Paced electrical alternans with syncope: Time for emergent pericardiocentesis?

Left bundle branch area pacing (LBBAP) is now a common implantation strategy for both pacing and resynchronization based device therapies. While LBBAP has been (somewhat paradoxically) demonstrated to be effective in cases of preexisting left bundle branch (LBB) block, and criteria for LBB capture have been established; minimal focus has been towards the intrinsic refractory rate of the LBB. Herein, we demonstrate a case of LBBAP rate exceeding the refractory period of the LBB, resulting in alternating LBBAP capture and non-selective septal pacing. Implicit understanding of this physiology has important sequel for accurate assessment of lead placement during implantation.