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PaCO2 and cerebral blood flow — FRCA Primary MCQ

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HardPhysiologyPaCO2 and cerebral blood flowFRCA Primary

A patient receives a volatile anaesthetic and develops increased cerebral blood flow with raised intracranial pressure. The anaesthetist increases minute ventilation, reducing PaCO2 from 5.3 to 4.0 kPa. What is the most likely cerebral vascular response?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — Cerebral vasodilatation with increased cerebral blood volume

Reducing PaCO2 causes cerebral vasoconstriction, lowering cerebral blood volume and potentially reducing intracranial pressure. The tempting error is to assume ventilation affects oxygenation alone; CO2 is a powerful cerebral vasodilator. Excessive hypocapnia can reduce cerebral blood flow to harmful levels. The pearl is to use ventilation deliberately and avoid severe hypocapnia.

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