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.