The heartbeat is controlled by the electrical system of the heart. This system is made up of several sequences that stimulate the heart muscle to contract. The SA node starts the heartbeat causing the atria, or upper chambers of the heart, to contract. The signal then travels to the AV node, bundle of His, bundle branches, and Purkinje fibers. This causes the ventricles, (lower chambers of the heart), to contract. The normal flow of electrical signals produces a normal heart rate. Normal heartbeats can be easily identified on an electrocardiogram (ECG).
An arrhythmia is an abnormal heartbeat that is too fast, too slow, or irregular. When an arrhythmia is present, the heart may seem to skip beats, or flutter. The term ‘bradycardia’ describes a heartbeat that is slow: less than 60 beats per minute. ‘Tachycardia’ usually refers to a resting heart rate that is fast: more than 100 beats per minute. ‘Fibrillation’ is an abnormal firing of signals within an area of the heart, causing a disorganized beat.