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What Causes Heart Murmurs?

Innocent Heart Murmurs

Why some people have innocent heart murmurs and others do not isn't known. Innocent murmurs are simply sounds made by blood flowing through the heart's chambers and valves, or through blood vessels near the heart.

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. This is because during pregnancy, women's bodies make extra blood. Most heart murmurs that occur in pregnant women are innocent.

Abnormal Heart Murmurs

Congenital heart defects or acquired heart valve disease often are the cause of abnormal heart murmurs.

Congenital Heart Defects

Congenital heart defects are the most common cause of abnormal heart murmurs in children. These defects are problems with the heart's structure that are present at birth. They change the normal flow of blood through the heart.

Congenital heart defects can involve the interior walls of the heart, the valves inside the heart, or the arteries and veins that carry blood to and from the heart. Some babies are born with more than one heart defect.

Heart valve problems, septal defects (also called holes in the heart), and diseases of the heart muscle such as hypertrophic cardiomyopathy are common heart defects that cause abnormal heart murmurs.

Examples of valve problems are 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. A hole in the septum between the heart's two lower chambers is called a ventricular septal defect.

Hypertrophic (hi-per-TROF-ik) cardiomyopathy (kar-de-o-mi-OP-ah-thee) (HCM) occurs if heart muscle cells enlarge and cause the walls of the ventricles (usually the left ventricle) to thicken. The thickening may block blood flow out of the ventricle. If a blockage occurs, the ventricle must work hard to pump blood to the body. HCM also can affect the heart’s mitral valve, causing blood to leak backward through the valve.

Heart Defects That Can Cause Abnormal Heart Murmurs

Figure A shows the structure and blood flow inside a normal heart. Figure B shows a heart with leaking and narrowed valves. Figure C shows a heart with a ventricular septal defect.

Figure A shows the structure and blood flow inside a normal heart. Figure B shows a heart with leaking and narrowed valves. Figure C shows a heart with a ventricular septal defect.

For more information, go to the Health Topics Congenital Heart Defects article.

Acquired Heart Valve Disease

Acquired heart valve disease often is the cause of abnormal heart murmurs in adults. This is heart valve disease that develops as the result of another condition.

Many conditions can cause heart valve disease. Examples include heart conditions and other disorders, age-related changes, rheumatic (ru-MAT-ik) fever, and infections.

Heart conditions and other disorders. Certain conditions can stretch and distort the heart valves, such as:

Damage and scar tissue from a heart attack or injury to the heart.

Advanced high blood pressure and heart failure. These conditions can enlarge the heart or its main arteries.

Age-related changes. As you get older, calcium deposits or other deposits may form on your heart valves. These deposits stiffen and thicken the valve flaps and limit blood flow. This stiffening and thickening of the valve is called sclerosis (skle-RO-sis).

Rheumatic fever. The bacteria that cause strep throat, scarlet fever, and, in some cases, impetigo (im-peh-TI-go) also can cause rheumatic fever. This serious illness can develop if you have an untreated or not fully treated streptococcal (strep) infection.

Rheumatic fever can damage and scar the heart valves. The symptoms of this heart valve damage often don't occur until many years after recovery from rheumatic fever.

Today, most people who have strep infections are treated with antibiotics before rheumatic fever develops. It's very important to take all of the antibiotics your doctor prescribes for strep throat, even if you feel better before the medicine is gone.

Infections. Common germs that enter the bloodstream and get carried to the heart can sometimes infect the inner surface of the heart, including the heart valves. This rare but sometimes life-threatening infection is called infective endocarditis (EN-do-kar-DI-tis), or IE.

IE is more likely to develop in people who already have abnormal blood flow through a heart valve because of heart valve disease. The abnormal blood flow causes blood clots to form on the surface of the valve. The blood clots make it easier for germs to attach to and infect the valve.

IE can worsen existing heart valve disease.

Other Causes

Some heart murmurs occur because of an illness outside of the heart. The heart is normal, but an illness or condition can cause blood flow that's faster than normal. Examples of this type of illness include fever, anemia (uh-NEE-me-eh), and hyperthyroidism.

Anemia is a condition in which the body has a lower than normal number of red blood cells. Hyperthyroidism is a condition in which the body has too much thyroid hormone.

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September 20, 2012 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.

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