After IRB approval and informed consent , 50 MO patients undergoing laparoscopic gastric bypass under volume-controlled ventilation ( tidal volume 6 ml kg ( -1 ) of IBW ) were randomly ventilated with either 10 cm HO PEEP or with 10 cm HO PEEP and one RM carried out after induction of pneumoperitoneum , and another after exsufflation .