Nine RCTs (3,596 participants) were included. All the trials reported random allocation and allocation concealment; none used blinding.
Lower versus higher tidal volume ventilation using similar PEEP values (four RCTs): Lower tidal volume ventilation significantly reduced hospital mortality (OR 0.75, 95% CI 0.58 to 0.96), but not mortality at follow-up or barotrauma.
Lower versus higher PEEP at low tidal volume ventilation (three RCTs): Hospital mortality and barotrauma did not differ significantly between groups. Based on two RCTs, higher PEEP reduced requirement for rescue therapy to prevent hypoxaemia (OR 0.51, 95% CI 0.36 to 0.71) and reduced mortality in patients who received rescue therapy (OR 0.51, 95% CI 0.36 to 0.71). Numbers of events presented in the paper for the lower PEEP strategy appeared to refer to the higher PEEP strategy and vice versa.
Combination of higher tidal volume and lower PEEP versus lower tidal volume and higher PEEP (two RCTs, n = 148): The combination of lower tidal volume and higher PEEP significantly reduced hospital mortality (OR 0.38, 95% CI 0.20 to 0.75) and barotrauma (OR 0.20, 95% CI 0.06 to 0.63).
Heterogeneity was not significant for the outcomes listed. Other outcomes and analyses were reported.