Heart Murmurs in Dogs

Estimated Reading Time 7 minutes
Heart Murmurs in Dogs

Normal hearts beat with a quiet flow of blood. A heart murmur occurs when blood flowing through the heart causes a sound that can be heard through a stethoscope and sometimes even felt by touch. The best way to describe this is like a wide, slow-moving river and suddenly a large obstruction such as a boulder rolls or falls into the river, causing the water to become turbulent as it flows around the stone, creating noise and sound. Keep reading to learn more about causes and what to do if your dog has been diagnosed with a heart murmur.

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A normal heartbeat emits two sounds - a lub and a dub- that are heard when listening to the heart using a stethoscope. When a heart murmur is present, your vet can hear an abnormal whooshing or swishing sound. This distinct change in the heart sound can be an incidental finding during your pet’s health and wellness check.

A heart murmur does not always mean your pet is ill, but it can be a red flag that there is something wrong with your pet’s heart. It is not a disease in itself, rather, it could be an important symptom of an underlying issue affecting the heart. Heart murmurs can be pathologic meaning the murmur is due to heart lesions that develop from the progression of heart disease and age. Heart murmurs can also be functional or physiologic and occur in young, growing dogs.

Causes of Heart Murmurs in Dogs

A heart murmur can be caused by a structural abnormality in the heart or it can indicate the presence of heart disease.

Pathologic heart murmurs can be caused by structural problems within the heart or they can be caused by other medical issues that are not caused by heart disease (also called ‘extra cardiac’).

When listening to your pet’s heart, your vet will observe the loudness of the heart murmur, what part of the chest it can be heard the loudest, when the murmur occurs during the contraction of the heart, and its length. Armed with this important information, your vet will be able to rate the heart murmur.

The intensity of heart murmur can be classified into several grades. Grade I and II murmurs are usually physiologic, while most heart murmurs that cause serious problems are at least a grade III. The louder the murmur, the higher the grade. Take note, however, that the grade of a murmur only gives a better understanding of the severity and does not show whether heart disease is present or not.

How are heart murmurs diagnosed in dogs?

A heart murmur is a clinical sign that can be present at birth (congenital) or may develop (acquired). Murmurs may be noticed during a routine physical exam in a dog that has no signs or symptoms of disease.

Sick dogs that have a heart murmur often need further diagnostic tests to determine if the murmur is associated with the illness. Dogs with symptoms of possible heart disease such as difficulty breathing, weakness, exercise intolerance, along with a heart murmur need further evaluation, including blood tests, x-rays, and echocardiogram (ultrasound of the heart).

Describing Heart Murmurs in Dogs

Veterinarians describe heart murmurs by listening to the heart and lungs of a dog using a stethoscope and define them using the following terms:

  • Grade from softest to strongest: Grade I-VI/VI (Grade 1-6/6)
  • Location where the murmur is heard the loudest on the chest (left-sided, right-sided, high, low, etc.) and if it can be heard in other areas of the chest or neck.
  • Murmurs are described by when they occur during the heart cycle (systole, diastole) or if they are continuously heard throughout the cycle.

Additional Testing

If your dog has a heart murmur, your vet will discuss additional testing based on your dog’s physical exam findings, health status, history of illness, age, and risk.

Veterinarians often begin with x-rays of your dog’s chest to evaluate their heart and lungs. Echocardiography is the imaging test of choice to determine what is causing the heart murmur. Other tests, including a complete blood count, chemistry panel, and urinalysis will help determine a baseline and if your dog has any underlying issues or disorders that may be contributing to the murmur. NT-pro-BNP testing is a cardiac biomarker that is often used to determine the severity of a patient’s heart disease. This test is recommended in dogs with a murmur but no symptoms of heart disease.

Heart murmurs associated with congenital heart disease include:

  • PDA (patent ductus arteriosis)
  • Aortic stenosis
  • Septal defects
  • Atrioventricular valve dysplasia
  • Tetralogy of Fallot
  • Other less common lesions

Heart murmurs associated with acquired heart disease include:

  • Mitral or tricuspid degenerative valvular heart disease
  • Cardiomyopathies
  • Anemia
  • Endocarditis
  • Diastolic dysfunction syndromes

Dog Breeds at Risk for Heart Murmurs

Different breeds of dogs are at risk for specific congenital heart malformations associated with heart murmurs. Some dog breeds are more likely to have physiologic murmurs, diseases of the heart, and the great vessels.

Small breed dogs such as Chihuahuas, Shih Tzus, and Toy Poodles are at risk for AVVI (atrioventricular valvular insufficiency), which is an acquired heart disease. Large breed dogs such as Great Danes, Doberman Pinschers, and Golden Retrievers can develop DCM (Dilated Cardiomyopathy).

Heart Murmurs in Young Dogs

In young dogs, soft heart murmurs (grade 1 or 2-3) are often called ‘innocent’ murmurs and don’t have any consequence to them. These murmurs usually resolve on their own when a puppy or kitten is about 14 weeks of age. However, a grade 3 to 6 murmur in puppies and soft murmurs that persist beyond 14-16 weeks old should be brought to the attention of a vet who may recommend the patient to a veterinary cardiologist. This may be an important symptom of congenital heart disease.

Following the basic diagnostic protocol for heart disease, the veterinary cardiologist will perform tests that include an echocardiogram, electrocardiogram, blood pressure, etc. to assess heart structure and function and determine if congenital heart disease is present.

Depending on the problem that is present, some congenital heart diseases can be treated or alleviated with specific procedures, such as surgery, catheter procedure, or medications.

Heart Murmurs in Older, Small Breed Dogs

The presence of heart murmurs in senior, small breed dogs may indicate the presence of a leaky heart valve located between the left atrium and left ventricle of the heart. This is called the ‘mitral valve’ and functions in allowing the flow of blood from the left atrium to the left ventricle and preventing any back-flow of blood from the left ventricle to the left atrium.

As dogs age, there is a consequent age-related degeneration of the mitral valve. When this happens, the valve is unable to keep the blood from leaking backward. The condition is known by several names - chronic valve disease, degenerative mitral valve disease, or endocardiosis.

Mild cases of degenerative mitral valve disease usually don’t result in a worrisome problem for an affected dog. But it’s a progressive disease and as the mitral valve continues to deteriorate, the leak can eventually worsen over time until it becomes severe enough to increase a dog’s risk of congestive heart failure.

Congestive heart failure happens when the leak through the mitral valve overwhelms the heart and fluid flows backward into the lungs instead of being pumped by the heart throughout the body. Dogs with congestive heart failure exhibit the following symptoms:

  • Cough that is usually more pronounced when at rest
  • Rapid breathing rate especially while at rest
  • Difficulty breathing
  • Fainting
  • Weakness
  • Lethargy
  • Poor exercise tolerance
  • Pot-bellied appearance

An echocardiogram can help determine the stage of chronic valve disease that is present. A chest x-ray can diagnose the presence or absence of congestive heart disease. If your dog is in congestive heart failure, your vet can prescribe medications to allow your dog have a good quality of life and prolong longevity.

Heart Murmurs in Middle-Aged to Older Large Breed Dogs

The presence of heart murmurs in these dogs may be a red flag for dilated cardiomyopathy. This is a problem of the ventricles, which are the pumping chambers of the heart. The muscles of the ventricle become weakened and affect the heart’s ability to contract when pumping blood. This can eventually lead to congestive heart failure.

In addition to exhibiting the typical signs of congestive heart failure, dogs with dilated cardiomyopathy may also experience fainting and can suddenly die from abnormal heart rhythm (cardiac arrhythmias).

While dilated cardiomyopathy can affect any middle-aged or older large breed dogs, cases are more predominant among Cocker Spaniels, Dalmatians, Great Danes, Irish Wolfhounds, Doberman Pinschers, Boxers, Portuguese Water Dogs, and Newfoundlands.

Other Risk Factors

Heart murmurs can happen secondary to non-cardiac problems such as fever, infection, anemia, hyperthyroidism, extreme thinness, obesity, and pregnancy.

It is important to understand that some heart diseases may not have a murmur, meaning the absence of a heart murmur does not mean that a patient does not have heart disease.

Is treatment available for dogs with heart murmurs?

Treating a heart murmur begins with determining the cause of the murmur. There are different medications available, depending on the diagnosis. Not all murmurs need treatment. Your vet will use their assessment of the severity or intensity of the murmur along with x-rays evaluating the heart size to determine the seriousness and outcome of the heart disease and necessary or recommended treatment(s).

Read more:

Your Pet’s Heart: A Guide to Understanding Heart Health in Dogs and Cats

How to Measure Resting Respiratory Rates in Dogs
Does my dog need a pacemaker?

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Published: 4/29/2021
Last updated: 12/13/2021
Dr. Denise Michanowicz, Veterinarian

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