On the basis of published studies in the literature (Squadrone et al. (16%), Lawrence et al.(4.5%), Hall et al.(15%), Mitchell et al.(11%) e Canet et al. (7.2%), the investigators supposed that the primary outcome (PaO2/FiO2 <300) will be around 7%. Using a 0.05 two tails alfa, we can obtain a precise two tails 95% confidence intervals (5.8% -8.4% according to the Wald adjusted method), obtaining a sample size of 1422 patients (software STATA 9.0).