MVD in dogs, formally called myxomatous mitral valve disease (MMVD), is a progressive condition where the mitral valve in the heart gradually deteriorates, allowing blood to leak backward instead of flowing forward through the body. It’s the most common heart disease in dogs, and certain breeds like Cavalier King Charles Spaniels are especially prone, with up to 90% developing it by age 10.
What Happens Inside the Heart
The mitral valve sits between the two left chambers of the heart, acting as a one-way door. When the heart squeezes to pump blood out to the body, the mitral valve closes to prevent blood from flowing backward into the upper chamber. In a healthy valve, thin, flexible leaflets snap shut tightly with each heartbeat.
In MVD, the tissue of these leaflets breaks down over time. The internal structure loses its organized layers of collagen and elastic fibers, replaced by a buildup of loose, spongy material called proteoglycans. The leaflets become thickened, floppy, and eventually start to bulge (prolapse) when the heart contracts. The tiny cord-like structures that anchor the leaflets in place, called chordae tendineae, can stretch and even snap. The result is a valve that no longer seals properly, so blood leaks backward with every heartbeat. This backward flow is called mitral regurgitation.
At first, the heart compensates by working harder and enlarging to handle the extra volume. Over months or years, this compensation fails, and fluid begins to back up into the lungs, producing the symptoms of congestive heart failure.
Which Dogs Are Most at Risk
MVD overwhelmingly affects small and medium-sized breeds. Cavalier King Charles Spaniels are by far the most susceptible, with the earliest onset and highest incidence of any breed. In one study, every Cavalier over age 7 already had evidence of the disease, and 50% of dogs under 2 years old showed early valve changes. Other commonly affected breeds include Dachshunds, Miniature Poodles, Chihuahuas, and Cocker Spaniels.
In larger breeds, MVD tends to appear later in life and progresses more slowly. Male dogs of any breed generally develop the disease earlier than females. The condition has a strong genetic component, which is why certain breeding lines are hit harder than others.
How Vets Detect and Stage MVD
The first sign a vet typically notices is a heart murmur, an abnormal whooshing sound heard through a stethoscope that indicates turbulent blood flow across the leaky valve. Not every murmur means your dog is sick right now, but it does signal that the valve isn’t closing perfectly.
Veterinary cardiologists use a staging system from the American College of Veterinary Internal Medicine (ACVIM) to classify how far the disease has progressed:
- Stage A: Dogs at high risk due to breed but with no detectable heart changes yet. A Cavalier with no murmur falls here.
- Stage B1: A murmur is present and the valve is leaking, but the heart hasn’t started enlarging in response. No treatment is typically needed.
- Stage B2: The leak has become severe enough that the heart is visibly enlarged on X-rays and ultrasound. This is where medication begins.
- Stage C: The dog has developed congestive heart failure, either currently or in the past. Symptoms like coughing, difficulty breathing, and exercise intolerance appear.
- Stage D: End-stage heart failure that no longer responds well to standard medications.
To distinguish B1 from B2, vets use echocardiography (heart ultrasound) and chest X-rays. The key measurements include a ratio comparing left atrial size to the aorta (1.6 or higher suggests significant enlargement), the internal diameter of the left ventricle during relaxation, and the overall size of the heart silhouette on X-rays. Meeting all of these thresholds, along with a murmur graded at least 3 out of 6 in loudness, qualifies a dog as Stage B2.
Signs Your Dog May Be Progressing
Many dogs live with MVD for years before showing any outward symptoms. The disease can be silently worsening while your dog seems perfectly normal. When symptoms do appear, they typically include a persistent cough (especially at night or after lying down), faster breathing at rest, reluctance to exercise, and tiring more easily on walks. Some dogs lose their appetite or seem restless at bedtime because lying flat makes breathing harder.
One of the most reliable ways to monitor your dog at home is by counting their sleeping or resting respiratory rate (SRR). When your dog is relaxed or asleep, count the number of breaths over 30 seconds and double it. A healthy rate with well-managed heart disease is typically 10 to 25 breaths per minute. If the rate climbs above 30 breaths per minute consistently, it may signal fluid building up in the lungs, which warrants a prompt call to your vet. Many owners find it helpful to track this number daily using a phone app or notebook so they can spot trends early.
Treatment at Each Stage
Dogs in Stage A and B1 don’t need medication. The focus is regular veterinary checkups and monitoring for progression. Your vet will likely recommend rechecking the heart every 6 to 12 months, depending on the breed and murmur grade.
Stage B2 is the turning point where treatment makes a real difference. A landmark clinical trial called the EPIC study found that starting a medication called pimobendan at this stage delayed the onset of heart failure by a median of 462 days, roughly 15 months of additional healthy time. Pimobendan works by helping the heart muscle contract more effectively while also relaxing blood vessels, reducing the heart’s workload. Dogs in the EPIC study who received pimobendan survived a median of 1,059 days from the start of treatment, compared to 902 days for those who did not.
Once a dog reaches Stage C (congestive heart failure), treatment expands to include a diuretic that helps the body eliminate excess fluid from the lungs, along with medications that reduce blood pressure and ease the strain on the heart. Most dogs with Stage C heart failure are managed on a combination of three or four medications taken daily. The goal is to keep the lungs clear of fluid, maintain comfortable breathing, and preserve quality of life for as long as possible.
Dietary adjustments also play a role. Dogs with mild disease benefit from modest sodium restriction, while dogs in heart failure need more significant sodium limits to help control fluid retention. Your vet can recommend specific diets formulated for cardiac patients.
Survival and Quality of Life
Prognosis varies widely depending on when the disease is caught and how well it responds to medication. Many dogs in Stages B1 and B2 live comfortably for years before ever developing heart failure, and some never progress to that point at all. For dogs that do reach Stage C, one study found a median survival time of about one year (368 days) from diagnosis, though individual outcomes ranged significantly based on the severity of complications.
Stage D, where heart failure becomes resistant to standard treatment, carries a shorter prognosis. At this point, the focus shifts toward keeping the dog comfortable and managing symptoms day to day.
Surgical Options
Open-heart mitral valve repair surgery exists for dogs and can be curative, but it is only available at a handful of specialized centers worldwide and costs tens of thousands of dollars. A newer, less invasive approach is showing early promise. A device called the canine V-Clamp clips the leaking valve leaflets together through a small incision, similar to a procedure widely used in human cardiology. In a recent feasibility study of 50 dogs, the procedure was successfully performed in 96% of cases with no deaths during the procedure, and the amount of blood leaking backward through the valve was roughly cut in half. Device-related complications occurred in about 6% of implants, all of which were resolved without fatal outcomes. This approach is still in its early stages and not yet widely available, but it represents a potential future alternative for dogs who don’t respond well to medications alone.