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Hypoxic Pulmonary Vasoconstriction — FRCA Primary MCQ

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HardPhysiology - RespiratoryHypoxic Pulmonary VasoconstrictionFRCA Primary

During one-lung ventilation, a volatile anaesthetic is increased from 0.5 to 1.5 MAC while cardiac output and inspired oxygen concentration are unchanged. Which physiological change most directly explains a fall in arterial oxygen tension?

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Correct answer: E — Attenuation of hypoxic pulmonary vasoconstriction in the non-ventilated lung

The best answer is “Attenuation of hypoxic pulmonary vasoconstriction in the non-ventilated lung”. Hypoxic pulmonary vasoconstriction diverts perfusion away from poorly ventilated alveoli. Volatile agents attenuate this matching response in a dose-related manner, increasing shunt through the non-ventilated lung and lowering PaO2. The key mechanism is perfusion redistribution, not an abrupt diffusion-membrane change or an anatomical-dead-space effect.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; Hypoxic pulmonary vasoconstriction: mechanisms and controversies: https://pmc.ncbi.nlm.nih.gov/articles/PMC1464287/; Lung circulation review: https://pmc.ncbi.nlm.nih.gov/articles/PMC7432532/