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Acute on Chronic Respiratory Failure — RACP Adult Medicine MCQ

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ModerateRespiratoryAcute on Chronic Respiratory FailureRACP Adult Medicine

A 55-year-old man with COPD presents with drowsiness. ABG: pH 7.28, PaCO₂ 72 mmHg, PaO₂ 55 mmHg, HCO₃⁻ 34 mmol/L. What is the acid-base disturbance?

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Correct answer: C — Chronic respiratory acidosis with acute decompensation

pH is low (acidaemic), PaCO₂ is elevated (respiratory acidosis), and HCO₃⁻ is elevated (renal compensation). Expected HCO₃⁻ for acute respiratory acidosis = 24 + (72-40)/10 × 1 = 27.2. The actual HCO₃⁻ of 34 exceeds this, indicating chronic compensation. However, the pH remains acidaemic, indicating acute-on-chronic decompensation.

Reference: eTG Respiratory – 2024 – ABG Interpretation