Heart Murmur in Dogs: Symptoms, Grades, and When to Worry

Name: Heart Murmur in Dogs: Symptoms, Grades, and When to Worry
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-03

A heart murmur in dogs is an extra sound your veterinarian hears through a stethoscope, usually a whoosh between the normal "lub" and "dub." Many murmurs cause no symptoms at all, especially early on, while others signal heart disease that needs monitoring or treatment. The murmur's grade, your dog's symptoms, and a few tests together tell your vet how much it matters.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

A murmur is a sound, not a diagnosis. It means blood is flowing in a turbulent way somewhere in the heart or nearby vessels.
Grades run from 1 (barely audible) to 6 (heard with the stethoscope barely touching the chest). Higher grades often mean more turbulence, but grade alone does not equal severity.
Many dogs with a low-grade murmur and no symptoms live normal lives with routine checkups.
Symptoms that matter most are coughing, fainting, exercise intolerance, fast or labored breathing, and a swollen belly.
Any dog with a new murmur should see a veterinarian. A cardiologist or an echocardiogram gives the clearest picture of what is happening.

What a Heart Murmur in Dogs Sounds Like and What Is Normal

A normal heartbeat has two main sounds, often described as "lub-dub." These are the valves closing as blood moves through the four chambers of the heart. The right side pumps blood to the lungs, and the left side pumps blood to the rest of the body [1].

A murmur is extra noise, usually a soft whoosh or swish, created when blood flow becomes turbulent. Some murmurs are innocent and harmless. Others are the first clue that a valve is leaking, a valve is narrowed, or there is an abnormal opening between chambers.

You cannot hear a murmur at home. Your vet hears it with a stethoscope during a physical exam. That is one reason yearly wellness visits matter, since a murmur often shows up before your dog acts sick.

The Grade Scale

Veterinarians grade murmurs from 1 to 6 based on how loud they are.

Grade 1: very soft, heard only in a quiet room with careful listening.
Grade 2: soft but easy to hear once you know where to listen.
Grade 3: moderate, heard immediately.
Grade 4: loud, but you still need the stethoscope to hear it.
Grade 5: very loud, and you may feel a vibration (a thrill) on the chest wall.
Grade 6: loudest, heard with the stethoscope barely touching the chest.

A grade 2 murmur can be serious, and a grade 4 murmur can sometimes be tolerated well for years. The grade is one piece of information, not the whole story. A study of 756 dogs found that murmur intensity is a sensitive indicator of several heart diseases, particularly myxomatous mitral valve disease, but accurate interpretation requires real clinical expertise [2].

What Is Normal

It is normal for a dog's heart rate to rise with excitement and slow at rest. Sinus arrhythmia, a rhythm that speeds up and slows down with breathing, is common and usually harmless [3]. What is not normal is a murmur that was not there before, a murmur that gets louder over time, or a murmur plus symptoms.

Main Causes of Heart Murmurs in Dogs

Murmurs fall into two broad groups. Some are harmless, often called innocent or physiologic murmurs, and are common in young puppies and in some athletic dogs. Others come from structural heart disease. Slightly more than 10% of animals examined by a veterinarian have some form of cardiovascular disease [4].

Cause	Typical clues	How urgent
Myxomatous mitral valve disease (MMVD)	Older small-breed dog, murmur loudest on the left side, often no symptoms early	See a vet soon. Monitor regularly, treat when the heart enlarges or symptoms start
Dilated cardiomyopathy	Larger breeds, weak or irregular heartbeat, exercise intolerance	See a vet soon. Can progress to heart failure
Congenital defects (present at birth)	Young dog, murmur found at a puppy visit, sometimes poor growth	See a vet soon. Some defects need surgery or procedures
Aortic or pulmonic stenosis	Young dog, murmur loudest at the heart base, possible fainting	See a vet soon. Severity decides the plan
Valve infections or unusual structural problems	New murmur in any age dog, sometimes fever or lethargy	See a vet promptly. These can worsen quickly
Innocent (physiologic) murmur	Puppy or young dog, soft grade 1 to 2, no symptoms, normal heart size	Routine visit. Often disappears with age

MMVD is the most common heart disease in dogs [5]. It causes the mitral valve to thicken and leak, so blood flows backward into the left atrium. Over time this can enlarge the heart and lead to congestive heart failure.

Congenital problems show up in young dogs. One six-week-old Labrador puppy with lethargy and a systolic murmur was found to have tricuspid valve dysplasia plus pulmonic stenosis [6]. Unusual structural issues can appear at any age. A 12-year-old dog with a soft grade 2 diastolic murmur was found to have a cyst on the aortic valve, and the dog stayed symptom-free eight months later [7]. A 10-year-old Shih Tzu with a continuous grade 4 murmur had a ruptured sinus of Valsalva aneurysm [8]. These are uncommon, but they show why imaging matters.

Heart Murmur in Dogs Symptoms: What Signals Progression

Early murmurs often cause no symptoms. That is the tricky part. Your dog may feel completely fine while the heart is quietly changing.

Symptoms usually appear when the heart can no longer keep up. Watch for these:

Coughing, especially at night or after resting
Faster breathing at rest, or breathing that looks effortful
Getting tired on walks that used to be easy
Fainting or collapsing
A swollen belly from fluid buildup
Less interest in food or gradual weight loss
Restlessness at night or trouble getting comfortable

A cough can come from heart disease or from many other causes, including airway disease. Your vet can help sort out which one your dog has.

Stages of Heart Disease

Veterinarians often use a staging system for MMVD. The stages run from dogs at risk, to dogs with a murmur but no heart enlargement, to dogs with heart enlargement but no symptoms, to dogs with symptoms of heart failure. The preclinical stage, meaning disease is present but the dog still acts normal, is when monitoring and sometimes early treatment matter most [5].

This is why a murmur found on a routine exam is not a reason to panic, but it is a reason to follow up. Catching the transition from preclinical to clinical disease lets your vet act at the right moment.

Red Flags That Make This an Emergency

Call an emergency clinic right away if your dog has any of these:

Struggling to breathe, or breathing with the belly and chest muscles working hard
Open-mouth breathing or gasping (dogs rarely pant this way when calm)
Collapse or fainting that does not fully resolve in a minute or two
Blue or gray gums
A resting breathing rate that stays high while your dog is asleep
Sudden weakness in the back legs, which can signal a clot
A swollen, tight belly along with heavy breathing

These signs can mean congestive heart failure or another crisis. They need same-day or immediate care.

What You Can Safely Do at Home

You cannot treat a murmur at home, but you can gather useful information and keep your dog comfortable.

Count breaths while your dog sleeps. Watch the chest rise and fall for 30 seconds and multiply by two. A normal sleeping rate is usually under 30 breaths per minute. Write the number down and bring it to visits.
Note any cough, especially its timing and whether it sounds wet or dry.
Track exercise tolerance. Has your dog slowed down on the usual route?
Keep your dog at a healthy weight. Extra weight makes the heart work harder.
Feed a complete, balanced diet. Ask your vet before adding supplements or "heart" products.
Do not start or stop any medication on your own. Heart medications are dosed by your veterinarian based on your dog's weight and health.
Keep routine vet visits even when your dog seems fine.

If your dog faints, keep them calm, check the gums, and call your vet or an emergency clinic.

What Your Veterinarian Will Check and Test

Your vet starts with a full physical exam, listening to the heart and lungs and checking pulses, gums, and breathing effort. They will note the murmur's grade, where it is loudest, and its timing.

From there, testing may include:

Chest X-rays to look at heart size and check for fluid in the lungs. X-rays often help diagnose heart disease, but they are less accurate than echocardiography for measuring specific heart chambers [3].
Electrocardiography (ECG) to record the heart's electrical activity and find arrhythmias or conduction problems [3].
Echocardiography (ultrasound of the heart), which gives the clearest view of valves, chamber size, and blood flow.
Blood and urine tests to check organ function and rule out other problems.
Cardiac biomarkers such as NT-proBNP, which can help identify dogs at higher risk of heart failure when echocardiography is not available or declined [5].

When echocardiography is not an option, your vet may combine auscultation, murmur intensity, heart size on X-rays, and biomarkers to judge risk and decide on early treatment [5].

If your dog needs a specialist, a veterinary cardiologist can perform a detailed ultrasound and recommend a plan. To prepare for that appointment, see What to Expect at Your Dog's Vet Visit for a Heart Murmur. You can also read more about how murmurs are graded and evaluated in Canine Cardiac Auscultation: Murmur Grading and Diagnostic Approach.

Monitoring and Prevention

You cannot prevent most heart disease in dogs, but you can catch it earlier and manage it better.

Keep up with wellness exams. A murmur is often found before symptoms appear.
Ask your vet how often to recheck. Dogs with a new murmur often need a recheck in a few weeks to a few months, depending on the grade and findings.
Follow the monitoring plan. Some dogs need only yearly visits. Others need rechecks every few months.
Give medications exactly as prescribed if your vet starts them.
Watch for the symptoms listed above and report changes early.

If your dog has been diagnosed with heart disease, learn the warning signs of advanced disease in Dog Heart Disease: How to Tell If Your Dog Is in Congestive Heart Failure. Early signs are covered in Dog Heart Disease: Recognizing the Early Signs and Knowing When to Act.

Limitations and When to Contact a Veterinarian

This article cannot tell you what your dog's specific murmur means. Only a veterinarian who examines your dog can do that.

Contact your vet the same day if your dog has a new cough, is tiring more easily, or has a resting breathing rate that seems higher than usual.

Go to an emergency clinic now if your dog is struggling to breathe, has collapsed, has blue gums, or has a swollen belly with heavy breathing.

Schedule a routine visit if your dog has a murmur but seems well, or if you noticed a change you want checked.

Frequently Asked Questions

Does a heart murmur always mean my dog has heart disease?

No. Some murmurs are innocent, especially in puppies and young dogs, and cause no problems. Others are the first sign of real heart disease. An exam and often an ultrasound are needed to tell the difference.

Can a murmur go away on its own?

Innocent murmurs in puppies often fade as the dog grows. Murmurs caused by structural heart disease generally do not go away and tend to change over time. Your vet will recheck to see which pattern your dog follows.

Will my dog need medication right away?

Not always. Many dogs with a murmur and no heart enlargement are monitored without treatment. Medication is often started when the heart enlarges or symptoms appear, and the choice depends on the specific disease.

Can I exercise my dog with a murmur?

Most dogs with a mild murmur and no symptoms can stay active. If your dog faints, tires quickly, or breathes hard with activity, ask your vet about safe limits before continuing the same routine.

How often should a dog with a murmur be rechecked?

It depends on the grade, the cause, and whether the heart is enlarged. Some dogs are rechecked yearly, while others need visits every few months. Your vet will set a schedule based on your dog's specific findings.

References

Introduction to Heart and Blood Vessel Disorders in Dogs - Dog Owners. Merck Veterinary Manual.
A machine-learning algorithm to grade heart murmurs and stage preclinical myxomatous mitral valve disease in dogs. Journal of veterinary internal medicine, 2024.
Diagnosis of Cardiovascular Disease in Dogs - Dog Owners. Merck Veterinary Manual.
Heart Disease in Dogs - Dog Owners. Merck Veterinary Manual.
Diagnosis and management of a more advanced stage of preclinical myxomatous mitral valve disease in dogs without echocardiography. Schweizer Archiv fur Tierheilkunde, 2024.
Histological evidence of myxomatous tissue in a Labrador puppy with pulmonic stenosis and tricuspid valve dysplasia. Journal of veterinary cardiology : the official journal of the European Society of Veterinary Cardiology, 2025.
Echocardiographic findings of a valvular aortic cyst in a dog. Journal of veterinary cardiology : the official journal of the European Society of Veterinary Cardiology, 2025.
Right sinus of Valsalva aneurysm ruptured into the right atrium in a dog. Journal of veterinary cardiology : the official journal of the European Society of Veterinary Cardiology, 2026.