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Diffusing Capacity

  • As described in our discussion of Fick's Law, the diffusion rate of a gas across the alveolar membrane depends on the surface area, thickness, and unique chemical relationship between the gas and membrane. Because these variables are impossible to measure directly, the concept of "Diffusing Capacity" has been developed as an empirically-measurable factor that combines the contribution of these variables. Consequently, the empirically-determined Diffusing Capacity (DL) is used to replace the D, A, and T terms of Fick's Law.
  • The Diffusing Capacity (DL) is designed to replace the contributions of the D, A, and T variables in Fick's Law; therefore, DL = D * A/T.
  • Replacing, the DL into Fick's Law yields the following equation.
    • V'gas = DL * ΔP
  • Rearranging this equation allows for determination of the DL using the experimentally measurable values for V'gas and ΔP of the gas across the pulmonary membrane, as follows:
    • DL = V'gas/(Palveolar - Pcapillary)
  • In reality, the diffusing capacity of the lung for a particular gas must be empirically determined for each, chemically-unique gas. In order to determine the DL for any particular gas the partial pressure gradient for the gas across the pulmonary membrane must be determined (ΔP) as well as its rate of transport (V'gas). Sadly, this is experimentally challenging to accomplish for the physiologically-important gases of oxygen and carbon dioxide in part because the partial pressure gradients for these gases changes as blood is transported through the pulmonary capillaries.
  • However, because Carbon Monoxide is rapidly bound by hemoglobin as it travels through the pulmonary capillary, its partial pressure in the pulmonary capillary blood remains negligible across the entire alveolar membrane and can largely be ignored. Consequently, the diffusing capacity for carbon monoxide can be determined simply by measuring the rate of carbon monoxide absorption by the lung (V'CO) as well as its partial pressure in the alveolar space PACO. Given certain controlled experimental conditions these values can be empirically determined, thus allowing for derivation of the lung's DLCO.
  • In reality, the DLCO variable is of little direct physiological significance since carbon monoxide is not a physiologically-relevant gas. However, pulmonary pathologies which affect the surface area, thickness, and chemical nature of the alveolar membrane, and thus change the value of DLCO, tend to produce proportional changes in the pulmonary diffusing capacity of oxygen and carbon dioxide. Because of this, measurement of the DLCO can be helpful in evaluating pulmonary pathologies which may affect the diffusing capacity of the physiologically-relevant gases, oxygen and carbon dioxide.