Although inotropes have a favorable effect on central hemodynamics in patients with heart failure, their effect on renal hemodynamics is incompletely defined. The purpose of this study is to evaluate the efficacy of a 75 min intravenous infusion of levosimendan compared to a 75 min infusion of dobutamine on renal hemodynamics and function in patients with chronic heart failure and signs of cardiorenal syndrome. The investigators hypothesis is that patients treated with levosimendan will show greater increases in renal blood flow and glomerular filtration rate (GFR) than those treated with dobutamine.