Abstract 10194: Pacing Site- and Rate-Dependent Shortening of Retrograde Conduction Time Over the Slow Pathway After Atrial Entrainment of Fast-Slow Atrioventricular Nodal Reentrant Tachycardia

Introduction: The entrainment rule by which the site of entrainment has no effect on the orthodromic conduction time across the reentry circuit immediately after cessation of pacing has not been unequivocally verified in atypical atrioventricular nodal reentrant tachycardia (AVNRT). We have recently observed a unique electrophysiological response upon atrial induction of fast-slow (F/S-) AVNRT, characterized by a shortening of the retrograde conduction time across the slow pathway (SP) (retro-SP-time). Hypothesis: Atrial entrainment pacing (EP) of a) the common-type (com-) F/S-AVNRT using a typical SP, or b) the superior-type (sup-) F/S-AVNRT using a superior SP, both might modify the retro-SP-time immediately after termination of EP. Methods: In 16 patients of com-F/S-AVNRT and 11 patients of sup-F/S-AVNRT in whom successful EP from the high right atrium (EP-HRA) and the proximal coronary sinus (EP-CS) was obtained at 2±2 rates, we evaluated the difference in the His-atrial (HA) interval measured immediately after cessation of each EP attempt (HA difference) between EP-HRA and EP-CS (HA[1]-HRA and HA[1]-CS, respectively) and also measured atrial-His and HA intervals in the 1st and 2nd cycles immediately after the EP, and during tachycardia stabilized. Results: >20 ms HA difference, defined as an atypical difference, at ≥1 rates of EP was observed in 16 patients (56%), including 7 with com- and 9 with sup-F/S-AVNRT. Irrespective of the EP rate, the atypical response of com-F/S-AVNRT was always due to a shorter HA[1]-CS than HA[1]-HRA, with a 39±22 ms HA difference, whereas the atypical response of sup-F/S-AVNRT was always due to a longer HA[1]-CS than HA[1]-HRA, with a 51±26 ms HA difference. Moreover, the atypical response disappeared within 2 cycles after the cessation of EP. The atypical difference, although its exact mechanism remains unclear, might attribute to a shortening of the retro-SP-time according to decremental properties of the SP with deeper antidromic penetration during EP provoked depending on the site and rate of pacing. Conclusions: We have identified a little-known pacing site- and pacing rate-dependent shortening of the retro-SP-time. This phenomenon may affect an interpretation of post-pacing interval after EP of F/S-AVNRT.