Patients in each subgroup will be randomized to one of two ventilatory strategies: (1) PEEP chosen by the PEEP titration procedure; (2) PEEP set at 4 cmH2O. After a new recruitment maneuver PEEP will be set at the designated value and the patient will be ventilated with an inspired oxygen fraction of 50% or greater in order to maintain peripheral oxygen saturation (SpO2) > 96%, a tidal volume of 6 mL/Kg and a respiratory rate to maintain an end tidal carbon dioxide (ETCO2) between 35-45.