Heart failure (HF) is a disease that is caused by a reduced heart muscle function. Reduced heart muscle function can occur as a consequence of reduced pumping activity from a weak heart muscle or because of a stiff heart muscle. This study is looking at the effectiveness of Natrecor (nesiritide) in patients that require hospitalization due to worsening heart failure as a result of a stiff or thickened heart muscle. Natrecor is a man-made version of a protein that my body makes on its own and has been approved for the treatment of patients requiring hospital admission for heart failure and have shortness of breath at rest or with minimal activity.