Transseptal activation delta: A novel electrocardiographic criterion for left bundle branch capture

Distinguishing left bundle branch capture from septal myocardial pacing during left bundle branch pacing implantation remains challenging. Several diagnostic criteria have been proposed, including right bundle branch block morphology in lead V1 and timing measures such as V6 R-wave peak time or the V6-V1 interpeak interval, but these parameters may depend on baseline conduction characteristics or pacing maneuvers not always available. We hypothesized that the difference between V6 R-wave peak time during monopolar pacing from the proximal (ring) and distal (tip) electrodes, termed transseptal activation delta (TAD), would be greater when the distal electrode captures the left bundle branch than during septal myocardial pacing.

This study aimed to evaluate the diagnostic accuracy of TAD in confirming the left bundle branch capture.

In this prospective, observational, single-center study, 122 consecutive patients who underwent left bundle branch pacing were included. In 20 patients, measurements were obtained during septal pacing and after confirmation of left bundle branch capture, yielding 142 pacing observations. Left bundle branch capture was defined by electrocardiographic criteria and dynamic pacing maneuvers. A separate prospective validation cohort of 30 additional patients was analyzed.

A TAD threshold of ≥25 ms identified left bundle branch capture with 78.9% sensitivity, 95.7% specificity, and 84.5% overall diagnostic accuracy. In the prospective validation cohort, this threshold retained diagnostic performance.

TAD is a practical, reproducible intraoperative criterion for identifying left bundle branch capture. A 25-ms threshold supports the role of TAD as a confirmatory marker of conduction system capture with high specificity.