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Chronic respiratory acidosis in COPD — FRCA Primary MCQ

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HardPhysiologyChronic respiratory acidosis in COPDFRCA Primary

A 63-year-old man with COPD has an ABG on air: pH 7.38, PaCO2 6.8 kPa, PaO2 7.6 kPa, HCO3- 31 mmol L-1. He is alert but dyspnoeic. What is the most likely acid-base interpretation?

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Correct answer: B — Acute respiratory alkalosis with renal compensation

An elevated PaCO2 with raised bicarbonate and near-normal pH indicates chronic respiratory acidosis with renal metabolic compensation. Acute respiratory acidosis would not usually have this degree of bicarbonate elevation. Metabolic alkalosis could raise PaCO2, but the clinical COPD context and primary hypercapnia favour chronic respiratory acidosis. The teaching point is to integrate pH, PaCO2, bicarbonate and clinical timescale.

Reference: RCoA Primary FRCA Syllabus 2026; Ganong's Review of Medical Physiology; West's Respiratory Physiology