Molecular Diffusion

The principal means of gas movement in the alveolar zone is molecular diffusion. In this zone, gas velocities approximate zero as a result of the very high total crosssectional area, with net gas transport best described by Fick’s law. The spontaneous gas particle mixing arising from Brownian motion is likely augmented by the increased turbulence of the gas molecules during HFOV [10].

Asymmetric Velocity Profiles

The gas velocity profile inside large- and middle-size airways is parabolic. The more central particles are propelled faster down the length of the airway, while the peripheral particles diffuse radially, promoting axial gas exchange with expired alveolar gas. This phenomenon is particularly evident at the airway bifurcations. The airway bifurcation phenomenon directs fresh, machine-delivered gas to the alveoli along a cone in the airway center, while exhaled gas moves out of the system along the outer airway wall [11].

Taylor Dispersion

Taylor dispersion is an effect in fluid mechanics in which a shear flow can increase the effective diffusivity of a fluid or gas mixture. In a shearing flow, adjacent layers of fluid move parallel to each other with different speeds. The shear acts to smear out the concentration distribution in the direction of the flow, enhancing the rate at which it spreads in that direction. A semiempiric analysis predicts that the combination of Taylor dispersion and molecular diffusion (augmented dispersion) accounts for almost all gas transport during HFOV [10].

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