Mexiletine action on the specialized tissues of the human heart

Electrophysiological properties of mexiletine (3 mg/kg i.v.) were studied in fifteen patients with various degrees of abnormalities in the specialized conduction system. Sinus cycle length was decreased in all patients; sinus node recovery time was increased in all patients, but the increment was not statistically significant. Atrial and atrioventricular (AV) nodal refractoriness were not modified. The relative refractory period of the His-Purkinje system was reduced in patients with normal intraventricular conduction on the surface electrocardiogram; no changes were noted in five patients with intraventricular conduction delay. In two patients, in whom AV nodal refractory period curves showed antegrade dual AV nodal pathways, mexiletine increased refractoriness of the fast pathway. This report points out that the drug is effective against arrhythmias sustained by a reentry mechanism, not only in the ventricles, but also in the AV node.