Electrophysiologic effects of mexiletine in children

The effects of intravenous administration of mexiletine on the refractory periods of the atrium and atrioventricular (AV) node were studied using His bundle recordings and the extrastimulus technique with atrial pacing. The drug was administered to 10 children with heart disease in an intravenous bolus dose of 3 mg/Kg, injected over a 5-min period, followed by an infusion of 1 mg/Kg/hr of mexiletine, achieving a mean therapeutic plasma concentration of 0.762 +/- 0.270 microgram/ml. The AH and HV intervals, the resting sinus cycle length, and the functional and effective refractory periods of the atrium and AV node were measured before and 5 min after initiation of the mexiletine infusion. The sinus cycle length was reduced significantly. However, neither the refractory periods nor the AH and HV intervals changed in a significant manner. Except for the heart rate, these results are similar to those reported in adults without conduction disturbances. In adults, mexiletine did not affect the sinus cycle length. However, in children, it consistently increased the heart rate. The lack of side effects in children is in contrast to adults, who usually suffer from gastrointestinal and neurologic symptoms upon receiving comparable doses parenterally.