Most dogs with a heart murmur live normal, comfortable lives for years, and many never develop symptoms at all. A heart murmur itself isn’t a disease. It’s a sound your vet hears through the stethoscope, caused by turbulent blood flow inside the heart. What matters is the underlying cause of that turbulence and whether it progresses enough to affect your dog’s health.
What Creates the Sound
Blood normally flows through the heart in smooth, silent streams. A murmur happens when something disrupts that flow, creating turbulence that vibrates enough to be heard. The three most common causes are a valve that doesn’t close properly (letting blood leak backward), a valve that’s too narrow (forcing blood through a tight opening), or an abnormal connection between heart chambers.
In some cases, especially in puppies, the murmur is “innocent,” meaning there’s no structural problem at all. These often disappear by four to five months of age as the puppy grows.
The Most Common Cause in Adult Dogs
Degenerative mitral valve disease accounts for roughly 75% of all cardiovascular disease in dogs. The mitral valve, which sits between the left atrium and left ventricle, gradually thickens and warps over time. As it loses its tight seal, blood leaks backward with each heartbeat. About 60% of affected dogs have only the mitral valve involved, while 30% also develop changes in the tricuspid valve on the right side of the heart.
The body compensates for this leak in a surprisingly effective way. The kidneys retain extra sodium and water, increasing blood volume so the heart can pump a larger total amount with each beat. Even though some blood flows backward, enough still moves forward to meet the body’s needs. This compensation is so successful that only about 30% of dogs with mitral valve disease ever develop congestive heart failure. The other 70% live out their lives with a murmur that never causes clinical problems.
Breeds at Higher Risk
Mitral valve disease eventually affects most older small-breed dogs, but certain breeds are hit harder and earlier. Cavalier King Charles Spaniels are the most notable example. The disease is 20 times more frequent in Cavaliers than in dogs overall, and virtually every Cavalier will develop it if they live long enough. While most breeds don’t see onset until their geriatric years, Cavaliers can develop murmurs as young as three years old. Even if they remain symptom-free for five years after that, they’re still only eight, leaving a long window where the disease could potentially progress.
Other commonly affected breeds include Dachshunds, Miniature Poodles, Chihuahuas, and Cocker Spaniels. Large-breed dogs develop heart murmurs too, but theirs are more often linked to a condition called dilated cardiomyopathy, where the heart muscle itself weakens and stretches.
What the Grade Means
Vets grade murmurs on a scale from 1 to 6 based purely on how loud they sound through the stethoscope:
- Grade 1: Extremely faint. Only detectable after listening intently for several minutes in a quiet room.
- Grade 2: Soft but easily heard after a few seconds.
- Grade 3: Moderate intensity, immediately obvious.
- Grade 4: Loud, but no vibration felt through the chest wall.
- Grade 5: Very loud with a vibration (called a “thrill”) you can feel by placing your hand on the chest.
- Grade 6: So loud it can still be heard even after the stethoscope is lifted off the chest, with a strong palpable thrill.
A higher grade generally means more turbulence, but the grade alone doesn’t tell you how sick your dog is or will become. A grade 3 murmur in an otherwise healthy dog with a normal-sized heart is a very different situation from a grade 3 murmur in a dog whose heart is already enlarged. That’s why further testing matters more than the number.
How Vets Determine Severity
The real question behind a murmur isn’t how loud it is but whether the heart has started to change shape in response to the extra workload. An echocardiogram (heart ultrasound) and chest X-rays are the key tools. Two measurements matter most: the ratio of the left atrium to the aorta (a value above 1.6 signals significant enlargement) and the size of the left ventricle adjusted for the dog’s weight.
Veterinary cardiologists use a staging system that ranges from A through D:
- Stage A: Dogs at high risk (like Cavaliers) but with no murmur yet. No treatment needed, just awareness.
- Stage B1: A murmur is present, but the heart hasn’t enlarged. No medication is typically recommended. Regular monitoring is all that’s needed.
- Stage B2: The murmur is present and the heart has enlarged enough to meet specific size thresholds. This is the point where starting medication can make a meaningful difference.
- Stage C: The dog has developed clinical signs of heart failure, either currently or in the past.
- Stage D: End-stage disease where symptoms persist despite aggressive treatment.
When Medication Makes a Difference
For dogs at Stage B2 (enlarged heart, no symptoms yet), a landmark clinical trial called the EPIC study showed that starting a specific heart medication extended the time before heart failure developed by about 15 months on average. Dogs in the treatment group went a median of 1,228 days before reaching heart failure or cardiac death, compared to 766 days in the placebo group. Treated dogs also lived roughly five months longer overall. This is why catching the transition from B1 to B2 through regular check-ups is so valuable: it’s the window where early intervention has the strongest evidence behind it.
Signs That a Murmur Is Progressing
Dogs in the early stages of heart disease look and act completely normal. As the disease progresses toward heart failure, the signs develop gradually. The most common ones to watch for are persistent coughing (especially in small dogs), tiring more quickly on walks, reluctance to exercise, and labored or rapid breathing even at rest.
The single most useful thing you can do at home is count your dog’s resting breathing rate. A healthy dog, whether or not they have heart disease, breathes between 15 and 30 times per minute while resting or sleeping. If that rate is consistently above 30 breaths per minute, it’s an early and reliable warning sign that fluid may be building up in or around the lungs. Count for 30 seconds and multiply by two. Doing this regularly gives you a baseline so you’ll notice a change before it becomes obvious.
Diet and Daily Life
Dogs with early-stage murmurs (B1) don’t need dietary changes or exercise restrictions. They can run, play, and eat their regular food. As the disease advances, moderate sodium restriction becomes helpful. Veterinary nutritionists recommend keeping sodium below 100 milligrams per 100 kilocalories of food for dogs with mild heart disease, dropping to below 80 milligrams per 100 kilocalories for moderate disease. In practice, this means avoiding high-sodium treats like deli meat, cheese, and many commercial dog treats, and switching to a heart-friendly diet your vet can recommend.
Exercise doesn’t need to stop entirely for most dogs with heart disease, but you should let your dog set the pace. If they slow down or seem winded, it’s time to rest. Forced high-intensity activity, like long runs alongside a bike, is worth avoiding once the heart is enlarged. Short, gentle walks remain beneficial and enjoyable for most dogs well into the later stages.
What This Means Day to Day
If your vet just told you your dog has a heart murmur, the most likely scenario is that your dog will need periodic check-ups (every 6 to 12 months for low-grade murmurs) and may eventually need an echocardiogram to check heart size. Many dogs stay at Stage B1 for years and never need medication. For those that do progress, early treatment can add over a year of healthy, symptom-free time. Keeping tabs on your dog’s breathing rate at home, staying on top of vet visits, and adjusting diet when recommended are the most impactful things you can do to keep your dog comfortable for as long as possible.