Ten patients with refractory recurrent
supraventricular tachycardia were found by electrophysiologic study to have bypass tracts and orthodromic
atrioventricular reentrant tachycardia. All had failed to respond to conventional antiarrhythmic
therapy and were therefore treated with oral
amiodarone (1,600 to 2,000 mg/day for 2 weeks, then 800 to 1,200 mg/day for another 2 weeks with subsequent 200 to 600 mg/day maintenance doses). During or after the fourth week of
therapy, electrophysiologic study was repeated. In 9 of 10 patients,
supraventricular tachycardia could not be reinduced by programmed stimulation. In the remaining patient, nonsustained
supraventricular tachycardia (greater than 10 beats, lasting less than 30 seconds) with a slower basic cycle length than that during the control period was provoked. Significant increases in the effective refractory period of the accessory pathway in both the anterograde (+26%, p less than 0.05) and retrograde (+40%, p less than 0.02) directions were noted, the magnitude of change being independent of the control effective refractory period. There were also significant increases in the effective refractory period of the right atrium (+24%, p less than 0.01) and the right ventricle (+15%, p less than 0.01) during long-term
therapy with
amiodarone. Over a mean follow-up period of 20 months, symptomatic control of the
arrhythmia occurred in all patients; in only one patient treatment with
amiodarone could not be continued because of side effects. These data establish the electrophysiologic basis for the effectiveness of
amiodarone in the prophylactic control of refractory
paroxysmal supraventricular tachycardia complicating the bypass tract syndromes.