Mode shift of an inhaled aerosol bolus is correlated with flow sequencing in the human lung

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.

Comparative Risks of Aldehyde Constituents in Cigarette Smoke Using Transient Computational Fluid Dynamics/Physiologically Based Pharmacokinetic Models of the Rat and Human Respiratory Tracts

Computational fluid dynamics (CFD) modeling is well suited for addressing species-specific anatomy and physiology in calculating respiratory tissue exposures to inhaled materials. In this study, we overcame prior CFD model limitations to demonstrate the importance of realistic, transient breathing patterns for predicting site-specific tissue dose. Specifically, extended airway CFD models of the rat and human were coupled with airway region-specific physiologically based pharmacokinetic (PBPK) tissue models to describe the kinetics of 3 reactive constituents of cigarette smoke: acrolein, acetaldehyde and formaldehyde. Simulations of aldehyde no-observed-adverse-effect levels for nasal toxicity in the rat were conducted until breath-by-breath tissue concentration profiles reached steady state. Human oral breathing simulations were conducted using representative aldehyde yields from cigarette smoke, measured puff ventilation profiles and numbers of cigarettes smoked per day. As with prior steady-state CFD/PBPK simulations, the anterior respiratory nasal epithelial tissues received the greatest initial uptake rates for each aldehyde in the rat. However, integrated time- and tissue depth-dependent area under the curve (AUC) concentrations were typically greater in the anterior dorsal olfactory epithelium using the more realistic transient breathing profiles. For human simulations, oral and laryngeal tissues received the highest local tissue dose with greater penetration to pulmonary tissues than predicted in the rat. Based upon lifetime average daily dose comparisons of tissue hot-spot AUCs (top 2.5% of surface area-normalized AUCs in each region) and numbers of cigarettes smoked/day, the order of concern for human exposures was acrolein > formaldehyde > acetaldehyde even though acetaldehyde yields were 10-fold greater than formaldehyde and acrolein.

Aerosol deposition in the human lung periphery is increased by reduced-density gas breathing

Aerosol mixing resulting from turbulent flows is thought to be a major mechanism of deposition in the upper respiratory tract (URT). Because turbulence levels are a function of gas density, the use of a low-density carrier gas should reduce deposition in the URT allowing the aerosol to reach more peripheral airways of the lung. We performed aerosol bolus tests on 11 healthy subjects to investigate the effect of reduced gas density on regional aerosol deposition in the human lung. Using both air and heliox (80% helium, 20% oxygen) as carrier gas, boluses of 1 and 2 microm-diameter particles were inhaled to five volumetric lung depths (V(p)) between 150 and 1200 mL during an inspiration from residual volume (RV) to 1 liter above functional residual capacity at a constant flow rate of approximately 0.50 L/sec, which was immediately followed by an expiration to RV at the same flow rate. Aerosol deposition and axial dispersion were calculated from aerosol concentration and flow rate measured at the mouth. For 1 microm-diameter particles, deposition was significantly reduced by 29 +/- 28% (mean +/- SD, p < 0.05) when breathing heliox instead of air at shallow V(p) (150 mL) and significantly increased by 11 +/- 9% at deep V(p) (1200 mL). For 2 microm-diameter particles, deposition was significantly higher at V(p) = 500 mL by 6 +/- 7% and the predicted V(p) to achieve 100% deposition was significantly lower with heliox (834 +/- 146 mL) compared to air (912 +/- 128 mL) (p < 0.05). Despite a decrease in deposition at shallow V(p), there was no change in axial dispersion, suggesting that other factors such as radial turbulent mixing result in decreased aerosol deposition. Our results suggested that heliox reduces upper airway deposition of 1 and 2 microm-diameter particles allowing more particles to penetrate and subsequently deposit in the peripheral lung.

Residual heterogeneity of intra- and interregional pulmonary perfusion in short-term microgravity

We hypothesized that the perfusion heterogeneity in the human, upright lung is determined by nongravitational more than gravitational factors. Twelve and six subjects were studied during two series of parabolic flights. We used cardiogenic oscillations of O2/SF6 as an indirect estimate of intraregional perfusion heterogeneity ( series 1) and phase IV amplitude (P4) as a indirect estimate of interregional perfusion heterogeneity ( series 2). A rebreathing-breath holding-expiration maneuver was performed. In flight, breath holding and expiration were performed either in microgravity (0 G) or in hypergravity. Controls were performed at normal gravity (1 G). In series 1, expiration was performed at 0 G. Cardiogenic oscillations of O2/SF6 were 19% lower when breath holding was performed at 0 G than when breath holding was performed at 1 G [means (SD): 1.7 (0.3) and 2.3 (0.6)% units] ( P = 0.044). When breath holding was performed at 1.8 G, values did not differ from 1-G control [2.6 (0.8)% units, P = 0.15], but they were 17% larger at 1.8 G than at 1 G. In series 2, expiration was performed at 1.7 G. P4 changed with gravity ( P < 0.001). When breath holding was performed at 0 G, P4 values were 45 (46)% of control. When breath holding was performed at 1.7 G, P4 values were 183 (101)% of control. We conclude that more than one-half of indexes of perfusion heterogeneity at 1 G are caused by nongravitational mechanisms.

Effect of 60 degrees head-down tilt on peripheral gas mixing in the human lung

The phase III slope of sulfur hexafluoride (SF6) in a single-breath washout (SBW) is greater than that of helium (He) under normal gravity (i.e., 1G), thus resulting in a positive SF6-He slope difference. In microgravity (microG), SF6-He slope difference is smaller because of a greater fall in the phase III slope of SF6 than He. We sought to determine whether increasing thoracic fluid volume using 60 degrees head-down tilt (HDT) in 1G would produce a similar effect to microG on phase III slopes of SF6 and He. Single-breath vital capacity (SBW) and multiple-breath washout (MBW) tests were performed before, during, and 60 min after 1 h of HDT. Compared with baseline (SF6 1.050 +/- 0.182%/l, He 0.670 +/- 0.172%/l), the SBW phase III slopes for both SF6 and He tended to decrease during HDT, reaching nadir at 30 min (SF6 0.609 +/- 0.211%/l, He 0.248 +/- 0.138%/l; P = 0.08 and P = 0.06, respectively). In contrast to microG, the magnitude of the phase III slope decrease was similar for both SF6 and He; therefore, no change in SF6-He slope difference was observed. MBW analysis revealed a decrease in normalized phase III slopes at all time points during HDT, for both SF6 (P < 0.01) and He (P < 0.01). This decrease was due to changes in the acinar, and not the conductive, component of the normalized phase III slope. These findings support the notion that changes in thoracic fluid volume alter ventilation distribution in the lung periphery but also demonstrate that the effect during HDT does not wholly mimic that observed in microG.