Q: Other than anxiety, what can cause your heart rate to increase when you're not exercising?

A: Rather than just listing the causes, here's my approach to investigating a rapid heartbeat.

First, I'd like to know if you have any other symptoms. That especially includes chest pain, shortness of breath, dizziness, and/or feeling faint. Any of these could suggest an abnormal heart rhythm. If you have any of these and a fast heartbeat, you should call your doctor right away.

Ideally, I'd want you to check your pulse at the time you feel your heart rate is high. You can check your pulse at the wrist or on the neck. At the wrist, lightly press the index and middle fingers of one hand on the opposite wrist, below the fat pad of the thumb. At the neck, lightly feel for your carotid pulse next to your windpipe. Count the number of beats in 15 seconds, and multiply by four. That's your heart rate.

Sometimes, it can feel like your heart is racing when actually it is in the normal range, which is less than 100 beats per minute. Doctors refer to a heart rate of 100 or higher as tachycardia.

As long as you feel perfectly fine, tachycardia rarely has a serious cause. But if you have other symptoms with tachycardia, you should see your doctor.

Your doctor would first determine if the heartbeat is regular (steady, like clockwork) or irregular (jumping around or skipping beats).

Causes of a regular rapid heartbeat (regular tachycardia) not related to exercise or anxiety include:

1. Fever

2. Dehydration

3. An overactive thyroid gland (hyperthyroidism)

4. A heart rhythm abnormality, such as atrial tachycardia, atrial flutter and ventricular tachycardia. However, these would almost always cause additional symptoms, not just a fast heart rate.

The most common cause of a very irregular, fast heartbeat is atrial fibrillation.

I suggest that you call your doctor to discuss your symptoms. If he or she has any concerns, the next step would be a routine EKG. Additional testing might include wearing a heart monitor, called a Holter or event monitor. This device records your heart rate and rhythm for 24 hours or longer.

(Howard LeWine, M.D., is a practicing internist at Brigham and Women's Hospital, Boston, Mass., and Chief Medical Editor of Internet Publishing at Harvard Health Publications, Harvard Medical School.)

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