The commonest form of persistent irregularity of the heart's action, accounting for about one-half of all the cases of irregular pulse. The atria do not empty themselves by the normal regular waves but contract partially in a rapid series of twitches; in consequence the ventricles do not receive the normal rhythmic stimulus to contract, and they also contract irregularly. It is serious, usually accompanies disease of the mitral value and is recognized by great irregularity in time and force of the pulse.
Atrial fibrillation is the cause of about one-third of strokes.