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.