Canine Heart Murmur Grades: What They Mean

Canine heart murmur grades are a 1-to-6 scale that tells a vet how loud and potentially significant a heart murmur is. Grade alone does not determine urgency, but it guides decisions about further testing, monitoring, and treatment.

Key takeaways:

Grades 1-2 are often innocent or minor; Grades 5-6 are almost always associated with significant structural disease
Myxomatous mitral valve disease (MMVD) is the most common cause in dogs, especially small breeds
Echocardiography is the only way to know whether a murmur is affecting heart function
Dogs with specific Grade 3-4 echo findings can now benefit from pimobendan even before symptoms appear
The detection of a cardiac murmur during a routine veterinary checkup often raises immediate concerns regarding the severity of the condition.
Clinical evaluation focuses on determining the underlying cause and the functional impact of the murmur (an abnormal “whooshing” or “swishing” sound heard between normal heartbeats, caused by turbulent blood flow within the heart).

What Is a Heart Murmur in Dogs?

A heart murmur in dogs is an abnormal sound heard between the normal “lub-dub” heartbeats. It is caused by turbulent blood flow within or near the heart, rather than smooth, laminar flow.

Not all murmurs indicate serious disease. Some are normal, especially in young puppies, and resolve on their own. Others signal structural heart disease that warrants investigation and treatment. A pet health expert needs to assess every new murmur, regardless of how soft it sounds.

How Veterinarians Grade Heart Murmurs in Dogs

Dog murmur grades are assigned using the six-point Levine scale, the standard system used in both human and veterinary cardiology. Canine heart murmur grades are assessed during a stethoscope examination in a quiet room, with the dog at rest.

Each grade reflects loudness and whether a thrill (a vibration palpable through the chest wall) is present. Grade alone does not reveal the cause. That requires further investigation.

Understanding Heart Murmur in Dogs: Grades 1 to 6

Canine heart murmur grades run from 1 (nearly inaudible) to 6 (heard with the stethoscope held off the chest). The grading system helps vets communicate severity and make consistent decisions about next steps.

Grade 1-2 Heart Murmurs

Grade 1 is the softest murmur possible, heard only by an experienced vet in complete quiet. Grade 2 is soft but audible on initial auscultation. No thrill is present at either grade.

In young puppies (typically under 16 weeks), Grade 1-2 murmurs are often innocent and resolve as the heart matures. In adult dogs, even soft murmurs warrant investigation. Regular echocardiographic monitoring allows any changes to be detected early.

Grade 3-4 Heart Murmurs

Grade 3 is a moderate murmur, easy to hear but without a palpable thrill. Grade 4 is louder and accompanied by a thrill at the point of maximum intensity.

In middle-aged and older small-breed dogs, such as Chihuahuas and Poodles, Grade 3-4 murmurs most commonly indicate myxomatous mitral valve disease (MMVD). Echocardiography is strongly recommended at this stage to assess cardiac structure.

The 2019 ACVIM Consensus Guidelines now recommend pimobendan for qualifying dogs, even before symptoms appear. The qualifying criterion is left atrial and ventricular enlargement on echo (Stage B2).

Grade 5-6 Heart Murmurs

Grade 5 is very loud, with a palpable thrill, and can be heard with the stethoscope lightly touching the chest. Grade 6 is the loudest possible murmur: audible with the stethoscope held off the chest, with a prominent thrill.

Grade 5-6 murmurs are almost always associated with significant structural heart disease. Most dogs at this grade have symptoms or are at high risk of developing them soon. Echocardiography, chest X-rays, and treatment planning are standard next steps.

Common Causes of Heart Murmurs in Dogs

Any canine heart murmur needs a cause to be identified, as treatment varies widely depending on what is driving it. Common causes include:

Myxomatous mitral valve disease (MMVD): the most common cause overall, especially in small breeds such as Cavalier King Charles Spaniels, Dachshunds, and Chihuahuas
Dilated cardiomyopathy (DCM): affects large breeds, including Dobermanns, Great Danes, Irish Wolfhounds, and Boxers
Pulmonic stenosis: a congenital narrowing of the pulmonary valve, common in Bulldogs and Bull Terriers
Patent ductus arteriosus (PDA): a congenital vessel abnormality that can lead to heart failure if left untreated
Aortic stenosis: narrowing of the aortic valve, seen in Golden Retrievers and Rottweilers
Innocent flow murmurs: heard in puppies, in anaemia, and in other high-output states

Symptoms That May Accompany a Heart Murmur

A dog murmur detected at a routine check-up is often caught before any symptoms develop. This is one of the most important reasons for annual auscultation at every vet visit.

When symptoms appear, they typically signal progression towards or into congestive heart failure:

Reduced exercise tolerance or tiring quickly on short walks
Coughing, particularly at night or in the early morning
Rapid or laboured breathing
Lethargy and weakness
Reduced appetite
Fainting or brief collapse (syncope)
Distended abdomen (ascites, from right-sided heart failure)

A resting respiratory rate above 30 breaths per minute is an early warning sign of fluid accumulating in the lungs. Pet parents can count this at home while their dog is asleep.

How Canine Heart Murmurs Are Diagnosed

Auscultation gives the vet the grade, location, timing (systolic or diastolic), and character of the murmur. This guides the next steps.

Echocardiography (ultrasound of the heart) is the gold standard for diagnosis. It shows valve anatomy, chamber sizes, and systolic function: none of these can be determined by grade alone. Chest X-rays assess heart size using the vertebral heart score and reveal changes in the lung fields. An ECG detects arrhythmias.

The NT-proBNP blood test measures a biomarker released in response to cardiac stress and helps distinguish cardiac from non-cardiac causes of symptoms. Blood pressure monitoring is also standard in older dogs with murmurs.

When a Heart Murmur Requires Treatment

Not every murmur needs medication. The decision depends on grade, echo findings, clinical signs, and underlying cause.

Canine heart murmur grades of 1-2 with no structural disease on echo: monitoring only. No medication is typically recommended at this stage.

For Stage B2 MMVD (specific echo criteria met), pimobendan is now recommended to delay heart failure onset, even without symptoms.

Dogs in congestive heart failure (Stage C or D): pimobendan combined with frusemide, with spironolactone added in many protocols. Congenital defects such as PDA may require interventional or surgical correction.

Living With a Dog Diagnosed With a Heart Murmur

For a dog diagnosed with a heart murmur, the focus shifts to monitoring and lifestyle adjustments. This does not always mean a drastic change in daily life, especially for lower-grade murmurs.

Key home management principles:

Avoid intense or prolonged exercise once cardiac disease is confirmed as significant
Maintain a lean body weight to reduce cardiac workload
Use a low-sodium diet for dogs in heart failure (a vet will advise when this becomes appropriate)
Count and record resting respiratory rate weekly: above 30 breaths per minute warrants a call to the vet
Keep all scheduled follow-up appointments

Prognosis and Long-Term Monitoring of Heart Murmurs in Dogs

Prognosis varies considerably by cause and stage. Healthy puppy murmurs have an excellent prognosis and typically resolve on their own. Dogs with MMVD detected early, especially those starting pimobendan in Stage B2, have significantly extended periods before heart failure develops.

Once congestive heart failure is present, the condition is managed rather than cured. Median survival with treatment is approximately nine to twelve months, though many dogs exceed this with careful management and monitoring.

Heart disease is progressive in most cases. Regular echocardiography (every six to twelve months, depending on stage) lets the experienced vet for dogs adjust treatment before the condition worsens.

Final Thoughts: About Canine Heart Murmur Grades

Canine heart murmur grades are one piece of a larger clinical picture, not a verdict in isolation. A Grade 2 murmur in a puppy is very different from a Grade 2 murmur in a seven-year-old Cavalier. Context matters. That context comes from a thorough veterinary assessment.

If your dog has been diagnosed with a heart murmur, book a consultation with a vet. Early detection and monitoring make the biggest difference to long-term outcomes.

FAQs: About Canine Heart Murmur Grades

Can a dog live a normal life with a heart murmur?

Many dogs with low-grade murmurs live normal, comfortable lives for years without ever developing symptoms.

Does every heart murmur require medication?

No. Grade 1-2 murmurs without structural disease on echocardiography are monitored rather than medicated. Grade 3-4 murmurs that meet specific echo criteria per ACVIM guidelines may benefit from pimobendan before symptoms appear.

Can heart murmurs get worse over time?

In most structural heart diseases, yes. MMVD is a progressive condition, and the murmur grade typically increases as the disease advances.

What grade of heart murmur is considered serious?

Grade 5-6 is almost always associated with significant structural disease. Grade 3-4 requires echocardiography to determine clinical significance. But even a Grade 1-2 murmur in a predisposed breed (such as a Cavalier King Charles Spaniel) deserves investigation.