Why is Mannitol administered to decrease ICP?

Answer 1

Osmosis! Remember that ICP is about fluid being in the wrong place, at the wrong time, and mannitol can be used to create an osmotic gradient in the opposite direction!

Answer 2

Mannitol is a diuretic (helps you lose fluid). Mannitol is also a sugar alcohol which does not easily cross the blood brain barrier (BBB) because the molecules in this medication are too large. This means that the large particles will create an osmotic gradient between the blood and the brain. This gradient has more molecules concentrated in the blood than in the brain, which forces fluid in the brain to move into the vasculature to be excreted to re-balance this gradient.

Basically, this drug helps to move excess water in the brain into the vasculature to be removed from the body through urination. This diuretic medication reduces the fluid volume of the brain to decrease the amount of pressure inside our skulls. Decreasing ICP is important to decrease cerebral edema (brain swelling) and associated brain injuries.

Bonus food for thought:

Why should mannitol be used extremely cautiously in cardiac patients?

Answer: The rapid fluid shift may overwhelm an unhealthy heart and cause more damage.