ELECTROCARDIOGRAPHY: NORMAL HEART, ACUTE ISCHEMIA, AND CHRONIC DISEASE
Normal Cardiac Conduction
Normal cardiac conduction follows a precisely coordinated pattern from the initiation of an electrical impulse to myocardial repolarization after ventricular contraction. Under normal conditions, the electrical cardiac cycle begins at the sinoatrial (SA) node—a collection of pacemaker cells located at the junction of the superior vena cava and the right atrium. These cells possess an intrinsic ability to initiate electrical impulses spontaneously. This is achieved by the depolarization of a polarized cell membrane, primarily from the flow of ions (Na+, K+, and Ca2+) through ion channels embedded in the membrane. The term “action potential” is used to describe the depolarization–repolarization cycle of a cell.
The SA node is the dominant pacemaker of the cardiac cycle and typically fires at a rate of 60–100 action potentials per minute. The rate is influenced by sympathetic and vagal stimulation. The action potential of one cell triggers the action potentials in adjacent cells. Thus, the impulse initiated by the SA node is propagated through nearby cells. This causes both contraction of the atrial myocardium (atrial systole) and further electrical impulse propagation through the cardiac conduction system. Specialized conducting cells are responsible for delivering this electrical signal to the remainder of the myocardium. Long, thin, and efficient in relaying electrical impulses, these cells are similar to wires carrying electricity. Once an electrical impulse reaches a myocardial cell, calcium ions are released within the cell causing contraction. This contraction, known as excitation–contraction coupling, is achieved through the coupling of actin and myosin molecules, which are abundant within the myocardial cell.
Starting at the SA node, an electrical impulse travels through internodal atrial pathways to the atrioventricular (AV) node. The AV node acts as a relay site by connecting atrial electrical impulses with the ventricular conducting system. Conduction through the AV node, which is located at the base of the atrial septum, is slowed momentarily, allowing for ventricular filling of blood prior to ventricular systole. Signal conduction is then accelerated rapidly through the bundle of His and its right and left bundle branches. The right bundle branch continues toward the apex of the right ventricle (RV), whereas the left bundle branch splits into left anterior superior and left posterior inferior fascicles. Electrical impulses progress through the fibers of the Purkinje system and are ultimately delivered to the ventricular myocardium that causes ventricular contraction (ventricular systole).
Action potentials in cardiac myocytes produce local currents that depolarize the membranes of adjacent cells and propagate electrical signals. Cell-to-cell conduction is also achieved by end-to-end connections called gap junctions. Reproduced, with permission, from Chapter 2. Characteristics of cardiac muscle cells. In: Mohrman DE, Heller LJ. eds. Cardiovascular Physiology. 7th ...