Carbonmonoxide diffusing capacity of lung | Respiratory physiology

The rate of diffusion of a gas across any membrane is given by Fick’s law which states that the diffusion rate is directly proportional to partial pressure gradient for the gas, directly proportional to the area of the membrane, inversely proportional to membrane thickness, and directly proportional to solubility of the gas and inversely proportional to square root of its molecular weight. If we combine the properties of lung and properties of gases together.. we call it diffusing capacity of lungs for that gas. Physiologically for the uptake of the gases from the alveoli, 2 things should happen: 1. The gases should cross the respiratory membrane 2. They should cross the plasma, enter into RBC and react with Hb (diffusion capacity of blood) When we determine diffusion capacity of lungs using a gas, the obtained value represents both the diffusion capacity of membrane and the diffusion capacity of blood so basically DL represents the entire diffusion distance which needs to be covered by a gas physiologically. Thats why its also known as transfer factor. Measurement of carbonmonoxide diffusion capacity This diffusion capacity is measured using a gas whose rate of diffusion is diffusion limited i.e carbonmonoxide in this case. To measure diffusion capacity of lungs for CO, we need to know rate of diffusion of gas and its partial pressure gradient. For this, single breath technique is used. In this the person first exhales maximally and then inhales maximally a dilute mixture of carbonmonoxide containing approximately 0.3% of CO which is diluted with helium, holds the breath for 10 sec to allow for diffusion of gas and then exhales. Then by determining the difference between the amount of gas inhaled and amount of gas exhaled, and the lung volume by helium dilution method, we can know the amount of gas which has diffused and also the alveolar partial pressure of carbonmonoxide. Partial prssure of carbonmonoxide in blood is zero. We then put those values in the formula and get the diffusion capacity or transfer factor.. Diffusion capacity of lungs for carbonmonoxide measured by this method comes to 25 ml/min/mmHg partial pressure difference. This value increases 2-3 times in exercise due to capillary recruitment and capillary distension. Decrease in CO diffusion capacity of lungs 1. Thickening of respiratory membrane as in pulmonary fibrosis, 2. Decrease in cross-sectional area of respiratory membrane as in emphysema…. 3. Since it also depends on diffusion capacity of blood i.e on blood volume and hemoglobin.. so if these are less..then also diffusion capacity will be less.