In diffusion capacity testing, which gas is used to estimate transfer from lung to blood?

In diffusion capacity testing, which gas is used to estimate transfer from lung to blood?

• A. Oxygen
• B. Hydrogen
• C. Nitrogen
• D. Carbon monoxide

Answer: (D)

Carbon monoxide is used in diffusion capacity testing to estimate transfer from lung to blood. CO binds rapidly and tightly to hemoglobin, so the amount that moves from the alveoli into the blood depends mainly on how easily gases diffuse across the alveolar–capillary membrane. Because the uptake is limited by diffusion rather than blood flow, CO serves as a sensitive tracer of the lung’s diffusing capacity (the surface area and thickness of the membrane). Using CO at low concentrations makes the test safe and provides a measurement that reflects the integrity of the alveolar-capillary barrier. Other gases don’t serve this purpose well: oxygen’s transfer involves more complex uptake dynamics, hydrogen isn’t taken up by blood in a way that reflects diffusion, and nitrogen isn’t absorbed to measure diffusion capacity.

Carbon monoxide is used in diffusion capacity testing to estimate transfer from lung to blood. CO binds rapidly and tightly to hemoglobin, so the amount that moves from the alveoli into the blood depends mainly on how easily gases diffuse across the alveolar–capillary membrane. Because the uptake is limited by diffusion rather than blood flow, CO serves as a sensitive tracer of the lung’s diffusing capacity (the surface area and thickness of the membrane). Using CO at low concentrations makes the test safe and provides a measurement that reflects the integrity of the alveolar-capillary barrier. Other gases don’t serve this purpose well: oxygen’s transfer involves more complex uptake dynamics, hydrogen isn’t taken up by blood in a way that reflects diffusion, and nitrogen isn’t absorbed to measure diffusion capacity.