Cerebral perfusion pressure

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

Definitions

From resistance

CPP can be defined as the pressure gradient causing cerebral blood flow (CBF) such that

CBF = CPP / CVR

where:

CVR is cerebrovascular resistance

By intracranial pressure

An alternative definition of CPP is:

CPP = MAP − ICP

where:

MAP is mean arterial pressure

ICP is intracranial pressure

This definition may be more appropriate if considering the circulatory system in the brain as a Starling resistor, where an external pressure (in this case, the intracranial pressure) causes decreased blood flow through the vessels. In this sense, more specifically, the cerebral perfusion pressure can be defined as either:

CPP = MAP − ICP (if ICP is higher than JVP)

or

CPP = MAP − JVP (if JVP is higher than ICP).

Physiologically, increased intracranial pressure (ICP) causes decreased blood perfusion of brain cells by mainly two mechanisms:

• Increased ICP constitutes an increased interstitial hydrostatic pressure that, in turn, causes a decreased driving force for capillary filtration from intracerebral blood vessels.
• Increased ICP compresses cerebral arteries, causing increased cerebrovascular resistance (CVR).

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, altough theclassic '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 cererbal autoregulation.

References

Others

• Sanders MJ and McKenna K. 2001. Mosby’s Paramedic Textbook, 2nd revised Ed. Chapter 22, "Head and Facial Trauma." Mosby.
• Walters, FJM. (1998). "Intracranial Pressure and Cerebral Blood Flow". Physiology (8, Article 4). http://www.nda.ox.ac.uk/wfsa/html/u08/u08_013.htm. Retrieved 10 February 2011.