Cough is initiated when irritant receptors in the mucousmembrane of the respiratory tract are stimulated. Coughis by far the most common respiratory symptom, andis characteristic in heavy smokers. Frequently, cough istriggered by the presence of sputum in the respiratorytract, and is useful in helping to clear infection from thebronchial tree. A wide variety of inhaled irritants in additionto cigarette smoke (e.g. noxious gases or cold air) maystimulate coughing, and this is more likely if the airwaysare already irritable because of inflammation as a consequenceof infection.
Similarly, the irritant receptors in thebronchial tree may be stimulated by tumours, inhaledforeign bodies, allergens and the asthmatic response,pulmonary oedema and external compression by lymphnodes. In non-smokers the most frequent causes of chroniccough are asthma, sinus disease and oesophageal reflux.With neurological disease laryngeal function may beimpaired or oesophageal motility abnormal (e.g. achalasia),and cough may be due to repeated aspiration.
A characteristic persistent dry cough canoccur with ACE inhibitors. Cough after drinking canalso indicate an oesophagobronchial fistula. In somepatients cough is worse at night, particularly in asthma orpulmonary oedema. Prolonged coughing reduces venousreturn, causes a transient fall in cardiac output and cerebraloxygenation, and leads to cough syncope. Damage tothe recurrent laryngeal nerve, commonly at the left hilumdue to bronchial carcinoma, leads to vocal cord paralysisand an inability to produce a normal explosive cough,which becomes ‘bovine’.
A dry cough, sometimes following an upper respiratorytract infection and often persisting for weeks or months,for which no cause can be found, is a common clinical problem.
In healthy subjects the bronchial tree produces approximately100 mL of mucus each day; this is carried upwards by ciliary action and is then unconsciously swallowed. This ‘escalator’ is a normal part of the mechanism for clearingdebris and pathogens from the bronchial tree. Indisease processes causing the production of excess mucus,irritant receptors are stimulated and sputum is coughedup.
Sputum is not described reliably by patients and it isalways best to inspect it. Sputum may be clear, white ormucoid, as in chronic bronchitis, or purulent, in which casepus is mixed with mucus and the sputum is yellow or green.Sputum may contain blood, which may be bright red (e.g.pulmonary infarction), a rusty colour (acute pneumonia)or pink (pulmonary oedema due to left heart failure). Inasthma the sputum may contain mucus plugs.
Microscopically,sputum may contain bacteria, pus cells, eosinophils(as in asthma and pulmonary eosinophilia) or malignantcells. It is helpful to know the volume of sputum producedeach day, and this can be particularly large: greater than20 mL in bronchiectasis, cystic fibrosis, and lung abscesswhen there is a bronchopulmonary fistula. Clinicalprogress can be monitored by documentation of sputumvolume. Occasionally patients with alveolar cell carcinomaproduce very large volumes of clear watery sputum (bronchorrhoea). Anaerobic infection results in foul smelling sputum.