The purpose of this study is to assess the safety and efficacy of istaroxime in patients hospitalized for Acute Decompensated Heart Failure (ADHF) not requiring inotropic therapy.This will be done by comparing the hemodynamic effect of a 24-hour infusion of three different doses of the drug versus placebo. Efficacy will be measured as a change in Pulmonary Capillary Wedge Pressure from pre-infusion to 6 hours after infusion start. Secondary objectives will include the evaluation of clinical efficacy and safety through assessment of cardiovascular and renal tolerability as well as changes in biological markers such as brain natriuretic peptide (BNP) and troponin I (TNI), and the neurohormones renin and aldosterone and also to assess the pharmacokinetics of istaroxime and its metabolites.