By:  Megan Poad, VMD, DACVIM (Cardiology)

A heart murmur is an abnormal heart sound that is heard when listening to the heart with a stethoscope. It is caused by abnormal turbulent blood flow. Abnormal turbulent blood flow can occur when blood passes across abnormal heart valves or across abnormal structures within the heart. It can also occur when blood flows very fast across normal structures (such as when an animal is excited). An echocardiogram (ultrasound of the heart) is a non-invasive test that is performed to determine the cause of the murmur.

Your veterinarian will grade the heart murmur out of 6, with 6/6 being the loudest murmur. In some instances, the louder the murmur, the more significant the abnormality within the heart.

If you have a new puppy or kitten:

Soft heart murmurs (grade 1/6 to 2-3/6) can occur in young animals and are of no consequence to them. These types of murmurs are called “innocent” murmurs. They are soft and usually go away by 14 weeks of age.

Loud heart murmurs (Grade 3-4/6 to 6/6) in a new puppy or kitty and soft heart murmurs that persist beyond 14-16 weeks of age should be evaluated by a cardiologist. This may indicate that congenital (present at birth) heart disease is present. The cardiologist will perform an echocardiogram as well as other tests (EKG, blood pressure, etc.) to assess the heart and determine if congenital heart disease is present. Depending on the disease present, some congenital diseases can be fixed or made better with an interventional catheter procedure, traditional surgery, or administration of cardiac medications.

If you have an older, small breed dog:

Heart murmurs in these dogs may indicate that these dogs have a leaky mitral valve (the heart valve in between the left atrium and left ventricle). The mitral valve’s job is to allow blood to flow from the left atrium to the left ventricle but not allow blood to flow backward from the left ventricle to the left atrium. This valve degenerates as dogs age and when it does, the older valve allows blood to leak backwards. We refer to this disease as chronic valve disease, endocardiosis or degenerative mitral valve disease.

Mild chronic valve disease that leads to small leaks across the mitral valve usually does not create a problem for a dog. The disease is slowly progressive in most cases and the leak will continue to worsen over months to years. If the disease becomes severe, the dog is at risk for developing congestive heart failure. Congestive heart failure means that the leak across the heart valve overwhelms the heart and fluid goes from the heart backwards into the lungs (instead of from the heart forward to the body). Signs of congestive heart failure include cough (especially a cough at rest), a fast breathing rate (especially at rest), difficulty breathing, fainting, weakness, lethargy, exercise intolerance and/or abdominal distension.

A cardiologist can perform an echocardiogram and determine the stage of chronic valve disease present. The presence or absence of congestive heart failure is diagnosed by X-ray of the chest. If congestive heart failure is present, oral medications can be prescribed to help maintain a good quality of life. Some dogs can do well with these medications for 1-2 years.

If you have a middle-aged to older large breed dog (especially a Doberman Pinscher, Irish Wolfhound, Great Dane, Boxer, St. Bernard, Newfoundland, Cocker Spaniel, Dalmatian or Portuguese Water Dog):

Heart murmurs in these dogs may indicate that they have a disease called dilated cardiomyopathy (with a subsequent leaky mitral valve). Dilated cardiomyopathy is a disease of the pumping chamber (ventricle) of the heart where the muscle becomes weakened and the contraction of the heart is decreased. When the ventricle is unable to pump blood to the body, fluid goes from the heart backwards into the lungs (instead of from the heart forward to the body). This is known as congestive heart failure. Signs of congestive heart failure include cough (especially a cough at rest), a fast breathing rate, difficulty breathing, fainting, weakness, lethargy, exercise intolerance and/or abdominal distension. In addition, because the heart is not pumping effectively, these dogs can develop profound weakness, lethargy, exercise intolerance and fainting. Unfortunately, they can also pass away suddenly from abnormal heart beats (arrhythmias) from the diseased heart chambers.

A cardiologist can perform an echocardiogram and determine if dilated cardiomyopathy is present in susceptible breeds. Oral medications can be initiated in the preclinical (asymptomatic) phase of dilated cardiomyopathy and in some cases, these medications can delay the progression of the disease to the development of congestive heart failure. If a dog develops clinical symptoms of congestive heart failure, X-rays are used to confirm the presence and determine the severity of the congestive heart failure. If congestive heart failure is present, oral medications can be prescribed to help maintain a good quality of life. Some dogs can do well with these medications for a period of months to 1-2 years.

If you have a middle-aged to older cat (especially a Maine Coon, Norwegian Forest Cat, Ragdoll, British Shorthair, American Shorthair, Sphynx or Devon Rex):

Heart murmurs in cats can be due to the excitement of the veterinary visit (physiologic heart murmurs) or due to underlying heart disease. An echocardiogram is necessary to differentiate between a heart murmur that is just due to excitement versus a heart murmur from underlying heart disease. Heart murmurs (or other abnormalities heard with a stethoscope) may indicate that cats have a disease called hypertrophic cardiomyopathy. This is the most common acquired heart disease seen in cats. Hypertrophic cardiomyopathy is a disease of the pumping chamber of the heart where the muscle does not relax well after it contracts. Blood is then unable to fill the heart normally and fluid goes from the heart backwards into the lungs (instead of from the heart forward to the body). This is known as congestive heart failure. Signs of congestive heart failure include a fast breathing rate, difficulty breathing, fainting, weakness, lethargy, hiding and a decreased appetite. Unfortunately, in addition, these cats can also pass away suddenly from abnormal heart beats from the diseased heart chambers. Some cats can develop signs of arterial thromboembolism, in which a clot forms within the heart, exits the heart, and interrupts blood flow to tissues/organs of the body (including the front or back legs).

A cardiologist can perform an echocardiogram and determine if hypertrophic cardiomyopathy or other structural heart disease is present in susceptible breeds and in cats that have abnormal heart sounds on physical examination. The progression of disease is different in every cat. Some cats develop congestive heart failure soon after their disease is diagnosed by echocardiogram. Other cats remain asymptomatic for their hypertrophic cardiomyopathy for many years before they develop congestive heart failure. Still others never develop congestive heart failure or any symptoms of their heart disease. Recheck cardiology visits and echocardiograms are necessary to determine the progression of disease in each patient. If a cat develops clinical symptoms of congestive heart failure, X-rays are used to confirm the presence and determine the severity of the congestive heart failure. If congestive heart failure is present, oral medications can be prescribed to help maintain a good quality of life, and some cats can do well with these medications for a period of months to 1-2 years.