Intracranial Pressure (ICP)

however, ICP does not rise initially due to compensatory mechanisms (autoregulation):

• • immediate: displacement of CSF to lumbar theca, displacement of blood from venous sinuses

  • ƒ delayed: displacement of ECF or ICF; displacement of brain tissue into compartments under less pressure (herniation)

Compensatory mechanisms of ↑ ICP

• Displacement of CSF, blood, extracellular fluid, ICF

• Herniation

once compensation is exhausted, ICP rises exponentially

Intracranial volume versus pressure.

Stage 1 (compensation)

↑ in 1 of 3 vol. (blood, CSF, or tissue) → compensated by ↓ in 1 or 2 other vol. Usually ↓CSF production or ↑ reabsorption → arterial constriction (↓ blood); Sx - drowsiness, slight confusion.

Stage 2

volume continues ↑ → constricting cerebral arteries → ↓ blood flow (hypoxia and hypercapnia) - Sx ↓ of consciousness, pathologic breathing, pupillary changes.

Stage 3 (decompensation)

hypoxia and hypercapnia → arteries reflex dilation → blood volume ↑ → ↑ ICP → compresses the arterioles and capillaries → ↑hypoxia and the hypercapnia → damage to brain tissue (↑ Sx) → reflexes ↑ BP → ↑ ICP

Stage 4

↑ swelling and pressure → herniation - When ICP = sBP → cerebral perfusion stops.