Efficacy of a mitral regurgitation severity index to predict long-term outcome in dogs with myxomatous mitral valve disease

Background: Predicting progression of myxomatous mitral valve disease (MMVD) in dogs can be challenging.Hypothesis/objectives: The mitral regurgitation severity index (MRSI) will predict time to congestive heart failure (CHF) and all-cause death in dogs with MMVD.Animals: Eight hundred sixty-nine client-owned dogs.Methods: Retrospective study pooling data from 4 previous samples including dogs with MMVD stage B2 or C. MRSI was calculated as: (heart rate [HR]/120) x left atrium-to-aorta ratio (LA:Ao) x (age in years/10) x 100. Alternative MRSI formulas substituting radiographic measures of left atrial size were also calculated. Cox proportional hazard modeling and time-dependent receiver-operator characteristic curves quantified prognostic performance.Results: For Stage B2 pooled samples, MRSI > 156 was predictive of time to CHF (median 407 vs 1404 days; area under the curve [AUC] 0.68; hazard ratio 3.02 [95% CI 1.9-4.9]; P < .001). MRSI > 173 was predictive of all-cause death (median survival 868 vs 1843 days; AUC 0.64; hazard ratio 4.26 [95% CI 2.4-7.5]; P < .001). MRSI showed superior predictive value compared to the individual variables of HR, LA:Ao, and age. Variations of the MRSI equation substituting radiographic vertebral left atrial size for LA:Ao were also significantly predictive of outcome in stage B2. MRSI was not consistently predictive of outcome in Stage C.Conclusions and clinical importance: MRSI was predictive of outcome (onset of CHF and all-cause death) in MMVD Stage B2, demonstrating utility as a useful prognostic tool. Echocardiographic LA:Ao can be effectively replaced by radiographically determined LA size in the MRSI formula.