Discussion
This is probably the first described case of diminishing cardiac systolic function after implantation of a direct left bundle branch pacing lead reversed by the introduction of a left ventricular pacing lead. Worsening of a cardiomyopathy substrate and subsequently heart failure with His bundle pacing has not been described. This particular case highlights an incomplete understanding of the relationship between simultaneous electrical activation and cardiac synchrony. Thus, a narrower QRS complex may not always correspond to better myocardial performance. In our case a QRS of 103 ms was achieved with direct left bundle branch pacing. This however, by exclusion, resulted in dyssynchronous left ventricular contraction, progressive cardiomyopathy and heart failure. The timeline of heart failure events in this patient strongly suggests a poor response to direct left bundle branch pacing confirmed by resolution after an upgrade to a biventricular system. Improvement of EF after the upgrade raises the possibility of an added benefit from the lateral left ventricular wall pre-excitation in restoring cardiac synchrony in some patients. It is unclear what characteristics may define poor responders to direct left bundle branch stimulation.