MINI-SYMPOSIUM: COUGHThe physiology 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
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Cough is an important component for lung health maintenance.
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Cough efficiency is dependent on airway characteristics and integrity of the neurophysiology of the cough pathway.
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Anti-tussive mediations may be counter productive.
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In a coughing illness such as an acute respiratory infection, various mechanisms account for up-regulation of the cough reflex.
RESEARCH DIRECTIONS
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Developmental aspects of cough eg does the plasticity of the cough reflex alter with age?
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Clinical studies on methods to improve efficiency in children.
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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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Airway Clearance in Tracheomalacia
2021, Seminars in Pediatric SurgeryCitation Excerpt :Features of airway clearance are shared across species and are impacted by multiple factors including mechanical and anatomic conditions as well as cellular and epithelial factors1. The physiology of airway clearance involves active components requiring conscious effort, such as forceful coughing, passive triggers built into brainstem function, as well as “passive active” components requiring no conscious effort2,3. Effective airway clearance is, therefore, under both conscious and unconscious control, and begins immediately at birth when the newborn must cough and clear any residual amniotic fluid in the tracheobronchial tree during the first cycles of respiration4,5.