A heart murmur in a dog can range from completely harmless to a sign of serious heart disease, depending on the underlying cause, the dog’s age, and whether the heart is physically changing shape. Many dogs live full, normal lives with a murmur and never need treatment. Others have a murmur that signals progressive valve disease requiring medication and monitoring. The murmur itself is just a sound, not a diagnosis, so what matters most is what’s causing it.
What a Heart Murmur Actually Is
A heart murmur is turbulent blood flow that a vet can hear through a stethoscope. In a healthy heart, blood moves smoothly and quietly through the chambers and valves. When something disrupts that flow, whether a leaky valve, a narrowed opening, or simply thin blood in a growing puppy, it creates an audible whooshing or swishing sound. The murmur is a clue, not a condition. Two dogs with identical-sounding murmurs can have completely different outlooks.
Murmur Grades: What the Numbers Mean
Vets grade murmurs on a scale of 1 to 6 based on how loud they are:
- Grade 1: Barely audible, only detectable in a quiet room
- Grade 2: Faint but easy to hear, limited to one spot on the chest
- Grade 3: Immediately audible when the stethoscope is placed in the right spot
- Grade 4: Loud and immediately obvious, but no vibration felt through the chest wall
- Grade 5: Very loud with a vibration (called a “thrill”) the vet can feel with their hand
- Grade 6: So loud it can be heard even with the stethoscope lifted slightly off the chest
Here’s the important nuance: loudness doesn’t reliably predict how sick the heart is. A grade 1 or 2 murmur usually means either no heart disease or very mild disease. Grades 3 and 4 are genuinely not predictive of severity at all. A very loud murmur (grade 5 or 6) is commonly associated with significant heart disease, but not always. The grading is also subjective, and vets themselves note that distinguishing between adjacent grades is difficult. So if your vet says “grade 3,” that alone doesn’t tell you whether your dog is in trouble. Further testing does.
Innocent Murmurs in Puppies
If your puppy was found to have a soft murmur, there’s a good chance it’s completely benign. Innocent murmurs are common in young, growing dogs. They tend to be soft (no louder than grade 2), occur during the part of the heartbeat when the heart contracts, and often have a musical quality. As puppies grow and their blood thickens with rising red blood cell counts, these murmurs typically disappear on their own.
That said, puppies can also be born with structural heart defects. If the murmur is louder than a grade 2, persists beyond a few months, or your puppy shows signs like exercise intolerance or slow growth, your vet will likely recommend further evaluation to rule out a congenital problem. Breeds like Bull Terriers, Dalmatians, German Shepherds, Golden Retrievers, Great Danes, Mastiffs, Newfoundlands, and Rottweilers carry higher risk for certain congenital valve abnormalities.
The Most Common Cause in Adult Dogs
In adult and older dogs, the most frequent reason for a heart murmur is myxomatous mitral valve disease (MMVD), a progressive degeneration of the valve that separates the left atrium from the left ventricle. Over time, the valve becomes thickened and misshapen, allowing blood to leak backward with each heartbeat. That backward flow is the murmur your vet hears.
MMVD is especially common in small and medium breeds. Cavalier King Charles Spaniels have a known genetic predisposition, and many develop murmurs by middle age. Larger breeds, meanwhile, are more prone to cardiomyopathy, a disease of the heart muscle itself that weakens the heart’s ability to pump effectively. Both conditions produce murmurs, but they progress differently and require different management.
The critical question with MMVD isn’t whether the murmur exists. It’s whether the leaky valve has forced the heart to enlarge. A dog can have a murmur for years with no heart enlargement and need nothing more than periodic checkups.
How Vets Determine the Real Severity
Veterinary cardiologists use a staging system that goes well beyond murmur grade:
- Stage A: Dogs at high risk (certain breeds) but with no murmur and no structural changes yet. No treatment needed.
- Stage B1: A murmur is present and there’s evidence of valve disease, but the heart hasn’t enlarged. These dogs feel fine, act normal, and don’t need medication. They need monitoring.
- Stage B2: The valve disease has become severe enough that the heart’s left side has noticeably enlarged on imaging. This is the stage where medication becomes beneficial, even though the dog may still seem perfectly healthy.
- Stage C: The dog has developed or is currently experiencing congestive heart failure, with symptoms like coughing, heavy breathing, reduced energy, or fluid buildup.
The distinction between B1 and B2 is one of the most important in canine cardiology, because it determines whether your dog needs to start treatment now or simply continue with regular rechecks.
Echocardiography (an ultrasound of the heart) is the gold standard for making this distinction. It lets the cardiologist see the valves, measure chamber sizes, and assess how blood is flowing. Chest X-rays help too, showing whether the heart silhouette has grown or if fluid is accumulating in the lungs. In clinics where echocardiography isn’t available, a blood test measuring a protein called NT-proBNP can help gauge severity. Elevated levels of this protein correlate with more advanced disease, though the test works best as a screening tool rather than a replacement for imaging.
What Treatment Looks Like
Dogs at Stage B1 typically don’t take any medication. Your vet will recommend rechecks, usually every 6 to 12 months, to watch for heart enlargement.
At Stage B2, a landmark clinical trial (the EPIC study) showed that starting a heart-supporting medication in dogs with enlarged hearts but no symptoms yet delayed the onset of heart failure by roughly 15 months. Dogs receiving the medication reached heart failure at a median of about 1,228 days, compared to 766 days in dogs that received a placebo. That’s a meaningful difference in a dog’s life. The medication reduced the risk of progressing to heart failure by about 36%.
Once a dog reaches Stage C, treatment typically involves a combination of medications to remove excess fluid, reduce the heart’s workload, and support its pumping ability. Many dogs stabilize well on medication and can maintain good quality of life for months to years, depending on how advanced the disease is when treatment begins.
Monitoring at Home
One of the most practical things you can do if your dog has a heart murmur is learn to count their sleeping respiratory rate. This is the number of breaths your dog takes per minute while fully asleep, not just resting. A healthy dog or a dog with well-controlled heart disease will typically breathe fewer than 30 times per minute during sleep.
To count it, watch your dog’s chest rise and fall while they’re sleeping. Count for 30 seconds and multiply by two. Do this a few times a week to establish a baseline. If the rate consistently climbs above 30 breaths per minute, or if you notice a sudden jump from your dog’s normal pattern, it can be an early sign that fluid is building up in the lungs, sometimes days before other symptoms appear. This simple check gives you a head start on getting your dog evaluated before a crisis develops.
Signs That Warrant Prompt Attention
A murmur alone, especially a low-grade one discovered at a routine exam, rarely requires emergency action. But certain symptoms alongside a known murmur suggest the heart may be struggling: persistent coughing (particularly at night or after lying down), rapid or labored breathing at rest, reluctance to exercise or tiring much faster than usual, fainting or collapsing, a swollen belly from fluid accumulation, or a bluish tint to the gums or tongue. Any of these paired with a known murmur points toward possible heart failure and should be evaluated quickly.
Dogs without any of these signs, even those with grade 3 or 4 murmurs, often live comfortably for years with appropriate monitoring. The seriousness of a heart murmur is almost never about the murmur itself. It’s about what the heart looks like on the inside and whether it’s beginning to fail.