Mitral Valve Disease in Dogs: A Road Map After a Heart Murmur
Mitral valve disease in dogs explained: what a heart murmur means, how staging works, treatment options, and monitoring your dog's breathing rate at home.
Bulut Veterinary Clinic Medical Team
Reviewed by Mücahid Çiftçioğlu, DVM
During a routine exam, your vet holds the stethoscope on your dog's chest a moment longer than usual and says the sentence: 'I can hear a heart murmur.' For many families that moment is frightening — yet a murmur is not the name of a disease, but a finding that deserves investigation. And its most common cause, mitral valve disease, can be managed for years with good quality of life when it's monitored properly and treated at the right time.
This article is a road map for the family of a dog with a newly discovered murmur: what goes wrong at the mitral valve, which stages the disease moves through, what each test tells you, when medication starts, and what to watch at home. We cover it at the depth you'd want from a consultation, in plain language.
What is the mitral valve — and what goes wrong?
On the left side of the heart sits a one-way door between the chamber that first collects oxygen-rich blood from the lungs (the atrium) and the chamber that pumps it out to the body (the ventricle): the mitral valve. A healthy valve snaps firmly shut when the ventricle contracts, so that every drop of blood moves forward into the body.
In mitral valve disease — what veterinarians call myxomatous mitral valve degeneration — these thin, flexible valve leaflets thicken and deform over the years until they can no longer close completely. The result is a leak: with every heartbeat, part of the blood slips backwards into the left atrium instead of moving forward (regurgitation — backward leakage of blood). The murmur is simply the sound of the turbulence created by that leaking blood.
While the leak is small, the heart compensates effortlessly. As it grows, the left atrium begins to enlarge to hold the returning blood, and the heart works harder. In advanced disease the pressure backs up into the vessels of the lungs and can lead to fluid accumulation in the lungs (pulmonary edema) — this is the point where the signs of heart failure appear.
Which dogs are at risk?
Mitral valve disease is the most common acquired (developing later in life) heart disease of dogs, and it is typically a disease of middle to older age. It is markedly more frequent in small and mid-sized breeds: the Cavalier King Charles Spaniel is especially known for its predisposition, and it's also common in breeds such as the Maltese, Shih Tzu, Chihuahua, Yorkshire Terrier, Poodle and Dachshund.
In predisposed breeds a murmur can sometimes be heard from middle age onward, which is why a careful cardiac auscultation at the annual exam is the most practical route to early detection — especially in small breeds. The disease is not contagious and is not caused by anything an owner did; the age-related degeneration of the valve tissue develops largely on a genetic background.
A murmur was heard: what does it actually mean?
A murmur is blood flow made audible, and its intensity is graded from 1 to 6 — grades 1-2 are very soft, while 5-6 are strong enough to be felt with a hand on the chest. An important truth: the loudness of the murmur does not always track the severity of the disease. The real question is how well the heart is compensating for the leak.
So the goal after finding a murmur is not panic but staging: has the heart enlarged or not? The answer to that question is the single most important piece of information for deciding whether treatment should begin — and it can only be answered with imaging. Not every dog with a murmur has to live as a heart patient; but every murmur deserves to be investigated properly once.
Signs: from the silent years to heart failure
The most deceptive feature of this disease is its silent phase, which can last for years: the dog runs, plays and eats well, and the only finding is the murmur heard on exam. Signs appear when the heart's ability to compensate starts to strain, and they can include:
- A cough that worsens with exertion or at night — may relate to the enlarged heart pressing on the windpipe, or to early fluid in the lungs
- Tiring sooner on walks, reduced stamina
- Faster breathing at rest or during sleep — one of the earliest and most reliable signs of fluid building up in the lungs
- Labored breathing, breathing with the belly muscles — needs urgent assessment
- Fainting episodes (syncope): brief loss of consciousness, often with exertion or excitement
- Loss of appetite and weight loss — in advanced disease
The diagnostic work-up: from stethoscope to echocardiography
The journey starts with careful auscultation: the location, intensity and character of the murmur give the first clues. Imaging then answers two fundamental questions. Chest X-rays show the overall size of the heart, whether the windpipe is being compressed, and — most importantly — whether there is fluid in the lungs.
The definitive assessment is made with a heart ultrasound (echocardiography — a live ultrasound study of the heart). It shows the structure of the valve leaflets, the size of the leak, and precise measurements of the left atrium and ventricle — in other words, the definitive answer to 'has the heart enlarged?'. Where needed, an ECG is added to assess heart rhythm, and blood tests to evaluate general health and kidney function — kidney values also matter for safely planning any future medication.
Staging: the disease's road map
The guidelines of the American College of Veterinary Internal Medicine (ACVIM) divide mitral valve disease into stages so that everyone speaks the same language — and treatment decisions hang on these stages:
- Stage A: No disease yet, but at risk due to breed predisposition — regular auscultation is recommended
- Stage B1: A murmur is present but heart dimensions are normal — no medication; monitored with periodic imaging
- Stage B2: A murmur with clear heart enlargement — treatment is recommended at this stage even without symptoms, to delay the onset of heart failure
- Stage C: Signs of congestive heart failure (fluid in the lungs) are present or have occurred — managed with combination medication
- Stage D: Advanced disease that no longer responds to standard treatment and needs intensified management
Treatment and management options
It's worth being honest about what treatment aims to do: medication does not repair the degenerating valve. It eases the heart's workload, controls fluid accumulation, and extends the time a dog lives well. Starting heart-supporting treatment in stage B2 — before any symptoms — can meaningfully delay the onset of heart failure, one of the most important advances in managing this disease in recent years. Once heart failure develops, diuretics (medication that removes excess fluid) and additional heart-support drugs join the plan. Which medications, and at what doses, is entirely individual and is decided only by your veterinarian — adjusting heart medication based on the internet or a neighbor's advice can cause serious harm in this disease.
Surgical valve repair, as performed in human medicine, does exist — but it is a highly specialized procedure offered by only a small number of centers worldwide; in practice, the vast majority of dogs are managed with medication and monitoring. Supporting measures include avoiding salty foods and salty treats, keeping weight under control, and maintaining regular but moderate exercise.
Monitoring at home: the sleeping respiratory rate
The most valuable monitoring tool in this disease is free and sits in your hands: the sleeping respiratory rate. While your dog is in deep sleep, count each rise-and-fall of the chest as one breath and record the number of breaths per minute. Values below 30 per minute during deep sleep are the generally accepted comfort zone; a number that creeps upward over days, or clearly exceeds 30, can be one of the earliest warnings of fluid building up in the lungs — and a reason to call your vet.
Logging a measurement once or twice a week on your phone gives your vet an invaluable trend line between visits. Beyond that, a cough that becomes more frequent, growing exercise intolerance, or a fainting-like episode are reasons to come in without waiting for the next scheduled check. Sudden, severe difficulty breathing is an emergency — our clinic is open 24/7, and early intervention in these situations saves lives.
The long term: hopeful and realistic
A diagnosis of mitral valve disease is not an ending. Many dogs in stage B1 live for years without a single symptom, and some live out their lives in that stage. Treatment started early in B2 stretches out the silent period; and a large share of dogs that do develop heart failure return to the walks and routines they enjoy once the right combination of medication is found.
The keys to the long term are consistency and partnership: check-ups and imaging at the intervals your dog's stage requires, medication given without interruption, breathing-rate logs at home, and changes reported early. The course of this disease genuinely differs from dog to dog — so trust your own dog's monitoring trend and your veterinarian's assessment more than any general statistic.
Frequently asked questions
A murmur was found in my dog — should we start medication right away?
No — a murmur alone is not a reason for medication. The decision depends on whether the heart has enlarged, which is determined with X-rays and a heart ultrasound. If heart dimensions are normal (stage B1), no medication is recommended and the dog is monitored; if there is clear enlargement (stage B2), treatment is recommended even before symptoms appear.
Does coughing in a dog with a murmur always mean heart failure?
No. In small-breed and older dogs, coughs very often have non-cardiac causes such as windpipe problems and chronic bronchitis; an enlarged heart can also press on the windpipe and trigger coughing without any failure. X-rays — and ultrasound where needed — make the distinction. If the cough is accompanied by fast breathing, don't wait to come in.
How do I measure the sleeping respiratory rate, and what should it be?
While your dog is deeply asleep, count the chest's rise-and-fall movements for one minute; each rise and fall is one breath. Below 30 breaths per minute in deep sleep is the generally accepted range. A value that climbs over days or clearly exceeds 30 is a finding to report to your vet — regular, logged measurements make monitoring much easier.
Can mitral valve disease be fixed with surgery?
Valve repair surgery is technically possible, but it's a demanding, costly procedure performed at only a few specialized centers worldwide. In practice, the vast majority of dogs are managed with medication and regular monitoring — and with that approach, a long stretch of good quality of life is realistic. Suitability varies case by case; it's worth discussing with your vet.
What can I do at home to slow the disease down?
Never skip medication, keep check-ups regular, and track the sleeping respiratory rate. Weight control, avoiding salty foods, and regular but moderate exercise all reduce the heart's workload. Reporting a worsening cough, reduced stamina or any fainting-like event early allows treatment to be updated in time.
I have a Cavalier — what should I do while my dog is still young?
In predisposed breeds the most valuable step is regular auscultation: have the heart listened to at least once a year, or more often if your vet advises. If a murmur is caught early, staging follows — and with it, the chance to start treatment at exactly the right time. Weight control and good general health also reduce the heart's long-term workload.
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If a murmur has been heard in your dog — or it's simply time for a heart check — let's plan the staging and follow-up together; our clinic is open 24/7.
This article is for general information only and is not a substitute for a veterinary examination. Always consult your veterinarian about decisions concerning your pet's health.