Case report of hidden (yet visible) systolic fascicular potentials in a patient with left bundle branch block during conduction system pacing implantation

Background: Left bundle branch pacing (LBBP) is being increasingly adopted as a more physiological alternative to conventional pacing. At implantation, appropriate lead depth can be guided by monitoring a number of parameters, including the presence of a fascicular potential during conducted rhythm. However, in patients with left bundle branch block (LBBB), a presystolic fascicular potential is most often absent, due to upstream conduction block. We present for the first time a case where delayed fascicular potentials were visible within the ventricular electrogram (EGM) of the pacing lead. Case summary: An 88-year-old patient with decompensated heart failure of ischaemic origin with a left ventricular ejection fraction of 0.30 and LBBB with a QRS of 180 ms was scheduled for LBBP pacemaker implantation. A pacing lead was screwed deep into the interventricular septum in the left bundle branch area. Upon close inspection, fascicular potentials within the ventricular EGM were visible during cycles with LBBB. A presystolic fascicular potential was visible during cycles with a narrow QRS following blocked atrial premature beats with a pause. Threshold testing revealed transition from non-selective to selective LBBP, confirming conduction system capture. Discussion: The present case illustrates that fascicular potentials, which are most probably retrograde (or alternatively anterograde with very slow conduction), may be visualized within the ventricular EGM during LBBP implantation in patients with LBBB. Implanting physicians should scrutinize the EGM for these potentials, as they provide valuable information that adequate lead depth has been reached. The prevalence of these findings needs to be further evaluated.