What are some causes for a rapid heart beat?

There can be several: Stress or any state where your body is producing Epinephrine (adrenaline) will increase your heart rate. Increased activity like exercise will alter your hr. Certain drugs like Cocaine can dramatically increase your hr. Also severe infections. Then there are rhythm problems of the heart like atrial fibrillation, svt and other abnormal electrical conduction can cause rapid heart rat.