Innocent heart murmurs are heard in 50 percent of children, most often between the ages of 3 and 8. Most children outgrow them as they enter adolescence and the chest wall becomes thicker, which makes the sound harder to hear. The percentage of adults with heart murmurs is much smaller than that of children, though most are still innocent in nature.
A heart murmur is generated by the flow of blood through the heart or its surrounding blood vessels. Murmurs typically have a whooshing or grating sound and are heard in between the “lub-DUB” sound you learned about in school. The lub-DUB comes from heart valves snapping shut as the heart muscle contracts.
In most cases, heart murmurs are the result of turbulence created as blood is being ejected from or returning to the heart. If you hold a water hose up to your ear and bend it a little, the flow of water out of the hose will speed up, creating a whooshing sound. As this happens, you will hear the water whooshing through the hose. The same principle creates heart murmurs.
There are several types of innocent heart murmurs in childhood.
A Still’s murmur is heard when the left ventricle contracts, forcing blood into the aorta, the large artery at the top of the heart that directs oxygen-rich blood to the body. Others can be heard when blood is pumped into the lungs and blood flows back to the heart from the veins in the neck. It is not known why some people have normal heart murmurs and others do not.
Normal heart murmurs can also be caused by exercise, fever, anemia and other physiologic changes that increase blood flow. These tend resolve when the underlying problem goes away.
Evaluating heart murmurs
When a pediatrician hears a heart murmur, he or she will listen carefully to determine if it sounds like a normal murmur or one that might be structural in nature. Normal murmurs usually have a musical or vibratory quality, are brief in duration and are heard in specific locations on a child’s chest. If there is a question about the nature of the murmur, or if the parents are worried about the finding, the child will usually be referred to a cardiologist for evaluation.
Sometime ago, I saw a newborn at Sibley Hospital for his first examination. The baby was 12 hours old, and the physical exam was normal except for a heart murmur. Babies often have a murmur in the first day because a blood vessel called the ductus arteriosus, which is important during the fetal stage, closes shortly after birth. (As the ductus arteriosis is obliterated, it can create turbulence the same way an anatomic defect does, causing a murmur.)