Q. My teenage son, who is in perfect health, was recently found to have an irregular heartbeat. His EKG (electrocardiogram) was abnormal, but his echocardiogram was normal. Oddly enough, his heartbeat seems to be irregular when he's not active, but it then becomes normal after he's been exercising. After our son had a 24-hour recording of his heartbeat, a cardiologist said that he didn't need to restrict his activities in any way, and that there was nothing to worry about.

Can you tell us what could be causing this problem and what the prognosis is? My son is very athletic, and we hope this condition won't interfere with his activities.

A. It sounds like your son has a type of harmless, slightly irregular heartbeat that commonly occurs in children and athletes. There are a few types, the most common of which is known as sinus arrhythmia. Once your son is checked and is found to have no signs of a more serious problem, you don't have to worry and he doesn't have to restrict his activities. In fact, you can think of his condition as a slight variation of normal.

The term arrhythmia means irregular heart rhythm, or heartbeat. The term sinus refers to the heart's internal pacemaker--the cells that trigger the heart to pump. With sinus arrhythmia, the pacemaker is in control, but the rhythm is slightly irregular. This differs from other forms of arrhythmia, in which cells other than the sinus cells take over control of the heart's rhythm.

It turns out that sinus arrhythmia is common in children. It almost seems as if the sinus cells have not quite settled down to a steady, predictable rhythm. In fact, the rhythm can vary with breathing, getting slightly slower or faster as the child breathes in and out. Other factors can alter the sinus cells' rhythm as well. For example, exercise can make a child's heartbeat go from regular to irregular. Or, as in your son's case, from irregular back to a regular rhythm.

In addition to sinus arrhythmia, there are a couple of other common types of irregular heartbeat in children. One of these is known as premature atrial contractions, or PACs. These are normal beats that occur sooner than expected in the heartbeat cycle. Almost all of the time, they are not a sign of anything wrong with the heart.

Another type of irregular heartbeat is one that varies depending on how fast the heart is beating. This is known as a rate-dependent arrhythmia. This means that the heart will beat regularly when going at one rate, but then get slightly irregular as it speeds up or slows down. The opposite can also happen, with the heartbeat becoming irregular when it goes faster or slower.

Athletes, especially well-conditioned ones, often have heartbeats that are slightly irregular. You may be familiar with the fact that athletes can have very slow heartbeats, sometimes around 40 beats a minute, instead of the more typical 70 or 80 beats a minute. They can also have slightly irregular heartbeats, either a sinus arrhythmia, PACs or another type. In almost all cases, these are harmless.

If your child has an irregular heartbeat, your doctor can check it out by doing an EKG (electrocardiogram), which makes a tracing of the heartbeat on paper. This will tell what kind of arrhythmia it is and what it probably means. Additional tests might include an echocardiogram, which takes a picture of the heart using sound waves and a Holter monitor. This device--much a like small tape recorder--makes a 24-hour recording of your heartbeat and examines it for any irregularities.

If everything checks out okay, then it's safe to say that your child's irregular heartbeat is harmless. It shouldn't cause him any problems, and it's likely that he'll outgrow it in time.

Jay Siwek, chairman of the department of family medicine at Georgetown University Medical Center, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, DC 20071. Questions cannot be answered personally.