Atrial tachycardia or atrial fibrillation happens when electrical signals in the atria are fired in a very fast and uncontrolled manner. This causes the atrial chambers of the heart to quiver instead of pump.
When this happens only part of the blood is forced out of the heart chamber. The blood pools inside the atrium and sometimes clots. These blood clots can cause a stroke if they break off and block an artery in the brain.
Although atrial fibrillation isn't life threatening, it can lead to other rhythm problems, chronic fatigue and congestive heart failure.