Diffusion Defect

Overview
  • Diffusion Defects refer to deficiencies in oxygen pulmonary gas exchange that yield abnormally low partial pressures of arterial oxygen. As described in oxygen pulmonary gas exchange, diffusion of oxygen from the alveolar space to the pulmonary capillaries in a healthy lung is normally perfusion-limited. This means that the partial pressure of alveolar oxygen largely equilibrates with the blood in the pulmonary capillaries by the time the ends of the capillaries are reached. In scenarios of defective diffusion, the rate of oxygen diffusion from the alveolar space to the blood in the pulmonary capillaries is slowed such that equilibration of the oxygen partial pressure does not occur. Consequently, oxygen diffusion in the context of a diffusion defect is diffusion-limited. Diffusion-limitation of oxygen may reach such an extent that significant declines in partial pressures of arterial oxygen may result, yielding hypoxemia.
Pathogenesis
  • The rate of oxygen diffusion across the alveolar membrane is determined by Fick's Law. Fick's Law states that the rate of gas diffusion across a permeable membrane is determined by the partial pressure gradient of the gas across the membrane, the thickness and surface area of the membrane, as well as certain chemical features of the gas and membrane. Theoretically, changes to any of these variables can occur in such a way as to reduce the rate of oxygen diffusion across the alveolar membrane enough to yield a diffusion defect. In practice, however, oxygen diffusion defects typically arise from increases in alveolar membrane thickness and/or reductions in total alveolar surface area
  • These changes can typically be measured directly as a significant increase in the diffusing capacity of the lung.
  • An important feature of diffusion defects is that they rarely, if ever, result in derangements of carbon dioxide gas exchange and thus do not affect the arterial partial pressure of carbon dioxide. This arises from the significantly greater diffusibility of carbon dioxide across the alveolar membrane compared to oxygen. Consequently, while partial pressures of arterial oxygen may be abnormally low in a lung with a diffusion defect, partial pressures of carbon dioxide are typically normal.
Diagnosis
  • A-a Gradient
    • Because partial pressures of arterial carbon dioxide are largely normal in scenarios of diffusion defects, the "Alveolar Gas Equation" described in alveolar oxygen calculates a largely normal theoretical value for the partial pressure of alveolar oxygen. However, as described above, diffusion defects lead to abnormal declines in the partial pressure of arterial oxygen. Consequently, diffusion defects are characterized by a widening of the A-a Gradient.
  • Response to Oxygen Therapy
    • As described above, Fick's Law states that diffusion of oxygen across the alveolar membrane is proportional to the partial pressure gradient of oxygen across the membrane. Oxygen Therapy yields a much higher partial pressure of inhaled oxygen and thus significantly enhances the partial pressure gradient of oxygen across the alveolar membrane. The consequent boost in the rate of oxygen diffusion across the alveolar membrane will in many cases correct any losses due to the reduce diffusing capacity of a lung suffering from a diffusion defect. Consequently, oxygen therapy will improve a hypoxemia caused by a diffusion defect.
Etiologies