Cough physiology and therapy tactics
Cough is a vital protective reflex that occurs in response to irritation of the respiratory mucosa. Its physiological purpose is to remove the irritant, whether it is excessive bronchial secretion (sputum) or a foreign body.
To understand the pharmacodynamics of drugs, it is important to know the structure of the reflex arc:
- Afferent component (signal input): begins with sensory receptors. Impulses from the larynx travel along fibers of the glossopharyngeal nerve (n. glossopharyngeus), and from the bronchi along sensory fibers of the vagus nerve (n. vagus).
- Central component: the signal enters the medulla oblongata, where stimulation of the cough reflex center occurs.
- Efferent component (signal output): signals are directed from the brain via somatic motor fibers to the respiratory muscles (diaphragm and intercostal muscles). A forced expiration is the result of their abrupt contraction.
Pharmacotherapy tactics strictly depend on the clinical picture:
- Productive ("wet") cough: accompanies inflammatory diseases with hypersecretion (bronchitis, tracheitis). It performs a protective function by draining the bronchi and accelerating recovery. Suppression of such a cough is contraindicated; expectorants are prescribed instead.
- Non-productive ("dry") cough: characteristic of pleurisy, neoplasms, and chronic inflammation. The protective function is lost; the symptom merely exhausts the patient and disrupts nocturnal sleep. In this case, it is advisable to use antitussive drugs.
Drug classification
According to their localization and mechanism of action, antitussives are divided into two large groups:
- Centrally acting agents (suppress the cough center in the brain):
- With narcotic effect: codeine, ethylmorphine.
- Non-narcotic: glaucine, oxeladin.
- Peripherally acting agents (reduce the sensitivity of respiratory tract receptors):
- Representative: prenoxdiazine.
Centrally acting agents
Narcotic antitussives These include opium alkaloids and their derivatives. Their mechanism of action is associated with the stimulation of opioid receptors in the medulla oblongata, which leads to a decrease in the excitability of the cough center.
- Codeine (methylmorphine). Available as the base or phosphate. Exerts pronounced antitussive and analgesic effects. In therapeutic doses, depression of the respiratory center is minimal (there is no direct correlation between cough suppression and respiratory depression). Often included in combination drugs ("Codelac", "Terpincode" tablets, Bechterew's mixture with adonis infusion and sodium bromide).
- Ethylmorphine (Dionine). A semisynthetic derivative of morphine. Prescribed orally for dry, exhausting coughs.
Risks: stimulation of opioid receptors in the mesolimbic and mesocortical systems causes euphoria, which accounts for high addiction potential (risk of dependence). Constipation occurs with systematic administration. Dispensing of these drugs is strictly regulated.
Non-narcotic antitussives They also directly suppress the cough center, but do not activate the opioidergic system. Consequently, they do not cause dependence and have a lesser effect on respiration.
- Glaucine (Glauvent). A plant alkaloid from yellow horned poppy. Well absorbed; the effect appears after 30 minutes and lasts about 8 hours. Side effects: lowered BP (hypotension), dizziness, nausea.
- Oxeladin (Tusuprex). A synthetic drug close in profile to glaucine. Rapidly and completely absorbed, peak concentration in the blood is reached in 4–6 hours.
Peripherally acting agents
These drugs work outside the CNS, acting directly on the respiratory tract.
- Prenoxdiazine (Libexin). Possesses a triple mechanism:
- Local anesthetic effect: reduces the sensitivity of bronchial mucosal receptors, preventing the initiation of the reflex.
- Bronchodilator effect: causes bronchial dilation.
- Anti-inflammatory effect.
The drug does not affect the CNS; the duration of its action is 3–4 hours. Due to local anesthesia, numbness of the tongue is a specific side effect. Dry mouth and diarrhea are also possible.
To memorize the effects of prenoxdiazine, use the "LBA" rule: Local anesthesia, Bronchodilation, Anti-inflammatory action. And the side effect is easily remembered via its mechanism — local anesthesia causes tongue numbness.
Class and exam questions
Why must codeine not be prescribed for a productive cough?
In productive (wet) cough, it is necessary to clear sputum. Suppressing the reflex with codeine will lead to the stagnation of infected secretions in the bronchi, depriving cough of its protective drainage function.
What is the main advantage of glaucine over ethylmorphine?
Glaucine is a non-narcotic agent. It does not activate opioid receptors, does not cause euphoria or drug dependence, and has a lesser effect on the respiratory center.
Which nerves form the afferent link of the cough reflex?
Sensory impulses from the larynx are transmitted via fibers of the glossopharyngeal nerve, and from the bronchi via fibers of the vagus nerve.
What else to study on this topic
- Pharmacokinetics and metabolism of codeine in the body
- Combination drugs with codeine (composition and synergy of components)
- Mechanisms of development of opioid dependence when taking central antitussives
- Comparative characteristics of glaucine and oxeladin
- Expectorants (as an alternative for productive cough)
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