Mannitol (Osmitrol)

ICP Reduction - Your primary intraoperative use. Draws free water out of brain tissue through osmotic gradient. Works best when the blood-brain barrier is intact. Give as a rapid infusion (15-20 min) for acute ICP crises.

IOP Reduction - Useful for acutely lowering IOP before ophthalmic surgery (e.g., open-globe repair). Give 30-60 min preoperatively for best effect.

Volume Shifts - Mannitol initially expands intravascular volume before the diuresis kicks in — watch your heart failure and volume-overloaded patients carefully. Pulmonary edema is a real risk if renal function is impaired. On the opposite spectrum, fluid losses resulting from mannitol administration, if not replaced, may result in hypovolemia.

Serum Osmolality Monitoring - Keep serum osmolality below 320 mOsm/kg to reduce the risk of osmotic nephrosis and AKI. If you're giving repeated doses, check an osmol gap.

Electrolyte Monitoring - Expect shifts in sodium and potassium — check labs with prolonged or repeated use. Hyperkalemia can occur from cellular dehydration releasing intracellular potassium.

Rebound ICP - Mannitol does NOT penetrate the blood-brain barrier (BBB). However In patients with a disrupted BBB (e.g., large contusion, tumor), mannitol can leak into brain tissue and reverse the osmotic gradient, worsening cerebral edema. Hypertonic saline may be preferable in these cases.

Urinary Catheter - Always place a foley before giving mannitol — you will have significant urine output and need to track it. Mannitol augments urinary output in the presence of hypovolemia, but output may be considerably less with severe glomerular or tubular injury.

Excessive Effect - Severe hyperosmolality or AKI: stop infusion, provide fluid resuscitation, monitor renal function and electrolytes closely. Dialysis may be needed in severe cases.

Drug Interactions - Additive diuresis with loop diuretics. May increase lithium toxicity by reducing renal clearance of lithium. Avoid co-administration with nephrotoxic agents.

Pediatric Implications - Dosing is weight-based (0.25-1 g/kg IV). Same osmolality monitoring applies. Use cautiously in neonates and infants due to limited renal reserve.

Obstetric Implications - Crosses the placenta and can cause fetal dehydration and electrolyte imbalance. Use only in life-threatening maternal situations (e.g., herniation). Not used routinely in obstetrics.

Absolute:

Anuria (no urine output despite adequate hydration)

Severe pulmonary edema or congestion

Active intracranial bleeding (relative — use clinical judgment)

Serum osmolality >320 mOsm/kg

Relative:

Severe dehydration or hypovolemia

Congestive heart failure (CHF)

Severe renal impairment

Caution:

Disrupted blood-brain barrier (increased ICP risk)

Patients requiring strict fluid balance

ICP/IOP reduction: 0.25-1 g/kg IV over 15-20 min (use 20% solution)

Freely filtered at the glomerulus but not reabsorbed — increases tubular osmolality, pulling water into the tubule and promoting diuresis. In the CNS, creates an osmotic gradient across an intact blood-brain barrier, drawing water from brain tissue into the vasculature and reducing cerebral edema.

Minimal hepatic (largely unmetabolized)

Renal

Available as 20% (200 mg/mL) and 25% (250 mg/mL) solutions — confirm concentration before dosing

May crystallize at low temperatures; warm and inspect for crystals before administration; use inline filter

Not a substitute for hypertonic saline (3%) — each has distinct indications and the choice matters clinically