Pressure Support Ventilation (PSV): patient's spontaneous activity will maintained and sedation will be maintained at a level of Richmond Assessment Sedation Scale (RASS) between -2 and -3. The level of pressure support (including PEEP) will be limited to ≤ 30 cmH2O; the pressure support level will ensure a tidal volume of 6 ml / Kg ideal body weight. PEEP, FiO2 and respiratory rate will be regulated according the ARDSnet protocol (33)