Forced expiratory volume in the first second (FEV1) and its ratio to forced vital capacity (FEV1/FVC) are two clinically relevant pulmonary function measures. While both FEV1 and FVC are influenced by lung size and can be reduced by restrictive lung diseases, obstructive lung disease leads to proportionately greater reduction in FEV1 than FVC. Therefore, a reduced FEV1/FVC, an indicator of airflow obstruction that is independent of lung size, is the primary criterion for defining an obstructive ventilatory defect1. Whereas low FEV1/FVC indicates the presence of airflow obstruction, FEV1 is used to classify severity and follow the progression of obstructive lung disease over time5,7,8.
