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Chronic Respiratory Acidosis — RACP Adult Medicine MCQ

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EasyRespiratoryChronic Respiratory AcidosisRACP Adult Medicine

A 60-year-old man with known COPD has an ABG showing pH 7.38, PaCO₂ 60 mmHg, PaO₂ 58 mmHg, HCO₃ 34 mmol/L. He is his usual self with no acute symptoms. What acid-base disturbance is present?

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Correct answer: C — Chronic compensated respiratory acidosis

The elevated PaCO₂ with appropriate metabolic compensation (expected HCO₃ increase of 3.5 mmol/L per 10 mmHg rise in PaCO₂ for chronic compensation: expected HCO₃ = 24 + [3.5 × 2] = 31 mmol/L – actual is 34, close to expected) and near-normal pH indicates chronic compensated respiratory acidosis. This is expected in stable severe COPD with chronic hypercapnia. The normal pH distinguishes chronic from acute respiratory acidosis (where pH would be significantly low). This patient's ABG is at his baseline.

Reference: eTG – 2025 – Respiratory