Thioridazine and QT interval prolongation

The use of thioridazine is contraindicated in patients with congenital or acquired QT interval prolongation syndromes or a history of cardiac arrhythmias. Thioridazine can prolong the QT interval of the electrocardiogram in a dose-related manner. The risk of torsade de pointes is progressively increased as the degree of prolongation becomes greater. In a crossover study of nine patients, a single 50 mg dose of thioridazine increased the QTc interval by a mean maximum of 23 msec compared to placebo. During postmarketing experience, torsade de pointes and sudden death have been reported, although a causal relationship has not been determined. The manufacturer states that thioridazine should not be administered in patients with a baseline QTc interval greater than 450 msec, and that it should be discontinued in patients with QTc measurements exceeding 500 msec during therapy. Electrolyte disturbances such as hypokalemia and hypomagnesemia may augment the prolongation effect of thioridazine on the QT interval and should be corrected prior to institution of therapy. Patients who experience frequent, severe, or prolonged diarrhea may be subject to electrolyte losses and should be followed closely and managed accordingly during therapy with thioridazine.