82 Autoregulation of Cerebral Blood

Cerebral blood flow per 100 g: normal value, autoregulation, CO₂ reactivity and ischaemic thresholds.

Cerebral blood flow per 100 g of tissue expresses brain perfusion independently of brain size. On average it is about 50–54 mL per 100 g per minute, higher in grey matter than in white matter. The brain needs a constant supply of oxygen and glucose, so its blood flow is tightly matched to its metabolic needs. Several mechanisms protect it: cerebral autoregulation (vessels constrict when blood pressure rises and dilate when it falls), the strong response to carbon dioxide (high CO₂ dilates and low CO₂ constricts cerebral vessels) and neurovascular coupling (active brain areas receive more blood).

BrainAutoregulationIschaemia

Average≈ 50–54 mL/100 g/min

Perfusion pressureCPP = MAP − ICP

Penumbra≈ 10–20 mL/100 g/min

Infarction riskbelow ≈ 10 mL/100 g/min

Figure 1. Approximate cerebral blood flow thresholds. In the ischaemic penumbra, neurons stop working but can recover if flow is restored in time.

What determines cerebral flow

Cerebral perfusion pressure (mean arterial pressure minus intracranial pressure)
Arterial CO₂ and O₂: hypercapnia and severe hypoxia dilate cerebral vessels; hyperventilation reduces flow
Blood viscosity: anaemia increases and polycythaemia decreases flow
Brain activity and temperature: seizures and fever increase, sedation and hypothermia decrease metabolism and flow

Autoregulation does not keep flow perfectly constant, and it is impaired in chronic hypertension (the range shifts to higher pressures), after stroke and brain injury, and in some older people. Lowering blood pressure too quickly in these patients can reduce cerebral flow.

Clinical significance

When flow falls to about 20 mL/100 g/min, neurons stop functioning; below about 10 mL/100 g/min they die within minutes. The zone between these thresholds, the ischaemic penumbra, is the target of urgent stroke treatment (thrombolysis, thrombectomy). Excessive flow (hyperaemia) can occur after reperfusion and in hypertensive crises.

Seek emergency care

immediately for sudden facial droop, arm or leg weakness, speech difficulty, loss of vision or sudden severe headache. Time to treatment determines how much brain can be saved.

References

Astrup J, Siesjö BK, Symon L. Thresholds in cerebral ischemia — the ischemic penumbra. Stroke. 1981;12(6):723–725. doi:10.1161/01.STR.12.6.723
Claassen JAHR, Thijssen DHJ, Panerai RB, Faraci FM. Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation. Physiol Rev. 2021;101(4):1487–1559. doi:10.1152/physrev.00022.2020

This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

Published on 1 May 2024