Effects of oral amiodarone on rate-dependent changes in refractoriness in patients with Wolff-Parkinson-White syndrome.

The refractory periods during cardiac pacing were studied in 13 patients with the Wolff-Parkinson-White syndrome. The right atrial and right ventricular refractory periods and refractory period of the accessory pathway (anterogradely and retrogradely) were studied at 2 pacing cycle lengths before and after therapy with oral amiodarone (8,400 to 11,200 mg given in 4 to 6 weeks). The right atrial and right ventricular effective refractory period shortened significantly when the pacing rate was increased during the control study, and also after oral amiodarone administration. The anterograde and retrograde effective refractory period of the accessory pathway also shortened significantly at control study, but not during treatment with oral amiodarone. This indicated that amiodarone blunted the rate-dependent shortening in the refractory period of the accessory pathway. The rate-dependent increase in refractoriness of the accessory pathway could not be predicted or determined in all patients. In 5 patients a rate-dependent increase in the effective refractory period of the accessory pathway was observed in the anterograde direction and in 3 patients in the retrograde direction while they were receiving oral amiodarone therapy. When these data were correlated with the mode of induction and termination of tachycardia, however, a possible effect was found in only 1 patient. Further investigation of new antiarrhythmic drugs should include the development of components resulting in a reliable and predictable increase in refractoriness when the heart rate increases. This would result in prompt termination of reentrant tachycardia by creating block for the circulating impulse.