Unidirectional block and longitudinal dissociation in an accessory pathway induced by radiofrequency

This report describes an unusual arrhythmia due to partial damage of an accessory pathway by radiofrequency energy delivered during a catheter ablation procedure. The following phenomena were observed after the first radiofrequency current application: a) manifest anterograde conduction over the Kent bundle was abolished, so that preexcitation disappeared; b) concealed anterograde conduction over the accessory pathway was interrupted, resulting in initiation of orthodromic re-entrant tachycardia by any sinus impulse; c) retrograde conduction through the accessory pathway was impaired, but still present, and a pattern of longitudinal dissociation manifested; this was suggested by alternation of the R-P intervals, that during orthodromic tachycardia were alternatively long and short. Following a second radiofrequency application, the R-P interval during orthodromic tachycardia became markedly prolonged (0.36 sec), to the extent that the pattern mimicked a form of atrial or sinus tachycardia. Retrograde accessory pathway conduction was totally interrupted following a third radiofrequency energy application.