A heart murmur is an abnormal sound — a whooshing or swishing — that your veterinarian hears through a stethoscope alongside the normal "lub-dub" of your dog's heartbeat. It means blood is flowing turbulently through the heart or past a valve. A murmur is not a diagnosis by itself. It is a sign that something in the heart's structure or flow dynamics has changed — and that something ranges from completely benign to serious progressive disease.

This article explains what heart murmur grades mean, when a murmur in a puppy is innocent and when it is not, what tests your veterinarian will recommend, the relationship between murmurs and myxomatous mitral valve disease (MMVD), and what questions to ask at your next visit.

Quick answer

A heart murmur is graded on a scale of I (very soft) to VI (very loud, palpable through the chest wall). The grade describes loudness, not necessarily disease severity. A grade I murmur in a healthy puppy may be innocent and resolve on its own. A new or worsening murmur in an older small-breed dog may be the first sign of myxomatous mitral valve disease — the most common acquired heart disease in dogs. The test that tells you what is actually happening inside the heart is an echocardiogram (cardiac ultrasound), not the stethoscope grade.

What causes a heart murmur in dogs

A murmur occurs when blood flows turbulently. This can happen for three broad reasons:

1. Structural heart disease (pathologic murmurs)

• Myxomatous mitral valve disease (MMVD): The most common cause of heart murmurs in adult dogs, especially small breeds. The mitral valve degenerates over time, becomes thickened and floppy, and leaks. Blood flows backward (regurgitates) through the leaky valve, creating the murmur. MMVD accounts for approximately 70–75% of all canine heart disease.
• Congenital heart defects: Present from birth. Examples include patent ductus arteriosus (PDA), subaortic stenosis (SAS), pulmonic stenosis, ventricular septal defect (VSD), and tricuspid valve dysplasia. These are more likely in puppies and certain breeds.
• Dilated cardiomyopathy (DCM): More common in large and giant breeds (Doberman Pinschers, Great Danes, Irish Wolfhounds, Boxers). The heart muscle weakens and stretches, causing poor pumping function and secondary valve leakage.

2. Flow-related (physiologic or innocent murmurs)

• Innocent puppy murmurs: Very common in young puppies, especially large breeds. These are soft (grade I–II), caused by the rapid flow of blood through a still-growing heart and vasculature. They typically appear at 6–8 weeks of age and resolve by 4–6 months. If a murmur persists beyond 6 months, is louder than grade II, or worsens over time, further workup is needed.
• Anemia: When red blood cell count drops significantly, blood becomes less viscous and flows faster, creating turbulence. The murmur resolves when the anemia is corrected.
• Fever or high cardiac output states: Hyperthyroidism, pregnancy, and significant physical exertion can increase blood flow velocity enough to cause a functional murmur.

3. Non-cardiac disease

• Anemia (as above)
• Hyperthyroidism (in cats; rare in dogs)
• Severe protein-losing disease

Murmur grading: What the numbers mean

Veterinarians grade murmurs on a I–VI scale based on loudness and physical characteristics:

What the grade does not tell you

• Grade does not equal severity. A loud murmur does not necessarily mean more serious heart disease. A dog with severe DCM may have a relatively quiet murmur, while a dog with early MMVD and a thickened but still reasonably functional valve may have a louder murmur because the jet of regurgitation happens to be very turbulent.
• Grade is subjective. It depends on the listener's experience, the dog's body condition (obesity muffles sound), the dog's cooperation, and ambient noise.
• Grade alone does not determine treatment. An echocardiogram — not the murmur grade — determines whether medication is needed.

When murmurs progress: MMVD staging

For the majority of older small-breed dogs with a new heart murmur, the underlying cause is myxomatous mitral valve disease. The American College of Veterinary Internal Medicine (ACVIM) has published consensus guidelines that stage MMVD from A through D:

Stage A

Dogs at risk for MMVD but with no current signs or murmur. Examples: Cavalier King Charles Spaniels, Dachshunds, Miniature Poodles, Chihuahuas — breeds with high MMVD prevalence. No treatment is needed. Regular annual exams.

Stage B1

Dogs with a murmur caused by MMVD but without evidence of cardiac enlargement on imaging. The murmur is present, but the heart is not yet remodeling in response to the valve leak. No medication is indicated. Monitoring with periodic exams and possibly thoracic radiographs.

Stage B2

Dogs with a murmur and documented cardiac enlargement on imaging, but no clinical signs of heart failure. This is a critical intervention point — the 2019 ACVIM guidelines recommend starting pimobendan (Vetmedin) at this stage based on the EPIC trial, which showed that early pimobendan therapy can delay the onset of congestive heart failure by up to 15 months and improve survival.

Diagnostic criteria for Stage B2 include:

• Echocardiogram: left atrium-to-aorta ratio (LA:Ao) ≥ 1.6 and normalized left ventricular internal diameter in diastole (LVIDdN) ≥ 1.7
• Radiographs: vertebral heart score (VHS) ≥ 11.5 or vertebral left atrial size (VLAS) ≥ 3.0

Monitoring: echocardiogram and/or radiographs every 4–6 months. Home monitoring of resting respiratory rate.

Stage C

Dogs with current or past clinical signs of congestive heart failure (CHF) caused by MMVD. Signs include cough, increased respiratory rate and effort, exercise intolerance, and potentially collapse. Treatment includes furosemide (a diuretic), pimobendan, an ACE inhibitor (e.g., enalapril or benazepril), and spironolactone. Hospitalization with injectable furosemide and oxygen may be needed for acute CHF episodes.

Stage D

Dogs with end-stage heart failure refractory to standard therapy. These dogs require escalating diuretic doses, additional medications (torasemide, sildenafil for pulmonary hypertension, antiarrhythmics), oxygen supplementation, and often repeated centesis for fluid accumulation. Specialty referral is strongly recommended.

Diagnostic tests

Echocardiogram (cardiac ultrasound)

The echocardiogram is the gold standard test for evaluating a heart murmur. It allows the veterinarian to:

• Visualize the heart chambers and valves in real time
• Measure chamber dimensions and wall thickness
• Assess valve function and regurgitation severity using Doppler
• Calculate fractional shortening and ejection fraction (measures of contractility)
• Estimate pulmonary artery pressure
• Diagnose congenital defects

An echocardiogram is non-invasive, does not require anesthesia in most dogs (mild sedation may be needed for anxious patients), and takes 15–30 minutes. It should be performed or reviewed by a veterinarian with advanced training in echocardiography — ideally a board-certified veterinary cardiologist.

Thoracic radiographs (chest X-rays)

Radiographs show:

• Overall heart size and shape (using vertebral heart score, or VHS)
• Evidence of left atrial enlargement (particularly using vertebral left atrial size, or VLAS)
• Pulmonary patterns consistent with heart failure (pulmonary venous congestion, pulmonary edema)
• Tracheal collapse or mainstem bronchial compression that may be causing cough

Radiographs are less expensive than echocardiography and are available in most general practices, but they cannot visualize internal cardiac structure or valve function. They complement — but do not replace — echocardiography.

Cardiac biomarkers: NT-proBNP

NT-proBNP (N-terminal pro–B-type natriuretic peptide) is a blood test that rises when cardiac muscle is stretched and stressed. It is useful as:

• A screening tool: a normal NT-proBNP in a dog with a murmur makes significant cardiac remodeling less likely.
• A monitoring tool: rising NT-proBNP over time suggests progressive cardiac stress.
• A triage tool: in a dog with respiratory signs, an elevated NT-proBNP supports cardiac (vs respiratory) disease as the cause.

NT-proBNP does not replace echocardiography — it is a complementary blood marker.

Electrocardiogram (ECG)

An ECG evaluates the electrical activity of the heart. It is primarily used to diagnose arrhythmias (irregular heart rhythms), not structural disease. A murmur without arrhythmia does not typically require an ECG, but one may be recommended if the dog has a rapid or irregular heartbeat, episodes of collapse, or if certain medications are being considered.

Breed predispositions

Small-breed dogs (MMVD)

Cavalier King Charles Spaniels, Dachshunds, Miniature and Toy Poodles, Chihuahuas, Pomeranians, Yorkshire Terriers, Shih Tzus, Maltese, Bichon Frises, and Cocker Spaniels are at high risk for MMVD. In Cavalier King Charles Spaniels, MMVD prevalence is exceptionally high — studies report that up to 90% of CKCS develop the disease by age 10.

Large-breed dogs (DCM)

Doberman Pinschers, Great Danes, Irish Wolfhounds, Boxers, and other large/giant breeds are at higher risk for dilated cardiomyopathy. DCM murmurs are often softer than MMVD murmurs despite more severe disease, which is why echocardiography is essential in these breeds even with a soft murmur.

Congenital defects

Certain breeds have higher rates of specific congenital defects: PDA in Maltese, Pomeranians, and German Shepherds; subaortic stenosis in Newfoundlands, Rottweilers, and Golden Retrievers; pulmonic stenosis in Bulldogs and terrier breeds.

Puppy murmurs vs adult-onset murmurs

Puppies

Most murmurs in puppies are innocent (physiologic) — soft, low-grade, and outgrown by 4–6 months. If your puppy has a murmur:

• A grade I–II murmur in a growing, active puppy with no other signs is usually monitored and rechecked. It often resolves.
• A murmur that is grade III or louder, persists beyond 6 months, is associated with poor growth or exercise intolerance, or worsens on recheck examination warrants echocardiography.
• If a congenital defect is found, some conditions (like PDA) are surgically correctable if identified early.

Adult dogs

A new murmur in an adult dog — particularly a small-breed dog over 5 years — is most likely MMVD until proven otherwise. The diagnostic pathway:

1. Auscultation: confirm the murmur, grade it, note its timing (systolic vs diastolic) and location (mitral vs other valve area).
2. Thoracic radiographs: assess heart size (VHS, VLAS) and look for pulmonary signs.
3. Echocardiogram: if radiographs show cardiomegaly, or if the murmur is loud (≥ grade III), or if the dog is showing any clinical signs (cough, exercise intolerance, increased respiratory rate).
4. NT-proBNP: optional adjunctive blood test to assess cardiac stress.
5. Recheck schedule: determined by stage and findings.

Pet insurance considerations

A heart murmur diagnosed before a pet insurance policy is active will almost certainly be classified as a pre-existing condition and excluded from coverage. This is one reason veterinarians often recommend:

• Enrolling in pet insurance early, before age-related conditions like MMVD develop.
• If a murmur is found at a routine exam, understanding that a future diagnosis of congestive heart failure related to that murmur will not be covered by a policy purchased after the murmur was documented.
• Some insurers differentiate between curable pre-existing conditions and incurable ones. A heart murmur caused by anemia may resolve when the anemia is treated; MMVD is chronic and progressive.

What to discuss with your veterinarian

• What grade is the murmur? Ask for the specific grade and where on the chest it is loudest. This gives you a baseline for comparison.
• Is this likely innocent or pathologic? The answer depends on age, breed, grade, and whether the murmur is new or changing.
• Should we do an echocardiogram? If your dog is an older small-breed dog with a grade III or louder murmur, an echocardiogram is generally recommended. If your dog is a puppy with a soft murmur, your veterinarian may recommend monitoring first.
• What stage is my dog's MMVD (if applicable)? The answer determines whether medication is needed now or monitoring is sufficient.
• What is the recheck schedule? Stage B1 dogs are typically rechecked every 6–12 months. Stage B2 dogs every 4–6 months.
• What should I monitor at home? Resting respiratory rate (RRR) is the most useful home monitoring tool. A sustained RRR above 30 breaths per minute while sleeping — in a dog with known heart disease — warrants a veterinary call.
• Does my dog need to see a cardiologist? General practitioners can diagnose and manage many cases of MMVD. Referral to a cardiologist is recommended for complex congenital disease, DCM, advanced heart failure (Stage C/D), or if the general practitioner is uncertain about staging or treatment.

Key points

• A heart murmur is turbulent blood flow — not a diagnosis. It requires investigation to determine the cause.
• Murmurs are graded I–VI based on loudness. The grade does not directly correlate with disease severity.
• In older small-breed dogs, a new murmur is most commonly caused by myxomatous mitral valve disease (MMVD).
• Echocardiography is the gold standard for evaluating a murmur. It shows structure, function, and determines ACVIM stage.
• Stage B2 MMVD (cardiac enlargement without clinical signs) is the critical intervention point where pimobendan is recommended to delay heart failure.
• Most puppy murmurs are innocent and resolve by 4–6 months. Murmurs that persist, worsen, or are grade III or louder should be evaluated.
• Home monitoring of resting respiratory rate is one of the most practical tools for dogs with known heart disease.

Sources

• Keene BW, Atkins CE, Bonagura JD, et al. (2019) — ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. Journal of Veterinary Internal Medicine, 33(3):1127–1140. https://pmc.ncbi.nlm.nih.gov/articles/PMC6524084
• Cornell University College of Veterinary Medicine — Get a Jump on Heart Murmurs. https://www.vet.cornell.edu/departments-centers-and-institutes/riney-canine-health-center/canine-health-information/get-jump-heart-murmurs
• Veterinary Partner (VIN) — Heart Murmurs in Dogs and Cats. https://veterinarypartner.vin.com/default.aspx?pid=19239&id=4952593
• VCA Animal Hospitals — Heart Murmurs in Dogs. https://vcahospitals.com/know-your-pet/heart-murmurs-in-dogs
• MSPCA-Angell — Beyond the Guidelines: Management of Advanced Heart Disease in Dogs. https://www.mspca.org/clinical/beyond-the-guidelines-management-of-advanced-heart-disease-in-dogs
• Boehringer Ingelheim — Vetmedin-CA1 (pimobendan) Product Page. https://animalhealth.boehringer-ingelheim.com/pets/canine/products/therapeutics/vetmedin-ca1
• Cardiac Education Group — ABCDs of Myxomatous Mitral Valve Disease. https://cardiaceducationgroup.org/wp-content/uploads/2024/09/ABCD-Canine-MMVD.pdf