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).

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

CVR = CPP / CBF

where:

CVR is cerebrovascular resistance

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 cerebro-blood volume (CBV), and therefore CBF remains constant (see as ref. Aaslid, Lindegaard, Sorteberg, and Nornes 1989: http://stroke.ahajournals.org/cgi/reprint/20/1/45.pdf).

The three pressures that can contribute to the CPP are:

• Mean arterial pressure (MAP)
• Intracranial pressure (ICP)
• Jugular venous pressure (JVP)

In many tissues, the perfusion pressure is just the pressure difference between the arterial side (the upstream pressure) and the venous side (downstream pressure). In some tissues there is a third pressure to consider and this is the pressure external to the blood vessels because if high this pressure can restrict flow through the tissue. This situation is known as a Starling resistor. Such a situation exists in the brain where the external pressure is the ICP. Consequently, the correct definition of CPP is:

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

or

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

Under normal circumstances (MAP between 60 to 150 mmHg and ICP about 10 mmHg) cerebral blood flow is relatively constant due to protective autoregulation.^[1][2] 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,^[3][4] although some authorities regard 50-150 mmHg as a normal range for adults.^[1] Children require pressures of at least 60 mmHg.^[3]

References

1. ^ ^{a b} Steiner LA, Andrews PJ (2006). "Monitoring the injured brain: ICP and CBF". British Journal of Anaesthesia 97 (1): 26–38. doi:10.1093/bja/ael110. PMID 16698860. http://bja.oxfordjournals.org/cgi/content/full/97/1/26. 
2. ^ Duschek S, Schandry R (2007). "Reduced brain perfusion and cognitive performance due to constitutional hypotension". Clinical Autonomic Research 17 (2): 69–76. doi:10.1007/s10286-006-0379-7. PMC 1858602. PMID 17106628. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1858602. 
3. ^ ^{a b} Tolias C and Sgouros S. 2003. "Initial Evaluation and Management of CNS Injury." Emedicine.com. Retrieved on March 19, 2007
4. ^ Czosnyka M, Pickard JD (2004). "Monitoring and interpretation of intracranial pressure". Journal of Neurology, Neurosurgery, and Psychiatry 75 (6): 813–821. doi:10.1136/jnnp.2003.033126. PMC 1739058. PMID 15145991. http://jnnp.bmj.com/cgi/content/full/75/6/813. 

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.