The investigators will prospectively study a total of 40 adult patients (> 18 years) divided into two subgroups: 20 laparoscopic surgery patients and 20 open surgery patients. After induction of anesthesia and neuromuscular blockade, all patients will be submitted to a recruitment maneuver in pressure-controlled ventilation (PCV) mode (PEEP = 20cmH2O, driving pressure = 20cmH2O, respiratory rate (RR) of 15 ipm and I:E ratio of 1:1) for 2 minutes followed by a decremental PEEP titration. The first step of the titration will start at a PEEP of 20 cmH2O and every 40 seconds PEEP will be decreased by 2 cmH2O, until a final PEEP of 4 cmH2O. A sub-sample of patients will have each step of the PEEP titration procedure recorded with an ultrasound and later evaluated by two different investigators. Optimal PEEP will be defined as that with the best compromise of atelectasis and overdistension as measured by EIT.