A 58-year-old man with COPD presents to the emergency department with shortness of breath and altered sensorium. Arterial blood gas shows pH 7.28, PaCO₂ 68 mmHg, HCO₃⁻ 30 mEq/L, and PaO₂ 52 mmHg. Which investigation is most appropriate to confirm the primary acid-base disorder and guide acute management?