4. Discussion
The paced morphology of the typical RBBB is considered as one of the necessary parameters during the LBBP implantation8-10. It is important to determine the LBBP lead depth into ventricular septum. As the advancement of the tip of the lead in the interventricular septum, the notch on the paced QRS in lead V1 will change from the LBBB morphology to the RBBB morphology4. We previously thought the pQRSd should not be shorter than the iQRSd in the patient with RBBB, as excitation is conducted forward through the LBB and Purkinje fibers to activate the LV first, and then to activate the RV through the intercalated discs of myocardium. In other words, paced morphology of the typical RBBB is formed because the LV is activated earlier than the RV (Figure 3 A). To our surprise, both selective and non-selective LBBP performed on patients with the RBBB caused a narrower QRS complex. The mechanisms by which this occurs have never been reported. In the following paragraphs, we discuss several possible mechanisms.