Cerebral autoregulation maintains constant cerebral blood flow (CBF) over a MAP range of approximately 60–160 mmHg. A patient with chronic hypertension has a rightward shift of this autoregulatory curve. What is the immediate clinical implication when their MAP drops to 70 mmHg (e.g., after aggressive antihypertensive therapy)?

• A CBF remains protected because 70 mmHg is still above the lower limit of the normal curve
• B CBF falls significantly because the lower limit of autoregulation in hypertensives is reset to a higher pressure, causing cerebral hypoperfusion ✓
• C CBF increases due to reactive hyperaemia as the pressure drops
• D CBF is maintained by increased cerebrovascular resistance

Explanation

In chronic hypertension, the autoregulatory curve shifts rightward — the lower limit of autoregulation rises from ~60 mmHg to approximately 80–100 mmHg, and the upper limit also shifts upward. Consequently, what is a 'safe' MAP of 70 mmHg in a normotensive person may fall below the lower autoregulatory threshold in a chronically hypertensive patient, causing pressure-passive CBF reduction and ischaemic symptoms. This is why acute antihypertensive treatment in hypertensive urgencies must be gradual, targeting a 15–25% MAP reduction rather than normalizing to 70 mmHg acutely.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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