Inotropic Action

INOTROPIC ACTION (literally "imparting force"), a change in the amplitude of heart contractions under the influence of various physiological and pharmacological agents. Positive inotropic action, i.e., an increase in the amplitude of heart contractions, is caused by stimulation of accelerators; negative inotropic action is obtained by stimulation of the vagus nerves. Corresponding effects are given by vagomimetic and sympathicomimetic poisons and salt ions. However, the inotropic action of a given agent depends on a number of conditions: pH, composition of the wash solution or blood, intracardiac pressure, heart rate, and therefore the observation of inotropic action requires work under constant conditions (artificially excited heart rhythm, etc.). The inotropy of different heart regions can change independently of the inotropy of the other regions. I. P. Pavlov succeeded in finding a branch in the cardiac plexus of a dog that gives a positive inotropic effect only on the left ventricle. The pathways of inotropic action have been studied in more detail by Hofmann: he found that the specific "inotropic nerves" of the frog heart are the nerves of the interventricular septum, stimulation of which gives a purely inotropic effect without chronotropic changes; after cutting these nerves, stimulation of the common vagosympathetic trunk no longer gives any inotropic action. Inotropic action of salts. Potassium salts exert negative inotropic action; this effect is not observed after atropinization. Sodium acts similarly in large concentrations; however, this action may depend on the fact that hypertonic solutions in general possess negative inotropic action. A decrease in the content of NaCl in the wash solution gives positive inotropic action. Lithium and ammonium salts exert positive inotropic action; rubidium acts similarly to potassium. Calcium acts positively inotropically and even leads to a systolic stoppage. The absence of calcium in the wash solution gives a negative inotropic effect. Barium and strontium act in general similarly to calcium. Magnesium acts antagonistically with respect to both calcium and potassium. Salts of heavy metals give negative inotropic action. However, the action of the aforementioned salts may be absent or perverted with changes in the pH of the wash solution and after preliminary treatment of the heart with other (often antagonistic) agents. Among anions, one can note the negative inotropic action of iodides, lactic and cyanic salts, small doses of which act however positively inotropically. Narcotics and alcohol act negatively inotropically; in very small doses they act positively inotropically. Carbohydrates (glucose) when added to the wash solution (as a source of energy) give positive inotropic action on an isolated heart. Digitalis influences inotropy not only indirectly (by acting on the vessels and the vegetative nervous system) but also directly on the heart muscle (small doses - positively, large doses - negatively), especially on the left ventricle. Adrenaline, by shortening the latent period of contraction and the systole, usually gives positive inotropic action; this effect is less pronounced in the frog than in warm-blooded animals. However, here, as in the case of many vegetative poisons, everything depends on the dose and on the condition of the heart. The action of camphor also depends on the dose: small doses give positive inotropic action, large doses give negative inotropic action; it is especially clearly expressed on pathologically altered hearts. Cocaine in very small doses acts positively inotropically, in large doses negatively. According to recent observations by Kisch, atropin in the first phase of its action excites the vagus nerve and therefore gives negative inotropic action. Muscarine group poisons act similarly to stimulation of the vagus nerve. Veratrin and strychnine, applied in small doses, give positive inotropic action. Caffein acts on inotropy mainly indirectly, changing the heart rate; but when applied in small doses to a fatigued heart, it acts directly on the heart muscle with positive inotropic action. (For the relationship between inotropic, dromotropic and chronotropic actions - see the corresponding words.)