Mechanism of induction of atrioventricular node reentry by simultaneous anterograde conduction over the fast and slow pathways

Introduction: AV node reentry (AVNRT) is typically induced with anterograde (Ant) block over the fast pathway (FP) and conduction over the slow pathway (SP), with subsequent retrograde (Ret) conduction over the FP. Rarely, a premature atrial complex (PAC) conducts simultaneously over the FP and SP to induce AVNRT (2 for 1). This study investigates the mechanism of 2 for 1 induction. Methods and Results: Of 192 consecutive patients (pts) undergoing posteroseptal radiofrequency ablation to treat AVNRT, 4 pts (2%) had 2 for 1 AVNRT induction. All needed isoproterenol for AVNRT initiation, and Ant conduction was over the SP during AVNRT. Controls (n = 15) were randomly selected from the remaining 188 pts and required isoproterenol to induce AVNRT with Ant block over the FP. For 2 for 1 versus control, respectively, there was no difference in mean age (55 vs. 46 yr), AVNRT cycle length (420 vs. 320 ms), or the Ant effective refractory period of the FP (320 vs. 344 ms). Of note, the PAC that induced AVNRT had a significantly longer AH interval over the SP in pts with 2 for 1 versus control (470 vs. 320 ms, P = 0.016), even though the A1A2 interval for induction was longer for 2 for 1 (315 vs. 260 ms, P = 0.003). Ret conduction over the SP was relatively poor in the 2 for 1 group as evidenced by 4/4 pts with induction of AVNRT during incremental ventricular pacing versus only 1/15 control pts (P < 0.001). Conclusion: The unique induction of AVNRT by a PAC with simultaneous conduction over the FP and SP is best explained by minimal to no retrograde invasion of the SP from the anterogradely conducted fast pathway impulse, and consistent with this observation is the initiation of slow/fast AVN reentry during incremental RV pacing.