Shah: Mechanical Ventilation, Focusing on the basics

Today we welcome back Dr. Nirav Shah, program director for the Pulmonary and Critical Care Fellowship program at the University of Maryland Medical Center. Today Dr. Shah takes us back to the basics of ventilator use. Even the experts in the audience will find tips and tricks to take back to the ICU in order to improve patient care. And for all the budding intensivists out there, this is a talk you cannot miss!!

Clinical Pearls (Assisted by Dr. Helen Prevas)

Respiratory failure:

  • Type 1: hypoxic
    • #1 cause V/Q mismatch
      • Decreased ventilation (airway, ILD)
      • Increased perfusion to normally ventilated lung (PE)
    • Shunt
      • ARDS, PNA, edema
  • Type 2: hypercarbic
    • Need to correct decreased alveolar hypoventilation
      • Increase minute ventilation +/- decrease dead space (if hypoxic, will correct with oxygenation)

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  • Treatment:
    • Reverse cause + positive pressure
      • Decrease work of breathing
      • Restore adequate gas exchange
    • Noninvasive (NIV)
      • CPAP, BiPAP, cuirass
    • Invasive (ETT or tracheostomy)
      • volume, pressure, hybrid, or “novel” modes (i.e., APRV, NAVA)

Goals:

  • Ventilation
    • Acceptable pCO2 + pH
    • Goal Pplat <30
    • Decrease auto-PEEP (breath stacking)
  • Oxygenation
    • Goal SpO2 >88% on FiO2 <60%
    • Optimal delivery of O2:  (DO2) = CO x (1.34 × Hb × SaO2) + (0.003 × PaO2)
  • ARDS
    • tidal volume ≤ 6 cc/kg IBW (based on height)
    • Conservative fluids 
  • Avoid VILI

Ventilator Definitions:

  • Control
    • How a breath is delivered (V vs. P vs. dual)
  • Triggering
    • When inspiration starts (flow or pressure)
  • Cycling
    • What determines switch from insp to exp
    • Time or flow sensed
  • Breaths
    • Mandatory, assisted, spontaneous
  • Flow pattern
    • Often set by the method of control
    • Sinusoidal, accelerating, constant (square), decelerating
  • Mode or breath pattern:
    • Spontaneous, assist control (AC), intermittent mandatory ventilation (IMV)
    • Pressure supported (PS) or volume supported (VS)
  • Scalar
    • Waveforms that plot pressure, flow, or volume vs time on the ventilator screen
  • Loops
    • Pressure or flow vs. volume (look like PFTs upside down)

Waveforms:

  • Allow provider to:
    1. Assess real time changes in patient condition
    2. Optimize vent settings and treatment
    3. Determine effectiveness of vent
    4. Detect adverse events
    5. Decrease risk of mechanical complications
  • Pressure waveforms
    • Diagnose: air trapping, obstruction, dyssynchrony, pressures (i.e., plateau or end inspiratory hold), triggering, bronchodilator response
    • Area under the pressure curve = Alveolar distending pressure
  • Flow waveforms
    • Detect air trapping, obstruction, bronchodilator response, triggering, dyssynchrony
    • Square
      • Pressure rises (higher peak inspiratory pressure)
    • Decelerating
      • Pressure constant
    • Plateau pressure is greater for Square vs. Decelerating with same volume
  • Volume waveform
    • Can detect air trapping, leak, tidal volume, dyssynchrony

Ventilator adjustments:

  • I (inspiratory) time
    • Set for PRVC and PC, VC on some vents (otherwise set flow)
      • PS: cycles once falls below set % of peak flow (decelerating flow)
    • Decrease Ti: less autoPEEP, decrease CV effects
    • Increase Ti: improves oxygenation
  • Trigger sensitivity
    • Increased sensitivity can lead to autotrigger/false triggering (water in circuit, heartbeat)
  • Rise time
    • Rate of rise of pressure (PC) or flow (VC)
      • Short: may be uncomfortable, but can lead to decreased inspiratory workload
        • Reduced VILI (biotrauma)
      • Long: decreased Tv in pressure mode (less time at set pressure) or increased pressure in volume mode (to reach target volume)

Troubleshooting:

  • Dx increased peak without change in Pplat
    • Increased resistance → obstruction, bronchospasm, biting, foreign body
  • Dx increase peak and Pplat
    • Decreased lung compliance (or increased Vt)
      • PTX, abdominal HTN, ARDS, edema
  • How to detect AutoPEEP
    • Flow not returning to 0 before next breath
    • Area under inspiratory curve not equal to area under expiratory curve
    • Dyssynchrony (double triggering)
  • Treating airway obstruction:
    • Bronchodilators
    • Suction
    • Prolong expiration
      • Decrease I time
      • Increase flow
    • Sedate if necessary
    • Increase extrinsic PEEP (2/3 intrinsic)

Suggested Reading

  1. Bone RC, Eubanks DH, Gluck E. Beyond the basics: operating the new generation of ventilator. A look at the features and functions of these units. J Crit Illn. 1992 May;7(5):770-82; 787-8.[PubMedLink]
  2. Bone RC, Eubanks DH. The basis and basics of mechanical ventilation. Dis Mon. 1991 Jun;37(6):321-406.[PubMed Link]
  3. Carbery C. Basic concepts in mechanical ventilation. J Perioper Pract. 2008 Mar;18(3):106-14.[Pubmed Link]
About the Author

Jim Lantry

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Just your average critical care doc: Wandering the ED and ICUs for the USAF down in the San Antonio Military Medical Center, traveling the globe to cannulate for ECLS wherever the need arises, and trying to keep up with great minds of today. E: JlantryMD@gmail.com

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Comments

  1. JY Kim

    Question on the last suggestion in Dr. Shah’s presentation regarding managing autoPEEP. I believe he was suggesting to increase extrinsic PEEP to more closely approximate the intrinsic PEEP and so reduce the negative pressure needed to be recruited by the patient to trigger a breath. Sounded like the patient was not receiving adequate breaths in addition to the problem of his autoPEEP?

    Wouldn’t it be easier to simply set the vent mode to SIMV or some other similar setting which would give the patient a minimum number of breathes. Although increasing extrinsic PEEP (but keeping it below the intrinsic PEEP) would probably not affect the patient’s ability to expire in any really significant way, it just seems counterintuitive to increase PEEP in a patient that is already having trouble expiring an adequate volume. Any clarification or thoughts would be appreciated. Thank you!

    1. Author
      Jim Lantry

      I will pass this question over to him and allow Dr. Shah to articulate a response.

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