In a patient under controlled ventilation, arterial PCO2 is acutely lowered from 40 mmHg to 30 mmHg. The expected change in cerebral blood flow is closest to:

• A No significant change, because autoregulation holds flow constant
• B An increase of about 30%
• C A decrease of about 90%
• D A decrease of about 30% ✓

Explanation

Between a PaCO2 of roughly 20 and 80 mmHg, cerebral blood flow changes by about 2 to 4% per mmHg of PaCO2, so a 10 mmHg fall produces roughly a 30% reduction in flow through hypocapnia-induced arteriolar constriction. Autoregulation refers to constancy of flow against changes in mean arterial pressure, not against PaCO2 changes, so option A is wrong. Flow does not approach zero because PaCO2 remains within the reactive range.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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