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PE Alveolar Dead Space — FRCA Primary MCQ

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HardPhysiology - RespiratoryPE Alveolar Dead SpaceFRCA Primary

During controlled ventilation, PaCO2 is 6.0 kPa and end-tidal CO2 is 3.6 kPa. Acute pulmonary embolism is suspected. Which change best explains this widened arterial-to-end-tidal CO2 gradient?

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Correct answer: D — Increased alveolar dead space from ventilated but underperfused alveoli

The best answer is “Increased alveolar dead space from ventilated but underperfused alveoli”. Pulmonary embolism creates ventilated alveoli with little or no perfusion, increasing alveolar dead space. Their low CO2 content dilutes expired alveolar gas and lowers end-tidal CO2 relative to PaCO2. Anatomical dead space is not the central change. Shunt causes hypoxaemia through the opposite V/Q extreme, while CO2 diffusion limitation is not the usual explanation.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; Lung circulation review: https://pmc.ncbi.nlm.nih.gov/articles/PMC7432532/