The skull is a rigid structure. This means that when the volume inside the skull increases, either due to bleeding, edema, or blocked drainage of cerebrospinal fluid (CSF), the pressure inside the skull (intracranial pressure, ICP) will also increase. This is a central mechanism in head injuries.

In acute injuries, we often refer to bleeding as the cause of increased pressure, but cerebral edema and impaired CSF drainage (e.g., hydrocephalus) also play a significant role, especially in the days and weeks after severe injury. The brain has some compensatory mechanisms that attempt to counteract an increase in pressure. An important strategy is to reduce the volume of cerebrospinal fluid, creating more space to handle bleeding or edema without the pressure rising too quickly. Venous blood volume can also be reduced to some extent.

What is cerebral perfusion pressure (CPP)?

For the brain to receive enough oxygen and nutrients, blood must be able to penetrate the brain tissue. This depends not only on blood pressure but also on the back pressure from inside the skull,
i.e., intracranial pressure (ICP). The relationship between these pressures is expressed through the concept of cerebral perfusion pressure (CPP).

CPP=MAP–ICP

MAP (Mean Arterial Pressure) is the average arterial blood pressure throughout the cardiac cycle.
ICP (Intracranial Pressure) is the pressure inside the cranial cavity.

CPP is thus the "driving pressure" that drives blood flow into the brain. A low CPP can lead to cerebral ischemia, while a high ICP can block blood flow even when blood pressure (MAP) appears sufficient.

Note: It is MAP, and not systolic blood pressure, that is important in this context, as MAP represents the average blood pressure.

Additional information:

If ICP is low (normal), central venous pressure (CVP) will constitute the actual back pressure against blood flow. The formula then becomes: CPP=MAP–CVP. In pathological
conditions such as cerebral hemorrhage, where ICP is significantly elevated and much higher than CVP, the original formula (CPP = MAP – ICP) is still used.