Cerebral perfusion pressure


Cerebral perfusion pressure, or CPP, is the net pressure gradient causing cerebral blood flow to the brain. It must be maintained within narrow limits because too little pressure could cause brain tissue to become ischemic, and too much could raise intracranial pressure.

Definitions

The cranium encloses a fixed-volume space that holds three components: blood, cerebrospinal fluid, and very soft tissue. While both the blood and CSF have poor compression capacity, the brain is easily compressible.
Every increase of ICP can cause a change in tissue perfusion and an increase in stroke events.

From resistance

CPP can be defined as the pressure gradient causing cerebral blood flow such that
where:

By intracranial pressure

An alternative definition of CPP is:
where:
This definition may be more appropriate if considering the circulatory system in the brain as a Starling resistor, where an external pressure causes decreased blood flow through the vessels. In this sense, more specifically, the cerebral perfusion pressure can be defined as either:
or
Physiologically, increased intracranial pressure causes decreased blood perfusion of brain cells by mainly two mechanisms:
FLOW
Ranging from 20ml 100g-1 min-1 in white matter to 70ml 100g-1 min-1 in grey matter.

Autoregulation

Under normal circumstances a MAP between 60 and 160 mmHg and ICP about 10 mmHg sufficient blood flow can be maintained with autoregulation. Although the classic 'autoregulation curve' suggests that CBF is fully stable between these blood pressure values, in practice spontaneous fluctuations can occur.
Outside of the limits of autoregulation, raising MAP raises CBF and raising ICP lowers it. In trauma some recommend CPP not go below 70 mmHg. Recommendations in children is 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, and therefore CBF will return to baseline value within seconds. These adaptations to rapid changes in blood pressure are known as dynamic cerebral autoregulation.

Footnotes