Mitral Valve Disease in Dogs: Stages, Signs, and Care

A veterinarian listens to a bright, middle-aged small dog, pauses, and mentions a soft new sound low on the left side. For many small-breed dogs that moment is the opening chapter of myxomatous mitral valve disease, the most common acquired heart condition of dogs.

Important: This article shares general educational information for pet owners and is not a substitute for an in-person veterinary examination, diagnosis, or treatment. For any medical concern or emergency, contact your veterinarian promptly.

Myxomatous mitral valve disease, usually shortened to MMVD, is not a single event but a long journey with milestones. Cardiologists map it using a staging framework from the American College of Veterinary Internal Medicine, and those stages turn an alarming word at a checkup into a plan.

This guide stays narrow on purpose: how the valve changes, which dogs it affects, what each stage means, the complications that belong to it, and when to seek help. For murmur fundamentals and daily life with heart failure, follow the links to the companion guides.

What Happens Inside a Degenerating Mitral Valve

The mitral valve sits between the left atrium and the left ventricle. Its thin leaflets are tethered from below by fine cords, the chordae tendineae, that stop them flipping backward when the ventricle contracts.

In myxomatous degeneration the leaflet tissue thickens and turns nodular and rubbery, the free edges curl, and the chordae stretch, so the valve no longer seals. With every contraction some blood is pushed backward into the left atrium.

That leak is called mitral regurgitation, and it drives everything that follows. The left atrium stretches to hold the extra volume, the ventricle enlarges, and pressure rises in the vessels behind it.

Why This Is a Dog Disease

Cats develop heart disease too, but the common feline conditions involve heart muscle rather than this pattern of valve degeneration. MMVD is a canine problem, and this article is about dogs.

The Dogs This Disease Tends to Affect

The breed and size signal here is one of the strongest in canine medicine. This is overwhelmingly a disease of small-breed, middle-aged to older dogs.

• The Cavalier King Charles Spaniel stands apart from every other breed. Murmurs often appear earlier in life, so the breed gets its own monitoring schedule.
• Dachshunds are a classic example. Standard and miniature varieties both appear.
• Chihuahuas are frequently affected in middle age. A small chest makes a soft murmur easy to hear.
• Miniature Poodles are a well-recognized group. Crosses of similar size share the profile.
• Miniature Schnauzers come up regularly too. Breed history is part of what your veterinarian weighs.
• Small mixed-breed dogs make up a large share of cases. Size and age matter more than pedigree.

Large-breed dogs are not immune, but a big dog with a new murmur usually prompts a different list of possibilities. Breed risk is a reason for attentive listening, never a diagnosis in itself.

The Murmur of Mitral Regurgitation and Its Limits

The sound a leaking mitral valve makes has a consistent signature. It is a systolic murmur, heard while the ventricle contracts and blood is forced backward, and it is loudest low on the left chest wall, over the heart's apex.

That murmur typically appears years before any sign of illness. A dog can run, eat, and behave normally while a murmur has been audible at every annual visit, so it is early information rather than a symptom.

Loudness Does Not Track Severity

A dog can have a loud murmur with a normal-sized heart, and an enlarged heart can produce a murmur that has not grown louder, because loudness reflects turbulence rather than chamber size. A murmur is therefore a prompt to image the heart, not a diagnosis; for the basics, see Heart Murmurs in Dogs and Cats.

The ACVIM Stages Your Veterinarian Works From

Staging exists because this disease is a continuum, and recommendations differ sharply depending on where a dog sits along it. Your veterinarian assigns the stage from examination plus imaging.

• Stage A describes risk alone. Predisposed breeds with no murmur and no structural change.
• Stage B covers dogs with the leak but no clinical signs. The dog looks normal, and this stage splits into two very different subgroups.
• Stage B1 means the heart has not enlarged. The valve leaks, but chamber measurements stay normal.
• Stage B2 means the heart has enlarged. The chambers have stretched past defined thresholds while the dog still looks well.
• Stage C means congestive heart failure has developed. Fluid has backed up, and treatment is directed by a veterinarian.
• Stage D describes failure that resists standard management. Care becomes intensive and closely supervised.

The stage cannot be worked out at home from how a dog looks. Ask which stage your dog is in and what evidence supported it, because that answer explains most of what follows.

Why the B1 to B2 Boundary Matters Most

If one thing deserves to be remembered from staging, it is what happens between B1 and B2. From the outside they look identical: the same cheerful dog, the same walks, the same murmur.

What changes is size, confirmed with chest radiographs and an echocardiogram measured against established criteria. Treatment changes there too: many stage B1 dogs are simply monitored, while stage B2 is where veterinarians consider treatment meant to delay heart failure.

Many dogs live for years in stage B1 with no treatment and a normal life, and some never progress further. Because the crossing into B2 is silent, the recheck schedule your veterinarian sets is the safety net.

Resting Breathing Rate and When It Starts to Matter

Counting the resting breathing rate is the most useful measurement an owner can contribute, but it matters differently at different points in the journey. Early on it is mostly baseline-gathering, teaching you what your own dog looks like when all is well.

It earns its reputation from stage B2 onward. Once the left atrium has enlarged, fluid backing into the lungs becomes possible, and a rising resting breathing rate is often the earliest change an owner can detect.

Count while your dog is asleep or fully at rest, never after exercise or in a warm room. Count each full rise and fall of the chest as one breath over a short fixed interval, convert to breaths per minute, and log it with the date.

This article deliberately does not say what a normal number is or what number should trigger a call. Ask your veterinarian for the baseline and for the change that should prompt contact.

Complications That Belong to This Disease

Rupture of a Chorda Tendinea

The stretched cords anchoring the leaflets can tear. When one gives way the leak worsens abruptly, a dog stable for years may develop breathing difficulty within hours, and that is an emergency.

Left Atrial Tear

A severely stretched atrium can tear in its wall, letting blood escape into the sac around the heart and compress it. Sudden weakness, collapse, and pale gums are the picture, and this is an emergency.

Pulmonary Hypertension

Pressure backing up behind the leaking valve can raise pressure in the lung blood vessels, a recognized companion problem in advancing disease. Owners may notice reduced stamina, labored breathing, or fainting, and it is identified on an echocardiogram.

A Cough That May Not Be the Lungs

A markedly enlarged left atrium sits beneath the main airway and can press on it, producing a dry, honking cough in a dog whose lungs are clear. Only imaging separates that from fluid or unrelated airway disease.

When Mitral Valve Disease Reaches Heart Failure

What is distinctive about this route into heart failure is that the left side gives way first. Pressure rises through the left atrium into the lung vessels, so fluid gathers in lung tissue and breathing changes lead the picture.

Most dogs also reach failure after a long, quiet stretch in stage B2, so families usually have warning. Daily life from that point is covered in Congestive Heart Failure in Dogs, and all of it applies here.

Surgical Mitral Valve Repair Is a Specialist Conversation

Almost everything a veterinarian does for this disease manages the consequences of the leak rather than the leak itself. One exception exists: open-heart repair on cardiopulmonary bypass, in which a surgeon reconstructs the leaflets and replaces the failed cords with artificial ones.

Availability is the context that matters. It is performed at only a very small number of centers worldwide and involves careful case selection, referral, and often travel, so it is a specialist conversation to open with a cardiologist rather than a decision to make from an article.

Myths That Follow a Mitral Valve Diagnosis

Some misunderstandings do real harm here. They frighten owners of dogs who are doing well, and they delay care for dogs who are not.

• A louder murmur means a worse heart. Loudness describes turbulence, not the size of the chambers behind the valve.
• A murmur means the dog is already in heart failure. Most dogs with this murmur are years away from failure, and many never reach it.
• Nothing can be done until the dog gets sick. Staging exists because recommendations change before any sign appears.
• A dog with a murmur should be kept quiet. Dogs in the earlier stages generally continue normal activity, and your veterinarian decides on limits.
• A cough always means fluid in the lungs. It can come from an enlarged atrium pressing on the airway, or from airway disease.

When to Call Your Veterinarian

Because this disease progresses quietly for so long, the changes that matter often arrive quickly. Treat the following as an emergency and go to your veterinarian or an emergency clinic now.

• Severe breathing difficulty is an emergency. Labored, open-mouthed, or panicked breathing, or a dog unable to settle.
• Blue or gray gums are an emergency. Any bluish or grayish tinge to gums or tongue means oxygen delivery is compromised.
• Collapse is an emergency. This includes fainting and sudden weakness, even if the dog recovers quickly.
• A sudden spike in the resting breathing rate is an emergency. A sharp jump above your dog's baseline needs attention now.

Other changes deserve a prompt call during office hours: a new cough at rest, reduced stamina on walks, restlessness at night, sleeping with the chest propped up, or a resting breathing rate drifting upward. None of these tell you what your dog has; they are reasons for an exam.

Frequently Asked Questions

Does a dog with a murmur always need an echocardiogram?

Your veterinarian weighs breed, age, murmur character, and the overall picture. Without imaging there is no stage at all, because the B1 and B2 distinction is defined by measurements, so the real question is whether staging would change what happens next.

Can this disease be reversed or slowed down?

Nothing reverses changes already present in the leaflets. Medical management addresses the consequences of the leak, while surgical repair is the only approach aimed at the valve itself.

How does a veterinarian tell a heart cough from airway disease?

Many small breeds prone to this disease are also prone to airway conditions, so a coughing small dog with a murmur may have both. Radiographs, an echocardiogram, and sometimes airway imaging separate them, which matters because the two are managed differently.

Should a breeding program screen for this?

In predisposed breeds the disease has a heritable component, and formal screening schemes based on auscultation by a cardiologist at defined ages exist in several countries. Anyone breeding such a dog should discuss current recommendations with a specialist.