Objective:To assess left ventricular(LV)systolic sequence and synchrony by speckle tracking imaging during right ventricular septum pacing and right ventricular apex pacing. Methods:Myocardial imaging of LV apical views and parasternal LV short axis(SAX)views was performed in 9 patients with right ventricular septum pacing(DDDspt group),15 patients with right ventricular apex pacing(DDDapx group)and 13 healthy subjects(control group).The time to peak systolic longitudinal strain(TsL)in the LV apical views,the time to peak systolic radial strain(TsR)and systolic radial strain rate(TsrR)in the LV SAX views were measured. The standard deviation of TsL,TsR and TsrR(TsL-SD,TsR-SD and TsrR-SD)and the maximal temporal difference of TsL,TsR and TsrR(TsL-d,TsR-d and TsrR-d)of 18 segments were used as an indicator of LV systolic dysynchrony,to evaluate the contraction sequence of the LV wall by analyzing TsL. Results:Compared with control group and the DDDspt group,TsL-SD,TsR-SD,TsrR-SD and TsL-d,TsR-d,TsrR-d were significantly higher in DDDapx group (P < 0.05). No significant difference was found between DDDspt group and control group. Left ventricular contraction sequence was similar in DDDspt group and control group,but it changed during right ventricular apical pacing. The contraction of the whole LV delayed in DDDapx group compared with DDDspt group and control group,most segments showed significant differences(P < 0.05). Conclusion:DDDapx group showed significant LV longitudinal,radial systolic dyssynchrony compared with DDDspt group,by inducing different electromechanical activation sequence. Speckle tracking imaging is a useful modality to evaluate systolic sequence and synchrony of the LV during pacing at variant right ventricular sites.