Kinetics and Disappearance of QRS Transition in Patients Undergoing Left Bundle Branch Pacing - A Novel Method for Classifying Microdislodgement.

Introduction: QRS transition during the threshold test is the gold standard for confirming direct capture of the Conduction System in patients with left bundle branch pacing (LBBP). Still, we have limited data on the kinetics of QRS transition over time. Microdislodgement is a known complication of left bundle branch area pacing (LBBAP); however, its true incidence depends on the thoroughness of follow-up. We aimed to evaluate the kinetics and disappearance of QRS transition in patients undergoing LBBP, assess the diagnostic yield of QRS transition at various time points, and characterize microdislodgement.

Methods: This prospective study included patients who underwent successful LBBP procedures at a tertiary center between January 2022 and February 2024. Based on the kinetics of QRS transition during intraoperative, postoperative, and follow-up threshold tests, microdislodgement was assessed.

Results: LBB capture was confirmed in 118 of 155 LBBAP patients (76.1%), which defined our LBBP population. Intraoperative QRS transition was observed in 86.4%, which decreased significantly postoperatively (47.0%) and at follow-up (33.0%)-in 92.0% of LBBP patients' capture of LBB remained, while microdislodgement occurred only in 5.0% of LBBP cases.

Conclusion: This study evaluates QRS transition kinetics in LBBAP patients, showing significant intraoperative detectability in patients with direct capture of LBB that decreases postoperatively and at first follow-up. Our classification of microdislodgement aids in understanding its impact on pacing outcomes.