Prolonged magnesium sulfate infusion in the treatment of ventricular tachycardia in acquired long QT syndrome

To confirm the antiarrhythmic effectiveness of magnesium sulfate given by slow infusion and evaluate its influence on ventricular repolarisation parameters, we enrolled 12 consecutive patients (9 men and 3 women; aged 49 to 78 years) with acquired prolonged QT interval (QTc = 500 to 640) and episodes of torsade de pointes (TdP). Five had chronic ischaemic heart disease, 2 acute myocardial infarction, 4 total atrioventricular block, and 1 systemic hypertension. Of the 12 patients, 5 were receiving class IA antiarrhythmic drugs (disopyramide in 2 patients; quinidine plus disopyramide in 1; quinidine in 2), one was treated with methyldopa plus chlorthalidone, and another was on diuretic therapy alone. Magnesium sulfate (MgSO4) infusions in 0.9% sodium chloride were instituted at 50 mg/min and continued for 2 hours after the disappearance of TdP. These were followed by prophylactic infusions of 30 mg/min for 90 minutes twice daily for the next 3 to 4 days. In all cases, episodes of arrhythmia disappeared within 20 to 30 minutes. However, arrhythmia suppression was not associated with significant modification of the QT interval. No adverse effects were observed, and both the heart rate and blood pressure remained unchanged from baseline values.