The primary objective of the VMAC study is to compare the blood flow and observe treatment and safety effects of the new dose of Natrecor to placebo, when added to standard care, in the treatment of acutely worsening CHF. The primary overall outcome that the study plan is based upon are the changes from the beginning of a study to 3 hours after the start of study drug, in pulmonary capillary wedge pressure (PCWP) (in subjects who have right heart catheters only) and the subject's self-evaluation of their breathing difficulties. The secondary objective is to compare the hemodynamic, (blood flow throughout the body) and clinical effects of Natrecor® with IV nitroglycerin and placebo. Additional objectives include a comparison of the use of other IV vasoactive agents and/or IV diuretics and the effects on other hemodynamic variables. The hypothesis of this study is that using the modified dose of Natrecor, (a 2-µg/kg bolus followed by a 0.01-µg/kg/min infusion) will achieve peak effects sooner than with previously studied doses, to sustain effects for at least 48 hours, and minimize excessive effects on blood pressure. Natrecor or placebo, administered as an intravenous 2-µg/kg bolus, followed by a fixed-dose infusion of 0.01-µg/kg/min.