In biology, diffusion capacity is a measurement of the lung's ability to transfer gases. Oxygen uptake may be limited by diffusion in circumstances low ambient oxygen or high pulmonary blood flow. Carbon dioxide is not limited by diffusion under most circumstances.
The "diffusing capacity" or "diffusion capacity" is part of a comprehensive test series of lung function called pulmonary function testing. In Europe, this is often called the "transfer factor".
Calculation
The rate of flow is directly proportional to the oxygen gradient (per Fick's law of diffusion), and the diffusion capacity (DL) can be calculated as the ratio of oxygen flux (J^*) across the alveolar-capillary membrane over the oxygen gradient between the alveoli ("A") and the alveolar capillaries ("c")
(The asterisk should be read as a dot over the letter, which is used to denote rate in respiratory physiology.)
QUESTION: What do the above variables represent, specifically?
However, because it is difficult to measure the capillary partial pressure of oxygen, carbon monoxide (CO) is used as the test gas to measure the diffusing capacity (Dlco). Because CO is so tightly bound to hemoglobin in the red blood cells, the partial pressure of CO in the capillaries is assumed to be zero. Thus,
Test Performance
In practice, the test is performed by having the test subject blow out all of the air that they can to reach residual volume. The person then takes a full vital capacity inhalation of a test gas mixture that contains a small amount of carbon monoxide (usually 0.3%) and some helium or other non-absorbed tracer gas. The test gas is held in the lung for about 10 seconds and then is exhaled from the lung. The first part of the expired gas is discarded and the next portion which represents gas from the alveoli is collected. By analyzing the concentrations of carbon monoxide and helium in the inspired gas and in the exhaled gas, it is possible to calculate how much carbon monoxide was taken up during the breath hold, and what the partial pressure of carbon monoxide was during the breath hold This method is known as the single-breath diffusing capacity test.
Other methods that are not so widely used at present can measure the diffusing capacity. These include the steady state diffusing capacity which is performed during regular breathing, or the rebreathing method that requires rebreathing from a reservoir of a gas mixtures.
Interpretation
The diffusing capacity (DLCO) is a test of the integrity of the alveolar-capillary surface area for gas transfer. It may be reduced in disorders that damage the alveolar walls (septa) such as emphysema. The DLCO is also reduced in disorders that thicken or damage the internal structure of the alveolar walls such as pulmonary fibrosis. Because the transfer of CO depends upon how much blood is traversing the lung, the DLCO can be reduced due to anemia. Some laboratories adjust or correct the DLCO to take into account the effect of anemia. Disorders of the pulmonary blood vessels such as thromboembolic disease or pulmonary hypertension also reduce the diffusing capacity. Cigarette smokers tend to have lower DLCO, which is partly due to damage to the lungs and partly due to the retention of carbon monoxide from the cigarettes. For these reasons, the DLCO is a sensitive test for disorders of the lung, but does not point toward a specific diagnosis.
External links
- Pulmonary+diffusing+capacity at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
- Template:GeorgiaPhysiology
 | This medical article is a stub. You can help Wikipedia by expanding it. |