Alvaro Marco del Castillo -

Introduction: Left bundle branch area pacing (LBBAP) includes different situations depending on the captured structure. Distinguishing among them may be difficult during daily practice as most criteria are based on intracavitary recordings or precise high-sensibility measurements. The present study aims to determinate new device-based criteria to easily establish left bundle branch and/or deep septal myocardium capture. Methods and Results: the timing of the far-field R-wave electrogram registered at the atrial channel after the onset of the ventricular pacing spike was registered and compared among the modalities of left bundle branch area pacing in consecutive patients receiving dual chamber pacemakers. 46 patients were included. Successful left bundle branch capture was achieved in 67% of the sample, with transition from non-selective to selective left bundle branch pacing (ns-LBBP to s-LBBP) being the most frequent pattern during ventricular threshold test (52%). In those patients, the right atrial appendage far-field R-wave time (RAA-FFT) abruptly prolonged by a mean of 21 ms when transitioning from ns-LBBP to s-LBBP. However, in those whose last captured structure during ventricular threshold test was the deep septal myocardium (uniquely left septal capture or ns-LBBP to left septal transition), no sudden prolongation of the RAA-FFT was observed, remaining unchanged with the shortest values (94 ms vs 115 ms; P <0.01). Conclusion: in patients receiving dual-chamber pacemaker with the ventricular lead in a LBBAP position, the presence of a RAA-FFT jump >10ms is 100% specific for s-LBBP, whereas a RAA-FFT <105ms indicates deep septal myocardium capture (alone or together with left bundle branch stimulation) with a 91% of probability.