Cardioversion is delivering an electrical shock to a person's heart to rapidly restore an abnormal heart rhythm back to normal.

There are two types of cardioversion -- external and internal.

EXTERNAL CARDIOVERSION

An external cardioversion is often performed in an emergency situation. When a person's heart is beating abnormally and inadequately pumping blood to the body, an electrical shock is given with a defibrillator to revert the heart rate to a normal rhythm.

Other times, an external cardioversion is scheduled ahead of time to treat an arrhythmia (most often atrial fibrillation) that is recent or has not responded well to medication. Prior to the scheduled cardioversion, it is often necessary to run tests to make sure that there are no blood clots in the heart. Some people need medication to "thin" their blood before having the cardioversion.

For a scheduled cardioversion, the person is usually sedated beforehand and his or her breathing is assisted. The doctor then delivers an electrical shock from a defibrillator to the heart using paddles on the patient's chest. The electrical shock usually makes the person's heart convert back to a normal rhythm. Afterwards, to decrease the likelihood of the arrhythmia returning -- or to prevent blood clots from entering the general circulation -- the doctor may recommend a medication regimen.

INTERNAL CARDIOVERSION

An internal cardioversion is delivered by a device similar to a pacemaker, called an implantable cardioverter defibrillator (ICD). This device is used for treating ventricular (lower heart chamber) arrhythmias, such as ventricular tachyarrhythmias or fibrillation. The severity of these arrhythmias can cause sudden death because of the dangerously fast heart rate.

Less often, internal cardioversion is needed to treat atrial (upper heart chamber)arrhythmias. In this case, the device is called an "atrial defibrillator".

Implantable defibrillators consist of a pulse generator that is implanted under the skin (just like a pacemaker) and electrodes (wires) that are placed into the heart. If the heart goes into an abnormal rhythm, the defibrillator will sense it and send an electrical shock to the heart, to make it start beating normally again.

The generator battery lasts for several years.

COMPLICATIONS

Possible complications, which are relatively uncommon for internal defibrillators, include:

• Bleeding
• Clot in a vein
• Perforation (tearing) of the heart
• Infection
• Lead dislodgement (wires not remaining where they were placed)
• Shocks delivered by the device in error