Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Grant TE 137/2020 Background. In primary mitral regurgitation (PMR) left ventricular (LV) remodeling is an adaptive process with prognostic significance. Aim. To characterize LV deformation parameters related to PMR severity for a better understanding of LV pathophysiology, in patients with mild to severe PMR and preserved LV ejection fraction (EF). Methods. 48 patients (56 ± 14 years, 42% men) with PMR (mild - 12 patients, moderate - 12 patients, severe - 24 patients) and 50 age and sex matched-controls (56 ± 12 years, 48% men) were analyzed by 2-dimensional speckle-tracking echocardiography (2D STE) for LV endo, mid, and epicardial longitudinal strain (LS); endo, mid, and epicardial circumferential strain (CS); and radial strain (RS). LV volumes and EF, and LA volumes and emptying fraction (EmF) were measured by 3D echocardiography. MR severity was quantified by vena contracta (VC), PISA radius, MR regurgitant volume, and EROA measurements, according to the 2021 ESC guidelines. Results. 3D LVEF had no significant difference between PMR patients and controls (p= 0.11). From control subjects to mild, moderate, and severe PMR, there was a progressive reduction of LS and RS (Figures 1 and 2), but not of CS. By 2D STE multilayer analysis, when compared to controls, PMR patients had decreased endo-, mid-, and epi-LS (p < 0.04), with a compensatory increase of endo-LS in mild PMR (Figure 2). Basal, middle, and apical segments LS decreased similarly and progressively from control subjects to PMR patients (p < 0.01). In addition, in patients with PMR only LS, but not RS or CS, correlated with MR severity (PISA radius, VC, and MR regurgitant volume: r= -0.43, r= -0.32, r= -0.38, all p < 0.01), and LA function (EmF: r = -0.41, p < 0.01). Conclusion. When compared to normal subjects, patients with PMR and preserved LVEF have altered LV longitudinal and radial deformation, with preserved circumferential strain. All longitudinal layers and all LV segments were similarly affected. Only longitudinal strain correlates with MR severity and atrial function. Our study suggests that a comprehensive evaluation of the LV deformation should be included in the assessment of patients with PMR, in order to identify early stages of LV dysfunction, starting even from mild MR. Abstract Figure. LV deformation - PMR vs controls Abstract Figure. Multilayer LS in PMR patients