Effect of Intraoperative Dynamic Compliance Guided Individualized Positive End-expiratory Pressure on Postoperative Atelectasis After Laparoscopic Bariatric Surgery

"Basic ventilation: Volume-controlled ventilation (VCV) mode with positive end-expiratory pressure (PEEP) of 8cm H2O after induction of anesthesia，~Recruitment maneuver (RM): Pressure-controlled ventilation (PCV) mode increasing PEEP from 10 to 25cmH2O in step of 5cmH2O per 30s.~PEEP-titration maneuver: ventilation parameters reset as basic ventilation with PEEP still 25cmH2O.Decrease PEEP to 5cmH2O in step of 2cmH2O per 10 respiratory cycles to confirm the highest dynamic lung compliance (Cdy).~After titration: A new RM will be performed and the final PEEP will be the one related to highest Cdy plus 2cm H2O.~Randomized after the second RM. Individualized PEEP arm: maintain individualized PEEP; PEEP8 arm: maintain a fixed PEEP of 8cm H2O.~VCV with other ventilation parameters the same as basic ventilation until extubation .~After discharged from postoperative anesthesia care unit (60 to 90 minutes after extubation): chest CT ."