Mechanical Ventilation

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Mechanical Ventilation
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Overview

• Intro
• NIV
• Basic Modes
• Settings
• Specific Conditions
• Ventilators
• Other modes

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Acute respiratory failure

• Hypoxia (PO2 lt 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 gt50mmHg)
• Hypoventilation
• Dead space ventilation
• Increased CO2 production

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Shunt
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Mechanical Ventilation

• Pump gas in and letting it flow out
• Function
• Gas exchange
• Manage work of breathing
• Avoid lung injury
• Physics
• 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

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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)

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Adverse Effects

• 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
• 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)
• Oxygen toxicity
• Acute lung injury due to high O2 concentrations

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Gas Trapping
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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

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NIV
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NIV

• Indicator of success
• Known benefits
• Younger age
• Lower APACHE score
• Cooperative
• Intact dentition
• Moderate hypercarbia (pHlt7.35, gt7.10)
• Improvement within first 2 hrs

• Contraindications
• Cardiac/Resp arrest
• Non-respiratory organ failure
• Encephalopathy GCS lt10
• GIH
• Haemodynamically unstable
• Facial or neurological surgery, trauma or
  deformity
• High aspiration risk
• Prolonged ventilation anticipated
• Recent oesophageal anastamosis

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NIV Benefits

• General
• COPD
• Cardiogenic pulmonary oedema
• Hypoxaemic respiratory failure
• Asthma
• Post-extubation
• Immunocompromised
• Other diseases

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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

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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
• 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

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VCV
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Pressure Control Ventilation

• 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

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PCV
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Pressure Support

• 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

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PS
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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

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SIMV
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VCV vs PCV
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VCV vs PCV
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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

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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

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Settings

• 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 - lt30cmH2O, sum of PEEP
  Pinsp
• Inspiratory time
• IE normally 12, simulates normal breathing
  synchrony
• PCV easy to set
• VCV complicated, Time Volume/Flow
• PEEP
• Start at 5cmH2O
• Higher APO, ARDS
• Lower asthma, COPD
• Triggering
• Flow triggering more sensitive, synchrony,
  -2cmH2O
• Pressure triggering
• Inappropriate triggering triggering when no
  patient effort
• Oxygenation
• ?FiO2, ?PEEP?, ?Insp Time, ?InspP, ?Insp pause
• Problems CVS effects, gas trapping, barotrauma

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Troubleshooting

• Airway pressure
• 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
• Tidal volume
• Reduced respiratory acidosis
• Monitor in PCV/PS
• Changes in compliance anywhere in system
• Expired Vt more accurate
• Minute ventilation determined by RR Vt
• Apnoea important in PS
• Intrinsic PEEP (gas trapping)
• Expiratory pause hold
• Hypotension after initiating IPPV
• Hypovolaemia/Reduced VR
• Drugs
• Gas trapping disconnect

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Troubleshooting

• Desaturation
• Patient causes
• All causes of hypoxic respiratory failure
• Endobronchial intubation, PTx, collapse, APO,
  bronchospasm, PE
• 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

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Ventilators

• Maquet
• VCV
• PCV
• PRVC
• PS/CPAP
• SIMV (VC) PS
• SIMV (PC) PS
• SIMV (PRVC) PS
• MMV
• NAVA

• Evita
• PS
• PCV
• SIMV
• PCVA
• Autoflow

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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)

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Adaptive Modes - PRVC

• Consistent tidal volumes
• Promotes inspiratory flow synchrony
• Automatic weaning
• Inappropriate increased respiratory drive, eg
  severe metabolic acidosis
• Evidence lower peak inspiratory pressures

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VCV vs PRVC
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Adaptive Modes - Autoflow

• First breath uses set TV I-time
• Pplateau measured
• 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)

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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

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Inverse Ratio Ventilation

• Increased mean airway pressure
• Prolonged IE ratio
• Improved oxygenation
• Reduced shunting
• Improved V/Q matching
• Decreased dead space
• Heavy sedation, paralysis
• Preferred PCV
• Benefit no effect in mortality in ARDS

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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

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Airway Pressure-Release Ventilation

• High constant PEEP intermittent releases
• Unrestricted spontaneous breaths reduced
  sedation
• Extreme form of inverse ratio ventilation
• EI 14
• Spontaneous breaths 10-40 total minute
  ventilation

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APRV

• Settings 2 pressure levels, 2 time durations
• Uses ALI, ARDS
• Caution COPD, increased respiratory drive

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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

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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

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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

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HFOV

• CO2 clearance
• Decrease oscillation frequency, increase
  amplitude, increase inspiratory time, increase
  bias flow (with ETT cuff leak)
• Oxygenation
• Mean airway pressure, FiO2
• Settings
• Airway pressure amplitude
• Mean airway pressure
• inspiration
• Inspiratory bias flow
• FiO2

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HFOV

• Applications
• ARDS
• Lung protection highest mean airway pressure
  lowest tidal volumes
• Ventilatory failure FiO2gt0.7, PEEPgt14, pH
  lt7.25, Vt gt6ml/kg, plateau pressure gt30)
• 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

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Mean Airway Pressure

• Main factor in recruitment and oxygenation
• Increased surface area for O2 diffusion
• Problems
• Barotrauma
• Haemodynamic instability
• Contraindicated patients
• Deep sedation, paralysis

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Specific Conditions

• ARDS
• Definition
• Diffuse bilateral pulmonary infiltrates
• No clinical evidence of Left Atrial Hypertension
  (CWPlt18mmHg)
• PaO2/FiO2 of 300 or less
• Exclusions
• 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 gt30cmH2O reduce Vt
• Lowest plateau pressure possible
• RR 6-35, aim pH 7.3-7.45
• Evidence improved mortality

FiO2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
PEEP 5 5-8 8-10 10 10-14 14 14-18 18-22
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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

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Pathways to VILI
End-Expiration
Moderate Stress/Strain
Tidal Forces (Transpulmonary and Microvascular
Pressures)
Extreme Stress/Strain
Rupture
Signaling
Mechano signaling via integrins, cytoskeleton,
ion channels
inflammatory cascade
Cellular Infiltration and Inflammation
Marini / Gattinoni CCM 2004
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Spectrum of Regional Opening Pressures (Supine
Position)
Superimposed Pressure

Lung Units at Risk for Tidal Opening Closure
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Lung Protection Strategies

• 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

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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

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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

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Three Types of Recruitment Maneuvers
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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
• Maximise expiratory time, low RR permissive
  hypercarbia
• Short inspiratory time
• High airway pressure - ?significance
• Expiratory hold
• Aim PEEPi lt 10cmH20, Pplat lt20cmH2O
• COPD
• Similar to asthma
• Bronchospasm not as great, reduced lung compliance

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Airway Obstruction

• Aim relieve work of breathing, minimise
  auto-PEEP
• Gas trapping
• 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

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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 diaphragms contraction signal
• Flexible response to effort
• Improves synchrony and weaning

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Neural Control of Ventilatory Assist (NAVA)
Ideal Technology
Central Nervous System ? Phrenic
Nerve ? Diaphragm Excitation ? Diaphragm
Contraction ? Chest Wall and Lung
Expansion ? Airway Pressure, Flow and Volume
New Technology
Ventilator Unit
Neuro-Ventilatory Coupling
Current Technology
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• References
• Cleveland clinic journal of medicine 2009 76(7)
  417-430
• UpToDate
• BASIC course notes
• Wests Respiratory Essentials
• Links
• http//emedicine.medscape.com/
• http//www.anaesthetist.com/anaes/vent/Findex.htm
  index.htm
• http//en.wikipedia.org/wiki/Mechanical_ventilatio
  n
• http//www.merck.com/mmpe/sec06/ch065/ch065b.html
• http//www.ccmtutorials.com/rs/index.htm
• http//www.aic.cuhk.edu.hk/web8/mechanical_ventila
  tion.htm