Discussion
The right panel in Figure 1 depicts the site of ablation of a concealed left free-wall accessory pathway (red arrow) by a retrograde transaortic approach. Ablation during ventricular pacing is preferable compared to during sustained tachycardia in view of the advantage of catheter stability, especially after elimination of the pathway during ablation. Notably, pacing was performed just below the RV basal outflow tract with the His bundle catheter pushed distally (white arrowhead, right-hand panels of Figure 1). The ablation signal in the left panel of Figure 2 marks the ‘A’ with an arrow, the small potential just after the ventricular electrogram in the RF distal channel. The early atrial activation along with the absence of isoelectric interval between V and A suggest that it is likely to be a successful ablation site.
With initiation of RF energy, there was an increase in the ventriculo-atrial interval in the distal coronary sinus (CS12) and RF distal channels with a change in atrial activation sequence (Figure 2, left panel). During continuation of ablation, the alternate wide and narrow QRS beats have the same retrograde concentric atrial activation pattern with a constant VA interval, as depicted in Figure 3. The narrower QRS complexes are associated with a reversal of the ventricular activation sequence in the coronary sinus channels. This may occur due to one of the following mechanisms: i) alternate His bundle capture and non-capture; ii) capture of the antero-basal left ventricle along with the RV on alternate beats; iii) RF catheter induced premature ventricular complexes (PVCs), causing fusion and narrowing of the alternate beats. His bundle capture is unlikely in this case as the catheter was placed deeper in the RV base and there was no His signal in the His distal channel at baseline. Capture of the antero-basal left ventricle on alternate beats resulting in narrow QRS is a possibility; however, the ventricular electrogram in the RF catheter would not be coincident with the pacing stimulus, seeing the large distance between the RF catheter and the His catheter (Figure 1, right-hand panel). Catheter induced PVCs have been reported to occur in a bigeminal pattern1 due to cardiac motion during tachycardia or ventricular pacing and then fuse with the paced beat, resulting in a narrower QRS complex. The earliest ventricular activation in the RF distal channel followed by ventricular activation wave front from distal to proximal in the CS suggests catheter induced PVCs as a putative mechanism in this case.
The change in the atrial activation pattern from eccentric to concentric is consistent with elimination of accessory pathway conduction. However, residual accessory pathway conduction may be missed by RV pacing. Interestingly, the phenomenon of alternating narrow and wide QRS complexes allows insight into the assessment of the success of RF ablation in this case. The atrial activation pattern and the stimulus to atrial activation interval remains exactly the same despite early left ventricular free wall capture. Note in the RF distal signal in Figure 3, the presence of four different potentials: the first immediately following the pacing spike is the local ventricular potential (V), follow by a likely artefact, followed by an atrial potential (A) and another potential (*). This additional electrogram marked by an asterisk is possibly a split atrial electrogram.
Acknowledgement: The authors wish to acknowledge the help in the interpretation provided by Prof Hein JJ Wellens prior to his unfortunate demise.