Mannitol (Surgical — ICP/Osmotherapy)

How it works

Being filtered but not reabsorbed by the kidney and largely confined to the extracellular space, mannitol raises plasma osmolality to draw water out of the brain and tissues and induces an osmotic diuresis.

Prescribing in practice

• Give through an in-line filter and assess renal function and volume status first, because mannitol can precipitate acute kidney injury, pulmonary oedema and dangerous fluid–electrolyte shifts — it is contraindicated in anuria, severe established renal failure, severe dehydration and decompensated cardiac failure.
• Inspect the solution and warm to redissolve any crystals before use, administer via a filtered set, and monitor serum osmolality and the osmolar gap to guide repeat dosing.
• Watch for rebound rises in intracranial pressure and correct hypovolaemia caused by the diuresis.

Monitoring

Monitor urine output, fluid balance, serum electrolytes, renal function and serum/calculated osmolality, alongside intracranial or intra-ocular pressure as appropriate.

Counselling the patient

• Usually given to critically ill or anaesthetised patients; explain to relatives it is a drip used to reduce pressure on the brain or eye.
• Where the patient is awake, mention it increases urine output markedly.

Evidence & guidelines

Mannitol is an established osmotherapy agent in neurosurgical and neurocritical-care guidance for raised intracranial pressure; the SPC details its renal, cardiac and electrolyte cautions.