Your Dog Has a Heart Murmur: What You Need to Know

The moment everything changes

It often happens during a routine visit. Your dog is at the vet for a wellness check or a vaccine appointment. The veterinarian places the stethoscope on your dog's chest, pauses, listens more carefully, and then says something like: "I'm hearing a heart murmur."

For most dog parents, those words trigger an immediate wave of fear. Heart murmur sounds serious. It sounds like heart disease. It sounds like something is terribly wrong.

Take a breath. A heart murmur is an important finding, but it is not, by itself, a diagnosis. It is a sound — a physical exam finding that tells your veterinarian something is worth investigating further. What comes next depends on your dog's age, breed, the characteristics of the murmur, and the results of additional testing.

This article will walk you through what a heart murmur actually is, what the grading system means, when a murmur is cause for concern, when it is not, and what to expect as you navigate the next steps.

What is a heart murmur?

A heart murmur is the sound of turbulent blood flow within the heart. In a healthy heart, blood flows smoothly and relatively silently through the chambers and valves. When something disrupts that smooth flow — a leaking valve, an abnormal opening, or even just a particularly vigorous blood flow — the turbulence creates a sound that can be heard with a stethoscope.

Think of it like water flowing through a pipe. When the pipe is smooth and properly sized, the water flows silently. But if there is a narrowing, an obstruction, or a leak, the water becomes turbulent and creates audible noise.

A heart murmur is not a disease. It is a sign — an audible clue that something may be affecting blood flow in the heart. That "something" could range from completely benign to clinically significant.

The grading system: I through VI

Veterinarians grade heart murmurs on a standardized scale from I to VI based on how loud the murmur is. Understanding this scale helps you put your dog's murmur in context.

Grade I

A grade I murmur is the softest detectable murmur. It is barely audible, heard only in a perfectly quiet room with careful, focused auscultation. Many veterinarians will listen multiple times to confirm they are truly hearing it. Grade I murmurs are easy to miss, and a busy or noisy clinic environment may prevent detection.

What it usually means: Very mild turbulence. In many dogs, this represents very early or very mild valve changes. In some cases, it may be an innocent or physiologic murmur.

Grade II

A grade II murmur is soft but readily audible with a stethoscope. There is no question that it is there — it can be heard consistently with each heartbeat.

What it usually means: Mild turbulence, often from early valve degeneration or a small amount of regurgitation. Grade II murmurs are extremely common in older small-breed dogs.

Grade III

A grade III murmur is moderately loud. It is easily heard and fills a noticeable portion of the cardiac cycle.

What it usually means: Moderate regurgitation or turbulence. At this grade, further evaluation with echocardiography is recommended if not already performed.

Grade IV

A grade IV murmur is loud and prominent. It dominates the heart sounds and is immediately apparent on auscultation.

What it usually means: Significant regurgitation or structural abnormality. Echocardiography is important to assess the severity of the underlying condition.

Grade V

A grade V murmur is very loud and is accompanied by a palpable thrill — a vibration that can be felt by placing a hand on the chest wall over the heart. This is a distinctive finding that signals substantial turbulence.

What it usually means: Severe regurgitation or significant structural disease. Referral to a veterinary cardiologist is strongly recommended.

Grade VI

A grade VI murmur is the loudest possible — it can be heard with the stethoscope held slightly away from the chest wall, without direct contact. A palpable thrill is always present.

What it usually means: Very severe regurgitation or structural abnormality. These murmurs indicate significant cardiac pathology.

Important: Murmur grade reflects loudness, which generally correlates with severity — but not perfectly. A grade III murmur can sometimes reflect more significant disease than a grade IV murmur, depending on the specific cause and the individual dog's anatomy. This is why echocardiography, not murmur grade alone, determines the true severity of heart disease.

Not all murmurs mean disease

This is one of the most important messages in this article. Not every heart murmur indicates heart disease. There are several situations where a murmur is detected but no underlying cardiac pathology exists.

Innocent murmurs in puppies

Puppies, particularly between 6 and 16 weeks of age, commonly have what are called innocent or physiologic murmurs. These are typically soft (grade I to II), heard best over a specific location on the chest, and are caused by the normal dynamics of a rapidly growing cardiovascular system.

Most innocent puppy murmurs resolve on their own by 4 to 5 months of age. Your veterinarian will monitor the murmur at subsequent puppy visits, and if it resolves, no further workup is needed.

However: Not all puppy murmurs are innocent. If a murmur is louder than grade II/III, persists beyond 5 months, or has characteristics suggesting a structural abnormality (such as being heard in an unusual location or having a particular quality), your veterinarian should recommend further evaluation to rule out congenital heart defects.

Physiologic murmurs

Some dogs — particularly thin-chested breeds or dogs with high cardiac output (such as during fever, anemia, or excitement) — can have flow murmurs that are not associated with any structural heart disease. These occur because blood is flowing faster or more forcefully than usual through a normal heart.

Causes of physiologic murmurs include:

• Anemia — Reduced blood viscosity causes faster, more turbulent flow.
• Fever or infection — Increased heart rate and cardiac output.
• Excitement or stress — Particularly in cats, but also in some dogs during veterinary visits.
• Pregnancy — Increased blood volume.
• Athletic heart — Some highly conditioned dogs may have flow murmurs.

If the underlying cause is identified and resolved, the murmur disappears.

How to tell the difference

The short answer is: you cannot tell from the stethoscope alone whether a murmur is innocent or pathologic with certainty. An echocardiogram (cardiac ultrasound) is the definitive way to distinguish between a murmur caused by structural heart disease and one that is benign. When there is any doubt — especially in breeds predisposed to heart disease — an echocardiogram provides the answer.

When to worry

While not every murmur requires alarm, certain situations should prompt further evaluation sooner rather than later:

New murmur in a middle-aged to older small-breed dog

This is the most classic presentation of Myxomatous Mitral Valve Disease (MMVD), the most common acquired heart disease in dogs. If your veterinarian detects a new murmur in a Cavalier King Charles Spaniel, Dachshund, Chihuahua, Poodle, Yorkshire Terrier, Pomeranian, or other small breed — particularly if the dog is 5 years or older — MMVD is the most likely cause.

This does not mean your dog is in immediate danger. Many MMVD dogs live years with only a murmur and no symptoms. But it does mean an echocardiogram is warranted to establish a baseline and determine the stage of disease.

Murmur grade is increasing

If your dog has had a known murmur that was previously grade II and is now grade IV, the underlying disease may be progressing. An updated echocardiogram is important to reassess.

Accompanying symptoms

If a murmur is detected alongside any of the following, prompt evaluation is essential:

• Increased breathing rate, especially at rest
• Coughing, particularly at night or when lying down
• Exercise intolerance — tiring more easily on walks or during play
• Fainting or collapse episodes
• Decreased appetite or weight loss
• Abdominal distension (possible fluid accumulation)

Murmur in a young dog that does not resolve

A persistent murmur in a dog under 1 year of age that does not resolve could indicate a congenital heart defect — a structural abnormality the dog was born with. Common congenital defects include:

• Patent ductus arteriosus (PDA) — An abnormal vessel that should have closed at birth remains open.
• Subaortic stenosis (SAS) — A narrowing below the aortic valve.
• Pulmonic stenosis (PS) — A narrowing of the pulmonic valve.
• Ventricular septal defect (VSD) — A hole between the two ventricles.

Many congenital defects are treatable, some with excellent outcomes, so early diagnosis through echocardiography is important.

What happens next: the echocardiogram

When a heart murmur warrants further investigation, the next step is almost always an echocardiogram — a cardiac ultrasound performed by a veterinary cardiologist or experienced ultrasonographer.

What an echocardiogram shows

An echocardiogram provides a detailed, real-time picture of:

• Heart chamber sizes — Are the atria and ventricles normal in size, or are they enlarged?
• Valve structure and function — Are the valves thickened, degenerated, or structurally abnormal? Is blood leaking backward through them?
• Blood flow patterns — Using Doppler ultrasound, the cardiologist can see and measure blood flow direction and velocity, identifying regurgitation and its severity.
• Heart muscle function — Is the muscle contracting normally? Is the heart pumping effectively?
• Other abnormalities — Congenital defects, masses, pericardial effusion, and other conditions.

What to expect during the exam

An echocardiogram is non-invasive and painless. Your dog lies on their side on a padded table, often with a cutout that allows the ultrasound probe to access the chest from below. Most dogs tolerate the exam well without sedation, though some anxious patients may benefit from mild sedation.

The exam typically takes 20 to 45 minutes, and your cardiologist will usually discuss the findings with you immediately afterward.

The information you will receive

After the echocardiogram, your cardiologist will give you:

• A diagnosis — what is causing the murmur
• A stage — if the diagnosis is MMVD, where your dog falls on the ACVIM staging scale (A through D)
• Specific measurements — chamber sizes, valve function metrics, and other quantitative data
• A treatment plan — based on the stage, this may range from "no treatment needed, recheck in 12 months" to medication recommendations
• A monitoring schedule — when to come back for follow-up

MMVD: the most common cause in small breeds

In the majority of cases, a heart murmur in a middle-aged to older small-breed dog is caused by Myxomatous Mitral Valve Disease. This is the single most common acquired heart disease in dogs, accounting for approximately 75 percent of all cardiac disease seen in veterinary practice.

What MMVD is

The mitral valve — the valve between the left atrium and left ventricle — degenerates over time. The valve leaflets become thickened and irregular, and the valve no longer closes tightly. Blood leaks backward through the valve with each heartbeat, creating the murmur your veterinarian hears.

Why there is reason for optimism

If your dog's murmur is caused by MMVD, the outlook is often far better than you might fear:

• MMVD is slowly progressive. Many dogs live years with a murmur and no symptoms.
• The staging system provides a clear roadmap. There is a well-defined framework for when to monitor, when to treat, and what to expect.
• The EPIC study proved that medication works. Pimobendan, started when the heart enlarges (Stage B2), delays heart failure by a median of approximately 15 months.
• Many MMVD dogs live full, normal lifespans. Particularly when the disease is detected early and monitored appropriately.

Key point: A diagnosis of MMVD is not a death sentence. It is a manageable condition with well-established treatment protocols and, in most cases, a favorable trajectory — especially with early detection and proper monitoring.

Other causes of heart murmurs in dogs

While MMVD is the most common cause of acquired heart murmurs in dogs, other conditions can also produce murmurs:

Congenital heart defects

As mentioned earlier, structural abnormalities present at birth — PDA, subaortic stenosis, pulmonic stenosis, ventricular septal defects — all produce murmurs with specific characteristics that an experienced veterinarian or cardiologist can often distinguish on auscultation alone, with echocardiography providing definitive diagnosis.

Dilated Cardiomyopathy (DCM)

In large and giant breed dogs, a murmur may be associated with DCM — a disease of the heart muscle rather than the valves. DCM murmurs are typically caused by the mitral valve leaking secondarily as the heart chambers dilate and the valve annulus stretches.

Endocarditis

Bacterial infection of the heart valves (infective endocarditis) can cause murmurs, typically in the context of a dog with fever, lethargy, and evidence of systemic infection. This is much less common than MMVD but is a serious condition requiring aggressive antibiotic therapy.

Pericardial disease

Conditions affecting the pericardium (the sac surrounding the heart) can sometimes produce abnormal heart sounds, though these are technically not murmurs in the classic sense.

Tumors

Cardiac tumors, such as hemangiosarcoma of the right atrium or aortic body tumors, can occasionally affect blood flow and produce murmurs, though this is relatively uncommon.

Living with a heart murmur

If your dog has been diagnosed with a heart murmur — whether it is caused by MMVD, a congenital defect, or another condition — here is what daily life looks like:

Activity

At early stages (B1 for MMVD, or mild congenital defects that do not require intervention), your dog can live a completely normal, active life. No restrictions on walks, play, or exercise. Your dog does not know they have a murmur, and there is no reason for their quality of life to change.

At later stages (B2 and early C), most dogs still do well with normal activity. Your cardiologist will guide you on any modifications if they become necessary.

Monitoring at home

The most important thing you can do at home is learn to count your dog's resting respiratory rate:

• Count breaths per minute when your dog is sleeping or deeply relaxed.
• Establish a baseline — most dogs breathe 15 to 25 times per minute at rest.
• Track it regularly — daily is ideal, several times a week at minimum.
• Call your veterinarian if the rate consistently exceeds 30 breaths per minute.

This simple act of counting breaths can detect heart failure development before obvious symptoms appear, buying valuable time for early intervention.

Dental care and anesthesia

A common concern for owners of dogs with heart murmurs is whether their dog can safely undergo anesthesia for dental cleanings or other procedures. In most cases, the answer is yes — particularly at earlier stages. Your veterinarian and cardiologist can assess the anesthetic risk based on the specific cardiac findings and recommend appropriate precautions.

Dogs with heart disease actually benefit from good dental health, as dental infections can potentially worsen cardiac conditions (particularly through the risk of endocarditis). Avoiding dental care entirely because of a murmur may do more harm than good.

Regular veterinary care

Continue all normal veterinary care — vaccinations, preventive medicine, dental cleanings, and wellness visits. A heart murmur does not change the need for comprehensive health care. If anything, it makes regular veterinary visits more important, as your veterinarian can monitor the murmur and detect changes early.

What not to do

Do not panic

A heart murmur, especially in a small breed dog, is an extremely common finding. It does not mean your dog is dying or that an emergency is imminent. Take the time to get proper evaluation before assuming the worst.

Do not skip the echocardiogram

Murmur grade alone does not tell you the full story. An echocardiogram is the only way to know what is actually happening inside the heart — how severe the valve leak is, whether the heart has enlarged, and what stage the disease is in. This information directly determines the treatment plan.

Do not start supplements or medications without veterinary guidance

Well-meaning internet advice may suggest various supplements, dietary changes, or medications for heart murmurs. While some supplements (like omega-3 fatty acids) may have modest benefits in certain cardiac conditions, self-medicating your dog based on online information can be harmful. Let your veterinarian or cardiologist guide the treatment plan.

Do not restrict your dog's life unnecessarily

Unless your cardiologist specifically recommends activity restriction, let your dog enjoy their normal routine. A dog with a grade II murmur and Stage B1 MMVD does not need to be carried up stairs or prevented from playing. Overrestricting activity based on fear rather than medical advice reduces quality of life without providing any cardiac benefit.

The bottom line

Hearing that your dog has a heart murmur is understandably frightening. But a murmur is a finding, not a verdict. It is the starting point of a conversation, not the end of one.

Here is what matters:

• A heart murmur is the sound of turbulent blood flow. It may or may not indicate significant heart disease.
• Not all murmurs are pathologic. Puppies often have innocent murmurs that resolve. Some murmurs are caused by non-cardiac factors like anemia or excitement.
• In small-breed dogs, MMVD is the most common cause — and it is a well-understood, treatable condition with often favorable outcomes.
• An echocardiogram is the essential next step. It tells you what is causing the murmur, how severe it is, and what — if anything — needs to be done.
• Many dogs with heart murmurs live long, happy, active lives. With appropriate monitoring and, when needed, medication, a heart murmur does not have to change the trajectory of your dog's life.

If your veterinarian heard a murmur today, the best thing you can do is schedule that echocardiogram, learn to count resting respiratory rates, and take it one step at a time. The odds are very much in your favor.