Cerebral Perfusion Pressure - Autoregulation

Autoregulation

Static autoregulation: Under normal circumstances (MAP between 60 to 150 mmHg and ICP about 10 mmHg), average cerebral blood flow (e.g. the average recorded over 5 minutes or over hours) is relatively constant due to protective autoregulation. However, although the classic 'autoregulation curve' proposed by Lassen et al. suggests that CBF is fully stable between these blood pressure values (known also as the limits of autoregualtion), in fact CBF may vary as much as 10% below and above its average within this range. Outside of the limits of autoregulation, raising MAP raises CPP and raising ICP lowers it (this is one reason that increasing ICP in traumatic brain injury is potentially deadly). CPP is normally between 70 and 90 mmHg in an adult human, and cannot go below 70 mmHg for a sustained period without causing ischemic brain damage, although some authorities regard 50-150 mmHg as a normal range for adults. Children require pressures of at least 60 mmHg.

Within the autoregulatory range, as CPP falls there is, within seconds, vasodilatation of the cerebral resistance vessels, a fall in cerebrovascular resistance and a rise in cerebral-blood volume (CBV), and therefore CBF will return to baseline value within seconds (see as ref. Aaslid, Lindegaard, Sorteberg, and Nornes 1989: http://stroke.ahajournals.org/cgi/reprint/20/1/45.pdf). These adaptations to rapid changes in blood pressure (in contrast with changes that occur over periods of hours or days) are known as dynamic cerebral autoregulation.