Expiratory airway obstruction
Expiratory airway obstruction is a condition characterized by increased resistance to air flow leading to diminished maximum expiratory flows. It is most often assessed from the FEV1 and the FVC. An abnormally low value then falls below an (arbitrary) lower limit, commonly derived from the distribution of FEV1 and FVC in a ‘normal’, asymptomatic population of non-smokers. The lower limit of FEV1, for example, may be selected so that a smaller value occurs in less 5% of the subjects in this ‘normal’ population. Airway obstruction may also lead to a diminished FVC, but this does not occur in all cases.
The larger the lung, the larger airway diameter ann hence the larger the volume that can be displaced during a forced expiration in one second. On that account it is useful to relate FEV1 to lung volume. This allows assessing whether a low FEV1 is due to a small lung or to obstruction to air flow. Lung volume (i.e. including residual volume) cannot be measured by standard spirometric tests; that is why the FVC or IVC is used as a substitute for lung volume to correct for volume dependence. The most popular index of airway obstruction is the FEV1/FVC ratio (also called forced expiratory ratio, FER). If obstructive lung disease causes the vital capacity to fall, the FVC is more affected than the IVC. In such cases, therefore, the FEV1/FVC ratio (=FER) is higher than the Tiffeneau index (FEV1/IVC).