Neuroscience Clerkship

CEREBRAL PERFUSION PRESSURE

Cerebral perfusion pressure (CPP) is the difference between mean arterial pressure (MAP) and intracranial pressure (ICP):

CPP = MAP – ICP        

In most organs, perfusion pressure is calcuated as arterial pressure minus venous pressure. However, in the upright position, the venous pressure in the head is essentially zero, due to the affect of gravity along with the fact that the brain is above the heart. Thus, in the brain for most cases, the exiting pressure is the intracranial pressure.

The gradient between systemic pressure and intracranial pressure, therefore, determines the magnitude of the force which drives the flow of blood to the brain. Increases in intracranial pressure or decreases in systemic pressure can result in impairment of the distribution of blood and oxygen to the brain. Conversely, CPP can be maintained by either raising MAP or lowering ICP.

Cerebral vessels can, however, respond to fluctuations in CPP with changes in vascular resistance. This cerebrovascular autoregulation is capable of maintaining a adequate cerebral blood flow over a range of systemic blood pressure from approximately 60 to 150 mm Hg (see figure above).

Hypotension beyond this autoregulatory range due to septic shock, for instance, can lower CPP and put the brain at risk for ischemia and infarction. Head trauma, CNS infection, and metabolic disturbances can impair normal cerebrovascular autoregulation and thereby threaten cerebral perfusion.

CPP monitoring is useful in the setting of closed head trauma, a situation in which the occurrence of cerebral ischemia due to elevated ICP can significantly influence outcome. Tissue which has pre-existing ischemia is especially vulnerable to damage in the setting of low CPP. Studies indicate that CPP should ideally be maintained at a minimum of 70 mm Hg following closed head injury. In the intensive care setting, CPP monitoring relies measuring both arterial pressure and intracranial pressure