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Electromechanical dyssynchrony occurs ubiquitously following tetralogy of Fallot (TOF) repair, manifesting electrically as a wide QRS duration and mechanically as a right-sided septal/apical flash. Early septal activation and prestretch of the right ventricular (RV) basal lateral wall followed by its postsystolic shortening contributes to inefficient RV mechanics. However, a right-sided septal flash is a dichotomous finding, and the severity of RV dyssynchrony as a continuous spectrum in relationship to RV dysfunction and clinical outcomes in patients with repaired TOF has not been studied. The aim of this study was to quantify the severity of electromechanical dyssynchrony in relation to RV remodeling and clinical outcomes in a pediatric cohort following TOF repair.A retrospective analysis was performed in 81 children with RV volume loading after TOF repair, aged 13.6 ± 2.9 years, and compared with 50 matched control subjects.Patients had higher RV basal-lateral prestretch and postsystolic strain amplitude and duration, RV mechanical dispersion, and basal lateral-septal wall delay compared with control subjects (P < .001 for all). All intra-RV dyssynchrony timing parameters were associated with reduced cardiac magnetic resonance–derived RV ejection fraction and/or echocardiography–derived RV longitudinal strain. Prestretch duration as a percentage of total shortening time and RV basal lateral–to–midseptal delay were independently associated with RV dysfunction. Postsystolic strain amplitude was higher in patients with ventricular arrhythmias compared with arrhythmia-free patients (7.8% [4.2%–13%] vs 2.0% [0%–12.5%], P = .03).RV prestretch duration, postsystolic strain, and RV lateral-septal delay quantify RV electromechanical dyssynchrony severity and reflect the underlying pathophysiology. The prestretch duration percentage and RV basal lateral–to–midseptal delay were independently associated with RV dysfunction, potentially providing a clinical tool to quantify RV electromechanical dyssynchrony.Electromechanical dyssynchrony is common after TOF repair.RV dyssynchrony is quantifiable by echocardiography.Electromechanical dyssynchrony parameters are associated with RV dysfunction.RV lateral-septal delay and prestretch amplitude are associated with RV remodeling.Electromechanical dyssynchrony is associated with exercise capacity and arrhythmia.