Intra Cranial Pressure (ICP)

Intracranial pressure (ICP) is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid (CSF). The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF. CSF pressure has been shown to be influenced by abrupt changes in intrathoracic pressure during coughing (intraabdominal pressure), valsalva (Queckenstedt's maneuver), and communication with the vasculature (venous and arterial systems). ICP is measured in millimeters of mercury (mmHg) and, at rest, is normally 7–15 mmHg for a supine adult, and becomes negative (averaging −10 mmHg) in the vertical position. Changes in ICP are attributed to volume changes in one or more of the constituents contained in the cranium.
Intracranial hypertension, commonly abbreviated IH or raised ICP, is elevation of the pressure in the cranium. ICP is normally 7–15 mm Hg; at 20–25 mm Hg, the upper limit of normal, treatment to reduce ICP may be needed.

http://en.wikipedia.org/wiki/Intracranial_pressure