Radiofrequency Ablation of Accessory Pathways: Characteristics of Transiently and Permanently Effective Pulses

The purpose of this study was to characterize and compare the radiofrequency current applications that produced permanent or transient accessory pathway conduction block. One hundred fifty-two radiofrequency energy applications that induced permanent (permanently effective pulses, n = 48J or transient (transiently effective pulses, n = 104) accessory pathway block in 57 patients with 60 accessory pathways were analyzed. The time from the onset of current application to disappearance of preexcitation or termination of supraventricular tachycardia by permanently effective pulses was 1-15 seconds (mean 3.6 ± 3.8 sec) compared to 2-29 seconds (mean 11.5 ± 7.5 sec) by transiently effective pulses (P < 0.01). After transiently effective pulses that induced block in accessory pathway, conduction resumed within 5 minutes while induced block by permanently effective pulses persisted in 44 of 48 patients (92%) during follow-up of 11 ± 12 months. The accessory pathway conduction returned in the remaining four patients after ablation 2 weeks to 7 months. After transiently effective pulses, 41 impulses were delivered to the same site using a higher power output (n = 32) and/or longer energy delivery duration (n = 20) without new mapping of accessory pathway location. Thirty-six of these impulses again resulted in transient accessory pathway block, four had no effect, only one impulse induced a permanent block in the accessory pathway. Pulses with higher power outputs tended to induce transient effects more frequently than pulses with lower energy. Thus, if block in the accessory pathway during radiofrequency ablation occurs within 5 seconds of current application, and if the accessory pathway block persists over 5 minutes, permanent block of accessory pathway conduction may be expected. If 15 seconds after onset of ablation still no effect is noted, it is unlikely that any permanent effect will occur at longer pulse duration. Therefore, it is not useful to repeat energy delivery after a transiently effective pulse with higher power settings and/or longer pulse duration to the same site without new mapping for localization of the accessory pathway. Further experimental studies are needed to explain the mechanisms of transient effects of radiofrequency pulses.