In the Search of Noninvasive Methods Delineating Left Bundle Branch Block Amendable With Conduction System Pacing

Conduction system pacing (CSP) is an emerging new method of cardiac resynchronization therapy (CRT), however, one third of patients with left bundle branch block (LBBB) have distal conduction disease, which is not amenable to correction with CSP. There is an emerging need for tailored analysis of ventricular depolarization patterns for patient selection for CRT pacing modality. We retrospectively analyzed 12 lead hrECGs, equivalent dipole (ED) trajectories and standard transthoracic echocardiograms of 18 heart failure patients fulfilling Strauss criteria for LBBB and indication for CRT randomized to the CSP arm of the ongoing CSP-Sync study (NCT05155865). Based on achievement of left bundle branch capture with shortening of left ventricular activation time, 12 patients had proximal LBBB (pLBBB group), and 6 had intact proximal LBBB conduction (dLBBB group) with similar average baseline QRS durations between the groups (179±14 ms in the pLBBB and 165±20 ms in the dLBBB group, p = 0.1). All patients fulfilled the Strauss criteria with no significant difference in the additional criterion (R wave > 0.1 mV in V1; p = 0.7). In the pLBBB group ED trajectory had an initial leftward direction (six vs. zero patients, p = 0.03) with a uniform (12 vs. one patient, p < 0.001) and slower (0.57 ± 0.12 m/s in the pLBBB vs. 0.75 ± 0.15 m/s in the dLBBB group, p = 0.01) velocity. After 6 months the pLBBB group achieved greater relative QRS duration shortening (26% ± 8% vs. 14% ± 9%; p < 0.02) and relative reductions in end left ventricle systolic volumes (41.3% ± 17.6% vs. 15.8% ± 6.1%; p = 0.004) with better improvement in ejection fraction (17.1% ± 11.0% vs. 5.5% ± 1.0%; p = 0.02). The ED trajectories from 12-lead hrECGs could better differentiate patients with proximal or distal LBBB than standard 12-lead ECG alone.