COUGHMEDICATIONS
The respiratory tract consists of the airways that conduct air to and from the oxygen and carbon dioxide gas-exchange surfaces within the lungs. The upper respiratory system consists of the nose, nasal cavity, and pharynx (throat), and the lower respiratory system includes the larynx (voice box), trachea (windpipe), bronchi, bronchioles, and alveoli of the lungs. A specialized respiratory mucosal layer, a type of protective epithelial tis- sue that may additionally contain mucus-secreting cells and cilia, lines the portion of the respiratory system that conducts air to the lungs. This airway conducting system begins at the entrance of the nasal cavity, extends through the pharynx and larynx, and continues along the trachea and bronchi to the bronchioles. The cilia continually propel mucus, loaded with dust particles and other types of debris, toward the pharynx and away from the lungs.
Although individuals can generally cough on purpose (a conscious cerebral brain mechanism), coughing is usually a parasympathetic reflex (an unconscious brainstem mechanism) of the body used to clear the lower respiratory system of aspirated particles (particles inadvertently breathed in), mucus, or excess fluids. This protective reflex operates when receptors located within the respiratory mucosa of the larynx, trachea, or bronchi are irritated by toxic vapors, chemical irritants, or mechanical stimuli. The irritation of the respiratory tract receptors results in a sudden explosive ejection of air from the lungs. This relatively complex visceral reflexive action is coordinated by the coughing center neurons located within the medulla oblongata of the brain. A cough begins with a deep breath in, at which point the opening between the vocal cords at the upper part of the larynx (called the glottis) shuts, trapping the air in the lungs. As the diaphragm and other muscles involved in breathing (e.g., intercostal muscles) press against the lung tissue, the glottis suddenly opens, producing an outflow of air that can reach speeds greater than 100 miles (160 kilometers) per hour. In normal situations, most individuals cough once or twice an hour during the day to clear the respiratory airway of irritants. However, when the level of irritants in the air becomes elevated, or when the respiratory system contains excess mucus or fluids, coughing may become frequent and prolonged, thereby potentially interfering with exercise or sleeping patterns. Excessive coughing may also lead to distress if accompanied by dizziness, chest pain, or breathlessness. The majority of coughs are related to the symptoms of the common cold virus or influenza.
Coughs are generally described as either dry or productive. A productive cough brings up sputum. Sputum is a secretion that is produced in the lungs and the bronchi (tubes that carry air to the lungs) and expectorated (spit out) through the mouth via deep coughing. It typically consists of discharges (e.g., mucus, irritants, etc.) from the respiratory passages combined with saliva. A nonproductive cough (also called a dry or hacking cough) fails to bring up sputum. Over-the-counter oral cough treatments can contain active ingredients such as expectorants (drugs that promote expectoration, the process of coughing up and spitting out). As an orally administered expectorant, guaifenesin [3-(2-methoxy- phenoxy)-1,2-propanediol; C10H14O4] stimulates receptors in the gastric
(stomach) mucosal tissue lining, which then initiates a reflex increase in
respiratory tract fluid secretions. This drug aids in loosening sputum and bronchial secretions by reducing adhesiveness and surface tension. By reducing the viscosity of secretions, guaifenesin increases the efficiency of the cough reflex and ciliary action in removing secretions from the trachea and bronchi, increases sputum volume, and lubricates irritated respiratory tract membranes through increased mucus flow. Thus, bronchial drainage is improved, and dry, nonproductive coughs become more pro- ductive and less frequent.
Nonproductive coughs attributable to colds or inhaled irritants, which interfere with rest and sleep, may also be treated with oral products containing antitussives. An antitussive is an agent that prevents or relieves coughing by depressing the cough center in the medulla oblongata or the cough receptors in the pharynx, trachea, or lungs. A centrally acting antitussive is an agent that specifically depresses the cough center located within the medulla oblongata. Dextromethorphan hydrobromide (3-methoxy-17- methyl-9a,13a,14a-morphinan hydrobromide monohydrate; C18H25NO·
HBr·H2O) is a synthetically produced non-narcotic centrally acting antitussive available in over-the-counter cough medications. Related to other centrally acting antitussives such as codeine, it is a salt of the methyl ether of the dextrorotatory isomer of levorphanol, a narcotic analgesic (a drug that relieves pain). While dextromethorphan hydrobromide has no analgesic or addictive properties like those of narcotic centrally acting antitussives, this drug suppresses the cough reflex by increasing the threshold for coughing to occur. When ingested at the proper therapeutic dosage, it does not depress respiration or inhibit ciliary action within the respiratory airways.
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