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Q: My father sometimes develops a very slow heart rate. He tells me not to worry because he feels OK. What happens inside the heart when the rate gets too low?

A: The steady beat of your heart depends on a crescent-shaped cluster of cells in the upper right portion of your heart. Called the sinoatrial or sinus node, it emits a tiny jolt of electrical current that triggers the heart to contract and pump blood throughout the body.

Because the sinus node determines the heart’s pace and rhythm, it is sometimes called the body’s natural pacemaker.

Age-related degeneration can affect the sinoatrial node and other parts of the heart’s conduction system. As a result, the heart’s normal rhythm (which typically ranges from 60 to 100 beats per minute) may drop below 60, known as bradycardia.

Bradycardia can occur when the sinoatrial node falters. If it suddenly stops working, however, the heart has “backup generators” that will take over.

One of these backups is the atrioventricular node, which can maintain a heart rate of about 50 to 60 beats per minute. If the electrical signals passing through this area are slowed or blocked, this is known as AV block or heart block.

Next in the conduction pathway are the left and right bundle branches, which are the final signals telling the ventricles to contract. If a slowdown or complete blockage happens over one of these paths, the other side can often compensate.

If all those backups fail, the heart muscle may still contract, but only about 30 to 40 beats per minute.

Get it checked out

Even though your father says he has no symptoms, he should make an appointment with his doctor to have an electrocardiogram. Sometimes older people blame fatigue and decreased exercise tolerance on just getting older when these symptoms are actually related to bradycardia.

Because episodes of bradycardia may come and go, the office electrocardiogram may not pick up the problem. Instead, people often need to wear a portable heart monitor that records the heart’s rhythm for a couple of weeks or longer.

Treating bradycardia will depend on the cause and severity. For example, it might be as simple as adjusting his medications. Beta blockers and calcium channel blockers frequently cause low heart rates.

If his medical evaluation shows irreversible damage to the heart’s electrical system, he will likely benefit from a permanent pacemaker. These devices are programmed to stimulate or “pace” the heart as needed to keep it beating normally.

As a result, pacemakers can help people stay active. And research suggests that people with pacemakers who do not have other serious health problems tend to live nearly as long as those without pacemakers.

Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, see health.harvard.edu.

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