What causes the heart rate to be high?
Several factors may increase heart rate.
Heart rate increases when there is --
stimulation by sympathetic nerves
suppression of parasympathetic activity
increase in secretion by adrenal medulla
increase in secretion of thyroxine hormone
ionic imbalance in tissue fluid.
Mental shock, anxiety, physical exercise, increased body temperature, various chemicals (stimulants) are known to cause an increase in heart rate.