Randomized clinical trial in which 136 patients underwent coronary artery bypass grafting between January 2011 and March 2012 were divided into three groups and admitted to mechanical ventilation with different positive end-expiratory pressure levels : Group A , 5 cmH2O ( n = 44 ) , Group B , 8 cmH2O ( n = 47 ) and Group C , 10 cmH2O ( n = 45 ) .