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Introduction In cardiac resynchronization therapy, pacing the left ventricle (LV) at sites of prolonged electrical delay is associated with better outcomes. We sought to characterize the interrelationships between intrinsic, right‐ventricular (RV)‐paced, and LV‐paced interventricular delays. Methods and Results The following electrical timings were measured at implantation for all electrodes of the LV quadripolar leads: QLV, interventricular delay in intrinsic rhythm (RVs‐LVs), in RV‐paced rhythm (RVp‐LVs), and in LV‐paced rhythm (LVp‐RVs). We included 32 patients (78% men, age 72 years, LV ejection fraction 29%, left bundle branch block 84%). QLV and RVs‐LVs were correlated (R2 = .72, p < .0001), as were RVs‐LVs and RVp‐LVs (R2 = .27, p = .002) and RVp‐LVs and LVp‐RVs (R2 = .60, p < .001). Direction of activation along the four LV lead electrodes was concordant between RVs‐LVs and RVp‐LVs in only 17 (53%) patients. The latest‐activated electrodes in RVs‐LVs and RVp‐LVs were concordant in 26 (81%) patients, adjacent in 3 (9%) patients, and remote in 3 (9%) patients. Biventricular‐paced QRS duration varied by more than 10 ms between the two electrodes in half of the patients with dissimilar latest electrodes. Among the seven echocardiographic nonresponders at 6 months, the programmed electrode was remote from the latest electrode in RVs‐LVs in five patients and in RVp‐LVs in three patients. Conclusion Intrinsic and RV‐paced interventricular electrical delays are correlated, but there is substantial heterogeneity between patients. The latest‐activated electrode may be different between RVs‐LVs and RVp‐LVs, and this might have important implications in selecting the optimal LV vector.

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