Shah: Mechanical Ventilation, Focusing on the basics

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)

• 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)