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What causes a heart murmur?

Andrea C. Bryan, MD
Cardiology (Cardiovascular Disease)
Heart murmurs are abnormal heart sounds, usually characterized by “whooshing or swishing.” These sounds are caused by turbulent blood in or near the heart and can be heard with a stethoscope. A heart murmur isn’t a disease but may represent an underlying heart problem. Most heart murmurs are harmless and don’t need treatment. However, some murmurs may require follow up tests to be sure that the murmur isn’t caused by a serious underlying heart condition.

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Anthony L. Komaroff, MD
Internal Medicine
A heart murmur is a sound caused by increased or turbulent flow of blood through heart chambers or valves. Some heart murmurs do not indicate heart problems, but others do. For example, certain heart murmurs can be normal when a woman is pregnant. But a heart murmur can also be caused indirectly by coronary artery disease if inadequate blood flow deprives the muscles controlling the heart's mitral valve of needed oxygen. The mitral valve may also have problems closing correctly if the heart has enlarged because of damage from a heart attack or other medical problem. If the valve fails to close completely during each contraction, blood can be squeezed backward through the valve -- a syndrome called mitral regurgitation. Unless the amount of mitral regurgitation is very minor or very pronounced, it will produce a murmur that can be detected with a stethoscope. Other valves in your heart -- the aortic, tricuspid, and pulmonary valves -- can also malfunction, causing murmurs. Valvular murmurs can result from regurgitation (leaky valve), stenosis (a tight valve, usually due to calcium or infection), or tears in the valve tissue.
An abnormal heart murmur can have several causes:
• The heart valves may be too narrow (called stenosis), causing them to limit the flow of blood through them.
• The heart valves might not be closing properly and instead are allowing blood to “leak” (regurgitate) backward in the heart.
• There may be a hole in the wall that separates the heart’s right and left chambers. A hole in the wall between the left and right upper chambers (the atria) is called an atrial septal defect (ASD). A hole in the wall between the left and right lower chambers (the ventricles) is called a ventricular septal defect (VSD).
• There may be abnormal blood flow between blood vessels, such as the ductus arteriosus and the aorta. The ductus arteriosus is normally present while a baby is growing in the mother’s womb, but it should naturally close soon after birth. A heart murmur could indicate the vessel has not closed as it should have.
• Muscular narrowings within the pumping chambers can cause a murmur as the heart tries to pump blood through these areas.
• Narrowed arteries within the lungs or the main body artery (aorta) may also cause a murmur.
Mark L. Cohen, MD
Cardiology (Cardiovascular Disease)
A heart murmur is a sound made from blood moving through the heart or one of the large blood vessels near the heart. It is heard with a stethoscope. All by itself, a heart murmur does not cause you to feel anything. Normally, the heart sounds occur only when the heart valves open and close. Blood flow through the heart usually doesn't cause any sound. When the sound is heard, it is a result of the blood flow not being entirely normal. Making the diagnosis of what is causing this is very important. In the old days, doctors were very, very good at making this diagnosis with a stethoscope and often times with feeling the heart through your skin or the pulsation of your arteries. In modern times, an echocardiogram is used and will virtually always make the diagnosis instantaneously. Many heart murmurs are absolutely nothing to worry about and are a result of totally normal blood flow. Some are caused by minor irregularities in blood flow that will not require treatment. Others may require followup and repeat echocardiograms. There are some types caused by abnormal heart valves or abnormalities of the heart itself that can require specific therapy. The echocardiogram will make the diagnosis and your doctor will be able to tell you exactly what is needed.
There are two types of heart murmurs caused by following reasons:
Innocent heart murmurs
Innocent heart murmurs are sounds heard when blood flows through a normal heart. These murmurs may occur when blood flows faster than normal through the heart and its attached blood vessels. Illnesses or conditions that may cause this to happen include fever, anemia, and hyperthyroidism (too much thyroid hormone in the body).
Extra blood flow through the heart also may cause innocent heart murmurs. After childhood, the most common cause of extra blood flow through the heart is pregnancy. Most heart murmurs found in pregnant women are innocent. They're due to the extra blood that women's bodies make while they're pregnant.
Changes to the heart that result from heart surgery or aging also may cause some innocent heart murmurs.
Abnormal heart murmurs

The most common cause of abnormal murmurs in children is congenital heart defects. These are problems with the heart's structure that are present at birth.
These defects can involve the interior walls of the heart, the valves inside the heart, or the arteries and veins that carry blood to the heart or out to the body. Some babies are born with more than one heart defect. Congenital heart defects change the normal flow of blood through the heart.
Heart valve defects and septal defects (also called holes in the heart) are common heart defects that cause abnormal heart murmurs.
Valve defects may include narrow valves that limit blood flow or leaky valves that don't close properly.
Septal defects are holes in the wall that separates the right and left sides of the heart.This wall is called the septum.
A hole in the septum between the heart's two upper chambers is called an atrial septal defect (ASD). A hole in the septum between the heart's two lower chambers is called a ventricular septal defect (VSD). ASDs and VSDs account for more than half of all abnormal heart murmurs in children.

This answer from the National Heart, Lung and Blood Institute has been reviewed and/or edited by Dr. William D. Knopf.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.