Diffusing capacity: Difference between revisions

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Revision as of 11:49, 1 September 2007

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In [[biology]], '''diffusion capacity''' is a measurement of the [[lung]]'s ability to absorbtransfer andgases. excreteOxygen gases,uptake notably,may [[oxygen]]be andlimited [[carbonby dioxide]].diffusion Itin iscircumstances partlow ofambient aoxygen comprehensiveor testhigh seriespulmonary ofblood lungflow. function calledCarbon [[pulmonarydioxide functionis testing]]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 flowflux (J^*) across the alveolar-capillary membrane over the oxygen gradient between the alveoli ("A") and the veinsalveolar capillaries ("vc"):

:<math>D_L = \frac {VJ^*_{O2}}{P_{AO2} - P_{vO2cO2}}</math>

:<math>D_L = \frac {V^*_{O2}}{P_{AO2} - P_{vO2}}</math>

(The asterisk should be read as a dot over the letter, which is used to denote [[rate]] in [[respiratory physiology]].)

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QUESTION: What do the above variables represent, specifically?

BecauseHowever, because it can beis difficult to measure thesethe quantities,capillary inpartial [[Dlco]],pressure aof small quantity ofoxygen, [[carbon monoxide]] is(CO) sometimesis used, underas the assummptiontest thatgas allto measure the diffusing capacity (Dlco). Because CO willis bindso tightly bound to hemoglobin in the red blood cells, thusthe simplifyingpartial pressure of CO in the equation:capillaries is assumed to be zero. Thus,

:<math>D_LD_LCO = \frac {VJ^*_{O2CO}}{P_{ACO2}}</math>

==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, 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 be used. These include the steady state diffusing capacity which is performed during regular breathing, or the rebreathing method that requires rebreathing from a reservoir.

==Interpretation==

:<math>D_L = \frac {V^*_{O2}}{P_{ACO2}}</math>

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. Thus, some laboratories adjust or correct the DLCO to take into account the effect of anemia. 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.

==External links==