You get a pop-up warning in the electronic medical record about potentially adverse interaction with a prolonged QT interval. What’s the risk, right? Afraid of a little torsades de pointes? Can’t we just give some prophylactic magnesium and call it a day? Let’s see if there’s any literature out there…

First off, let’s review magnesium. A prominent role of magnesium in the body is functioning as a de facto calcium antagonist which will inhibit the torsades de pointes mechanism.1 Furthermore, it’s part of the treatment for digoxin-induced arrhythmias in addition to the antibody fragments. Though not classically an antiarrhythmic, magnesium may convert some arrhythmias and prior work shows low magnesium may be proarrhythmogenic.2

Hypermagnesium is relatively uncommon, especially in the critically ill. However, if it does occur, toxicity manifests as neuromuscular symptoms or EKG changes, starting with widening of the QRS.1 Severe magnesium toxicity may lead to cardiac arrest but can be treated with calcium gluconate or dialysis in addition to standard resuscitative interventions.

Hypomagnesium is more common and is seen in 65% of intensive care patients.3 More pertinent to the undifferentiated ED patient, we consider administering magnesium when attempting to avert ventricular arrhythmias. For torsades de pointes, 2g of magnesium sulfate is the drug of choice. Though solid clinical data is lacking, small studies have demonstrated suppression of monomorphic ventricular tachycardia.4 A meta-analysis by Shiga et al in 2004 showed how it has potential (at least in the cardiac surgery population) to be used prophylactically to avert supraventricular and ventricular arrhythmias.5 So, in short, it’s unclear how low our threshold for giving prophylactic magnesium should be when there’s risk of prolonged QT. Strong evidence is lacking; however, the cardiac surgery population appears to get some benefit.

References
1. Herroeder S, Schönherr ME, De hert SG, Hollmann MW. Magnesium–essentials for anesthesiologists. Anesthesiology. 2011;114(4):971-93.
2. Moran JL, Gallagher J, Peake SL, Cunningham DN, Salagaras M, Leppard P: Parenteral magnesium sulfate versus amiodarone in the therapy of atrial tachyarrhythmias: A prospective, randomized study. Crit Care Med 1995; 23:1816–24
3. Rubeiz GJ, Thill-Baharozian M, Hardie D, Carlson RW: Association of hypomagnesemia and mortality in acutely ill medical patients. Crit Care Med 1993; 21:203–9Rubeiz, GJ Thill-Baharozian, M Hardie, D Carlson, RW
4. Ceremuzynski L, Gebalska J, Wolk R, Makowska E: Hypomagnesemia in heart failure with ventricular arrhythmias. Beneficial effects of magnesium supplementation. J Intern Med 2000; 247:78–86
5. Shiga T, Wajima Z, Inoue T, Ogawa R. Magnesium prophylaxis for arrhythmias after cardiac surgery: a meta-analysis of randomized controlled trials. Am J Med. 2004;117(5):325-33.

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