Timing: respiratory failure within 1 week of a known insult or new and/or worsening respiratory symptoms
Origin: respiratory failure not fully explained by cardiac function or volume overload (need objective criterion such as echocardiography to exclude hydrostatic oedema if no risk factor is present)
Imaging: bilateral opacities on chest radiograph or CT not fully explained by effusion, collapse or nodules
Oxygenation: acute onset of hypoxaemia defined as PaO2/FiO2 <300 mmHg on at least PEEP 5 cmH2O
PaO2/FiO2 of 200–300 (using mmHg) is mild ARDS
PaO2/FiO2 of 100–200 (using mmHg) is moderate ARDS
