Pressure or volume
• Breath sequence
– Continuous mandatory
– Intermittent mandatory
– Continuous spontaneous
• Targeting scheme (settings)
– Vt, inspiratory time, frequency, FiO2, PEEP, flow
trigger
Volume Control Ventilation
• Set tidal volume
• Minimum respiratory rate
• Assist mode – both ventilator and patient can initiate
breaths
• Advantage
– Simple, guaranteed ventilation, rests respiratory muscle
• Disadvantages
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Not synchronised – ventilator breath on top of patient breath
Inadequate flow – patient sucks gas out of ventilator
Inappropriate triggering
Decreased compliance – high airway pressure
Requires sedation for synchrony
VCV
Pressure Control Ventilation
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Set inspiratory pressure
Constant pressure during inspiration
High initial flow
Inspiratory pause – built in
Advantages
– Simple, avoids high inspiratory pressures, improved
oxygenation
• Disadvantages
– Not synchronised
– Inappropriate triggers
– Decreased compliance – reduced tidal volume
PCV
Pressure Support
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Set inspiratory pressure
Patient initiates breath
Back-up mode – apnoea
Cycle from inspiration to expiration
– Inspiratory flow falls below set proportion of peak
inspiratory flow
• Advantages
– Simple, avoids high inspiratory pressure, synchrony,
less sedation, better haemodynamics
• Disadvantages
– Dependent on patient breaths
– Affected by changes in lung compliance
PS
Synchronised Intermittent
Mandatory Ventilation
• Mandatory breaths – VCV, PCV
• Patient breaths – depends on SIMV cycle
– Synchronised mandatory breath
– Pressure support breath
• Advantages
– Synchrony, guaranteed minute ventilation
• Disadvantages
– Sometimes complicated to set
SIMV
VCV vs PCV
VCV vs PCV
VCV vs PCV - Advantages
• PCV + PS
– Variable flow
– Reduced WOB
– Max Palveolar = Max
Pairway (or less)
– Palveolar controlled
– Variable I-time &
pattern (PS)
– Better with leaks
• VCV
– Consistent TV
• changing
impedance
• Auto-PEEP
– Minimum min. vent.
(f x TV) set
– Variety of flow waves
VCV vs PCV - Disadvantages
• PCV + PS
– Variable tidal volume
• Too large or too small
• No alarm/limit for
excessive TV (except
some new gen. vents)
– Some variablity in
max pressures (PC,
expir. effort)
• VCV
– Variable pressures
• airway
• alveolar
– Fixed flow pattern
– Variable effort = variable
work/breath
– Compressible vol.
– Leaks = vol. loss
Settings
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FiO2 – start at 1.0
RR – average 12, higher for those with sepsis/acidosis
Tidal volume – 500ml, 8ml/kg, smaller volumes in ARDS
Inspiratory pressure - <30cmH2O, sum of PEEP + Pinsp
Inspiratory time
– I:E – normally 1:2, simulates normal breathing – synchrony
– PCV – easy to set
– VCV – complicated, Time = Volume/Flow
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PEEP
– Start at 5cmH2O
– Higher – APO, ARDS
– Lower – asthma, COPD
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Triggering
– Flow triggering – more sensitive, synchrony, -2cmH2O
– Pressure triggering
– Inappropriate triggering – triggering when no patient effort
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Oxygenation
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Ventilation
– FiO2, PEEP, Insp Time, InspP, Insp pause
– Problems – CVS effects, gas trapping, barotrauma
– Tidal volume, RR, eliminate dead space
– Problems – barotrauma, gas trapping (reduced minute ventilation)
Troubleshooting
Airway pressure
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Tidal volume
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Minute ventilation – determined by RR + Vt
Apnoea – important in PS
Intrinsic PEEP (gas trapping)
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Expiratory pause hold
Hypotension – after initiating IPPV
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Reduced – respiratory acidosis
Monitor in PCV/PS
Changes in compliance – anywhere in system
Expired Vt – more accurate
Hypovolaemia/Reduced VR
Drugs
Gas trapping – disconnect
Tension pneumothorax
Dysynchrony
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Patient factors
Ventilator – settings, eg I:E
PS > SIMV > PCV/VCV
Total PEEP
PEEPe
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Ventilator – settings, malfunction
Circuit – kinking, water pooling, wet filter
ETT – kinked, obstructed, endobronchial intubation
Patient – bronchospasm, compliance (lungm, pleura, chest wall), dysynchrony, coughing
Inspiratory pause pressure - Estimate of alveolar pressure
Pressure
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PEEPi
Time
Troubleshooting
• Desaturation
– Patient causes
• All causes of hypoxic respiratory failure
• Endobronchial intubation, PTx, collapse, APO,
bronchospasm, PE
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Equipment causes
FIO2 1.0
Sat O2 waveform
Chest moving?
• Yes – Examine patient, treat cause
• No – Manually ventilate
– No – ETT/Patient problem
– Yes – Ventilator problem – setting, failure, O2 failure
Ventilators
• Maquet
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VCV
PCV
PRVC
PS/CPAP
SIMV (VC) + PS
SIMV (PC) + PS
SIMV (PRVC) + PS
MMV
NAVA
• Evita
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PS
PCV+
SIMV
PCV+A
Autoflow
Adaptive Modes - PRVC
• PCV unable to deliver guaranteed minimum
minute ventilation
• Changing lung mechanics + patient effort
• Pressure controlled breaths with target tidal
volume
• Inspiratory pressure adjusted to deliver minimum
target volume
• Not VCV - average minimum tidal volume
guaranteed
• Like PCV – constant airway pressure, variable
flow (flow as demanded by patient)
Adaptive Modes - PRVC
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Consistent tidal volumes
Promotes inspiratory flow synchrony
Automatic weaning
Inappropriate – increased respiratory drive, eg severe
metabolic acidosis
• Evidence – lower peak inspiratory pressures
VCV vs PRVC
Adaptive Modes - Autoflow
• First breath uses set TV & I-time
– Pplateau measured
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Pplateau then used
V/P measured each breath
Press. changed if needed (+/- 3)
Dual mode similar to PRVC
– Targets vol., applies variable press. based on mechanics
measurements
– Allows highly variable inspiratory flows
– Time ends mandatory breaths
• Adds ability to freely exhale during mandatory inspiration
(maintains pressure)
PCV + Assist
• Like PCV, flow varies automatically to
varying patient demands
• Constant press. during each breath variable press. from breath to breath
• Mandatory + patient breaths the same
Inverse Ratio Ventilation
• Increased mean airway pressure
• Prolonged I:E ratio
• Improved oxygenation
– Reduced shunting
– Improved V/Q matching
– Decreased dead space
• Heavy sedation, paralysis
• Preferred PCV
• Benefit – no effect in mortality in ARDS
Other Modes
• Adaptive support ventilation
– Mandatory minute ventilation
– Adaptive pressure control
• Proportional assist ventilation
– Pressure support (spontaneous breaths)
– Pressure applied function of patient effort
• Automatic tube compensation
– adjusts its pressure output in accordance with
flow, theoretically giving an appropriate
amount of pressure support
Airway Pressure-Release
Ventilation
• High constant PEEP + intermittent
releases
• Unrestricted spontaneous breaths –
reduced sedation
• Extreme form of inverse ratio ventilation
• E:I – 1:4
• Spontaneous breaths – 10-40% total
minute ventilation
APRV
• Settings – 2 pressure levels, 2 time
durations
• Uses – ALI, ARDS
• Caution – COPD, increased respiratory
drive
APRV
• Increase mean airway pressure
– Alveolar recruitment, improve oxygenation
• Promote spontaneous breathing
– Improved V/Q match, haemodynamics
• Improved synchrony
• Evidence – no difference in mortality,
decreased duration of ventilation
High-Frequency Ventilation
• 4 types
– High frequency jet ventilation
• Ventilation by jet of gas
• 14-16G cannula, specialised ventilator
• 35 psi, RR100-150, Insp 40%
– High frequency oscillatory ventilation
– High frequency percussive ventilation
• HFV + PCV
• HFOV – oscillating around 2 pressure levels
• Less sedation, better clearance of secretions
– High frequency positive pressure ventilation
• Conventional ventilation at setting limits
High Frequency Oscillatory
Ventilation
• Ventilator delivers a constant flow (bias flow)
• Valve creates resistance – maintain airway
pressure
• Piston pump oscillates 3-15Hz (RR160-900)
• “Chest wiggle” – assess amplitude
• Tidal volumes – less than dead space
• Ventilation – achieved by laminar flow
• Deep sedation, paralysis
HFOV
• CO2 clearance
– Decrease oscillation frequency, increase amplitude,
increase inspiratory time, increase bias flow (with ETT
cuff leak)
• Oxygenation
– Mean airway pressure, FiO2
• Settings
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Airway pressure amplitude
Mean airway pressure
% inspiration
Inspiratory bias flow
FiO2
HFOV
• Applications
– ARDS
– Lung protection – highest mean airway pressure + lowest tidal
volumes
– Ventilatory failure – FiO2>0.7, PEEP>14, pH <7.25, Vt >6ml/kg,
plateau pressure >30)
• Contraindicated
– Severe airflow obstruction
– Intracranial hypertension
• Evidence
– Animal models – less histologic damage + lung inflammation
– Better oxygenation as rescure therapy in ARDS
– No difference in mortality
Mean Airway Pressure
• Main factor in recruitment and oxygenation
• Increased surface area for O2 diffusion
• Problems
– Barotrauma
– Haemodynamic instability
– Contraindicated patients
– Deep sedation, paralysis
Specific Conditions
• ARDS
– Definition
• Diffuse bilateral pulmonary infiltrates
• No clinical evidence of Left Atrial Hypertension (CWP<18mmHg)
• PaO2/FiO2 of 300 or less
– Exclusions
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Unilateral lung disease
Children (wt less than 25kg)
Severe obstructive lung disease (asthma, COPD)
Raised intracranial pressure
High PEEP, low volumes + pressure
SIMV(PRVC) + PS
Vt 6ml/gk – check plateau pressure
Pins >30cmH2O – reduce Vt
Lowest plateau pressure possible
RR 6-35, aim pH 7.3-7.45
Evidence – improved mortality
FiO2
0.3
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0.5
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0.9
1.0
PEEP
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5-8
8-10
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10-14
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14-18
18-22
Ventilator Induced Lung Injury
• Excessive inflation pressure
• Mechanical tissue damage
• Inflammation – mechano-signaling due to
tensile forces
• Overstretching of lung units
• Shear force at junction of open and
collapsed tissue
• Repeated opening and closing of small
airways under high pressure
Pathways to VILI
End-Expiration
Extreme Stress/Strain
Tidal Forces
Moderate Stress/Strain
(Transpulmonary and
Microvascular
Pressures)
Rupture
Signaling
Mechano signaling via
integrins, cytoskeleton, ion channels
inflammatory cascade
Cellular Infiltration and Inflammation
Marini / Gattinoni CCM 2004
Spectrum of Regional Opening Pressures
(Supine Position)
Opening
Pressure
Superimposed
Pressure
Inflated
0
Small Airway
Collapse
10-20 cmH2O
Alveolar Collapse
(Reabsorption)
Consolidation
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Lung Units at Risk for Tidal
Opening & Closure
20-60 cmH2O
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Lung Protection Strategies
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Heterogenous lung units
PEEP
Tidal volume
Keep the lung as open as possible without
generating excessive regional tissue
stresses is a major goal of modern
practice
Prone Ventilation
• Homogenise transpleural pressure
• Compression – reduced compression from heart
+ abdomen
• Improved recruitment
• Increase in FRC
• Decreased shunt
• Benefit
– Improved oxygenation in 60-80% patient, even on
return to supine position
– No change mortality
Recruitment Manoeuvres
• Open collapsed lung tissue so it can remain open during
tidal ventilation with lower pressures and PEEP, thereby
improving gas exchange and helping to eliminate high
stress interfaces
• Although applying high pressure is fundamental to
recruitment, sustaining high pressure is also important
• Methods of performing a recruiting maneuver include
single sustained inflations and ventilation with high
PEEP
Three Types of Recruitment Maneuvers
Specific Conditions
• Unilateral lung disease
– Similar approach to ARDS
– Increase Insp time – improve gas distribution
– Lateral position – normal lung down
• Reduce shunt
• Reduce normal lung compliance
• Risk of contamination
– Independent lung ventilator
• Asthma
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Maximise expiratory time, low RR – permissive hypercarbia
Short inspiratory time
High airway pressure - ?significance
Expiratory hold
Aim – PEEPi < 10cmH20, Pplat <20cmH2O
• COPD
– Similar to asthma
– Bronchospasm not as great, reduced lung compliance
Airway Obstruction
• Aim – relieve work of breathing, minimise auto-PEEP
• Gas trapping
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Increases work of breathing
Haemodynamic compromise
Predisposes to barotrauma
Decreases ventilation
• PEEP
– Effects Depend on Type and Severity of Airflow Obstruction
– Generally Helpful if PEEP  Original Auto-PEEP
– Potential Benefits
• Decreased Work of Breathing
• Increased VT
• Improved Distribution of Ventilation
NAVA
• Neurally adjusted ventilatory assist
• Controls ventilator output by measuring
the neural traffic to the diaphragm
• NAVA senses the desired assist using an
array of esophageal EMG electrodes
positioned to detect the diaphragm’s
contraction signal
• Flexible response to effort
• Improves synchrony and weaning
Neuro-Ventilatory Coupling
Neural Control of Ventilatory Assist (NAVA)
Central Nervous System
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Phrenic Nerve
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Diaphragm Excitation
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Diaphragm Contraction
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Chest Wall and Lung
Expansion
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Airway Pressure, Flow and
Volume
Ideal
Technology
New
Technology
Current
Technology
Ventilator
Unit
• References
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Cleveland clinic journal of medicine 2009; 76(7): 417-430
UpToDate
BASIC course notes
Wests Respiratory Essentials
• Links
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http://emedicine.medscape.com/
http://www.anaesthetist.com/anaes/vent/Findex.htm#index.htm
http://en.wikipedia.org/wiki/Mechanical_ventilation
http://www.merck.com/mmpe/sec06/ch065/ch065b.html
http://www.ccmtutorials.com/rs/index.htm
http://www.aic.cuhk.edu.hk/web8/mechanical_ventilation.htm