Introduction: Permanent deep septal stimulation with capture of the left bundle branch (LBB) enables maintenance/restoration of the physiological activation of the left ventricle. However, it is almost always accompanied by the simultaneous engagement of the local septal myocardium, resulting in a fused (nonselective) QRS complex, therefore, confirmation of LBB capture remains difficult. Methods: We hypothesized that programmed extrastimulus technique can differentiate nonselective LBB capture from myocardial-only capture as the effective refractory period (ERP) of the myocardium is different from the ERP of the LBB. Consecutive patients undergoing pacemaker implantation underwent programmed stimulation delivered from the lead implanted in a deep septal position. Responses to programmed stimulation were categorized on the basis of sudden change in the QRS morphology of the extrastimuli, observed when ERP of LBB or myocardium was encroached upon, as: "myocardial," "selective LBB," or nondiagnostic (unequivocal change of QRS morphology). Results: Programmed deep septal stimulation was performed 269 times in 143 patients; in every patient with the use of a basic drive train of 600 milliseconds and in 126 patients also during intrinsic rhythm. The average septal-myocardial refractory period was shorter than the LBB refractory period: 263.0 ± 34.4 vs 318.0 ± 37.4 milliseconds. Responses diagnostic for LBB capture ("myocardial" or "selective LBB") were observed in 114 (79.7%) of patients.Conclusions: A novel maneuver for the confirmation of LBB capture during deep septal stimulation was developed and found to enable definitive diagnosis by visualization of both components of the paced QRS complex: selective paced LBB QRS and myocardial-only paced QRS. K E Y W O R D S effective refractory period, electrocardiogram, left bundle branch pacing, nonselective capture, refractoriness

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Introduction: One of the challenges of left bundle branch (LBB) pacing is to place the pacing lead deep enough in the septum to obtain capture of the LBB, yet not too deep to avoid perforation. We hypothesized that the occurrence of the ectopic beats of qR/rsR morphology in V1 lead (fixation beats) during the lead fixation would predict that the final desired intraseptal lead depth was just reached, while the lack of fixation beats would indicate too shallow position, and need for more lead rotations. Methods: Consecutive patients during LBB pacing device implantation were analyzed retrospectively and then prospectively with respect to the occurrence of the fixation beats during each episode of lead rotation. We compared the presence of fixation beats during the lead rotation event directly before the LBB capture area depth was reached versus during the events before intermediate/unsuccessful positions. Results: A total of 339 patients and 1278 lead rotation events were analyzed. In the retrospective phase, the fixation beats were observed in 327/339 of final lead positions and in 9/939 of intermediate lead positions (p<0.001). The sensitivity, specificity, positive and negative predictive value of the LBB area fixation beats as a marker for reaching the LBB capture area was 96.4%, 99.0%, 97.3% and 99.0%, respectively. In the prospective, fixation-beats-guided, implantation phase the fixation beats were observed in all patients and only at the LBB capture depth. Conclusions: Monitoring fixation beats during deep septal lead deployment can facilitate the procedure and possibly increase the safety of lead implantation.