A 75-year-old woman with a 20-year history of chronic cough and dyspnea has had bilateral lower extremity edema for the past 3 years. Over the past 5 days, her cough and dyspnea worsened despite antibiotic and diuretic use. She now presents with insomnia and agitation. Blood gas analysis: pH 7.35, PaO₂ 55 mmHg, PaCO₂ 74 mmHg, AB 42 mmol/L, serum chloride (Cl) 80 mmol/L. The most likely diagnosis is: