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Mannitol Mechanism of Action — FRCA Final MCQ

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ModerateNeuroanaesthesiaMannitol Mechanism of ActionFRCA Final

A 60-year-old man (ASA III) is in ICU following a traumatic brain injury. His ICP is 22 mmHg and MAP is 90 mmHg. You administer 200 mL of 20% mannitol over 15 minutes. What is the mechanism of action of mannitol in reducing ICP?

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Correct answer: A — Osmotic gradient draws water from brain parenchyma into the intravascular compartment

Mannitol is an osmotic agent that creates an osmotic gradient between the intravascular space (high osmolality) and the brain parenchyma (lower osmolality), drawing water from the brain tissue into the blood. This reduces brain water content and ICP. The effect requires an intact blood-brain barrier (BBB). In areas with disrupted BBB, mannitol may leak into the brain tissue and worsen oedema with repeated doses (rebound effect). Dose: 0.25-1 g/kg. It also has a rheological effect (reducing blood viscosity, improving cerebral blood flow) which occurs more rapidly.

Reference: Brain Trauma Foundation – 2016 – TBI guidelines; NACCS – 2023 – Neuroanaesthesia