Background: Conventional right ventricular (RV) apical pacing may have detrimental effects on cardiac structure and left ventricular (LV) function, which are associated with the development of heart failure. It remains uncertain if the deterioration of LV function as noted in a proportion of patients receiving RV apical pacing is directly related to chronic induced LV dyssynchrony. Methods: 37 patients with single or dual chamber pacemaker (PM) at least 12 months, previously diagnosed with sick sinus syndrome with intact atrioventricular conduction, were enrolled. Atrial lead was positioned in the right atrial appendage and ventricular lead in the RV apex (RVA). Transthoracic two-dimensional echocardiography, tissue synchronization imaging (TSI) and strain echocardiography were performed to determine the chamber size, standard deviation of the time to peak systolic velocity in ejection period (SD-Ts) among the 12 LV segments, myocardial performance index, and global LV strain. All the parameters were compared with 30 healthy controls. Results: Ejection fraction and mean global LV strain were significantly lower in patients with RVA pacing. Myocardial performance index and LV filling pressure represented by E/E��� were significantly increased in patients with RVA pacing. In addition, there was significant difference in the intraventricular delays of SD-Ts (dyssynchrony index) (Table) Conclusion: LV mechanical dyssynchrony was significantly induced by long-term RVA pacing with definite deterioration of LV function represented by ejection fraction and myocardial performance index. Moreover, significant impairment of LV longitudinal deformation may suggest global LV dysfunction, which would be predisposing factor in development of heart failure.