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Background Cardiac resynchronization therapy (CRT) is an effective electrical therapy for patients with moderate to severe heart failure and cardiac dyssynchrony. This study aimed to investigate the degree of acute left ventricular (LV)resynchronization with biventricular pacing (BVP) at different LV sites and to examine the feasibility of performing transthoracic echocardiography (TTE) to quantify acute LV resynchronization during CRT procedure.Methods Fourteen patients with NYHA Class ⅢⅣ heart failure, LV ejection fraction ≤35%, QRS duration ≥120 ms and septal-lateral delay (SLD) ≥60 ms on tissue Doppler imaging (TDI), underwent CRT implant. TDI was obtained from three apical views during BVP at each accessible LV site and SLD during BVP was derived. Synchronicity gain index (Sg) by SLD was defined as (1+(SLD at baseline - SLD at BVP)/SLD at baseline).Results Seventy-two sites were studied. Positive resynchronization (R+, Sg>1) was found in 42 (58%) sites. R+ was more likely in posterior or lateral than anterior LV sites (66% vs. 36%, P <0.001). Concordance of empirical LV lead implantation sites and sites with R+ was 50% (7/14).Conclusions The degree of acute LV resynchronization by BVP depends on LV lead location and empirical implantation of LV lead results in only 50% concordance with R+. Performing TTE during CRT implantation is feasible to identify LV sites with positive resynchronization.