Effect of airway dynamics on gas transport and particle deposition in the human respiratory tract

Theoretical deposition models of inhaled particles in the human lung developed in the past have always assumed that the airways expand uniformly. This is a questionable assumption since different parts of the lung have different airway compliances and resistances and as a result different ventilations. In this study, mathematical models of ventilation distribution and particle deposition were developed for a five-lobe lung based upon airway dynamics and aerosol mechanics. The calculated ventilation and deposition results from the models compared favorably with existing experimental data. Comparisons were also made with the results from an earlier model based on an approximated lung geometry.; Ventilation results from the model show that different lobes receive different ventilation per unit lung volume and the distribution depends on gravity and flow rate. At normal or high gravity, the ventilation per unit lung volume was found to be higher in the lower lobes than in the upper lobes due to the larger airway compliance of the lower lobes. Airway resistance was found to play a meaning role in ventilation distribution only at very high flow rates. Because the lower lobes have a larger airway resistance, increasing flow rate would reduce ventilation to the lower lobes thus resulting in a more uniform ventilation distribution to all lobes. A ventilation reversal can occur if the flow rate is extremely high. When the gravity is absent, only airway resistance controls ventilation distribution and in this case a more uniform distribution was found at low flow rates.; Ventilation distribution was also found to be affected by the properties of the inspired gas. Ventilation results were calculated from the model for the SF₆-O₂ and He-O₂ mixtures and compared with the air. There is a preferential distribution of ventilation to the upper lobes if the density of the inspired gas is greater than the air.; The ventilation results was incorporated into a particle deposition model to examine the effect of ventilation distribution on deposition fractions in the five lobes. The model accounts for the nonuniformity of ventilation distribution while an earlier lobe deposition model provides only the uniform ventilation results. A specific lobe deposition fraction (deposition fraction per unit lung volume in a lobe) was introduced to evaluate the differences in lobe deposition for different particle sizes, flow rates, as well as gravity conditions. The two upper lobes were found to have smaller specific deposition fractions than the middle and two lower lobes for all particle sizes. The ratio of the specific lobe deposition fraction to that obtained from the uniform ventilation model was greater than 1 for the middle and two lower lobes and less than 1 for the upper lobes. This nonuniformity became greater at a higher gravity. However, at high flow rates, the specific deposition ratios were found to move closer to the uniform ventilation results.; For hygroscopic particles such as NaCl, there is a lesser deposition of small particles and more deposition of the large particles in all lobes than the nonhygroscopic counterparts because of particle growth. The nonuniform ventilation effect was found to further enhance this result, especially for large particles.