Tachycardia is a heart rate that exceeds the normal range. A heart rate over 100 beats per minute is generally accepted as tachycardia. Tachycardia can be caused by various factors which often are benign. However, tachycardia can be dangerous depending on the speed and type of rhythm. Tachycardia comes from the Greek words tachys (rapid or accelerated) and cardia (of the heart).