Hearing that your dog has a heart murmur is unsettling, and the first number you’re usually given — the grade — is the one that matters least for deciding what to do next.
A murmur’s grade, from 1 to 6, describes how loud it is through a stethoscope. What decides treatment is something the stethoscope can’t measure: whether the heart has started to enlarge. In the dogs where it has, a large clinical trial found that starting medication before any symptoms delayed heart failure by about 15 months. That is why the most important next step is usually a test of heart size, not a wait-and-see recheck.
What Usually Causes a Murmur in an Adult Dog
In older small and medium-sized dogs, the most common cause by far is myxomatous mitral valve disease (MMVD), also called degenerative mitral valve disease. Over years, the valve between the left chambers of the heart thickens and stops sealing properly, so blood leaks backwards with each beat. That leak is the murmur.
It is especially common in breeds such as Cavalier King Charles Spaniels, Chihuahuas, Dachshunds, Poodles, Shih Tzus and Maltese, and Cavaliers tend to develop it years earlier than other breeds. The disease usually progresses slowly, and many dogs never develop heart failure from it.
Two other situations look different:
- Puppies. Soft murmurs in young puppies are often “innocent” and fade by around four to five months of age. A murmur that persists, or a loud one at any age, is worth investigating for congenital heart disease.
- Large and giant breeds. In breeds like Dobermans, Boxers and Great Danes, the concern is more often heart muscle disease (dilated cardiomyopathy) than valve disease, and it can be present with only a soft murmur or none at all. Breed-specific screening is a separate conversation with your vet.
The rest of this page is about the common case: an adult small or medium dog with mitral valve disease.
The Grade Is Loudness, Not Severity
Murmurs are graded on a 1–6 scale:
| Grade | Roughly means |
|---|---|
| 1–2 | Soft; may need a quiet room to hear |
| 3–4 | Moderate to loud; easy to hear |
| 5–6 | Very loud; a vibration can be felt on the chest wall |
For mitral valve disease, louder murmurs do tend to go with more advanced disease, which is why grade matters as a trigger. Veterinary cardiology guidance strongly recommends an echocardiogram when a murmur is grade 3/6 or louder, to find out whether the heart has enlarged. But two dogs with the same grade can have very different hearts, and the grade alone doesn’t tell you which treatment stage your dog is in.
The Stages That Actually Drive Treatment
Veterinary cardiologists use a staging system from the American College of Veterinary Internal Medicine (ACVIM):
| Stage | What it means | Typical plan |
|---|---|---|
| A | At high risk (for example, a Cavalier) but no detectable disease | No treatment; routine checks |
| B1 | Leaky valve and murmur, heart not enlarged enough to meet treatment criteria | No heart medication; recheck heart size every 6–12 months |
| B2 | Leaky valve and heart enlarged past set measurements, no symptoms | Pimobendan recommended |
| C | Past or current heart failure | Heart failure medications, including pimobendan |
| D | Heart failure no longer responding to standard treatment | Specialist management |
The line between B1 and B2 is the decision point. It is set by measurements on an echocardiogram: the size of the left atrium compared with the aorta (LA:Ao of 1.6 or more) together with the size of the left ventricle adjusted for body weight. If an echocardiogram isn’t available, chest X-rays can be used for dogs with a murmur of grade 3/6 or louder. The guidance gives X-ray cut-offs for overall heart size and for left atrial size.
Why Treating Before Symptoms Works Here
It seems backwards to medicate a dog that looks completely well. The evidence for doing it at stage B2 comes from the EPIC trial, published in 2016 in the Journal of Veterinary Internal Medicine:
- Who: 360 dogs enrolled across 36 centers in 11 countries, aged 6 or older, weighing about 4–15 kg (9–33 lbs), each with a murmur of grade 3/6 or louder and an enlarged heart on echocardiogram and X-ray, but no signs of heart failure. Nearly half were Cavaliers.
- What: Half received pimobendan, half a placebo.
- Result: Dogs on pimobendan went a median of 1,228 days (about 41 months) before heart failure or cardiac death, against 766 days (about 25 months) on placebo — a delay of roughly 15 months. The trial was stopped early because the benefit was clear.
There is no equivalent evidence for starting pimobendan at stage B1, before the heart has enlarged. So the medication only helps in the right dogs, and an echocardiogram (or X-rays) is how you find out whether yours is one of them. Starting it too early costs money without proven benefit. Never checking can mean missing a 15-month advantage.
What It Costs
Published US price ranges:
| Item | Typical cost |
|---|---|
| Cardiology consultation, first visit | $200–$500 |
| Echocardiogram (heart ultrasound) | $300–$800 |
| Chest X-rays | $125–$350 |
| Recheck echocardiogram (stage B1, every 6–12 months) | Similar to the first echo; ask about recheck pricing |
| Pimobendan | About $30–$80 a month for small and medium dogs; more for large dogs |
| Diuretic such as furosemide (stage C) | About $10–$30 a month |
Many specialist clinics price the consultation and echocardiogram together. Pimobendan is dosed by body weight, so its monthly cost scales with the dog. A Chihuahua and a 60-lb dog on the same drug can have very different bills.
Put the numbers next to the trial result and the case for checking is strong. For a dog that turns out to be stage B2, a few hundred dollars of diagnostics leads to a medication associated with a median of about 15 extra months before heart failure. For a dog that turns out to be B1, the same test means you can skip daily medication for now and recheck on a schedule.
If cost is the obstacle, ask whether your general vet can take chest X-rays as a first step. It is the accepted alternative when echocardiography isn’t possible.
What to Watch at Home
Once a murmur has been found, the most useful thing you can do between visits takes thirty seconds a day: count your dog’s breaths while they sleep. In dogs with heart disease, a rising sleeping breathing rate is often the earliest sign that fluid is starting to build up in the lungs, before coughing or obvious distress.
- About 15–30 breaths a minute while asleep is typical.
- Consistently above about 30 is the point to recount and call your vet.
Our Sleeping Respiratory Rate Counter times the count for you and keeps a log on your phone that you can show your vet.
Also tell your vet about:
- tiring more easily on walks, or stopping to rest
- a new cough, especially at night or after lying down
- restlessness at night or difficulty settling
- fainting or brief collapse
Laboured breathing, pale or bluish gums, or collapse are emergencies. Go to an emergency vet rather than waiting for an appointment.
Frequently Asked Questions
My dog has a grade 3 heart murmur. What happens next? Ask about checking the heart’s size. Veterinary cardiology guidance strongly recommends an echocardiogram (heart ultrasound) when a murmur is grade 3 out of 6 or louder, because the treatment decision depends on whether the heart is enlarged, not on the grade itself. If an echocardiogram isn’t possible, chest X-rays measuring heart size are an accepted alternative for dogs with a murmur of that loudness.
When should a dog with a heart murmur start pimobendan? Under the ACVIM staging system used by veterinary cardiologists, pimobendan is recommended once mitral valve disease has enlarged the heart past set measurements, called stage B2, even if the dog has no symptoms. In the EPIC trial, dogs at this stage given pimobendan went a median of about 15 months longer before heart failure or cardiac death than dogs given a placebo. There is no equivalent evidence for starting it earlier, in stage B1, when the heart is not yet enlarged.
How much does it cost to check a dog’s heart murmur? Published price ranges are roughly $200–$500 for a first cardiology consultation, $300–$800 for an echocardiogram, and $125–$350 for chest X-rays. Many clinics bundle the consultation and echocardiogram. If heart medication is started, pimobendan typically costs about $30–$80 a month for small and medium dogs and more for large dogs, because it is dosed by weight.
How long can a dog live with a heart murmur? It depends on the stage, not the murmur. Many dogs with mitral valve disease and a normal-sized heart live for years without ever developing heart failure. In the EPIC trial, which studied dogs whose hearts were already enlarged, the dogs given pimobendan went a median of about 41 months before heart failure or cardiac death. Once heart failure develops, outlook depends heavily on how well it responds to treatment.
What should I watch for at home if my dog has a murmur? The single most useful home check is the sleeping breathing rate: count breaths while your dog is asleep, and contact your vet if it stays consistently above about 30 a minute. Also watch for tiring more easily on walks, a new cough, restlessness at night, or fainting. Laboured breathing, pale or bluish gums or collapse are emergencies.
Related Articles
- Sleeping Respiratory Rate Counter — the daily home check, with a log for your vet
- Cat Heart Murmur: What It Does and Doesn’t Mean — why cats are a different story
- How Often Should You Take Your Dog to the Vet? — where murmurs usually get picked up
- Senior Pet Care Checklist