The effect of prone positioning in acute respiratory distress syndrome or acute lung injury: a meta-analysis. Areas of uncertainty and recommendations for research

Objective: To compare the effects of ventilation in prone and in supine position in patients with acute lung injury/acute respiratory distress syndrome (ALI/ARDS). Design: Meta-analysis of randomised controlled trials. Data sources: BioMedCentral, PubMed, CINAHL, and Embase ( to November 2007), with additional information from authors. Measurements and results: From selected randomised controlled trials comparing positioning in ALI/ARDS we extracted data concerning study design, disease severity, clinical outcomes, and adverse events. Five trials including 1,372 patients met the inclusion criteria for mortality analysis; one trial was added to assess the effects on acquisition of ventilator-associated pneumonia ( VAP). The included trials were significantly underpowered and enrolled patients with varying severity. Prone positioning duration and mechanical ventilation strategy were not standardised across studies. Using a fixed-effects model, we did not find a significant effect of prone positioning (proning) on mortality ( odds ratio 0.97, 95% confidence interval 0.77-1.22). The PaO2/FiO(2) ratio increased significantly more with proning ( weighted means difference 25 mmHg, p < 0.00001). Proning was associated with a non-significant 23% reduction in the odds of VAP ( p = 0.09), and with no increase in major adverse airway complications: OR 1.01, 95% CI 0.71-1.43. Length of intensive care unit stay was marginally and not significantly increased by proning. Conclusions: Prone position is not associated with a significant reduction in mortality from ALI/ARDS despite a significant increase in PaO2/FiO(2), is safe, and tends to decrease VAP. Published studies exhibit substantial clinical heterogeneity, suggesting that an adequately sized study optimising the duration of proning and ventilation strategy is warranted to enable definitive conclusions to be drawn.