In this episode I’ll discuss the use of magnesium sulfate in critical care.

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Shout out to listener “Pharmacy Alex” for suggesting this topic!

When I think of magnesium in critical care, 3 uses come to mind:

1. Prevention of recurrent eclamptic seizures

2. Adjunctive therapy in life-threatening asthma exacerbations

3. Treatment of torsades / polymorphic ventricular tachycardia

Prevention of recurrent eclamptic seizures

Magnesium sulfate IV is the drug of choice for prevention of recurrent eclamptic seizures. Efficacy of magnesium for this indication was established by The Eclampsia Trial Collaborative Group. 1687 women with eclampsia were recruited into an international multicentre randomized trial comparing magnesium vs phenytoin and magnesium vs diazepam. Women allocated magnesium sulfate had a 52% lower risk of recurrent convulsions than those allocated diazepam. Women allocated magnesium sulfate had a 67% lower risk of recurrent convulsions than those allocated phenytoin.

Mechanism

The specific mechanism by which magnesium prevents recurrent eclamptic seizures is unclear. Magnesium has effects on peripheral and cerebral vasodilation, blood-brain barrier protection, and as an anticonvulsant.

Dosing

For prevention of recurrent eclamptic seizures, magnesium sulfate is given 4-6 g IV over 15 minutes followed by a 2 g / hour infusion. IM injection is also an option, although it is painful and slower acting.

Adjunctive therapy in life-threatening asthma exacerbations

Guidelines recommend considering IV magnesium sulfate in severe asthma exacerbations that are not responding to bronchodilators and corticosteroids. Not all studies of magnesium in asthma have found positive results.

Mechanism

IV magnesium sulfate has bronchodilator activity in acute asthma. This may be due to inhibition of calcium influx into airway smooth muscle cells.

Dosing

For adjunctive treatment of life-threatening asthma exacerbation, give magnesium sulfate 2 g IV over 20 minutes.

Torsade de pointes

IV magnesium is frequently thought of as treatment for torsades. However there are discrepancies between references on the role of magnesium in torsades and the evidence to support it.

UpToDate states:

Intravenous magnesium sulfate is first-line therapy, being highly effective for both the treatment and prevention of recurrence of long QT-related ventricular ectopic beats or TdP.

The ACLS guidelines state:

Although magnesium is commonly used to treat torsades de pointes VT (polymorphic VT associated with long QT interval), it is supported by only 2 observational studies that showed effectiveness in patients with prolonged QT interval.

Dosing

The ACLS guidelines recommend electricity (defibrillation) before magnesium for torsades. For a pulseless adult with torsades & prolonged QT interval give 2 g magnesium IV push.

Review article

A good review article of magnesium in critical care can be found here.

Did I miss an indication for magnesium in critically ill patients? Let me know here!

If you like this post, check out my book – A Pharmacist’s Guide to Inpatient Medical Emergencies: How to respond to code blue, rapid response calls, and other medical emergencies.

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