Discussion
Conduction system pacing especially LBA pacing has recently gained
substantial traction as an alternative to traditional coronary sinus
pacing for CRT. 2-4 There are several potential
advantages of LBA pacing that have been demonstrated in multiple
studies. Numerous reports have shown impressive QRS shortening and
improvement in heart failure together with good pacing parameters in
patients who previously failed standard CS pacing and continued to have
wide QRS post standard biventricular pacing. 2-4However, there are still significant number of patients in whom LBA
cannot correct the underlying LBBB, like the case presented here.
The most interesting and somewhat unexpected finding of this case is
that when combined with the coronary sinus pacing lead located in the
posterolateral position, the apparently “failed” LBA pacing lead
resulted in marked shortening of the paced QRS duration, improved
electric synchronization and led to CRT super-response. A recent study
has demonstrated that incorporation of an CS lead into an existing
functional LBA pacing system may result in additional QRS shortening
compared to LBA pacing alone.5 However, our case
appears to be the first one that showed even a “failed” LBA pacing
lead, when combined with an existing CS lead that failed to confer CRT
benefit, may result in marked synergistic shortening of the QRS and
ultimately improved clinical outcome.
The exact mechanism underlying this synergistic QRS shortening between
the CS lead and LBA pacing lead is not entirely clear. Because the LBA
pacing lead in this case failed to correct the LBBB and significantly
narrow the QRS, the added benefit of pacing through this lead versus the
more apically located RV defibrillation lead may be related to the
anatomic position of the LBA lead in relation to the CS lead. Having the
RV lead in a basal and anterior position creates excellent separation
from the CS lead located in the posterolateral aspect of the left
ventricle on fluoroscopic views (figure 2). The pacing vector between
the CS and LBA leads appears to cover a much larger bulk of left
ventricle compared to the one between the CS and RV defibrillation lead.
In conclusion, simultaneous LBA and CS pacing (in the posterolateral
position) may achieve substantial synergy and improve CRT outcome in
patients who failed either LBA and/or CS pacing alone. We are currently
in the process of conducting a larger cohort study to help further
confirm this finding and elucidate the underlying mechanism.