Cerebral perfusion pressure

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Cerebral perfusion pressure, or CPP, is the net pressure gradient causing blood flow to the brain. 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

$CBF = CPP/CVR$

where:

CVR is cerebrovascular resistance

The three pressures that can contribute to the CPP are:

• Mean arterial pressure (MAP)
• 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).

Raising MAP raises CPP and raising ICP lowers it (this is one reason that increasing ICP in traumatic brain injury is potentially deadly). CPP, or MAP minus ICP, 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. Children require pressures of at least 60 mmHg.

References

• 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. Issue 8, Article 4. Available. Retrieved on March 19, 2007.