- Severe chronic respiratory disease (i.e., chronic hypercapnia [PaCO2 greater than 45 mmHg], chronic hypoxemia [PaO2 less than 55 mmHg on FiO2 = 0.21], hospitalization within the last 6 months for respiratory failure [PaCO2 greater than 50 mm Hg and/or PaO2 less than 55 mmHg on 0.21 FiO2], secondary polycythemia, severe pulmonary hypertension [mean PAP (pulmonary artery pressure) greater than 40 mmHg], or ventilator dependency)