Apparent increase in intracranial volume Fig. 5.1. Intracranial compliance curve.

hematoma. This "compliance" can be quantified by adding or withdrawing a known volume of saline to the CSF via a ventriculostomy, and noting the rise in ICP - the volume pressure response.

Elevation of ICP decreases cerebral perfusion, and may result in secondary ischemia, as the net cerebral perfusion pressure (CPP) is determined by the difference between mean arterial blood pressure (MAP) and ICP. (CPP = MAP - ICP or MAP - JVP, where JVP = jugular venous pressure. This applies when the cranium is open and ICP is zero.) In addition to its effect on CPP, an elevated ICP can also result in herniation. Although a clear-cut threshold cannot be determined, an elevated ICP >30 mmHg is associated with an increased risk of transtentorial or brain-stem herniation. Thus monitoring and treatment of ICP is of paramount importance in the ICU.

In patients with spinal cord injury, the perfusion pressure to the cord is similarly determined by the difference between MAP and CSF pressure. Although most cord-injured patients present with complete lesions, anatomical disruption is rare, and maintenance of adequate perfusion is important to preserve cord function to ischemic regions proximal to the main site of injury.

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