Expectorants and Mucolytic Drugs
Expectorants and mucolytic drugs are used to increase the output of bronchial secretions, enhance the clearance of bronchial exudate, and promote a productive cough. Saline expectorants are promoted to stimulate bronchial mucous secretions via a vagally mediated reflex action on the gastric mucosa. However, there are no well-designed studies that support these claims. Examples of these drugs include ammonium chloride, ammonium carbonate, potassium iodide, calcium iodide, and ethylenediamine dihydroiodide. Iodine-containing products should not be administered to pregnant, hyperthyroid, or milk-producing animals.
Direct stimulants of respiratory secretions include the volatile oils, such as eucalyptus oil and oil of lemon. They are believed to directly increase respiratory tract secretions. Their efficacy in animals is unknown.
Guaifenesin (glyceryl guaiacolate) is a centrally acting muscle relaxant that may also have an expectorant effect. It may stimulate bronchial secretions via vagal pathways. The volume and viscosity of bronchial secretions does not change, but particle clearance from the airways may accelerate. It is a common component of human cold remedies in combination with dextromethorphan.
N-acetylcysteine is available as a 10% solution that can be nebulized. Its mucolytic effect is the result of the exposed sulfhydryl groups on the compound, which interact with disulfide bonds on mucoprotein. Acetylcysteine helps to break down respiratory mucus and enhance clearance. It may also increase the levels of glutathione, which is a scavenger of oxygen-free radicals. Aerosolization of acetylcysteine can cause reflex bronchoconstriction due to irritant receptor stimulation, so its use should be preceded by bronchodilator therapy.
Dembrexine is a phenolic benzylamine available in some countries for respiratory disease in horses. The proposed effect is through an alteration of the constituents and viscosity of abnormal respiratory mucus and an improved efficiency of respiratory clearance mechanisms. It also has an antitussive action and enhances concentrations of antibiotics in lung secretions. It is supplied as a powder that is sprinkled on the feed at a dosage of 0.33 mg/kg, bid.