Bohr dead space calculation — FRCA Primary MCQ

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HardPhysiologyBohr dead space calculationFRCA Primary

A 40-year-old patient is ventilated with tidal volume 500 mL. Mixed expired CO2 is 3.2 kPa and arterial CO2 is 5.3 kPa. What is the approximate physiological dead space fraction using the Bohr equation?

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Correct answer: E — 0.20

Bohr dead space fraction is (PaCO2 - PECO2)/PaCO2, which is (5.3-3.2)/5.3, approximately 0.40. The common mistake is to divide by mixed expired CO2 rather than arterial CO2. A high dead space fraction implies substantial ventilation of poorly perfused lung units. This calculation is particularly relevant in pulmonary embolism and severe lung disease.

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