Quantitative mitral regurgitation and conventional echocardiographic variables in canine myxomatous mitral valve disease

Introduction Disease severity in dogs with myxomatous mitral valve disease (MMVD) is commonly assessed using conventional echocardiographic variables and staging systems based on cardiac remodeling and heart failure status. Multiparametric scoring systems incorporating variables such as early diastolic transmitral velocity (E-wave) have also been recently proposed. However, these approaches primarily reflect secondary morphological and loading changes rather than directly quantifying severity of mitral regurgitation (MR). Therefore, quantitative regurgitant indices may provide additional information regarding the functional severity of MR. Methods This retrospective study included 167 predominantly small-breed dogs with MMVD evaluated at Japanese referral centers, classified according to the American College of Veterinary Internal Medicine (ACVIM) staging system. Body weight–normalized regurgitant volume (RVol/kg) and regurgitant fraction (RF) were calculated using volumetric methods. Group comparisons among ACVIM stages, correlation analysis, logistic regression analysis, and receiver operating characteristic analysis were performed to evaluate associations between regurgitant indices and conventional echocardiographic variables. Results RVol/kg and RF increased significantly with advancing disease stage (both p < 0.001). However, no significant differences were observed between stages B2 and C/D and severe MR (RF ≥ 50%) was identified in some dogs classified as stage B1. Quantitative regurgitant indices showed significant correlations with the left atrium-to-aorta ratio, body-weight–normalized left ventricular internal diameter in diastole (LVIDDN), and E-wave, whereas correlations with fractional shortening were weak. The mitral-aortic velocity-time integral (VTI) ratio demonstrated the strongest correlations with both RVol/kg and RF (both r = 0.75; p < 0.001) and showed the highest diagnostic performance in identifying severe MR (area under the curve = 0.89). In the multivariable analysis, LVIDDN and the mitral-aortic VTI ratio remained independent predictors of severe MR. Discussion Quantitative regurgitant indices were associated with conventional echocardiographic variables and may provide complementary information regarding MR severity in dogs with MMVD. In this predominantly small-breed Japanese MMVD population, the mitral-aortic VTI ratio showed strong associations with quantitative regurgitant indices and may be a clinically useful Doppler-derived surrogate for MR severity, although prospective studies are required to confirm these findings.