During a 48 hrs pre-randomization period, control mechanical ventilation will be set with a Tidal Volume (VT) of 6 ml / kg (for ideal weight) and a Pplat limited to 30 cm H2O; inspiratory flow 50-70 l / min with end-of-breath pause of 0.2-0.5 sec, ratio I: E from 1: 1 to 1: 3, respiratory frequency of 20-35 steps to maintain 7.3 <pH <7.5. If the pH is <7.30 the respiratory frequency will increase up to 35 / min; If the pH is> 7.5, the respiratory rate will be progressively reduced to the target pH range. FiO2 and The PEEP will be set according to the ARDSnet table (33)