In normal hearts, the upper two chambers (‘atria’) pump blood in a coordinated fashion into the two bottom chambers (‘ventricles’) which then pump this blood to the rest of the body. This relationship is maintained by a regular electrical impulse from the upper chamber (normal sinus rhythm). This usually occurs at the rate of between 60 and 100 beats per minute.
In AF, the normal electrical impulse in the 2 atria is disrupted and is replaced by chaotic electrical activity. The atria now ‘fibrillate’ at a rate of 300-400 times a minute and the ventricles follow in a rapid irregular fashion at rates up to 200 beats per minute.