In assessing lung function, what does a decreased FEV1/FVC ratio typically indicate?

A decreased FEV1/FVC ratio is indicative of obstructive lung disease. This measurement reflects the proportion of a person's forced expiratory volume in one second (FEV1) to their forced vital capacity (FVC). In obstructive lung diseases, such as asthma or chronic obstructive pulmonary disease (COPD), the airway is narrowed, leading to a reduction in the amount of air that can be forcibly exhaled in one second compared to the total volume of air that the patient can exhale forcefully. As a result, the FEV1 is significantly decreased while the FVC may be preserved or only slightly reduced, leading to a reduced FEV1/FVC ratio.

The significance of this ratio lies in its ability to differentiate between different types of lung pathologies. In restrictive lung disease, for instance, both FEV1 and FVC are reduced, often preserving the ratio or even showing an increased ratio because the reduction in FEV1 is proportionate to the reduction in FVC. Therefore, a decreased FEV1/FVC ratio specifically points to obstructive conditions rather than restrictive or mixed conditions.