What Causes Heart Murmurs?

Innocent Heart Murmurs

Innocent murmurs are heard when blood moves noisily through a normal heart. Sometimes these murmurs occur when:

• Blood is flowing faster than usual through the heart and blood vessels attached to the heart.
• An increased amount of blood is flowing through the heart.

Illnesses or conditions that can cause blood to flow faster than usual through the heart include:

• Fever
• Anemia
• Too much thyroid hormone in the body (hyperthyroidism)

Many, if not most, children will have a heart murmur heard by their doctor at some time in their lives. After childhood, the most common cause of an increased amount of blood flowing through the heart is pregnancy. Most murmurs found in pregnant women are innocent. They are due to the extra blood that women's bodies make while they are pregnant.

Innocent murmurs are sometimes due to changes to the heart resulting from heart surgery or from aging.

Abnormal Heart Murmurs

The most common cause of abnormal murmurs is congenital heart defects. Congenital heart defects occur when the heart, heart valves, or blood vessels attached to the heart do not develop normally before a baby is born. Some babies are born with a combination of heart defects. Common defects that cause murmurs include:

• Congenital septal defects, which are holes in the wall (septum) that separates the right and left sides of the heart. They account for more than half of abnormal murmurs in children.
• Congenital valve defects, which include narrow valves that do not allow enough blood to flow through them and leaking valves that do not close properly.

Figure A shows the normal anatomy and blood flow of the interior of the heart. Figure B shows a heart with a murmur caused by leaking and narrowed valves. Figure C shows a murmur caused by a ventricular septal defect. Please note that many heart murmurs occur in normal hearts and occur without underlying heart disease. These are known as innocent heart murmurs.

Infections and other conditions that damage heart valves or other structures of the heart also may cause murmurs. These include:

• Rheumatic fever, a serious illness that can develop after a person has an untreated or incompletely treated infection caused by the bacteria that cause strep throat or scarlet fever. Rheumatic fever can lead to permanent damage to the heart. If your doctor diagnoses strep throat, be sure your child takes all of the antibiotics prescribed, even if he or she feels better before the antibiotics run out.
• Endocarditis, an inflammation of the inner lining of the heart and valves that is usually caused by a bacterial infection. Endocarditis is a serious disease that can lead to permanent heart damage and other complications. Endocarditis usually occurs in an abnormal heart.
• Calcification (hardening and thickening) of valves as a result of aging. The hardened and thickened heart valves do not work as they should.