What causes atrial fibrillation?

By Mayo Clinic Staff

Atrial fibrillation is an irregular and often rapid heart rate that occurs when the two upper chambers of your heart experience chaotic electrical signals.

Your heart consists of four chambers — two upper chambers (atria) and two lower chambers (ventricles). Within the upper right chamber of your heart (right atrium) is a group of cells called the sinus node. This is your heart's natural pacemaker. The sinus node produces the impulse that normally starts each heartbeat.

Normally, the impulse travels first through the atria and then through a connecting pathway between the upper and lower chambers of your heart called the atrioventricular (AV) node. As the signal passes from the sinus node through the atria, they contract, pumping blood from your atria into the ventricles below. As the signal passes through the AV node to the ventricles, it signals the ventricles to contract, pumping blood out to your body.

In atrial fibrillation, the atria experience chaotic electrical signals. As a result, they quiver. The AV node — the electrical connection between the atria and the ventricles — is bombarded with impulses trying to get through to the ventricles. The ventricles also beat rapidly, but not as rapidly as the atria, as not all the impulses get through. The reason is that the AV node is like a highway on-ramp — only so many vehicles can get on at one time.

The result is a fast and irregular heart rhythm. The heart rate in atrial fibrillation may range from 100 to 175 beats a minute. The normal range for a heart rate is 60 to 100 beats a minute.

Possible causes of atrial fibrillation

Abnormalities or damage to the heart's structure are the most common cause of atrial fibrillation. Possible causes of atrial fibrillation include:

  • High blood pressure
  • Heart attacks
  • Abnormal heart valves
  • Heart defects you're born with (congenital)
  • An overactive thyroid or other metabolic imbalance
  • Exposure to stimulants, such as medications, caffeine or tobacco, or to alcohol
  • Sick sinus syndrome — improper functioning of the heart's natural pacemaker
  • Lung diseases
  • Previous heart surgery
  • Viral infections
  • Stress due to pneumonia, surgery or other illnesses
  • Sleep apnea

However, some people who have atrial fibrillation don't have any heart defects or damage, a condition called lone atrial fibrillation. In lone atrial fibrillation, the cause is often unclear, and serious complications are uncommon.

May. 01, 2014 See more In-depth