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Capnogram slope — FRCA Primary MCQ

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HardPhysicsCapnogram slopeFRCA Primary

On a capnogram from a patient with severe COPD, phase III rises slowly rather than forming a flat plateau. What best explains this?

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Correct answer: B — Uneven alveolar emptying from different lung-unit time constants

The best answer is “Uneven alveolar emptying from different lung-unit time constants”. A sloping alveolar plateau reflects uneven emptying of lung units with different time constants, producing sequential exhalation of gas with differing carbon dioxide concentrations. This is typical of obstructive lung disease and is not explained by vapour pressure, ultrasound or a heat-and-moisture exchanger.

Reference: RCoA, Primary FRCA Syllabus: https://rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf