Your vet heard something during a routine check-up a soft murmur, nothing your senior dog seems to notice at all. He's still eating, playing, greeting you at the door with the same energy. And yet the word "murmur" has been sitting in your chest ever since. Here's something genuinely worth knowing before you spiral: this is, by a wide margin, the single most common heart finding in dogs, and there's an actual staging system your vet is quietly working through in their head right now one that tells you far more than the word "murmur" alone ever could.

Mitral valve disease in dogs is usually myxomatous mitral valve disease (MMVD), a progressive wear-and-tear change of the valve between the heart's left atrium and left ventricle, and it's the most common acquired heart disease in dogs, making up roughly 75% of all canine heart disease. As the valve thickens and loses its normal shape, it stops sealing tightly, letting some blood leak backward with every heartbeat. The genuinely important thing to understand upfront: a murmur is very often the first and only sign for months or years, and many dogs feel completely normal well before anything resembling heart failure ever develops.

Key Takeaways

• Mitral valve disease (MMVD) is a progressive, degenerative thickening of the mitral valve it's an age-related wear-and-tear change, not an infection, and nothing a pet parent caused.
• It's the most common acquired heart disease in dogs, accounting for roughly 75% of cases, and small and toy breeds are affected far more often than large breeds.
• A heart murmur is usually the first sign, and many dogs feel entirely normal for months to years before any signs of heart failure appear.
• Vets stage MMVD using the ACVIM system (B1, B2, C, D), and the B1-to-B2 transition matters enormously, since that's typically when pimobendan is first recommended.
• The EPIC study found that pimobendan delayed the onset of congestive heart failure or cardiac death by about 15 months in Stage B2 dogs with an enlarged heart.
• A resting or sleeping breathing rate consistently above about 30–35 breaths per minute, especially with effort or coughing, is a genuine reason to contact your vet promptly.

What Is Mitral Valve Disease, Really?

Mitral valve disease in dogs is usually myxomatous mitral valve disease (MMVD, often just called MVD), a progressive wear-and-tear change of the mitral valve the valve that sits between the left atrium and left ventricle. As it thickens and loses its normal shape, it no longer seals tightly, so some blood leaks backward each time the heart contracts.

This is genuinely worth stating plainly: MMVD is a degenerative disease, not an infection, and not something a pet parent causes. The valve tissue and its supporting structures gradually change over time, and while the exact trigger isn't fully understood, genetics clearly play a role in many dogs.

Who Is Most at Risk?

The biggest risk factors are breed, age, and body size. Small-breed and toy-breed dogs are affected far more often than large dogs, risk rises as dogs get older, and males may be affected earlier or more severely in some populations. Breeds commonly associated with this disease include Cavalier King Charles Spaniels, Dachshunds, Chihuahuas, Miniature Poodles, and Cocker Spaniels and for predisposed breeds, especially Cavaliers, early screening genuinely matters, since this breed is known to develop the disease earlier and more severely than most.

Why Does This Get Missed for So Long?

A heart murmur is often the very first finding, and many dogs feel completely normal for months to years before any signs of congestive heart failure appear. This isn't a coincidence or a case of a dog "hiding" symptoms the heart genuinely compensates well in the earlier stages, which is exactly why a murmur discovered at a routine check-up, in a dog who seems perfectly fine, isn't something to panic about, but also isn't something to ignore either.

How Do Vets Stage Mitral Valve Disease?

This is genuinely the most useful framework to understand if your dog has been diagnosed, because treatment decisions are built directly around it. Treatment depends on ACVIM stage a widely used staging system developed by the American College of Veterinary Internal Medicine.

Stage B1 describes dogs with a murmur but no heart enlargement this stage is usually just monitored, with no medication needed yet.

Stage B2 describes dogs with a murmur and a heart that has begun to enlarge (cardiomegaly), even though they still show no outward symptoms. This stage often starts pimobendan.

Stages C and D describe dogs who have developed, or are no longer responding well to standard treatment for, congestive heart failure these stages need heart-failure medications and considerably closer follow-up.

Why the B1-to-B2 Transition Matters So Much

The B1-to-B2 transition is especially important, because that's the stage where pimobendan is commonly recommended and there's genuinely strong evidence behind starting it at this specific point. The EPIC study found that pimobendan delayed the onset of congestive heart failure or cardiac death by about 15 months in dogs with Stage B2 disease and cardiomegaly. That's a genuinely meaningful extension of good-quality time, and it's exactly why catching the B1-to-B2 transition early through imaging, not just listening for symptoms matters so much. If your dog has a new murmur, asking your vet whether an echocardiogram is needed is genuinely one of the most useful next steps you can take.

How Is Mitral Valve Disease Diagnosed and Staged?

The most useful test for confirming MMVD and deciding its stage is an echocardiogram, which shows the valve directly, measures chamber size, and helps determine whether the heart is enlarged enough to change the treatment plan. Chest X-rays help assess heart size and check for pulmonary edema if congestive heart failure is suspected. Blood pressure, bloodwork, and sometimes NT-proBNP or cardiac troponin can add useful context. An ECG may be recommended specifically if your dog has fainting, an irregular rhythm, or more advanced disease.

A Simple Home-Monitoring Habit Worth Learning

This is genuinely practical, actionable information worth memorising. A normal resting or sleeping respiratory rate is often around 15 to 30 breaths per minute. A consistent trend above roughly 30 to 35 breaths per minute especially alongside visible effort or coughing deserves prompt veterinary guidance rather than a wait-and-see approach.

When Should You Worry More?

If your dog is breathing faster than usual while asleep, working hard to breathe, cannot lie down comfortably, faints, or has blue-grey gums, see your vet immediately. These specific signs genuinely mean something has shifted, and they warrant same-day attention rather than a routine appointment.

Does Diet Need to Change?

Most dogs do not need sodium restriction in preclinical disease (Stages B1 and B2) unless your vet specifically advises it this is a genuinely common point of confusion, since "heart disease" makes many pet parents assume an immediate need for a strict low-salt diet. Once congestive heart failure actually develops, diet and medication routines become considerably more important, and your vet will guide you through the specific changes that make sense at that stage.

Living With a Dog Diagnosed With Mitral Valve Disease

• Ask your vet directly which ACVIM stage your dog is in, and what that specifically means for monitoring frequency and treatment timing this single question does more to clarify the situation than almost anything else.
• If your dog is a high-risk breed (Cavalier King Charles Spaniel especially) and your vet hears a murmur, ask about timing for an echocardiogram rather than waiting for symptoms to appear first.
• Learn to count your dog's resting or sleeping breathing rate, and track it consistently as covered above, this is one of the most useful, low-effort tools for catching real changes early.
• Don't assume sodium restriction is automatically needed. As covered above, most dogs in earlier stages don't need it unless your vet specifically says so.
• Build on the fuller heart-health picture. Our guide on acquired heart diseases in dogs covers MMVD alongside the other acquired conditions dogs can develop, and our guide on heart failure in dogs explains what genuinely changes once a dog progresses into Stage C or D.
• Understand the diagnostic and treatment tools your vet is using. Our guides on how vets diagnose heart disease in dogs and how heart disease in dogs is treated cover the imaging and medications referenced throughout this piece in more depth.
• Ask your vet whether a general cardiovascular support supplement fits your dog's care plan, always as a genuine complement to prescribed treatment, never a replacement for it.

FAQ

Does every dog with mitral valve disease eventually develop heart failure?
No many dogs remain stable in Stage B1 for months to years, and some never progress to symptomatic heart failure at all. Progression varies significantly by individual dog, which is exactly why regular staging through imaging matters more than assuming a fixed timeline.

Is a heart murmur the same thing as mitral valve disease?
Not necessarily a murmur is a sound your vet hears, and MMVD is one possible (and the most common) cause of it in older, small-breed dogs specifically. Confirming the actual cause and stage needs proper imaging, not just the presence of a murmur alone.

When should my dog start pimobendan?
This is typically considered once a dog transitions from Stage B1 to Stage B2 meaning a murmur is present alongside a heart that has begun to enlarge since that's the specific point where the EPIC study demonstrated a genuine survival and quality-of-life benefit. Your vet will confirm this transition through imaging, not symptoms alone.

Can mitral valve disease be cured or reversed?
No it's a progressive, degenerative condition, and current treatment focuses on slowing progression, managing symptoms, and extending good-quality time rather than reversing the valve changes themselves.

My dog has a murmur but seems completely healthy should I still worry?
Not necessarily worry, but it's genuinely worth following up rather than ignoring it. Many dogs with an early-stage murmur feel completely normal for a long time, but knowing your dog's actual stage through an echocardiogram if your vet recommends one gives you real information rather than guessing.

References

1. Mitral Valve Disease (MVD) in Dogs: Stages & Treatment. SpectrumCare.