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Background: Optimal timing of the atrioventricular delay in cardiac resynchronization therapy (CRT) can improve synchrony in patients suffering from heart failure. The purpose of this study was to evaluate the impact of SyncAV™ on electrical synchrony as measured by vectorcardiography (VCG) derived QRS metrics during bi-ventricular (BiV) pacing., Methods: Patients implanted with a cardiac resynchronization therapy (CRT) device and quadripolar left ventricular (LV) lead underwent 12‑lead ECG recordings. VCG metrics, including QRS duration (QRSd) and area, were derived from the ECG by a blinded observer during: intrinsic conduction, BiV with nominal atrioventricular delays (BiV Nominal), and BiV with SyncAV programmed to the optimal offset achieving maximal synchronization (BiV + SyncAV Opt)., Results: One hundred patients (71% male, 40% ischemic, 65% LBBB, 32 ± 9% ejection fraction) completed VCG assessment. QRSd during intrinsic conduction (166 ± 25 ms) was narrowed successively by BiV Nominal (137 ± 23 ms, p < .05 vs. intrinsic) and BiV + SyncAV Opt (122 ± 22 ms, p < .05 vs. BiV Nominal). Likewise, 3D QRS area during intrinsic conduction (90 ± 42 mV ∗ ms) was reduced by BiV Nominal (65 ± 39 mV ∗ ms, p < .05 vs. intrinsic) and further by BiV + SyncAV Opt (53 ± 30 mV ∗ ms, p = .06 vs. BiV Nominal)., Conclusion: With VCG-based, patient-specific optimization of the programmable offset, SyncAV reduced electrical dyssynchrony beyond conventional CRT., Competing Interests: Declaration of competing interest JM, NB and LM are employees of Abbott. FP, CP, BT and LC received research grants and/or are consultants with Abbott. NV reports consulting fees/honoraria from Abbott, Boston Scientific, Biotronik, and Medtronic. This study was funded by Abbott., (Crown Copyright © 2019. Published by Elsevier Inc. All rights reserved.)