Intervention (if applicable): no intervention Main study parameters/endpoints: The primary endpoint of the study is to describe the pharmacokinetic profile by determining plasma levels of levosimendan and its metabolites during 12 days following a 24-hour infusion in children with heart failure in different age groups. Secondary endpoints are the clinically measured hemodynamic variables (heart rate, bloodpressure, lactate, troponin, pro-BNP, venous saturation) and echocardiographic variables (ejection fraction, shortening fraction, tissue Doppler) of all patients.