Pediatric patients, especially infants undergoing open heart surgery have a predictable fall in cardiac index 6 to 18 hours after surgery, the so-called low cardiac output syndrome (LCOS). Patients, who have LCOS require more monitoring, more medication and a longer stay in intensive care unit. To prevent LCOS the phosphodiesterase inhibitor milrinone is routinely used during the first 24 hours after surgery. Levosimendan, a calcium- sensitizer improves cardiac muscle contractile force, vascular smooth muscle relaxation and coronary blood flow through calcium sensitization of the myocardial contractile filaments and opening of potassium channels without increasing oxygen consumption of the heart muscle cells. As the myocardium of infants is more calcium dependent than in later life, levosimendan should be of special benefit in this age group. The purpose of this study is to investigate whether levosimendan is superior to milrinone in preventing LCOS in infants after corrective open heart surgery.