Causes and Risk Factors for Heart Murmur

Innocent heart murmurs can be caused by exercise, pregnancy, and fever. Aging and heart surgery can cause changes in the heart that may result in an innocent murmur. Other conditions that can cause innocent heart murmurs include anemia (low red blood cell count; lack of red blood cells to transport oxygen to other parts of the body) and hyperthyroidism (excessive production of thyroid hormone).

A number of medical conditions can cause abnormal heart murmurs. In children, the most common cause is a congenital heart defect, which develops before birth and affects the blood flow through the heart. In some cases, infants are born with more than one heart defect.

Common congenital heart defects include the following:

  • Atrial septal defect (ASD) is a hole in the septum (wall) that divides the heart's upper chambers (atria).
  • Ventricular septal defect (VSD) is a hole in the septum that divides the heart's lower chambers (ventricles). More than half of all abnormal heart murmurs in children are caused by ventricular septal defects.
  • Patent ductus arteriosus (PDA) occurs when the ductus arteriosus does not close after birth. A developing fetus does not breathe while in the uterus; therefore, its lungs do not require a blood supply. Before birth, a channel called the ductus arteriosus diverts blood around the lungs by connecting the pulmonary arteries, which supply blood to the lungs after birth, to the aorta, which supplies blood to the rest of the body. After birth, when the infant is breathing and the lungs fill with air, the ductus arteriosus normally closes, allowing blood to flow through the pulmonary arteries to the lungs. In a baby with PDA, the channel stays open and blood continues to pass by the lungs.
  • Cardiomyopathy causes the heart muscle to become too thick or too weak to pump blood normally.
  • Stenosis occurs when heart valves are "sticky" or too narrow, affecting blood flow into or out of the heart.
  • Regurgitation occurs when heart valves do not close completely and blood leaks backward into the heart.

In adults and older children, heart murmurs are typically caused by heart valve problems related to infections, illness, or aging. These conditions include the following:

  • Rheumatic fever is an inflammatory illness that can develop when strep throat infection is not properly treated. Rheumatic fever may eventually damage heart valves permanently.
  • Endocarditis is a bacterial infection that attacks the inner lining of the heart. Left untreated, it can damage or destroy heart valves, block blood flow, or cause blood to leak backward.
  • Mitral valve prolapse occurs when the mitral valve does not close properly. The mitral valve is located between the left atrium and left ventricle. When this valve doesn't close as it should, the "leaflets" of the valve expand back into the atrium when the left ventricle contracts. Mitral valve prolapse sometimes is a congenital heart defect that is not discovered until adulthood.
  • Valve calcification may develop as people age. In this condition, the heart valves harden or thicken, making it more difficult for blood to move through them.
  • Cardiac myxoma, which is the most common type of heart tumor in adults, can grow inside of the heart and block blood flow.
  • Hypertrophic cardiomyopathy (also called idiopathic hypertrophic subaortic stenosis) is a condition that causes the muscle inside the left ventricle to thicken, narrowing the path for blood flow.

Risk factors for heart murmurs are associated with the underlying cause. For example, patients who have heart abnormalities are more likely to develop endocarditis (inflammation of the heart lining) following certain medical or dental procedures. Intravenous drug use also can increase the risk for severe cardiovascular complications and heart murmurs.

The risk for valve calcification increases with age. People who have rheumatic fever as children are more likely to have heart valve abnormalities in adulthood. Some causes for heart murmurs (e.g., cardiac myxoma, hypertrophic cardiomyopathy, congenital heart disease) have genetic (hereditary) risk factors.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 19 Nov 2008

Last Modified: 01 Sep 2010