Tachycardia

TACHYCARDIA, a significant acceleration of cardiac activity, a phenomenon caused by the heart's reaction to a range of physiological and pathological influences. The site of origin of the accelerated impulses causing the acceleration of the cardiac rhythm can be various parts of the heart; in connection with this, two forms of T. are distinguished—one in which the impulses originate in the sinus node, from which, in the norm, impulses to the contraction of the heart also originate (nomotopic, sinus T.), and another in which the impulses originate in other parts of the heart (heterotopic T.). The latter has a paroxysmal character, begins and ends suddenly, and usually has a very high frequency of cardiac contractions (up to 200 and more beats in 1 min.) (see Paroxysmal tachycardia). Nomotopic T., on the contrary, are characterized usually by a lower frequency of cardiac contractions (90-170 beats in 1 min.), a gradual beginning and end. Nomotopic T. are very difficult to distinguish from fluctuations in the frequency of cardiac contractions during physiological accelerations of the cardiac rhythm. For example, in a newborn the pulse frequency is 136 in 1 min.; up to 25 years the pulse gradually slows down, from 25 to 60 years it is 70-72, after 60 years it again slightly accelerates. In women the pulse is more frequent than in men by 5-10 beats in 1 min.; when changing the position of the body from lying to standing, the pulse accelerates on average by 10 beats (the so-called orthostatic T.). During physical exertion, various T. of different degrees and duration are observed, with an increase in the number of cardiac contractions against the norm by 20-100 and more in 1 min., depending on the degree and nature of the physical exertion, constitutional features of the organism, state of the nervous system, etc. Mental exertions, meals, swallowing, sexual act, consumption of taste substances (alcohol, tea, coffee, etc.), an increase in the external temperature also cause an acceleration of the pulse. The influence of all the listed physiological factors in the sense of greater or lesser acceleration of the cardiac rhythm and in the sense of the duration of this acceleration is especially sharply manifested in persons with an especially excitable nervous system, in anemia, with shifts in the endocrine-autonomic system with an increase in sympathetic excitability. Acceleration of cardiac contractions above the usual ones observed in physiological fluctuations in conditions often reveals constitutional features of the organism—'latent neurosis', as well as hidden organic lesions of one or another nature. In pathological conditions, nomotopic T. are caused by febrile states, diseases of the cardiovascular and nervous systems, disturbances in the correlation of the glands of internal secretion, diseases of the internal organs, etc.-Febrile T. According to the law of Liebermeister, the number of heart beats for each degree above 37° increases against the norm by 8 beats in 1 min. Most infectious diseases cause such febrile T. The exception is typhoid fever, in which the pulse frequency lags behind the degree of increase in temperature, as well as tuberculous meningitis, in the early stages of which bradycardia is noted. Other diseases, such as scarlet fever, diphtheria, some forms of influenza, lobar pneumonia, can be accompanied by T. that significantly goes beyond the Liebermeister formula. A particularly pronounced tendency to T. is found in streptococcal diseases, erysipelas, postpartum peritonitis, septicemia, sometimes giving T. up to 160-200 beats in 1 min., as well as tuberculosis of the lungs, in which T. can reach high degrees even in afebrile periods (140 and more beats in 1 min.).-T. in diseases of the cardiovascular system. Various T. of different degrees and duration are observed in acute and chronic myocarditis, acute endo- and pericarditis, heart defects (mainly mitral), especially in the stage of decompensation, acute and chronic aortitis, sclerosis of the coronary arteries of the heart (in the most severe cases bradycardia is often present), and during acute insufficiency of a healthy heart under the influence of physical overexertion. T. in most of the listed cases is characterized by inconstancy, lability, often disappears at rest and appears during physical exertion, excitement, and often to such a degree as they never cause in the norm. A drop in blood pressure during acute and significant blood loss, shock, peritonitis, etc., is usually also accompanied by pronounced tachycardia. T. in disturbances in the sphere of endo- and autonomic nervous system. T. is especially clearly revealed in hyperfunction of the thyroid gland (Basedow's disease), in a less expressed form in hyper- and hypofunction of the ovaries (sexual maturation, climacteric, after ovariectomy).-Reflex T. are often caused by severe pain, for example renal, hepatic colic, chronic lung diseases, diseases of the gastrointestinal tract (dyspepsia, especially meteorism).-Toxic T. can be caused both by medications (atropine, adrenaline, caffeine, digitalis) and by taste substances (coffee, tea, alcohol). In poisoning by mushrooms (champignons, muscarine), in beriberi, diabetic coma, uremia, T. also partly has a toxic character.-Compression T. are caused by compression of the vagus nerve along its entire extent from the medulla oblongata to the heart by various pathological processes.-T. in organic diseases of the nervous system can be of bulbar origin (lesion of the center of the vagus nerve in the medulla oblongata in late stages of meningitis, with increasing hydrocephalus, tumors of the medulla oblongata and cerebellum, progressive bulbar paralysis) and peripheral (tabes, polyneuritis, etc.). The mechanism of origin of T. is complex and diverse. Physiological T. are caused first of all by disturbances in the balance in the sphere of the autonomic nervous system due to a decrease in the tone of the vagus nerve or an increase in the tone of the accelerator nerve, or due to both of these. These disturbances in balance explain T. during physical and mental exertion, during the act of swallowing, orthostatic T., etc. The basis of constitutional T. is assumed to be congenital disturbances in the balance in the sphere of the endocrine-autonomic nervous apparatus in favor of sympathetic innervation. T. caused by the introduction of exogenous poisons (coffee, nicotine, alcohol) into the organism, as well as medicinal substances, depend either on the inhibition of the endings of the vagus nerve (atropine), or on the elimination of its central influence (nicotine), or on the excitation of the sympathetic nerve (adrenaline), or on the direct effect on the nervous-muscular tissue of the sinus node (caffeine). The pathogenesis of reflex T. is extremely complex. Such T. can be caused by the reflex effect of a irritating factor on the corresponding centers of the vagus and sympathetic nerves, or indirectly by a primary change in the frequency of breathing, blood pressure, activity of the adrenal glands, which in turn cause an acceleration of the cardiac rhythm. In infectious diseases, not complicated by weakening of cardiac activity or a drop in blood pressure, T. is caused both by an increase in the temperature of the blood and by the peculiarities of specific microbes and the quality of the toxins they produce, affecting either the contractility and tone of the myocardium or the system producing and conducting impulses, or the nervous apparatus of the heart. The latter peculiarities explain the apparently more significant acceleration of the pulse in some infections, especially streptococcal. T. that occur during septic diseases accompanied by a drop in temperature are particularly serious, as are long T. in convalescents, since they indicate a lesion of the myocardium of inflammatory, degenerative, etc. character, especially its specific system (see Myocarditis). The T. developing in a number of cases of typhoid fever, especially if it occurs suddenly, is usually caused either by intestinal hemorrhages or perforated peritonitis (drop in blood pressure) or by weakening of cardiac activity. Bradycardia of the early stages of tuberculous meningitis, caused by irritation of the vagus nerve, is replaced by T. in later stages due to its inhibition. T. in tuberculosis, especially in its afebrile forms, is explained by some authors by compression of the vagus nerve or by irritation of the sympathetic nerve by enlarged mediastinal and bronchial glands. Other T. in tuberculosis is explained by tuberculous intoxication acting either on the bulbar centers (paralysis of the vagus nerve) or on sympathetic innervation. Finally, a third T. in tuberculosis is explained by an increase in the excitability of the cardiac muscle in connection with general exhaustion of the organism or a drop in blood pressure.-T. in diseases of the cardiovascular system has a complex origin. There are two complex reflex mechanisms regulating the frequency of the cardiac rhythm. According to the law of Marey, the frequency of the cardiac rhythm changes proportionally to the height of the arterial blood pressure (with an increase in pressure—slowing of the rhythm, with a decrease—acceleration). On the contrary, an increase