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Q: What would cause a person to have a slow heartbeat?

A: First, let's clarify what we mean by slow heartbeat. Doctors say someone has a slow heart rate ("bradycardia") when the pace is fewer than 60 beats per minute.

The most common and best reason to have a slow heart rate is to be physically fit. If you exercise more, your heart can supply your body when it is at rest with fewer beats per minute. High-performance athletes often have heart rates in the 40s. There is essentially no heart rate that is too slow—as long as your body is getting all the blood it needs.

How can you tell if your body is not getting all the blood it needs? Unfortunately, the first sign is passing out. Fainting spells are a sign that the heart is going abnormally slow, and the brain is not getting enough oxygen.

In the elderly, a heart attack or some other disease that causes scarring within the heart can damage the heart's electrical system. Sometimes, as the heart's electrical system is slowly damaged, the heart will have periods when it races, and other times when it goes very slowly. People with this problem often need pacemakers to keep the heart rate up during the slow periods.

The heart rate also slows down with a few other medical conditions. These do not originate in the heart, but they do affect it. For example, people with low thyroid conditions will often have a very slow heartbeat. People who are very cold also have slow heartbeats. In these conditions, the slow heart rate is usually not the most serious problem—fixing the underlying medical condition is the immediate concern.

When people come to see me concerned about a slow heartbeat, I first ask them if they have had fainting spells, or do not have enough energy to do their daily activities. If they answer "no" to those questions, I next see if I can make their heart speed up by having them walk up a flight or two of stairs. If they can do that activity, and their heart speeds up, I reassure them that their slow heart rate probably just means they are in good shape.