The purpose of this study is to compare the hemodynamic (blood flow) and clinical effects of the study drug, Natrecor (nesiritide, a recombinant form of the natural human peptide normally secreted by the heart in response to heart failure) to those of intravenous nitroglycerin or placebo, when added to the standard care therapy that is usually administered in the treatment of patients with worsening congestive heart failure.