 2. Experimental design: Patients undergoing routine cardiac catheterization will be studied. A thermodilution pulmonary artery catheter will be inserted through a venous sheath and threaded into the pulmonary artery. After several minutes of quiet rest, baseline hemodynamics will be obtained, including a determination of cardiac output using thermodilution techniques and 30 seconds of pressure recordings from the left ventricle and proximal aorta. In addition, a brief echocardiogram will be performed, to determine ejection fraction and indices of diastolic filling. Finally, arterial tonometry will be obtained in quick succession from the brachial, radial, femoral, and carotid arteries. Following the collection of baseline data, a bolus and infusion of nesiritide will be started. After 10 minutes of nesiritide infusion, with the drug still infusing, a full set of invasive hemodynamics will again be obtained, as well as brief echocardiographic and arterial tonometry examinations. 