Current guidelines state that CRT implantation is less beneficial for patients with RBBB compared to those with LBBB (2). Current evidence suggests that frequent or persistent RV-only pacing in patients with narrow QRS and LV dysfunction can be harmful due to ventricular desynchronization attributable to RV apex pacing (3-5).
Studies suggest that LV-only pacing is not inferior to BIV pacing in sinus rhythm patients with LV dysfunction and LBBB and ICD back up (6,7). A novel algorithm of CRT by delivering synchronized LV pacing with the intrinsic conduction in patients with severe LV systolic dysfunction and LBBB (adaptive CRT) has been demonstrated to show that it is at least as effective as protocol-driven echocardiographic optimization. It also has shown a significant reduction in the probability of 30-day readmission for both HF and all-cause hospitalizations, 46% reduced incidence of AF compared to conventional CRT, and prolongation of CRT device battery life (8-10). Patients who had an adaptive CRT algorithm that provided \> 50% synchronized LV pacing or had normal AV conduction with the adaptive CRT algorithm had decreased risk of death or heart failure hospitalization when compared to those with \<50% synchronized LV pacing or echocardiography-optimized BIV pacing respectively (11). In another acute study the LV dP/dtmax was higher with LV than BIV pacing when LV pacing was associated with ventricular fusion caused by intrinsic activation (12).
Several studies have suggested that optimization of the programmed atrioventricular delay (AVD) and interventricular delay (VVD) delays may incrementally improve the long-term outcome of BIV pacing (13-15).
The lateral LV wall contracts early in patients with RBBB when compared to LBBB so LV pacing in the CRT devices is less likely to be beneficial. . In an experimental study of 12 dogs with tachycardia-induced cardiomyopathy and RBBB (6 dogs) or LBBB (6 dogs) (16), RV-only pacing enhanced LV function and synchrony as seen by dP/dtmax measured by catheter tip placed in the LV chamber and synchrony evaluation by cardiac MRI in the RBBB group as well or better than BIV pacing. LV-only pacing worsened function in the RBBB group.
RV-only pacing was also evaluated in a prospective study of 7 patients with RBBB and RV dysfunction with most patients having congenital heart disease. LV global function was intact at baseline (17). Sequential atrioventricular RV pacing with an atrioventricular delay of 90% of the PR interval was superior to atrial-only pacing for both improvement in RV dP/dtmax and LV cardiac index as seen at cardiac catheterization. The RV dP/dtmax increased by 22% in RBBB patients with RV pacing and QRS decreased from 163 +/- 39 to 126 +/- 31 ms. The two studies together suggest that most of the benefit from BIV pacing in patients with RBBB and HF is due to the RV pacing component and the benefit might be reduced due to the LV pacing component. Better timing of RV pacing in patients with RBBB might significantly decrease desynchronization from RV pacing and maximize the benefit of RV pacing as seen in adaptive LV-pacing CRT in patients with LBBB and LV systolic dysfunction.
These studies have led to the current hypothesis that adaptive RV pacing using RV-only pacing synchronized to LV activation when intrinsic AV conduction is normal is more physiological and will improve RV and LV function by Echocardiography parameters and on Medtronic ECG belt for activation in patients with RBBB and LV dysfunction because of improved synchronization and narrowing the QRS duration.
As a secondary goal, battery life with BIV pacing will be compared to adaptive RV pacing assuming at least 50% of RV-only pacing will be achieved with adaptive CRT for RBBB.
