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.
From the sinus node, the signal travels through bundles of cells. 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.
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.
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.