Introduction

Cough is probably the most powerful and commonest normal physiological reflex. It is essential for the clearance of the respiratory tract, but in disease it may become pathological such that it impairs bodily functions and becomes an embarrassment for the patient. It is characterized by a violent expiration, which provides the high flow rates that are required to shear away mucus and remove foreign particles from the larynx, trachea and large bronchi. Its function to expel excess secretions and inhaled irritants from the airways is immediately obvious. However, the causes of cough are not necessarily associated with excessive bronchial secretions, as for example in chronic bronchitis, but are often related to lung diseases such as asthma and viral infection of the upper respiratory tract. Intensive and frequent cough may impair breathing and cardiac circulation, increase oxygen consumption and interfere with eating, sleep and rest.

Coughing is initiated when sensory receptors in the respiratory tract receive stimuli of sufficient intensity to evoke an increase in afferent nerve impulse activity. Cough reflexes can be provoked easily by mechanical and chemical stimuli applied to the epithelium of either the larynx or tracheobronchial tree. There are three main groups of airway sensory receptors which may be involved in the cough reflex initiated from these sites: the slowly adapting stretch receptors (SARs), the rapidly adapting stretch receptors (irritant, RARs) and the pulmonary and bronchial C-fibre receptors. Each is distributed throughout the tracheobronchial tree and the last group is also present in the alveolar wall.