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Authors' Contributions 1.JF -Substantial contributions to acquisition, analysis and interpretation of data, drafting the work, final approval of the version to be published and agreement to be accountable for all aspects of the work. 2.AO -Substantial contributions to the conception and design of the work, analysis and interpretation of data, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 3.TP -Substantial contributions to the conception and design of the work, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 4.SW -Substantial contributions to the conception and design of the work, analysis and interpretation of data, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 5.AL -Substantial contributions to the conception and design of the work, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 6.NL -Substantial contributions to the conception and design of the work, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 7.AB -Substantial contributions to the conception and design of the work, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 8.RP -Substantial contributions to the conception and design of the work, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 9.LK -Substantial revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 10.SG -Substantial contributions to the conception and design of the work, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 11.IP -Substantial contributions to the conception and design of the work, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 12.IQ -Substantial contributions to revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable for all aspects of the work. 13.RJ -Substantial contributions to the conception and design of the work, revising it critically for important intellectual content, final approval of the version to be published and agreement to be accountable ...

Background - Left bundle branch pacing (LBBP) is a novel pacing method and has been observed to have low and stable pacing thresholds in prior small short-term studies. The objective of this study was to evaluate the feasibility and safety of LBBP in a large consecutive diverse group of patients with long-term follow up. Methods - This study prospectively enrolled 632 consecutive pacemaker patients with attempted LBBP from April 2017 to July 2019. Pacing parameters, complications, ECG, and echocardiographic measurements were assessed at implant, and during follow-up of 1, 6, 12 and 24 months. Results - LBBP was successful in 618/632 (97.8%) patients according to strict criteria for LBB capture. Mean follow-up time was 18.6±6.7 months. 231 patients had follow-up over 2 years. LBB capture threshold at implant was 0.65±0.27 mV@0.5ms and 0.69±0.24 mV@0.5ms at 2-year follow-up. A significant decrease in QRS duration was observed in patients with LBBB (167.22 ± 18.99ms vs. 124.02 ± 24.15ms, p<0.001). Post implantation left ventricular ejection fraction improved in patients with QRS≥120ms (48.82±17.78 % vs. 58.12±13.04 %, p<0.001). The number of patients with moderate and severe tricuspid regurgitation decreased at 1-year. Permanent right bundle branch injury occurred in 55 (8.9%) patients. LBB capture threshold increased to more than 3 V or loss of bundle capture in 6 patients (1%), 2 patients of them had loss of conduction system capture. Two patients required lead revision due to dislodgement. Conclusions - This large observational study suggests that LBBP is feasible with high success rates and low complication rates during long term follow up. Therefore, LBBP appears to be a reliable method for physiological pacing for patients with either a bradycardia or heart failure pacing indication.