Diffusing Capacity of the Lung for Carbon Monoxide - DLCO

In an effort to explain a little about the tests we perform in our lab I have previously discussed Spirometry, Bronchial Provocation and MIPS/MEPS and thought a blog about DLCO may be interesting.
The primary function of the lungs is to exchange gases. The two processes involved are the exchange of gas between the blood and the alveoli and the ventilation of the alveoli with air.
Carbon dioxide (CO2) is very soluble in water and moves rapidly across the alveolar membranes so that the pressure of CO2 in the alveoli is virtually identical to that in the blood. The rate-limiting step in expiring CO2 is the ventilation of the lung with atmospheric air (which contains virtually no CO2).
For oxygen, which is poorly soluble in water, the diffusion across the alveolar membrane is much slower and so oxygen uptake is more sensitive to alveolar disease than is CO2 removal. Although ventilation is also important for replenishing alveolar O2 supplies, the rate-limiting step in O2 exchange is usually exchange across the alveolar membrane.

The most popular method of measuring the diffusing capacity (DCO) is the single breath method whereby the patient takes a vital capacity breath of a mix of 0.3% Carbon monoxide, O2 21% in N2 and a tracer gas delivered via the automated valve from the spirometer followed by a 10 second breath hold then exhales. The first 750ml of dead space contaminated gas or washout is discarded and the next liter collected and analyzed in the alveolar sampling device. During exhalation the concentration of both Carbon monoxide (CO) and tracer gas fall with dead space washout. The tracer gas shows a plateau as alveolar gas is exhaled. CO will show a similar pattern but with a lower concentration because of diffusion during the breath hold.

The inert tracer gas we use in our lab, Methane (CH4) measures dilution of the inhaled CO and estimates the alveolar volume (VA). It distinguishes between the dilution of CO with air in the lungs and the uptake of CO into the blood. The change in CH4 concentration reflects the dilution of the gas remaining in the lungs (i.e. residual volume). This change is used to determine the initial CO concentration before diffusion and to determine the lung volume at which the breath hold occurred. The Krogh constant (KCO or DLCO/VA is an attempt to adjust the DLCO for differences in lung volume. It follows that when a KCO does not correct to normal there is likely to be an abnormality of the lung tissue.

Carbon monoxide (CO) is used to measure diffusing capacity (DCO). CO has similar physical properties to oxygen in terms of it solubility and ability to diffuse across the alveolar membrane into the plasma. The CO effectively follows the same path as oxygen, even combining and competing for the same binding sites on the Hb molecule as oxygen to form carboxyhaemoglobin (COHb) Reported in milliliters of CO/minute/mm of Hg, dry i.e. STPD. The affinity of CO for hemoglobin is approximately 210 times that of oxygen. The CO transferred across the alveolar wall is retained within the circulation and not exhaled.

The uptake of CO is determined by two things.
The diffusion properties of the blood-gas barrier
The rate of combination of CO with blood
The diffusion properties of the alveolar membrane depend on its thickness and area. Thus, the diffusing capacity is reduced by diseases in which the thickness is increased, including diffuse interstitial fibrosis, sarcoidosis and asbestosis and in a patient who has had a pneumonectomy where the surface area is reduced.

The rate of combination of CO with blood is reduced when the number of red cells in the capillaries is reduced as in anaemia and where the capillary blood volume is reduced in pulmonary embolism. At our lab we correct the DLCO for haemoglobin.

We know that the diseased lung tends to empty unevenly therefore the expired liter of gas that is analyzed for CO is probably not representative of the whole lung. For this reason the diffusing capacity is sometimes referred to as the transfer factor (TLCO) to emphasize that it is more a measure of the lung’s overall ability to transfer gas into the blood than a specific test of diffusion characteristics. In spite of this, the test has a definite place in the pulmonary function laboratory and is frequently useful in assessing the severity and type of lung disease.

Vanessa