A 72-year-old woman with chronic obstructive pulmonary disease (COPD, FEV₁ 35% predicted) and severe osteoarthritis of the knees is admitted to hospital with acute exacerbation of COPD. Her baseline respiratory rate is 18/min, PaO₂ 65 mmHg, and PaCO₂ 48 mmHg on supplemental oxygen. She has been on morphine 10 mg every 6 hours for pain control at home. The admitting physician prescribes morphine 15 mg IV stat for acute pain, followed by 10 mg IV every 4 hours. Within 2 hours of the first dose, the patient's respiratory rate drops to 10/min, PaO₂ falls to 52 mmHg, and she becomes drowsy. What is the most appropriate immediate management?