Magnesium (MgSO₄) in Emergency and Critical Care: Complete Guide for Clinicians

 

Magnesium (MgSO₄) in Emergency and Critical Care: Complete Guide for Clinicians

Magnesium sulphate (MgSO₄) is one of the most versatile and lifesaving medications used in emergency medicine, cardiology, and critical care. Despite being inexpensive and widely available, it is often under-utilized or incorrectly dosed. This comprehensive guide covers everything you need to know about its indications, dosing strategies, preparation, mechanisms, side effects, and the clinical scenarios where it truly saves lives.

Magnesium plays a central role in neuromuscular stability, cardiac conduction, bronchial smooth muscle relaxation, and cellular enzymatic processes. In acute care settings, timely administration can improve outcomes and even reverse life-threatening arrhythmias.

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What is Magnesium Sulphate (MgSO₄)?

Magnesium sulphate heptahydrate is a formulation where 1 gram ≈ 4 mmol of Mg²⁺. In hospital practice, it is commonly stocked as Magnesium sulphate 50% solution, where:

10 ml contains 20 mmol of Mg²⁺ (equivalent to 5 g MgSO₄).

This concentrated form requires dilution prior to administration depending on the clinical indication.

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Key Clinical Indications for Magnesium Use

1. Severe Hypomagnesaemia

Hypomagnesaemia can lead to:

Ventricular arrhythmias

QT prolongation

Refractory hypokalaemia

Muscle weakness and tetany

Infusion dose:

2–4 g (8–16 mmol Mg²⁺) infused over 60 minutes.

Magnesium correction is essential when potassium or calcium correction is failing, as magnesium deficiency prevents cellular uptake of both electrolytes.

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2. Torsades de Pointes (TDP)

Torsades de pointes is a polymorphic ventricular tachycardia associated with prolonged QT interval. Magnesium works even when the serum level is normal because it stabilizes myocardial electrical activity.

Bolus dose:

2 g (8 mmol) over 10 minutes.

May repeat once if arrhythmia persists.

Mechanism:

Shortens QT interval

Suppresses early after-depolarisations

Helps terminate TDP and prevent recurrence

Even in drug-induced QT prolongation, magnesium remains a first-line therapy.

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3. Acute Severe Asthma

Magnesium is a potent smooth muscle relaxant and reduces airway hyperresponsiveness.

Infusion dose:

1.2–2 g over 20 minutes.

Benefits:

Reduces hospitalization

Improves airflow

Enhances response to bronchodilators

It is particularly useful in patients who are not responding to repeated nebulisations.

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Pharmacological Mechanism of Action

Magnesium acts on multiple systems simultaneously:

Cardiac System

Stabilises myocardial cell membranes

Inhibits calcium influx

Reduces arrhythmogenicity

Respiratory System

Relaxes bronchial smooth muscle

Decreases acetylcholine release at neuromuscular junctions

Neuromuscular System

Acts as a physiological calcium antagonist

Reduces neuroexcitation and muscle spasm

Electrolyte Role

Enhances renal conservation of potassium

Needed for PTH release and calcium metabolism

This multi-system action explains why magnesium is so beneficial in arrhythmias, asthma, eclampsia, and metabolic derangements.

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Dose and Administration (Practical Guide)

Hypomagnesaemia

2–4 g (8–16 mmol) over 1 hour

Torsades de Pointes

2 g IV over 10 minutes

Repeat once if needed

Acute Severe Asthma

1.2–2 g over 20 minutes

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How to Prepare Magnesium for Clinical Use

For a 2 g dose:

1. Draw 4 ml of magnesium sulphate 50% (equals 2 g / 8 mmol).

2. Dilute in 100 ml Normal Saline.

3. Infuse using a pump at the rate appropriate for your indication.

This ensures accurate delivery and reduces risk of side effects.

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Monitoring During Magnesium Administration

Magnesium must be infused in a monitored environment such as a resuscitation bay.

Monitoring includes:

Continuous ECG

Blood pressure

Respiratory status

Urine output

Watch for signs of magnesium toxicity:

Loss of patellar reflexes

Weakness

Drowsiness

Warmth/flushing

Nausea

Double vision

Slurred speech

Severe toxicity may lead to respiratory depression or hypotension; calcium gluconate is the antidote.

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Additional Drugs Required in Associated Conditions

In Torsades de Pointes

Magnesium is essential, but you may also need:

Intravenous potassium if the patient is hypokalaemic

Correction of underlying triggers (QT-prolonging drugs, electrolyte imbalance)

Consideration of temporary pacing in refractory cases

In Acute Severe Asthma

Administer along with:

Salbutamol and Ipratropium nebulisers

Systemic corticosteroids (prednisolone or hydrocortisone)

IM adrenaline 0.5 mg if anaphylaxis is suspected

Magnesium is an adjunct, not a standalone therapy, in asthma.

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Advantages of Using Magnesium in Emergency Medicine

Rapid onset of action

Useful even when serum magnesium is normal (e.g., TDP)

Favourable safety profile in controlled settings

Cheap and readily available

Requires no specialized storage

Effective across multiple systems

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Common Myths and Clarifications

Myth: Magnesium works only when levels are low.

Fact: Magnesium works in Torsades even with normal levels.

Myth: All magnesium doses must be slow.

Fact: In arrhythmias like TDP, a fast bolus is required.

Myth: Oral magnesium is enough.

Fact: Oral replacement is too slow and unreliable in acute settings.

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Clinical Tips and Pitfalls

Always assess for hypokalaemia and hypocalcaemia—these often coexist.

Deep tendon reflexes begin to depress at magnesium levels > 4 mmol/L.

Rapid IV push outside Torsades management may cause hypotension.

Severe asthma patients benefit most when magnesium is given early.

Avoid mixing magnesium with bicarbonate in the same line—precipitation risk.

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References (Recommended)

1. European Resuscitation Council (ERC) Guidelines for Advanced Life Support.

2. Global Initiative for Asthma (GINA) Management Guidelines.

3. American Heart Association ACLS Arrhythmia Guidelines.

4. StatPearls: Magnesium Sulfate Pharmacology.

5. British National Formulary (BNF): Magnesium Sulphate IV Use.

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Final Thoughts

Magnesium sulphate is a powerful, cost-effective, and often lifesaving medication in emergency and critical care. Whether managing a patient in refractory asthma, stabilizing a life-threatening arrhythmia like Torsades de Pointes, or correcting severe electrolyte disturbances, magnesium should always be on your therapeutic radar. Understanding its dosing, dilution, monitoring, and early use can significantly improve patient outcomes.