Acute respiratory failure may occur early in the postoperative course, requiring endotracheal intubation and mechanical ventilation in selected patients, thus increasing morbidity and mortality and prolonging intensive care unit (ICU) and hospital stay.We will perform a multicenter, prospective, randomized clinical trial to compare the efficacy of non invasive ventilation (NIV) which associated pressure support ventilation (PSV: 5 to 15 cmH2O) and positive end expiratory pressure (PEEP: 5 to 10 cmH2O) with standard oxygen therapy in the treatment of postoperative acute respiratory failure. We also set out to examine the hypothesis that early application of NIV may prevent intubation and mechanical ventilation in patients who develop acute respiratory failure after abdominal surgery.