Heart Murmur in Dogs

A heart murmur in dogs is an abnormal sound your vet hears with a stethoscope during the heartbeat. Instead of the normal “lub-dub,” there is an added whooshing or swishing sound caused by turbulent blood flow. Murmurs are usually graded by loudness on a scale from 1 to 6, but the grade does not always match how serious the underlying disease is. A soft murmur can still matter, and a loud murmur does not always mean advanced heart failure.

A murmur is a clinical finding, not a final diagnosis. Some dogs have innocent or physiologic murmurs, especially puppies, and these may disappear as they mature. Others develop murmurs because of structural heart disease such as myxomatous mitral valve disease, dilated cardiomyopathy, pulmonic stenosis, subaortic stenosis, or patent ductus arteriosus. Murmurs can also happen when the heart itself is not the main problem, such as with anemia, fever, pregnancy, low blood protein, or high thyroid hormone levels.

In adult dogs, the most common acquired cause of a murmur is mitral valve leakage, especially in older small-breed dogs. In puppies and young dogs, congenital heart defects are a bigger concern. Because the causes vary so much, the next step after hearing a murmur is to work with your vet on a plan that fits your dog’s age, breed, symptoms, and exam findings.

Many dogs with a murmur feel normal at first. That is why routine wellness visits matter. Early detection gives your vet a chance to monitor changes over time, recommend the right tests, and discuss treatment options before breathing problems or heart failure develop.

Diagnosis starts with a full physical exam. Your vet will listen to where the murmur is loudest, when it occurs in the heartbeat, and whether there are other abnormal sounds such as arrhythmias or a gallop rhythm. They will also consider your dog’s age, breed, body condition, gum color, pulse quality, breathing pattern, and whether symptoms are present at home. This first step helps decide how urgent the workup should be.

From there, testing is chosen to answer a more important question: what is causing the murmur? Common tests include chest X-rays to look for heart enlargement or fluid in the lungs, blood pressure measurement, ECG to assess rhythm, heartworm testing when appropriate, and bloodwork to check for anemia or other non-cardiac causes. The most useful test for many dogs is an echocardiogram, which is an ultrasound of the heart. It can show valve leakage, chamber enlargement, congenital defects, and pumping function.

Not every dog needs every test on day one. A bright, symptom-free puppy with a soft murmur may need rechecks first, while an older dog with cough or fast breathing may need imaging right away. Referral to a veterinary cardiologist is often recommended when the murmur is moderate to loud, the dog has symptoms, the breed is high risk for inherited heart disease, or a congenital defect is suspected.

In practical terms, many pet parents start with an exam and chest X-rays, then move to echocardiography if the murmur persists or if symptoms suggest true heart disease. That stepwise approach can be reasonable in stable dogs, but dogs with breathing trouble, collapse, or suspected heart failure usually need a faster and more complete workup.

The causes of a heart murmur fall into two broad groups: cardiac and non-cardiac. Cardiac causes include leaking valves, narrowed valves or outflow tracts, holes between heart chambers, abnormal blood vessels, and diseases that weaken or enlarge the heart muscle. In adult dogs, myxomatous mitral valve disease is the most common acquired cause. In puppies and young dogs, congenital defects such as patent ductus arteriosus, pulmonic stenosis, subaortic stenosis, ventricular septal defect, and valve dysplasia are important possibilities.

Breed, age, and body size all matter. Older small-breed dogs are more likely to develop degenerative mitral valve disease. Larger breeds may be at higher risk for dilated cardiomyopathy, while certain breeds are overrepresented for congenital problems such as subaortic stenosis or pulmonic stenosis. A murmur in a young puppy may be innocent, but it should still be monitored because some congenital defects are first noticed this way.

Not all murmurs mean primary heart disease. Turbulent blood flow can also happen with anemia, fever, pregnancy, excitement, low blood protein, or other conditions that change blood viscosity or circulation. Heartworm disease can also contribute to cardiac changes and abnormal heart sounds in some dogs. That is one reason your vet may recommend lab work and parasite testing along with heart-focused diagnostics.

Risk goes up when a dog already has symptoms like cough, exercise intolerance, fainting, or fast breathing at rest. A newly detected murmur in a senior dog deserves attention, but so does a persistent murmur in a growing puppy. The key is not to guess from the sound alone. Your vet needs to match the murmur with the rest of the clinical picture.

Not every heart murmur can be prevented. Congenital defects are present from birth, and age-related valve disease becomes more common in many dogs as they get older. Still, there are practical steps that may lower risk or help your vet catch problems earlier. Routine wellness exams are one of the most important tools because many murmurs are first found before a dog shows any symptoms.

Keeping your dog at a healthy weight can reduce strain on the heart and lungs. Good dental care also matters because severe dental disease can contribute to bacteria entering the bloodstream, which is relevant in dogs at risk for infective endocarditis. Year-round heartworm prevention is another key step in most parts of the United States, since heartworm disease can damage the heart and lungs.

If your dog belongs to a breed with known inherited heart disease, ask your vet whether earlier screening makes sense. Breeding dogs should be screened according to breed-club and cardiology recommendations. For dogs already diagnosed with a murmur, prevention shifts toward slowing progression and avoiding crises. That may include regular follow-up, medication compliance, weight control, and watching for subtle changes in breathing or stamina.

Prevention is really about partnership. Your vet cannot stop every heart condition, but they can help you choose a monitoring plan that fits your dog’s risk level and your household budget.

Prognosis depends on the cause of the murmur, not the murmur alone. Dogs with innocent puppy murmurs often do very well and may outgrow the sound by about 16 weeks of age. Dogs with mild valve disease can remain stable for months to years before developing symptoms. Some congenital defects are highly manageable, especially when identified early and treated appropriately.

Once a murmur is linked to progressive heart disease, the outlook becomes more variable. Dogs with myxomatous mitral valve disease may live comfortably for a long time with monitoring and, when indicated, medication. Others progress to congestive heart failure and need long-term management. In some conditions, such as severe subaortic stenosis or significant rhythm disturbances, the risk of collapse or sudden death is higher.

Recovery also depends on whether the dog is currently stable, in heart failure, or recovering from an interventional procedure. A dog hospitalized for fluid in the lungs may improve quickly with oxygen and diuretics, but still need lifelong follow-up. A dog treated for a correctable congenital defect may have a much different path than one with chronic degenerative valve disease.

The most useful way to think about prognosis is in stages. Ask your vet what disease stage your dog is in now, what changes would mean progression, and what monitoring schedule makes sense. That gives you a clearer picture than murmur grade alone.