How the dose is calculated
This calculator covers the weight-based paediatric severe asthma dose. Adult obstetric and cardiac indications use fixed regimens, not weight-based dosing, and are listed separately below.
Weight-based dosing (paediatric)
| Indication | Dose | Max single dose |
|---|---|---|
| Severe asthma exacerbation | 40 mg/kg IV over 20 min | 2 g |
Fixed adult regimens (not weight-based)
| Indication | Regimen |
|---|---|
| Eclampsia / severe pre-eclampsia (Zuspan regimen) | 4 g IV loading dose over 15-20 min, then 1-2 g/hr IV maintenance infusion |
| Torsades de pointes | 1-2 g IV bolus over 5-15 min |
Magnesium toxicity (loss of deep tendon reflexes, respiratory depression, cardiac arrest) is dose- and renal-function-dependent — monitor reflexes, respiratory rate, and urine output during infusion, especially in renal impairment. Calcium gluconate is the antidote for magnesium toxicity.
Clinical use
- Standard of care for seizure prophylaxis/treatment in eclampsia and severe pre-eclampsia — reduces the risk of eclamptic seizures more effectively than other anticonvulsants in this setting.
- An effective adjunct bronchodilator in severe asthma exacerbations not responding adequately to first-line bronchodilators and steroids.
- First-line treatment for torsades de pointes, regardless of the patient's serum magnesium level.
Frequently asked questions
What is magnesium sulfate used for?
Seizure prevention/treatment in eclampsia and severe pre-eclampsia, an adjunct bronchodilator in severe asthma, and first-line treatment for torsades de pointes.
Why isn't the eclampsia dose weight-based?
The Zuspan and Pritchard regimens for eclampsia/pre-eclampsia use fixed adult loading and maintenance doses established by clinical trials, rather than a per-kg calculation — this calculator's weight-based function is used for the paediatric asthma indication instead.
What is the antidote for magnesium toxicity?
IV calcium gluconate reverses the cardiac and neuromuscular effects of magnesium toxicity and should be available at the bedside whenever magnesium sulfate is infused.
References
- American College of Obstetricians and Gynecologists. Gestational Hypertension and Preeclampsia. ACOG Practice Bulletin.
- Global Initiative for Asthma (GINA). Global Strategy for Asthma Management and Prevention.
- American Heart Association. Advanced Cardiovascular Life Support (ACLS) Guidelines.