Heart murmurs in dogs are caused by turbulent blood flow through the heart or major blood vessels, producing a whooshing sound your veterinarian can hear through a stethoscope. The most common cause by far is degenerative valve disease, which accounts for roughly 75% of all cardiovascular disease in dogs. But murmurs can also result from birth defects, heart muscle disease, parasitic infections, or even non-cardiac conditions like anemia.
A murmur itself isn’t a disease. It’s a sign that something is affecting how blood moves through the heart. The underlying cause can range from completely harmless to life-threatening, which is why identifying the source matters more than simply knowing a murmur exists.
Degenerative Valve Disease
The single most common reason adult dogs develop heart murmurs is myxomatous mitral valve disease (MMVD). Over time, the mitral valve, which sits between the left atrium and left ventricle, degenerates. Its surface becomes bumpy and irregular, and it no longer closes completely. Each time the heart beats, blood leaks backward through the faulty valve, creating the turbulence your vet hears as a murmur.
The exact cause of this degeneration isn’t fully understood, but in Cavalier King Charles Spaniels and Dachshunds it’s a known inherited trait. Small and medium breeds are affected far more often than large breeds, and the condition typically worsens with age. Many dogs live with a low-grade murmur from valve disease for years before it progresses enough to cause symptoms like coughing, exercise intolerance, or labored breathing. In some dogs, the heart eventually enlarges to compensate for the leaking valve, and that enlargement is one of the criteria veterinary cardiologists use to decide when to start medication to delay heart failure.
Congenital Heart Defects in Puppies
When a puppy has a heart murmur, the most likely explanation is a structural defect that was present at birth. The four most common congenital heart defects in dogs are patent ductus arteriosus (PDA), subaortic stenosis, pulmonic stenosis, and ventricular septal defect.
Patent Ductus Arteriosus
Before birth, all puppies have a small vessel connecting the aorta and the pulmonary artery. This connection normally closes shortly after birth. When it stays open, blood flows abnormally between these two major vessels, creating a distinctive murmur. PDA is more common in female puppies. It’s often caught during a routine vaccination visit when the vet first listens to the puppy’s heart. Without treatment (usually a minimally invasive procedure to close the vessel), 40 to 70 percent of dogs with PDA will die within their first year.
Pulmonic Stenosis
In pulmonic stenosis, the valve between the right side of the heart and the artery leading to the lungs is abnormally narrow. Blood has to squeeze through a tighter opening than normal, which creates turbulence and a murmur. English Bulldogs, Schnauzers, Beagles, Chihuahuas, terrier breeds, Cocker Spaniels, Samoyeds, and Basset Hounds are more commonly affected. Many puppies with this condition show no symptoms at first. An echocardiogram (cardiac ultrasound) is the best way to see the narrowed valve and determine how severe it is.
Dilated Cardiomyopathy
Dilated cardiomyopathy (DCM) is a condition where the heart muscle weakens and the chambers stretch out, making the heart less effective at pumping blood. It’s recognized as a genetic condition that primarily affects large and giant breeds like Doberman Pinschers, Great Danes, and Irish Wolfhounds. Males are more commonly affected, and the disease typically appears in middle-aged to older dogs. As the heart chambers enlarge, the valves may no longer seal properly, allowing blood to leak and producing a murmur. DCM is much less common in small and medium breeds.
Heartworm Disease
Heartworms are parasites that live in the heart and pulmonary arteries. In more advanced infections, the worms physically interfere with the tricuspid valve on the right side of the heart, preventing it from closing properly. This creates a murmur that a vet can hear on the right side of the chest. In severe cases known as caval syndrome, a large mass of worms disrupts the valve apparatus enough to cause sudden, life-threatening heart failure. Heartworm-related murmurs are entirely preventable with year-round heartworm prevention.
Non-Cardiac Causes
Not every murmur points to a heart problem. Conditions outside the heart can change blood flow enough to produce an audible murmur. These “physiologic” murmurs can result from:
- Anemia: When a dog’s red blood cell count drops significantly, blood becomes thinner and flows faster, creating turbulence
- Fever: Elevated body temperature increases heart rate and blood flow velocity
- High blood pressure
- Pregnancy: Increased blood volume changes flow dynamics
- Low blood protein levels
In puppies, innocent flow murmurs are also common. These are soft, low-grade murmurs with no underlying disease. They often disappear on their own by about four to five months of age as the puppy grows. The key distinction is that physiologic murmurs resolve once the underlying condition is treated or the puppy matures, while structural murmurs persist or worsen.
How Murmurs Are Graded
Veterinarians grade murmurs on a scale of 1 to 6 based on how loud they are. A grade 1 murmur is barely audible, even in a quiet room. A grade 6 is so intense that the vet can feel the vibration (called a “thrill”) through the chest wall with their fingertips. The grade gives a rough sense of how much turbulence is present, but it doesn’t always correlate directly with severity of disease. A dog with a grade 2 murmur from a congenital defect can be in more danger than a dog with a grade 4 murmur from mild valve degeneration. That’s why further testing matters.
How the Cause Is Identified
Hearing the murmur is just the first step. Your vet will note where on the chest it’s loudest, when during the heartbeat it occurs, and how intense it is. These clues help narrow down the possibilities, but a definitive answer usually requires additional diagnostics.
An echocardiogram is the gold standard. This ultrasound of the heart lets a cardiologist watch the valves open and close in real time, measure chamber sizes, and assess how well the heart is pumping. Chest X-rays show whether the heart is enlarged and whether fluid has started accumulating in or around the lungs. Blood tests can rule out non-cardiac causes like anemia or detect markers of heart muscle stress. For heartworm, a simple blood test confirms or rules out infection.
The combination of murmur characteristics, imaging, and blood work typically gives a clear picture of both the cause and how far the condition has progressed. From there, your vet can recommend monitoring intervals or treatment based on the specific diagnosis and your dog’s stage of disease.