Intracranial Pressure (ICP)

ICP is the pressure that exists within the cerebrospinal fluid, or CSF (see CSF). The CSF is secreted in the ventricles of the brain (see “Ventricles) and circulates within and around the brain in the subarachnoid space (see “Subarachnoid Space). The ICP was first measured in the late 19th century by puncturing the reservoir of CSF in the lumbar spine (see “Lumbar Spine”) with a special needle. This procedure was called a lumbar puncture (LP) or spinal tap.

Other methods developed to measure ICP included tapping the ventricles of the brain with a blunt needle or a catheter (see “Ventricles”), implanting a pressure monitor within the substance of the brain (intraparenchymally), implanting a catheter or similar device within the spinal fluid over the convexity of the brain (see “Convexity”), or implanting a specially designed hollow bolt into a fine opening drilled into the skull (the Richmond Screw). The bolt is placed such that CSF flows freely into and through its bore.

Because all these procedures require tapping into the CSF, and all, save the LP, require surgery and inpatient management in a critical care unit, they are categorized as “invasive.” The LP is less invasive than the other methods.

ICP can be measured in terms of a height of a column of water (H2O) in millimeters (mm) of water (mm H2O) or, like blood pressure (BP), in terms of the height of a column of mercury (Hg) in mm Hg.

ICP was traditionally measured by tapping into the CSF and allowing it to find its level freely inside a sterile, upright, calibrated glass or plastic tube called a manometer. The manometer was attached to the needle or catheter used to tap into the CSF. The pressure was determined by measuring the height column of CSF. Because the physical properties of CSF closely resemble those of water, pressure was expressed in terms of mm H2O.

The convention developed for intraparenchymal monitoring is different. ICP is expressed using the mmsame terms of reference as blood pressure (BP) – the height of a column of mercury (Hg) in mm Hg. One mm Hg is equal to 13.6 mm H2O. The conversion factor between mmHg and mm H2O, therefore, is 13.6

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