Aims: Although electrocardiographic and echocardiographic findings with right ventricular (RV) pacing mimic intrinsic left bundle branch block (LBBB), left ventricular (LV) mechanical activation pattern may differ. We compared mechanical activation pattern of the LV in RV-pacing-induced LBBB with intrinsic LBBB in symptomatic chronic heart failure patients.
Methods and results: We studied 37 patients with chronic RV-pacing and 37 patients with intrinsic LBBB who were referred for cardiac resynchronization therapy. Echocardiographic study including 2D speckle tracking longitudinal strain and M-mode were performed at baseline. Patients with intrinsic LBBB were younger, had higher prevalence of ischaemic heart disease, and had more severe depressed LV function. The basal-septal segments were the earliest activated sites in 11% of patients in RV-pacing-induced LBBB compared with 30% in intrinsic LBBB (P= 0.04). The mid- and basal-lateral segments were the latest activated sites in 57% of patients in RV-pacing-induced LBBB compared with 30% in intrinsic LBBB (P = 0.03). Left ventricular dyssynchrony, using longitudinal strain, time delay ≥ 130 ms between either mid-septal or apico-septal and either basal or mid-lateral segments was present in 71% of patients with RV-pacing-induced LBBB compared with 59% in intrinsic LBBB (P = 0.03). Using M-mode, LV dyssynchrony was present in 11% of patients with RV-pacing-induced LBBB compared with 59% in intrinsic LBBB (P = 0.02).
Conclusion: Right ventricular-pacing results in less early basal activation and more often early mid-septal and late lateral wall activation in comparison with intrinsic LBBB. Imaging techniques that only visualize the basal- or mid-part of the LV may result in a serious underestimation of dyssynchrony in patients with pacing-induced LBBB.