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).