A normal heartbeat produces a regular, identifiable pattern: the P wave, the QRS complex, and the T wave. These segments show the electrical activity of specific parts of the heart.
The normal pattern of electrical activity that is seen on an electrocardiogram or ECG is called a sinus rhythm The SA node is the normal pacemaker of the heart; it begins the heartbeat by producing an electrical signal. This signal spreads through the atria, or upper chambers of the heart, causing them to contract. Electrical activity in the atria is seen on the ECG as the P wave. The signal then reaches the AV node, which transmits the electrical signal that continues through the bundle of His, the bundle branches, and the Purkinje fibers. This causes the ventricles, the lower chambers of the heart, to contract. Electrical activity in the ventricles is seen as the QRS and the T wave. For each beat, the cycle begins again at the SA node.