The heart is a pumping machine that moves blood to the lungs, and then once the blood is filled with oxygen, to the body's organs. But the heart is also a rhythm machine with finely tuned electrical circuits. It is actually this electrical system that starts and keeps the pumping mechanism moving.
The heart's electrical circuit begins in the right atrium, one of the heart's two upper chambers, with a signal originating in a group of specialized cells called the sinoatrial node or SA node. The SA node is often called the heart's natural pacemaker because it literally sets the pace at which the heart beats.
The electrical signal spreads from the right atrium to the left atrium and then travels down to the ventricles, the heart's two lower chambers. In response to the electrical signal, the ventricles' muscle cells contract and pump blood into the arteries. (See "How the Heart Works" for more information).
When the heart is healthy, it has a normal rhythm between 60 and 100 beats per minute (bpm). When the heart's rhythm is slower or faster than normal, the rhythm is considered to be abnormal and is called an arrhythmia. Arrhythmias can occur in either the ventricles or the atria, and sometimes occur in both the upper and lower heart chambers.
Atrial fibrillation -- also called AF or A-Fib -- is the most common arrhythmia. In the next section, we'll see what what it is.