Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Ventricular Fibrillation


L. Brent Mitchell

, MD, Libin Cardiovascular Institute of Alberta, University of Calgary

Last full review/revision Feb 2021| Content last modified Feb 2021
Click here for the Professional Version
Topic Resources

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 impulses.

  • Ventricular fibrillation causes unconsciousness in seconds, and if the disorder is not rapidly treated, death follows.

  • Electrocardiography helps determine if the cause of cardiac arrest is ventricular fibrillation.

  • Cardiopulmonary resuscitation (CPR) must be started within a few minutes, and it must be followed by defibrillation (an electrical shock delivered to the chest) to restore normal heart rhythm.

In ventricular fibrillation, the ventricles merely quiver and do not contract in a coordinated way. No blood is pumped from the heart, so ventricular fibrillation is a form of cardiac arrest. It is fatal unless treated immediately.

The most common cause of ventricular fibrillation is a heart disorder, particularly inadequate blood flow to the heart muscle due to coronary artery disease, as occurs during a heart attack. Other causes include the following:

Symptoms of Ventricular Fibrillation

Ventricular fibrillation causes unconsciousness in seconds. If untreated, the person usually has a brief seizure and then becomes limp and unresponsive. People develop irreversible brain damage after about 5 minutes because oxygen no longer reaches the brain. Death soon follows.

Diagnosis of Ventricular Fibrillation

  • Electrocardiography

Cardiac arrest is diagnosed when a person suddenly collapses, turns deathly pale, stops breathing, and has no detectable pulse, heartbeat, or blood pressure. Ventricular fibrillation is diagnosed as the cause of the cardiac arrest by electrocardiography (ECG).

ECG: Reading the Waves

An electrocardiogram (ECG) represents the electrical current moving through the heart during a heartbeat. The current's movement is divided into parts, and each part is given an alphabetic designation in the ECG.

Each heartbeat begins with an impulse from the heart's pacemaker (sinus or sinoatrial node). This impulse activates the upper chambers of the heart (atria). The P wave represents activation of the atria.

Next, the electrical current flows down to the lower chambers of the heart (ventricles). The QRS complex represents activation of the ventricles.

The electrical current then spreads back over the ventricles in the opposite direction. This activity is called the recovery wave, which is represented by the T wave.

Many kinds of abnormalities can often be seen on an ECG. They include a previous heart attack (myocardial infarction), an abnormal heart rhythm (arrhythmia), an inadequate supply of blood and oxygen to the heart (ischemia), and excessive thickening (hypertrophy) of the heart's muscular walls.

Certain abnormalities seen on an ECG can also suggest bulges (aneurysms) that develop in weak areas of the heart's walls. Aneurysms may result from a heart attack. If the rhythm is abnormal (too fast, too slow, or irregular), the ECG may also indicate where in the heart the abnormal rhythm starts. Such information helps doctors begin to determine the cause.

ECG: Reading the Waves

Treatment of Ventricular Fibrillation

  • Cardiopulmonary resuscitation

  • Preventing further episodes

Ventricular fibrillation must be treated as an extreme emergency. Cardiopulmonary resuscitation (CPR) must be started as soon as possible. It must be followed by defibrillation (an electrical shock delivered to the chest), as soon as the defibrillator is available. Drugs used to treat abnormal heart rhythms (see table Some Drugs Used to Treat Arrhythmias) may then be given to help maintain the normal heart rhythm.

When ventricular fibrillation occurs within a few hours of a heart attack in people who are not in shock and who do not have heart failure, prompt cardioversion restores normal rhythm in 95% of people, and the prognosis is good. Shock and heart failure suggest severe damage to the ventricles. If the ventricles are severely damaged, even prompt cardioversion has only a 30% success rate, and 70% of people who are resuscitated die without regaining normal function.

People who are successfully resuscitated from ventricular fibrillation and survive are at high risk of another episode. If ventricular fibrillation is caused by a reversible disorder, that disorder is treated. Otherwise, most people have an implantable cardioverter-defibrillator (ICD) surgically implanted to correct the problem if it recurs. ICDs continually monitor the rate and rhythm of the heart, automatically detect ventricular fibrillation, and deliver a shock to convert the arrhythmia back to a normal rhythm. Such people are often also given drugs to prevent recurrences.

More Information

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Test your knowledge

Fainting (syncope) is sudden, brief loss of consciousness in which a person falls to the ground or slumps in a chair and then regains consciousness. The cause of fainting may be something dangerous or relatively harmless. For which of the following groups are dangerous causes of fainting most common? 
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID