A grade 4 heart murmur in dogs is moderately severe. On the six-point scale veterinarians use, it sits just past the midpoint, loud enough to be heard easily with a stethoscope and often radiating across a wide area of the chest. But the grade alone doesn’t tell you how sick your dog is. What matters far more is the underlying heart condition causing the murmur and whether the heart has started to change shape or size in response.
What a Grade 4 Murmur Actually Means
Veterinarians grade murmurs on a scale from 1 (barely audible, only heard in a quiet room with careful listening) to 6 (so loud it can be heard with the stethoscope barely touching the chest). A grade 4 murmur is loud and immediately obvious, radiating beyond the specific point where it’s loudest. It does not, however, produce a “thrill,” which is a vibration you can feel by placing your hand on the dog’s chest wall. That physical vibration is the dividing line between a grade 4 and a grade 5.
A louder murmur generally means more turbulent blood flow inside the heart, which often correlates with more significant disease. But grading is somewhat subjective. It depends on the vet’s hearing, the room’s noise level, and even the dog’s body condition. A thin dog’s murmur sounds louder than the same murmur in an overweight dog with more tissue dampening the sound. That’s why a grade 4 murmur is a starting point for investigation, not a diagnosis by itself.
Common Conditions Behind a Grade 4 Murmur
In older small-breed dogs, the most common cause is myxomatous mitral valve disease (MMVD), a progressive condition where the valve between the left atrium and left ventricle thickens and degenerates, allowing blood to leak backward. Cavalier King Charles Spaniels, Dachshunds, and Chihuahuas are especially prone. A grade 4 murmur from MMVD typically means the valve leak is significant enough that the heart may already be enlarging to compensate.
In puppies and young dogs, a grade 4 murmur more often points to a congenital defect, meaning a structural problem the dog was born with. The most common include:
- Subaortic stenosis: a narrowing below the aortic valve, which can lead to heart failure, fainting, and in some cases sudden death
- Pulmonic stenosis: a narrowing of the valve leading to the lungs, which strains the right side of the heart
- Patent ductus arteriosus (PDA): a fetal blood vessel that should close at birth but stays open, overloading the left heart
- Ventricular septal defect: a hole between the two main pumping chambers
Large-breed dogs can also develop dilated cardiomyopathy, where the heart muscle weakens and stretches. This condition is more common in Dobermans, Great Danes, and Boxers, though murmurs from it tend to be softer. A grade 4 murmur in a large breed raises particular concern for subaortic stenosis.
Why the Grade Alone Doesn’t Predict Outcome
Two dogs with identical grade 4 murmurs can have completely different futures. One might live comfortably for years; another might develop heart failure within months. The difference comes down to what’s happening inside the heart, which is why imaging is essential after a grade 4 murmur is detected.
An echocardiogram (heart ultrasound) is the gold standard. It lets a veterinarian or cardiologist see the heart pumping in real time: how the valves are moving, whether chambers are enlarged, how strongly the muscle contracts, and whether blood is flowing in the wrong direction. Doppler technology maps blood flow with color, revealing leaks and abnormal jets. A chest X-ray complements this by showing the overall heart shape, measuring its size relative to the spine (called the vertebral heart score), and checking the lungs for fluid buildup that signals heart failure.
Together, these tests place your dog into a clinical stage that’s far more useful than the murmur grade.
Clinical Staging and What It Means for Your Dog
Veterinary cardiologists use a staging system developed by the American College of Veterinary Internal Medicine (ACVIM) to guide treatment decisions for dogs with MMVD. For a dog with a grade 4 murmur, the most relevant stages are B2 and C.
Stage B2 describes a dog with no outward symptoms but whose heart has already enlarged enough on imaging to meet specific thresholds. The criteria include a murmur of at least grade 3 out of 6 plus measurable enlargement of the left atrium and left ventricle on echocardiography or X-ray. This is a critical stage because treatment at B2 has been shown to delay the onset of heart failure. Dogs at this stage are typically started on a medication called pimobendan, which helps the heart contract more efficiently and relaxes blood vessels so the heart works less hard. It’s given twice daily, about an hour before meals.
Stage C means the dog has developed clinical signs of heart failure, either currently or in the past. At this point, treatment becomes more aggressive, usually adding a diuretic to remove excess fluid from the lungs. Once a dog reaches Stage C, survival time is generally 6 to 14 months, though individual variation is wide. Some dogs do well for longer with careful medication management. Stage D represents heart failure that no longer responds well to standard treatment, and the median survival drops to around nine months.
A dog with a grade 4 murmur that’s still in Stage B1, with no significant heart enlargement on imaging, may not need medication yet but should be monitored regularly.
Signs That Heart Failure Is Developing
The transition from a stable murmur to active heart failure can be subtle at first. The earliest sign is an increase in breathing rate, which most owners don’t notice because it happens gradually. A healthy dog at rest breathes fewer than 30 times per minute. If your dog’s resting or sleeping breathing rate climbs above 35 to 40 breaths per minute, especially with visible effort in the belly muscles, that’s a red flag for fluid building up in or around the lungs.
You can track this at home by counting your dog’s breaths while they’re relaxed or asleep. One rise and fall of the chest equals one breath. Count for 30 seconds and multiply by two. Doing this regularly gives you a baseline so you’ll notice changes early. Several smartphone apps designed for this purpose can help you log trends over time.
Other warning signs include reduced interest in walks or play, tiring quickly during activities that used to be easy, a new persistent cough (especially at night or after lying down), restlessness or difficulty settling, loss of appetite, and fainting or collapse episodes. Cold ears and paws can indicate the heart isn’t pumping enough blood to the extremities. Abdominal swelling suggests fluid accumulation from right-sided heart failure. Any of these changes in a dog with a known grade 4 murmur warrants a veterinary visit promptly.
What Treatment Looks Like Day to Day
For dogs at Stage B2 or beyond, medication becomes a daily routine. Pimobendan is the cornerstone, given as a chewable tablet twice a day on an empty stomach. When combined with a diuretic and sometimes an additional heart medication in dogs with more advanced disease, studies have shown improved quality of life and extended survival.
Expect rechecks every few months, with periodic X-rays or echocardiograms to track how the heart is responding. Your vet will likely adjust diuretic doses over time because heart failure is progressive and requires escalating treatment. If your dog has been on the same medication dose for a long stretch and is doing fine, that’s actually a reassuring sign that the disease may be stable.
Activity restriction is usually unnecessary in the early stages. Most cardiologists encourage dogs to stay as active as they want to be, letting the dog set its own pace. Forced intense exercise isn’t wise, but walks and play are generally fine and beneficial. Sodium-restricted diets are sometimes recommended for dogs in heart failure, though evidence on their benefit varies.
The Bottom Line on Severity
A grade 4 murmur is significant enough to take seriously but not an automatic death sentence. Many dogs with grade 4 murmurs live comfortably for years, particularly when the underlying cause is identified early and managed with appropriate medication. The murmur’s grade tells you there’s notable turbulence in the heart. The echocardiogram tells you what’s actually going on and what to do about it. If your vet has detected a grade 4 murmur, the single most important next step is cardiac imaging to determine the cause and the stage of disease, because that’s what drives every treatment decision that follows.