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?