Magnesium Sulphate - Sulfate
Related Subjects:
|Calcium Physiology
|Magnesium Physiology
|Calcitonin
|Hypocalcaemia
|Hypercalcaemia
|Hypomagnesaemia
|Hypermagnesaemia
|Primary Hyperparathyroidism
|Familial hypocalciuric hypercalcaemia (FHH)
|Sarcoidosis
⚠️ Caution: For peripheral administration, magnesium sulfate 50% must be diluted to ≤5% (20 mmol in 100 mL) to avoid pain, thrombophlebitis, or tissue necrosis.
Use central access if higher concentrations are required.
💊 About
- Magnesium sulfate is a highly soluble inorganic salt used in emergency medicine, obstetrics, and critical care.
- About 60% of body magnesium is stored in bone; the remainder is intracellular - essential for enzyme function, ATP metabolism, and cell membrane stability.
- Always check the BNF entry for updated dosing guidance.
⚙️ Mechanism of Action
- Acts as a physiological calcium antagonist - blocks calcium influx at voltage-gated channels, relaxing smooth muscle.
- Stabilises excitable membranes, reducing neuronal excitability and arrhythmogenicity.
- Modulates NMDA receptors in the CNS, producing anticonvulsant and neuroprotective effects.
- Acts as a cofactor in >300 enzymatic reactions including ATP synthesis and energy metabolism.
🎯 Indications
- 🌬️ Acute severe asthma (FEV₁ < 25% predicted).
- ❤️ Torsades de Pointes and other ventricular arrhythmias.
- 💧 Hypomagnesaemia (especially in malnutrition, alcoholism, diuretics).
- 🤰 Pre-eclampsia & eclampsia – prevention and treatment of seizures (RCOG 2023).
- 🧠 Neuroprotection in imminent preterm birth (24–29 weeks gestation) per NICE NG25.
💉 Parenteral Doses (Magnesium sulfate 1 g = 4 mmol)
| Indication | Dose | Infusion Rate | Comments |
| Torsades de Pointes |
8 mmol (2 g) IV |
Over 10–15 min in 100 mL 5% Dextrose / NaCl |
Then 72 mmol (18 g) / 24 h infusion if recurrent. |
| Acute Severe Asthma |
8 mmol (2 g) IV |
Over 20 min |
Single dose; improves FEV₁ and reduces admission risk. |
| Eclampsia |
16 mmol (4 g) loading → 96–192 mmol (12–24 g) / 24 h |
Loading over 20 min; maintenance infusion 1 g / h |
Use Pritchard or Zuspan regimens; monitor reflexes, RR, and urine output. |
| Hypomagnesaemia |
20 mmol (5 g) |
Over 1–3 h |
Max 50 mmol / day; up to 160 mmol over 5 days if persistent deficiency. |
💉 IM Injection
- Used only if IV access unavailable; inject deep IM (buttock or thigh).
- Small risk of pain and sterile abscess formation - use caution.
🧮 Oral Replacement
- Replace orally if serum Mg < 0.7 mmol/L and symptoms present (e.g. tremor, arrhythmia, weakness).
- Magnesium glycerophosphate: 4 mmol per tablet, 2 tabs TDS (≈ 24 mmol daily).
- GI side effect: diarrhoea - titrate to tolerance.
⚕️ Interactions & Precautions
- Prolongs the action of non-depolarising muscle relaxants - monitor in anaesthesia or ITU.
- Potentiates CNS depression when combined with benzodiazepines or opioids.
- Adjust dose in renal impairment - accumulation leads to respiratory depression or cardiac arrest.
🚫 Contraindications
- Severe renal failure - do not exceed 20 g / 48 h.
- Myasthenia gravis (may worsen weakness).
- Heart block or profound hypotension.
⚠️ Adverse Effects
- 🩸 Hypotension (especially with rapid bolus).
- 💤 CNS depression and loss of reflexes with excessive serum levels.
- 🫁 Respiratory depression in overdose.
- 🧠 Flushing, warmth, and sedation are early signs of rising magnesium levels.
☠️ Magnesium Toxicity
- Therapeutic range: 2–3.5 mmol/L. Toxicity > 5 mmol/L → respiratory depression, cardiac arrest.
- ❌ Do not continue loading rates beyond the recommended period.
- Max bolus: 6 g over 15–20 min; maintenance ≤ 3 g / h.
- Antidote: 10 mL of 10% calcium gluconate IV over 10 min → repeat as required.
- Monitor RR > 12 / min, urine > 100 mL / 4 h, and preserved patellar reflexes during infusion.
📚 References
