FiO2 — Fraction of Inspired Oxygen

FiO2 ranges from 0.21 (room air) to 1.0 (100% oxygen). The SICS module recommends starting at FiO2 1.0 on admission to ICU and weaning rapidly using SpO2 as a guide. The target SpO2 in most patients is 94–98%. If the patient is already established at lower FiO2 from theatre and oxygenation is adequate, there is no need to increase it. High FiO2 contributes to oxygen toxicity and atelectasis (absorption atelectasis — nitrogen washes out and alveoli collapse).

PEEP — Positive End-Expiratory Pressure

PEEP maintains a positive pressure in the airway at the end of each breath, preventing alveolar collapse. It improves oxygenation by increasing functional residual capacity (FRC), recruiting collapsed alveoli, and reducing shunt. It also reduces atelectrauma by preventing the cyclical opening and closing of alveoli. Starting PEEP is typically 5–10 cmH2O for most patients; in ARDS, higher PEEP (10–15 cmH2O or more) is used to maintain alveolar recruitment. Excessive PEEP, however, can cause barotrauma, impair cardiac output by reducing venous return, and worsen oxygenation by overdistending already-open alveoli and diverting blood flow to collapsed units.

Tidal Volume

Tidal volume (VT) is the volume delivered per breath. For lung-protective ventilation, VT should be set at 6–8 mL/kg of predicted body weight (PBW, calculated from height and sex — not actual weight). This is particularly important in ARDS, where the accessible lung volume is dramatically reduced and conventional tidal volumes cause regional overdistension. The ARDSNet trial demonstrated a 22% relative reduction in mortality with 6 mL/kg versus 12 mL/kg VT in ARDS, establishing lung-protective ventilation as the standard of care.

Respiratory Rate

Respiratory rate is set to control PaCO2. The initial rate is typically 14–18 breaths per minute. PaCO2 is inversely related to minute ventilation (VT × rate), so increasing the rate lowers PaCO2 and increasing VT also lowers PaCO2. Rates above 20/min should be discussed with a senior clinician — very high rates can cause breath stacking (auto-PEEP) in obstructive lung disease and may generate dangerously high airway pressures.

Airway Pressures

Plateau pressure — measured during an inspiratory hold in volume-controlled ventilation — reflects alveolar pressure and should be kept below 30 cmH2O. Driving pressure (plateau pressure minus PEEP) is increasingly recognised as the most important predictor of VILI and should ideally be below 15 cmH2O. Peak airway pressure is higher than plateau pressure due to airway resistance and is less useful as a measure of alveolar stress, though very high peak pressures (>40 cmH2O) warrant investigation.

Parameter	Recommended Initial Setting	Key Rationale
FiO2	1.0; wean rapidly to SpO2 94–98%	Start safe; reduce to minimise O2 toxicity
PEEP	5–10 cmH2O (higher in ARDS)	Prevents atelectrauma; improves oxygenation
Tidal volume	6–8 mL/kg predicted body weight	Lung-protective; reduces VILI
Respiratory rate	14–18 /min	Titrate to PaCO2 target 4.5–6.0 kPa
Plateau pressure	Target <30 cmH2O	Limits barotrauma
Driving pressure	Target <15 cmH2O (plateau − PEEP)	Independent predictor of outcome in ARDS
I:E ratio	1:2 standard; 1:3–4 in obstruction	Allows adequate expiratory time