lung injury of acute onset, within 1 week of an apparent clinical insult and with progression of respiratory symptoms bilateral opacities on chest imaging (chest radiograph or CT) not explained by other lung pathology (e.g. effusion, lobar/lung collapse, or nodules) respiratory failure not explained by heart failure or volume overload decreased PaO 2/FiO 2 ratio (a decreased PaO 2/FiO 2 ratio indicates reduced arterial oxygenation from the available inhaled gas): mild ARDS: 201 – 300 mmHg (≤ 39.9 kPa) moderate ARDS: 101 – 200 mmHg (≤ 26.6 kPa) severe ARDS: ≤ 100 mmHg (≤ 13.3 kPa) Note that the Berlin definition requires a minimum positive end expiratory pressure (PEEP) of 5 cmH 2O for consideration of the PaO 2/FiO 2 ratio. This degree of PEEP may be delivered noninvasively with CPAP to diagnose mild ARDS.