What Causes a Heart Murmur?

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 (uh-NEE-me-eh), 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.

Heart Defects That Can Cause Abnormal Heart Murmurs

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

Conditions that damage heart valves or other structures of the heart also may cause abnormal heart murmurs. These include rheumatic (ru-MAT-ik) fever, endocarditis (EN-do-kar-DI-tis), calcification (KAL-si-fi-KA-shun), and mitral (MI-tral) valve prolapse (MVP). Heart murmurs due to these problems are more common in adults.

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 a person has an untreated or not fully treated strep infection.

Rheumatic fever can lead to permanent heart damage. If you or your child has strep throat, take all of the antibiotics prescribed, even if you feel better before the medicine runs out.

Endocarditis

Endocarditis is a serious infection of the heart valves or lining of the heart. A bacterial infection usually causes endocarditis, and it usually occurs in an abnormal heart. Endocarditis can lead to permanent heart damage and other health problems.

Calcification

Calcification occurs when the heart’s valves get hard and thick as a result of aging. When this happens, the valves don’t work as they should.

Mitral Valve Prolapse

MVP is a condition in which the heart’s mitral valve doesn’t work properly. In MVP, when the left ventricle contracts, one or both flaps of the mitral valve flop or bulge back (prolapse) into the left atrium. This can cause a heart murmur.