MINI-SYMPOSIUM: COUGH
The physiology of cough

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Summary

Cough is comprised of three phases (inspiratory, compressive and expiratory) and serves as a vital defensive mechanism for lung health. It prevents pulmonary aspiration, promotes ciliary activity and clears airway debris. The importance of an intact cough mechanism is reflected in the occurrence of pulmonary problems when cough is inefficient. Cough efficiency is dependent on physical/mechanical aspects (respiratory muscles, mucus, airway calibre and larynx) and integrity of the neurophysiological pathway of cough. The understanding of the latter has progressed significantly (albeit mostly in animals) with the discovery of vanniloid receptors (and subtypes) and, more recently, by the characterisation of distinct cough receptors. However, the relative contributions of previously described airway afferents/receptors to cough are still disputed. Plasticity of the peripheral and central afferent pathways in cough has recently been shown to be important in pathological states associated with increased cough. To date, little is known of the developmental aspects of cough.

Section snippets

INTRODUCTION

Cough, the most common symptom seen by general practitioners, has important defensive roles in health and disease. Ineffective cough is associated with respiratory morbidity such as recurrent pneumonia. However, chronic cough can be troublesome. It impairs the quality of life of adults1, 2 (no paediatric data) and significantly worries the parents of coughing children.3, 4 Coughs are easily recognisable and, unlike the symptom of wheeze,5 parents are almost as good as clinicians at recognising

COUGH MECHANICS AND SOUNDS

Physiologically, cough has three phases: inspiratory, compressive and expiratory.7 This physiological definition appears to be unimportant clinically but is essential in animal studies where cough sounds are non-existent or difficult to identify. The inspiratory phase consists of inhaling a variable amount of air that serves to lengthen the expiratory muscles, optimising the length–tension relationship. The compressive phase consists of a very brief (200 ms) closure of the glottis to maintain

COMPONENTS OF THE COUGH PATHWAY

The knowledge of cough neurophysiology has advanced significantly in recent years, although much of the work is based on animal models and may have limited applicability to humans due to significant interspecies differences.15 Furthermore, much of these works were performed in animals in an altered conscious state (e.g. under anaesthesia16) or in vitro.17 Readers are referred to recent reviews15, 16, 18, 19, 20, 21, 22 for in-depth aspects of cough-related neurophysiology. A summary of the

COUGH EFFICIENCY

An effective cough is dependent on generation of high linear velocities and interaction between flowing gas and mucus in the airways.7 This is dependent on the integrity of the mechanisms described above. Other physical characteristics also influence cough efficiency, including adequate airway calibre (efficiency decreased in the presence of flow limitation,29 e.g. severe malacia), mucus properties (sputum tenacity, adhesiveness, water content etc.)7 and respiratory muscle strength.7 When the

DEVELOPMENTAL ASPECTS OF COUGH

The central pathway for cough is a brainstem reflex, linked to control of breathing28 which undergoes a maturation process so that the reference values for normal respiratory rate in children are different to adults.33 In early life, cough is related to primitive reflexes (laryngeal chemoreflex) that undergo maturation resulting in significant differences in swallowing between young children and adults.34 Plasticity (modulation) of the cough reflex has been shown in animals,16, 28 although it

COUGH PHYSIOLOGY IN PATHOLOGICAL STATES

Clinical states reflecting pathophysiology of the cough pathway can be divided into: (a) increased (in response to a trigger, e.g. a respiratory infection) or ‘excessive’ (irritating cough with little physiological value) cough; and (b) decreased cough (Table 1). In conditions related to increased cough, triggers often involve several components of the cough pathway, e.g. tobacco smoke can cause cough through its influence on cough epithelium (ciliary, globet cells etc.) but also through the

CLINICAL IMPLICATIONS

Knowledge of the physiology of cough is clinically relevant. For example, in conditions where cough is inefficient, recognition of the likelihood of poor mucociliary clearance may prompt the use of other mucociliary clearance techniques. Based on the knowledge that the inspiratory phase of cough is important for cough efficiency, air stacking or mechanical insufflation (to increase lung volume prior to the compressive phase) has been used in patients with muscle weakness to improve cough

PRACTICE POINTS

  • Cough is an important component for lung health maintenance.

  • Cough efficiency is dependent on airway characteristics and integrity of the neurophysiology of the cough pathway.

  • Anti-tussive mediations may be counter productive.

  • In a coughing illness such as an acute respiratory infection, various mechanisms account for up-regulation of the cough reflex.

RESEARCH DIRECTIONS

  • Developmental aspects of cough eg does the plasticity of the cough reflex alter with age?

  • Clinical studies on methods to improve efficiency in children.

  • Mechanisms of down and up regulation of the cough reflex in children.

ACKNOWLEDGMENTS

A.B. Chang is funded by a Practitioner Fellowship from the National Health and Medical Research Council, Australia and by the Royal Children's Hospital Foundation, Brisbane. Dr. McElrea and Dr. van Asperen's helpful comments on this manuscript are acknowledged and appreciated.

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