The primary objective of this study is to establish that Nesiritide (Natrecor) is effective in reducing pulmonary hypertension (PHTN) acutely as measured by a 20% reduction in the mean pulmonary arterial (PA) pressure. The secondary objectives will include: improvement in pulmonary vascular resistance (PVR), patient symptoms, exercise tolerance, frequency of toxicity, and surgeon's willingness to proceed with operation.