Which lung volume measurement method is least affected by obstructive lung disease?

The body box measurement, also known as plethysmography, is the lung volume measurement method that is least affected by obstructive lung disease. This technique measures the total lung capacity and can distinguish between lung volumes more effectively than other methods, especially in patients with obstructive conditions, such as asthma or COPD.

The reason body box measurement is reliable in the presence of obstructive lung disease lies in its principle of measuring pressure changes within a sealed chamber as the patient breathes. It captures total lung capacity and can assess any changes in lung volume resulting from air trapping, which is commonly seen in obstructive diseases.

In contrast, methods like helium dilution and single-breath nitrogen washout can be influenced by lung perfusion and the ability to fully exhale, which may be compromised in obstructive lung disease. These methods primarily rely on diffusion and may not capture the true lung volume changes occurring in patients with airway obstruction. Water displacement, while a classic method, is often less practical and less accurate in adult patients compared to the body box measurement.

Thus, due to its ability to accurately measure lung volumes by accounting for trapping and static lung mechanics, the body box measurement stands out as the most effective technique in the context of obstructive lung disease.