There are two types of arrhythmias.

Tachycardias ("tachy" means fast; "cardia" means heart) make the heart beat too quickly or, in some cases, cause an irregular heartbeat. When at rest, the normal heart rate is between 60 to 100 beats per minute. Tachycardia is the most common type of arrhythmia. It can start in the atria (the two upper chambers of the heart) or the ventricles (the two lower chambers of the heart).

• Ventricular fibrillation ("fibrillation" means twitching) is the most serious form of tachycardia. When this happens, the ventricles quiver and are unable to pump blood. It is extremely dangerous and if left untreated can cause death within minutes.
• Torsades de pointes also starts in the ventricles. This is an irregular speeding up of the heartbeat and can turn into ventricular fibrillation if untreated. Unlike other arrhythmias common in people with heart disease, torsades de pointes can happen to young healthy individuals.
• Atrial fibrillation causes the atria to quiver and produce uncoordinated beats. However, it is less dangerous than ventricular fibrillation.

Bradycardias ("brady" means slow) make the heart beat too slowly. This condition is less common and it can be harmful because the heart is not pumping blood fast enough to deliver enough oxygen and nutrients to the entire body.