Tachycardia refers to rapid beating of the heart. By convention it's defined as a heart rate greater than 100 beats per minute in adults. Tachycardia may be normal, such as in exercise and stress, or abnormal, such as in cardiac arrhythmias. However, depending on the mechanism of the tachycardia and the health status of the person, tachycardia may be harmful and require medical treatment. In extreme cases, tachycardia can be life threatening. Tachycardia can be harmful in three ways. First, if… (More on Tachycardia)