An excited fusion of the LV and local RV septum
SLBBP and NSLBBP are two subgroups of LBBP, the fundamental difference between them is the region captured. During low output pacing at near‐threshold, SLBBP capturing only the LBB is characterized by an isoelectric interval between the pacing stimulus and the onset of surface QRS complex. With increasing pacing output, SLBBP is converted to NSLBBP with a narrower QRS complex and no isoelectric interval, which means both LBB and local adjacent septal myocardium are captured4,11. Chen et al. described the changes of paced morphology of QRS obtained by NSLBBP under different pacing outputs, showing a narrow paced QRS complex (QR with narrow R wave in lead V1, and narrow S wave in lead I, V5 and V6) 12. We hypothesize that under NSLBBP, myocardial excitation no longer follows the sequence of excitation of the LV first and then the RV, but follows the excitation of the LV along with the local RV septum. In this condition, excitation would be transmitted from the LBB area to the local RV septum through the intercalated discs of septal myocardium, leading to a fusion of the excitation of the LV, LBB area and local RV septum, which eventually forms a narrow paced QRS complex (Figure 3 B).