Heart Murmurs in Dogs

What is a heart murmur?

A heart murmur is an abnormal heart sound, usually detected when your veterinarian listens to your dog’s heart with a stethoscope.

What causes a heart murmur?

A heart murmur is caused by turbulent blood flow in the heart. Some murmurs are “innocent” or “physiologic,” meaning they are not caused by disease. Others are “pathologic,” meaning they are caused by disease. These murmurs may be caused by structural heart disease or a problem outside the heart (an extracardiac problem).

Do all murmurs sound the same?

No. Murmurs vary in how loud they sound, and the loudness reflects the amount of turbulence in the heart. However, the loudness of a heart murmur does not always indicate how severe the underlying disease is.

Murmurs are graded by their intensity, usually on a scale of 1 to 6. A Grade 1 murmur is very quiet, may only be heard intermittently, and is usually only heard in one location on the chest. A Grade 6 murmur is very loud, heard everywhere that the heart can be heard, and can be felt when a person places their hand on the chest near the heart.

Your veterinarian also assesses when the murmur occurs during the heartbeat, how long it lasts, and where it is loudest. Most murmurs in dogs occur during systole, the phase when the heart contracts to pump blood out. These characteristics, along with any signs of illness your dog may have, help your veterinarian to determine what is causing the murmur.

What is an innocent or physiologic heart murmur?

An innocent or physiologic heart murmur is benign (not harmful) and has no impact on your dog's health. One type is often found in young, growing puppies, particularly large-breed puppies that are growing rapidly. The murmur may first appear at six to eight weeks of age, and a puppy with an innocent heart murmur will usually outgrow it by about four to five months of age. In general, these murmurs are quiet (usually Grade I to 2) and do not cause any signs of illness.

What structural heart problems cause a heart murmur?

Structural heart problems can disturb blood flow, creating turbulence. These problems may include a leaky heart valve, a thickening or narrowing of a valve or large blood vessel, or an abnormal hole between the heart chambers or between two arteries that are not normally connected. Structural heart problems may be congenital (present at birth) or acquired later in life.

The heart is composed of four chambers: the left atrium, the left ventricle, the right atrium, and the right ventricle. Blood flows from the right side of the heart, through the pulmonary artery, to the lungs, where it is oxygenated. Then the oxygenated blood goes through the left side of the heart and into the aorta, where it is pumped to the rest of the body. Between each of the chambers and main blood vessels there is a valve that prevents blood from flowing back into the chamber as the heart pumps.

In dogs, some congenital heart diseases are hereditary (genetic), including subaortic stenosis, pulmonic stenosis, and patent ductus arteriosus. Others are not documented as being hereditary, including ventricular septal defects, atrial septal defects, and Tetralogy of Fallot.

The most common cause of an acquired heart murmur in dogs is mitral insufficiency (also called mitral regurgitation), a condition in which the mitral valve becomes thickened and begins leaking. Mitral insufficiency tends to be more common in small-breed dogs. Other causes of an acquired heart murmur in older dogs include dilated cardiomyopathy (DCM) and bacterial endocarditis, which is caused by a bacterial infection that localizes on a heart valve.

What problems outside the heart cause a heart murmur?

Problems outside the heart (extracardiac problems) that can cause a heart murmur include anemia (low levels of red blood cells), hypoproteinemia (low protein levels in the blood), fever or infection, pregnancy, obesity, or emaciation (extreme thinness). Murmurs caused by these conditions are often called functional heart murmurs. 

In young puppies, anemia and hypoproteinemia can be caused by a heavy infestation of parasites such as intestinal worms, blood parasites, fleas, or ticks. Adult dogs may become anemic because of blood loss or a serious underlying disorder.

How will my veterinarian determine whether the murmur is caused by disease?

If your veterinarian detects a heart murmur, they will ask you about your dog's health and perform a physical examination. These findings will help them determine whether further testing is needed.

If your puppy has a quiet murmur, your veterinarian may recommend a re-check in a few weeks to see whether it has disappeared. This may indicate that it was an innocent murmur. Similarly, if your adult dog appears extremely stressed during an examination, your veterinarian may recommend a re-check when your dog is calmer.

A dog with a heart murmur that is caused by a structural heart disease or an extracardiac problem will generally have some symptoms or clinical signs that can be attributed to the disease. However, the clinical signs may be subtle and not noticeable until the disease becomes advanced. The most common symptoms in a dog with a clinically significant heart murmur include:

• Poor appetite
• Weight loss (or stunted growth in a puppy)
• Breathing problems (often occurring in the middle of the night or after the dog has been lying down for a while)
• Pale gums
• Poor exercise tolerance
• Collapse or fainting spells
• Coughing

An abnormal heart rhythm or weak pulse may suggest an underlying problem. If your veterinarian suspects structural heart disease or an extracardiac problem, they will recommend further testing. In most cases, further testing should be performed immediately so that treatment can be started as soon as possible.

What other tests may be recommended?

Depending on the examination findings, your veterinarian may recommend X-rays, an electrocardiogram (ECG), or an ultrasound examination of the heart (echocardiogram). If your veterinarian suspects that the heart murmur is secondary to another disease, they may recommend blood tests or other tests, such as blood pressure measurements. Your veterinarian may refer your dog to a veterinary cardiologist for further testing.

The most useful test to determine the location of a heart murmur is an echocardiogram that includes a Doppler examination. An echocardiogram produces images of the beating heart, allowing the veterinarian to assess its size and movement. A Doppler examination measures the speed and direction of blood flow through the heart’s chambers and valves. Together, these tests help identify the source of the turbulence causing the murmur. 

How is a heart murmur treated?

Treatment depends on the underlying cause of the heart murmur. Physiologic heart murmurs do not require any treatment, but regular monitoring is advised. If the murmur is caused by an underlying problem, treatment depends on the diagnosis and may include specialized diets, medications, and supportive care.

Some congenital heart defects can be surgically corrected, including pulmonic stenosis and patent ductus arteriosus. Your veterinarian will advise you on the appropriate course of action to meet your dog's needs.

What is the long-term outlook?

The long-term outlook ranges from excellent to very poor, depending on the cause of the murmur. 

• If the murmur is physiologic, the outlook is good to excellent. 
• If the murmur is caused by a problem outside the heart that can be treated, the murmur may resolve over time. 
• If the murmur is caused by congenital heart disease, the outlook depends on the type of heart defect present. If the defect can be surgically corrected, the outlook is very good.

For acquired heart disease, the outlook varies with the condition:

• A dog with mitral insufficiency can usually be managed with long-term medications. 
• A dog with dilated cardiomyopathy has a variable outlook; if they show signs of heart failure, the outlook is very poor. 
• A dog with bacterial endocarditis has a variable outlook, depending on the severity of the infection and the valve affected. 

Each case is different. Your veterinarian will discuss the treatment options and outlook for your dog, along with any monitoring and follow-up testing needed.