Your dog went in for a booster or a nail trim, the vet listened to the chest for a few seconds longer than usual, and then said the words heart murmur. Nothing about your dog changed on the drive home, and that is worth holding onto: a murmur is a sound, not a diagnosis, and many dogs carry one for years while living completely normal lives. What it does mean is that the next step is looking at the heart rather than just listening to it.

What a murmur actually is

Blood moving smoothly through a healthy heart is almost silent. When it becomes turbulent, usually because a valve is leaking, that turbulence makes an audible whoosh between the normal heart sounds. That whoosh is the murmur.

Vets grade murmurs from 1 to 6 by how loud they are. Grade 1 is barely detectable in a quiet room; grade 5 and 6 are loud enough to feel through the chest wall as a vibration. Loudness is a rough guide, not a verdict. A grade 5 murmur can belong to a dog with a normal-sized heart, and a dog with significant disease can have a surprisingly quiet one, particularly once the heart is struggling.

Why small dogs get them so often

The overwhelming cause in older small dogs is myxomatous mitral valve disease, sometimes written as MMVD or degenerative valve disease. The mitral valve sits between the two left-sided chambers. Over years, its leaflets thicken and become nodular, they stop sealing cleanly, and blood leaks backwards with every beat.

Cavalier King Charles Spaniels are the classic example, with a strong genetic predisposition that can show up years earlier than in other breeds. Dachshunds, Chihuahuas, poodles, terriers, and most small-breed crossbreeds are also over-represented. It is a slow disease. The gap between the first faint murmur and any real trouble is frequently measured in years.

Not every murmur is valve disease, which is why the workup matters:

  • Innocent puppy murmurs that fade as the heart grows, usually gone by four to six months.
  • Congenital defects in young dogs, such as a persistent duct or a narrowed outflow, which behave very differently from valve disease.
  • Anaemia, fever, or being very thin, all of which can make blood flow turbulent without any structural heart problem.
  • Dilated cardiomyopathy in large breeds, where the muscle rather than the valve is the problem.

What the imaging adds

Chest X-rays answer the practical questions: how big is the heart, has the left atrium enlarged, are the lung vessels distended, and is there fluid in the lungs. Vets often measure the vertebral heart score, which compares heart size against the dog's own spine so that breed differences do not skew the result. X-rays are also the fastest way to tell whether a breathless dog is in heart failure or has an airway problem instead.

An echocardiogram shows the valve moving in real time, measures the chambers directly, and quantifies how much blood is leaking backwards. It is the most accurate way to confirm the diagnosis and, more importantly, to decide whether it is time to start treatment. The same technique is what confirms the very different heart disease that affects cats, described in our article on feline HCM on echocardiogram.

The staging system your vet is using

Veterinary cardiologists group valve disease into stages, and knowing which one applies to your dog explains almost every recommendation you will hear.

  • Stage B1. A murmur is present, the heart is still a normal size. No medication. Recheck imaging in roughly six to twelve months.
  • Stage B2. The heart and left atrium have measurably enlarged, but your dog still feels fine. This is the decisive stage.
  • Stage C. Congestive heart failure has occurred, meaning fluid has built up in the lungs. Treated, and often controlled well for a long time.
  • Stage D. Heart failure that no longer responds to standard doses and needs a more intensive plan.

Stage B2 gets the attention because of what a large international trial demonstrated: starting pimobendan once the heart has enlarged, but before any symptoms appear, delayed the onset of heart failure by around 15 months on average. That is a substantial stretch of good life, and it is only available to dogs whose enlargement is caught while they still seem completely well. It is also the reason your vet may want imaging on a dog who, to you, has nothing wrong with him.

What to watch for at home

The single most useful thing you can do is count your dog's sleeping breathing rate. Wait until your dog is genuinely asleep, watch the chest rise and fall for 30 seconds, and double it. Most healthy dogs sit under 30 breaths per minute and many are closer to 15 or 20. Write the number down once or twice a week so you know your own dog's baseline.

Call your vet promptly if you see:

  • A sleeping breathing rate that climbs consistently above 30, or a clear rise from your dog's usual number
  • Faster or more effortful breathing at rest, or a belly that heaves with each breath
  • Restlessness at night, or refusing to settle lying down
  • Tiring quickly on familiar walks, or stopping to sit
  • Fainting or collapse, which always warrants a same-day call
  • A cough that is new, moist, or paired with any of the above

On that last point, be careful with cough alone. The small breeds most affected by valve disease are also the breeds most prone to collapsing trachea and chronic bronchitis, which produce a dry honking cough that has nothing to do with the heart. Dogs in heart failure usually breathe fast rather than cough dramatically. Cats confuse this further, since their breathing problems are more often airway-related, as described in our piece on feline asthma on chest X-ray.

Treatment, in plain terms

In stage B1 the treatment is patience and rechecks. There is no evidence that starting drugs before the heart enlarges helps, and it exposes a well dog to medication for no gain.

In stage B2, pimobendan is the mainstay. It helps the heart contract more efficiently and eases the load on it. Some cardiologists add other medication depending on the individual case.

In stage C, furosemide or torasemide is added to clear fluid from the lungs, usually alongside pimobendan and an ACE inhibitor, and often spironolactone. Most dogs improve dramatically within a day or two of the first dose. A moderately salt-restricted diet is generally advised, and any dog on diuretics needs periodic kidney and electrolyte checks.

Surgical mitral valve repair now exists at a small number of specialist centres worldwide and has good results in selected dogs, but it is expensive and not widely available. For most dogs, medical management remains the plan.

Why a second read can help

Much of this hinges on measurements taken from images: heart size on a radiograph, chamber dimensions on an echo, whether that hazy lung pattern is fluid or an old airway change. Those judgements can differ between readers, and a borderline call is exactly what decides whether your dog starts medication now or in a year. If your dog's X-rays or scan report sit in that grey zone, an independent radiology read is a reasonable thing to ask for. DocOrbit lets your vet or you upload the images and receive an expert second opinion to bring back into the conversation with your own veterinary team.

Is a heart murmur in a dog always serious?

No. Plenty of dogs live for years with a soft murmur and never develop heart failure, and some murmurs are not caused by valve disease at all. Puppies can have innocent murmurs that disappear as they grow, and anaemia or fever can produce one temporarily. What matters is not the sound itself but what is happening to the heart behind it, which is why your vet will want imaging rather than repeated listening.

How long can a dog live with a heart murmur?

Often years. Many small dogs are diagnosed in middle age and stay entirely well for a long time before anything changes, and some never progress to heart failure at all. Once congestive heart failure does develop, the outlook is measured in months to a couple of years with good treatment, but reaching that point can take a very long time. Regular rechecks are what keep the timeline in your favour.

Does a dog with a heart murmur need medication right away?

Not always, and this is where the imaging matters. If the heart is still normal in size, no medication is recommended and the plan is monitoring. If the scan shows the heart and left atrium have started to enlarge, starting pimobendan at that point has been shown to delay the onset of heart failure by roughly 15 months on average. Treating too early has no proven benefit, and treating too late loses that window.

Is coughing always a sign of heart failure in dogs?

No, and this trips up many owners. In the small breeds that get mitral valve disease, chronic airway problems such as a collapsing trachea or bronchitis are extremely common, and they cause a honking, dry cough that has nothing to do with the heart. Heart failure in dogs tends to show up as fast or laboured breathing rather than as coughing alone. A chest X-ray usually separates the two.

What is a normal sleeping breathing rate for a dog?

Count the breaths while your dog is fully asleep, watching the chest rise and fall for 30 seconds and doubling it. Most healthy dogs sit comfortably under 30 breaths per minute, and many are around 15 to 20. A dog with known valve disease that starts consistently sleeping above 30, or whose usual number climbs over several days, should be seen promptly. It is the single most useful thing an owner can track at home.

Does my dog need an echocardiogram or is an X-ray enough?

They answer different questions. The X-ray shows the size and shape of the heart and whether fluid is building in the lungs, which is what decides whether a breathless dog is in heart failure. The echocardiogram shows the valve itself and measures the chambers directly, which is the most accurate way to confirm mitral valve disease and to decide whether treatment should start. Many dogs have both, often the X-ray first.

Key takeaways

  • A murmur is a sound caused by turbulent blood flow, and its loudness does not reliably predict how serious the underlying disease is.
  • In older small breeds the usual cause is degenerative mitral valve disease, which typically progresses slowly over years.
  • X-rays and an echocardiogram answer different questions; together they decide which stage your dog is in.
  • Medication starts when the heart enlarges, not when the murmur appears, and starting at that point buys meaningful extra time.
  • A sleeping breathing rate consistently above 30 is the home sign that should prompt a call.

This article is for general information only and is not veterinary advice. Always discuss your animal's imaging results and next steps with a qualified veterinarian.