17.02.4 Cough

A cough is a physiological mechanism to clear respiratory passages of foreign material and excessive secretions. Thus, when something is irritating the bronchial mucosa this leads to bronchoconstriction, and the bronchoconstriction stretches the respiratory stretch receptors, which are also known as the cough receptors (Figure 17.5). The stretch receptors send a message to central nervous system and there is a reflex cough to clear the substance causing the irritation.

Figure 17.5 The cough reflex (Copyright Sheila Doggrell, QUT)

Thus, a cough performs a useful physiological function, and it is only when cough is chronic and/or unproductive that it becomes annoying and fatiguing.

An anti-tussive is an agent used to relieve or prevent a cough. For the cough associated with bronchial asthma, the selective b₂-adrenoceptor agonists (e.g. salbutamol) are used. Salbutamol causes bronchodilation to reduce the stimulation of the respiratory stretch receptors. Menthol vapour also reduces the sensitivity of the respiratory peripheral stretch/cough receptors.

There are also centrally acting anti-tussive agents including codeine and dextromethorphan. Codeine is analgesic and anti-tussive. Methorphan is an analog of codeine. The L-isomer of methorphan is analgesic and addictive. The D-isomer of methorphan (dextromethorphan) is not analgesic or addictive, but is anti-tussive. Thus, dextromethorphan is the agent used to prevent cough.