COMMENTARY:
The differential diagnosis for the long RP tachycardia with a 1:1 AV
relationship includes a permanent form of junctional reciprocating
tachycardia, atrial tachycardia and atypical atrioventricular nodal
re-entrant tachycardia (AVNRT). The VA intervals prior and to after the
VPB are same which suggests VA linking and therefore the diagnosis of
atrial tachycardia is less likely (which also correlated with the
pseudo-V-A-A-V response on ventricular entrainment). The resetting of
tachycardia with identical atrial activation sequences pre- and post-VPB
suggests the presence of a retrograde accessory pathway and evidences
its participation in tachycardia. Induction of His refractory VPB during
tachycardia with a coupling interval of 272 ms resulted in
post-excitation of atrial signal and suggests a decrementing nature of
the retrograde limb of the tachycardia circuit. The second VPB in the
tracing, likely catheter induced from the same site as the induced one
but shows narrower morphology suggesting engagement of the septum or
conduction system but with a longer coupling interval of 302ms, also
showed post-excitation of the atrial impulse. The magnitude of the
post-excitation is higher (26ms) during the induced VPB rather than the
catheter-induced VPB of (14ms). The varying degree of post-excitation
can be explained by the distance from the VPC origin to the site of the
ventricular end of insertion of the accessory pathway and the coupling
interval. The morphology of the VPB being narrow but similar to the
paced beat makes this as a cause for incremental post-excitation less
likely. Hence the degree of post-excitation in the decrementally
conducting retrograde limb of the tachycardia circuit is most probably
related to the coupling intervals of the VPB. Post-excitation will be
higher when the VPB coupling interval is shorter. This phenomenon
re-iterates the decremental nature of the retrograde limb of the
tachycardia circuit further confirming the diagnosis of permanent form
of junctional reciprocating tachycardia (PJRT).
In this case, successful radiofrequency ablation of the decrementally
conducting accessory pathway was done outside the coronary sinus ostium
with no inducible tachycardia thereafter.