A regular heart rate of 100 or more beats a minute, caused by increased electrical activity in the SINOATRIAL NODE (see also HEART). This level of tachycardia is normal during and just after exercise, and may also be caused by stress or anxiety. If tachycardia persists when the person is resting, it may be due to underlying disease such as thyrotoxicosis (see under THYROID GLAND, DISEASES OF) and investigation is advisable.