Expectorants

EXPECTORANTS (expectorantia) facilitate the removal of secretions from the respiratory passages. They include a range of substances from the groups of alkaloids, glycosides, terpenes, essential oils, salts, alkalis, aromatic compounds, etc. The mechanism of action of expectorants is insufficiently studied experimentally, and the corresponding theories are based primarily on clinical material. The main group of expectorants consists of emetics used in small doses to promote expectoration without causing vomiting or nausea. The action of these agents manifests, among other things, in increased secretion of many glands, and first and foremost the glands of the mucous membrane of the respiratory tract (the initial period of the so-called nausea). The mechanism of this expectorant action can be of various kinds: either direct stimulation of the centers of bronchial secretion (e.g., apomorphine), the proof of which is the presence of an effect even after ligation of the vessels supplying the mucous glands, or reflex action depending on irritation of the gastric mucosa (e.g., ipecacuanha, senega, emetic stone). A third possible mechanism is the action of expectorants on the secretory apparatus of the mucous membrane of the respiratory tract (on the cells or on the nerve endings). This view on the mechanism of action finds support in experiments with the action of expectorants after cutting the secretory nerves of the mucous glands; there are indications that some of the mentioned substances exhibit this peripheral action during their excretion from the body through the respiratory tract. As a result of the expectorant action, regardless of the mechanism, bronchial secretion becomes abundant, liquid, and easily expelled outward. The greatest therapeutic value as expectorants is possessed by those emetics which are characterized by a prolonged period of nausea (ipecacuanha, senega). Another large group of expectorants consists of substances for which there is the assumption that they are excreted from the body, among other things, through the mucous membrane of the respiratory tract. They can be subdivided into 2 subgroups: 1) substances with a saline action (e.g., iodides, sodium chloride); they cause secretion of liquid mucus due to a change in the ratio in the osmotic equilibrium between blood and lymph, on the one hand, and cell plasma on the other; 2) substances that increase the alkaline reaction inside the secretory cells, which leads to the appearance of abundant, liquid mucus; examples can be alkalis. According to their mechanism of action, ammonium salts, terpenes, essential oils, aromatic compounds of the guaiacol type, wood tar, etc., should be referred to the above group of expectorants. In addition to the above views on the mechanism of expectorant action, there are experimental data speaking for the existence of another mechanism. For example, according to the experiments of Robert and Kochmann, some expectorants possess an exciting action on the ciliated epithelium of the upper respiratory tract; this circumstance can have value only with simultaneous liquefaction of the mucus. Others attribute a certain role to the smooth muscle of the bronchioles and alveoli; from this point of view, substances that dilate the bronchial lumen have some value; dilation can occur either from stimulation of the endings of the bronchial dilator (sympathetic nerve) or from inhibition of the constricting fibers (vagus nerve). Dilation of the bronchial lumen and peristalsis of the muscle can, of course, facilitate the secretion of mucus. A certain role may be played in this by independent rhythmic contractions of the bronchial muscle, observed for the first time by Einthoven. The question remains open whether such phenomena occur in the whole organism; in any case, data obtained by some authors in experiments in vitro cannot be entirely transferred to conditions in vivo. The increased cough reflex observed with some expectorants also apparently does not remain without influence on the results of their action (emetine, apomorphine, senega). The main indication for the use of expectorants is inflammation of the respiratory tract and lungs in the presence of cough with thick, difficult-to-expectorate sputum. The mucus secreted during the action of expectorants serves as protection for the inflamed mucous membrane from cold; this can also lead to weakening and disappearance of the cough reflex; in cough, expectorants are often combined with analgesic agents (e.g., opium).