Convective mixing in human respiratory tract: estimates with aerosol boli

Heyder, J.; Blanchard, James; Feldman, Henry A.; Brain, Joseph D.

doi:10.1152/jappl.1988.64.3.1273

Cited by 130 publications

(79 citation statements)

References 21 publications

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“…Note that because of the fluid no-slip conditions, the perimeter of the blue͞white interface remains in contact with the wall of the tracheal cannula. It follows that the color interface is enormously stretched by the end of the inspiration, entering multiple airway pathways where about 2 23 distal acinar ducts (for human lungs; ref. 34) are connected to a single trachea sampling millions of alveolar spaces.…”

Section: Discussionmentioning

confidence: 99%

“…This assumption implies that there is negligible flow-induced mixing between inhaled particles and the alveolar residual gas (17)(18)(19)(20)(21) and consequently little deposition of fine particles (18,21). Experimental studies, however, consistently demonstrate appreciable aerosol mixing (23,24) and deposition (25) deep in the lung, which cannot be accounted for by any known mechanism such as inertial streamline crossing, gravitational sedimentation, or diffusion within the context of reversible acinar flow. These unresolved basic contradictions between predictions and experimental observations necessitate a re-examination of our current understanding and a new approach to the identification of causal links between aerosol exposure and its health effects.…”

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confidence: 99%

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Chaotic mixing deep in the lung

Tsuda

Rogers

Hydon

et al. 2002

Proc. Natl. Acad. Sci. U.S.A.

114

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Our current understanding of the transport and deposition of aerosols (viruses, bacteria, air pollutants, aerosolized drugs) deep in the lung has been grounded in dispersive theories based on untested assumptions about the nature of acinar airflow fields. Traditionally, these have been taken to be simple and kinematically reversible. In this article, we apply the recently discovered fluid mechanical phenomenon of irreversible low-Reynolds number flow to the lung. We demonstrate, through flow visualization studies in rhythmically ventilated rat lungs, that such a foundation is false, and that chaotic mixing may be key to aerosol transport. We found substantial alveolar flow irreversibility with stretched and folded fractal patterns, which lead to a sudden increase in mixing. These findings support our theory that chaotic alveolar flow-characterized by stagnation saddle points associated with alveolar vortices-governs gas kinematics in the lung periphery, and hence the transport, mixing, and ultimately the deposition of fine aerosols. This mechanism calls for a rethinking of the relationship of exposure and deposition of fine inhaled particles.P athogenic aerosols play a major role in causing or exacerbating many pulmonary diseases such as lung cancer, bronchitis, emphysema, and asthma (1-4) and in spreading infectious diseases such as influenza, anthrax, and tuberculosis (5-8). The long-term effects of particulate air pollution on public health are striking; they have been shown to be equivalent to a shortening of life expectancy of approximately 2 years in the United States (9, 10). The magnitude of this effect is comparable to the deaths caused by all cancers (11). Fine particles-less than a few micrometers in diameter-are of great concern because they can penetrate deep into the pulmonary acinus (i.e., the gas exchange region of the lung). The critical factor that determines the fate of inhaled fine particles-whether deposited deep in the lung or exhaled in the next breath-is the kinematic interaction of inhaled and residual alveolar gas, but the underlying mechanism of this interaction is still not well understood.Because gas flow in the alveolar region occurs at very low Reynolds number (12) and the wall motion of the lung during breathing is essentially reversible (13-16), it has long been assumed that flow patterns deep in the lung are well described as a kinematically reversible Stokes flow (17-21). This means that, apart from the effects of molecular diffusion, a fluid ''particle'' that is transported by Stokes flow returns [by reversing the flow (22)] to its original position at the completion of one breathing cycle. This assumption implies that there is negligible flow-induced mixing between inhaled particles and the alveolar residual gas (17-21) and consequently little deposition of fine particles (18, 21). Experimental studies, however, consistently demonstrate appreciable aerosol mixing (23, 24) and deposition (25) deep in the lung, which cannot be accounted for by any known mechanism such as inerti...

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Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

Chaotic mixing deep in the lung

Tsuda

Rogers

Hydon

et al. 2002

Proc. Natl. Acad. Sci. U.S.A.

114

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“…Our laboratory's recent findings (37)(38)(39)(40)(41), as well as those of others (8,18), however, have questioned the validity of the basic assumptions that formed the basis of the dispersive theories. We have demonstrated that, because of the peculiar geometry of the alveolated duct and its time-dependent motion associated with tidal breathing, under certain conditions, alveolar flow can be chaotic (16,40).…”

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confidence: 91%

Kinematically irreversible acinar flow: a departure from classical dispersive aerosol transport theories

Henry

Butler

Tsuda

2002

Journal of Applied Physiology

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irreversible acinar flow: a departure from classical dispersive aerosol transport theories. J Appl Physiol 92: 835-845, 2002. First published October 26, 2001 10.1152/japplphysiol. 00385.2001.-Current theories describe aerosol transport in the lung as a dispersive (diffusion-like) process, characterized by an effective diffusion coefficient in the context of reversible alveolar flow. Our recent experimental data, however, question the validity of these basic assumptions. In this study, we describe the behavior of fluid particles (or bolus) in a realistic, numerical, alveolated duct model with rhythmically expanding walls. We found acinar flow exhibiting multiple saddle points, characteristic of chaotic flow, resulting in substantial flow irreversibility. Computations of axial variance of bolus spreading indicate that the growth of the variance with respect to time is faster than linear, a finding inconsistent with dispersion theory. Lateral behavior of the bolus shows fine-scale, stretch-and-fold striations, exhibiting fractal-like patterns with a fractal dimension of 1.2, which compares well with the fractal dimension of 1.1 observed in our experimental studies performed with rat lungs. We conclude that kinematic irreversibility of acinar flow due to chaotic flow may be the dominant mechanism of aerosol transport deep in the lungs. lung; deposition; chaos; fractal; particulate pollution CONVECTION AND DIFFUSION ARE the two major mechanisms of mass transport for gas molecules and submicrometer aerosols in the pulmonary acinus. For gas transport, diffusion dominates at distances comparable to acinar size and over times comparable to breathing frequencies, and, therefore, theories based on diffusion are probably adequate. By contrast, the particle diffusivity of submicrometer-sized aerosols is very small, and, therefore, acinar convection, even though it is in a quasi-Stokes viscous flow regime (26), is correspondingly more important and may dominate aerosol transport. However, current theories describe aerosol transport as a dispersion (diffusion-like) process (e.g., Refs. 7,10,11,19,34). These theories are based on the following two key assumptions: 1) acinar flow is basically kinematically reversible (i.e., during expiration each fluid particle retraces the path taken during inspiration) (9, 45), and 2) all processes (including the coupling of Brownian diffusivity with the convective flow field and any kinematic irreversibility that may be present) that contribute to irreversible aerosol bolus spreading can be characterized as axial mixing with an effective longitudinal diffusivity (D eff ). The first assumption is based on classical fluid mechanics (36), and the second assumption is substantially equivalent to Taylor dispersion (35). As most aerosol studies are currently interpreted in the framework of these dispersion theories, experimental data are often reduced and analyzed through the use of some D eff (e.g., Ref. 30), and many of the recent theoretical research efforts are focused on refining D eff for...

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“…12 Heyder et al performed mixing estimates with aerosol bolus consisting of ϳ1 m particles. 13 The dispersion of the inhaled bolus increased with increasing penetration volume. The net transport of particles from the ͑particle-laden͒ inspired air to the residual air was shown to occur as a result of irreversible processes whose origins were unknown.…”

Section: Introductionmentioning

confidence: 98%

Steady streaming: A key mixing mechanism in low-Reynolds-number acinar flows

et al. 2011

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Study of mixing is important in understanding transport of submicron sized particles in the acinar region of the lung. In this article, we investigate transport in view of advective mixing utilizing Lagrangian particle tracking techniques: tracer advection, stretch rate and dispersion analysis. The phenomenon of steady streaming in an oscillatory flow is found to hold the key to the origin of kinematic mixing in the alveolus, the alveolar mouth and the alveolated duct. This mechanism provides the common route to folding of material lines and surfaces in any region of the acinar flow, and has no bearing on whether the geometry is expanding or if flow separates within the cavity or not. All analyses consistently indicate a significant decrease in mixing with decreasing Reynolds number ͑Re͒. For a given Re, dispersion is found to increase with degree of alveolation, indicating that geometry effects are important. These effects of Re and geometry can also be explained by the streaming mechanism. Based on flow conditions and resultant convective mixing measures, we conclude that significant convective mixing in the duct and within an alveolus could originate only in the first few generations of the acinar tree as a result of nonzero inertia, flow asymmetry, and large Keulegan-Carpenter ͑K C ͒ number.

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Convective mixing in human respiratory tract: estimates with aerosol boli

Cited by 130 publications

References 21 publications

Chaotic mixing deep in the lung

Chaotic mixing deep in the lung

Kinematically irreversible acinar flow: a departure from classical dispersive aerosol transport theories

Steady streaming: A key mixing mechanism in low-Reynolds-number acinar flows

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