The applied physiology of cough.

The cough reflex is such an important defense mechanism of the respiratory tract that a thorough knowledge of its applied physiology in respiratory disease is essential for clinicians. It is a well-integrated reflex, which has afferent limb consisting of receptors and afferent nerves, the central cough center in the brainstem and the efferent limb consisting of motor nerves supplying the muscles of coughing. The cough process consists of inspiratory phase, compressive phase and expiratory phase. Stimuli that can initiate the cough process can be central or peripheral in the lungs or outside the lungs. The important function of the cough reflex is to maintain the airways and alveoli clear and healthy. The mechanisms by which it achieves this are (i) high velocity of expiratory gas flows produced in the expiratory phase (ii) the compression of the lungs and airways by high positive pleural pressure generated in the compressive phase. Cough threshold can be determined by giving acetic acid, citric acid or capsaicin inhalation challenge test. This has practical application in clinical research. The pathophysiological basis of different types of cough encountered in clinical practice is discussed.