In restrictive pulmonary disorders, how is total lung capacity (TLC) typically affected?

• A. Increased compared to predicted value
• B. Normal compared to predicted value
• C. Decreased compared to predicted value
• D. Varies widely among patients

Answer: (C)

In restrictive pulmonary disorders, total lung capacity (TLC) is typically decreased compared to the predicted value. This decrease occurs because restrictive diseases, which include conditions such as pulmonary fibrosis, sarcoidosis, and obesity hypoventilation syndrome, restrict the expansion of the lungs. The stiffness of lung tissue or the inability to expand the thoracic cavity limits the amount of air the lungs can hold, resulting in a reduced TLC.

In restrictive disorders, although the airways are not obstructed, the volume of air that can be inhaled is significantly less than normal, indicating that the lungs cannot fill to their full capacity. As a result, when measured, the TLC is often found to be lower than the predicted values based on individual characteristics such as age, sex, height, and ethnicity. Understanding this concept is crucial for evaluating and managing patients with restrictive lung disease and interpreting their pulmonary function tests accurately.