Tachycardia is a very fast heart rate – over 100 beats per minute. It is normal for the heart to beat quickly during exercise or emotional stress. However, if tachycardia occurs at rest, it may be a symptom of a serious health disorder. Tachycardia may affect the upper or lower heart chambers, called the atria and ventricles. Atrial tachycardia (atrial flutter or fibrillation) is not life threatening but may have serious consequences for your health, such as a high risk of stroke. Ventricular tachycardia (ventricular flutter) can be life-threatening. Ventricular tachyarrhythmia (ventricular fibrillation) is the most dangerous type of tachycardia. The heart muscle cells contract chaotically, which can result in death within a matter of minutes. This is one of the most common causes of sudden cardiac death .