We quantitatively pooled the results of individual trials when possible. We expressed the treatment effect as a relative risk for dichotomous outcomes and as a weighted mean difference for continuous outcomes with 95% confidence intervals. We considered effects statistically significant if P < 0.05. A z test was used to statistically test the estimates of treatment effect between groups [16] . We assessed heterogeneity among trials using the I 2 statistic, and considered an I 2 value greater than 50% to indicate substantial heterogeneity [17] . RevMan 4.2 software and a random effects model were used to perform the analyses [18] . We chose the random effects model because it gives a more conservative estimate of the precision of the treatment effects and because the true effect of the intervention probably varies given the different populations enrolled in these trials [19] . A subgroup analysis was planned based on the aetiology of respiratory failure (trials enrolling exclusively patients with respiratory syncytial virus (RSV)/ severe bronchiolitis compared with all other trials) if sufficient data were available, because these trials were likely to enrol a younger, more homogeneous, population with a lower predicted risk of mortality. We also planned sensitivity analysis based on methodological features of the included trials (trials reporting adequate allocation concealment compared with all other trials).