- Severe chronic respiratory disease, including chronic hypercapnia (arterial carbon dioxide pressure [PaCO2] greater than 45 mm Hg), chronic hypoxemia (PaO2 less than 55 mm Hg or FiO2 = 0.21), hospitalization within the 6 months prior to study entry for respiratory failure (PaCO2 greater than 50 mm Hg and/or PaO2 less than 55 mm Hg on 0.21 FiO2), secondary polycythemia, severe pulmonary hypertension (mean pulmonary arterial pressure [PAP] greater than 40 mm Hg), or ventilator dependency