Mitral valve disease is the most common heart condition in dogs, accounting for roughly 75% of all canine heart disease cases. It occurs when the mitral valve, which sits between the left atrium and left ventricle of the heart, gradually deteriorates and stops closing properly. Blood leaks backward with each heartbeat, forcing the heart to work harder over months and years until it can no longer keep up. The full clinical name is myxomatous mitral valve disease (MMVD), and while it progresses slowly in most dogs, it can eventually lead to congestive heart failure.
What Happens Inside the Heart
The mitral valve has two thin flaps (leaflets) that open and close with every heartbeat to keep blood flowing in one direction. In a healthy dog, blood moves from the left atrium into the left ventricle, and the valve snaps shut to prevent backflow. In mitral valve disease, the leaflets undergo structural changes at a cellular level: the strong collagen layer that gives them their shape breaks down, and the tissue fills with spongy material that makes the leaflets thick, lumpy, and unable to seal tightly.
The result is mitral regurgitation, where blood leaks backward into the left atrium each time the ventricle contracts. Early on, the leak is small and the heart compensates easily. Over time, the constant extra volume stretches both the left atrium and left ventricle, a process called cardiac remodeling. In advanced stages, the supporting cords that anchor the valve leaflets can snap (chordal rupture), the atrial wall can tear, and fluid backs up into the lungs. That fluid buildup is congestive heart failure.
Breeds and Risk Factors
Small-breed dogs are far more susceptible than large breeds, and certain breeds carry dramatically higher risk. In a large study of dogs seen at primary-care veterinary practices in England, Cavalier King Charles Spaniels had roughly 47 times the odds of being diagnosed compared to the reference population. King Charles Spaniels followed at about 36 times the odds. Chihuahuas, Whippets, and Poodles also showed significantly elevated risk, at 6, 5, and 3 times the odds, respectively.
Age is the other major driver. The prevalence climbs steadily as dogs get older and can approach 100% in geriatric populations of high-risk breeds. Male dogs tend to develop clinically significant disease earlier than females. Crossbred dogs are not immune; they made up the second most frequently diagnosed group in the same study, likely reflecting how common the condition is across all small dogs as they age.
How Vets Stage the Disease
Veterinary cardiologists use a four-stage system (A through D) developed by the American College of Veterinary Internal Medicine (ACVIM) to classify how far the disease has progressed. Each stage guides treatment decisions differently.
- Stage A: Dogs at high risk (such as Cavalier King Charles Spaniels) but with no detectable murmur or structural changes yet. No treatment is needed.
- Stage B1: A heart murmur is present, but the heart has not yet enlarged. These dogs are monitored but typically not medicated.
- Stage B2: The heart has measurably enlarged. On echocardiography, this means a left atrium-to-aorta ratio of 1.6 or greater, along with enlargement of the left ventricle. On chest X-rays, the vertebral heart score exceeds 10.5 (breed-adjusted). This is the stage where medication begins.
- Stage C: The dog has developed signs of congestive heart failure, such as coughing, rapid breathing, exercise intolerance, or fluid in the lungs.
- Stage D: Heart failure that no longer responds adequately to standard medications.
The distinction between B1 and B2 matters enormously because it determines when treatment starts. Your vet will use a combination of chest X-rays, echocardiography (heart ultrasound), and physical exam findings to pinpoint which stage your dog is in.
Signs You Might Notice at Home
In the early stages, most dogs show no symptoms at all. The first clue is often a heart murmur detected during a routine exam. As the disease progresses, you may notice your dog tiring more quickly on walks, coughing (especially at night or after lying down), or breathing faster than usual while resting.
One of the most useful things you can do at home is count your dog’s resting respiratory rate. While your dog is sleeping or resting calmly, count the number of breaths in 60 seconds. A normal resting rate is generally under 30 breaths per minute. If it consistently exceeds 35 breaths per minute, that can signal fluid building up in the lungs and warrants a call to your vet. Labored breathing with visible belly movement is especially concerning. Tracking this number regularly gives you and your vet an early warning system for heart failure progression.
Medical Treatment by Stage
Dogs in Stage B2 benefit from starting a medication called pimobendan, which strengthens the heart’s contractions and dilates blood vessels to reduce the workload on the heart. The landmark EPIC trial demonstrated that starting pimobendan in dogs with enlarged hearts but no symptoms delayed the onset of heart failure by approximately 15 months compared to placebo. The median time to heart failure or cardiac death was 1,228 days in treated dogs versus 766 days in untreated dogs. That is a substantial difference in a dog’s lifespan.
Once a dog reaches Stage C and is in active heart failure, treatment expands to include a diuretic (most commonly furosemide), which helps the body clear excess fluid from the lungs. This is the cornerstone of heart failure management. Dogs typically start at a moderate dose given twice daily, with adjustments based on how they respond. Because diuretics push fluid out, they can cause side effects like dehydration, increased thirst and urination, and drops in potassium and sodium levels. Your vet will monitor bloodwork periodically to catch these imbalances early. Additional medications to manage blood pressure and reduce the heart’s workload are often layered in at this stage as well.
Dogs in Stage D, where standard treatment is no longer controlling symptoms, may need higher or more frequent diuretic doses and additional medications. The treatment goal at this point shifts toward maintaining comfort and quality of life.
What to Expect Over Time
The overall trajectory varies widely depending on when the disease is caught and how the dog responds to treatment. Many dogs live comfortably for years in the early stages with no medication at all. Dogs diagnosed at Stage B2 and started on pimobendan can often go three or more years before developing heart failure.
Once congestive heart failure develops (Stage C), the timeline shortens considerably. Most dogs with medically managed Stage C disease experience cardiac-related death within 6 to 14 months of their first heart failure episode. Some dogs do better, and some progress faster, but that range gives a realistic frame of reference. Close monitoring and medication adjustments can make those months significantly more comfortable.
Surgical Options
Open-heart mitral valve repair is a real option, though it is available only at a small number of specialized centers worldwide. The procedure involves cardiopulmonary bypass (a heart-lung machine), placement of artificial cords to support the valve leaflets, and tightening of the valve ring (annuloplasty). The results are striking: in a large study of over 1,000 dogs, surgical survival was above 95% across all disease stages, including dogs already in heart failure. The surgery essentially fixes the structural problem rather than just managing symptoms.
The major barriers are cost and access. The procedure typically costs $30,000 to $50,000 or more, requires a highly specialized surgical team, and involves a recovery period with close postoperative monitoring. For dogs who are good surgical candidates and whose owners can pursue it, the outcomes can be excellent. For the majority of dogs, medical management remains the standard approach.
Diet and Daily Life
Nutrition plays a supporting role in managing heart disease, though dietary needs shift as the disease progresses. In the early stages, a high-quality, balanced diet is generally sufficient. Once heart failure develops, moderate sodium restriction becomes more relevant. Research shows that sodium intake among dogs with heart disease varies enormously, and dogs in heart failure tend to consume more sodium than those without. Avoiding high-sodium treats and table scraps is a practical first step, though extreme sodium restriction is not recommended either, as it can reduce appetite and cause other problems.
Maintaining a healthy body weight matters. Extra weight forces the heart to pump harder, while muscle wasting (cardiac cachexia) in advanced disease saps a dog’s strength and quality of life. Gentle, regular exercise is typically fine for dogs in the early stages. As the disease advances, let your dog set the pace. Short, easy walks are better than pushing through fatigue.