Discussion:
Focusing solely on the surface ECG findings, a deflection consistent
with a retrograde P wave is observed at the terminal end of the QRS,
although it is challenging to confirm. A very late coupled premature
ventricular contraction (PVC) is applied, and it demonstrates evident
QRS fusion. This must be by consequence “His refractory”. This PVC
delays the subsequent QRS complex. Thus, one observes fusion and reset
of the tachycardia, most consistent with macroreentry. The observation
suggests that the retrograde limb of the circuit is an accessory pathway
(AP).[1] It is worth noting that the ability of a very late coupled
PVC from the RV to reset the tachycardia implies that the RV is part of
the circuit or has excellent access to it. Possible explanations for
this include the presence of a right AP, right paraseptal AP or a right
nodoventricular AP.[2, 3]
The intracardiac tracings (Figure 1) reveal that the prevailing SVT is
an orthodromic reciprocating tachycardia (ORT), and the retrograde limb
is a left lateral AP with eccentric atrial activation.[4] However,
this has to be reconciled with the observation that a “very late”
coupled PVC from the RV resets and terminates the SVT, making a left
lateral AP quite unlikely, as it would require a closer coupled PVC from
the RA to access the circuit.
Figure 2 illustrates that the RVa PVC advanced the right atrial
electrograms (EGMs) (His A-A 399ms), but not the left atrial EGMs (CS
7-8 A-A 437ms). Simultaneously, it caused a delay in the subsequent QRS
and termination of the SVT. This is best explained by a second right AP
that did not participate directly in the SVT, as depicted in the
schematic diagram (Figure 2).
The cumulative evidence supports the existence of a “double loop”
mechanism, in which the anterograde conduction occurs over the normal
atrioventricular nodal (AVN) conduction, while retrograde conduction is
over either the right or left AP. The left AP circuit activates the
atrium ahead of the right AP, preventing the anterograde wave
originating from the right AP from conduction into the AVN. However, the
right AP still serves as a substrate for the double loop. When the PVC
conducts retrogradely over the right AP, it enters the circuit and
reveals the right loop. The advanced atrial activation via the right AP
subsequently conducts with a long PR interval (HIS A-V 372ms), likely
over a slow AVN pathway, and the tachycardia terminates. Consequently,
the right AP does not directly participate in the observed SVT
(bystander), but it possesses the capability to reset and terminate it.
It is important to note that the tracings do not exclude the possibility
of a right nodoventricular connection.
It is speculated that the termination of the tachycardia is due to the
effective refractory period (ERP) of the AVN being reached, possibly
because of an aborted AVN echo occurring retrogradely over the fast
pathway. This renders the fast pathway refractory (retrograde concealed
conduction).
After mapping the left lateral AP during RV pacing, radiofrequency
ablation was carried out. Subsequent testing confirmed the presence of a
right AP with retrograde conduction but without any inducible
tachycardia and a long anterograde ERP.