Causes

A heart murmur may happen:

• When the heart is filling with blood (diastolic murmur)
• When the heart is emptying (systolic murmur)
• Throughout the heartbeat (continuous murmur)

A heart murmur may be innocent or abnormal.

Innocent heart murmurs

A person with an innocent murmur has a normal heart. This type of heart murmur is common in newborns and children.

An innocent murmur can occur when blood flows more rapidly than normal through the heart. Conditions that may cause rapid blood flow through your heart, resulting in an innocent heart murmur, include:

• Physical activity or exercise
• Pregnancy
• Fever
• Not having enough healthy red blood cells to carry adequate oxygen to your body tissues (anemia)
• Too much thyroid hormone in your body (hyperthyroidism)
• Phases of rapid growth, such as adolescence

Innocent heart murmurs may disappear over time, or they may last your entire life without ever causing further health problems.

Abnormal heart murmurs

In adults, abnormal murmurs are most often due to acquired heart valve problems. In children, abnormal murmurs are usually caused by structural problems of the heart (congenital heart defects).

Common congenital defects that cause heart murmurs include:

• Holes in the heart. Known as septal defects, holes in the heart may or may not be serious, depending on the size of the hole and its location.
• Cardiac shunts. Cardiac shunts occur when there's an abnormal blood flow between the heart chambers or blood vessels, which may lead to a heart murmur.
• Heart valve problems present from birth. Examples include valves that don't allow enough blood through them (stenosis) or those that don't close properly and leak (regurgitation).

In older children and adults, causes of abnormal heart murmurs include infections and conditions that damage the structures of the heart. For example:

• Valve calcification. This hardening or thickening of valves, as in mitral stenosis or aortic valve stenosis, can occur as you age. Valves may become narrowed (stenotic), making it harder for blood to flow through your heart, resulting in murmurs.

• Endocarditis. This infection of the inner lining of your heart and valves typically occurs when bacteria or other germs from another part of your body, such as your mouth, spread through your bloodstream and get stuck in your heart.

  Left untreated, endocarditis can damage or destroy your heart valves. This condition usually occurs in people who already have heart valve problems.

• Rheumatic fever. Although now rare in the United States, rheumatic fever is a serious condition that can occur when you don't receive prompt or complete treatment for a strep throat infection. It can permanently affect the heart valves and interfere with normal blood flow through your heart.