Overview
Pressure Physiology and Why Perfusion Falls
The cranial vault is a fixed compartment containing brain tissue, blood, and cerebrospinal fluid (CSF).
The cranial vault is a fixed compartment containing brain tissue, blood, and cerebrospinal fluid (CSF). A rise in one component is initially buffered by displacement of CSF and venous blood. Once that reserve is exhausted, a small additional volume can produce a steep rise in intracranial pressure (ICP). Common mechanisms include a hematoma, tumour, abscess, cerebral edema, hydrocephalus, or impaired venous drainage. The consequence is not simply a high number on a monitor. Cerebral perfusion pressure (CPP) falls as ICP rises: CPP = MAP − ICP A patient can therefore become ischemic from either increased ICP or inadequate mean arterial pressure (MAP). Ischemia worsens cellular swelling, which raises ICP further. This pressure–perfusion cycle can eventually force brain tissue through rigid intracranial openings, producing herniation and brainstem compression.