Mexiletine and tocainide: does response to one predict response to the other?

Mexiletine and tocainide were administered to 79 patients to determine whether the response to one of these drugs would predict the effect of the other. In 57 patients, the two agents were evaluated noninvasively with monitoring and exercise testing, and efficacy was judged by the suppression of spontaneous ventricular arrhythmia. In the remaining 22 patients, electrophysiologic testing was performed and efficacy was defined as the inability to induce more than two repetitive ventricular premature beats. An equal number of patients responded to mexiletine and tocainide (38 versus 39%). However, in only 42 patients (53%) were the results concordant. There was no difference in concordance when the results were analyzed by method of drug evaluation, left ventricular ejection fraction or etiology of presenting arrhythmia. It is concluded that mexiletine and tocainide have different clinical effects and must be evaluated individually.