Measuring Pulmonary Diffusion
While oxygen is normally perfusion limited, some diseases, particularly restrictive diseases can cause diffusion limitations. In order to measure alveolar diffusion, we use a diffusion limited gas such as carbon monoxide that creates a bottleneck at the membrane.
Here, DL is the combination of terms from Fick's Law. c = capillary and is assumed to be 0 for carbon monoxide. It is important not to smoke before this test so this is true.
A concentration of CO is inhaled and held for 10 s, then expired and PACO is measured to ascertain CO uptake.
Normal DLCO is 20-40 ml/min/mm Hg; abnormal is less than 80% of predicted. A change in DLCO will result from changes in [Hgb] and must be taken into consideration.
Oxygen must cross the alveolar membrane and bind with hemoglobin, and diffusion depends on the rates of both.
