Loss of consciousness is the most common sign of ventricular fibrillation.

Early signs and symptoms

A condition in which the lower chambers of your heart beat too rapidly (ventricular tachycardia, or VT) can lead to ventricular fibrillation. Signs and symptoms of VT include:

• Chest pain
• Rapid heartbeat (tachycardia)
• Dizziness
• Nausea
• Shortness of breath
• Loss of consciousness

To understand ventricular fibrillation, consider a normal heartbeat.

What's a normal heartbeat?

When your heart beats, the electrical impulses that cause it to contract follow a precise pathway through your heart. Interruption in these impulses can cause an irregular heartbeat (arrhythmia).

Your heart is divided into four chambers. The chambers on each half of your heart form two adjoining pumps, with an upper chamber (atrium) and a lower chamber (ventricle).

During a heartbeat, the smaller, less muscular atria contract and fill the relaxed ventricles with blood. This contraction starts after the sinus node — a small group of cells in your right atrium — sends an electrical impulse causing your right and left atria to contract.

The impulse then travels to the center of your heart, to the atrioventricular node, which lies on the pathway between your atria and your ventricles. From here, the impulse exits the atrioventricular node and travels through your ventricles, causing them to contract and pump blood throughout your body.

What causes ventricular fibrillation?

The cause of ventricular fibrillation isn't always known. The most common cause is a problem in the electrical impulses traveling through your heart after a first heart attack or problems resulting from a scar in your heart's muscle tissue from a previous heart attack.

Some cases of ventricular fibrillation begin as a rapid heartbeat called ventricular tachycardia (VT). This rapid but regular beating of the heart is caused by abnormal electrical impulses that start in the ventricles.

Most VT occurs in people with a heart-related problem, such as scars or damage from a heart attack. Sometimes VT can last less than 30 seconds (nonsustained) and may not cause symptoms. But VT may be a sign of more-serious heart problems.

If VT lasts more than 30 seconds, it will usually lead to palpitations, dizziness or fainting. Untreated VT will often lead to ventricular fibrillation.

Most cases of ventricular fibrillation are linked to some form of heart disease.

Factors that may increase your risk of ventricular fibrillation include:

• A previous episode of ventricular fibrillation
• A previous heart attack
• A heart defect you're born with (congenital heart disease)
• Heart muscle disease (cardiomyopathy)
• Injuries that cause damage to the heart muscle, such as electrocution
• Use of illegal drugs, such as cocaine or methamphetamine
• Significant electrolyte abnormalities, such as with potassium or magnesium

Ventricular fibrillation is the most frequent cause of sudden cardiac death. The condition's rapid, erratic heartbeats cause the heart to abruptly stop pumping blood to the body. The longer the body is deprived of blood, the greater the risk of damage to your brain and other organs. Death can occur within minutes.

The condition must be treated immediately with defibrillation, which delivers an electrical shock to the heart and restores normal heart rhythm. The rate of long-term complications and death is directly related to the speed with which you receive defibrillation.