ICP is pressure in the intracranial space. It is normally 0-10mmHg.
Kelly-Monroe doctrine: the skull is fixed in volume and contents are minimally compressible fluids. Normally contains
Increase in any of these volumes must be matched by equal decrease in another, or ICP will rapidly increase, unless skull disrupted (open fracture, decompressive craniotomy).
Brain volume
\(\uparrow\) in oedema, space-occupying lesion
\(\downarrow\) with age (atrophy)
CSF volume
High ICP drives reabsorption \(\to\) homeostasis
\(\uparrow\) with efflux failure (SAH, obstructive hydrocephalus)
\(\downarrow\) with drainage or decreased production (low ICP, diuretics, carbonic anhydrase inhibitors)
Blood volume
\(\uparrow\) with cerebral vasodilation (hypoxia, hypercapnia, fever, high cerebral metabolic rate incl. seizure)
\(\downarrow\) with cerebral vasoconstriction
\(\Delta\)MAP outside autoregulatory range (or with failure of autoregulation)
Venous outflow obstruction (head down, ETT ties, c-spine collar, SVC syndrome, sinus venous thrombosis)