The right bundle branch originates from the His bundle and forms three branches at the base of the tricuspid anterior papillary muscle. It runs on the low position of the ventricular septum and the anterior wall of the right ventricle, the free wall of the right ventricle and the posterior papillary muscle, and the lower right posterior part of the ventricular septum. The electrical excitement is quickly transmitted to each segment of the right ventricular wall through the three branches, ensuring the interventricular and intraventricular synchronous contraction. However, because the right bundle branch is slender, superficial, mostly supplied by a single branch of the left anterior descending branch, the right bundle-branch conduction system is prone to conduction disorders. Therefore, RBBB is common in clinical practice, with an incidence of 8%, which increases with age[6, 7]. Herein, the incidence of RBBB in patients undergoing pacemaker implantation is not supposed to be low. The optimal pacing method for bradyarrhythmia with RBBB remains to be explored. His-Bundle pacing, cardiac resynchronization therapy, and right bundle-branch pacing can correct RBBB morphology, but it has the disadvantages of a complicated operation, an unstable pacing threshold, and a low success rate[8-10]. At present, LBBAP is a pacing method with a high success rate, a stable pacing threshold, and physiological pacing for bradyarrhythmia. A case report showed that LBBAP could eliminate the RBBB morphology in patient with RBBB. However, there is currently a lack of echocardiographic techniques to study the ventricular function and synchronization of RBBB patients after LBBAP.
In this study, we performed LBBAP on patients with and without RBBB who had no statistical difference in general conditions. After optimization of the atrioventricular interval, we used the IVMD, TMAD, and TDI to evaluate the synchronization of left and right ventricular myocardial movements. Through these three methods, we found the following: 1) The movement of the right ventricular myocardium in the RBBB group after LBBAP treatment was slightly later than that of the left ventricular myocardium; 2) TMAD and TDI techniques are easier to find a nuance of biventricular desynchrony than IVMD. In this study, we used unipolar LBBAP pacing. There are two possibilities for pacing signals to be transmitted to the right ventricle: 1) The right ventricular myocardium in the region of the left bundle branch is stimulated, and the right ventricle is stimulated through intercellular conduction. This conduction method may cause asynchrony between the right and left chambers (Figure. 4A); 2) In previous studies, the existence of interconnection fibers (TFs) between the left and right bundle branches was also proposed. The pacing signal of the left ventricle may be transmitted to the right bundle branch through TFs, which in turn stimulates the right ventricle physiologically (Figure 4B). However, in the presence of RBBB, the pacing signal of the left ventricle may not be able to transmit to the right ventricle through TFs to achieve physiological pacing of the right ventricle (Figure. 4C), so the synchronization of the two ventricles will be inconsistent. This argument was verified by our study. By the application of TMAD and TDI, we detected a slight difference between RBBB group and non-RBBB group. The maximum systolic displacement time and peak time of systolic velocity of TV side wall is slow than that of MV in RBBB group in the same cardiac cycle. The reasons why IVMD did not find the discrepancy are probably as follows: 1) PPEI and APEI come from different cardiac cycles, which is an important cause of error. 2) IVMD evaluates biventricular synchronization by hemodynamic method. While TMAD and TDI directly evaluate the mechanical myocardial synchronization of the ventricles. We supposed that the desynchrony of biventricular myocardial movement happen before the biventricular hemodynamics. Through this study, we found that patients with RBBB still had asynchrony of left and right ventricular contractions after LBBAP.
According to the study by Mou Junyu et al, SDt-R of complete RBBB is different from that of the control group, with 49.89±4.79ms VS 8.90±1.67 respectively.Compared with this study, our data did not find a distinct difference in right ventricular asynchrony in two groups. Nor did we find difference in two groups in the aspect of right ventricular function, namely TAPSE, TV-s’, RVFAC and LS-RV, as well as in the aspect of left ventricular function and intraventricular asynchrony, namely LVEF, GLS and SDt-L. It suggests that at least RBBB did not exacerbate the asynchrony and dysfunction of right ventricle in LBBAP patients. This may be because of the activation of right ventricular myocardial cells in the adjacent area during LBBAP pacing, which accelerates the depolarization process of the right ventricle and thereby may help to ameliorate right ventricle desynchrony in RBBB patients. A lot of researches have testified that LBBAP could ensure the physiological pacing of the left ventricle. The morphology and function of the left and right ventricles affect each other. Insufficiency or asynchronization of the left heart can cause changes in the pulmonary artery pressure due to the increase in left atrial pressure, leading to an increase in the afterload of the right heart; Insufficiency or asynchrony of the right heart can cause the right heart to enlarge. In the limited volume of the pericardium, the enlarged right heart causes the diastolic restriction of the two ventricles. Studies have shown that right ventricular dysfunction or asynchrony of exercise is an early warning indicator of poor prognosis for heart failure and non-response of CRT[13, 14].Herein, in the selection of pacing strategy, the function and synchronization of the two ventricular should all be taken into account. In the context of that RBBB do not bring more damage to right ventricular synchronization and function to RBBB patient receiving LBBAP than those without RBBB, LBBAP could realize the left ventricular physiological pacing of RBBB patients. From this early assessment, we deduce that LBBAP could be safely applied in the patients with RBBB.
This study also had certain limitations. First, the small sample size may have caused some errors in statistics. Second, because the right ventricle has an irregular crescent shape, we only studied the strain of the right ventricle in the apical four chambers; Finally, the follow-up time was short, so this study could only reflect the short-term impact of LBBAP on right ventricular function and synchronization. With a prolonged implantation time, whether the mechanical and electrical remodeling of the heart will have a long-term improvement effect on RBBB remains unknown. Whether it can reduce atrial fibrillation and cardiovascular adverse events caused by RBBB [7, 15] also remains to be further studied.