Those Two Words That Stop Your Heart
"Your dog has a heart murmur." I say those words to owners regularly, and I can see the color drain from their faces every single time. A heart murmur sounds terrifying. It sounds like heart disease. It sounds like a ticking clock. And sometimes it is serious. But often, especially when caught early, it's manageable and your dog can live a full, happy life with one.
Let me break down what a heart murmur actually is, what it means for your specific dog, and what happens next.
What a Heart Murmur Actually Is
A heart murmur is a sound. That's it. It's an abnormal whooshing or swishing sound heard through a stethoscope during the heartbeat, caused by turbulent blood flow within the heart or major blood vessels. Normal blood flow is smooth and relatively quiet. When something causes that flow to become turbulent, it creates the sound we call a murmur.
A murmur is not a disease in itself. It's a finding that points toward a possible underlying condition. Some murmurs are completely benign. Others indicate structural heart problems that need management.
The Grading System
Murmurs are graded on a scale of 1 to 6:
- Grade 1: Very soft, barely audible, often intermittent. Sometimes heard only in a quiet room with a very good stethoscope.
- Grade 2: Soft but consistently audible.
- Grade 3: Moderate intensity.
- Grade 4: Loud, easily heard.
- Grade 5: Very loud, can be felt through the chest wall as a vibration (called a "thrill").
- Grade 6: Extremely loud, audible even with the stethoscope slightly off the chest.
Grade alone doesn't tell the whole story. A grade 2 murmur from a leaking heart valve can be more clinically significant than a grade 3 "innocent" murmur in a puppy. The grade tells us about the murmur's intensity, not its cause or severity.
The Most Common Causes
Myxomatous Mitral Valve Disease (MMVD)
This is the most common heart disease in dogs, particularly in small breeds (Cavalier King Charles Spaniels, Chihuahuas, Dachshunds, Miniature Poodles). The mitral valve, which separates the left atrium from the left ventricle, gradually degenerates and becomes leaky. Blood flows backward through the valve during each heartbeat, creating the murmur sound. MMVD is progressive but often slow, and many dogs live for years with a murmur before (if ever) developing clinical signs of heart failure.
Dilated Cardiomyopathy (DCM)
More common in large breeds (Dobermans, Great Danes, Boxers, Irish Wolfhounds), DCM involves the heart muscle weakening and the chambers enlarging. This can produce a murmur and eventually lead to heart failure. The progression tends to be faster than MMVD, which is why early detection and monitoring are especially important in predisposed breeds.
Congenital Defects
Some puppies are born with structural heart abnormalities (patent ductus arteriosus, pulmonic stenosis, subaortic stenosis). These are usually detected at the first puppy visits. Some require surgical intervention, while others can be monitored.
Innocent or Physiologic Murmurs
Young, growing puppies sometimes have low grade murmurs that resolve on their own as they mature. These "innocent" murmurs are caused by the dynamics of blood flow in a growing heart, not by structural disease. They're usually grade 1 to 2 and absent by six months of age.
What Happens After a Murmur Is Found
Your vet will likely recommend some or all of the following, depending on the murmur's characteristics and your dog's overall health:
Echocardiogram (Cardiac Ultrasound)
This is the gold standard for evaluating a heart murmur. An echocardiogram lets us see the heart's structure and function in real time. We can measure chamber sizes, evaluate valve function, assess the heart muscle's strength, and determine exactly what's causing the murmur. If your vet recommends an echo, this is typically done by a veterinary cardiologist.
Chest X rays
X rays show the overall size and shape of the heart and the condition of the lungs. An enlarged heart silhouette on X ray indicates that the heart is compensating for a problem. Fluid in the lungs would indicate congestive heart failure.
Blood Pressure
High blood pressure can worsen heart disease and cause a murmur on its own. Checking blood pressure is a simple, non invasive part of a cardiac workup.
Monitoring
For low grade murmurs with no clinical signs and normal imaging, monitoring at regular intervals (every 6 to 12 months) is often the recommended approach. Tracking changes over time tells us whether the condition is stable or progressing.
Treatment
Treatment depends on the cause and stage. A dog with a grade 2 murmur from early MMVD and normal heart size may not need any medication yet. A dog in congestive heart failure needs urgent medical management including diuretics, heart medications, and monitoring.
For MMVD specifically, a landmark 2019 study called the EPIC trial demonstrated that starting the medication pimobendan before the onset of heart failure (at a specific stage of disease) delayed the onset of heart failure by about 15 months. This was a game changer in veterinary cardiology. It means that monitoring and staging the disease matters, because there's a window where proactive treatment makes a significant difference.
What You Can Do at Home
- Learn to count your dog's resting respiratory rate. When your dog is sleeping, count the number of breaths in 15 seconds and multiply by 4. Normal is under 30 breaths per minute. A consistently elevated resting respiratory rate can be the first sign of fluid building up in the lungs. Many vets recommend tracking this weekly for dogs with known heart murmurs.
- Maintain a healthy weight. Extra weight means the heart has to work harder to circulate blood through more tissue.
- Keep exercise appropriate. Dogs with heart murmurs don't necessarily need to stop exercising, but avoid extreme exertion. Your cardiologist or vet can guide you on what's appropriate for your dog's specific condition.
- Monitor for signs of heart failure: coughing (especially at night or when lying down), exercise intolerance, labored breathing, fainting or collapse, distended abdomen, and loss of appetite.
Perspective
A heart murmur is not a death sentence. Many dogs live full, happy lives with murmurs. What matters is identifying the cause, staging the disease appropriately, and managing it proactively. The dogs who do best are the ones whose owners detect the murmur early, follow up with appropriate diagnostics, monitor at home, and work closely with their veterinary team. You can be that owner. Start by taking a breath (one of the things your dog's heart is working to support), and then take the next step your vet recommends.
