Cardiac dysrhythmia (or arrhythmia) is a disturbance in the rate
of cardiac muscle contractions, or any variation from the normal
rhythm or rate of heart beat. The term encompasses abnormal
regular and irregular rhythms as well as loss of rhythm. Cardiac
dysrhythmias are found in a vast range of conditions and may be
defined in a number of ways, including by site of origin (e.g.,
supraventricular, ventricular, atrial), mechanism of disturbance
(e.g., fibrillation, automaticity, re-entry or triggered
activity), rate of disturbance (e.g., tachycardia, bradycardia)
and electrocardiogram appearance (e.g., long QT syndrome).
Dysrhythmias may be acute or chronic, and some (especially
ventricular arrhythmias) may be life-threatening. Sudden cardiac
death is the most severe manifestation of ventricular
arrhythmias (e.g., ventricular fibrillation).