Neurocritical care is dependent on the ability to accurately monitor cerebral physiology. One of the most important physiologic variables to consider is cerebral blood flow (CBF) because neuronal survival is directly linked to an appropriate match between metabolic demand and metabolic substrate. CBF depends on cerebral perfusion pressure (CPP), defined as the mean arterial blood pressure (mABP) minus the mean intracranial pressure (ICP).

There are many ways (invasive and noninvasive) to measure CBF and ICP in the ICU setting. This chapter reviews techniques for measuring both these parameters and considers the benefits and pitfalls of the different techniques, along with a basic consideration of their interpretation. The reader is directed to relevant sections elsewhere in this text for a detailed consideration of ICP and CBF physiology.

MEASUREMENT OF INTRACRANIAL PRESSURE

Physical Principles

ICP can be measured in several anatomic spaces:

1. ▪ Intraventricular

2. ▪ Intraparenchymal

3. ▪ Subdural

4. ▪ Subarachnoid

5. ▪ Epidural

Intracranial pressure is synonymous with CSF pressure. ICP depends on several parameters:

1. ▪ Internal (intracranial) volume

2. ▪ Elastance of the system

3. ▪ Contribution from the atmosphere

4. ▪ Orientation of the craniospinal axis relative to the gravitational vector

The traditional reference point for ICP measurement is the foramen of Monro, the location of which is estimated by the external acoustic meatus (EAM). ICP is variably expressed in either mm Hg, or in mm H2O. Dividing the pressure in mm H2O by 13.6 yields the equivalent pressure in mm Hg; this constant is based on the ratio of the density of water to mercury.