Beneficial effect of amiodarone on pacing induced terminability of reentrant ventricular tachycardia

A 33 year-old woman was referred to our hospital for further treatment of v entricular tachycardia (VT). During treatment with amiodarone (200 mg/day), clinical VT at the cycle length of 510 ms was induced. During the VT, rapi d ventricular pacing was repeated at progressively shorter cycle lengths af ter a decrement of 10 ms steps. The VT was entrained by the rapid pacing an d reproducibly terminated at a. paced cycle length of 380 ms. Four weeks af ter reducing the amiodarone to 100 mg/day, programmed stimulation was repea ted. The VT with the same morphology but with a slightly shorter cycle leng th of 480 ms was again induced. However, at this time, rapid pacing from th e same site could not terminate VT and transient acceleration developed at a shorter paced cycle length of 260 ms. The QT (QTc) interval, effective re fractory period at the pacing site and width of the paced QRS complex were similar before and after changing the amiodarone treatment. The most charac teristic change of VT in the second study was a widening of the entrainment zone, which was calculated as the difference between VT cycle length and t he longest pacing cycle length which interrupts VT during the entrainment ( from 130 to > 220 ms), and it may be explained by the preferential shorteni ng of the action potential duration and/or facilitation of the depressed ce ll to cell conduction within the reentry circuit. Amiodarone must exert a p referential action in the reentry circuit and modulate the conduction prope rty as well as the effective refractory period. We should pay close attenti on to the efficacy of antitachycardia pacing during the modification of ami odarone treatment.