[Effects of mexiletine in the Wolff-Parkinson-White syndrome]

The effects of intravenous mexiletine were studied by His bundle recordings and programmed stimulation in 7 patients aged from 20 to 40 years old (average age : 32 years), 5 of whom had an overt WPW syndrome (4 type A, and 1 type B), and 2 of whom had concealed pre excitation. All had reciprocating tachycardia. Electrophysiological investigation was performed under basal conditions, and then several times in the hour following intravenous mexiletine (3,5 mg/Kg in 5 minutes) followed by an infusion of 0,07 mg/Kg/min. The following results were obtained : 1. mexiletine did not cause any significant changes in the normal AV conduction pathway; 2. the refractory periods of the atria and ventricles were unaffected by the drug; 3. pre excitation regressed in 2 of the 5 patients with overt WPW. In a third patient, the effective anterograde refractory period of the accessory pathway increased by 210 ms. The retrograde refractory period of the accessory pathways increased in three patients and remained unchanged in the others; 4. there was little change in the ability to induce reciprocating tachycardia by stimulation. However, in two patients, the attacks were shortened, terminating spontaneously within a few seconds. This study shows the electrophysiological basis of the use of mexiletine in the WPW syndrome. Although the results do show some variability, they justify the use of mexiletine in patients with paroxysmal tachycardia in the WPW syndrome.