Any variation from the normal rhythm of the heart beat. When the heart beats too fast, it's called tachycardia. When it beats too slow, it's called bradycardia. An arrhythmia can also mean that heart beats irregularly (skips a beat or has an extra beat). At some time or another, most people have felt their heart race or skip a beat. These occasional changes can be brought on by strong emotions or exercise. They usually are not a cause for alarm. Arrhythmias that occur more often or cause symptoms may be more serious and need to be discussed with a doctor.