Acute respiratory distress syndrome (ARDS)
Onset within one week of a known clinical insult or new or worsening respiratory symptoms.
Chest imaging (that is, X‐ray, computed tomography scan, or lung ultrasound): bilateral opacities, not fully explained by volume overload, lobar or lung collapse, or nodules.
Origin of pulmonary infiltrates: respiratory failure not fully explained by cardiac failure or fluid overload. Need objective assessment (for example, echocardiography) to exclude hydrostatic cause of infiltrates/oedema if no risk factor present.
Oxygenation impairment in adults:
mild acute respiratory distress syndrome (ARDS): 200 mmHg < ratio of arterial oxygen partial pressure/fractional inspired oxygen (PaO2/FiO2) ≤ 300 mmHg (with positive end‐expiratory pressure (PEEP) or continuous positive airway pressure (CPAP) ≥ 5 cmH2O, or non‐ventilated);
moderate ARDS: 100 mmHg < PaO2/FiO2 ≤ 200 mmHg (with PEEP ≥ 5 cmH2O, or non‐ventilated);
severe ARDS: PaO2/FiO2 ≤ 100 mmHg (with PEEP ≥ 5 cmH2O, or non‐ventilated);
when PaO2 is not available, SpO2/FiO2 ≤ 315 mmHg suggests ARDS (including in non‐ventilated patients).
Oxygenation impairment in children: note OI = Oxygenation Index and OSI = Oxygenation Index using SpO2. Use PaO2‐based metric when available. If PaO2 not available, wean FiO2 to maintain SpO2 ≤ 97% to calculate OSI or SpO2/FiO2 ratio:
bilevel (non‐invasive ventilation or CPAP) ≥ 5 cmH2O via full‐face mask: PaO2/FiO2 ≤ 300 mmHg or SpO2/FiO2 ≤ 264;
mild ARDS (invasively ventilated): 4 ≤ OI < 8 or 5 ≤ OSI < 7.5;
moderate ARDS (invasively ventilated): 8 ≤ OI < 16 or 7.5 ≤ OSI < 12.3;
severe ARDS (invasively ventilated): OI ≥ 16 or OSI ≥ 12.3.