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Tidal volume (VT)
  • Initially choose 8–10 mL/kg IBW
  • Avoid high volumes to prevent barotrauma
  • VT = ↑ MV and ↓ Paco2 and ↑ pH
  • If ARDS/ALI is present, use 6 mL/kg IBW
Respiratory rate (RR)
  • 12–14 breaths/min usually adequate
  • ↑ RR = ↑ MV and ↓ Paco2 and ↑ pH, but beyond a certain point, dead space ventilation and risk of breath stacking
Fraction of inspired oxygen (FiO2)Start at 1.0 and taper down quickly to achieve a goal Pao2 of >60 mm Hg and O2 saturation of >90%
Inspiratory flow
  • Usually 40–60 L/min
  • ↑ Flow = ↓ inspiratory time and ↑ expiratory time, thus ↓ I:E ratio
  • Useful in obstructive airways disease to decrease auto-PEEP, be careful though about the increase in peak airway pressure
Positive end-expiratory pressure (PEEP)
  • Typically set at 5 cm H2O
  • ↑ PEEP (up to 20–24 cm H2O) = ↑ oxygenation in ALI/ARDS
  • May lead to decreased venous return/hypotension, increased plateau pressure/barotrauma, may increase ICP (in theory)

IBW, ideal body weight; ARDS, acute respiratory distress syndrome; ALI, acute lung injury; I:E ratio, inspiratory to expiratory ratio; ICP, intracranial pressure.

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  • Trigger: What signals the ventilator to initiate the inspiration?
    • Time or patient effort (pressure or flow)
  • Target (limit): What limits/governs the airflow during inspiration?
    • Pressure or flow or volume (not time)
  • Termination (cycle): What signals the ventilator to stop the inspiration?
    • Time or pressure or flow or volume

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  • Mandatory—The machine triggers and/or cycles the breath
  • Spontaneous—Patient triggered and cycled (the patient determines the VT); may be assisted or unassisted
  • Assisted—The patient triggers the spontaneous breath and the machine does at least some of the work. The airway pressure rises above the baseline pressure (e.g., pressure support ventilation)

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  • Volume-cycled ventilation:
    • Controlled mechanical ventilation (CMV)—rarely used nowadays in the ICU
      • Time triggered, preset RR and VT, the patient cannot trigger any extra breaths
      • The patient should be heavily sedated/paralyzed
    • Assist control/volume control (AC/VC)—most common mode

      Time and patient triggered, preset RR and VT. Patients can trigger additional set tidal volumes; they do not necessarily need to be sedated or paralyzed. Beware of hyperventilation and respiratory alkalosis.

    • Intermittent mandatory ventilation (IMV)/synchronized intermittent mandatory ventilation (SIMV)—may also be used as a weaning mode (not routinely used today, may actually prolong the weaning process)

      IMV: time-triggered mandatory breaths, preset RR and VT. In between breaths, the patient can take additional spontaneous assisted breaths with a chosen pressure support (the VT varies with the effort).

      May lead to breath stacking (mandatory breath on top of a spontaneous breath).

      SIMV: similar to IMV, but the mandatory breaths are machine (time) or patient triggered, providing synchrony with the patient's effort and eliminating breath stacking.

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  • Flow-cycled ventilation: ...

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