Diffusing Capacity - Test Performance

Test Performance

The single-breath diffusing capacity test is the most common way to determine . The test is performed by having the subject blow out all of the air that he/she can, leaving only the residual lung volume of gas. The person then inhales a test gas mixture rapidly and completely, reaching the total lung capacity as nearly as possible. This test gas mixture contains a small amount of carbon monoxide (usually 0.3%) and a tracer gas that is freely distributed throughout the alveolar space but which doesn't cross the alveolar-capillary membrane. Helium and methane are two such gasses. The test gas is held in the lung for about 10 seconds during which time the CO (but not the tracer gas) continuously moves from the alveoli into the blood. Then the subject exhales.

The anatomy of the airways brings with it complications, since the inspired air must pass through the mouth, trachea, bronchi and bronchioles before it gets to the alveoli where gas exchange will occur; on exhalation, alveolar gas must return along the same path, and so the exhaled sample will be purely alveolar only after a 500 to 1,000 ml of gas has left the subject. While it is algebraically possible to approximate the effects of anatomy (the three-equation method), disease states introduce considerable uncertainty to this approach. Instead, the first 500 to 1,000 ml of the expired gas is disregarded and the next portion which contain gas that has been in the alveoli is analyzed. By analyzing the concentrations of carbon monoxide and inert gas in the inspired gas and in the exhaled gas, it is possible to calculate according to Equation 2. First, the rate at which CO is taken up by the lung is calculated according to:

.

( 4 )

The pulmonary function equipment monitors the change in the concentration of CO that occurred during the breath hold, and also records the time .

The volume of the alveoli, is determined by the degree to which the tracer gas has been diluted by inhaling it into the lung.

Similarly,

.

( 5 )

where

is the initial alveolar fractional CO concentration, as calculated by the dilution of the tracer gas.

is the barometric pressure

Other methods that are not so widely used at present can measure the diffusing capacity. These include the steady state diffusing capacity that is performed during regular tidal breathing, or the rebreathing method that requires rebreathing from a reservoir of gas mixtures.