Delayed left ventricular septal activation relative to apical right ventricular contraction: a novel echocardiographic indicator of absent septal conduction in left bundle branch block

Background Left bundle branch (LBB) area pacing has recently emerged as a promising alternative to conventional cardiac resynchronization therapy (CRT) in patients with major left bundle branch block (LBBB) and left ventricular (LV) systolic dysfunction. The LBB forms a subendocardial trifascicular network composed of septal, anterior, and posterior fascicles. In clinical practice, the presence of major LBBB on surface ECG is often interpreted as complete interruption of conduction through all three fascicles. However, growing evidence indicates that conduction through the septal fascicle frequently remains persists. Purpose To evaluate the integrity of residual LBB conduction in patients with major LBBB using echocardiography and to investigate the diagnostic utility of a novel echocardiographic parameter—the mechanical delay between apical right ventricular (RV) and LV septal contraction—as a potential marker for identifying true complete LBBB and improving selection for CRT or conduction system pacing. Methods In this observational study, echocardiographic data from 60 patients with major LBBB undergoing conventional CRT or conduction system pacing were retrospectively analyzed. Standard echocardiographic parameters included LV dimensions, systolic function and established dyssynchrony indices. The interval between the onset of RV apical free wall contraction and LV septal contraction was measured in the apical four-chamber view using Tissue Doppler Imaging (using EchoPAC software), allowing high temporal resolution analysis of regional myocardial velocities, facilitating objective quantification of interventricular mechanical delay as an indirect marker of septal conduction integrity. Results In 42 patients (70%), contraction of the LV septal wall and RV apex occurred simultaneously, consistent with preserved conduction through the septal fascicle of the LBB. In 18 patients (30%), a distinct delay (average time delay of 32.8 ms) in LV septal activation relative to RV apical contraction was identified, suggesting loss of septal fascicular conduction and the presence of a complete LBBB pattern. Considering a realistic null hypothesis of near-complete LBBB (expected ≤5% preserved septal conduction), exact binomial testing (p < 10−40) confirmed that most patients labeled as having major LBBB on surface ECG do not exhibit complete interruption of septal conduction. Conclusion Preservation of septal fascicular conduction is frequent among patients diagnosed with major LBBB. The detection of a delay between RV apical and LV septal contraction on echocardiography may represent a practical, noninvasive marker of absent septal conduction, thus identifying patients with true complete LBBB. This novel parameter holds potential clinical value for refining the diagnosis of complete LBBB and optimizing patient selection for CRT and conduction system pacing strategies.