Particulate deposition in the human lung under lunar habitat conditions

Space Medicine: Inspiring a new generation of physicians

Space medicine is critical in enabling safe human exploration of space. The discipline focuses on supporting human survival, health, and performance in the austere environment of space. It is set to grow ever more important as significant transitions in the standard of space operations in the suborbital, low earth orbit (LEO) and beyond LEO domains will take place in the coming years. NASA along with their international and commercial partners have committed to returning to the Moon through the Artemis missions in this decade with the aim of achieving a permanent sustainable human presence on the lunar surface. Additionally, the development of reusable rockets is set to increase the number and frequency of humans going to space by making space travel more accessible. Commercial spaceflight and missions beyond LEO present many new challenges which space medicine physicians and researchers will need to address. Space medicine operates at the frontier of exploration, engineering, science and medicine. Aviation and Space Medicine (ASM) is the latest specialty to be recognised by the Royal College of Physicians and the General Medical Council in the UK. In this paper, we provide an introduction to space medicine, review the effects of spaceflight on human physiology and health along with countermeasures, medical and surgical issues in space, the varied roles of the ASM physician, challenges to UK space medicine practice and related research, and finally we explore the current representation of space medicine within the undergraduate curriculum.

The Effect of Aging on Aerosol Bolus Deposition in the Healthy Adult Lung: A 19-Year Longitudinal Study

Background: While it is recognized that peripheral lung structure and ventilation heterogeneity change with age, the effects of age on aerosol deposition in the healthy adult lung is largely unknown. Methods: A series of aerosol bolus inhalations were repeatedly performed in four healthy subjects over a period of 19 years (years = 0, 9, 15 and 19). For each series, a bolus of 1 μm particles was inhaled at penetration volumes (V_p) ranging from 200 to 1200 mL. Aerosol bolus deposition (DE), dispersion (H), and mode shift (MS) were calculated along with the rate of increase in these parameters with increasing V_p (slope-DE, slope-H, and slope-MS). Results: Slope-DE significantly increased from 0.040 ± 0.014 (mean ± standard deviation) at year 0 to 0.069 ± 0.007%/mL at year 19 (p = 0.02) with no significant difference in DE at shallow depth (V_p = 200 mL; 14% ± 4% at year 0 vs. 15% ± 7% at year 19, p = 0.25). There was no significant effect of age on either slope-H (0.44 ± 0.05 at year 0 vs. 0.47 ± 0.09 mL/mL at year 19, p = 0.6) or dispersion at shallow depth (192 ± 36 mL at year 0 vs. 220 ± 54 mL at year 19, p = 0.2). Slope-MS became significantly more negative with increasing age (-0.096 ± 0.044 at year 0 vs. -0.171 ± 0.027 mL/mL at year 19, p = 0.001) with no significant difference in MS at shallow depth (12 ± 10 at year 0 vs. 7 ± 15 mL at year 19, p = 0.3). Conclusions: These data suggest that (1) peripheral deposition increases with aging in the healthy lung, likely as a result of increasing closing volume with age; (2) alterations in the mechanical properties of healthy adult lungs with age occur uniformly; and (3) the significant increase in the magnitude of MS-slope with age is likely due to the concomitant increase in peripheral deposition and possible alterations in flow sequencing.

Effects of Partial Gravity on the Function and Particle Handling of the Human Lung

Purpose of Review

The challenges presented to the lung by the space environment are the effects of prolonged absence of gravity, the challenges of decompression stress associated with spacewalking, and the changes in the deposition of inhaled particulate matter.

Recent Findings

Although there are substantial changes in the function of the lung in partial gravity, the lung is largely unaffected by sustained exposure, returning rapidly to a normal state after return to 1G. Provided there is adequate denitrogenation prior to a spacewalk, avoiding the development of venous gas emboli, the lung copes well with the low pressure environment of the spacesuit. Particulate deposition is reduced in partial gravity, but where that deposition occurs is likely in the more peripheral airspaces, with associated longer retention times, potentially raising the toxicological potential of toxic dusts.

Summary

Despite its delicate structure the lung performs well in partial gravity, with the greatest threat likely arising from inhaled particulate matter (extra-terrestrial dusts).

Total and regional deposition of inhaled aerosols in supine healthy subjects and subjects with mild-to-moderate COPD

Despite substantial development of sophisticated subject-specific computational models of aerosol transport and deposition in human lungs, experimental validation of predictions from these new models is sparse. We collected aerosol retention and exhalation profiles in seven healthy volunteers and six subjects with mild-to-moderate COPD (FEV1 = 50-80%predicted) in the supine posture. Total deposition was measured during continuous breathing of 1 and 2.9 μm-diameter particles (tidal volume of 1 L, flow rate of 0.3 L/s and 0.75 L/s). Bolus inhalations of 1 μm particles were performed to penetration volumes of 200, 500 and 800 mL (flow rate of 0.5 L/s). Aerosol bolus dispersion (H), deposition, and mode shift (MS) were calculated from these data. There was no significant difference in total deposition between healthy subjects and those with COPD. Total deposition increased with increasing particle size and also with increasing flow rate. Similarly, there was no significant difference in aerosol bolus deposition between subject groups. Yet, the rate of increase in dispersion and of decrease in MS with increasing penetration volume was higher in subjects with COPD than in healthy volunteers (H: 0.798 ± 0.205 vs. 0.527 ± 0.122 mL/mL, p=0.01; MS: -0.271±0.129 vs. -0.145 ± 0.076 mL/mL, p=0.05) indicating larger ventilation inhomogeneities (based on H) and increased flow sequencing (based on MS) in the COPD than in the healthy group. In conclusion, in the supine posture, deposition appears to lack sensitivity for assessing the effect of lung morphology and/or ventilation distribution alteration induced by mild-to-moderate lung disease on the fate of inhaled aerosols. However, other parameters such as aerosol bolus dispersion and mode shift may be more sensitive parameters for evaluating models of lungs with moderate disease.

Increase in relative deposition of fine particles in the rat lung periphery in the absence of gravity

While it is well recognized that pulmonary deposition of inhaled particles is lowered in microgravity (μG) compared with gravity on the ground (1G), the absence of sedimentation causes fine particles to penetrate deeper in the lung in μG. Using quantitative magnetic resonance imaging (MRI), we determined the effect of gravity on peripheral deposition (DEPperipheral) of fine particles. Aerosolized 0.95-μm-diameter ferric oxide particles were delivered to spontaneously breathing rats placed in plethysmographic chambers both in μG aboard the NASA Microgravity Research Aircraft and at 1G. Following exposure, lungs were perfusion fixed, fluid filled, and imaged in a 3T MR scanner. The MR signal decay rate, R2*, was measured in each voxel of the left lung from which particle deposition (DEP) was determined based on a calibration curve. Regional deposition was assessed by comparing DEP between the outer (DEPperipheral) and inner (DEPcentral) areas on each slice, and expressed as the central-to-peripheral ratio. Total lung deposition tended to be lower in μG compared with 1G (1.01 ± 0.52 vs. 1.43 ± 0.52 μg/ml, P = 0.1). In μG, DEPperipheral was larger than DEPcentral (P < 0.03), while, in 1G, DEPperipheral was not significantly different from DEPcentral. Finally, central-to-peripheral ratio was significantly less in μG than in 1G (P ≤ 0.05). These data show a larger fraction of fine particles depositing peripherally in μG than in 1G, likely beyond the large- and medium-sized airways. Although not measured, the difference in the spatial distribution of deposited particles between μG and 1G could also affect particle retention rates, with an increase in retention for particles deposited more peripherally.

Aerosol deposition in the human lung in reduced gravity

The deposition of aerosol in the human lung occurs mainly through a combination of inertial impaction, gravitational sedimentation, and diffusion. For 0.5- to 5-μm-diameter particles and resting breathing conditions, the primary mechanism of deposition in the intrathoracic airways is sedimentation, and therefore the fate of these particles is markedly affected by gravity. Studies of aerosol deposition in altered gravity have mostly been performed in humans during parabolic flights in both microgravity (μG) and hypergravity (~1.6G), where both total deposition during continuous aerosol mouth breathing and regional deposition using aerosol bolus inhalations were performed with 0.5- to 3-μm particles. Although total deposition increased with increasing gravity level, only peripheral deposition as measured by aerosol bolus inhalations was strongly dependent on gravity, with central deposition (lung depth<200 mL) being similar between gravity levels. More recently, the spatial distribution of coarse particles (mass median aerodynamic diameter≈5 μm) deposited in the human lung was assessed using planar gamma scintigraphy. The absence of gravity caused a smaller portion of 5-μm particles to deposit in the lung periphery than in the central region, where deposition occurred mainly in the airways. Indeed, 5-μm-diameter particles deposit either by inertial impaction, a mechanism most efficient in the large and medium-sized airways, or by gravitational sedimentation, which is most efficient in the distal lung. On the contrary, for fine particles (~1 μm), both aerosol bolus inhalations and studies in small animals suggest that particles deposit more peripherally in μG than in 1G, beyond the reach of the mucociliary clearance system.