Cerebral Pefusion Pressure

What is cerebral perfusion pressure (CPP)?

Cerebral Perfusion Pressure (CPP) is the pressure gradient between the systemic blood pressure and the pressure in the cranial compartment.  The pressure difference is the gradient that is necessary to "drive" blood from the aorta into the cranial compartment.  Blood flow and perfusion to the brain depend upon an adequate blood pressure gradient.

How is it calculated?

Cerebral Perfusion Pressure (CPP) is calculated as the mean arterial blood pressure (MBP) - mean
intracranial pressure (ICP). 

CPP = MABP - ICP 

What is a normal CPP?

Normal CPP is 60-80 mmHg. 

What does a low CPP indicate?

A decrease in the CPP suggests that the gradient required to push blood towards the brain is not being maintained.  This can cause brain ischemia from reduced cerebral blood flow.

What can cause the CPP to be too low?

The CPP will fall if the MABP drops, or if the ICP rises without a corresponding rise in systemic blood pressure. 

If the ICP rises, a normal compensatory response is to increase the systemic BP in an attempt to maintain cerebral perfusion pressure.  Hypertension in the presence of brain injury may be a life saving response, and frequently indicates raised intracranial pressure.

Treatment for a low CPP should be aimed at correcting the underlying cause (i.e. by increasing the blood pressure or lowering the ICP).
 

Is a high CPP harmful?

When brain injury occurs, cerebral capillaries can become "leaky" or more permeable to water.  In addition, cerebral blood vessels may dilate in response to brain tissue injury, hypoxemia, hypercarbia, acidosis or hypotension.  If the BP becomes elevated, the increased CPP can lead to increased cerebral blood flow.  When combined with increased capillary permeability or cerebral vasodilation, blood flow can increase to the point where brain edema worsens.

The goal for maintaining the CPP is to ensure that the CPP is high enough to perfuse the brain; while preventing excessive CPP elevations that might worsen any cerebral edema.

The degree of elevation above normal, and the duration of time that the ICP is high are important indicators of brain injury.