A patient with obstructive lung disease, compared to a healthy patient, would have a decreased:

In patients with obstructive lung disease, the forced expiratory volume in one second (FEV1) is significantly impacted due to the narrowing and blockage of the airways, which hinders airflow when exhaling. This results in a lower measurement of FEV1 when compared to healthy individuals, as they can typically exhale more air within the first second of a forced breath. The reduction in FEV1 is a hallmark of obstructive conditions such as asthma, chronic obstructive pulmonary disease (COPD), and emphysema, and it serves as an important diagnostic criterion for evaluating the severity of these conditions.

The comparison to healthy patients also illustrates that while other lung volumes might be affected, they do not show the same direct and defining decrease such as FEV1. For example, functional residual capacity (FRC), inspiratory capacity (IC), and tidal volume (VT) can be altered in response to obstructive lung disease, but the defining feature and primary indicator of obstruction is the reduction in FEV1. Thus, FEV1 is the most critical measurement that illustrates the impact of obstructive lung disease on respiratory function.