Intracranial pressure

Monro-Kellie doctrine

• Brain is enclosed in rigid bony skull.
• Fluid is incompressible.
• Changes in the volume of intracranial content lead to rapid increase in pressure

However, when content increases (e.g. tumour growth)

--> Pressure increases slowly initially
* Because of translocation of CSF out of ventricles and into extracranial subarachnoid space

When this buffer is gone, then pressure increases very abruptly, as per Monro-Kelli doctrine.

Normal intracranial content

By volume:

• Brain 85%
  * Tissues 20%
  * Water 65%
• CSF 10%
• Blood 5%

Increase in cerebral blood flow

--> Increased intracranial blood volume 
+ Possible increase in CSF production

--> Increase in ICP

But, CSF translocation and increased CSF resorption limits rise in ICP.

NB:

At 112mmCSF, absorption and production of CSF are roughly equal
* [WG21:p617]

Techniques used to manipulate CBF and ICP

• Head up tilt
• No obstruction to venous drainage
• Avoidance of cerebral vasodilators (e.g. halothane)
• Hyperventilation
  * via decreased pCO2
• Mannitol
  * via decreased brain water content
• Controlled hypotension
  * via decreased intracranial blood volume