A fast heartbeat, or tachycardia, occurs when something disrupts the normal activity of the heart's electrical system, according to Mayo Clinic. Many things disrupt the heart's electrical signals, from consumption of too much caffeine to the presence of congenital heart defects.
The heart's electrical system, also called the cardiac conduction system, contains the sinoatrial node, atrioventricular node and His-Purkinje system. The SA node sends out a signal prompting the heart to start beating. This is why it is sometimes called the heart's natural pacemaker, reports the National Heart, Lung, and Blood Institute. A person's heart rate corresponds to the number of signals the SA node sends out every minute. When the signal arrives at the AV node, it slows down so the ventricles of the heart have time to fill with blood. From there, the signal travels to the His-Purkinje system.
In some cases, a fast heartbeat is temporary and caused by drinking caffeinated beverages or exercising. Smoking, drinking too much alcohol and abusing drugs also cause fast heartbeat, notes Mayo Clinic. Some babies are born with abnormal electrical pathways in their hearts, resulting in tachycardia later in life. Heart disease, hyperthyroidism and electrolyte imbalances also cause tachycardia. Prescription medications and pacemakers are used to treat abnormal heart rhythms caused by medical conditions. If these treatments do not work, surgery is also an option.