What Is a Heart Murmur in Dogs?
A heart murmur is an abnormal sound heard through a stethoscope during the heartbeat cycle. Rather than the clean "lub-dub" of a healthy heart, a murmur produces a whooshing or swishing noise caused by turbulent blood flow. Murmurs are not a diagnosis in themselves — they are a clinical finding that signals your vet should investigate further.
Understanding the grading system, the type of murmur, and the most likely underlying causes will help you navigate the next steps with confidence.
The Heart Murmur Grading Scale: Grade 1 to Grade 6
Vets use a standardised grading scale from 1 to 6 to describe how loud and intense a murmur is. The grade does not always directly correlate with disease severity, but it provides useful information about how advanced things may be.
- Grade 1: Very faint and barely audible, even with a stethoscope in a quiet room. Often detected incidentally during routine examinations.
- Grade 2: Soft but consistently audible with a stethoscope.
- Grade 3: Moderately loud, heard easily. Roughly the dividing line between mild and significant murmurs.
- Grade 4: Loud murmur, still requiring a stethoscope to detect.
- Grade 5: Very loud, and a "precordial thrill" — a vibration felt by placing a hand on the chest wall — is present.
- Grade 6: The loudest category. Audible without a stethoscope placed directly on the chest, and a palpable thrill is always present.
Higher grades are more likely to be associated with structural heart disease, but a Grade 2 murmur caused by severe mitral valve disease can still be clinically significant.
Systolic vs Diastolic Murmurs
Murmurs are also classified by when they occur in the cardiac cycle. Systolic murmurs occur during the contraction phase of the heart and are the most common type in dogs. They are typically associated with mitral or tricuspid valve disease, or outflow obstructions. Diastolic murmurs occur during the filling phase and are far less common in dogs, often pointing to aortic or pulmonary valve insufficiency. Continuous murmurs are heard throughout the cycle and can indicate a patent ductus arteriosus, a congenital defect.
Left-Sided vs Right-Sided Murmurs
The location where the murmur is loudest gives important clues about its origin. Left-sided murmurs are the most common and are typically heard over the mitral valve area on the left chest wall. They are most often caused by myxomatous mitral valve disease (MMVD). Right-sided murmurs, heard over the right chest, more commonly indicate tricuspid valve disease, which can also occur alongside MMVD.
Innocent and Physiological Murmurs in Puppies
Not all murmurs indicate serious disease. Young puppies, typically between 6 and 12 weeks of age, may have what is called an innocent or physiological murmur. These are caused by rapid growth, a relatively thin chest wall, and turbulent blood flow through a still-developing cardiovascular system. They are typically Grade 1 to 2, located on the left side, and carry no clinical significance.
In the vast majority of cases, innocent puppy murmurs resolve on their own by 4 to 6 months of age. If a murmur persists beyond this point, a more thorough cardiac evaluation is warranted to rule out congenital heart defects.
Main Causes of Heart Murmurs in Adult Dogs
Myxomatous Mitral Valve Disease (MMVD)
MMVD is by far the most common cause of acquired heart murmurs in dogs, accounting for the majority of cases seen in general practice. The mitral valve, which sits between the left atrium and left ventricle, gradually degenerates. The valve leaflets thicken, become nodular, and fail to close properly, allowing blood to leak backwards — a process called regurgitation.
MMVD predominantly affects small to medium breeds, including Cavalier King Charles Spaniels, Dachshunds, Miniature Schnauzers, and Poodles. It is a progressive disease that can eventually lead to congestive heart failure if left unmanaged.
Dilated Cardiomyopathy (DCM)
DCM is more common in large and giant breeds, including Dobermanns, Great Danes, Irish Wolfhounds, and Boxers. In DCM, the heart muscle weakens and the chambers enlarge, reducing the heart's ability to pump effectively. Murmurs in DCM dogs tend to be softer and caused by mitral or tricuspid regurgitation secondary to the enlarged, distorted heart chambers rather than primary valve disease.
Diagnosing the Cause: The Role of Echocardiography
A stethoscope examination can detect a murmur and estimate its grade, but it cannot determine the underlying cause or assess how much the heart has been affected. An echocardiogram — an ultrasound of the heart — is the gold standard for diagnosis. It allows a cardiologist or trained vet to visualise the valve structures, measure chamber sizes, assess the pumping function, and detect fluid accumulation. Chest X-rays can show cardiac enlargement and fluid in the lungs, and an electrocardiogram (ECG) can detect arrhythmias.
The ACVIM Staging System
The American College of Veterinary Internal Medicine (ACVIM) has established a consensus staging system for MMVD that guides treatment decisions:
- Stage A: Breeds at high risk of developing MMVD but with no murmur detected yet. No treatment required, but regular monitoring recommended.
- Stage B1: Murmur present but no cardiac enlargement on X-ray or echocardiogram. No treatment currently recommended; monitoring every 12 months.
- Stage B2: Murmur present with evidence of cardiac enlargement. Pimobendan is now recommended at this stage, as studies have shown it significantly delays the onset of heart failure.
- Stage C: Current or past signs of congestive heart failure. Active treatment with diuretics, pimobendan, and ACE inhibitors is required.
- Stage D: Refractory heart failure that no longer responds adequately to standard treatments. Palliative care and advanced management options are considered.
The Cavalier King Charles Spaniel MMVD Health Scheme
Because Cavalier King Charles Spaniels are uniquely predisposed to early-onset MMVD, a dedicated health scheme exists in the UK to reduce the prevalence of the disease through responsible breeding. Under this protocol, both parents must be heart-examined by a cardiologist at 2.5 years of age or older and found to be free of a murmur before they are used for breeding. Dogs from health-tested parents tend to develop MMVD later in life, demonstrating the positive impact of this programme.
If you are considering purchasing a Cavalier, always ask breeders for up-to-date heart certificates for both parents.
Living With a Dog That Has a Heart Murmur
Many dogs with low-grade murmurs live long and comfortable lives without ever developing heart failure. Regular veterinary monitoring, appropriate exercise, a balanced diet, and weight management all play important roles. If medication has been prescribed, administering it consistently and attending follow-up appointments is essential. Recognising early warning signs — coughing, increased breathing rate at rest, reduced exercise tolerance, or swollen abdomen — means you can seek help before a crisis develops.