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The cardiac conduction system refers to a unique group of cells within the heart that generate and propagate the electrical signal that causes the heart muscle to contract and relax. Five units make up the cardiac conduction system — the sinoatrial node (SA node), the atrioventricular node (AV node), the bundle of His, the Purkinje fibers, and the right and left bundle branches. The primary pacemaker of the heart is the SA node, which generates electrical impulses at a regular rate to cause contraction and emptying of the upper heart chambers, the atria. Next, the electrical signal passes to the AV node, where it is delayed slightly to allow complete emptying of the atria before channeling through the bundle of His and the Purkinje fibers to cause contraction of the bottom heart chambers, the ventricles. The cardiac conduction system precisely times the sequential contraction of the heart chambers to provide an efficient pump action to propel the blood throughout the body.
With a rate in accordance with the body's demand for oxygen, the SA node of the cardiac conduction system sends its electrical stimulus to the 100 million cells of the atria. Contraction of these cells occurs almost instantaneously, in less than one-third of a second. As the atria contract, blood pushes through the valves between the atria and the ventricles and fills the ventricles. After the atria discharge fully, the valves between the chambers close and the atrial cells relax. At this point, the SA node recharges for its next impulse.
The AV node serves as a way station for the electrical signal, timing it precisely to produce ventricular contraction once the ventricles are filled. Like high-speed railways, the bundle of His and the left and right bundle branches disperse the electrical signal to the ventricles. Purkinje fibers further disseminate the signal to the farthest reaches of the cardiac conduction system, much like small side railway systems. Ventricular contraction occurs when the signal stimulates the 400 million ventricular cells. The AV node recharges during the next ventricular filling.
Sinus rhythm is the normal heart rhythm produced by the cardiac conduction system. The electrocardiogram (ECG) is a graphical representation consisting of five waves that represent the electrical events of the heartbeat. Comprising the first hump on the graph, the P wave represents the stimulation of the atrial cells to contract, while the next inverted hump, the Q wave, represents stimulation to the bundle of His in the walls separating the right and left sides of the heart. Following closely, the large, spiking R wave reflects stimulation to the ventricles, and the inverted S wave signifies excitement of the Purkinje fibers. Finally, the T wave marks the relaxation of the ventricles and recharging of the conduction system for the next contraction.