Under normal circumstances the heart beats 60 to 80 times per minute, with co-ordinated electrical surges that keep the rhythm steady. But sometimes the heart beats irregularly—either too fast, too slow or in an erratic manner. These abnormalities, called arrhythmias, can cause disruptive and frightening symptoms.

In the worst-case scenario, they can trigger a potentially fatal cardiac arrest (sudden cessation of an effective heartbeat).

A normal heartbeat begins with an electrical signal generated by cells in the upper right part of the heart—a region called the sinus node. The sinus node is the heart's natural pacemaker, initiating the heartbeat. Arrhythmias occur when these signals or their transmission go awry.

The various types of arrhythmias are categorized according to two features: their site of origin and their effect on heart rate.

Arrhythmias can originate in the atria (the two upper chambers of the heart); the atrioventricular (AV) node (a cluster of cells in the center of the heart that conducts electrical impulses from the atria to the ventricles); or the ventricles (the pumping chambers in the lower part of the heart). Arrhythmias can also produce a heart rate that is too slow (bradycardia—under 60 beats a minute) or too fast (tachycardia—over 100 beats a minute at rest).

Publication Review By: Gary Gerstenblith, M.D., and Simeon Margolis, M.D., Ph.D.

Published: 05 Jul 2103

Last Modified: 05 Jul 2013