At rest, the heart beats somewhere between 60 and 100 times per minute. Tachycardia is defined as more than 100 beats per minute, while bradycardia is defined as fewer than 60 beats per minute. Both tachycardia and bradycardia can occur in surges or as persistent conditions. Most arrhythmias fall into the tachycardia category. Some originate in the atria, some in the ventricles.
Ventricular arrhythmias are typically more serious conditions.
In fact, most cases of sudden cardiac death are due to ventricular arrhythmias, not heart attack, as was once thought. Particularly dangerous is ventricular fibrillation, in which activity within the ventricles is so uncoordinated that the ventricles are reduced to twitching, and virtually no pumping takes place. Ventricular fibrillation will cause death within a few minutes unless treated with CPR and defibrillation as soon as possible. Bradycardias can be due to problems with nerves that control heartbeat, with the sinus node, where the heart's electrical impulses originate, or with actual transmission of those impulses through the heart. In the latter condition, known as heart block, electrical signals fail to reach the ventricles from the atria because of some conduction problem.