Mitral Valve Disease (MMVD)

MMVD progresses through stages, and the clinical picture changes as it advances. In early stages there may be no symptoms at all — a murmur on a routine exam is often the first and only finding for years:

• Heart murmur — detected during a physical exam; typically a left-sided systolic murmur that grows louder as the valve leaks more; often present for two to five years before any other signs appear
• Coughing — particularly at night or after lying down for a period; caused by an enlarged left atrium compressing the main airway or by early pulmonary edema (fluid in the lungs)
• Elevated resting respiratory rate — counting your dog's breaths per minute while asleep is one of the most sensitive home indicators that fluid is beginning to accumulate
• Reduced exercise tolerance — the dog tires more quickly on walks or shows less interest in play
• Fainting or collapse (syncope) — may occur with exertion or during excitement when the heart cannot meet sudden demand
• Rapid or labored breathing — a sign that congestive heart failure has developed and requires urgent evaluation

Many dogs with stage B1 disease (murmur, no heart enlargement) feel completely well and show no symptoms for years. Regular monitoring — rather than alarm — is the appropriate response to a newly detected murmur.

MMVD has a strong hereditary component, particularly in small-breed dogs, though the exact genetic pathways are still being characterized in most breeds:

• Cavalier King Charles Spaniels — have an exceptionally strong hereditary predisposition; nearly all Cavaliers develop a mitral valve murmur by middle age, often earlier than other breeds; breed-specific screening and breeding protocols have been established to delay its onset
• Other small and toy breeds — Dachshunds, Chihuahuas, Miniature Schnauzers, Poodles, Maltese, and Shih Tzus are all over-represented; roughly 30–40% of small dogs over ten years old have some degree of MMVD
• Age — the disease is progressive and age-related; it is rare in dogs younger than five and increasingly prevalent beyond eight years
• Male dogs — statistically affected at a slightly earlier age and with faster progression than females, though both sexes are susceptible
• Large breeds — MMVD can occur but is far less common; giant breeds are more prone to dilated cardiomyopathy as their primary cardiac condition

The ACVIM (American College of Veterinary Internal Medicine) staging system — B1, B2, C, and D — guides every treatment decision in MMVD. Accurate staging requires an echocardiogram, not just a stethoscope:

• Cardiac auscultation (listening) — grades the murmur intensity (1–6 scale) and identifies the timing and location; a murmur's grade alone does not tell us the size of the heart or how the valves look
• Echocardiogram (cardiac ultrasound) — the gold standard for MMVD staging; performed on-site by Dr. D'Urso; measures left atrial and ventricular dimensions against body size to determine whether enlargement has reached the threshold where pimobendan (Vetmedin) is recommended. The EPIC trial established specific measurement criteria for this threshold, and applying them correctly requires a board-certified cardiologist.
• Chest radiographs (X-rays) — assess overall heart size and detect pulmonary edema when there is clinical suspicion of early heart failure
• Blood pressure — elevated blood pressure increases the regurgitant leak and accelerates progression; it is measured at every cardiac visit
• NT-proBNP — a blood biomarker that rises as the heart comes under greater stress; useful as a supplement to imaging for trending disease severity over time

Treatment is stage-dependent. Many dogs with MMVD require no medication for years — the goal is to intervene at the right moment, not prematurely:

• Stage B1 (murmur, no heart enlargement) — monitoring only; echocardiogram and exam every six to twelve months to detect the transition to B2. No medication is currently recommended.
• Stage B2 (murmur plus measurable heart enlargement meeting EPIC criteria) — pimobendan (Vetmedin) is now strongly evidence-based and recommended before any symptoms develop. The EPIC trial demonstrated that starting pimobendan at this stage delays the onset of heart failure by a median of 15 months.
• Stage C (active congestive heart failure) — pimobendan plus diuretics (furosemide or torasemide), ACE inhibitors (enalapril, benazepril), and spironolactone; acute episodes may require hospitalization for oxygen and injectable diuretics
• Stage D (refractory CHF, inadequate response to standard therapy) — dose escalation and addition of second-line diuretics or other vasodilators, guided by Dr. D'Urso
• Sodium-restricted diet — introduced at stage C to help limit fluid retention
• Home resting respiratory rate monitoring — once a dog reaches B2 or C, daily breath counts while asleep allow early detection of fluid accumulation between hospital visits

A newly detected heart murmur in a dog of any age warrants an echocardiogram to establish a baseline and determine the ACVIM stage — this single piece of information shapes years of monitoring and treatment decisions. If your dog is a Cavalier King Charles Spaniel or another small breed known to be predisposed, annual cardiac screening is valuable even before a murmur is heard. Dogs already at stage B2 should be rechecked every six months to track progression, and any dog showing coughing, labored breathing, or exercise intolerance needs prompt evaluation for heart failure. Dr. D'Urso's on-site echocardiography means staging is accurate and results are available the same day. Book an appointment to learn your dog's ACVIM stage today.