A pacemaker is a medical device that it inserted in the chest. It uses electrical pulses to regulate the human heartbeat. Pacemakers are used to treat arrhythmia, which is an irregular heartbeat. A arrhythmia can either be tachycardia, which means the heart is beating too quickly, or brachycardia, which means that the heart is beating too slowly. Causes of arrhythmia's include heart attack, abnormal levels of blood potassium, congenial heart disease, enlarged heart, overactive thyroid, and heart failure. Arrhythmia's can also be caused by the use of certain substances, including stimulants, beta-blockers, and nicotine.