Discussion
In this case, we found the two distinct retrograde atrial activations during AVNRTs of which the earliest site was in the HB region (Figure 1A) or the inferior lateral mitral annulus (Figure 3B and Suppl. Figure 1). SVT1 was diagnosed as a fast-slow AVNRT using the fast pathway as an antegrade limb and the SSP as a retrograde limb. Fast-slow AVNRT using an SSP is known to usually have a short AH interval, but in this case, since the AH and HA intervals during the tachycardia were nearly identical, a typical long RP pattern was not observed. Kaneko et al. have reported that not all cases of AVNRT via an SSP necessarily exhibit a short AH and long RP pattern, suggesting that there may be inter-individual diversity in the electrocardiographic appearance probably due to a balance between the antegrade and retrograde conductivities during the tachycardia 7,8. They also reported that a double atrial response characterized by two types of retrograde conduction near the superior region of the AV node4 is recognized in some cases, but it was not observed in this case. We found several important findings of SVT1 such as atrial inducibility of the tachycardia without a jump in the AH interval, the EAA around the superior AV node, and no effect of ablation on the rightward inferior extension. According to those findings, we considered the fast pathway as an antegrade conductive pathway and the SP as the retrograde pathway before the first application of the RF delivery. Moreover, the fact that ventricular overdrive pacing during the tachycardia exhibited VA dissociation excluded the involvement of the accessory pathway and a concealed node-ventricular Mahaim fiber for the establishment of the tachycardia. Kaneko, et al. reported that VA dissociation during ventricular entrainment pacing of fast-slow AVNRT using an SSP, although not specific, was commonly observed due to block at the lower common pathway 4. This phenomenon observed in this case, was also consistent with this type of AVNRT. Finally, differential atrial overdrive pacing excluded the AT for a diagnosis of the tachycardia. In terms of catheter ablation, since the EAA site during retrograde conduction via the SSP was close to the HB region, ablation of the SSP from the right atrium had a risk of AV node injury. A previous report noted that ablation from the NCC would be safe for the SSP ablation without the risk of AN node damage4. Ablation of the SVT1 was finally successful from the NCC without AV node injury.
The diagnosis of SVT2 as AVNRT was also made by ruling out the presence of an accessory pathway and atrial tachycardia during an electrophysiological study repeated after the ablation of the SPP. Interestingly, serial ventricular pacing during SVT2 could entrain the tachycardia, but also exhibited VA dissociation in some cases. This difference may have at least in part depended on a temporal change in the autonomic tone or blood concentration of isoproterenol that modified the conductivities of the lower common pathway. Alternately, the conductivities of the lower common pathway might become altered depending on the type of AV nodal circuit. The earliest activation site during SVT2 was observed at the mitral valve annulus in the LA, suggesting the ILA-SP as a retrograde limb of fast-slow AVNRT. Moreover, the development of junctional rhythm observed during the RF application with an atrial breakthrough to the earliest activation site of SVT2, followed by termination of SVT2 may have also suggested the presence of the ILA-SP along the mitral annulus in the LA 1,9 that might have formed from AV nodal cell tissue10. A trans-septal approach to the LA might be better in patients with difficulty in terminating a tachycardia by ablating from the CS5.
Considering these results of the electrophysiologic studies and ablation, the retrograde conductivities of the ILA-SP were present before the ablation of the SSP. During SVT1, retrograde conduction via the ILA-SP was not manifested probably because the retrograde penetration into the ILA-SP encountered an antergrade wavefront into the ILA-SP due to a slightly longer VA conduction time of the ILA-SP than the that of the SSP. As a result, we could mainly observe retrograde conduction via the SSP before the NCC ablation (Suppl. Fig 3). Collectively, this is, to the best of our knowledge, the first report showing the involvement of the ILA-SP as a bystander pathway during fast-slow AVNRT using an SSP.