Impact of Cardiac Dimensions and Patient Phenotype on Electrocardiographic Criteria for Left Bundle Branch Capture

Electrocardiographic (ECG) criteria are commonly used to confirm left bundle branch (LBB) capture during conduction system pacing, but the influence of structural determinants on their performance remains uncertain. We aimed to assess the relationship of cardiac dimensions and patient phenotype with paced ECG parameters during confirmed LBB capture, and to determine whether ventricular size modifies the diagnostic performance of ECG-based criteria. Single-center prospective cohort including consecutive patients with LBB capture confirmed by differential capture. Associations between paced ECG parameters and clinical, phenotypic, and echocardiographic variables were tested using linear regression analysis. The effect of left ventricular end-diastolic diameter (LVEDD) on V6-RWPT diagnostic performance was evaluated across LVEDD strata, by generalized estimating equations, and by receiver-operating characteristic analysis. A total of 307 patients with unequivocal LBB capture were included. LVEDD was independently associated with longer paced V6-RWPT, V1-RWPT, and paced QRS duration, but not with the V6-V1 interpeak interval. In contrast, patient phenotype was not independently associated with paced ECG parameters. Each 10-mm increase in LVEDD was associated with a 2.13-ms increase in paced V6-RWPT. The sensitivity of established V6-RWPT cutoffs decreased progressively with increasing LVEDD, and LVEDD significantly modified the relationship between V6-RWPT and LBB capture (interaction B = -0.003; P = 0.022). Receiver-operating characteristic analyses also showed lower discrimination in larger ventricles, particularly in diseased LBB. Left ventricular size is an important determinant of paced ECG characteristics during LBB capture and influences the diagnostic performance of V6-RWPT. These findings support interpreting ECG criteria for LBB capture according to ventricular size.