Cerebral perfusion pressure (CPP) is the driving pressure for blood flow into the brain. It is defined as the difference between the mean arterial pressure (MAP) and the intracranial pressure (ICP):

This equation has immediate clinical utility: if CPP falls to dangerous levels, it can be because MAP has fallen (hypotension), because ICP has risen, or both. The treatment strategy must address whichever component — or both — is responsible. Raising MAP with vasopressors, lowering ICP with osmotherapy or ventilation adjustments, or both simultaneously may be required.

Normal ICP in a healthy supine adult is 7–15 mmHg. Values consistently above 20–22 mmHg in a patient with severe traumatic brain injury are considered pathological and warrant treatment. Values above 40 mmHg are associated with very poor outcomes and often represent exhaustion of all compensatory mechanisms.

In the context of raised ICP, simply maintaining a normal MAP is insufficient. If ICP is 30 mmHg and MAP is 80 mmHg, CPP is only 50 mmHg — below the autoregulation range and likely to cause ischaemia. This is why MAP targets in patients with raised ICP are typically higher than in other ICU patients.