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Purpose: Cardiac resynchronization therapy (CRT) has evolved to an important therapy in adult heart failure patients with left ventricular failure. Recently CRT has been introduced in the paediatric population with promising results, however, no guidelines for patient selection for CRT are available for this population. Speckle tracking strain imaging could assist in defining guidelines for CRT in children by comprehensive assessment of the synchrony in LV mechanics using LV dyssynchrony parameters. The purpose of this study is to establish age-related reference values of time to peak systolic strain and left ventricular (LV) dyssynchrony parameters in children using speckle tracking strain imaging.Methods: Healthy patients (aged 0-18 years) with structurally normal hearts were prospectively included. In each subject images of the LV apical 4-chamber view and of the parasternal short-axis view at the level of the papillary muscle were recorded. Longitudinal, radial and circumferential speckle tracking strain analysis was performed and yielded values for time to peak systolic-strain (tS). Furthermore, LV longitudinal strain mechanical dyssynchrony parameters, including septal-lateral delay (S-L delay) and the longitudinal SDt-6, were calculated; SDt-6 being defined as the standard deviation of time to peak-systolic strain of the six segments in one strain analysis. In addition, radial and circumferential strain dyssynchrony parameters were calculated, including anteroseptal-posterior delay (AS-P delay) and the SDt-6 of both strain types. The relation of these parameters with age was assessed using regression analysis.Results: A total of 108 healthy individuals (63 male) were included. Global tS increased with age in all imaging planes (R>0.58; p<0.001). The median (interquartile range) longitudinal S-L delay was 22.5 (13.0-38.5)ms and the longitudinal SDt-6 was 30.4 (23.3-35.6). The median radial AS-P delay was 23.5 (13.0-52.0)ms and the radial SDt-6 was 15.4 (7.9-26.2), while the median circumferential AS-P delay was 34.0 (14.0-65.0)ms and the circumferential SDt-6 was 27.0 (16.5-37.7). No significant linear relation between dyssynchrony parameters and age was found.Conclusions: A significant positive linear relation was shown between tS and age. In contrast, LV mechanical dyssynchrony parameters, defined using speckle tracking strain imaging, were not age-related during childhood. This implies that in paediatric patients with heart failure, irrespective of age, this set of normal values of dyssynchrony parameters can possibly assist in defining guidelines for CRT.