A decreased forced expiratory volume in 1 second (FEV1) primarily reflects difficulty in expelling air rapidly due to airway obstruction. In obstructive lung diseases such as asthma, chronic bronchitis, and COPD, airway narrowing leads to increased resistance during expiration, thus reducing FEV1 significantly. This contrasts with restrictive lung diseases where FEV1 and forced vital capacity (FVC) both decrease proportionally, often maintaining a normal or high FEV1/FVC ratio. Therefore, a decreased FEV1 is most indicative of obstructive pathology impairing airflow.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 20th Edition.