The assessment of airflow obstruction: Role of measurements of airways resistance and of tests of forced expiration

Abstract

1. 1. The term ‘airflow obstruction’ is commonly used to describe two different aspects of airways disease: ( a) a rise in airways resistance during ordinary quiet tidal breathing and ( b) a reduction in maximum expiratory flow. In severe air-ways obstruction there is characteristically both a rise in airways resistance during quiet breathing and a fall in maximum expiratory flow. The structural and functional changes responsible for each of these two features of airflow obstruction are not identical and so differences can occur between the magnitudes of the increase in resistance and the reduction in maximum expiratory flow. Such differences are particularly obvious when the abnormality of the lung is not severe and therefore are of importance in the early stages of airways disease, in ageing and in studying the effects of drugs or irritants on normal lungs. 2. 2. Measurements of total airways resistance need careful standardization for lung volume and flow rate. They are greatly influenced by the dimensions of extrathoracic airways and of the larynx. They are sensitive to generalized air-way reactions (such as those dependent on vagal reflexes), but are probably less sensitive to early bronchial disease for this may be confined to parts of the bronchial tree which contribute little to the total airways resistance. 3. 3. Tests of maximum expiratory flow rate such as the FEV 1 directly reflect the effects both of changes in the elastic recoil of the lungs and of changes in intrapulmonary airways resistance. These tests are in most instances not influenced by the resistance of extrathoracic airways. They have particular advantages in the assessment of patients with severe intrapulmonary airways obstruction, since in such patients they are hardly dependent on the expiratory effort applied and are closely related to the events of tidal breathing. 4. 4. In the early stages of airways disease both tests of maximum expiratory flow and of airways resistance may be within normal limits. Maximum expiratory flow in the middle of the vital capacity is probably the most sensitive simple test of forced expiration for detecting the early stages of airways disease. The best tests for detecting the earliest stages of airway disease may be those which depend on regional abnormalities in the distribution of ventilation. 5. 5. Comparison of maximum inspiratory and maximum expiratory flow indicates the extent of expiratory dynamic compression in intrathoracic airways obstruction but provides little help in distinguishing asthma from emphysema. The absence of expiratory dynamic compression when airways resistance is raised suggests that the obstruction is extrathoracic. 6. 6. The elastic recoil pressure of the lung has a profound effect on the dimensions of the airways and on maximum expiratory flow.