A heart murmur in dogs is an extra sound between the normal heartbeat, caused by turbulent blood flow inside the heart. It’s not a disease itself but a sign that something is disrupting the smooth flow of blood, whether that’s a leaky valve, a narrowed vessel, or a structural abnormality. Some murmurs are harmless and resolve on their own, while others signal progressive heart disease that needs monitoring or treatment.
How a Heart Murmur Works
A healthy heart produces two distinct sounds: a “lub” and a “dub,” created by valves snapping shut as blood moves through the four chambers. Blood normally flows smoothly through these chambers in one direction. When something forces blood through a narrow opening, or when a valve fails to close completely and allows blood to leak backward, the flow becomes turbulent. That turbulence creates vibrations your vet can hear through a stethoscope as a whooshing or swishing sound between the normal lub-dub.
The heart has four one-way valves designed to keep blood moving forward. If any of these valves don’t open or close properly, they disrupt the flow. Abnormal connections between blood vessels or holes between heart chambers can also create murmurs.
What Causes Murmurs in Dogs
The single most common cause is degenerative valve disease, which accounts for roughly 75% of all cardiovascular disease in dogs. The mitral valve, which sits between the left atrium and left ventricle, takes the brunt of the damage. About 60% of affected dogs have only the mitral valve involved, while 30% have both the mitral and tricuspid valves affected.
This type of valve disease is strongly tied to age and breed. Older, small-breed dogs are most commonly affected. In Cavalier King Charles Spaniels and Dachshunds, it’s an inherited condition that can appear earlier in life. Over time, the valve leaflets thicken and warp, losing their ability to seal tightly. Blood leaks backward with each heartbeat, and the heart gradually enlarges to compensate.
Other causes include congenital heart defects (problems a dog is born with), dilated cardiomyopathy (where the heart muscle weakens and stretches), heartworm disease, and conditions outside the heart like severe anemia that force blood to move faster than usual.
Innocent Murmurs in Puppies
Not every murmur means trouble. Puppies sometimes have soft, quiet murmurs that vets call “innocent” or “physiologic.” These are caused by the cardiovascular system still maturing, not by any structural problem. Most puppies outgrow these murmurs by 6 months of age. If your vet detects a soft murmur in a young puppy and isn’t concerned, a recheck in a few months is typically all that’s needed to confirm it has resolved.
How Murmurs Are Graded
Vets rate murmurs on a scale from 1 to 6, with 1 being the faintest and 6 being so loud you can feel the vibration through the chest wall. In practice, the grading is subjective. The meaningful distinctions are between soft murmurs (grades 1-2), moderately loud to loud murmurs (grades 3-4), and murmurs intense enough to produce a palpable “thrill,” a buzzing sensation your vet can feel with their fingertips on the chest (grades 5-6).
A louder murmur generally indicates more turbulence, which often correlates with more significant disease. But volume alone doesn’t tell the whole story. A grade 2 murmur in a 12-year-old Cavalier King Charles Spaniel may warrant closer investigation than a grade 3 murmur in a growing puppy.
Stages of Heart Disease
Veterinary cardiologists classify heart disease into four stages, which helps determine when treatment should begin:
- Stage A: Dogs at high risk (due to breed or family history) but with no detectable heart changes yet.
- Stage B1: A murmur is present, but the heart is still normal in size. No treatment is typically needed, just periodic monitoring.
- Stage B2: The heart has begun to enlarge. The left atrium and left ventricle are bigger than they should be, even though the dog still feels fine. This is the stage where medication may begin.
- Stage C: The dog has developed congestive heart failure. Fluid builds up in or around the lungs, causing visible symptoms.
- Stage D: Heart failure that no longer responds adequately to standard treatment.
Many dogs with murmurs live at Stage B1 for years without ever progressing. The jump from B1 to B2, and from B2 to C, is what vets track through regular checkups and imaging.
Signs to Watch For
Dogs in the early stages of heart disease often show no symptoms at all. The murmur is discovered incidentally during a routine exam. As the disease progresses and the heart struggles to pump efficiently, you may notice your dog tiring more quickly on walks, breathing faster than usual at rest, or developing a persistent cough, especially at night or after lying down. Some dogs lose their appetite or seem restless and unable to settle comfortably.
One of the most reliable things you can track at home is your dog’s sleeping respiratory rate. Count the number of breaths your dog takes per minute while deeply asleep (one rise and fall of the chest equals one breath). Establish a baseline over several nights. If that rate climbs more than 20 to 30 percent above normal over three consecutive days, or consistently exceeds 35 breaths per minute, that’s a sign fluid may be accumulating in the lungs and your dog needs prompt veterinary attention.
How Murmurs Are Diagnosed
Hearing the murmur is just the starting point. To understand what’s causing it and how far it has progressed, vets use several tools. An echocardiogram (heart ultrasound) is the gold standard. It shows the size of each heart chamber, how well the valves are functioning, and whether blood is leaking in the wrong direction. Chest X-rays reveal whether the heart has enlarged and whether fluid has started collecting in the lungs.
A blood test measuring a protein called NT-proBNP can also help gauge severity. This protein rises as the heart becomes more stressed. Values below 500 pmol/L suggest significant cardiac disease is unlikely. Levels between 500 and 1,100 may warrant further testing depending on the dog’s symptoms. Values above 1,100 are strongly associated with meaningful heart disease, and levels exceeding 1,725 suggest congestive heart failure is likely already present. Dogs with NT-proBNP above 1,500 have roughly six times the odds of developing heart failure compared to those with lower levels.
Treatment and What to Expect
Treatment depends entirely on the stage. Dogs with a murmur but a normal-sized heart (Stage B1) typically don’t need medication. They need monitoring: periodic exams, and possibly echocardiograms every 6 to 12 months to catch any changes early.
Once the heart has enlarged (Stage B2), medication can make a significant difference. A large study found that dogs with loud murmurs and heart enlargement who started a heart-strengthening medication had a 34% chance of developing heart failure within five years, compared to 56% in dogs who didn’t receive it. That translates to roughly 311 additional days free of heart failure over a five-year period. Starting treatment at the right time, before symptoms appear, is one of the most impactful decisions in managing the disease.
Dogs who progress to congestive heart failure (Stage C) need additional medications to remove excess fluid from the lungs and reduce the workload on the heart. Many dogs at this stage still enjoy a good quality of life for months to years with proper management. Stage D, where the heart no longer responds well to standard therapy, carries a more guarded outlook, but even then, adjustments to medication can provide comfort and extend time.
Living With a Heart Murmur
If your dog has been diagnosed with a murmur, the most useful thing you can do is learn their normal. Get familiar with their resting breathing rate, their typical energy level, and how they behave after exercise. These baselines make it far easier to spot subtle changes early, when intervention is most effective.
Moderate exercise is fine and even beneficial for most dogs with murmurs, unless your vet advises otherwise. You don’t need to wrap your dog in bubble wrap. Dogs with early-stage disease can live completely normal lives. Maintaining a healthy weight matters because excess body weight puts additional strain on an already compromised heart. A diet moderate in sodium may help in later stages, though extreme salt restriction isn’t necessary for most dogs with early murmurs.
Regular veterinary checkups are the backbone of managing this condition. The transition from a stable murmur to progressive disease can be gradual and invisible from the outside. Catching heart enlargement before symptoms develop gives you the best chance of keeping your dog comfortable and active for as long as possible.