They were allocated into four different groups : 1 ) Gpre : treated with positive pressure in the BiPAP mode ( Bi-Level Positive Airway Pressure ) before surgery for one hour ; 2 ) Gpos : BIPAP after surgery for one hour ; 3 ) Gintra : PEEP ( Positive End Expiratory Pressure ) at 10 cmH2O during the surgery ; 4 ) Gcontrol : only conventional respiratory physiotherapy .