Heart Murmur in Dogs: Symptoms, Stages, and What To Expect

A heart murmur in dogs is a sound, not a diagnosis. It is simply an extra whooshing noise your veterinarian hears between the normal beats of your dog's heart, usually picked up during a routine physical exam. Hearing those words at a checkup can be unsettling, but a murmur on its own does not tell you how serious, if at all, the underlying cause is. Many dogs live full, active lives for years with one.

If your dog's vet mentioned a murmur, you are probably trying to figure out what it means for your dog's future. The honest answer is that it depends: a murmur is a clue, not a verdict. The same soft whooshing sound can point to something as minor as a growing puppy's cardiovascular system catching up with a growth spurt, or to a condition that needs ongoing monitoring for years. The grading, the type of murmur, and your dog's age and breed all shape what your vet recommends next.

What a Heart Murmur Actually Is

A heart murmur is an extra sound a stethoscope picks up between the normal "lub-dub" of a heartbeat. It happens when blood moves through the heart with turbulence instead of a smooth, quiet flow, similar to water rushing through a narrowed section of pipe instead of a wide-open one.

Blood normally moves through the heart in an orderly loop: into the left atrium, down into the left ventricle, and out to the body, with the heart valves opening and closing to keep everything moving one way. A murmur is the sound of that blood flow turning turbulent. Either a heart valve is not sealing cleanly, a chamber or vessel is narrowed, or the blood is thinner than usual, and the turbulence becomes audible.

Vets also note when the sound occurs in the heartbeat. Systolic murmurs happen while the heart contracts and are by far the most common in dogs. Diastolic murmurs happen while the heart relaxes and refills, and they are uncommon enough that hearing one usually prompts a referral. A continuous murmur, audible through both phases, points toward a specific congenital problem covered below.

A murmur by itself is not a disease. It is a physical exam finding, just as an elevated heart rate or a fever is. Some murmurs are harmless quirks of a growing puppy's heart, while others are the first sign of a structural change that a vet will want to track over time. The murmur tells your vet where to look next, not what they will find.

How Vets Grade a Heart Murmur in Dogs

Once a vet hears a murmur, the next step is grading its loudness on a scale of Grade I through Grade VI:

• Grade I to II: soft, and only heard in a quiet room with focused listening
• Grade III: moderate, and easy to hear as soon as the stethoscope touches the chest
• Grade IV: loud, and able to radiate to other areas of the chest
• Grade V to VI: very loud, sometimes strong enough to feel as a vibration, called a thrill, with a hand on the chest wall

Here is the part that surprises a lot of pet parents: a higher grade does not automatically mean more serious heart disease. Grade reflects how much turbulence a vet can hear, not how advanced the underlying condition is. A dog with a Grade II murmur from meaningful valve disease and a dog with a Grade V murmur from a well-compensated leaky valve can both be doing fine day to day. Grade is one data point your vet weighs alongside breed, age, and any symptoms at home, not a standalone severity score.

Innocent Murmurs in Puppies

Puppies, especially fast-growing large breeds, often develop a soft murmur around 6 to 8 weeks of age as their cardiovascular system catches up with a growth spurt. These are called innocent or physiologic murmurs, and they commonly resolve on their own within the first four to six months of life as the heart finishes maturing.

If a puppy's murmur has not faded by the time they are done growing, or if it is graded higher than a II, a vet will usually recommend an echocardiogram to rule out a structural cause rather than assume it will resolve on its own.

Congenital Heart Defects Behind a Puppy's Murmur

When a murmur does not fade on schedule, the question becomes whether a congenital heart defect is behind it. These are structural problems a dog is born with, and a handful turn up often enough that a veterinary cardiologist looks for them by name:

• Patent ductus arteriosus (PDA): a fetal blood vessel that should close soon after birth stays open, letting blood flow the wrong way between two major arteries. This is the classic cause of a continuous murmur.
• Subaortic stenosis: a narrowing just below the aortic valve, which makes the left ventricle work harder to push blood past the obstruction.
• Pulmonic stenosis: the same idea on the right side of the heart, a narrowing at the pulmonic valve that strains the right ventricle.
• Ventricular septal defect: an opening in the wall between the lower heart chambers, letting blood cross where it should not.

Each of these creates the turbulence your vet is hearing. Some are managed medically and monitored, while others can be corrected with a procedure; PDA, in particular, has considerably better outcomes when it is caught early. That is why a persistent puppy murmur warrants a proper cardiac evaluation rather than another six months of waiting.

Common Causes of Acquired Murmurs in Adult and Senior Dogs

When a murmur shows up later in life instead of during puppyhood, it is called an acquired murmur, and the likely cause often tracks with a dog's size.

In small and toy breeds, especially as they move into middle age and beyond, the most common cause by a wide margin is myxomatous mitral valve disease (MMVD), also called degenerative mitral valve disease or chronic valve disease, a gradual thickening and degeneration of the mitral valve, the flap between the heart's left chambers. It accounts for roughly three out of every four cases of canine heart disease, and it shows up especially often in Cavalier King Charles Spaniels and Dachshunds.

In large and giant breeds, a murmur is more often linked to dilated cardiomyopathy (DCM), a disease in which the heart muscle itself weakens and the chambers stretch out, pumping less efficiently. Breeds like Doberman Pinschers, Great Danes, and Irish Wolfhounds are more commonly affected.

Less commonly, a murmur can also show up secondary to something other than a degenerating valve, such as anemia, fever, pregnancy, or heartworm disease. Non-cardiac murmurs are sometimes called functional murmurs, and they tend to fade once the underlying issue is treated.

The Stages of Mitral Valve Disease

For dogs diagnosed with MMVD, veterinary cardiologists use a staging system from the American College of Veterinary Internal Medicine (ACVIM) to describe how far the disease has progressed:

• Stage A: A dog at breed-related risk, with a normal heart and no murmur yet
• Stage B1: A murmur is present, but the heart is still a normal size on imaging
• Stage B2: A murmur is present, and the heart has started to enlarge on X-ray or echocardiogram, even though the dog shows no outward symptoms
• Stage C: The dog has current or past signs of congestive heart failure, such as coughing, labored breathing, or reduced stamina
• Stage D: Heart failure that no longer responds well to standard medications

A dog can sit comfortably in Stage B1 for years with nothing more than a periodic recheck. Movement between stages, not the murmur grade by itself, is what your vet is actually tracking over time.

Grain-Free Diets and DCM: What We Actually Know

You may have come across headlines linking grain-free dog food to DCM. The FDA's Center for Veterinary Medicine opened an investigation into a possible connection between certain diets and cases of non-hereditary DCM, after receiving reports that often involved legume-heavy, grain-free formulas. Its most recent public findings describe the issue as complex and possibly involving multiple factors, and the agency has been clear that it has not established a direct causal relationship between grain-free food and DCM.

In other words, the investigation is real and worth knowing about, but "grain-free food causes heart disease" oversimplifies where the science currently stands, and so does "grain-free food has been proven safe." If your dog eats a grain-free or legume-heavy diet and you are concerned, the most useful step is a conversation with your veterinarian about your dog's specific diet history, not a diet swap based on a headline.

Symptoms That Mean It Is Time to Call the Vet

A murmur alone rarely causes symptoms you would notice at home. What matters more is watching for signs that the heart itself is starting to struggle, including:

• A new or worsening cough, especially at night or after lying down
• Rapid or labored breathing, even at rest
• Tiring more easily on walks or play sessions your dog used to handle with ease
• Fainting or collapsing episodes
• Pale gums, or gums with a bluish tinge

None of these symptoms confirm heart disease on their own, but any of them is worth a same-week call to your vet, and labored breathing at rest or a collapsing episode warrants urgent care the same day.

What This Means for Your Dog

If your dog was just diagnosed with a murmur, the most useful next step is usually an echocardiogram, which gives your vet an actual picture of the heart's structure instead of just its sound. Depending on what they hear, that may sit alongside other diagnostic tests: chest X-rays to assess heart size and check the lungs for fluid, a blood pressure measurement, and routine bloodwork. Together, those results separate a murmur that simply needs monitoring from one that needs treatment now. From there, most dogs settle into a monitoring routine built around periodic rechecks, a comfortable weight, and regular exercise unless your vet advises otherwise.

Many dogs found to have an early-stage murmur are otherwise ordinary, happy pets who need the same everyday care every dog benefits from as they get older, including a healthy weight and consistent vet visits. As your dog ages, staying on top of routine wellness exams is what catches most murmurs while they are still easy to track, long before any symptoms would show up at home.

A heart murmur diagnosis is not a sentence; it is information. Most dogs found to have one, whether it is the physiologic kind in a puppy or an early-stage MMVD murmur in an older dog, go on to live comfortably for a long time with nothing more than a vet keeping an eye on things. Under the Weather formulates its everyday digestive and wellness products in Vermont with veterinarians, but a heart finding like this is squarely a conversation for your vet, not something any supplement is designed to address. Keep up with routine visits, and let your vet's exam guide what happens next.