Surgical repair is the primary therapy for congenital heart defects in the newborn. The neonatal cardiovascular system is at particular risk to develop the surgery-related low cardiac output syndrome (LCOS), thus vasoactive agents are routinely used in the postoperative management. Systematic research on the efficacy of these drugs is scarce in the newborn. As LCOS pathophysiology joints impaired myocardial contractility and the peripheral effects of ischemia/reperfusion injury on the endothelium, early use of inodilators (IND) are strongly recommended to reduce afterload and improve contractility. This study aims to test the equivalence in dose-dependent hemodynamic effects of 2 IND, Milrinone and Levosimendan, used early without loading dose in the preoperative period to prevent LCOS. By means of non-invasive technology the investigators will assess cardiac function (serial structural and functional echocardiography), the cerebral and peripheral perfusion and oxygenation (continuous near-infrared monitoring), cerebral function (continuous amplitude integrated EEG monitoring), will rule out CNS acquired lesions (serial transfontanelar echo-Doppler studies), and will follow up different biochemical markers of myocardial stress and apoptosis. Pharmacokinetic studies will be also performed.