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: We tested our hypothesis that atrial entrainment pacing (EP) of a) the common-type (com-) fast-slow (F/S-) atypical atrioventricular nodal reentrant tachycardia (AVNRT) using a typical slow pathway (SP), or b) the superior-type (sup-) F/S-AVNRT using a superior SP, both modify the retrograde conduction time across the SP immediately after termination of EP (retro-SP-time). Methods: We measured the difference in the His-atrial interval (HA difference) immediately after cessation of EP, performed at 2 ± 2 rates from the high right atrium (HA[1]-HRA) versus from the proximal coronary sinus (HA[1]-CS) in 17 patients with com-F/S-AVNRT and 11 patients with sup-F/S-AVNRT. We also measured the atrial-His and HA intervals of the first and second cycles immediately after cessation of EP and during stable tachycardia. Results: Unequal responses, defined as a ≥ 20-ms HA difference at ≥1 EP rates, were observed in 16 patients (57%), including 7 with com- and 9 with sup-F/S-AVNRT. Irrespective of the EP rate, all unequal responses of com-F/S-AVNRT were due to a shorter HA[1]-CS than HA[1]-HRA, with a mean 34 ± 11 ms HA difference, whereas all unequal responses of sup-F/S-AVNRT were due to a longer HA[1]-CS than HA[1]-HRA, with a mean 49 ± 25 ms HA difference. The unequal responses resolved within two cycles after the cessation of EP. Conclusions: We have identified a little-known pacing site- and pacing rate-dependent shortening of the retro-SP-time.