Cardiogenic mixing increases aerosol deposition in the human lung in the absence of gravity.

RATIONALE: Exposure to extraterrestrial dusts is an almost inevitable consequence of any proposed planetary exploration. Previous studies in humans showed reduced deposition in low-gravity compared with normal gravity (1G). However, the reduced sedimentation means that fewer particles deposit in the airways, increasing the number of particles transported to the lung periphery where they eventually deposit albeit at a smaller rate than in 1G. In this study, we determined the role that gravity and other mechanisms such as cardiogenic mixing play in peripheral lung deposition during breath holds.
METHODS: Eight healthy subjects inhaled boluses of 0.5 μm-diameter particles to penetration volumes (Vp) of 300 and 1200ml that were followed by breath holds of up to 10 sec. Tests were performed in 1G and during short periods of microgravity (μG) aboard the NASA Microgravity Research Aircraft. Aerosol deposition and dispersion were calculated from these data.
RESULTS: Results show that, for both Vp, deposition in 1G was significantly higher than in μG. In contrast, while dispersion was significantly higher in 1G compared to μG at Vp=1200ml, there was no significant gravitational effect on dispersion at Vp=300ml. Finally, for each G level and Vp, deposition and dispersion significantly increased with increasing breath-hold time.
CONCLUSION: The most important finding of this study is that, even in the absence of gravity, aerosol deposition in the lung periphery increased with increasing residence time. Because the particles used in this study were too large to be significantly affected by Brownian diffusion, the increase in deposition is likely due to cardiogenic motion effects.