Prone ventilation can improve refractory hypoxaemia in ARDS but its effects on mortality have not been impressive in some studies which may be underpowered or include patients with less severe hypoxaemia. An updated meta-analysis showed significantly reduced ICU mortality in the four recent studies that enrolled only patients with ARDS, as opposed to ARDS/ALI (odds ratio = 0.71; 95% confidence interval = 0.5 to 0.99; P = 0.048; number needed to treat = 11). There may also be benefit from a greater duration of prone positioning.

An updated study-level meta-analysis of randomised controlled trials on proning in ARDS and acute lung injury
Critical Care 2011, 15:R6 Full text