Background: Different ventilatory management strategies could possibly prevent the damaging effects of Cardiopulmonary Bypass (CPB). We conducted this trial to compare the effects of different continuous positive airway pressures (CPAP) applied during CPB on postoperative pulmonary function.Methods: We randomly assigned 45 patients undergoing coronary artery bypass graft (CABG) surgery into 3 groups. Patients will either be disconnected from mechanical ventilation (MV), or receive either 5 or 10 Cm H2O of CPAP by passive air insufflation during CPB. Alveolar recruitment with 30 Cm H2O applied for 30 seconds was done forall patients before reinstitution of MV.Results: PaO2 was significantly (P<0.05) higher in the group receiving 10 Cm H2O compared to the other two groups at every point of measurement other than baseline.There was no significant difference in compliance, time to weaning from MV, time to extubation, ICU and total hospital length of stay among the 3 groups.Conclusion: Patients who received higher CPAP during CPB had significant improvement of oxygenation, probably secondary to avoidance of atalectasis during CPB; this however was not associated with significant improvement of compliance or other clinical parameters.