What lung function parameter is primarily reduced in restrictive lung disease?

In restrictive lung disease, the primary lung function parameter that is reduced is Vital Capacity (VC). This reduction occurs because restrictive lung conditions lead to a decreased ability of the lungs to expand fully, resulting in a smaller overall lung volume. As a consequence, the total volume of air that can be forcibly exhaled after a deep breath—encapsulated by Vital Capacity—is diminished.

Ventilation mechanics are affected by the inability of the lungs or chest wall to expand adequately. Conditions contributing to this limitation can include pulmonary fibrosis, pleural disease, or scoliosis, among others, which all result in reduced lung volumes.

The FEV1/FVC ratio may remain normal or even increase because both the FEV1 (Forced Expiratory Volume in one second) and FVC (Forced Vital Capacity) decrease proportionately in restrictive disease. While Peak Expiratory Flow may also be reduced due to decreased effort or lung volume, it is not the primary parameter measured for the diagnosis of restrictive disease. Lastly, Residual Volume (RV) might actually be preserved or even increased in various restrictive pathologies; therefore, it does not directly indicate the restrictive process as clearly as Vital Capacity does.