A tachycardia is defined as a very fast heartbeat of more than 100 beats per minute. Physical effort, excitement or stress naturally accelerates the heartbeat without any underlying health problem. Up to a certain point, acceleration of the heart´s activity increases cardiac output, or the volume of blood pumped by the heart. Beyond this point, however, the heartbeat is too fast to fill the ventricles with enough blood and cardiac output is reduced. Tachycardia may occur only in the atria (the heart’s upper two chambers) or only in the ventricle (the heart’s lower two chambers). Atrial tachycardia is not in itself life threatening, but has serious health consequences and is linked to a high risk of stroke. If the atrial tachycardia is still rhythmic, it is called atrial flutter. During an atrial fibrillation , the activity of the heart muscle cells is arrhythmic and chaotic. During a ventricular tachyarrhythmia, better known as ventricular fibrillation, the heart chambers can no longer contract. They lose their ability to pump blood due to the disorderly and irregular electrical activity in the muscle cells. Ventricular fibrillation causes cardiac arrest within minutes and is one of the most common causes of sudden cardiac death.