BACKGROUND: Electrical and mechanical dyssynchrony of the ventricles in patients who had permanent pacemaker have been commonly found in patients with high percentage of ventricular pacing and heart failure was frequently followed. We postulated that newly developed a notch at the downstroke of paced QRS during follow up can be a marker of mechanical LV dyssynchrony and subsequent left ventricular failure, irrespective of width of paced QRS. METHODS: A total of 15 patients with permanent pacemaker implantation and normal left ventricular systolic function were retrospectively evaluated. Twelve-lead electrocardiogram was used for identifying newly developed downstroke notch of paced QRS. Complete echocardiographic examination was done at pre-implantation and during follow-up for all patients. RESULTS: 15 patients {10 of patients with atrioventricular block and 5 of patient with SSS} 9 male (60%) and 6 female (40%)} with mean age of 64.0짹15.7 years were enrolled in this study and mean follow-up was 76.0짹40.2 months. Underlying heart disease included hypertension (47.7%), diabetes mellitus (27.7%), and coronary heart disease (13.3%). Newly developed downstroke notch of paced QRS was identified in 33.0% of the patients in any of two precordial or limb leads. 1. Decreased ejection fraction was more significantly noted in patients with newly developed downstroke notch than those without (-10.8짹8.6% vs -3.2짹7.0% p=0.006), but there was no significant difference of width of QRS (168.5짹19.5 vs 168.9짹18.5 p=0.57) and incidence of ventricular pacing between two groups. 2. LV systolic and diastolic dimension during follow-up were significant greater in patients with newly developed downstroke notch than those without (LV diastolic dimension; 52.8짹5.9 vs 49.4짹4.1mm,p=0.03 LV systolic dimension; 34.4짹3.2 vs 29.6짹1.0mm, p=0.005), but there was no significant difference in the maximal intrinsicoid deflection between two groups. 3. Neither maximal intrinsicoid deflection (ID) nor change in ID during follow up were different between two groups (Change of ID; 9.7짹13.7% vs -0.6짹10.6 p>0.05). CONCLUSIONS: There was a good correlation between newly developed downstroke notch of paced QRS and decreased EF with dilation of the LV in patients with pacemaker. Downstroke notch of paced QRS might be a marker of newly developed mechanical LV dyssynchrony or dilatation, independent on the width of paced QRS.