We assessed the renoprotective effect of the highest dose of N-acetylcystein sanctioned for clinical use in a prospective , double-blind , placebo-controlled study including 70 chronic kidney disease patients , stage 3 or 4 , who underwent coronary artery bypass graft surgery , on cardiopulmonary bypass ( CPB ) and off CPB , and were randomly allocated to receive either N-acetylcystein 150 mg/kg followed by 50 mg/kg for 6 hours in 0.9 % saline or only 0.9 % saline .