Unique electrophysiological properties of fast‐slow atrioventricular nodal reentrant tachycardia characterized by a shortening of retrograde conduction time via a slow pathway manifested during atrial induction

Electrophysiological properties of reentry circuits of fast‐slow atrioventricular nodal reentrant tachycardia (F/S‐AVNRT) may contribute to cyclic variability after atrial induction. Methods In 156 atrial inductions of 33 patients with F/S‐AVNRT, we measured the atrio‐His (AH) and His‐atrial (HA) intervals in the first cycle after the induction (AH[1] and HA[1], respectively), those in the second cycle (AH [2] and HA [2], respectively), and those during tachycardia that maintained a stable cycle length AH[T] and HA[T], respectively), and calculated the value of AH(1) minus AH(T) [ΔAH] and the value of HA(1) minus HA(T) [ΔHA] in each induction. According to the sum of ΔAH and ΔHA, tachycardia variability was classified as incremental (<−20), balanced (−20 to 20), or decremental (>20). Results ΔAH and ΔHA were significantly different between the three responses: 6 ± 28 and −67 ± 39 in 55 inductions (35%) with an incremental response, 20 ± 10 and −23 ± 28 in 59 (38%) with a balanced response, and 54 ± 44 and 4 ± 50 in 42 (27%) with a decremental response, respectively. Incremental response was reproducibly and consistently observed in 33% of patients. HA(2) was similar to HA(T) in inductions with an incremental response. These results suggest that incremental response can be manifested only in the first cycle when HA(1) is excessively shortened, approximating a retrograde conduction time over a slow pathway, in contrast, and far superior to a decremental delay of AH(1). Conclusion In specific patients with F/S‐AVNRT, poorly recognized, electrophysiological properties of shortening a retrograde conduction time over a slow pathway was manifested during atrial induction.