Differentiating between airway obstruction and lung restriction
Airway obstruction is a condition characterized by impediment to inspiratory and/or expiratory airflow. What is a normal flow depends on lung volume: smaller lungs generate smaller flows. A low FEV1/FVC ratio (forced expiratory ratio, FER)or FEV1/IVC ratio (Tiffeneau index), indices in which time-averaged expiratory flow has been more or less normalized for lung size, is therefore regarded as a hallmark of airway obstruction.
Restriction is a condition where the maximum achievable lung volume has diminished. By definition this implies that the total lung capacity is too small. This may be due to various underlying processes, such as intrapulmonary disease (for example pulmonary fibrosis) or extrapulmonary abnormalities (such as Bechterew’s disease which limits the excursions of the chest due increased stiffness of the thoracic cage).