Choose, dose and monitor mannitol and hypertonic saline safely.
At the bedside
- 1
Choose agent
Hypovolaemic/hypotensive or AKI: hypertonic saline. Hypervolaemic/heart failure or hypernatraemic: mannitol (with caution) or diuresis. Either for herniation.
- 2
Mannitol dosing
0.25-1 g/kg of 20% over 15-20 min; repeat every 4-6 h as needed.
- 3
Hypertonic saline dosing
3% NaCl 150-250 mL (2-5 mL/kg) bolus over 10-20 min, or 23.4% 30 mL via central line for herniation.
- 4
Monitor before each repeat dose
Mannitol: serum osmolality < 320 mOsm/kg and osmolar gap < 20 (measured - calculated osmolality). HTS: Na < 155-160 mmol/L.
- 5
Replace urine losses after mannitol
Matched isotonic fluid to prevent hypovolaemia and hypotension.
- 6
Wean slowly
Avoid rapid Na fall after prolonged HTS (rebound oedema): reduce Na by ≤ 8-10 mmol/L/day.
Osmotherapy
Mannitol 20%
- Dose
- 0.25-1 g/kg over 15-20 min
- Range
- 0.25–1 g/kg
- Worked example
- 70 kg at 0.5 g/kg = 35 g = 175 mL of 20%
Osmotherapy
Hypertonic saline (3% or higher)
- Dose
- 3% 2-5 mL/kg bolus; 23.4% 30 mL (central) for herniation
- Range
- 2–5 mL/kg
- Worked example
- 70 kg: 3% 200 mL over 15 min, recheck Na in 4 h
Mannitol vs hypertonic saline
| Feature | Mannitol 20% | Hypertonic saline 3% |
|---|---|---|
| Volume effect | Diuresis → hypovolaemia | Volume expansion |
| Best for | Euvolaemic/hypervolaemic patients | Hypotension, hypovolaemia, hyponatraemia |
| Limits | Osmolality > 320 / gap > 20, AKI | Na > 155-160, heart failure |
| Side effects | AKI, hypotension, rebound | Hypernatraemia, hyperchloraemic acidosis, overload |
| Indian availability | Universal, cheap | 3% widely available |
Calculators for this topic
References
- 1
- 2
Pathophysiology
An intact blood-brain barrier is required for osmotic agents to work; where it is disrupted, agents can leak into the injured brain, which is one reason for rebound. Mannitol also reduces blood viscosity, improving microcirculatory flow.
Evidence
- Meta-analyses suggest hypertonic saline may lower ICP slightly more than equiosmolar mannitol, without clear outcome difference [SIBICC 2019].
Controversies
- Continuous HTS infusion to target Na 145-155 (COBI trial, 2021: no improvement in functional outcome).
🇮🇳 India notes
- Mannitol 20% is cheap and stocked everywhere; 3% saline bottles are increasingly available. 23.4% saline is rarely available outside large centres.
References
- 1
- 2
Test yourself
Module quiz · score 70% to complete the module
Educational reference for trained clinicians. Check doses against your hospital protocol and the patient in front of you.