Murmur intensity in small-breed dogs with myxomatous mitral valve disease reflects disease severity

OBJECTIVES: To determine whether murmur intensity in small-breed dogs with myxomatous mitral valve disease reflects clinical and echocardiographic disease severity. METHODS: Retrospective multi-investigator study. Records of adult dogs <= 20 kg with myxomatous mitral valve disease were examined. Murmur intensity and location were recorded and compared with echocardiographic variables and functional disease status. Murmur intensities in consecutive categories were compared for prevalences of congestive heart failure, pulmonary hypertension and cardiac remodelling. RESULTS: 578 dogs [107 with "soft" (30 Grade I/VI and 77 II/VI), 161 with "moderate" (Grade III/VI), 160 with "loud" (Grade IV/VI) and 150 with "thrilling" (Grade V/VI or VI/VI) murmurs] were studied. No dogs with soft murmurs had congestive heart failure, and 90% had no remodelling. However, 56% of dogs with "moderate", 29% of dogs with "loud" and 8% of dogs with "thrilling" murmurs and subclinical myxomatous mitral valve disease also had no remodelling. Probability of a dog having congestive heart failure or pulmonary hypertension increased with increasing murmur intensity. CLINICAL SIGNIFICANCE: A 4-level murmur grading scheme separated clinically meaningful outcomes in small-breed dogs with myxomatous mitral valve disease. Soft murmurs in small-breed dogs are strongly indicative of subclinical heart disease. Thrilling murmurs are associated with more severe disease. Other murmurs are less informative on an individual basis.