What happens to the FEV1/FVC ratio in obstructive lung disease?

In obstructive lung disease, the FEV1/FVC ratio decreases. FEV1, or Forced Expiratory Volume in one second, measures the volume of air that can be forcefully exhaled in the first second of the expiration phase. FVC, or Forced Vital Capacity, is the total volume of air that can be exhaled after taking the deepest breath possible.

In obstructive lung conditions, such as asthma or chronic obstructive pulmonary disease (COPD), the airways are narrowed, leading to increased resistance to airflow. As a result, patients have difficulty expelling air quickly, which leads to a greater reduction in FEV1 compared to FVC. While both values may decrease, the FEV1 decreases more significantly, resulting in a lower FEV1/FVC ratio.

This decreased ratio is a hallmark feature of obstructive lung disease and is critical for differentiating it from restrictive lung diseases, where the FEV1 and FVC both decrease, but the ratio remains normal or may even increase.