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In patients with obstructive lung disease, the primary lung function value that is affected is the forced expiratory volume in 1 second (FEV1). Obstructive lung diseases, such as asthma and chronic obstructive pulmonary disease (COPD), are characterized by increased resistance to airflow due to inflammation, mucus production, or airway narrowing. As a result, these patients experience difficulty in expelling air from the lungs quickly, particularly in the first second of forced expiration, which is what FEV1 measures.

The FEV1 value is crucial in diagnosing and assessing the severity of obstructive lung disease. A decreased FEV1 results in a reduced ratio of FEV1 to forced vital capacity (FVC), often indicating obstruction. This contrasts with restrictive lung diseases, where the FEV1 may be decreased but the FEV1/FVC ratio typically remains normal or even increased.

In summary, the reason FEV1 is primarily affected is due to the nature of obstructive pathology that compromises airflow, leading to a significant decrease in the volume of air expelled in the first second of forced expiration. Understanding this relationship helps clinicians diagnose, classify, and manage obstructive lung diseases effectively.