Influence of bronchial asymmetry on cardiogenic gas mixing in the lung.

To examine whether diffusive interaction in an asymmetrical bronchial tree contributes to cardiogenic mixing, we simulated the changes in nitrogen concentration (FN2) in an asymmetrical two-trumpet model of the lung based on canine anatomy during an inspiration of 100% oxygen and a subsequent breathhold. Results were obtained by solving a second-order differential gas-transport equation for simultaneous convection and diffusion with the assumption that velocity profiles were blunt. Cardiogenic flow pulses were simulated by periodic flow oscillations superimposed on constant respiratory flows. Both during inspiration and breath-holding the simulated pattern of FN2 within conducting airways was qualitatively similar to that observed in open-chested dogs. Despite the assumption of blunt velocity profiles, the apparent enhancement of gas mixing by oscillatory flows was comparable to that attributed to cardiogenic mixing in experimental studies. The results suggest that cardiogenic flow pulses in an asymmetrical bronchial tree may enhance gas mixing within the conducting airways by causing phasic displacements of diffusion fronts toward branchpoints subtending lung units of unequal volume. Diffusion of gas from the smaller units accelerates the decrease in inspired gas concentration within conducting airways.