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