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.