Patients with abnormal kidney function are at increased risk for complications following heart surgery, including worsening kidney function possibly requiring dialysis, a prolonged stay in the critical care unit and hospital, and the increased risk of death. Prior attempts at kidney protection for heart surgery patients have had mixed results. Two medicines, fenoldopam and N-acetylcysteine, have been shown to protect kidney function in other circumstances that cause kidney stress. The purpose of this study is to determine whether these medications will help to maintain the function of diseased kidneys during heart surgery.