Atrial Fibrillation and the Heart

Atrial fibrillation occurs when the electrical system that controls your heartbeat is no longer working properly (see Human Heart). Normally, after an electrical signal is sent from the SA (sinoatrial) node, it travels across the atria and causes the muscles in both chambers to contract uniformly. In people who have atrial fibrillation, this signal from the SA node generates several more electrical signals that chaotically crisscross the atria, causing different patches of the atrial muscle to contract at different times. This causes the atria to quiver, or fibrillate, sometimes as fast as 350 to 700 times a minute -- much faster than a normal heart rate.

 

When atrial fibrillation occurs, there's a lack of coordination between the patches of muscle, which means that the whole chamber doesn't contract at the same time. As a result, less blood is squeezed into the ventricle from the atrium. Because of this, blood can get backed up in the atrium, then into the blood vessel that leads into it, and, with time, even farther back throughout the rest of the body.

 

Normally, the electrical signals from the SA node continue through the AV (atrioventricular) node to the ventricles, causing them to contract as well. But in atrial fibrillation, the signals to the ventricles are either irregular or incomplete, so the contractions of the atria and the ventricles are no longer synchronized. This causes the ventricles to beat more rapidly and irregularly, too. They may contract before they have filled with blood, and this means that your body gets less oxygen and nutrients.

 

Atrial fibrillation can be fast or slow. The normal heart rate is considered to be between 60 and 100 beats per minute. Usually, people with atrial fibrillation who have a heart rate below 100 beats per minute have fewer symptoms of atrial fibrillation than those with heart rates greater than 100 beats per minute.