The purpose of this study is to gain experience in administering nitric oxide (NO) to patients with critical pulmonary hypertension resistant to conventional therapy, and to develop criteria for predicting the likelihood of a patient deriving benefit from inhaled NO. Patients who will receive NO will be ventilator-dependent. Since NO in unstable in air or oxygen, it must be added to the inspired gas at a point where it is in contact with air for only a few seconds before being inhaled. The process of administering the NO to the inspiratory gas will naturally be affected by the ventilator, the mode of ventilation, and the mechanics of the total gas flow through the ventilator. Additionally, the act of mechanical ventilation will affect the pulmonary and hemodynamic physiology of the patient. Different modes of mechanical ventilation are selected in individual patients based on predicting pulmonary and hemodynamic effects of that mode of ventilation and the patients' individual pathophysiology. Addition of NO to the inspired gases may have an impact upon the patients' response to various modes of ventilation. This investigation will use patients as their own controls in a longitudinal fashion and will consider only whether NO is effective in lowering the pulmonary artery pressure, or in redistribution pulmonary blood flow so that arterial oxygen saturation is improved and/or pulmonary shunt decreased. Efficacy of inhaled NO will be correlated with the nature and severity of the patient's disease, the type of respiratory support the patient is receiving, and whatever concomitant therapy the patient is receiving.