The purpose of this randomized single center study is to determine the individual PEEP value that produces the best possible compromise of lung collapse and lung hyperdistention. Patients submitted to general anesthesia and mechanical ventilation during surgery (laparoscopy and open surgery) will participate. A PEEP titration procedure will be performed and the "optimal PEEP" value will be determined by electrical impedance tomography (EIT). An ultrasound will be used to record each step of the PEEP titration procedure in a sub-sample of patients. A total of 40 patients will be mechanically ventilated using physiological tidal volume (TV=6mL/kg of IBW) and fraction of inspired oxygen (FIO2) of 0.5 and will be randomized to one of two groups: "optimal PEEP" or a "low PEEP" (4cmH2O). Lung collapse and mechanics will be monitored by EIT throughout the intraoperative period. After extubation, a lung CT will be performed to evaluate the amount of lung collapse.