Evaluation of factors that might affect the grading of heart murmurs in dogs with myxomatous mitral valve disease

Abstract: Myxomatous mitral valve disease (MMVD) is the most common heart disease in dogs. The first sign of MMVD is usually a heart murmur, which most commonly is detected before clinical signs of the disease potentially develop. The preclinical period of MMVD is usually long and can be prolonged with medical treatment. Murmur grading and, primarily, diagnostic imaging techniques are used to determine if medical treatment is indicated. Many veterinarians use a 6-level scale to grade heart murmurs. The grades of the 6-level scale are, however, defined differently in various literature. Several studies have shown a relatively good agreement between murmur grade and disease severity based on echocardiographic assessment in dogs with MMVD. However, the agreement is not complete. The aim of this study was to evaluate potential factors, such as dog and mitral regurgitation (MR) characteristics, that might impact murmur assessment, as well as to evaluate the utility of various murmur grading scales in a group of dogs with MMVD. Evaluation of the utility of various murmur grading scales could potentially help optimize existing murmur grading scales. Moreover, knowledge about factors impacting murmur grading could potentially lead to a better understanding of the association between murmur grades and disease severity (based on echocardiographic assessment). Client-owned dogs presenting with a heart murmur at the Cardiology Clinic at the University Animal Hospital at the Swedish University of Agricultural Sciences (SLU) in Uppsala, Sweden, were prospectively recruited to the study if they, based on echocardiographic findings, had a diagnosis of MMVD. Information about dog characteristics, data from clinical examination (including murmur characteristics), and echocardiographic findings were collected. The MMVD severity staging system (including stages A, B1, B2, C, and D) developed by the American College of Veterinary Internal Medicine (ACVIM) Specialty of Cardiology Consensus panel was used. A total of 53 dogs were recruited to the study. A systolic heart murmur could be heard when listening with the ear close to the chest wall (without a stethoscope) in 44%, 86%, and 100% of dogs in stages B1, B2, and C-D, respectively. The murmurs remained audible when the stethoscope was lifted slightly off the chest wall in 11%, 18%, and 63% of dogs in stages B1, B2, and C-D, respectively. None of the dogs in stage B1 had a precordial thrill, whereas 43% and 78% of dogs in stage B2 and stages C-D, respectively, had precordial thrills. The murmur grade increased with ACVIM stage. Most dogs (65% and 83%, respectively) in stages B1 and B2 had an MR jet with a lateral direction, while most dogs (60%) in stages C-D had an MR jet with a central direction. In conclusion, whether the murmur can be heard without a stethoscope and specific intensity descriptions for each grade might be redundant criteria in murmur grading scales. Both the 4-level and 6-level scales might be useful in dogs with MMVD, but more research is needed to further evaluate the utility of these and other scales. Furthermore, no firm conclusions about if the factors investigated impact the agreement between murmur grading and disease severity can be drawn from the results in this study. Factors such as body condition score, murmur characteristics, and the direction of the MR jet are still considered potential factors that could impact murmur grading, but further research with a larger study population is needed.