From Big Medical Encyclopedia

PACEMAKER (English pacemaker the setting speed, a pacemaker) — the source of periodically arising extending excitement involving any excitable system in the uniform specific act of life activity. It is necessary to distinguish artificial and natural Items.

Artificial P. are the electropulse devices allowing to impose an artificial rhythm of reductions to various natural excitable educations. Electronic stimulators of action of the heart, breath, digestive, urinogenital and other systems are developed, in particular. Artificial P. use as ritmovoditel of heart and breath for stimulation of reductions of ventricles of heart in beat of auricles, for urezhayu-Russian cabbage soup of cardiac activation, etc. E.g., an opportunity to impose an artificial rhythm of reductions to ventricles of heart by means of artificial P. at cross to a heart block (see) it was highly appreciated by clinical physicians and it is recognized as the main method of treatment of similar patients. Further improvement of methods of electrostimulation, creation of new types of artificial P. allowed to pass to wide use of the electrostimulators implanted into a body of the patient (see. Cardiostimulation , Electrostimulation ).

A methodological basis of creation of artificial P. is studying of features of functioning of natural P. normal and at various forms of pathology. Natural P. represent set of the nervous and muscle cells setting a rhythm of activity to any system or body. The physiological role of natural P. consists in ensuring automatism and regulation of intensity of work of these or those systems of an organism (e.g., cardiovascular, respiratory and t) due to change of the frequency (rhythmics) of excitement. Distinctiveness of the functional organization P. is ability to self-excitation (see. Automaticity ), to participation in development of the extending excitement, coming from other Item.

From a set of the potential P. providing reliability of work of body allocate the true P. involving in the extending excitement other P. of any body. Key feature of true P. is the advancing formation of self-excitation by it. At the same time these or those structures carry out a role of a source of the extending excitement only on condition of readiness of other elements of vozbudikhmy system for carrying out the created Item. excitement (see).

Own frequency of P. for lack of external influences is rather constant, by means of mechanisms neurohumoral regulation (see) frequency of categories P. according to the level of requirements of an organism is regulated at present. So, accumulation in blood of end products of exchange and carbonic acid is followed by increase of a heart rhythm and the breath providing their removal. Periodic vibrations of frequency of excitement of P. serve, besides, as informative expression of a condition of the central mechanisms of regulation. The condition of an anesthesia, action of a number of ganglioblokator, a functional overstrain of cardiovascular system reduce duration of an oscillatory cycle. Mechanisms of self-control of activity of any body working in the automatic mode, its works providing optimization of a rhythm at the level set by an organism closely interact with the mechanism of the central regulation. Mechanisms of self-control carry out optimum setup of the working body depending on requirements and functionality of system. Thus, the coordinate mutually-equilibration of frequency and the rhythm generated by the Item is provided.

Conscious (strong-willed) management extends preferential to P. of behavioural reactions — in particular motivational excitement (see. Motivations ) and on evolutionarily youngest P., e.g. P. of breath. In the course of an ontogeny accrue, and then the limits of fluctuations of frequency of cordial and respiratory rhythms accompanying physical and psychoemotional tension decrease. Functional characteristics of homologous P. at animal different types differ among themselves on the frequency, a gradient and speed of carrying out excitation waves, duration of the refractory period.

Allocate inside - and Extra organ Items. As P. of skeletal, respiratory muscles the centers located in the highest departments of c act respectively motive and respiratory. N of page. True P. of heart is the sinus (sinus and atrial) node which is localized within body. The possibility of the central imposing is shown, however, to heart of the managed rhythm, napr, by irritation of a vagus nerve by electric impulses, similar to the impulses observed in this nerve at natural action of the heart. In a complete organism of P. of these or those functional systems are in a hierarchical taxonomy. E.g., in P.'s heart of the first order is a sinus and atrial node; Items of the second and third orders are located in the carrying-out system of ventricles.

Transition of a role of true P. to elements of lower order can be observed at damage or P.'s oppression of the highest order. So, the transition of heart to a ventricular rhythm registered in a wedge, practice is noted at the full block of carrying out excitement in an atrioventricular node, or a ventriculonector — a sick sinus syndrome (see. Heart ), and in an experiment — at artificial damage of the last. The similar phenomena can take place during the strengthening of the braking parasympathetic influences covering P. of heart of the highest order at simultaneous sympathetic activation of ventricular Items. Parallel activity of P. of different levels arises at lacks of coordination of activity of departments and sites of body. Atrioventricular blockade with preservation of a sinoatrial rate of activity of auricles and the simultaneous slowed-down ventricular rhythm is observed at disturbances of atrioventricular carrying out. The highest form of disintegration of activity of excitable system is parallel uncoordinated activity of a set of P. of the lowest orders. Nek-ry types of fibrillation of ventricles can serve as an example, at to-rykh there are multiple ectopic centers.

P.'s excitement depends on emergence of an impulse in one or is more often to group of electroexcitable cells of identical level of excitability. Distribution of categories P. in time is described by the curves characterizing and defining activity of the corresponding functional systems. Frequency of generation of P. of action potentials is directly connected with the speed of slow diastolic polarization of a cell of P. and a difference between the level of transmembrane potential reached at the end re polarization by the maximum diastolic potential, and critical potential, at Krom there occurs the regenerative as avalanche accruing depolarization.

Artificial P. are constantly improved, in particular increase in adequacy of artificial P. to natural is reached by rapprochement of points of stimulation with natural sources of excitement and ways of spread of activation, increase in «physiology» of a configuration of the irritating incentives, their approval in time of phases of activity of natural P. and other ways.

See also Cardiostimulation , Electrostimulation .

Bibliography: Andreyev S. V. Recovery of action of the heart of the person after death, M., 1955; Anokhin P. K. System mechanisms of higher nervous activity, Chosen works, M., 197 9; B of p e d both to and with Yu. Yu. Electric cardiac activation in clinical practice, M., 1967, bibliogr.; it, Electric cardiac activation at tachycardias and tachyarrhythmias, M., 1976; Goffman B. and Kreynfild P. Elektrofiziologiya of heart, the lane with English, M., 1962; Goodwin B. The temporary organization of a cell, the lane with English, M., 1966; To au-sitsky G. I. Afferent systems of heart, M., 1975; P and p and I am V. I, and T pieces of e e in A. A. Some disturbances of a heart rhythm at asynchronous electrocardiostimulation and bneupravlyaemy stimulation, Tbilisi, 197 7; Problems of the general and clinical physiology of cardiovascular system, under the editorship of B. I. Hodorova, Kiev, 1976; Roshchevsky M. P. Evolutionary electrocardiology, L., 1972; Udelnov M. G. Nervous control of heart, M., 1961; At d e of l N about in M. G. and Sukhov of G. S. Avtomatiya of heart, Usp. fiziol, sciences, t. 5, No. 1, page 82, 1974; Physiology of blood circulation, Physiology of heart, under the editorship of E. B. Babsky, L., 1980.

A. N. Medelyanovsky.