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a method of investigating the heart muscle by recording bioelectric potentials. Contractions of the heart (systole) are preceded by excitation of the myocardium, accompanied by the passage of ions across the membranes of the myocardial cells, causing the potential difference between the external and internal surfaces of the membranes to change. Measurements made by means of microelectrodes show that the change in potential is about 100 microvolts (mv). Under normal conditions, the divisions of the human heart become excited in succession so that the changing potential difference between the already excited and still unexcited divisions of the heart is recorded on its surface. Owing to the electroconductivity of tissues, these electric processes can be detected even when the electrodes are placed on the body in places where the change in the potential difference is 1–3 mv.
Electrophysiological experiments were first performed on the heart in the 19th century, but the methods were not given medical application until the conclusion of Einthoven’s studies (1903–24). Einthoven used a quick-response string galvanometer. He suggested the names of the elements in the recorded curve, and he proposed a standard recording system and the main criteria for evaluation. The method’s great informational value, comparative technical simplicity, and safe and easy application guaranteed its widespread use in medicine and physiology.
The principal elements of a modern electrocardiograph are an amplifier, a galvanometer, and a recording device. A curve, or electrocardiogram (EKG), with acute and round waves that recur during every systole is obtained when the changing pattern of distribution of electric potentials is recorded on moving paper. The waves are usually designated by the letters P, Q, R, S, T, and U. The P wave is associated with the activity of the atrium, and the remaining waves with that of the ventricles. The shape of the waves in the different leads is generally different. The EKG’s of different persons can be compared if standard recording conditions are maintained and if the method of placing the electrodes on the limbs and chest (12 leads are normally used) is the same. The conditions are determined by the sensitivity of the apparatus (1 mm = 0.1 mv) and by the rate at which the paper moves (25 or 50 mm/sec). The examinee is usually recumbent and resting (in the presence of special indications, after physical activity or the administration of a drug). Analysis of an EKG reveals the presence, size, shape, and width of the waves and the intervals between them. The information is used to evaluate the electric processes in the heart as a whole and, to a certain extent, the electric activity of fairly circumscribed portions of the myocardium.
Electrocardiography is of the greatest importance for diagnosing disturbances of the cardiac rhythm and for detecting myocardial infarction and other diseases. However, changes in the EKG reflect only the nature of the disturbance of the electrical processes and are generally not strictly specific for a certain disease. Changes can result not only from disease but also from ordinary daily activity, diet, medication, and other causes. Therefore, the physician bases his diagnosis not on the EKG alone but on all clinical and laboratory signs of the disease. The diagnostic value of the EKG increases when several are taken in succession—at intervals of several days or a week—and compared.
Electrocardiographs are also used in cardiac monitors, making possible round-the-clock observation of the severely ill. In clinical, sports, and space medicine the devices are used for tele-metric control of a person at work; the electrodes are applied by special methods, and radio communication is set up between the galvanometer and the recording device.
The bioelectric activity of the heart can be recorded by other means. The potential difference is characterized by an intensity and direction specific for a particular moment. In other words, it is a vector and can be arbitrarily represented by an arrow occupying a certain position in space. The characteristics of the vector change during the cardiac cycle in such a manner that its initial point remains fixed, while the terminal point describes a complex closed curve. When projected to a flat surface, the curve, called a vectorcardiogram (VKG), resembles a series of loops. The VKG can be constructed approximately by graphic means based on the EKG in different leads. It can also be obtained directly by using a special apparatus, a vectorcardiograph, in which a cathode-ray tube is the recording device and two pairs of electrodes placed on the examinee in the corresponding plane are used as leads. By changing the position of the electrodes one can obtain a VKG in different planes and derive a more complete spatial representation of the electric processes. Vectorcardiography sometimes supplements electrocardiography as a diagnostic tool. Study of electrophysiological principles and the clinical use of electrocardiography and vectorcardiography, along with improvement of apparatus and recording techniques, are the concerns of a special branch of medicine called electrocardiology.
In veterinary medicine, electrocardiography is used to diagnose changes in the heart of large and small animals (chiefly horses, cattle, and dogs) that result from some noncontagious or infectious diseases. Disturbances of the cardiac rhythm, enlargement of divisions of the heart (atria, ventricles), and other changes can be detected in animals by means of electrocardiography. The procedure is of value in monitoring the effect on the heart muscle of drugs currently in use or undergoing testing.
REFERENCESIsakov, I. I., M. S. Kushakovskii, and N. B. Zhuravleva. Klinicheskaia elektrokardiografiia. Leningrad, 1974.
Sumarokov, A. V., and A. A. Mikhailov. Klinisheskaia elektrokar-diografiia, 3rd ed. Moscow, 1975.
Friedman, H. H. Diagnostic Electrocardiography and Vectorcardiography. New York, 1971.
Chung, E. K. Electrocardiography: Practical Applications With Vectorial Principles. New York, 1974.
A. A. MIKHAILOV