Diffusing capacity: Difference between revisions

Revision as of 21:34, 21 February 2009

Revision as of 19:23, 4 May 2009

Line 4:

==Calculation==

The rate of flow is directly proportional to the oxygen gradient (per [[Fick's law of diffusion]]), and the diffusion capacity (DLO_O2) can be calculated as the ratio of oxygen flux (J^*) across the alveolar-capillary membrane over the oxygen [[partial pressure]] gradient between the alveoli ("A") and the alveolar capillaries ("ca")

:<math>D_L = \frac {J^*_{O2}}{P_{AO2} - P_{cO2aO2}}</math>

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

J^*_O2 is the uptake of oxygen (ml/min). P_AO2 is the alveolar partial pressure of oxygen. P_cO2aO2 is the [[partial pressure]] of oxygen in the alveolar capillary. Thus, the higher the diffusing capacity DL, the more gas will be transferred across the alveolar-capillary membrane for a given gradient in partial pressure (or concentration) of the gas.

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), in Europe this is called transfer factor (TLco). 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,