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					Mechanical Ventilation
                 Overview
•   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


 450          0
mmHg         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
   –   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
•   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
•   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                • Contraindications
   – Known benefits                      – Cardiac/Resp arrest
   – Younger age                         – Non-respiratory organ failure
   – Lower APACHE score                  – Encephalopathy GCS <10
   – Cooperative                         – GIH
   – Intact dentition                    – Haemodynamically unstable
   – Moderate hypercarbia                – Facial or neurological surgery,
     (pH<7.35, >7.10)                      trauma or deformity
   – Improvement within first 2 hrs      – High aspiration risk
                                         – Prolonged ventilation
                                           anticipated
                                         – Recent oesophageal
                                           anastamosis
               NIV Benefits
•   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
   –   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
•   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
•   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                 • VCV
  – Variable flow            – Consistent TV
  – Reduced WOB                  • changing
  – Max Palveolar = Max            impedance
    Pairway (or less)            • Auto-PEEP
  – Palveolar controlled     – Minimum min. vent.
  – Variable I-time &          (f x TV) set
    pattern (PS)             – Variety of flow waves
  – Better with leaks
VCV vs PCV - Disadvantages

 • PCV + PS                      • VCV
   – Variable tidal volume         – Variable pressures
      • Too large or too small         • airway
                                       • alveolar
      • No alarm/limit for
        excessive TV (except       – Fixed flow pattern
        some new gen. vents)       – Variable effort = variable
                                     work/breath
   – Some variablity in
                                   – Compressible vol.
     max pressures (PC,
                                   – Leaks = vol. loss
     expir. effort)
                                       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 - <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
•   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
•   Ventilation
     – Tidal volume, RR, eliminate dead space
     – Problems – barotrauma, gas trapping (reduced minute ventilation)
                                         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




                                                       Pressure
•   Hypotension – after initiating IPPV                                           Total PEEP
     –   Hypovolaemia/Reduced VR
     –




                                                                  PEEPe
         Drugs
     –   Gas trapping – disconnect                                                   PEEPi
     –   Tension pneumothorax
•   Dysynchrony
     –   Patient factors                                                  Time
     –   Ventilator – settings, eg I:E
     –   PS > SIMV > PCV/VCV
                      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
                 Ventilators
• Maquet                 • Evita
  –   VCV                  –   PS
  –   PCV                  –   PCV+
  –   PRVC                 –   SIMV
  –   PS/CPAP              –   PCV+A
  –   SIMV (VC) + PS       –   Autoflow
  –   SIMV (PC) + PS
  –   SIMV (PRVC) + PS
  –   MMV
  –   NAVA
     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
• 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
•   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
  –   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
       •   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   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
 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
       End-Expiration                                        Pathways to VILI


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      10-20 cmH2O
                                        Collapse


                                     Alveolar Collapse
                                      (Reabsorption)     20-60 cmH2O


                                      Consolidation         
  =   Lung Units at Risk for Tidal
      Opening & Closure
      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
            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
   –   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
   –   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
Neural Control of Ventilatory Assist (NAVA)

                                                              Ideal
                                Central Nervous System
  Neuro-Ventilatory Coupling


                                                           Technology
                                           
                                    Phrenic Nerve
                                           
                                                             New
                                 Diaphragm Excitation                   Ventilator
                                                          Technology     Unit
                                Diaphragm Contraction
                                           
                                  Chest Wall and Lung
                                      Expansion
                                                            Current
                               Airway Pressure, Flow and   Technology
                                        Volume
• 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_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

				
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posted:12/17/2011
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