Mitral Valve Disease in Dogs: The Slow Leak a Stethoscope Can Catch Years Early

Here is the short version: the most common heart problem your dog can develop is a slowly leaking valve, and the first sign is usually a soft murmur your vet hears through a stethoscope long before your dog feels anything. It mostly shows up in older small-breed dogs, it can sit quietly for years, and there is now a well-run trial showing that catching it at the right moment delays heart failure by more than a year. This guide walks through what the leak is, which dogs get it, where the treatable window sits, and the one number you can track at home.

It is the most common heart disease in dogs

The condition has a mouthful of a name, myxomatous mitral valve disease, and a few plainer ones: degenerative valve disease, chronic valvular disease, or endocardiosis. Whatever you call it, it is the single most common heart disease in dogs. Veterinary references put it at roughly 75 percent of all canine cardiovascular disease.¹ It almost always begins after about age eight, and it is far more common in small breeds than large ones.²

~75%

Kittleson, Merck Veterinary Manual, 2023

The mitral valve sits between the two left chambers of the heart and is supposed to snap shut with every beat so blood moves one way, forward and out to the body. In this disease the valve tissue thickens and deforms over time, so it no longer seals. A little blood sloshes backward with each beat. That backward jet is the murmur, and it is why vets grade these murmurs on a scale and listen for them at every senior checkup.

Why a leak you cannot hear still matters

The leak is small at first, and the heart compensates well, which is exactly why dogs stay symptom-free for so long. The problem is what compensation costs over years. To keep enough blood moving forward despite the backward leak, the left side of the heart gradually enlarges. The left atrium stretches, the main pumping chamber works harder, and pressure backs up toward the lungs. When that pressure finally pushes fluid into the lungs, the dog tips into congestive heart failure, and the quiet phase is over.

So the murmur is not the emergency. It is the early-warning signal. A dog can carry a soft mitral murmur for years and feel completely normal, and plenty never progress to failure at all. The reason a murmur is worth taking seriously is that it tells you which dog to watch, and watching is what makes the treatable window findable.

Some dogs are practically built for it

Small breeds dominate, and one breed sits in a category of its own. In Cavalier King Charles Spaniels the disease is inherited and starts unusually early. Roughly half of Cavaliers have a heart murmur by age five, and nearly all surviving Cavaliers have at least a low-grade murmur by age ten.³ Dachshunds, Miniature Poodles, Chihuahuas, and other small breeds are also over-represented, and the Merck veterinary reference notes the disease behaves as an inherited trait in Cavaliers and Dachshunds in particular.¹

If you have a Cavalier, this is not a someday concern. It is a plan-for-it concern, and many cardiologists recommend starting heart screening in this breed earlier than in dogs generally.

The trial that found a window

For a long time the honest advice was to wait, because nothing had been proven to help before symptoms started. That changed in 2016 with a study called EPIC, a randomized trial of 360 dogs published in the Journal of Veterinary Internal Medicine.⁴ It asked a sharp question: if a dog's heart has already enlarged from the leak but the dog still feels fine, does starting the drug pimobendan then change anything?

It did. Dogs given pimobendan went a median of 1228 days before reaching the study's main endpoint of heart failure or cardiac death, versus 766 days for dogs on placebo.⁴ The authors summed it up as prolonging the symptom-free period by roughly 15 months, which they called a substantial clinical benefit.⁴ For a disease of older dogs, fifteen extra months of normal life is a lot.

~15 months

Boswood et al., J Vet Intern Med, 2016

The catch is that the benefit was specific to a defined stage, not to every dog with a murmur. This is where the staging system matters.

The stages, in plain language

Cardiologists use a shared map, the ACVIM consensus stages, that runs from A to D.⁵ Stage A is a dog at risk because of its breed, with a normal heart and no murmur. Stage B1 is a murmur with a heart that has not yet enlarged much. Stage B2 is a murmur with a heart that has clearly enlarged but still no symptoms. Stage C is current or past heart failure, and Stage D is end-stage disease that no longer responds to standard treatment.

The EPIC benefit lives in Stage B2, the narrow band where the heart has grown but the dog still feels fine. The consensus panel defines that stage with specific measurements: a murmur of at least 3 out of 6, a left-atrium-to-aorta ratio of 1.6 or more, a normalized left-ventricle dimension of 1.7 or more, and a vertebral heart score above 10.5.⁵ Those numbers come from an echocardiogram and a chest X-ray, which is the reason a murmur alone is not enough to start treatment. The imaging is what tells you whether the dog is in the window.

How it is diagnosed

Diagnosis is a short ladder, and most dogs do not climb all of it at once.² It starts with the stethoscope, because nearly every dog with a meaningful leak has a murmur over the left side of the chest. From there, chest X-rays show the size and shape of the heart and whether fluid is collecting in the lungs. An echocardiogram, a heart ultrasound, is the gold standard: it shows the valve itself, measures the chambers, and is what assigns the stage. A blood test called NT-proBNP can support the picture, especially when a vet is sorting out whether a coughing dog is coughing from the heart or the airways.

What it costs

The screening tests are modest. Treatment, once it is needed, is a lifelong daily medication rather than a single procedure, so the cost is spread out.

• Cardiology consult with an echocardiogram: commonly about $500 to $1,500, often near $800 to $1,200 for a full workup with a board-certified cardiologist.
• Lifelong pimobendan: often roughly $40 to $80 a month, depending on your dog's size and where you fill the prescription, with generic versions lowering that.
• Managing active heart failure: more, because it usually adds a diuretic and other medications, periodic rechecks, and sometimes an emergency visit when the disease first decompensates.

$500–$1,500

Aggregated 2024–2026 U.S. veterinary cost guidance; figures vary by region and clinic

The honest part about how long

Owners always want a number, and the honest answer is that the silent murmur stage can last years. Once a dog reaches actual heart failure, median survival on treatment is commonly cited at around 9 to 12 months, but the range is wide and individual dogs beat it regularly.⁶ Experienced cardiologists are careful here, because survival varies so much that a single firm number can mislead more than it helps. The useful takeaway is the direction of the lever, not a countdown: the earlier the disease is found and staged, the more of those good years are still ahead.

The one number to watch at home

There is a genuinely useful home test, and it costs nothing. Count your dog's breaths while it is asleep, one breath being one rise and fall of the chest, over a full minute. Healthy dogs and dogs with well-controlled heart disease almost always sleep at fewer than 30 breaths per minute.⁷ A sustained climb above 30, or a clear upward drift from your own dog's normal, is one of the earliest signs that fluid is starting to back up into the lungs, and it is a reason to call your vet promptly rather than wait for a cough.

What to do this week

1. If your dog is a senior or a small breed, make sure a vet listens to its heart at least once a year, and ask directly whether they hear a murmur and how they grade it.
2. If you have a Cavalier King Charles Spaniel, ask your vet when to begin cardiac screening, since this breed develops the disease early.
3. If a murmur has already been heard, ask whether an echocardiogram is worth doing to find out the stage, because the stage is what decides whether treatment helps now.
4. Learn your dog's normal sleeping breathing rate this week, while it is healthy, so you have a baseline to compare against later.

A leaking valve sounds alarming, and the name certainly does, but this is one of the conditions where paying quiet attention pays off. The murmur arrives early, the home test is free, and the science has found a real window. Most of the work is just noticing in time.