|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%|
- 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)
- 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
- 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)
- 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.
- Flow-cycled ventilation:
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