In this prospective, randomized trial, including patients scheduled for a major open or laparoscopic abdominal surgery (duration >2 hours) under general anesthesia , the investigators will compare 2 strategies of protective mechanical ventilation: a fixed intraoperative PEEP of 6 cmH2O and an individualized intraoperative PEEP according to the driving pressure.

Patients will be randomized into 2 groups:

• Fixed PEEP group: will receive protective intraoperative mechanical ventilation: tidal volume: 6 ml/kg of PBW, recruitment maneuvers every immediately after tracheal intubation and every 2 hours and a fixed PEEP of 6 cmH2O
• Individualized PEEP: protective mechanical ventilation with the same modality with an individualization of PEEP levels hourly in order to achieve the lowest driving pressure. PEEP levels will be adjusted after intubation and hourly by increasing or decreasing PEEP levels every 10 respiratory cycles.

The primary outcome: incidence of post operative pulmonary complications during the first post operative days.

Inclusion Criteria:

• BMI<35, ARISCAT score > 26, patients scheduled for major abdominal surgery

Exclusion Criteria:

• unplanned post operative ICU stay and mechanical ventilation