Cough is a protective reflex, its purpose being expulsion of respiratory secretions of foreign particles from air passages. It occurs due to stimulation of mechano-or chemoreceptors in throat, respiratory passages or stretch receptors in the lungs. Cough may be useful or useless.
Useless (nonproductive/Dry) cough should be suppressed. Useful (productive/Wet) cough serves to drain the air way, its suppression is not desirable, may even be harmful, except if the amount of expectoration achieved is small compared to the effort of continuous coughing.
Apart from specific remedies (antibiotic, etc.), cough may be treated as a symptom (nonspecific therapy) with:
‘’The leading cause of coughs is colds and flus, but there are numerous other causes that trigger coughing attacks,’’ says Dr. Robert McLean. “Chronic coughs that last more than eight weeks can be a sign of a more serious lung condition such as asthma, COPD and pulmonary fibrosis.”
Lozenges, cough drops, linctus containing syrup, glycerine, liquorice.
a) Bronchial secretion enhancer:
Sodium or Potassium citrate, Potassium iodide, Guaiphenism, Balsum of Tolu, Vasaka, Ammonium chloride.
Bromhexine, Ambroxol, Acetyl cysteine, Carbocisteine.
3. Antitussives (Cough center suppressant)
a) Opiods: Codeine, Ethylmorphine, Pholcodeine.
b) Nonopiods: Dextromethorphan, Nascapine, Chlophedianol.
c) Antihistamine: Chlorpheniramine, Diphehydramine, Promethazine.
d) Peripherally acting: Prenoxdiazine.
Bronchodilators: Salbutamol, Terbutalin.
Demulcents sooth the throat and reduce afferent impulses from the inflamed/irritated pharyngeal mucosa, thus provide symptomatic relief in dry cough arising from throat.
Are drugs believed to increase bronchial secretion or reduce viscosity, facilitating its removal by coughing.
Sodium or Potassium citrate are considered to increase bronchial secretion by salt action.
Potassium iodide is secreted by bronchial glands can irritate the airway mucosa. Prolonged use can affect thyroid function and produce iodism. It is not used now.
Guaphenism, Vasaka, Tolu balsum are plant products which are supposed to enhance bronchial secretion and mucociliary function while being secreted by tracheobronchial glands.
Ammonium salts are nauseating-reflexly increase respiratory secretions. A variety of expectorant formulations containing an assortment of the above ingredients, often in combination with antitussives/antihistaminic are marketed and briskly promoted, but objectives evidence of efficacy of these is non-conclusive.
The US-FDA has stopped marketing of all expectorants, except Guaphenism. Steam inhalation and proper hydration may be more helpful in clearing airway mucus.
These are drugs that act in the CNS to raise the threshold of cough Centre or act peripherally in the respiratory tract to reduce tussals impulses or both these actions. Because they aim to control rather than eliminate cough, antitussives should be used only for dry nonproductive cough or if cough is unduly tiring, disturbs sleep or is hazardous (hernia, piles, cardiac disease, ocular surgery).
COMMON DRUGS FOR COUGH
A derivative of the Alkaloid vasine obtained from Adhatoda vasica (Vasaka), is a potent mucolytic and mucokinetic, capable of inducing thin copious bronchial secretion. It is particularly useful if mucous plugs are present.
Side effects are rhinorrhea and lacrimation, nausea gastric irritation, hypersensitivity.
A metabolic of bromhexine having similar mucolytic action, uses and side effects.
An opium alkaloid, qualitatively similar to and less potent than morphine, but is more selective for cough Centre. Codeine is regarded as the standard antitussive; suppresses cough for about 6 hours.
The antitussive action is blocked by naloxone indicating that it is exerted through opioid receptors in the brain. Abuse liability is low, but present; constipation is the chief drawback. At higher doses respiratory depression and drowsiness can occur, especially in children.
Driving may be impaired. Like morphine, it is contraindicated in asthmatics and in patients with diminished respiratory reserve; should be avoided in children.
Dextromethorphan does not depress mucociliary function of the airway mucosa and is practically devoid of constipating action.
Though considered nonaddicting, some drugs abusers indulge in it. The antitussive action of dextromethorphan has been rated equivalent to codeine, but some clinical studies have found it to be no better than placebo.
Side effects: Dizziness, nausea, drowsiness; at high doses hallucinations and ataxia may occur.
Many H1 antihistamines have been conventionally added to antitussive/expectorant formulations. They afford relief in cough due to their sedative and anticholinergic actions, but lack selectivity for the cough Centre. They have no expectorant property, may even reduce secretions by anticholinergic action. They have been specially promoted for cough in respiratory allergic states, though their lack of efficacy in asthma is legendary.
Bronchospasm can induce or aggravate cough. Stimulation of pulmonary receptors can trigger both cough and bronchoconstriction, especially in individual with bronchial hypersensitivity.
Bronchodilators relieve cough in such individuals and improve the effectiveness of cough clearing secretions by increasing surface velocity of airflow during the act of coughing. They should be used only when an element of bronchoconstriction is present and not routinely.
Their fixed dose combinations with antitussives are not satisfactory because of differences in time course of action of the components and liability for indiscriminate use.