Mechanical Ventilation-101 Carey Thomson, MD, MPH Critical

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Mechanical Ventilation-101

• Carey Thomson, MD, MPH
• Critical Care Services
• Mount Auburn Hospital

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Mechanical Ventilation

• Individualized approach
• Anatomy of mechanical ventilation
• Modes the basics
• Modes --the sophisticated
• Waveforms and Mechanics
• Liberation from mechanical ventilation

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Individualize your approach
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   Solution

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Goals When Setting The Ventilator

• Avoid alveolar over-distension
• Provide adequate alveolar ventilation
• Promote patient-ventilator synchrony
• Apply PEEP to maintain alveolar recruitment
• Provide adequate oxygenation
• Use the lowest possible FIO2
• Avoid auto-PEEP

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Ventilator-Induced Lung Injury
Gas Exchange
Setting the Ventilator
Patient Comfort
Hemodynamics
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Anatomy of a Vent

• Triggering
• Flow pattern
• Cycling
• Modes
• Waveforms and Mechanics

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Breath Types Delivered By Vents

• Assisted either triggered by the patient or
  cycled by the ventilator
• Volume-controlled
• Pressure-controlled
• Mixed modes
• Spontaneous triggered and cycled by the patient
• CPAP
• Pressure support

Back-up rate

No back-up rate
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Triggering start inspiration

• Triggered based on time, pressure, or flow
• Vent initiated Time
• The vent cycles based on a set rate (if pt does
  not initiate)
• Patient initiated (i.e. inspiratory effort
  maintained)
• Negative Pressure
• Patient's effort sensed by a decrease in the
  baseline pressure
• Unnecessary work
• Replaced by Flow triggers
• Flow triggers (flow by)
• Inspiration deflects the flow of continuous gas
  traveling around the system

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Triggering

• Pitfalls
• If the trigger is too sensitive the patient
  overtriggers and hyperventilates
• If it is not sensitive enough, the patient
  becomes dysynchronousair hunger
• Example PCP/ARDS with PTX and 60/minute

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Flow pattern

• Decelerating
• High initial flow
• Inspiration slows down as alveolar pressure
  increases
• results in a lower peak airway pressure than
  constant and accelerating flow
• better distribution characteristics
• Can be used in both pressure targeted and volume
  targeted ventilation
• Constant constant rate until the set tidal
  volume is delivered
• Accelerating flow increases progressively
• This should not be used in clinical practice.
• Sinusoidal spontaneous breathing and CPAP

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Peak inspiratory flow rate

• How quickly the breath is delivered
• Default 60L/min
• Ti (inspiration) VT/Flow Rate
• Pitfalls
• Flow rate is too high gt rapid delivery to most
  compliant at higher peak pressures
• Peak flow is too lowgt pt demands more?
  dysynchrony and Higher Ti Lower Te autopeep
• Pressure augmentation automatic increased flow
  when the patients demands exceed the peak flow.

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Setting the VentilatorPEEP

• PEEP
• Prevent alveolar collapse improve oxygenation
• Alveolar recruitment may decrease the risk of
  VILI
• Evidence is lacking that higher levels of PEEP,
  compared to modest levels of PEEP, decrease
  mortality (N Engl J Med, 2004351327)
• Counter-balance auto-PEEP, thus improving the
  ability to trigger
• Improve cardiac performance by decreasing venous
  return and left ventricular afterload
• Pitfalls reduced venous return, hypotension,
  interpretation of wedge readings inaccurate

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Modes
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Pressure Support Ventilation

• Spontaneous breathing mode
• Patient triggered
• Pressure limited (inspiration)
• Flow cycled
• Vent cycles to expiration when flow decreases by
  a preset amount (i.e. 25 of peak flow).
• Pitfall
• Not guaranteed minute ventilation
• COPD patients with slow flow changes

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Pressure Control Ventilation

• Flow occurs until a preset peak pressure is met
  over a fixed inspiratory period
• Set Ti too shorttoo rapid breath too long
  autoPeep (short expir time)
• Longer Tihigher mean airway pressures
• Flow waveform always decelerating (flow slows as
  it reaches the pressure limit)
• Good
• Patient synchrony/comfort
• Noncompliant lung units are more likely to be
  inflated in PCV due to decelerating flow and
  maintenance of airway pressure over time.
• Pitfall
• Change in compliance/mechanics change in
  delivered tidal volume.
• Autopeep decreased driving pressure

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Volume Controlled Ventilation

• Fixed peak flow rate (60L/min)
• Specified flow pattern (constant or decelerating)
• Fixed Tidal Volume
• Fixed inspiratory time VT/Flow rate
• Pressure varies with lung mechanics
• Pitfall
• Respiratory alkalosis
• Airway pressures may be too high (ARDS)

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SIMV

• Fixed volume delivered at a set rate
• Spontaneous breaths can be taken and/or supported
• Breaths synchronized to prevent "stacking
• Good set minute ventilation
• Pitfalls Difficulty adapting to intermittent
  nature of assistance..decrease wob less than
  desired

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Work of Breathing and SIMV
Esoph balloon -1cmH20, 10ml/kg, 60L/min
WorkInflation pressure/volume change compared
between passive and active conditions Increased
WOB with demand-valve circuitry Respiratory
sensor output does not adjust breath to breath
change in respiratory load IMV may contribute to
respir muscle fatigue or prevent recovery from it
Marini et al, Am Rev Respir Dis 1988 1381169
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SIMV
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New and Salvage Modes

• Airway pressure release ventilation bilevel
• Pressure-regulated volume control
• Proportional Assist Ventilation
• Automatic Tube compensation

None have been shown to improve patient outcomes
compared to traditional modes
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Airway Pressure Release Ventilation

• The ventilator cycles between an upper and lower
  CPAP level
• SEVERE inverse IE Usually of 8-91 SHORT expir
  time is key (cant allow full exhalation), and
  longer Timore oxygenation
• The pressure is intermittently released to a
  lower level, thus eliminating waste gas
• Time-triggered, pressure-limited, time-cycled
  mode
• The key element of APRV is that the baseline
  airway pressure is the upper CPAP level

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APRV evidence?

• Improves aeration (Wrigge. Anesth. 200399376)
• 24pts with ARDS and APRV vs PSV (Purensen, AJRCCM
  1999159
• reduced shunt, dead space, VQmismatch
• 30 pts with ARDS after trauma (Putensen, AJRCCM
  2001)
• Improved LOS (23 vs 30 p0.032), Length of vent
  support (15 vs 21 p0.032)
• Increased Crs, PaO2, CI, reduced shunt

Chiang AA. CCM 199422 (9)1431
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BILEVEL

• Bilevel CPAP or BIPAP is APRV with spontaneous
  breathing.
• Can also be used with PCV
• A valve allows the patient to breath
  spontaneously at either CPAP/PEEP levels, and
  partial assistance (pressure support or automatic
  tube compensation) is used to assist breaths.
• Well tolerated

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Pressure Regulated Volume Control

• Form of assist-control ventilation.
• Breaths can be ventilator initiated (control
  breath) or patient initiated (assist breath)
• Constant pressure applied throughout inspiration
  (like pressure control)
• Ventilator adjusts pressure from breath to
  breath, as patient's airway resistance and
  respiratory system compliance changes, in order
  to
• deliver the set tidal volume
• If the delivered volume is too low
• it increases the inspiratory pressure
• on the next breath.
• If it is too high it decreases
• the pressure

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Proportional Assist Ventilation

• Vent guarantees the of work which it does in
  the face of changes in respiratory system
  (elastance, resistance, flow demand by patient)
• NO PRE-SET TV, flow, or pressure
• Flow Asssist (FA cmH20/L/s) or Volume Assist
  (VA cmH20/L) are delivered
• Varies from breath to breath based on patient
  effort and positive pressure at the airway
  opening based on the levels of set VA or FA
• This mode is interactive,
• as the ventilator varies its
• output to maintain its
• proportion of the workload

Ranieri V. J Appl Physiol 19968142636. Wrigge
H. Intensive Care Med 1999257908. Giannouli E.
Am J Respir Crit Care Med 1999159171625. Gras
so S. Am J Respir Crit Care Med 200016181926.
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High Frequency Oscillation

• High Peep, TINY tidal volume (1-5ml/kg) at high
  RR (60-300)
• Avoid over-distention
• High rate clears CO2
• Some Case series in adults have reported some
  efficacy in salvage cases
• Improved PaO2 and PaCO2 with lower FIO2 with
  apparent safety
• Multiple trials show no benefit over mechanical
  ventilation

Forte, CCM 1997 25937 Mehta, CCM
2001291360 Derdak, AJRCCM 2002166801
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Othermainly in salvage ARDS

• Inverse ratio ventilation (IEgt1)
• Reduce cardiac output (high pressures, autopeep)
• NO improves PaO2 but not mortality in ARDS
  (Dellinger, CCM 19982615)
• Higher PEEP increases PaO2/FIO2 and compliance
  but not mortality (NEJM 2004351327-336)
• Prone positioning improves PaO2 but not mortality
  
• (Gattinoni, NEJM 2001345568)
• ECMO lung to rest while on bypass
• Heroic salvage
• European trial ongoing CESAR

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Trouble shooting
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Monitoring Respiratory Mechanics

• Assessment of mechanics is useful in vented
  patients
• Insights into the pathophysiology
• Pressure, flow, and volume in ventilator circuit
• Derived measures
• Compliance
• Resistance
• Time-based graphics (waveforms)
• Pressure
• Flow
• Volume

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Assessing Mechanics

• Typical Mechanics/Use of waveforms
• PIP, Pplat, RAW
• auto-PEEP
• Compliance CL, CCW, Crs

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Case

• PC IP 20, TV 400ml, RR set 28
• Gas PaCO2 60, PaO2 60
• Resident increases RR to 32
• 1hr later, you are called to a pt with an SpO2
  88, TV now 260ml
• ABG PaCO2 88, PaO2 55
• What do you think?
• Compliance problem and needs increased IP?

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Waveform analysis
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Waveform analysis
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Measuring auto PEEP End Expiratory pause No
active exhalation or inspiratory effort Treats
lungs as single compartment
PIP
PIP
pressure
auto PEEP
set PEEP
time
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Flow Waveform
flow
inhalation
time
0
exhalation
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Auto-peep

• Gas trapped at end expiration
• Insufficient E time
• Inability to trigger (multiple attempts leads to
  autoPeep and high elastic recoil)
• Raises the baseline pressure decreased driving
  pressure
• Further adds to difficulty in triggering
• Problem
• Poor ventilation (lower driving pressure) and
  oxygenation
• Increased work of breathing Tachypnea,
  Dyssynchrony
• High airway pressures?pneumothorax hemodynamic
  effects
• Solution
• Reduce RR
• Bronchodilate, clear airway
• ?Other factors agitation, pain, fever
• Applied PEEP
• Possibly Increase inspiratory flow rate (to
  decrease I time and increase E time)but may lead
  to tachypnea

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Auto-PEEP and Triggering
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Case

• You are called to assess a patient who has a RR
  of 45 and is described as guppy breathing.
• The nurse has tried sedation, suctioning, and the
  RR was increased to 35 by the resident, but the
  pattern continues.

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Waveform analysis
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Waveform analysis

• Inspiratory flow not high enough scooped inward
• Patient attempting to trigger more and unable too
  (wants more breath)

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Increasing peak flow

• Peak flow should be roughly four times that of
  the minute ventilation (MV 15L requires PFgt60L)
• If the patient is breathing spontaneously, adjust
  to match their efforts
• In panel 1 the peak flow (PF) is 40 l/min and the
  patient is slightly dysynchronous.
• In the middle panel the PF is 50 l/min and the
  patient remains dysynchronous.
• In the last panel the PF is 100 l/min, but the
  peak airway pressure is too high.

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Respiratory System Compliance
Decreased with

• mainstem intubation
• congestive heart failure
• ARDS
• atelectasis
• consolidation
• fibrosis
• hyperinflation

• tension pneumothorax
• pleural effusion
• abdominal distension
• chest wall edema
• thoracic deformity

nl 50 100 mL/cm H2O (intubated patients)
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Pressure
PEEP
time
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PIP
No active breathing Treats lung as single unit
Pplat
PEEP
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Pplat Palv Pplat Transpulmonary Pressure?
transpulmonary pressure 15 cm H2O
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Pplat Palv Pplat Transpulmonary Pressure?
transpulmonary pressure 45 cm H2O
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Pplat Palv Pplat Transpulmonary Pressure?
Risk of VILI may be different with the same Pplat
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?Peso ?Ppl
Benditt, Respir Care 2005 5068
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Transpulmonary Pressure
ventilator support
patient effort
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Full Ventilator Support
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PIP
Ppl (Peso)
Palv (Pplat)
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Inspiratory Resistance
nl 5 10 cm H2O/L/s (intubated patients)
measure with 60 L/min (1 L/s) constant flow
Increased with

• Secretions
• Bronchospasm
• Small endotracheal tube

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Goals When Setting The Ventilator

• Avoid alveolar over-distension
• Provide adequate alveolar ventilation
• Promote patient-ventilator synchrony
• Apply PEEP to maintain alveolar recruitment
• Provide adequate oxygenation
• Use the lowest possible FIO2
• Avoid auto-PEEP

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WEANING
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Approach to Weaning
Full or partial ventilatory support
Disease resolution Adequate gas
exchange Hemodynamic stability Ability to breathe
determine cause of failure
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Most Patients Dont Need Weaning

• Normal Crs
• Normal Raw
• Awake
• They need to be liberated!

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Weaning Approaches

• Spontaneous breathing trial with T-piece
• Spontaneous breathing trial on ventilator
• Pressure support ventilation (PSV)
• Synchronized intermittent mandatory ventilation
• SIMV PSV
• AC modes titrated down (ACVC, PC)

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Work of Breathing and SIMV
Esoph balloon -1cmH20, 10ml/kg, 60L/min
WorkInflation pressure/volume change compared
between passive and active conditions Increased
WOB with demand-valve circuitry Respiratory
sensor output does not adjust breath to breath
change in respiratory load IMV may contribute to
respir muscle fatigue or prevent recovery from it
Marini et al, Am Rev Respir Dis 1988 1381169
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Comparison of Weaning Methods

• Prior Trials with IMV
• Schacter et alIMV vs other no difference
• Retrospective, nonuniform gps,poor protocol
• Hastings SBT vs IMV at rate 4
• Post-op cardiac little difficulty expected
• Linson IMVSBT
• Weighted toward short term support
• 2/3 weaned in 2hrs and had vent support lt72hrs
• Others with poor protocols/retrospective, etc

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Comparison of Weaning Methods

• Brochard, Am J Respir Crit Care Med 1994 150896
• 456 Patients screened via Tpiece 75 extubated
• SBT via Tpiece could be done up to 8x/day and
  potentially not extubated until 3 trials of 2hrs
  had been achieved !!!!
• increased demand/wobfatigue

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Comparison of Weaning Methods

• Esteban et al, N Engl J Med 1995 332 345
• 546 Patients (7.5/-6days) screened via T-piece
  ?75 extubated
• Greatest Success with SBT
• IMV was favored with least days (6.5) on vent vs
  10.8 PSV, 11.5 SBT x 2hrs vs 8.4 SBT x 30min

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Evidence Based Recommendation

• Patients receiving mechanical ventilation for
  respiratory failure should undergo a formal
  assessment of discontinuation potential if the
  following criteria are met
• Evidence for some reversal of the underlying
  cause for respiratory failure
• Adequate oxygenation and pH
• Hemodynamic stability (absence of active
  myocardial ischemia, absence of significant
  hypotension)
• Capabile of initiating inspiratory efforts

EB Guidelines for weaning and discontinuation of
ventilatory support. Task force ACCP, AA Respir
Care, AC of CCM Chest, December 2001
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Evidence Based Recommendation

• Patients should be assessed with a daily
  spontaneous breathing trial (SBT)
• An initial brief period of spontaneous breathing
  can be used to assess the capability of
  continuing on to a formal SBT
• Consider ventilator discontinuation after a
  well-tolerated SBT of 30-120min

Chest, December 2001
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Evidence Based Recommendation

• Criteria for tolerance
• Respiratory pattern
• Subjective comfort
• Hemodynamic stability
• Adequacy of gas exchange
• Weaning Parameters are not predictive!
• Minute ventilation, RR, TV, MIP all have
  sensitivity from 0.76-1.0 but POOR specificities
  (0.11-0.54) Yang and Tobin. NEJM 19913241447
• RSBI best (sens 0.97 spec 0.64)-lt105 on 0/0

Yang and Tobin. NEJM 19913241447 Epstein,
AJRCCM.199515254S Chatila, AJM
199610161 Epstein. AJRCCM 19961541647 Jacob,
CCM 199725253 Meade Chest 2001120400S-424S
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Approach to Weaning
Full or partial ventilatory support
Disease resolution Adequate gas
exchange Hemodynamic stability Ability to breathe
determine cause of failure
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Weaning Summary

• Weaning parameters are of limited usefulness
• SBT is the best means of determining when a
  patient is able to sustain spontaneous breathing
• Weaning success is lower with SIMV than trials of
  spontaneous breathing or PSV
• The need for a ventilator should be separated
  from the need for an airway

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Avoid Ventilator-Induced Lung Injury
Gas Exchange
Hemodynamics
Setting the Ventilator
Triggering Cycling Flow
Patient Comfort Individual Issues
Modes
Use Waveforms
Liberation From the Ventilator SBT!
Understand Mechanics