To understand how ventricular fibrillation happens, consider what should happen during a normal heartbeat.

What's a normal heartbeat?

When your heart beats, the electrical impulses that cause it to contract must follow a precise pathway through your heart. Any 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?

It's not always known what causes ventricular fibrillation. But 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 fast, regular beating of the heart is caused by abnormal electrical impulses that start in the ventricles.

Most VT occurs in people with some form of heart-related problem, such as scars or damage within the ventricle muscle from a heart attack. Sometimes VT can last for 30 seconds or less (nonsustained) and may not cause any symptoms, although it causes inefficient heartbeats. 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.

In ventricular fibrillation, rapid, chaotic electrical impulses cause your ventricles to quiver uselessly instead of pumping blood. Without an effective heartbeat, your blood pressure plummets, instantly cutting off blood supply to your vital organs — including your brain. Most people lose consciousness within seconds and require immediate medical assistance, including cardiopulmonary resuscitation (CPR). Your chances of survival are better if CPR is delivered until your heart can be shocked back into a normal rhythm with a device called a defibrillator. Without CPR or defibrillation, death results in minutes. Most cases of ventricular fibrillation are linked to some form of heart disease.

Nov. 01, 2011