ECG tutorial: Electrical components of the ECG
- Jordan M Prutkin, MD, MHS, FHRS
Jordan M Prutkin, MD, MHS, FHRS
- Associate Professor of Medicine, Division of Cardiology, Electrophysiology Section
- University of Washington
The electrocardiogram (ECG) provides a graphic record of the electrical activity of the heart. (See "Basic principles of electrocardiographic interpretation".) Each cardiac cell generates an action potential as it becomes depolarized and then repolarized during a normal cycle (movie 1). (See "Myocardial action potential and action of antiarrhythmic drugs".)
Depolarization of cardiac cells proceeds in an orderly fashion in the normal situation beginning in the sinus node, and then spreading sequentially through the atria, atrioventricular (AV) node, and ventricles. (See "ECG tutorial: Physiology of the conduction system".) The electrical signal spreads through the heart as a wavefront(s) of depolarization. This wavefront(s) results in a minute electrical field that can be detected at the body's surface as an ECG. (See "Left bundle branch block" and "Right bundle branch block" and "ECG tutorial: Intraventricular block".) In particular, the surface ECG records the activation and recovery signals of working myocardial cells, not of the pacemaker cells of the SA and AV nodes, or of the specialized conduction system.
All electrical fields have two important associated parameters: magnitude and direction. The standard electrocardiogram (ECG) is simply a graphical representation of the direction and magnitude of the electrical field of the heart as it changes with time; each lead “looks at” this electrical field from a different angle.
One can imagine, for example, a strip of cardiac muscle that is being stimulated by an external source. Initially, the inside of the cell is negative while the outside is relatively positive. When the muscle is stimulated and sodium ions flow into the cell, the outside becomes negative with reference to the inside of the cell (which is now positive). However, the section of the myocardial membrane that has not yet become depolarized (or is in a resting state) remains positive. Thus, this sequence establishes a dipole that has direction and magnitude (figure 1).
The direction of the dipole is simply the net direction of the positive charge relative to the negative charge; this corresponds to the direction of the wavefront of depolarization for the muscle strip. The magnitude of the dipole is determined by the amount of positive sodium ions that flow into the cell (eg, how positive the inside of the cell becomes). Since the heart is a complex three-dimensional structure consisting of millions of cells, the direction of the dipole changes with time and is the result of a summation of all the instantaneous dipoles. At any point in time, the mean dipole can be determined. This changing direction of the electrical impulse is the basis of vectorcardiography.
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