A 64-yeаr-оld, 197-pоund, 6-fоot 1-inch mаle pаtient was admitted 3 days ago for a COPD exacerbation. He is currently receiving supplemental oxygen via nasal cannula at 3 L/min, albuterol and ipratropium every 4 hours, and an intravenous corticosteroid and antibiotic. He has bilaterally diminished breath sounds, with rhonchi in both bases and a weak, nonproductive cough. A chest x-ray from this morning shows bibasilar infiltrates. The current arterial blood gas results reveal pH 7.32, PaCO2 57 mm Hg, PaO2 54 mm Hg, SaO2 85%, and HCO3 – 29 mEq/L.