When the ventricles begin to quiver and do not employ coordinated contractions, the heart is said to be fibrillating. In this condition the ventricles cannot pump blood from the heart. Ventricular fibrillation (V-fib) is the worst kind of abnormal heart rhythm, and is a form of cardiac arrest. It involves the pumping of the lower chambers of the heart, while atrial fibrillation involves the upper chambers.

Ventricular fibrillation often is associated with acute ischemic events (ischemia involves the deprivation of oxygenated blood to an area of tissue), and with chronic ischemic heart disease. It is frequently seen immediately following a heart attack. It also may develop during hypoxia, atrial fibrillation, or improper grounding of electrical devices. An extremely low level of potassium in the blood also can cause ventricular fibrillation.

The first, and usually the only, symptom of V-fib is sudden unconsciousness.

When an individual suddenly collapses, the possibility of ventricular fibrillation should be considered immediately. A quick assessment usually shows no pulse or heartbeat. The diagnosis of ventricular fibrillation is confirmed with an electrocardiogram.

Basic life support with standard cardiopulmonary resuscitation (CPR) must be started within a few minutes, followed as soon as possible with cardioversion. Cardioversion is an electric shock delivered to the heart to stop the fibrillating. Early defibrillation is the key to survival. If left untreated, irreversible brain damage, due to lack of oxygen to the brain, occurs after about five minutes. After the heart resumes its normal rhythm, medications are given to help maintain the rhythm.

Research continues into methods to deliver defibrillation as soon as possible to those experiencing ventricular fibrillation. One of the studies addressed in 2003 researched various clinical trials that implanted defibrillators into patients to prevent sudden cardiac death. The devices worked in many instances but more proof of their success was needed for widespread use.

Early and effective CPR may provide the time necessary for medical personnel to arrive with a defibrillator. If a defibrillator is able to promptly restore a normal rhythm, up to 25% of victims are able to leave the hospital without evidence of brain damage.

If ventricular fibrillation occurs in the hospital in conjunction with a heart attack, defibrillation has a 95% success rate. If shock and heart failure are present at the time, even with immediate defibrillation, only about 30% of those stricken are successfully restored to a normal heart rate.

A healthy lifestyle to reduce the risk of heart diseases which lead to ventricular fibrillation is the best prevention. For people who have experienced an episode of V-fib, an internal cardioverter-defibrillator may prevent further episodes.