Which gas is used to measure diffusion capacity of the lung?

• A. O2
• B. N2O
• C. CO
• D. CO2

Answer: (C)

The gas used to measure the diffusion capacity of the lung is carbon monoxide (CO). This measurement is often referred to as the diffusing capacity of the lungs for carbon monoxide (DLCO). The reason CO is utilized for this test lies in its unique characteristics: it binds to hemoglobin in red blood cells with a significantly higher affinity than oxygen, which allows for accurate measurement of how well gases can transfer from the alveoli into the bloodstream.

During the test, a small amount of CO is inhaled, and the concentration of CO in the exhaled air is measured. The difference in concentration helps to determine how effectively the lungs facilitate the transfer of gases into the blood. This measurement is particularly important in assessing conditions that may impair gas exchange, such as pulmonary fibrosis, emphysema, and other restrictive or obstructive lung diseases.

The other gases mentioned do have their purposes in pulmonary function tests, but they are not used for this specific measure of diffusion capacity. Oxygen, for instance, is essential for respiration and the body’s metabolism, while nitrous oxide (N2O) is used primarily for assessing ventilation-perfusion ratios; meanwhile, carbon dioxide (CO2) plays a role in measuring other aspects of pulmonary function but does not bind to