A multi-compartment model for slow bronchial clearance of insoluble particles--extension of the ICRP human respiratory tract models

Theoretical and experimental approaches to the deposition and clearance of ultrafine carcinogens in the human respiratory tract

INTRODUCTION

  Although inhaled ultrafine particles (UFPs) represent serious lung burdens and are thus responsible for a remarkable number of respiratory diseases (including cancer), only limited information on their deposition and clearance in the lung compartments is available. The study presented here tries to overcome this deficit by using a detailed theoretical approach to UFP behavior in the lungs.

METHODS

  The deposition model used in this context is based upon a stochastic lung geometry and the generation of single-particle trajectories in the tracheobronchial tree according to the random walk algorithm. Simulation of UFP clearance is conducted with the help of a multi-compartment model that considers cellular/non-cellular sites of temporary particle storage as separate compartments.

RESULTS

  As predicted by the models and confirmed by experimental findings, deposition of UFPs by Brownian motion takes place in both the upper and lower compartments of the respiratory tract. Alveolar accumulation of particulate mass increases proportionally with the inhalative flow rate. Clearance of UFPs is chiefly dominated by slow mechanisms with respective half-times ranging from several days to months.

DISCUSSION

  Modeling of UFP behavior in the respiratory tract represents an appropriate tool for forthcoming medical studies on this particle class, but it needs to be subjected to further refinements. •  As outlined by this study, alveolar deposition of UFPs, correlating with a noticeable risk of malignant transformations and cancer development, is determined by a number of factors, including effective particle size and velocity of particle transport in the conducting airways. •  With the help of appropriately validated models, respective predictions on the pulmonary burdens of UFP after short-term or long-term exposure can be made. In the case of subjects suffering from bronchial and/or alveolar UFP overloads, respective clearance approaches may be applied to simulate particle removal scenarios.

Deposition and cellular interaction of cancer-inducing particles in the human respiratory tract: Theoretical approaches and experimental data

Inhaled particles that are deposited on the epithelial surface of the human respiratory tract (HRT) may act as serious health hazards, in the worst case inducing the development of various types of lung cancer. In the past, several particle types, such as asbestos fibers, hard wood dust and cigarette smoke were identified and classified as human carcinogens. Due to their different physical and chemical properties these particles are characterized by remarkable discrepancies concerning their transport, deposition, and epithelial interaction in the HRT. In order to continuously increase the knowledge on carcinogenic particle behavior in the HRT, theoretical models describing single stages of particulate action in the lung airways were developed over the last few decades. With the help of these mathematical approaches physical characteristics of aerosolized drugs as well as protocols of inhalative therapies for the treatment of lung diseases could be significantly optimized. In addition, new experimental setups for the enlightenment of possible mechanisms underlying particle-lung interaction were, among other things, founded upon the results of theoretical computations. This review summarizes the efforts and advances of theoretical lung modeling from the early 1970s till today, thereby mainly directing the attention to the simulation of carcinogenic particle behavior in the HRT.

Bioaerosols in the lungs of subjects with different ages-Part 2: clearance modeling

BACKGROUND

The present contribution deals with theoretical aspects regarding biogenic particle clearance from various lung structures of probands with different ages (1, 5, 15, 20 y). With reference to part 1 of the study, particles varying in size and shape are subject to a detailed analysis. The main goal of the investigation consists in an increase of our knowledge concerning the clearance behaviour of bioparticles and its dependence upon various physiological and anatomical factors.

METHODS

Theoretical clearance of biogenic particles was subdivided into four main phases, namely fast bronchial clearance, slow bronchial clearance, fast alveolar clearance, and slow alveolar clearance. All of these phases were simulated by using a well validated stochastic modeling approach, where the main focus is set on the randomly varied particle mass transfer between main compartments of the human respiratory tract. Whilst effects of particle geometry on clearance were approximated by application of the projective-diameter concept, age dependence of the particle removal process was expressed by the experimentally proven relationship between bronchial mucus velocities and morphometry of the airway tree.

RESULTS

According to the results of the theoretical simulations efficiency of fast bronchial clearance, expressed by the 24-h-retention value, exhibits a negative correlation with proband's age, whereas the other clearance phases are characterized by a rather conservative behaviour among the different age categories. Highest clearance rates may be observed for very fine (<0.01 µm) and very coarse particles (>5 µm) preferentially deposited in the upper bronchial airways, whilst large particles accumulated in the alveoli may be stored there for several months to years.

CONCLUSIONS

The study comes to the conclusion that infants and children dispose of an enhanced bronchial clearance efficiency with respect to adolescents and adults, which results in a faster removal of particulate substances accumulated in the upper bronchial regions. Particles escaping from the natural filtering process in the upper airways and undergoing alveolar deposition are subject to identical clearance scenarios among the age groups and may represent remarkable health hazards.

Local lung deposition of ultrafine particles in healthy adults: experimental results and theoretical predictions

BACKGROUND

Ultrafine particles (UFP) of biogenic and anthropogenic origin occur in high numbers in the ambient atmosphere. In addition, aerosols containing ultrafine powders are used for the inhalation therapy of various diseases. All these facts make it necessary to obtain comprehensive knowledge regarding the exact behavior of UFP in the respiratory tract.

METHODS

Theoretical simulations of local UFP deposition are based on previously conducted inhalation experiments, where particles with various sizes (0.04, 0.06, 0.08, and 0.10 µm) were administered to the respiratory tract by application of the aerosol bolus technique. By the sequential change of the lung penetration depth of the inspired bolus, different volumetric lung regions could be generated and particle deposition in these regions could be evaluated. The model presented in this contribution adopted all parameters used in the experiments. Besides the obligatory comparison between practical and theoretical data, also advanced modeling predictions including the effect of varying functional residual capacity (FRC) and respiratory flow rate were conducted.

RESULTS

Validation of the UFP deposition model shows that highest deposition fractions occur in those volumetric lung regions corresponding to the small and partly alveolated airways of the tracheobronchial tree. Particle deposition proximal to the trachea is increased in female probands with respect to male subjects. Decrease of both the FRC and the respiratory flow rate results in an enhancement of UFP deposition.

CONCLUSIONS

The study comes to the conclusion that deposition of UFP taken up via bolus inhalation is influenced by a multitude of factors, among which lung morphometry and breathing conditions play a superior role.

Total deposition of ultrafine particles in the lungs of healthy men and women: experimental and theoretical results

BACKGROUND

Inhaled ultrafine particles (UFP) may induce greater adverse respiratory effects than larger particles occurring in the ambient atmosphere. Due to this potential of UFP to act as triggers for diverse lung injuries medical as well as physical research has been increasingly focused on the exact deposition behavior of the particles in lungs of various probands. Main purpose of the present study was the presentation of experimental and theoretical data of total, regional, and local UFP deposition in the lungs of men and women.

METHODS

Both experiments and theoretical simulations were carried out by using particle sizes of 0.04, 0.06, 0.08, and 0.10 µm [number median diameters (NMD)]. Inhalation of UFP took place by application of predefined tidal volumes (500, 750, and 1,000 mL) and respiratory flow rates (150, 250, 375, and 500 mL·s(-1)). For male subjects a functional residual capacity (FRC) of 3,911±892 mL was measured, whereas female probands had a FRC of 3,314±547 mL. Theoretical predictions were based on (I) a stochastic model of the tracheobronchial tree; (II) particle transport computations according to a random walk algorithm; and (III) empirical formulae for the description of UFP deposition.

RESULTS

Total deposition fractions (TDF) are marked by a continuous diminution with increasing particle size. Whilst particles measuring 0.04 µm in size deposit in the respiratory tract by 40-70%, particles with a size of 0.10 µm exhibit deposition values ranging from 20% to 45%. Except for the largest particles studied here TDF of female probands are higher than those obtained for male probands. Differences between experimental and theoretical results are most significant for 0.10 µm particles, but never exceed 20%. Predictions of regional (extrathoracic, tracheobronchial, alveolar) UFP deposition show clearly that females tend to develop higher tracheobronchial and alveolar deposition fractions than males. This discrepancy is also confirmed by airway generation-specific deposition, which is permanently higher in women than in men.

CONCLUSIONS

From the experimental data and modeling predictions it can be concluded that females bear a slightly higher potential to develop lung insufficiencies after exposure to UFP than males. Besides higher deposition fractions occurring in female subjects, also total lung deposition dose is noticeably enhanced.

Bioaerosols in the lungs of subjects with different ages-part 1: deposition modeling

BACKGROUND

In this contribution the inhalation and deposition of bioaerosols including particles with various shapes and sizes were investigated for probands with different ages (1, 5, 15 and 20 y). The study should help to increase our knowledge with regard to the behavior of variably shaped and sized particles in lungs being subject to different developmental stages.

METHODS

Simulation of particle transport and deposition in single structures of the respiratory tract was conducted by using a stochastic model of the tracheobronchial tree and well-validated analytical and empirical deposition formulae. Possible effects of particle geometry on deposition were taken into consideration by application of the aerodynamic diameter concept. Age-dependent lung morphometry and breathing parameters were computed by using appropriate scaling factors.

RESULTS

Theoretical simulations came to the result that bioparticle deposition in infants and children clearly differs from that in adolescents and adults insofar as the amount of deposited mass exhibits a positive correlation with age. Nose breathing results in higher extrathoracic deposition rates than mouth breathing and, as a consequence of that, lower particle amounts are enabled to enter the lung structures after passing the nasal airways. Under sitting breathing conditions highest alveolar deposition rates were calculated for particles adopting aerodynamic diameters of 10 nm and 4 µm, respectively.

CONCLUSIONS

The study comes to the conclusion that bioparticles have a lower chance to reach the alveoli in infants' and children's lungs, but show a higher alveolar deposition probability in the lungs of adolescents and adults. Despite of this circumstance also young subjects may increasingly suffer from biogenic particle burden, when they are subject to a long-term exposure to certain bioaerosols.

Clearance of carbon nanotubes in the human respiratory tract-a theoretical approach

INTRODUCTION

Theoretical knowledge of carbon nanotube clearance in the human respiratory tract represents an essential contribution to the risk assessment of artificial airborne nanomaterials. Thus, single phases of nanotube clearance were simulated with the help of a theoretical model.

METHODS

In this study, clearance of single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) was simulated by using a validated mathematical approach that includes all clearance mechanisms known hitherto. Fast mucociliary clearance is approximated by a steady-state steady-flow mucus model, whereas slow clearance mechanisms are modeled by definition of related clearance half-times.

RESULTS

Clearance may be subdivided into three phases, including fast bronchial clearance (mucociliary escalator), slow bronchial clearance (particle uptake by airway macrophages, transcytosis), and alveolar clearance (phagocytosis by alveolar macrophages, endocytosis by alveolar epithelium). According to the clearance model used in this study, mucociliary clearance is completed within the first 24 h after exposure, whereas slow bronchial clearance is characterized by a half-time of 5 d. Alveolar clearance is marked by half-times >100 d. As a result of their different deposition patterns, SWCNT and MWCNT show some discrepancies with regard to their clearance insofar as long SWCNT reside significantly longer in the lungs than MWCNT. This circumstance is among other expressed by higher 24-h, 10-d, and 100-d retentions computed for SWCNT compared to MWCNT.

DISCUSSION AND CONCLUSIONS

Due to their partly high residence times in distal lung regions, carbon nanotubes may bear the potential to act as triggers of inflammatory reactions or fibrotic modifications of the lung structure. Further they may also induce malignant transformations of lung cells, resulting in the development of lung tumours.

A theoretical approach to the deposition and clearance of fibers with variable size in the human respiratory tract

In the study presented here, a mathematical approach for the deposition and clearance of rigid and chemically stable fibers in the human respiratory tract (HRT) is described in detail. For the simulation of fiber transport and deposition in lung airways an advanced concept of the aerodynamic diameter is applied to a stochastic lung model with individual particle trajectories computed according to a random walk algorithm. Interception of fibrous material at airway bifurcations is considered by implementation of correction factors obtained from previously published numerical approaches to fiber deposition in short bronchial sequences. Fiber clearance is simulated on the basis of a multicompartment model, within which separate clearance scenarios are assumed for the alveolar, bronchiolar, and bronchial lung region and evacuation of fibrous material commonly takes place via the airway and extrathoracic path to the gastrointestinal tract (GIT) or via the transepithelial path to the lymph nodes and blood vessels. Deposition of fibrous particles in the HRT is controlled by the fiber aspect ratio beta in as much as particles with diameters <0.1 microm deposit less effectively with increasing beta, while larger particles exhibit a positive correlation between their deposition efficiencies and beta. A change from sitting to light-work breathing conditions causes only insignificant modifications of total fiber deposition in the HRT, whereas alveolar and, above all, tubular deposition of fibrous particles with a diameter >or=0.1 microm are affected remarkably. For these particles enhancement of the inhalative flow rate results in an increase of the extrathoracic and bronchial deposition fractions. Concerning the clearance of fibers from the HRT, 24-h retention is noticeably influenced by beta and, not less important, by the preferential deposition sites of the simulated particles. The significance of beta with respect to particle size may be regarded as similar to that determined for the deposition scenarios, while breathing conditions do not have a valuable effect on clearance.

Effect of particle size on slow particle clearance from the bronchial tree

The Human Respiratory Tract Model of the International Commission on Radiological Protection assumes that a fraction of particles deposited in the bronchial tree clears slowly, this fraction decreasing with increasing particle geometric diameter. To test this assumption, volunteers inhaled 5-microm aerodynamic diameter 111In-polystyrene and 198Au-gold particles simultaneously, as a 'bolus' at the end of each breath to minimize alveolar deposition. Because of the different densities (1.05 versus 19.3 g cm3), geometric diameters were about 5 and 1.2 microm, respectively, and corresponding predicted slowly cleared fractions were about 10% and 50%. However, lung retention of the 2 particles was similar in each subject. Retention at 24 hours, as a percentage of initial lung deposit (mean +/- SD) was 34 +/- 12 for polystyrene and 31 +/- 11 for the gold particles.

Immunohistochemical and immunocytochemical detection of SchS34 antigen in Stachybotrys chartarum spores and spore impacted mouse lungs

The purpose of this study was to evaluate the distribution of a 34 kD antigen isolated from S. chartarum sensu lato in spores and in the mouse lung 48 h after intra-tracheal instillation of spores by immuno-histochemistry. This antigen was localized in spore walls, primarily in the outer and inner wall layers and on the external wall surfaces with modest labelling observed in cytoplasm. Immuno-histochemistry revealed that in spore impacted mouse lung, antigen was again observed in spore walls, along the outside surface of the outer wall and in the intercellular space surrounding spores. In lung granulomas the labelled antigen formed a diffusate, some 2-3x the size of the long axis of spores, with highest concentrations nearest to spores. Collectively, these observations indicated that this protein not only displayed a high degree of specificity with respect to its location in spores and wall fragments, but also that it slowly diffuses into surrounding lungs.