The heartbeat is controlled by the electrical system of the heart. This system is made up of several parts that tell the muscle of the heart when to contract. The SA node starts the heartbeat, causing the atria, or upper chambers of the heart, to contract. The signal then travels through the AV node, bundle of His, bundle branches, and Purkinje fibers. This causes the ventricles, the lower chambers of the heart, to contract. The flow of electrical signals produces a normal heartbeat. Normal heartbeats can be seen on an Electrocardiogram or ECG.
Atrial fibrillation is a type of heart arrhythmia, or irregular heartbeat. In atrial fibrillation, disorganized electrical signals originate in the heart’s upper chambers, or atria, causing the rhythm to be irregular. These signals come from small areas in the heart represented here by stars. Irregular heartbeats can be seen in an Electrocardiogram or “ECG”. Because contractions are not coordinated as in the normal heartbeat, the heart does not pump blood effectively to the rest of the body.