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.
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.
- Bumetanide
Inhibits the Na-K-2Cl cotransporter in the thick ascending limb → potent diuresis. ~40× more potent than furosemide (1 mg bumetanide ≈ 40 mg furosemide).
Browse all classes: /reference/drugs
Suggested reading
- •Stoelting & Hines, Pharmacology & Physiology in Anesthetic Practice, 6e
- •Miller's Anesthesia, 9e
- •FDA package insert
