Mannitol

Elevated intracranial pressure often follows a traumatic brain injury. One way to lower pressure in the brain is to administer mannitol intravenously. In the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss mannitol.

The converse of this effect [removal of urea during renal dialysis] is to inject into the blood urea or mannitol, another molecule that does not readily cross the blood–brain barrier. This lowers the driving pressure of water within the blood, and water flows from the brain into the blood. Although the effects do not last long, this technique is sometimes used as an emergency treatment for cerebral edema.

Mannitol (C₆H₁₄O₆) has a similar size, structure, and chemical formula as glucose (C₆H₁₂O₆). It is metabolically inert in humans. A 10% solution consists of 100 g of mannitol per liter (1000 g) of water. Mannitol has a molecular weight of 182 g/mole, implying an osmolarity of (100 g/liter)/(182 g/mole) = 0.55 moles/liter, or 550 mosmole. Blood has an osmolarity of about 300 mosmole, so 10% mannitol is significantly hypertonic. Problem 3 in Chapter 5 asks you to calculate the osmotic pressure produced by mannitol.

Problem 3 Sometimes after trauma the brain becomes very swollen and distended with fluid, a condition known as cerebral edema. To reduce swelling, mannitol may be injected into the bloodstream. This reduces the driving force of water in the blood, and fluid flows from the brain into the blood. If 0.01 mol l⁻¹ of mannitol is used, what will be the approximate osmotic pressure?

Mannitol works best for short-term reduction of intracranial pressure. If it is administered continuously, eventually some of the mannitol may cross the blood-brain barrier, reducing its osmotic effect (Wakai et al., 2008). Then, when mannitol administration is discontinued, the mannitol that crossed into the brain can actually have the opposite effect of osmotically drawing water from the blood into the brain.

Interestingly, if you give a high enough concentration of mannitol, the osmotic shrinking of endothelia cells can disrupt the blood brain barrier. Sometimes mannitol is used to ensure that certain drugs are able to pass from the blood into the brain.