Abstract Body (Do not enter title and authors here): Background: New onset left bundle branch block (LBBB) is the most common complication following transcatheter aortic valve replacement (TAVR), occurring in up to 35% of cases. Previous case studies have shown that in some patients with new onset LBBB, the QRS duration is positively correlated with the preceding RR interval. This unique phenomenon, known as inverse decremental conduction (IDC) or Yan conduction, prompted us to investigate its clinical features and implications.

Methods: We reviewed 12-lead ECGs and available in-patient telemetry for patients with new onset LBBB within 14 days following any types of cardiac surgery from 2018 to 2023 at two medical centers. Patients with ≥3 different degrees of LBBB in response to different preceding RR on 12-lead ECGs or on telemetry were included in the analysis.

Results: A total of 22 patients (19 males) with a mean age of 75±8 years were identified to exhibit varying degrees of LBBB, with QRS duration that was positively correlated to the preceding RR interval within a certain range of heart rates. Among these patients, 17 had undergone TAVR, while the remaining 5 underwent surgical aortic valve replacement (AVR) and/or mitral valve replacement. 13 of the 22 patients were in sinus rhythm with premature atrial beats or sinus arrhythmias, while the remaining 9 patients were in atrial fibrillation. Figure 1 shows the typical manifestations of IDC in the left bundle on ECG in a patient post TAVR, with a positive correlation between QRS duration and the preceding RR (r = 0.87, p < 0.01). IDC in the left bundle in patients with new-onset LBBB occurred within three days following cardiac surgeries in 20 of the 22 patients, while the remaining 2 patients exhibited IDC in the left bundle > 7 days after the surgeries. IDC in the left bundle is a temporary phenomenon, commonly lasting <24 hours in most cases, and evolving into either a normal QRS or permanent LBBB. Interestingly, alternating LBBB occurred in 5 patients with IDC in the left bundle as shown in Figure 2.

Conclusion and Comment: 1) there is a positive correlation between the preceding RR interval and QRS duration, indicating IDC in left bundle, in the patients with new onset of LBBB following cardiac surgeries particularly AVR; 2) this unique phenomenon is often short-lived; 3) theoretically, IDC in left bundle can translate into PR interval changes consistent with IDC in patients with preexisting right bundle branch block, leading to paroxysmal AV block.