Exposures

Oxygen exposure variables were collected from the onset of ARDS for 7 days, or until unassisted breathing (defined as extubation, or breathing without ventilatory assistance with a PEEP ≤ 5 cm H₂O for one whole calendar day) or death (whichever occurred earliest). These variables were collected for up to 7 days because early oxygen exposure has previously been demonstrated to have significant effect on outcomes in other cohorts of critically ill patients [5, 11, 19]. The primary exposure of interest was the average time-weighted PaO₂ calculated over a maximum of 7 days from meeting ARDS criteria [20]:

T-PaO₂ = mean PaO₂ × time (between consecutive timepoints where PaO₂ was measured). T-Total = time between the first and last PaO₂ measurement during the period of mechanical ventilation after meeting ARDS criteria, up to 7 days.

Hyperoxaemia was defined prospectively as a PaO₂ greater than 14 kPa (105 mmHg) [11]. Excess oxygen exposure was defined as any FiO₂ > 0.5 in patients with a PaO₂ > 10.7 kPa (80 mmHg), and was calculated at 24-h intervals with a cumulative value obtained [16].