Normally about 100 ml of fluid is produced from the respiratory tract each day and most of it is swallowed. Respiratory mucus consists largely of water and its slimy character is due to glycoproteins cross-linked together by disulphide bonds. In pathological states much more mucus may be produced; an exudate of plasma proteins which bond with glycoproteins and form larger polymers results in the mucus becoming more viscous. Patients with chest diseases such as cystic fibrosis and bronchiectasis have difficulty in clearing their chest of viscous sputum by cough because the bronchial cilia are rendered ineffective. Drugs that liquefy mucus can provide benefit.
Carbocisteine and mecysteine have free sulphydryl groups that open disulphide bonds in mucus and reduce its viscosity. They are given by orally or by inhalation (or instillation) and may be useful chiefly where particularly viscous secretion is a problem (cystic fibrosis, care of tracheostomies). Mucolytics may cause gastrointestinal irritation and allergic reaction.
Water inhalation as an aerosol (breathing over a hot basin), though cheap, is not to be despised, and is good expectorant therapy in bronchiectasis. Simply hydrating a dehydrated patient can have a beneficial effect in lowering sputum viscosity.
Dornase alfa is phosphorylated glycosylated recombinant human deoxyribonuclease. It is given daily by inhalation of a nebulised solution containing 2500 units (2.5 mg). It is of modest value only in patients with cystic fibrosis, whose genetic defect in chloride transport causes particularly viscous sputum. The blocked airways, as well as the sputum itself, are a trap for pathogens and the lysis of invading neutrophils leads to substantial levels of free and very viscous DNA within the CF airways.
As always, it is necessary to have a clear Idea of the underlying problem before starting to any therapy For example, the approach to cough due to invasion of a bronchus by a neoplasm differs from that due to postnasal drip from chronic sinusitis or to that due to chronic bronchitis.The following are general recommendations:
• Simple suppression of useless cough
Codeine, pholcodine, dextromethorphan and methadone fetuses can be used in large, infrequent doses. In children, cough is nearly always useful and sedation at night is more effective to give rest than is codcine. A sedative antihistamine is convenient (e.g. promethazine), although sputum thickening may be a disadvantage. In pertussis infection (whooping cough), codeine and atropine methonitrate may be tried.
• To increase bronchial secretion slightly and to liquefy what is there
Water aerosol with or without menthol and benzoin inhalation, or menthol and eucalyptus inhalation may provide comfort harmlessly.
Carbocysteine or another mucolytic orally may occasionally be useful.
Preparations containing any drug having antimuscarinic action arc undesirable because it thickens bronchial secretion. Oxygen inhalation dries secretions, so rendering them even more viscous: oxygen muse be bubbled through water and patients having oxygen may need measures to liquefy sputum.
• Cough originating in the pharyngeal region
Glutinous sweets or lozenges (demulcents), incorporating a cough suppressant or not, as appropriate, are used.
These are said to encourage productive cough by increasing the volume of bronchial secretion; however there is little clinical evidence to support this, and they may be of no more value than placebo. The group includes squill, guaiphenesin, ipecacuanha, creosotes and volatile oils.
Every formulary is replete with combinations of antitussives, expectorants, mucolytics, broncho-dilators and sedatives. Although choice is not critical, a knowledge of the active ingredients is important, for some contain sedative anti-muscarinic antihistamines or phenypropanolamines (which may antagonise antihypertensives). Use of glycerol or syrup as a demulcent cough preparation, or of simple linctus (citric acid) is probably defensible. The rationale for compound linctus (dextromethorphan, pseudoephredrine, triprolid-ine) is dubious.
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