Atrial fibrillation: A disorder found in about 2.2 million Americans. In it the heart's two upper chambers (the atria) cease to contract effectively. Atrial fibrillation may result from multiple reentrant waves (multiple spiral waves as shown in the figure) within the atria or from the interactions of repetitive electrical activity from one or more areas of the atria. The irregular electrical activity results in ineffective atrial contraction.
In atrial fibrillation, the ventricles do not beat as rapidly as the atria because the AV node, acting as a safety valve, prevents most of the atrial beats from exciting the ventricles. Although the atria do not contract effectively, this type of arrhythmia is not immediately life-threatening. However, it can lead to other problems, such as chronic fatigue, congestive heart failure, and increased risk of stroke. The likelihood of developing atrial fibrillation increases with age. Three to five percent of people over 65 have atrial fibrillation.