Makindo Medical Notes

⚠️ 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)

💉 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

• BNF: Magnesium Sulfate
• ICID Salisbury: IV Magnesium Sulfate Guidelines
• RCHT Hypomagnesaemia Guideline (NHS Cornwall)
• RCOG Green-top 10(A): Pre-Eclampsia and Eclampsia (2023)
• NICE NG25: Preterm Labour and Birth (2022)