ELECTROCARDIOGRAPHY

The conducting system of the heart showing the source of the electrical impulses (P, QRS complex, method of graphic tracing (electrocardiogram; ECG or EKG) of the electric current generated by the heart muscle during a heartbeat. The tracing is recorded with an electrocardiograph (actually a relatively simple string galvanometer), and it provides information on the condition and performance of the heart. Electrocardiograms are made by applying electrodes to various parts of the body to lead off the tiny heart current to the recording instrument. The four extremities and the chest wall have become standard sites for applying the electrodes. After the electrodes are in place, held with a salt paste, a millivolt from a source outside the body is introduced so that the instrument can be calibrated. Standardizing electrocardiograms makes it possible to compare them as taken from person to person and from time to time from the same person. The normal electrocardiogram shows typical upward and downward deflections that reflect the alternate contraction of the atria (the two upper chambers) and of the ventricles (the two lower chambers) of the heart. The first upward deflection, P, is due to atrial contraction and is known as the atrial complex. The other deflections, Q, R, S, and T, are all due to the action of the ventricles and are known as the ventricular complexes. Any deviation from the norm in a particular electrocardiogram is indicative of a possible heart disorder. Information that can be obtained from an electrocardiogram includes whether the heart is enlarged and where the enlargement occurs, whether the heart action is irregular and where the irregularity originates, whether a coronary vessel is occluded and where the occlusion is located, and whether a slow rate is physiological or caused by heart block. The presence of high blood pressure, thyroid disease, and certain types of malnutrition may also be revealed by an electrocardiogram. The Dutch physiologist Willem Einthoven developed the first electrocardiograph in 1903. During the late 1960s, computerized ECGs came into use in many of the larger hospitals.