The purpose of the study is to compare the effect of fenoldopam and ketanserin on kidney function preservation in patients at high risk for renal failure after cardiac surgery. Acute, oliguric renal failure develops in up to 2% of patients undergoing cardiac surgery. Some of them require renal replacement therapy and despite that mortality in this group exceeds 30-60%. The investigators await that the use of fenoldopam and/or ketanserin may decrease the rate of severe renal failure.