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

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.