The epidemiology of chronic respiratory disease.

Chronic respiratory disease (CRD) is usually understood to include chronic bronchitis, emphysema, asthma, and a number of other chronic bronchopulmonary diseases. This paper will concentrate on the first two. The epidemiology of asthma differs in many ways from that of chronic bronchitis and emphysema, for example, in its age and sex incidence, relationships to social class, allergy, and time trends. But many of the clinical features are common to all three diseases with the result that diagnosis may be difficult. Furthermore the extent to which asthma contributes to emphysema is as yet uncertain. While it would be desirable to consider the epidemiology of the components of CRD separately this is unfortunately not yet possible. A number of conferences and organizations (2,17,104) have suggested definitions and classifications of these diseases. The following definitions, essentially those of the American Thoracic Society, would probably be generally accepted. Chronic bronchitis is a clinical condition characterized by excessive mucous secretion in the bronchi. It is manifested by chronic or recurrent productive cough not attributable to other lung or heart disease. Emphysema refers to an anatomical alteration of the lung characterized by an abnormal enlargement of the air spaces distal to the terminal, nonrespiratory bronchiole, usually accompanied by destructive changes in the alveolar walls. Asthma is a condition characterized by increased responsiveness of the trachea and bronchi to various stimuli. It is manifested by widespread narrowing of the airways which changes in severity either spontaneously or as a result of treatment. During the past 20 years epidemiologists have developed standardized methods for studying CRD (3,35,36,65). Precisely worded questions about respiratory symptoms and chest illnesses have been formulated, simple tests of ventilatory lung function, notably the forced expiratory volume (FEV) and forced vital capacity (FVC), have been widely applied, various objective methods, such as measurement of the volume and quality of the morning sputum during the first hour or half hour after rising, have been recommended (93). Bronchial reactivity has occasionally been measured. More sophisticated testing of lung function and chest radiography have been included in some surveys. The result is a considerable body of knowledge on chronic bronchitis but rather less on emphysema and asthma.