Case Presentation
46-year-old women who had a history of recurrent palpitations was cardioverted elsewhere for a broad QRS tachycardia of left bundle branch block (LBBB) morphology (Figure 1a). On evaluation, her baseline ECG showed SR with narrow QRS which changed to LBBB morphology (Figure 1b) at heart rates greater than 90bpm which was suggestive of acceleration dependent block.1 Echocardiogram showed severe left ventricular (LV) dysfunction. Cardiac MRI was done which showed no scar or myocardial edema. She underwent a coronary angiogram which showed normal coronary arteries. Levophase of the coronary angiogram showed atresia of the coronary sinus [CS] os and it drained partially through the small cardiac vein into the right atrium (RA) and partially through a perisistent left SVC.
She was taken up for an electrophysiology study with the intent of radiofrequency ablation. Three quadripolar catheters were placed in the His, right ventricular apex, and right atrial appendage [RAA]. As the CS os was atretic, it could not be cannulated. Baseline AH (Atrial-His 70ms) and HV (His-Ventricular 45ms) intervals were normal. On atrial pacing, there were two subtly different LBBB morphology QRS seen (Figure 1c and Figure 1d). QRS in lead II in Figure 1d was biphasic similar to the tachycardia ECG in Figure1a while it was monophasic in Figure 1b and 1c. On pacing from the RAA, predominant QRS morphology was LBBB with a duration of 130ms with a positive HV interval of 45ms – suggestive of aberrant conduction (Figure 1c). On pacing from lateral RA, the LBBB widened to 150ms and the His was retrogradely activated with HV being negative and VH interval fixed at 10ms (figure 1d). On incremental atrial pacing, the AV interval was decremental and VH interval remained the same, suggestive of atriofascicular pathway. With programmed ventricular extra stimulus, a broad QRS tachycardia of LBBB morphology (Figure 2) with a QRS as in Figure 1d was induced. During the tachycardia, His was retrogradely activated, VH was 10ms and atrial activation was earliest in the His region. Septal refractory APC ( in Figure 2) from lateral RA was delivered. There seems to be an AVVA response followed by an antegrade His (marked as H in figure 2) with positive HV of 45ms. This was followed by a resumption of the original tachycardia. What is the likely mechanism for the response seen?