Electrophysiologic effects of mexiletine in children.

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Abstract
The effects of i.v. administration of mexiletine on the refractory periods of the atrium and atrioventricular (AV) node were studied by using His bundle recordings and the extrastimulus technique with atrial pacing. The drug was administered to 10 children with heart disease in an i.v. bolus dose of 3 mg/kg injected over a 5-min period; this was followed by an infusion of 1 mg/kg per h of mexiletine, this achieving a mean therapeutic plasma concentration of 0.762 .+-. 0.270 .mu.g/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. 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. In children, it consistently increased the heart rate. The lack of side effects in children is in contrast to that in adults, who usually suffer from gastrointestinal and neurologic symptoms upon receiving comparable doses parenterally.

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