Expansion
Carbon dioxide is the most powerful cerebral vasodilator
Expansion
Cerebral blood flow is about 50 ml per 100 g per minute, roughly 15 per cent of cardiac output for 2 per cent of body weight.
Determinants, in order of importance:
- Carbon dioxide - the most potent influence; flow changes roughly linearly with the arterial partial pressure between about 3 and 10 kPa. Hypercapnia dilates, hypocapnia constricts
- Oxygen - only below about 6.7 kPa (50 mmHg), when vasodilatation occurs sharply
- Autoregulation - constant flow between mean pressures of about 60 and 160 mmHg
- Neural and metabolic coupling - local activity increases regional flow
Cerebral perfusion pressure = mean arterial pressure - intracranial pressure, and should be kept above about 60 mmHg in brain injury.
Deliberate hyperventilation lowers carbon dioxide and shrinks cerebral blood volume, temporarily reducing intracranial pressure; it is a short-term rescue measure only, since sustained vasoconstriction risks ischaemia.