Cerebral autoregulation maintains constant cerebral blood flow (CBF) over what mean arterial pressure (MAP) range, and which of the following conditions impairs this autoregulation?

• A MAP 70–200 mmHg; impaired only in severe hypertensive encephalopathy
• B MAP 50–150 mmHg; this is impaired in severe traumatic brain injury, malignant hypertension, and acute ischaemic stroke ✓
• C MAP 30–100 mmHg; impaired in any vasodilatory state including simple fever
• D MAP 60–180 mmHg; impaired specifically by hypercapnia above 60 mmHg PaCO2

Explanation

Cerebral autoregulation maintains constant CBF (~50 mL/100 g/min) over a MAP range of approximately 50–150 mmHg through myogenic and metabolic mechanisms. Below 50 mmHg, vasodilation is maximal and CBF falls passively with MAP (ischaemia). Above 150 mmHg, autoregulation breaks down and 'forced vasodilation' occurs (breakthrough hyperperfusion, risk of hypertensive encephalopathy). Conditions that impair autoregulation include: severe traumatic brain injury, acute ischaemic stroke, subarachnoid haemorrhage, malignant hypertension, severe hypercapnia, and severe hypoxia. This knowledge is critical in TBI management to maintain adequate cerebral perfusion pressure (CPP = MAP − ICP).

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

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