FEV1 is a common index used in pulmonary function measurements. It is usually expressed in two ways. 1. by comparing the measured FEV1 with the FVC, which is called the one-second rate, or FEV1%. 2. FEV1 is compared to its own predicted value, i.e. FEV1 as a % of the predicted value. How should the above two indicators be applied clinically? It is generally believed that in the early stage of obstructive ventilation disorder, FEV1 decreases first, while FVC does not, at this time, the application of FEV1/FVC can reflect the early airflow obstruction more sensitively. In the more severe stage of airway obstruction, not only FEV1 decreases, but also FVC, which shows that FEV1/FVC may not decrease with the increase of airway obstruction. In contrast, the ratio of FEV1 to its expected value decreases with increasing airflow obstruction. Therefore, the ratio of FEV1 to its expected value is a reliable indicator of the severity of airflow obstruction, while FEV1/FVC is a sensitive indicator of obstructive ventilation dysfunction, but in severe obstructive ventilation dysfunction, because not only FEV1 decreases but also FVC, it may appear that the ratio of FEV1/FVC is normal instead, but then FEV1 to expected value must be significantly lower.