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Cerebral Blood Flow Regulation — FRCA Primary MCQ

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HardPhysiology - NeurophysiologyCerebral Blood Flow RegulationFRCA Primary

A ventilated patient has PaCO2 reduced from 5.3 to 3.3 kPa for ten minutes while PaO2 and mean arterial pressure remain stable. Which accompanying cerebral change is most likely?

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Correct answer: E — Cerebrovascular resistance increases and cerebral blood flow falls

The best answer is “Cerebrovascular resistance increases and cerebral blood flow falls”. A fall in PaCO2 produces cerebral vasoconstriction, raising cerebrovascular resistance and reducing cerebral blood flow. That is why hypocapnia may transiently reduce intracranial blood volume but can compromise perfusion if excessive. The response is not caused by a rapid reset of pressure autoregulation, nor does hypocapnia intrinsically raise cerebral metabolic demand.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; Regulation of cerebral blood flow in humans: https://pmc.ncbi.nlm.nih.gov/articles/PMC8576366/