Causes

To understand how ventricular fibrillation occurs, it may be helpful to know how the heart normally beats.

The normal heartbeat

Your heart is made up of four chambers — two upper chambers (atria) and two lower chambers (ventricles).

The heart's rhythm is normally controlled by a natural pacemaker (the sinus node) in the right upper chamber (atrium). The sinus node sends electrical signals that normally start each heartbeat. These electrical signals move across the atria, causing the heart muscles to squeeze (contract) and pump blood into the ventricles.

Next, the signals arrive at a cluster of cells called the AV node, where they slow down. This slight delay allows the ventricles to fill with blood. When the electrical signals reach the ventricles, the lower heart chambers contract and pump blood to the lungs or to the rest of the body.

In a healthy heart, this heart signaling process usually goes smoothly, resulting in a normal resting heart rate of 60 to 100 beats a minute.

The heart during ventricular fibrillation

Ventricular fibrillation is caused by either a problem in the heart's electrical properties or by a disruption of the normal blood supply to the heart muscle. Sometimes, the cause of ventricular fibrillation is unknown.