Which gas is primarily used to measure the diffusing capacity of the lungs?

The diffusing capacity of the lungs is predominantly measured using carbon monoxide (CO) because of its unique properties that facilitate this specific assessment. Carbon monoxide is utilized due to its high affinity for hemoglobin, which allows it to effectively measure how well gases diffuse from the alveoli into the blood.

During the test, a small amount of carbon monoxide is inhaled, and the decrease in its concentration in the alveolar gas is measured. This decrease reflects how much carbon monoxide has moved across the alveolar-capillary membrane into the bloodstream, providing an indicator of the diffusing capacity.

Other gases listed, such as nitrogen and oxygen, do not have the same binding affinity to hemoglobin and are not as effective in distinguishing between impaired diffusion and other respiratory mechanics. Carbon dioxide is also not used for this measurement because its primary role is in the regulation of blood pH and respiratory drive, rather than assessing the diffusion properties of the lungs. Thus, carbon monoxide stands out as the ideal choice for this specific pulmonary function test.