A 58-year-old man with chronic obstructive pulmonary disease (COPD) presents to the emergency department with acute dyspnea and altered mental status. Arterial blood gas analysis reveals: pH 7.28, PaCO₂ 65 mmHg, HCO₃⁻ 28 mEq/L, PaO₂ 45 mmHg. Hemoglobin is 14 g/dL with normal oxygen saturation by pulse oximetry (SpO₂ 88%). His wife reports he has been increasingly somnolent over the past 6 hours. Which mechanism best explains his altered mental status despite "adequate" SpO₂?