A grade 3 heart murmur in dogs is a moderate-intensity abnormal sound heard through a stethoscope, sitting right in the middle of the 1-to-6 grading scale veterinarians use to rate murmur loudness. It’s loud enough to be detected immediately, but not so loud that it produces a vibration you can feel through the chest wall. A grade 3 murmur is a clinically significant finding, and veterinary cardiologists generally recommend further evaluation at this level or higher.
How the 6-Point Grading Scale Works
Veterinarians grade heart murmurs on a scale from 1 to 6 based purely on how loud they sound through a stethoscope. A grade 1 murmur is so faint that a vet needs several minutes of careful listening in a quiet room to detect it. A grade 2 is soft but noticeable within seconds. Grade 3 is where the murmur becomes moderate in intensity, clearly audible without effort.
At grade 4, the murmur is loud but still only audible. Grades 5 and 6 are loud enough to produce a “thrill,” a vibration that the vet can feel with their hand pressed against the dog’s chest. A grade 6 murmur is so intense it can still be heard even after lifting the stethoscope slightly off the body.
The grade tells you about volume, not necessarily about how serious the underlying problem is. A grade 3 murmur can be associated with a range of conditions, from mild valve leakage to something that needs closer monitoring. That’s why the grade alone doesn’t determine treatment. It’s one piece of the puzzle.
What Causes a Grade 3 Murmur
The sound itself comes from turbulent blood flow inside the heart. In a healthy heart, blood moves smoothly through the chambers and valves. When a valve doesn’t close properly, or a structure is abnormally narrow, blood swirls or leaks backward, creating the whooshing sound a vet picks up.
The most common cause in adult and older dogs is myxomatous mitral valve disease (MMVD), a progressive condition where the mitral valve (between the heart’s left chambers) gradually degenerates and allows blood to leak backward. Small breeds are especially prone to this. In puppies, a grade 3 murmur more often points to a congenital defect. Large breeds like golden retrievers, Rottweilers, and Newfoundlands are more prone to subaortic stenosis, a narrowing near the aortic valve. Small breeds and bulldogs are more prone to pulmonic stenosis, a narrowing of the valve leading to the lungs.
Less commonly, grade 3 murmurs can result from dilated cardiomyopathy (where the heart muscle weakens and stretches), heartworm disease, or anemia severe enough to change how blood flows through the heart.
Why Grade 3 Is a Key Threshold
Grade 3 is the point at which veterinary guidelines start paying closer attention. The American College of Veterinary Internal Medicine (ACVIM) uses a staging system for MMVD that ranges from Stage A (breeds at risk but no disease yet) through Stage D (end-stage heart failure). One of the criteria for Stage B2, the stage where medication may be started before symptoms appear, is a murmur intensity greater than 3 out of 6.
Stage B2 also requires imaging findings that show the heart is enlarging in response to the valve leak. Specifically, vets look at the ratio of the left atrium to the aorta on an ultrasound (above 1.6 is significant) and whether the heart appears enlarged on chest X-rays. So a grade 3 murmur alone doesn’t automatically mean your dog needs medication, but it does mean the vet will want to check whether the heart is remodeling.
This matters because there is a medication that, when started at the right time in Stage B2, delays the onset of heart failure. The decision to start it depends on those imaging measurements, not the murmur grade alone. If the chest X-ray shows a borderline heart size, echocardiography is strongly recommended to guide the decision.
What Tests to Expect
After detecting a grade 3 murmur, your vet will likely recommend an echocardiogram, which is essentially an ultrasound of the heart. This test shows how the valves are moving, whether blood is leaking backward, how large the heart chambers are, and how strongly the heart muscle is contracting. It’s noninvasive, and most dogs tolerate it without sedation or anesthesia.
Chest X-rays help assess overall heart size and check for fluid in or around the lungs, which would signal heart failure. A blood test called pro-BNP measures a protein that rises when the heart muscle is under stress. It’s useful as a monitoring tool over time, since rising levels can signal progression before symptoms appear. Repeating the echocardiogram annually is a standard approach to tracking how the condition evolves.
Signs to Watch at Home
Many dogs with a grade 3 murmur show no symptoms at all. The murmur may be found incidentally during a routine exam, and the dog may go months or years before anything changes. But knowing what to watch for helps you catch progression early.
The most telling signs include breathing problems (particularly at night or after lying down for a while), coughing, reduced willingness to exercise or play, fainting or collapsing episodes, poor appetite, unexplained weight loss, and pale gums. In puppies, a grade 3 murmur that’s causing problems may show up as stunted growth.
One of the simplest things you can do at home is count your dog’s resting respiratory rate while they’re sleeping. A consistent rate above 30 to 40 breaths per minute can be an early indicator that fluid is building up in the lungs. Tracking this number over weeks gives you and your vet useful data.
Breeds at Higher Risk
Small and toy breeds are disproportionately affected by MMVD. Cavalier King Charles spaniels are particularly well known for developing the condition, often at a younger age than other breeds. Dachshunds, miniature poodles, and Chihuahuas are also commonly affected. In these dogs, a grade 3 murmur discovered in middle age is a familiar finding.
Large and giant breeds face different risks. Golden retrievers, Rottweilers, and Newfoundlands are more susceptible to congenital narrowing near the aortic valve. Doberman pinschers, boxers, and Great Danes are predisposed to dilated cardiomyopathy. The breed context shapes what a grade 3 murmur likely means and what diagnostic path makes the most sense.
What Grade 3 Means for Your Dog’s Outlook
A grade 3 murmur is not a diagnosis on its own. It’s a physical finding that tells your vet something abnormal is happening with blood flow in the heart. Some dogs live comfortably for years with a stable grade 3 murmur and never develop heart failure. Others progress, particularly if the underlying cause is a degenerative valve disease that worsens with age.
The key factor in prognosis isn’t the murmur grade itself but what the imaging reveals. A dog with a grade 3 murmur and a normal-sized heart on echocardiogram (Stage B1) typically just needs monitoring. A dog with the same murmur grade but clear heart enlargement (Stage B2) benefits from starting treatment to delay heart failure. This is why the echocardiogram matters so much, and why veterinary cardiologists recommend it as soon as a grade 3 murmur is identified.
Early detection and regular monitoring give you the best chance of catching the disease at the point where intervention makes the biggest difference. Dogs whose treatment begins at the right stage often gain meaningful time before heart failure develops, if it develops at all.