In adults with polymorphic ventricular tachycardia, what is the magnesium sulfate dose when prolonged QT is observed after cardioversion?

• A. 2 gm IV/IO bolus over five minutes
• B. 4 gm IV/IO bolus over five minutes
• C. 2 gm IV/IO bolus over ten minutes
• D. 1 gm IV/IO bolus over five minutes

Answer: (A)

When a patient with polymorphic ventricular tachycardia presents after cardioversion with a prolonged QT, the treatment focus is torsades de pointes prevention and stabilization. Magnesium sulfate is given to rapidly stabilize cardiac membranes and suppress early afterdepolarizations that drive the arrhythmia, even if magnesium levels are normal. The best dose is a 2 g IV or IO bolus delivered over about five minutes to quickly raise magnesium and help restore stable rhythm. If torsades recurs, another bolus may be given and a continuous infusion can be started (0.5–1 g per hour) with close monitoring. Dosing higher or slower, such as 4 g, or starting with a smaller 1 g bolus, is not appropriate for the immediate need to rapidly counteract the arrhythmia.