Mannitol
Osmitrol
Osmotic diuretic
Freely filtered, not reabsorbed. Pulls water from tissues into vascular space (transient ↑intravascular volume) then drives osmotic diuresis. Reduces ICP by drawing water across an intact blood-brain barrier.
Indications
- •Acute reduction of intracranial pressure (TBI, intracranial mass, herniation)
- •Acute reduction of intraocular pressure
- •Forced diuresis in rhabdomyolysis (controversial — fluid + bicarb is primary)
- •Renal protection during AAA cross-clamp / cardiac surgery (controversial)
Dosing
| Context | Adult | Pediatric |
|---|---|---|
| ICP / cerebral edema | 0.25–1 g/kg IV over 15–20 min; repeat q4–6 h | 0.25–1 g/kg IV |
| Acute IOP | 1.5–2 g/kg IV over 30 min | — |
| Pre-renal-clamp | 12.5–25 g IV over 20 min before clamping | — |
Pharmacokinetics
Onset 15 min ICP reduction. Peak ICP effect 30–60 min. Duration 4–6 h. Renal elimination unchanged.
Hemodynamic effects
Transient volume expansion in first 30 min — caution in CHF, may precipitate pulmonary edema. Then diuresis → hypovolemia + hypotension if not replaced.
Respiratory effects
Pulmonary edema risk during the volume-expansion phase; especially relevant in patients with poor LV function.
Side effects
- !Pulmonary edema (early, from volume shift)
- !Hypotension after diuresis (volume depletion)
- !Hypernatremia + hyperosmolarity (paradoxical — water leaves cells faster than mannitol clears)
- !Acute kidney injury if serum osm > 320 mOsm/kg or osmolar gap > 55 (osmotic nephrosis)
- !Hyperkalemia transient
Contraindications
- ×Anuria
- ×Severe pulmonary edema or active CHF
- ×Active intracranial hemorrhage with disrupted BBB (mannitol leaks into brain → worsens edema)
- ×Serum osm > 320 mOsm/kg
Clinical pearls
- ★ICP RULE: filter-needle every dose (mannitol crystallizes at room temp — never skip the filter; crystals = capillary obstruction).
- ★OSMOLALITY TARGET: keep serum osm < 320 mOsm/kg; > 320 = AKI risk. Check osm + Na before each dose.
- ★DOUBLE THE DOSE doesn't double the effect — diminishing returns above 1 g/kg. Add hypertonic saline 3% as second-line.
- ★DISRUPTED BBB: with skull fracture or herniation through dural defect, mannitol can leak into brain parenchyma and pull water IN, worsening edema. Hypertonic saline is preferred when BBB integrity is in question.
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Other drugs in Diuretics
- Furosemide
Inhibits the Na-K-2Cl cotransporter in the thick ascending loop of Henle. Massive natriuresis + kaliuresis + magnesiuresis. Also a venodilator — drops preload within minutes, before diuresis kicks in.
- Acetazolamide
Reversibly inhibits carbonic anhydrase in proximal renal tubule, choroid plexus, ciliary body, RBC. Renal effect: ↓Na/H exchange → bicarbonate diuresis + mild Na/water loss + metabolic acidosis. CNS effect: ↓CSF production. Eye effect: ↓aqueous humor.
- Spironolactone
Competitive antagonist at the mineralocorticoid receptor in the distal nephron → inhibits aldosterone-mediated Na reabsorption + K secretion. Net effect: mild Na/water loss + K retention + Mg retention. Also a weak androgen-receptor antagonist.
Browse all classes: /reference/drugs