A 58-year-old man with chronic obstructive pulmonary disease (COPD) presents to the emergency department with acute dyspnea and altered mental status. His wife reports he has been increasingly somnolent over the past 3 days. On examination, he is cyanotic and has pursed-lip breathing. Arterial blood gas analysis shows: pH 7.28, PaCO₂ 72 mmHg, HCO₃⁻ 32 mEq/L, PaO₂ 48 mmHg. What is the primary acid-base disturbance?