The heart normally beats at a rate of about 60 to 100 beats per minute at rest. A rate faster than 100 beats a minute in an adult is called tachycardia. Most people experience transient rapid heartbeats, called sinus tachycardia, as a normal response to excitement, anxiety, stress, or exercise. If tachycardia occurs at rest or without a logical cause, however, it is considered abnormal.

The two main types of tachycardia are abnormal supraventricular tachycardias (which originate in the upper...