What could be causing my slow heartbeat and is it dangerous?

DEAR DOCTOR K:

Last time I went to the doctor, my heart rate was 55 beats per minute. What could be causing my slow heartbeat? Is it dangerous?

DEAR READER:

A normal heart rate at rest is between 60 and 100 beats per minute. A slow heart rate, of less than 60 beats per minute, is called bradycardia. You have just a slight bradycardia.

Bradycardia can be normal if you’re a well-conditioned athlete. A patient of mine bicycles 50 miles a week and has a resting heart rate of 50. Regular exercise improves the heart’s ability to pump blood efficiently. As a result, the heart doesn’t need to beat as often to supply the body’s needs.

But sometimes bradycardia is caused by an abnormal heart condition. To understand what can go wrong, you need to understand how the heart is supposed to work.

The heart is a fist-sized muscle. It has four chambers, each with walls of muscle. Each chamber pumps when the muscle of the chamber gets an electrical signal to do so.

When everything is working properly, an electrical signal initiates a heartbeat. This signal comes from the heart’s sinus node, located in the right atrium (one of the two upper chambers of the heart, the atria).

From the sinus node, the signal travels through bundles of cells that carry the signals as if they were a copper wire. The signal first travels to the atrioventricular (A-V) node, located between the atria.

Then the signal travels through another bundle of cells (called the bundle of His) that is located between the ventricles (the heart’s lower chambers). From there, the signal travels to the muscular walls of the ventricles, which respond to the signal by pumping, here is an illustration of this process:

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A problem in the sinus node can cause bradycardia. Or it can result when signals don’t properly pass through the A-V node and bundle of His. Bradycardia also can be a side effect of certain medications.

Treatment depends on what is causing the bradycardia. If your heartbeat is slow because you’re a healthy athlete, you probably don’t need treatment. Most people don’t need treatment unless they have bothersome symptoms. When I see a patient with an unusually slow pulse at rest, I often have him walk up two flights of stairs. If his heart is healthy, the heart rate will jump in response to exercise.

Bradycardia can be caused by certain medicines (the most common of which are beta-blockers), or by certain diseases (such as an underactive thyroid, or hypothyroidism).

Bradycardia resulting from certain cardiac arrhythmias can be treated with a permanent pacemaker. When implanted in the body, this device generates electrical impulses that regulate the heartbeat.

When it’s abnormal, a slow heart rate almost always has an effective treatment. But I’ll bet it’s not abnormal in you, just healthy.