LAUSR

A 46‐year‐old lady with nonischemic dilated cardiomyopathy, severe left ventricular (LV) dysfunction and underlying complete left bundle branch block with QRS duration of 165ms and HV interval of 70 ms (Figure 1), has undergone left bundle branch pacing (LBB) with 4.1‐ F diameter 3830 Select SecureTM pacing lead (Medtronic Inc.). As surrogates of LBB capture cannot be demonstrated in this case programmed deep septal stimulation is performed. Programmed stimulation is done in unipolar mode with output of 5 V at 0.4 ms when the final deep intraseptal lead position is reached. Premature beats are introduced after an 8‐beat basic drive train of 400 ms. The coupling interval is decreased by 10‐ms steps, starting from 350 ms. An interesting phenomenon is observed during the programmed deep septal stimulation (Figure 2A,B). What is the mechanism?