A heartbeat that is too fast is called tachycardia (TAK-ih-KAR-de-ah). A heartbeat that is too slow is called bradycardia (bray-de-KAR-de-ah).
Although arrhythmias are considered harmless, certain arrhythmias can be serious or even life threatening. Depending on the arrhythmia, the heart may not be able to properly pump blood throughout the body. Lack of blood flow can cause damage to major organs, including the brain, heart and other organs.