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Aims - Baseline mechanical intraventricular dyssynchrony showed only a weak correlation with response to CRT in HF patients with wide QRS. We aimed to evaluate the effects of RV pacing on baseline intraventricular dyssynchrony in patients submitted to CRT. Methods - In 40 consecutive HF patients (LBBB, sinus rhythm, normal PR interval, 22 ischemic etiology, 65.5±10.7 years, 21 women, NYHA class 3.3 ± 0.5, LV ejection fraction 20.1±4.1%), speckle tracking radial strain was performed during sinus rhythm (ODO mode) and during RV pacing (DDD with optimum AV interval) one week after biventricular device implantation. RV lead was placed on interventricular septum (RVS, n=30) and RV apex (RVA, n=10). Patients had significant baseline intraventricular dyssynchrony, (i.e. =130 ms time difference in peak septal wall to infero-lateral wall strain). Maximum LV delay area (MDA) was defined as the segment with the latest systolic peak from the 6 regional color-coded time-strain curves. Midventricular global radial strain (mGRS) was determined averaging the segmental radial strain values. Results - Overall, RV pacing did not significantly increased intraventricular dyssynchrony (350±98 ms vs. 322±90 ms during SR, p=0.08). However, RVA pacing significantly increased LV dyssynchrony (367±58 ms vs. 312±60 ms during SR, p<0.001). mGRS was significantly reduced during RV pacing (13.3±8.5% vs. 18.3±7.4% during SR, p<0.001). The location of MDA shifted during RV pacing in 31 out of 40 patients (77%). Conclusions - In HF patients with wide QRS submitted to CRT, RV pacing alters the pattern of intraventricular dyssynchrony and impairs LV strain. [ABSTRACT FROM AUTHOR]