Mitral Valve Disease in Dogs: Causes, Stages, Symptoms

When to call the vet: mitral valve disease usually progresses slowly, but some changes need same day attention. Call your veterinarian promptly if your dog develops a new or worsening cough, tires much faster than usual on walks, or is breathing faster than normal while genuinely asleep. Go to an emergency clinic straight away, at any hour, if your dog is struggling to breathe, breathing with an open mouth at rest, standing with elbows out and neck extended to get air, has pale, blue or grey gums, faints or collapses, or cannot settle and lie down. A dog who looked stable last week can decompensate suddenly, and breathing difficulty is always an emergency rather than something to watch until morning.

Mitral valve disease in dogs, known clinically as myxomatous mitral valve degeneration or MMVD, is a slow, progressive thickening of the small valve that separates the left atrium from the left ventricle. As the leaflets stiffen and curl, they stop sealing cleanly, so a little blood leaks backwards with every heartbeat. That leak is what a veterinarian hears through the stethoscope as a heart murmur. It is the most common acquired heart condition in adult dogs, it is strongly linked to small breeds and increasing age, and it moves through a well defined set of stages that veterinary cardiologists use to decide when treatment should begin.

The single most useful thing to understand on the day of diagnosis is this: a murmur is not an emergency, and many dogs live comfortably for years in the early stages with nothing more than monitoring.

How the valve fails, and why some dogs are prone to it

The mitral valve has two thin leaflets anchored by fine cords called chordae tendineae. When the left ventricle contracts, the leaflets slam shut so blood is pushed forward into the body rather than backward into the left atrium.

In myxomatous degeneration, the connective tissue of those leaflets changes character. The tissue becomes thickened, nodular and rubbery, and the edges curl away from each other. The seal fails. Blood regurgitates into the left atrium, the atrium stretches to accommodate the extra volume, and the left ventricle enlarges as it works harder to keep forward output normal. For a long stretch of the disease, that compensation works beautifully and the dog looks completely well.

Trouble starts when the left atrium can no longer absorb the extra pressure. Pressure backs up into the pulmonary veins, fluid seeps into the lung tissue, and the dog develops congestive heart failure. In some dogs a chorda tendinea ruptures, which can cause a sudden decompensation in a dog who seemed stable the week before.

What causes it, and how dogs get it

Mitral valve disease is degenerative and strongly genetic. Dogs do not get it from food, from being overweight, from too much exercise or from too little. It is not an infection, which distinguishes it from endocarditis, and it is not the same disease as dilated cardiomyopathy, which typically affects large and giant breeds.

Small breeds carry most of the risk. Cavalier King Charles Spaniels are affected earliest and most severely, with an inherited pattern that veterinary researchers consider polygenic. Dachshunds, Chihuahuas, Miniature and Toy Poodles, Maltese, Shih Tzus, Papillons and Yorkshire Terriers are all over-represented. Age is the other major factor, because the valve changes accumulate over years. A middle-aged small dog who had a clean heart last year can arrive at the next annual exam with a soft new murmur.

That is the honest answer to how dogs get mitral valve disease. They are born with the predisposition, and time does the rest.

Symptoms owners notice first

In the earliest phase there are no symptoms at all. The murmur is almost always an incidental finding at a routine or pre-anaesthetic exam, picked up in a dog who is eating, running and sleeping normally.

As the disease advances, the signs owners report most often are:

• A soft, persistent cough, often worse at night, after lying down, or during excitement
• Faster breathing at rest, which is usually the earliest objective change
• Reduced stamina on walks, stopping earlier, lagging behind, or refusing stairs
• Restless, broken sleep, or an unwillingness to lie flat on one side
• Fainting or collapse, usually brief, often after exertion or a coughing fit
• Reduced appetite and gradual loss of muscle over the topline and hind end
• A swollen or fluid-filled abdomen in advanced right-sided involvement

The most valuable thing an owner can do at home is count sleeping respiratory rate. Watch the chest rise and fall while the dog is genuinely asleep, count for thirty seconds and double it. Veterinary cardiologists commonly ask owners to log this number daily and to call if it climbs consistently above roughly thirty breaths per minute, because a rising trend often precedes visible distress. Labored breathing, blue or grey gums, or collapse are emergencies and need a clinic the same hour, not the next morning.

The ACVIM stages, from A to D

Veterinary cardiology uses a staging system published by the American College of Veterinary Internal Medicine. It is the vocabulary your vet will use, and it determines what happens next.

Stage	What it means	What typically happens
A	At-risk breed, no murmur, no structural change	Annual auscultation, no medication, no imaging needed
B1	Murmur present, heart not enlarged on imaging	Monitoring, repeat chest radiographs or echocardiogram on a set interval
B2	Murmur present, left atrium and ventricle measurably enlarged	Cardiac medication is usually started, monitoring becomes more frequent
C	Clinical signs of congestive heart failure, current or previously treated	Prescribed cardiac medication, home respiratory rate logging
D	Heart failure no longer controlled by standard treatment	Advanced or referral-level management, close quality-of-life discussion

Why stage B2 is the one that changes the plan

Stage B2 is the dividing line, and it is the stage owners search for most. A dog in B2 still looks well and still has no symptoms, but imaging shows the heart has enlarged past defined measurement thresholds, assessed on echocardiogram and chest radiographs. That matters because of the EPIC study, a large multicentre clinical trial published in the Journal of Veterinary Internal Medicine, which found that starting cardiac medication in dogs with stage B2 delayed the onset of congestive heart failure compared with placebo. This is why your vet may want an echocardiogram on a dog who seems completely normal. The measurement, not the behaviour, decides whether medication starts.

Moving from B1 to B2 is not a crisis. It is the point at which a plan becomes active rather than observational.

Does mitral valve disease hurt dogs?

The degenerating valve itself does not generate pain. Valve tissue is not richly supplied with pain receptors, and a dog in stage B1 or B2 is not in discomfort because of the murmur. Owners who ask whether their dog is suffering during the long quiet stages can usually be reassured that the answer is no.

What does cause distress is congestive heart failure. Fluid in the lungs produces breathlessness, and breathlessness is frightening and exhausting for a dog in the same way it is for a person. A dog who cannot settle, who stands with elbows out and neck extended, or who pants through the night is distressed even if nothing is strictly painful. Coughing fits are tiring. Cardiac medication prescribed by a veterinarian exists in large part to relieve exactly that, and many dogs are noticeably more comfortable once fluid is cleared, although results vary from dog to dog. Pain and distress are different things, and the second one is very much treatable.

What management looks like day to day

Treatment is prescription-led and belongs entirely to your veterinarian or a veterinary cardiologist. Once imaging shows the heart has enlarged past the defined thresholds, your vet may prescribe medication. Once congestive heart failure appears, your vet may prescribe further medication to clear fluid from the lungs and reduce the workload on the heart. Sodium is usually moderated rather than eliminated, and appetite is protected, because muscle loss worsens outcomes, but any change to diet or feeding should happen under your vet's direction rather than on your own. Surgical mitral valve repair exists and is performed at a small number of specialist centres. Whether a particular dog is a candidate is a decision for the veterinary cardiologist and surgical team, weighed against travel, anaesthetic risk and the individual dog, and it is not a judgement to settle from home.

Monitoring is the other half of the job: repeat imaging, periodic bloodwork to check kidney values while on heart medication, and your daily sleeping respiratory rate log. Never stop, reduce or skip a cardiac medication because your dog seems better, and never add anything new to a cardiac patient's routine without clearing it with the veterinarian who prescribed those medications.

Supporting the rest of an aging body

A dog in stage B1 or B2 is very often also a dog with a stiff back end. The same twelve-year-old small breed carrying a murmur is losing cartilage, losing muscle and shortening their stride. Cardiac medication does nothing for that, and the mobility decline is frequently what the owner actually notices first at home.

There is no single answer that covers the heart and the joints at once, so it is worth raising mobility at the same cardiac appointment rather than waiting for a separate visit months later. Tell your vet what the dog can no longer do: the stairs that are now refused, the sofa that used to be a single jump, the walk that ends halfway down the street. Mention whether stiffness is worst first thing in the morning, whether it eases once the dog warms up, and whether the back feet slip or scuff on hard floors. Those details help a veterinarian separate orthopaedic pain, neurological weakness and the exercise intolerance that comes from the heart itself, because from the outside they can look very similar.

Practical household changes sit comfortably alongside a cardiac plan: non-slip runners over slick floors, a ramp instead of a jump, food and water at an easy height, shorter and more frequent walks on cooler parts of the day, and a soft bed the dog can get in and out of without a scramble. Anything you want to add to an older cardiac dog's routine, including anything bought over the counter, should be cleared first with the veterinarian who prescribed the heart medication, so the whole plan stays in one set of hands.

What to note for your vet, and when it becomes an emergency

The appointment goes better when you arrive with a record rather than an impression. Keep a short daily log, on paper or on your phone, and bring it to every recheck. Useful things to note:

• The sleeping respiratory rate, logged daily, so the trend over weeks is visible rather than a single number
• Coughing: how often, what time of day, whether it happens on waking, on exertion, or at night
• Exercise tolerance: how far the dog walks before stopping, and whether that distance is shrinking
• Appetite, drinking and body weight, including any muscle loss along the spine and hind end
• Any fainting or collapse: what the dog was doing beforehand, how long it lasted, and how quickly recovery happened
• Gum colour if you have checked it, and any abdominal swelling or sudden weight change
• Exactly which medications were given and when, and any doses that were missed or vomited back up

Take a short phone video of anything you cannot easily describe, such as a coughing fit, a collapse, or the way your dog is breathing. Breathing patterns are hard to recall accurately and a video often answers a question faster than a paragraph.

Treat the following as reasons to go straight to an emergency clinic rather than wait: breathing that is laboured, noisy or effortful at rest, open-mouth breathing in a dog who is not hot or excited, a sleeping respiratory rate that climbs and stays up, gums that look pale, blue, grey or muddy, collapse or fainting, a dog who will not or cannot lie down and settle, a swollen abdomen appearing over a day or two, a sudden weakness or coldness in the hind limbs, or a cough that becomes constant and distressing. If you are unsure, phone the clinic and describe what you are seeing. Veterinary teams would far rather triage a call that turns out to be nothing.

Key Takeaways

• Mitral valve disease is degenerative thickening of the mitral valve, the most common acquired heart disease in adult dogs, and it is driven by genetics and age rather than lifestyle.
• Small breeds dominate the risk pool, with Cavalier King Charles Spaniels affected earliest and most severely.
• The ACVIM stages run A, B1, B2, C and D. Stage B2 means measurable heart enlargement without symptoms, and it is usually when medication begins.
• The disease itself is not painful, but congestive heart failure causes real distress, which is why treatment matters.
• A daily sleeping respiratory rate count is the single most useful home monitoring tool an owner has.
• Mobility decline in an older cardiac dog is a separate problem with separate options, and both belong in the same conversation with your vet.
• Laboured breathing, blue or grey gums, collapse or an inability to settle are emergencies and need immediate veterinary care.

Your veterinarian owns this diagnosis. They decide the staging, order the imaging, choose the medication and set the recheck interval, and a veterinary cardiologist should be the one interpreting an echocardiogram. Everything an owner does at home, the respiratory rate log, the symptom notes, the non-slip floors and the steady routine, works around that plan rather than in place of it. Bring the questions, bring the log, and keep the plan in one set of hands.

Nothing in this article treats, cures or prevents any condition. Every decision about your dog's care belongs with your veterinarian.

Related guides: What Happens If a Dog Has Heart Disease? (Signs to Watch) and Doberman Heart Problems : Symptoms and Breed Risks.