Heart murmurs in dogs are one of the most common findings during routine veterinary examinations, particularly in older dogs and certain predisposed breeds. A heart murmur is not a disease in itself. It is an abnormal sound, detected through a stethoscope, caused by turbulent blood flow within or around the heart. The significance of a murmur ranges from entirely benign to an indicator of significant underlying cardiac disease, and this is why any murmur detected by a veterinarian deserves proper investigation. Understanding what a murmur means, what causes it, and what to do about it helps pet parents make informed and confident decisions about their dog’s care.
What Is a Heart Murmur in Dogs?
In a healthy heart, blood flows smoothly through the chambers and valves in a coordinated, laminar pattern that produces no audible sound beyond the normal heartbeat. A murmur occurs when blood flow becomes turbulent, creating vibrations in the surrounding tissue that the veterinarian can hear through a stethoscope placed against the chest.
Turbulent flow can be caused by a defective valve that no longer closes properly and allows blood to leak backwards (regurgitation), a narrowed valve or vessel that forces blood through a restricted opening (stenosis), an abnormal connection between chambers or vessels that should not exist, reduced blood thickness (anaemia), or hyperdynamic states that increase the speed of blood flow through normal structures.
Murmurs are described by their timing within the cardiac cycle. Systolic murmurs occur during the contraction phase of the heartbeat and are by far the most common type seen in dogs. Diastolic murmurs occur during the relaxation phase and are less common. Continuous murmurs span both phases and are characteristic of specific conditions such as patent ductus arteriosus.
Grades and Types of Heart Murmurs in Dogs
When a veterinarian detects a heart murmur in a dog, they assign it a grade from I to VI based on the loudness and characteristics of the sound. This grading system provides a standardised way to document and track the murmur over time:
| Grade | Description | Clinical Significance |
|---|---|---|
| Grade I | Very soft, only detectable in a quiet room after careful auscultation | Often physiological or very early disease; monitor |
| Grade II | Soft but consistently heard on first placement of the stethoscope | May be innocent or early valve disease; echocardiogram recommended |
| Grade III | Moderate intensity, clearly audible | Likely indicates significant valve disease; echocardiogram indicated |
| Grade IV | Loud, easily heard; no palpable vibration (thrill) yet | Significant underlying disease; echocardiogram and staging important |
| Grade V | Very loud; palpable thrill (vibration) felt on chest wall | Advanced disease; echocardiogram and treatment planning required |
| Grade VI | Loudest possible; audible with stethoscope held just off the chest | Severe pathology; urgent echocardiographic evaluation needed |
Murmur grade does not always directly correlate with the severity of underlying disease. A Grade III murmur from mitral valve disease in a dog that has been compensating well for several years may produce fewer clinical signs than a lower-grade murmur in a dog with rapidly progressing disease. This is why echocardiography, not murmur grade alone, is the gold standard for assessing the true severity and stage of the underlying cardiac condition.
Innocent murmurs, also called physiological murmurs, are soft murmurs (typically Grade I to II) heard in puppies and young dogs that are not associated with any structural abnormality. They are thought to result from the rapid heart rate and slightly hyperdynamic circulation of young animals. Most innocent murmurs disappear entirely by 4 to 6 months of age. Any murmur that persists beyond this age warrants echocardiographic evaluation to confirm that it is truly innocent.
Symptoms of Heart Murmurs in Dogs
Many dogs with heart murmurs, particularly those with mild or early-stage disease, show no symptoms at all. The murmur is discovered during a routine examination, often before the dog shows any signs of cardiac compromise. This asymptomatic period can last for months to years depending on the underlying condition.
As the underlying disease progresses, symptoms may develop. These include:
- A persistent, soft cough that tends to be worse at night or after lying down
- Faster breathing at rest, which you may notice when watching your dog sleep
- Reduced exercise tolerance, tiring more quickly than usual during walks
- Restlessness at night or difficulty settling into a comfortable sleeping position
- Fainting or brief collapse during or after activity
- Progressive abdominal distension in cases where right-sided heart failure develops
- General lethargy and reduced interest in daily activities
- Weight loss and muscle wasting in more advanced stages
The absence of symptoms does not mean the murmur is insignificant. It means the dog is compensating well for now. Regular echocardiographic monitoring is the only way to track whether that compensation is being maintained or beginning to break down.
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Causes of Heart Murmurs in Dogs
Heart murmurs in dogs arise from a range of underlying causes, and identifying the specific cause guides both the treatment approach and the prognosis:
- Degenerative Mitral Valve Disease (DMVD): The most common cause of heart murmurs in dogs, particularly in small and medium breeds above 5 to 7 years of age. The mitral valve leaflets thicken and distort progressively, allowing blood to leak backwards with each heartbeat and producing the characteristic systolic murmur heard loudest over the left heart base.
- Dilated Cardiomyopathy (DCM): As the heart muscle weakens and dilates, the mitral and tricuspid valves may no longer close properly, producing secondary regurgitation murmurs. Murmurs in large breeds with DCM tend to be softer than those seen in mitral valve disease.
- Congenital Heart Defects: Conditions such as pulmonic stenosis, subaortic stenosis, ventricular septal defect, and patent ductus arteriosus all produce characteristic murmurs detectable from birth or early puppyhood.
- Infective Endocarditis: Bacterial infection of the heart valves causes structural valve damage and results in regurgitation murmurs. It is often accompanied by systemic signs of infection and fever.
- Anaemia: Reduced haemoglobin concentration decreases blood viscosity, increasing flow velocity and producing a soft, functional murmur in the absence of structural heart disease. These murmurs typically resolve when the anaemia is treated.
- Heartworm Disease: Heavy heartworm infestation can affect the right side of the heart and produce murmurs related to tricuspid regurgitation and elevated pulmonary artery pressure.
How Veterinarians Diagnose Heart Murmurs in Dogs
Detecting a murmur is the beginning of the diagnostic process, not the end. The next steps depend on the murmur grade, the dog’s age and breed, and whether any clinical signs are present:
1. Physical Examination and Auscultation: The vet characterises the murmur by its location, grade, timing, and quality. The location of the murmur on the chest wall helps identify which valve is most likely affected, as different valves are best heard at specific points.
2. Echocardiography: An echocardiogram is the most important diagnostic tool for evaluating heart murmurs in dogs. It directly visualises the valve structure, measures the degree of regurgitation, assesses chamber dimensions, and evaluates the heart’s pumping function. Echocardiography stages the severity of the underlying disease and determines when and what treatment is needed.
3. Chest X-Rays: Thoracic radiographs assess heart size and detect early signs of pulmonary oedema. An enlarged cardiac silhouette on X-ray is an important indicator of disease progression and triggers a change in management, including the introduction of pimobendan in dogs with mitral valve disease.
4. ECG: An electrocardiogram identifies arrhythmias that may accompany structural heart disease.
5. Blood Tests: Cardiac biomarkers (NT-proBNP, troponin I), complete blood count (to assess for anaemia), and organ function tests provide additional information about cardiac stress, overall health, and medication readiness.
Treatment for Heart Murmurs in Dogs
Treatment for heart murmurs in dogs depends entirely on the underlying cause and the stage of disease at the time of diagnosis:
1. No treatment for innocent or low-grade murmurs: Dogs with innocent murmurs or mild, asymptomatic early-stage disease require only monitoring. Periodic auscultation, and echocardiography at agreed intervals, confirm whether the disease is stable or progressing.
2. Pimobendan for preclinical mitral valve disease: The EPIC trial, a landmark clinical study, demonstrated that beginning pimobendan in dogs with mitral valve disease at a specific echocardiographic threshold, before any clinical signs of heart failure develop, significantly delays the onset of congestive heart failure and extends survival time. This is now a standard recommendation in dogs who meet the criteria.
3. Diuretics, ACE inhibitors, and pimobendan for established heart failure: Once congestive heart failure develops, a combination of medications to remove accumulated fluid, reduce cardiac workload, and improve pumping strength is used. This combination significantly improves both quality and length of life after the onset of failure.
4. Treatment of underlying cause: Murmurs caused by anaemia resolve when the anaemia is treated. Murmurs caused by infective endocarditis require prolonged antibiotic therapy. Congenital defects may be surgically correctable in selected cases.
Dogs being monitored for heart murmurs, particularly those with repeated echocardiograms and medication adjustments, can develop mild but genuine anxiety around veterinary visits. A calm, consistent home environment between appointments is an important part of cardiac care. VOSD Anxiety Care is gently formulated to support dogs experiencing stress and unsettled behaviour during extended monitoring periods. Always consult your vet before introducing any supplement alongside cardiac medications.
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Prognosis: Can Dogs Live With Heart Murmurs?
Yes. Many dogs with heart murmurs live entirely normal lives for years without ever developing clinical signs of heart disease. The prognosis depends primarily on the underlying cause, the murmur grade, and how closely the dog is monitored:
- Dogs with innocent or physiological murmurs that resolve in puppyhood have an entirely normal life expectancy.
- Dogs with mild mitral valve disease and no echocardiographic threshold criteria may remain stable and asymptomatic for two to five years or longer with periodic monitoring.
- Dogs that meet the criteria for pimobendan therapy, started at the appropriate stage, have a meaningfully extended period before congestive heart failure develops.
- Once congestive heart failure develops, appropriate treatment commonly maintains good quality of life for 12 months or longer.
Caring for a Dog with a Heart Murmur
- Attend all scheduled echocardiographic rechecks: The interval between echocardiograms is guided by the current stage of disease. These rechecks allow medication to be started or adjusted at exactly the right time.
- Monitor resting respiratory rate at home: Count your dog’s breaths per minute while they are sleeping deeply. A consistent rate above 30 per minute warrants same-day veterinary contact.
- Give medications consistently: If your dog is on cardiac medication, dose timing and consistency are essential. Never skip or adjust doses without veterinary guidance.
- Maintain a healthy body weight: Excess weight increases cardiovascular workload. Your vet will advise on appropriate nutrition.
- Exercise moderately: Gentle, regular walks are appropriate for most dogs with heart murmurs. Strenuous exercise or extreme excitement should be avoided once significant disease is identified.
When Should You See a Vet?
Seek veterinary assessment promptly if your dog develops any of the following:
- A new or worsening persistent cough, particularly at night
- Faster breathing at rest that has developed or worsened over days
- Any episode of fainting or collapse
- Significant reduction in exercise tolerance over weeks
- Pale, grey, or bluish gums at any time
Heart murmurs in dogs are a common finding that, with proper monitoring and timely treatment, can be managed very effectively. Many dogs with murmurs live long, comfortable, and happy lives. The key is early detection, regular echocardiographic assessment, and acting on the findings at the right time. If your dog has been diagnosed with a murmur, the most important next step is finding out exactly what lies behind it and establishing an appropriate monitoring plan with your veterinarian.
Helping Rescue Dogs with Medical Needs
In rescue settings across India, heart murmurs are regularly discovered in older dogs during intake examinations, often in animals with no prior veterinary history. For many of these dogs, a murmur is the first indication that their heart health needs attention. With echocardiographic staging, appropriate medication when indicated, and consistent follow-up care, many rescue dogs with heart murmurs go on to be successfully rehomed into families who continue their cardiac monitoring. Early diagnosis in the rescue setting, rather than after symptoms develop, meaningfully improves outcomes for these dogs.
