What pulmonary function test result is typically decreased in intrapulmonary disorders?

• A. Vital capacity (VC)
• B. Diffusing capacity (DLCO)
• C. Inspiratory reserve volume (IRV)
• D. Expiratory reserve volume (ERV)

Answer: (B)

Intrapulmonary disorders, such as interstitial lung disease, pneumonia, or pulmonary fibrosis, typically lead to a decreased diffusing capacity (DLCO). The diffusing capacity test assesses how well oxygen and carbon dioxide can cross the alveolar-capillary membrane.

In intrapulmonary disorders, structural changes occur in the lung parenchyma, which can impair gas exchange. For instance, the thickening or destruction of the alveolar walls reduces the surface area available for gas diffusion, thereby lowering the DLCO values. This decrease indicates a compromised ability of the lungs to transfer gases effectively into the bloodstream.

While conditions may also affect other pulmonary function parameters like vital capacity, inspiratory reserve volume, and expiratory reserve volume, these changes are not as specifically indicative of intrapulmonary disorders as the diffusing capacity. A reduction in vital capacity can be seen in both restrictive and obstructive conditions, and inspiratory or expiratory reserve volumes can be affected by various other factors. Thus, the key change in gas exchange efficiency in intrapulmonary disorders specifically relates to a decline in DLCO.