There are many causes of abnormal heart rhythms Causes Abnormal heart rhythms (arrhythmias) are sequences of heartbeats that are irregular, too fast, too slow, or conducted via an abnormal electrical pathway through the heart. Heart disorders are... read more 
(arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle, such as avoiding alcohol, caffeine (in beverages and foods), and smoking. Other arrhythmias are dangerous or bothersome enough to need treatment. Cardioversion-defibrillation is one type of treatment. Other treatments for arrhythmias include insertion of a pacemaker Artificial Pacemakers There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more 
or an implantable cardioverter-defibrillator (ICD) Implantable Cardioverter-Defibrillator (ICD) There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more , use of antiarrhythmic drugs Drugs to Treat Abnormal Heart Rhythms There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more , or destruction of a small area of heart tissue that is responsible for the arrhythmia (ablation Destroying Abnormal Heart Tissue (Ablation) There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more ).
Cardioversion-defibrillation involves giving an electrical shock to the heart. Sometimes this shock can stop a fast arrhythmia and restore normal rhythm. The shock briefly stops the heart from beating and, after a second or two, it starts beating again on its own. Often, it starts back in a normal rhythm but sometimes the arrhythmia starts again. However, electrical shocks cannot restart a heart that has no electrical activity at all (asystole).
Giving the heart such an electrical shock is called cardioversion, or defibrillation, depending on the type of abnormal rhythm for which it is used.
Cardioversion is giving an electrical shock timed (synchronized) to a specific part of the heart beat. It may be used for arrhythmias starting in the atria (such as atrial fibrillation Atrial Fibrillation and Atrial Flutter Atrial fibrillation and atrial flutter are very fast electrical discharge patterns that make the atria (upper chambers of the heart) contract very rapidly, with some of the electrical impulses... read more ) or the ventricles (such as ventricular tachycardia Ventricular Tachycardia Ventricular tachycardia is a heart rhythm that originates in the ventricles (lower chambers of the heart) and produces a heart rate of at least 120 beats per minute (the normal heart rate is... read more 
) as long as there is some type of organized electrical activity in the heart.
Defibrillation is giving an electrical shock during ventricular fibrillation Ventricular Fibrillation Ventricular fibrillation is a potentially fatal, uncoordinated series of very rapid, ineffective contractions of the ventricles (lower chambers of the heart) caused by many chaotic electrical... read more 
. During ventricular fibrillation, there is no organized electrical activity in the heart to which the shock can be timed.
The machine that delivers the shock is called a defibrillator, even though it is used for both defibrillation and cardioversion. Defibrillators can be used by a team of doctors and nurses, by paramedics, or by firefighters.
An automated external defibrillator (AED) Automated External Defibrillator: Jump-Starting the Heart 
can automatically detect the presence of an arrhythmia, determine if a shock is advisable, and deliver the proper strength shock. Thus, using an AED requires only minimal training, such as that provided in a first-aid course. . AEDs are present in many public places, such as airports, sports arenas, hotels, and shopping malls.
