Cerebral perfusion pressure (CPP) is maintained at 70 mmHg by autoregulation. If mean arterial pressure suddenly drops to 50 mmHg and ICP is 15 mmHg, what is the CPP and what autoregulatory response is expected?

• A CPP = 35 mmHg; cerebral vasodilation occurs to maintain perfusion, but below 50 mmHg MAP the autoregulatory range is exceeded ✓
• B CPP = 35 mmHg; cerebral vasoconstriction preserves blood-brain barrier integrity
• C CPP = 65 mmHg; normal autoregulation maintains flow as ICP buffer compensates
• D CPP = 35 mmHg; no autoregulatory response occurs below 60 mmHg CPP

Explanation

CPP = MAP – ICP = 50 – 15 = 35 mmHg. Cerebral autoregulation normally maintains CBF constant for CPP between 50-150 mmHg by adjusting cerebrovascular resistance (vasodilation when CPP falls). However, at CPP of 35 mmHg, the lower limit of autoregulation (approximately 50 mmHg CPP) has been breached — cerebral blood flow becomes pressure-passive and falls precipitously, risking cerebral ischemia. The vasodilatory response is maximal but ineffective. Option B (vasoconstriction) would worsen perfusion; option C miscalculates CPP; option D incorrectly states no response occurs.

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

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