Limited effect of magnesium sulphate on torsades de pointes ventricular tachycardia.

A patient with thioridazine-induced torsades de pointes ventricular tachycardia treated with magnesium sulphate is presented. Due to incessant recurrence of tachycardias it was possible to observe the time course of the effect of 1 g intravenous doses. Suppression of torsades de pointes lasted at best for no more than 18 min and a maintenance infusion was not effective without overdrive pacing. Magnesium sulphate may have value as first aid therapy for drug-induced torsades de pointes ventricular tachycardia. However, its effect disappears rapidly, and, therefore, should not alone be relied on as prophylactic treatment.