Tachycardia is the racing of the heart beat to more than a 100 beats per minute in relation to the normal 60-80 beats. Rapid heart beat, in its most benign form, is due to emotional disturbance. It may also be due to overactivity of the thyroid gland as in thyrotoxicosis. A more common cause is fever -- a rise in temperature is always accompanied by a rise in heart rate. The most common cause is a failing heart.