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Gas flow and mixing in the airways

Objective: To survey the current state of scientific knowledge of gas flow and mixing in pulmonary airways, especially at high frequencies. Data Sources: Results from the authors' own laboratory studies and Western bioengineering literature on respiratory fluid mechanics. Study Selection: This survey concentrates on understanding the principal physical mechanisms that underlie the enhancement of airway gas transport in high‐frequency oscillation. The results of experimental, computational, and mathematical studies are described. Data Synthesis: The topic was covered by seven presenters at the Münster Meeting on High Frequency Ventilation, January 31 to February 2, 1993. Six of these presentations are summarized in the six sections of this paper under the following headings: Introductory Survey; Three‐Dimensional Numerical Simulation of Inspiratory and Expiratory Flows in Small Airways; Computational and Experimental Models of High‐Frequency Oscillation; Unsteady Gas Mixing in Airways; Gas Dispersion From the Lagrangian Viewpoint; and Soluble Gas Mass Transfer in Tubes and Airways. Conclusions: The dominant mechanism for the enhancement of gas transport along airways at high frequency is likely to lie in the coupling of the secondary motions caused by airway curvature with the oscillatory longitudinal flow. However, laboratory experiments and theoretical analyses have led only to an accurate simulation of the phenomena in idealized geometries, not in real lungs. (Crit Care Med 1994; 22:S24‐S36)