MVD, or mitral valve disease, is the most common heart condition in dogs. It occurs when the mitral valve, which separates the left atrium from the left ventricle, gradually deteriorates and stops closing properly. This allows blood to leak backward with each heartbeat instead of flowing forward to the body. Over months to years, that backward leak forces the heart to work harder, enlarge, and eventually fail.
What Happens Inside the Heart
The mitral valve is made of thin, flexible flaps (called leaflets) held in place by tiny cord-like structures called chordae tendineae. In MVD, the tissue of these leaflets and cords undergoes a process called myxomatous degeneration. The supportive collagen layer breaks down and gets replaced by a spongy, gel-like material. This makes the leaflets thick, floppy, and misshapen, so they no longer seal tightly when the heart pumps.
The damage typically starts at the edges of the leaflets where the cords attach, then spreads inward. Early on, the changes are patchy and mild. Over time, the entire valve becomes distorted. The cords themselves can also degenerate, and in advanced cases, they may rupture entirely. When this happens, the valve loses even more of its function and heart failure can develop rapidly.
The backward flow of blood, called mitral regurgitation, is the central problem. It creates a volume overload: the left atrium swells with extra blood, and the left ventricle has to pump harder to compensate. Over time, both chambers enlarge and remodel. In severe cases, the atrial wall can tear, fluid backs up into the lungs, and congestive heart failure sets in.
Breeds and Age of Onset
MVD overwhelmingly affects small to medium-sized breeds. Cavalier King Charles Spaniels are by far the most susceptible. Nearly all Cavaliers develop some degree of mitral valve degeneration by age 7, and many show signs years earlier. A Cavalier with an audible heart murmur at age 4 is considered to have severely premature disease. Other commonly affected breeds include Dachshunds, Miniature Poodles, Chihuahuas, Cocker Spaniels, and Shih Tzus.
In most small breeds, MVD tends to appear in middle to older age, typically after age 8 or so. Large-breed dogs can develop the condition too, though it’s far less common. Because the disease has a strong genetic component, particularly in Cavaliers, breeding programs that screen for early heart murmurs play an important role in reducing its prevalence.
Signs to Watch For
MVD is a slow-progressing disease, and many dogs live with it for years before showing any symptoms at all. The first sign is usually a heart murmur detected during a routine veterinary exam. At this stage, your dog likely feels completely normal.
As the disease advances and the heart starts to struggle, the earliest noticeable symptom is often reduced exercise tolerance. Your dog may tire more quickly on walks, pant more than usual, or seem less interested in play. Coughing is another hallmark, particularly during or after activity and sometimes at night. This cough develops because the enlarged heart presses on the airways or because fluid begins to accumulate in the lungs.
More advanced heart failure brings labored breathing, reluctance to lie down, restlessness (especially at night), loss of appetite, and weight loss. Some dogs develop fluid in the abdomen, and others experience episodes of sudden weakness or fainting caused by irregular heart rhythms. One of the most useful things you can do at home is monitor your dog’s sleeping respiratory rate. A consistent resting rate above 30 breaths per minute can be an early signal that heart failure is developing or worsening.
How Vets Diagnose and Stage MVD
Diagnosis starts with a stethoscope. The characteristic whooshing sound of a heart murmur is often the first clue. But determining how far the disease has progressed requires imaging. Echocardiography (an ultrasound of the heart) is the gold standard. It lets the vet see the valve leaflets, measure how much blood is leaking backward, and assess whether the heart chambers have enlarged. Chest X-rays complement the ultrasound by showing overall heart size and revealing fluid in the lungs.
Veterinary cardiologists use a staging system developed by the American College of Veterinary Internal Medicine (ACVIM) to classify the disease into five stages, which directly guide treatment decisions:
- Stage A: Dogs at high risk due to breed predisposition, but with no detectable disease yet. No treatment needed, just monitoring.
- Stage B1: A murmur is present, but imaging shows the heart hasn’t changed size. These dogs need regular checkups but no medication.
- Stage B2: The leak is severe enough that the heart has started to enlarge, even though the dog still feels fine. This is the critical threshold where treatment begins.
- Stage C: The dog has developed clinical signs of heart failure, either currently or in the past.
- Stage D: End-stage disease where heart failure symptoms persist despite aggressive treatment.
To qualify for Stage B2, and therefore to start medication, a dog typically needs to meet specific imaging thresholds: a vertebral heart score above 10.5 on X-ray, and on echocardiography, a left atrium-to-aorta ratio of 1.6 or greater along with an enlarged left ventricle. If echocardiography isn’t available, a vertebral heart score of 11.5 or higher on X-ray alone may be enough to justify starting treatment.
Treatment at Each Stage
There is no cure for MVD, but the right medications at the right time can significantly extend a dog’s comfortable life. The treatment approach escalates with each stage.
Stage B2: Delaying Heart Failure
The landmark EPIC trial showed that starting a medication called pimobendan at this stage delays the onset of heart failure by a median of about 15 months. Pimobendan works by strengthening the heart’s contractions and relaxing blood vessels, reducing the workload on the heart. It’s given twice daily and is now the standard of care for B2 dogs. Some cardiologists also add an ACE inhibitor at this stage, though the evidence for that is less definitive.
Stage C: Managing Heart Failure
Once heart failure develops, the treatment plan expands considerably. In an acute crisis, the priority is clearing fluid from the lungs with injectable diuretics. Once stabilized, dogs go home on a daily regimen that typically includes pimobendan, an oral diuretic to prevent fluid buildup, an additional diuretic called spironolactone, and often an ACE inhibitor. If the dog develops an irregular heart rhythm like atrial fibrillation, additional medications to control heart rate may be added.
Stage D: Refractory Heart Failure
At this stage, standard doses of medication are no longer controlling symptoms. Vets increase diuretic doses, may switch to a more potent diuretic, and sometimes add extra doses of pimobendan. The goal shifts toward maintaining comfort and quality of life.
Diet and Nutrition
Dietary management becomes increasingly important as MVD progresses. Moderate sodium restriction is recommended, generally in the range of 50 to 100 milligrams per 100 kilocalories of food, depending on disease severity. Cutting sodium too aggressively can backfire by triggering hormonal changes that actually worsen fluid retention, so work with your vet to find the right balance rather than simply switching to the lowest-sodium food available.
Maintaining body weight is equally critical. Dogs with advanced heart disease often lose muscle mass, a process called cardiac cachexia. Feeding a nutrient-dense, highly palatable, and easily digestible diet helps offset this. Dogs that maintain or gain weight after a heart failure diagnosis tend to survive longer than those that lose weight. Omega-3 fatty acids from fish oil are sometimes added for their anti-inflammatory benefits, though the evidence in dogs is still evolving.
Surgical Options
Open-heart mitral valve repair is possible in dogs, but it remains rare and highly specialized. The surgery involves placing artificial cords to restore valve function, and outcomes in experienced hands can be remarkable. In one study of 25 dogs, the amount of blood leaking backward dropped from about 73% before surgery to just 2% at one month post-op, and those results held at 12 months. However, these procedures require cardiopulmonary bypass and are available at only a handful of centers worldwide, with most expertise concentrated in Japan and a few locations in the U.S. and Europe. Cost typically runs into the tens of thousands of dollars.
What to Expect Over Time
MVD is a progressive disease, but the timeline varies enormously. Some dogs live with a murmur for years and never develop heart failure. Others progress quickly. Once heart failure symptoms appear (Stage C), many dogs live 6 to 12 months with medication and dietary management. A smaller number do well for 18 months or longer. The dogs that tend to do best are those whose disease was caught early, who started pimobendan at the B2 stage, and whose owners monitor breathing rate and appetite closely at home so medication adjustments happen quickly when needed.
Regular veterinary rechecks, typically every 3 to 6 months for B2 dogs and more frequently once heart failure develops, allow your vet to adjust medications before symptoms spiral. Tracking your dog’s resting respiratory rate daily at home is one of the simplest and most powerful monitoring tools available.