The normal heartbeat originates from the heart's normal pacemaker, called the sinus node. A fast arrhythmia occurs when the electrical activity of the heart originates from a location other than this normal pacemaker or when the normal pacemaker activates the heart at an abnormally rapid rate.

Abnormal locations of rhythm origin can be in the upper or lower chambers of the heart, or it can be a "circuit" composed of parts of the upper chambers, the lower chambers, or both.