Electrocardiograma

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ECG tutorial: Electrical components of the ECG
Author
Philip J Podrid, MD
Section Editor
Ary L Goldberger, MD
Deputy Editor
Gordon M Saperia, MD, FACC
Disclosures
Last literature review version 19.2: May 2011 | This topic last updated: April 19, 2011
INTRODUCTION — The electrocardiogram provides a graphicrecord 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 situationbeginning in the sinus node, and then spreading sequentially through the atria, AV node, and ventricles. (See "ECG tutorial: Physiology of the conduction system".) The electrical signal spreads through the heart as a wavefront of depolarization. This wavefront results in a minute electrical field that can be detected at the body's surface as an electrocardiogram. (See "Anatomy andelectrophysiology of the sinoatrial node" and "Electrocardiographic interpretation of left bundle branch block" and "Electrocardiographic interpretation of right bundle branch block".)
ELECTRICAL FIELDS — All electrical fields have two important associated parameters, magnitude and direction. The standard electrocardiogram is simply a graphical representation of the direction and magnitude of the electricalfield 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 membrane is relatively positive. When the muscle is stimulated and sodium ions flow into the cell, the membrane becomes negative withreference 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; thiscorresponds 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 asummation 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.
The mean direction of the electrical field during the entire cardiac cycle is called the electrical axis. The magnitude of the electrical field as measured by an electrode is proportional to the distance between theelectrode and the dipole, and to the size of the dipole; on the surface ECG, the latter is related to the mass of tissue being depolarized.
Three electrodes placed along the muscle strip will record different electric signals:
* The first electrode is located at the site of origin of depolarization. The dipole is immediately adjacent to this electrode at the start of depolarization; thus, astrong negative charge is recorded. The electrode continually sees a negative charge throughout depolarization of the strip since the dipole is constantly moving away from it until depolarization of the muscle strip is completed, there is no further movement of an electrical charge, and an isoelectric baseline is restored.
* The second electrode records a dipole as seen from the middle of the...