Under normal physiological conditions, cerebral blood flow (CBF) is maintained at a remarkably constant level of approximately 50 mL per 100g of brain tissue per minute across a wide range of systemic blood pressures. This phenomenon — cerebral autoregulation — operates through myogenic mechanisms (blood vessels constrict when perfusion pressure rises and dilate when it falls) and metabolic mechanisms (local metabolic products including CO2, H+, and adenosine regulate arteriolar tone in proportion to metabolic demand).

Autoregulation is effective across a MAP range of approximately 50–150 mmHg in a healthy normotensive adult. Below MAP 50 mmHg, cerebrovascular resistance can no longer compensate and CBF falls passively with perfusion pressure, causing ischaemia. Above MAP 150 mmHg, forced vasodilation occurs, causing hyperaemia and potentially breakthrough oedema — the mechanism of hypertensive encephalopathy.

This autoregulation curve is not fixed. In patients with chronic hypertension, it shifts rightward — the same patient who maintains CBF at a MAP of 65 mmHg when normotensive may require a MAP of 80 mmHg to maintain adequate CBF after years of hypertension have reset their autoregulatory range. This has important implications for targets during resuscitation.

Brain injury frequently impairs or abolishes autoregulation. After severe TBI, SAH, large ischaemic strokes, and other major neurological events, CBF may become entirely pressure-passive — rising and falling directly with systemic blood pressure. In this state, hypotension causes ischaemia and hypertension causes haemorrhagic transformation or worsening oedema. The clinical message is to assume autoregulation is impaired after any significant brain injury and to maintain blood pressure meticulously within a defined target range.

The Role of Carbon Dioxide

Carbon dioxide is the most potent physiological regulator of cerebral blood flow. The relationship is approximately linear over the clinically relevant range: for each 1 kPa rise in PaCO2, CBF increases by approximately 25–30%. Hypercapnia causes cerebrovascular vasodilation, increasing cerebral blood volume and therefore ICP. Conversely, hypocapnia causes vasoconstriction, reducing cerebral blood volume and lowering ICP.

This relationship is exploited therapeutically in the acute management of dangerously raised ICP: brief controlled hyperventilation to a PaCO2 of 3.5–4.0 kPa can reduce ICP rapidly by causing cerebral vasoconstriction. However, this is strictly a temporising measure. Sustained hypocapnia causes ischaemia — the vasoconstriction that lowers ICP also reduces CBF below the threshold needed to maintain tissue viability. Hyperventilation should not be used for more than 30–60 minutes without definitive measures (osmotherapy, surgical decompression) being implemented.