Pulmonary penetration and deposition of aerosols in different gases: Fluid flow effects

Steady state mouth breathing deposition experiments were carried out in normal human subjects in two different gases, air and 80% He-20% O₂ using monodisperse aerosols of diameter 1.7 μm. The overall deposition of particles, occurring primarily in the pulmonary spaces, was found in every case to be greater for the He-O₂ gas compared with air at the same breathing conditions. Since intrinsic particle transport is equal at this diameter for the two gas mixtures, differences must be due to fluid flow effects. On the basis of its lower Reynolds number (Re_air/Re_He-O2 = 3.80), the He-O₂ gas is believed to have a more rapid profile development in the airways which carries the aerosol front deeper into the pulmonary spaces. This produces a larger interface between tidal and reserve air leading to greater deposition. Differences in mechanical mixing at the bronchial or alveolar level do not explain the observed differences in these experiments, since they would predict the opposite effect.