Measurements of airway dimensions and calculation of mass transfer characteristics of the human oral passage

In silico investigation of inhalation condition impacts on hygroscopic growth and deposition of salbutamol sulphate in human airways

The objective of this study is to explore the transport, size growth, and deposition of Salbutamol Sulphate (SS) using Computational Fluid Dynamics (CFD). A CT-based realistic model of human airways from the oral cavity to the 5th generation of the lung was utilized as the computational domain. Four Test Cases (TC) with varying temperature and relative humidity (RH) under two inspiratory waveforms were considered to completely evaluate the impact of inhalation conditions on particle growth. Salbutamol Sulphate (SS) is a β2-adrenergic agonist and has been extensively used for asthma treatment. A monodispersed distribution of SS particles with an initial diameter of 167 nm was considered at the mouth inlet based on pharmaceutical data. Results indicated that inhalation of saturated/supersaturated air (RH>100%) leads to significant hygroscopic growth of SS particles with a factor of 10. In addition, the deposition efficiency of SS particles under the Quick and Deep (QD) inhalation profile was enhanced as the flow temperature and humidity increased. However, the implementation of Slow and Deep (SD) inspiratory waveform revealed that the same particle size growth is achieved in the respiratory system with lower deposition efficiency in the mouth-throat (less than 3%) and tracheobronchial airway (less than 2.18%). For the escaped particles form the right lung, in the SD waveform under TC 3, the maximum particle size distribution was for 600 nm particles with 25% probability. In the left lung, 30% of the particles were increased up to 950 nm in size. For the QD waveform in TC 3 and TC4, the most frequent particles were 800 nm with 36% probability. This holds practical significance in the context of deep lung delivery for asthmatic patients with enhanced deposition efficiency and large particle size. The findings of the present study can contribute to the development of targeted drug delivery strategies for the treatment of pulmonary diseases using hygroscopic dry powder formulations.

A revision of the multiple-path particle dosimetry model focusing on tobacco product aerosol dynamics

To assess the health impact of inhaled aerosols, it is necessary to understand aerosol dynamics and the associated dosimetry in the human respiratory tract. Although several studies have measured or simulated the dosimetry of aerosol constituents, the respiratory tract focus areas have been limited. In particular, the aerosols generated from tobacco products are complex composites and simulating their dynamics in the respiratory tract is challenging. To assess the dosimetry of the aerosol constituents of tobacco products, we developed a revised version of the Multiple-Path Particle Dosimetry (MPPD) model, which employs (1) new geometry based on CT-scanned human respiratory tract data, (2) convective mixing in the oral cavity and deep lung, and (3) constituent partitioning between the tissue and air, and clearance. The sensitivity analysis was conducted using aerosols composed of four major constituents of electronic cigarette (EC) aerosols to investigate the parameters that have a significant impact on the results. In addition, the revised model was run with 4 and 10 constituents in ECs and conventional cigarettes (CCs), respectively. Sensitivity analysis revealed that the new modeling and the physicochemical properties of constituents had a considerable impact on the simulated aerosol concentration and dosimetry. The simulations could be carried out within 3 min even when 10 constituents of CC aerosols were analyzed simultaneously. The revised model based on MPPD is an efficient and easy-to-use tool for understanding the aerosol dynamics of CC and EC constituents and their effect on the human body.

A path forward in the development of new aerosol drug delivery devices for pediatrics

Inhaled medications are widely accepted as being the optimal route for treating pediatric respiratory diseases, a leading cause of hospitalization and death. Despite jet nebulizers being the preferred inhalation device for neonates and infants, current devices face performance issues with most of the drug never reaching the target lung location. Previous work has aimed to improve pulmonary drug deposition, yet nebulizer efficiency remains low. The development of an inhalant therapy that is efficacious and safe for pediatrics depends on a well-designed delivery system and formulation. To accomplish this, the field needs to rethink the current practice of basing pediatric treatments on adult studies. The rapidly evolving pediatric patient (i.e. neonates to eighteen) needs to be considered because they are different from adults with respect to airway anatomy, breathing patterns, and adherence. Previous research approaches to improve deposition efficiency have been limited due to the complexity of combining physics, which drives aerosol transport and deposition, and biology, especially within the area of pediatrics. To address these critical knowledge gaps, we need a better understanding of how patient age and disease state affect deposition of aerosolized drugs. The complexity of the multiscale respiratory system makes scientific investigation very challenging. The authors have simplified the complex problem into five components with these three areas as ones to address first: how the aerosol is (i) generated in a medical device, (ii) delivered to the patient, and (iii) deposited inside the lung. In this review, we discuss the technological advances and innovations made from experiments, simulations, and predictive models in each of these areas. In addition, we discuss the impact on patient treatment efficacy and recommend a clinical direction, with a focus on pediatrics. In each area, a series of research questions are posed and steps for future research to improve efficacy in aerosol drug delivery are outlined.

A Novel Trans-Tracheostomal Retrograde Inhalation Technique Increases Subglottic Drug Deposition Compared to Traditional Trans-Oral Inhalation

Subglottic stenosis represents a challenging clinical condition in otolaryngology. Although patients often experience improvement following endoscopic surgery, recurrence rates remain high. Pursuing measures to maintain surgical results and prevent recurrence is thus necessary. Steroids therapy is considered effective in preventing restenosis. Currently, however, the ability of trans-oral steroid inhalation to reach and affect the stenotic subglottic area in a tracheotomized patient is largely negligible. In the present study, we describe a novel trans-tracheostomal retrograde inhalation technique to increase corticosteroid deposition in the subglottic area. We detail our preliminary clinical outcomes in four patients treated with trans-tracheostomal corticosteroid inhalation via a metered dose inhaler (MDI) following surgery. Concurrently, we leverage computational fluid-particle dynamics (CFPD) simulations in an extra-thoracic 3D airway model to gain insight on possible advantages of such a technique over traditional trans-oral inhalation in augmenting aerosol deposition in the stenotic subglottic region. Our numerical simulations show that for an arbitrary inhaled dose (aerosols spanning 1–12 µm), the deposition (mass) fraction in the subglottis is over 30 times higher in the retrograde trans-tracheostomal technique compared to the trans-oral inhalation technique (3.63% vs. 0.11%). Importantly, while a major portion of inhaled aerosols (66.43%) in the trans-oral inhalation maneuver are transported distally past the trachea, the vast majority of aerosols (85.10%) exit through the mouth during trans-tracheostomal inhalation, thereby avoiding undesired deposition in the broader lungs. Overall, the proposed trans-tracheostomal retrograde inhalation technique increases aerosol deposition rates in the subglottis with minor lower-airway deposition compared to the trans-oral inhalation technique. This novel technique could play an important role in preventing restenosis of the subglottis.

In Silico Quantification of Intersubject Variability on Aerosol Deposition in the Oral Airway

The extrathoracic oral airway is not only a major mechanical barrier for pharmaceutical aerosols to reach the lung but also a major source of variability in lung deposition. Using computational fluid dynamics, deposition of 1−30 µm particles was predicted in 11 CT-based models of the oral airways of adults. Simulations were performed for mouth breathing during both inspiration and expiration at two steady-state flow rates representative of resting/nebulizer use (18 L/min) and of dry powder inhaler (DPI) use (45 L/min). Consistent with previous in vitro studies, there was a large intersubject variability in oral deposition. For an optimal size distribution of 1−5 µm for pharmaceutical aerosols, our data suggest that >75% of the inhaled aerosol is delivered to the intrathoracic lungs in most subjects when using a nebulizer but only in about half the subjects when using a DPI. There was no significant difference in oral deposition efficiency between inspiration and expiration, unlike subregional deposition, which shows significantly different patterns between the two breathing phases. These results highlight the need for incorporating a morphological variation of the upper airway in predictive models of aerosol deposition for accurate predictions of particle dosimetry in the intrathoracic region of the lung.

Lower Inspiratory Breathing Depth Enhances Pulmonary Delivery Efficiency of ProAir Sprays

Effective pulmonary drug delivery using a metered-dose inhaler (MDI) requires a match between the MDI sprays, the patient’s breathing, and respiratory physiology. Different inhalers generate aerosols with distinct aerosol sizes and speeds, which require specific breathing coordination to achieve optimized delivery efficiency. Inability to perform the instructed breathing maneuver is one of the frequently reported issues during MDI applications; however, their effects on MDI dosimetry are unclear. The objective of this study is to systemically evaluate the effects of breathing depths on regional deposition in the respiratory tract using a ProAir-HFA inhaler. An integrated inhaler mouth-throat-lung geometry model was developed that extends to the ninth bifurcation (G9). Large-eddy simulation (LES) was used to compute the airflow dynamics due to concurrent inhalation and orifice flows. The discrete-phase Lagrangian model was used to track droplet motions. Experimental measurements of ProAir spray droplet sizes and speeds were used as initial and boundary conditions to develop the computational model for ProAir-pulmonary drug delivery. The time-varying spray plume from a ProAir-HFA inhaler into the open air was visualized using a high-speed imaging system and was further used to validate the computational model. The inhalation dosimetry of ProAir spray droplets in the respiratory tract was compared among five breathing depths on a regional, sub-regional, and local basis. The results show remarkable differences in airflow dynamics within the MDI mouthpiece and the droplet deposition distribution in the oral cavity. The inhalation depth had a positive relationship with the deposition in the mouth and a negative relationship with the deposition in the five lobes beyond G9 (small airways). The highest delivery efficiency to small airways was highest at 15 L/min and declined with an increasing inhalation depth. The drug loss inside the MDI was maximal at 45–60 L/min. Comparisons to previous experimental and numerical studies revealed a high dosimetry sensitivity to the inhaler type and patient breathing condition. Considering the appropriate inhalation waveform, spray actuation time, and spray properties (size and velocity) is essential to accurately predict inhalation dosimetry from MDIs. The results highlight the importance of personalized inhalation therapy to match the patient’s breathing patterns for optimal delivery efficiencies. Further complimentary in vitro or in vivo experiments are needed to validate the enhanced pulmonary delivery at 15 L/min.

Effects of chlorine particle concentration on the human airway

For COVID-19, chlorine has lately been utilised as a home disinfectant. Given that chlorine is hazardous to the human airway, the current research investigates the effects of chlorine mass fraction and droplet size on the human airway. The effects are investigated at chlorine mass ratios of 2% (24 ppm), 10% (120 ppm), 15% (180 ppm), and 20% (240 ppm), as well as chlorine particle diameters of 10 nm, 20 nm, 30 nm, and 50 nm, and three inhalation rates (15 l/min, 30 l/min, and 60 l/min). The results reveal that when the chlorine mass fraction is 2% and the inhalation rate is low, the chlorine volume fraction decreases. Furthermore, at 2% chlorine and a rapid breathing rate, chlorine particles are accelerated to escape into the lungs.

Evaluation of Impulse Oscillometry in Respiratory Airway Casts with Varying Obstruction Phenotypes, Locations, and Complexities

The use of impulse oscillometry (IOS) for lung function testing does not need patient cooperation and has gained increasing popularity among both young and senior populations, as well as in patients with breathing difficulties. However, studies of the IOS sensitivity to regional lung obstructions are limited and have shown mixed results. The objective of this study was to evaluate the performance of an IOS system in 3D-printed lung models with structural abnormalities at different locations and with different severities. Lung trees of two complexity levels were tested, with one extending to the sixth generation (G6) and the other to G12. The IOS responses to varying glottal apertures, carina ridge tumors, and segmental bronchial constrictions were quantified in the G6 lung geometry. Both the G6 and G12 lung casts were prepared using high-resolution 3D printers. Overall, IOS detected the progressive airway obstructions considered in this study. The resonant frequency dropped with increasing obstructions for all three disease phenotypes in the G6 lung models. R20Hz increased with the increase in airway obstructions. Specifically, R20Hz in the airway model with varying glottal apertures agreed reasonably well with complementary measurements using TSI VelociCalc. In contrast to the high-resistance (R) sensitivity to the frequency in G6 lung models, R was nearly independent of frequency in G12 lung models. IOS R20Hz demonstrated adequate sensitivity to the structural remodeling in the central airways. However, the changes of R5Hz and X5Hz vs. airway obstructions were inconclusive in this study, possibly due to the rigid lung casts and the difference of a container–syringe system from human lungs.

Aerosol Particle Transport and Deposition in Upper and Lower Airways of Infant, Child and Adult Human Lungs

Understanding transportation and deposition (TD) of aerosol particles in the human respiratory system can help clinical treatment of lung diseases using medicines. The lung airway diameters and the breathing capacity of human lungs normally increase with age until the age of 30. Many studies have analyzed the particle TD in the human lung airways. However, the knowledge of the nanoparticle TD in airways of infants and children with varying inhalation flow rates is still limited in the literature. This study investigates nanoparticle (5 nm ≤ dp ≤ 500 nm) TD in the lungs of infants, children, and adults. The inhalation air flow rates corresponding to three ages are considered as Qin=3.22 L/min (infant), 8.09 L/min (Child), and Qin=14 L/min (adult). It is found that less particles are deposited in upper lung airways (G0–G3) than in lower airways (G12–G15) in the lungs of all the three age groups. The results suggest that the particle deposition efficiency in lung airways increases with the decrease of particle size due to the Brownian diffusion mechanism. About 3% of 500 nm particles are deposited in airways G12–G15 for the three age groups. As the particle size is decreased to 5 nm, the deposition rate in G12–G15 is increased to over 95%. The present findings can help medical therapy by individually simulating the distribution of drug-aerosol for the patient-specific lung.

In vitro delivery efficiencies of nebulizers for different breathing patterns

Background

Nebulizers are medical devices that deliver aerosolized medication directly to lungs to treat a variety of respiratory diseases. However, breathing patterns, respiration rates, airway diameters, and amounts of drugs delivered by nebulizers may be respiratory disease dependent.

Method

In this study, we developed a respiratory simulator consisting of an airway model, an artificial lung, a flow sensor, and an aerosol collecting filter. Various breathing patterns were generated using a linear actuator and an air cylinder. We tested six home nebulizers (jet (2), static (2), and vibrating mesh nebulizers (2)). Nebulizers were evaluated under two conditions, that is, for the duration of nebulization and at a constant output 1.3 mL using four breathing patterns, namely, the breathing pattern specified in ISO 27427:2013, normal adult, asthmatic, and COPD.

Results

One of the vibrating mesh nebulizers had the highest dose delivery efficiency. The drug delivery efficiencies of nebulizers were found to depend on breathing patterns.

Conclusion

We suggest a quantitative drug delivery efficiency evaluation method and calculation parameters that include considerations of constant outputs and residual volumes. The study shows output rates and breathing patterns should be considered when the drug delivery efficiencies of nebulizers are evaluated.

Polydisperse Aerosol Transport and Deposition in Upper Airways of Age-Specific Lung

A comprehensive understanding of airflow characteristics and particle transport in the human lung can be useful in modelling to inform clinical diagnosis, treatment, and management, including prescription medication and risk assessment for rehabilitation. One of the difficulties in clinical treatment of lung disorders lies in the patients’ variable physical lung characteristics caused by age, amongst other factors, such as different lung sizes. A precise understanding of the comparison between different age groups with various flow rates is missing in the literature, and this study aims to analyse the airflow and aerosol transport within the age-specific lung. ANSYS Fluent solver and the large-eddy simulation (LES) model were employed for the numerical simulation. The numerical model was validated with the available literature and the computational results showed airway size-reduction significantly affected airflow and particle transport in the upper airways. This study reports higher deposition at the mouth-throat region for larger diameter particles. The overall deposition efficiency (DE) increased with airway size reduction and flow rate. Lung aging effected the pressure distribution and a higher pressure drop was reported for the aged lung as compared to the younger lung. These findings could inform medical management through individualised simulation of drug-aerosol delivery processes for the patient-specific lung.

Role of CFD based in silico modelling in establishing an in vitro-in vivo correlation of aerosol deposition in the respiratory tract

Effective evaluation and prediction of aerosol transport deposition in the human respiratory tracts are critical to aerosol drug delivery and evaluation of inhalation products. Establishment of an in vitro-in vivo correlation (IVIVC) requires the understanding of flow and aerosol behaviour and underlying mechanisms at the microscopic scale. The achievement of the aim can be facilitated via computational fluid dynamics (CFD) based in silico modelling which treats the aerosol delivery as a two-phase flow. CFD modelling research, in particular coupling with discrete phase model (DPM) and discrete element method (DEM) approaches, has been rapidly developed in the past two decades. This paper reviews the recent development in this area. The paper covers the following aspects: geometric models of the respiratory tract, CFD turbulence models for gas phase and its coupling with DPM/DEM for aerosols, and CFD investigation of the effects of key factors associated with geometric variations, flow and powder characteristics. The review showed that in silico study based on CFD models can effectively evaluate and predict aerosol deposition pattern in human respiratory tracts. The review concludes with recommendations on future research to improve in silico prediction to achieve better IVIVC.

A 3D printed human upper respiratory tract model for particulate deposition profiling

Pulmonary route is the main route of drug delivery for patients with asthma and chronic obstructive pulmonary diseases, offering several advantages over the oral route. Determining the amount of drug deposited onto various parts of the respiratory tract allows for a good correlation to clinical efficacy of inhalation drug devices. However, current in vitro cascade impactors measure only the aerodynamic particle size distribution, which does not truly represent the in vivo deposition pattern in human respiratory tract. In this study, a human upper respiratory tract model was fabricated using a 3D printer and subsequently characterized for its dimensional accuracy, surface finishing and air leaking. The effects of using a spacer and/or various airflow rates were also investigated. To assess this in vitro model, the deposition pattern of a model drug, namely, salbutamol sulphate, was tested. The resultant deposition pattern of salbutamol sulphate from a metered dose inhaler at 15 L per minute with the spacer, showed no significant difference from that of a published radiological in vivo study performed in adult humans. In addition, it was also found that the deposition pattern of salbutamol at 35 L per minute was comparable to the results of another published study in human. This in vitro model, showing reasonable in vitro-in vivo correlation, may provide opportunities for personalized medicine in special populations or disease states.

SARS COV-2 virus-laden droplets coughed from deep lungs: Numerical quantification in a single-path whole respiratory tract geometry

When an infected person coughs, many virus-laden droplets will be exhaled out of the mouth. Droplets from deep lungs are especially infectious because the alveoli are the major sites of coronavirus replication. However, their exhalation fraction, size distribution, and exiting speeds are unclear. This study investigated the behavior and fate of respiratory droplets (0.1-4 μm) during coughs in a single-path respiratory tract model extending from terminal alveoli to mouth opening. An experimentally measured cough waveform was used to control the alveolar wall motions and the flow boundary conditions at lung branches from G2 to G18. The mouth opening was modeled after the image of a coughing subject captured using a high-speed camera. A well-tested k-ω turbulence model and Lagrangian particle tracking algorithm were applied to simulate cough flow evolutions and droplet dynamics under four cough depths, i.e., tidal volume ratio (TVR) = 0.13, 0.20. 0.32, and 0.42. The results show that 2-μm droplets have the highest exhalation fraction, regardless of cough depths. A nonlinear relationship exists between the droplet exhalation fraction and cough depth due to a complex deposition mechanism confounded by multiscale airway passages, multiregime flows, and drastic transient flow effects. The highest exhalation fraction is 1.6% at the normal cough depth (TVR = 0.32), with a mean exiting speed of 20 m/s. The finding that most exhaled droplets from deep lungs are 2 μm highlights the need for more effective facemasks in blocking 2-μm droplets and smaller both in infectious source control and self-protection from airborne virus-laden droplets.

Computational fluid dynamic models as tools to predict aerosol distribution in tracheobronchial airways

Aerosol and pollutants, in form of particulates 5-8 μm in main size face every day our respiratory system as natural suspension in air or forced to be inhaled as a coadjutant in a medical therapy for respiratory diseases. This inhalation happens in children to elderly, women and men, healthy or sick and disable people. In this paper we analyzed the inhalation of aerosol in conditions assimilable to the thermal therapy. We use a computational fluid dynamic 3D model to compute and visualize the trajectories of aerosol (3-7-10-25 µm) down to the sixth generation of bronchi, in a steady and dynamic condition (7 µm) set as breath cycle at rest. Results, compared to a set of milestone experimental studies published in literature, allow the comprehension of particles behavior during the inhalation from mouth to bronchi sixth generation, the visualization of jet at larynx constriction and vortices, in an averaged characteristic rigorous geometrical model including tracheal rings. Results on trajectories and deposition show the importance of the including transient physiological breath cycle on aerosol deposition analyses. Numerical and graphical results, may enable the design of medical devices and protocols to make the inhalations more effective in all the users' population.

Evaluation of wearing comfort of dust masks

Dust masks are widely used to prevent the inhalation of particulate matter into the human respiratory organs in polluted air environments. The filter of a dust mask inherently obstructs the natural respiratory air flows, and this flow resistance is mainly responsible for the discomfort experienced when wearing a dust mask. In atmospheric conditions seriously contaminated with fine dust, it is recommended that common citizens wear a dust mask in their everyday lives, yet many people are reluctant to wear a dust mask owing to the discomfort experienced when wearing it for a long time. Understanding of physical reasons for the discomfort is thus crucial in designing a dust mask, but remains far from clear. This study presents a technique to quantify the wearing comfort of dust masks. By developing a respiration simulator to measure the pressure loss across a dust mask, we assessed the energy costs to overcome flow resistance when breathing through various types of dust masks. The energy cost for a single inhalation varies with the mask type in a range between 0 and 10 mJ. We compared the results with the survey results of 40 people about the wearing comfort of the dust masks, which revealed that the wearing comfort crucially depends on the energy cost required for air inhalation though the dust mask. Using the measured energy cost during inhalation as a parameter to quantify the wearing comfort, we present a comprehensive evaluation of the performance of dust masks in terms of not only the filtering performance but also the wearing comfort. Our study suggests some design principles for dust mask filters, auxiliary electric fans, and check valves.

Cascade Impactor Performance of Commercial pMDI Formulations Using Modified Induction Ports

The induction port (IP) for aerosol analysis with the Next Generation Pharmaceutical Impactor as monographed in the United States and European pharmacopoeia (USPIP) lacks physiological relevance, which, amongst other reasons, has been identified as critical for the predictability of in vitro aerosol data to lung deposition observed in vivo. In this publication, we report the impact of replacing the USPIP with two modified induction ports, which were designed based around geometries derived from a computer tomographic scan of a human trachea and the distal section of the USPIP. Test formulations were selected on the basis of availability of in vivo lung deposition data so that results obtained in vitro could be evaluated for their predictability. All formulations assessed showed increased deposition in the modified induction ports, and different mechanisms of particle deposition have been identified. In vitro predictions of the lung deposition were found to correlate well with the in vivo observations reported using the modified induction ports. Furthermore, the quality of the correlation was found superior to the one achieved with the USPIP with an average deviation of the predicted from observed values (n = 10) of 6 ± 4, 12 ± 6, and 16 ± 6% for the modified induction ports (mIP and mIPext) and the USPIP, respectively, when using a fine particle fraction (FPF) cutoff value of 5 μm. Using a FPF cutoff value of 3 μm yielded a more accurate in vitro-in vivo correlation with an average deviation of the predicted from observed values of 5 ± 4, 7 ± 5, and 8 ± 4% for the mIP, mIPext, and USPIP, respectively. For both FPF size cutoff values, the mIP yielded the most accurate in vitro-in vivo correlation.

Effect of inspiration cycle and ventilation rate on heat exchange in human respiratory airways

A transient three dimensional (3D) theoretical axisymmetric model is developed for heat exchange across the human respiratory tract during inspiration phase and applied to study the changes in the airway temperature and velocity profile for varying ventilation rates and inhalation temperatures. A multi-compartment approach is used to study the same to avoid the airway scaling problem from micro to nano scale. This analysis also includes the role of water evaporation in mucus and non perfused tissue layers and the role of capillary bed in thermal variations during respiration. The results of heat transfer in airway and mucus layer depend on the local morphological parameters. The results are compared with the case of hypothetical regular geometry to show the significance of local morphology. The location where the inhaled air gets saturated with the body core temperature is computed to estimate the saturation distance of air. The complete analysis is made for two breathing cycles with different inhalation to exhalation ratios. The results indicate that decreasing the ventilation rate and increasing the respiration cycle can avoid the deep penetration of heat into the tract and consequently tissue thermal injury can be avoided. We have also explained numerically the role of mucus layer in avoiding tissue injury in intra-thoracic airways. We have also observed a significant difference in results for high ventilation rates between the cases of actual (cast replica) and regular airway geometry. The numerical results are in good adjustment with existing experimental data and thus validate our approach.

Regional aerosol deposition in the human airways: The SimInhale benchmark case and a critical assessment of in silico methods

Regional deposition effects are important in the pulmonary delivery of drugs intended for the topical treatment of respiratory ailments. They also play a critical role in the systemic delivery of drugs with limited lung bioavailability. In recent years, significant improvements in the quality of pulmonary imaging have taken place, however the resolution of current imaging modalities remains inadequate for quantifying regional deposition. Computational Fluid-Particle Dynamics (CFPD) can fill this gap by providing detailed information about regional deposition in the extrathoracic and conducting airways. It is therefore not surprising that the last 15years have seen an exponential growth in the application of CFPD methods in this area. Survey of the recent literature however, reveals a wide variability in the range of modelling approaches used and in the assumptions made about important physical processes taking place during aerosol inhalation. The purpose of this work is to provide a concise critical review of the computational approaches used to date, and to present a benchmark case for validation of future studies in the upper airways. In the spirit of providing the wider community with a reference for quality assurance of CFPD studies, in vitro deposition measurements have been conducted in a human-based model of the upper airways, and several groups within MP1404 SimInhale have computed the same case using a variety of simulation and discretization approaches. Here, we report the results of this collaborative effort and provide a critical discussion of the performance of the various simulation methods. The benchmark case, in vitro deposition data and in silico results will be published online and made available to the wider community. Particle image velocimetry measurements of the flow, as well as additional numerical results from the community, will be appended to the online database as they become available in the future.

Numerical simulations of aerosol delivery to the human lung with an idealized laryngeal model, image-based airway model, and automatic meshing algorithm

The authors proposed a new method to automatically mesh computed tomography (CT)-based three-dimensional human airway geometry for computational fluid dynamics (CFD)-based simulations of pulmonary gas-flow and aerosol delivery. Traditional methods to construct and mesh realistic geometry were time-consuming, because they were done manually using image-processing and mesh-generating programs. Furthermore, most of CT thoracic image data sets do not include the upper airway structures. To overcome these issues, the proposed method consists of CFD grid-size distribution, an automatic meshing algorithm, and the addition of a laryngeal model along with turbulent velocity inflow boundary condition attached to the proximal end of the trachea. The method is based on our previously developed geometric model with irregular centerlines and cross-sections fitted to CT segmented airway surfaces, dubbed the "fitted-surface model." The new method utilizes anatomical information obtained from the one-dimensional tree, e.g., skeleton connectivity and branch diameters, to efficiently generate optimal CFD mesh, automatically impose boundary conditions, and systematically reduce simulation results. The aerosol deposition predicted by the proposed method agreed well with the prediction by a traditional CT-based model, and the laryngeal model generated a realistic level of turbulence in the trachea. Furthermore, the computational time was reduced by factor of two without losing accuracy by using the proposed grid-size distribution. The new method is well suited for branch-by-branch analyses of gas-flow and aerosol distribution in multiple subjects due to embedded anatomical information.

Validating Whole-Airway CFD Predictions of DPI Aerosol Deposition at Multiple Flow Rates

BACKGROUND

The objective of this study was to compare aerosol deposition predictions of a new whole-airway CFD model with available in vivo data for a dry powder inhaler (DPI) considered across multiple inhalation waveforms, which affect both the particle size distribution (PSD) and particle deposition.

METHODS

The Novolizer DPI with a budesonide formulation was selected based on the availability of 2D gamma scintigraphy data in humans for three different well-defined inhalation waveforms. Initial in vitro cascade impaction experiments were conducted at multiple constant (square-wave) particle sizing flow rates to characterize PSDs. The whole-airway CFD modeling approach implemented the experimentally determined PSDs at the point of aerosol formation in the inhaler. Complete characteristic airway geometries for an adult were evaluated through the lobar bronchi, followed by stochastic individual pathway (SIP) approximations through the tracheobronchial region and new acinar moving wall models of the alveolar region.

RESULTS

It was determined that the PSD used for each inhalation waveform should be based on a constant particle sizing flow rate equal to the average of the inhalation waveform's peak inspiratory flow rate (PIFR) and mean flow rate [i.e., AVG(PIFR, Mean)]. Using this technique, agreement with the in vivo data was acceptable with <15% relative differences averaged across the three regions considered for all inhalation waveforms. Defining a peripheral to central deposition ratio (P/C) based on alveolar and tracheobronchial compartments, respectively, large flow-rate-dependent differences were observed, which were not evident in the original 2D in vivo data.

CONCLUSIONS

The agreement between the CFD predictions and in vivo data was dependent on accurate initial estimates of the PSD, emphasizing the need for a combination in vitro-in silico approach. Furthermore, use of the AVG(PIFR, Mean) value was identified as a potentially useful method for characterizing a DPI aerosol at a constant flow rate.

A computational prediction for the effective drug and stem cell treatment of human airway burns

Burns in the airway from inhaling hot gases lead to one of the most common causes of death in the United States. In order to navigate tissues with large burn areas, the velocity, temperature, and heat flux distributions throughout the human airway system are computed for the inhalation of hot air using the finite-element method. From there, the depth of burned tissue is estimated for a range of exposure times. Additionally, the effectiveness of drug or stem cell delivery to the burned airway tissue is considered for a range of drug or cell sizes. Results showed that the highest temperature and lowest heat flux regions are observed near the pharynx and just upstream of the glottis. It was found that large particles such as stem cells (>20 μm) are effective for treatment of the upper airways, whereas small particles (<10 μm) such as drug nanoparticles are effective in the lower airways.

A method for in vitro regional aerosol deposition measurement in a model of the human tracheobronchial tree by the positron emission tomography

Researchers have been studying aerosol transport in human lungs for some decades. The overall lung deposition can be predicted with sufficient precision nowadays. However, the prediction of local deposition remains an unsolved problem. Numerical modeling of aerosol transport can provide detailed data with such precision and spatial resolution which were unavailable in the past. Yet, the necessary validation of numerical results represents a difficult task, as the experimental data in a sufficient spatial resolution are hardly available. This article introduces a method based on positron emission tomography, which allows acquisition of detailed experimental data on local aerosol deposition in a realistic model of human lungs. The method utilizes the Condensation Monodisperse Aerosol Generator modified for a safe production of radioactive aerosol particles and a special measuring rig. The scanning of the model is performed on a positron emission tomography–computed tomography scanner. The evaluation of aerosol deposition is based on a volume radioactivity analysis in a specialized, yet publicly available software. The reliability of the method was tested and its first results are discussed in the article. The measurements performed using the presented method can serve for validation of numerical simulations, since the presented lung model digital geometry is available.

Validating CFD Predictions of Pharmaceutical Aerosol Deposition with In Vivo Data

PURPOSE

CFD provides a powerful approach to evaluate the deposition of pharmaceutical aerosols; however, previous studies have not compared CFD results of deposition throughout the lungs with in vivo data.

METHODS

The in vivo datasets selected for comparison with CFD predictions included fast and slow clearance of monodisperse aerosols as well as 2D gamma scintigraphy measurements for a dry powder inhaler (DPI) and softmist inhaler (SMI). The CFD model included the inhaler, a characteristic model of the mouth-throat (MT) and upper tracheobronchial (TB) airways, stochastic individual pathways (SIPs) representing the remaining TB region, and recent CFD-based correlations to predict pharmaceutical aerosol deposition in the alveolar airways.

RESULTS

For the monodisperse aerosol, CFD predictions of total lung deposition agreed with in vivo data providing a percent relative error of 6% averaged across aerosol sizes of 1-7 μm. With the DPI and SMI, deposition was evaluated in the MT, central airways (bifurcations B1-B7), and intermediate plus peripheral airways (B8 through alveoli). Across these regions, CFD predictions produced an average relative error <10% for each inhaler.

CONCLUSIONS

CFD simulations with the SIP modeling approach were shown to accurately predict regional deposition throughout the lungs for multiple aerosol types and different in vivo assessment methods.

Mechanisms of pharmaceutical aerosol deposition in the respiratory tract

Aerosol delivery is noninvasive and is effective in much lower doses than required for oral administration. Currently, there are several types of therapeutic aerosol delivery systems, including the pressurized metered-dose inhaler, the dry powder inhaler, the medical nebulizer, the solution mist inhaler, and the nasal sprays. Both oral and nasal inhalation routes are used for the delivery of therapeutic aerosols. Following inhalation therapy, only a fraction of the dose reaches the expected target area. Knowledge of the amount of drug actually deposited is essential in designing the delivery system or devices to optimize the delivery efficiency to the targeted region of the respiratory tract. Aerosol deposition mechanisms in the human respiratory tract have been well studied. Prediction of pharmaceutical aerosol deposition using established lung deposition models has limited success primarily because they underestimated oropharyngeal deposition. Recent studies of oropharyngeal deposition of several drug delivery systems identify other factors associated with the delivery system that dominates the transport and deposition of the oropharyngeal region. Computational fluid dynamic simulation of the aerosol transport and deposition in the respiratory tract has provided important insight into these processes. Investigation of nasal spray deposition mechanisms is also discussed.

Creation of an in vitro biomechanical model of the trachea using rapid prototyping

Previous in vitro models of the airways are either rigid or, if flexible, have not matched in vivo compliance characteristics. Rapid prototyping provides a quickly evolving approach that can be used to directly produce in vitro airway models using either rigid or flexible polymers. The objective of this study was to use rapid prototyping to directly produce a flexible hollow model that matches the biomechanical compliance of the trachea. The airway model consisted of a previously developed characteristic mouth-throat region, the trachea, and a portion of the main bronchi. Compliance of the tracheal region was known from a previous in vivo imaging study that reported cross-sectional areas over a range of internal pressures. The compliance of the tracheal region was matched to the in vivo data for a specific flexible resin by iteratively selecting the thicknesses and other dimensions of tracheal wall components. Seven iterative models were produced and illustrated highly non-linear expansion consisting of initial rapid size increase, a transition region, and continued slower size increase as pressure was increased. Thickness of the esophageal interface membrane and initial trachea indention were identified as key parameters with the final model correctly predicting all phases of expansion within a value of 5% of the in vivo data. Applications of the current biomechanical model are related to endotracheal intubation and include determination of effective mucus suctioning and evaluation of cuff sealing with respect to gases and secretions.

Targeting aerosol deposition to and within the lung airways using excipient enhanced growth

BACKGROUND

Previous studies have characterized the size increase of combination submicrometer particles composed of a drug and hygroscopic excipient when exposed to typical airway thermodynamic conditions. The objective of this study was to determine the deposition and size increase characteristics of excipient enhanced growth (EEG) aerosols throughout the tracheobronchial (TB) airways and to evaluate the potential for targeted delivery.

METHODS

Submicrometer particles composed of a poorly water-soluble drug (insulin) and hygroscopic excipient (sodium chloride) were considered at drug:excipient mass ratios of 50:50 and 25:75. A previously validated computational fluid dynamics model was used to predict aerosol size increase and deposition in characteristic geometries of the mouth-throat (MT), upper TB airways through the third bifurcation (B3), and remaining TB airways through B15. Additional validation experiments were also performed for albuterol sulfate:mannitol particles. Both growth of combination particles and deposition are reported throughout the conducting airways for characteristic slow and deep (SD) and quick and deep (QD) inhalations.

RESULTS

For all EEG cases considered, MT deposition was less than 1% of the drug dose, which is at least one order of magnitude lower than with state-of-the-art and conventional inhalers. Final aerosol sizes exiting the TB region and entering the alveolar airways were all greater than 3 μm. For SD inhalation, deposition fractions of 20% were achieved in the lower TB region of B8-B15, which is a factor of 20-30×higher than conventional delivery devices. With QD inhalation, maximum alveolar delivery of 90% was observed.

CONCLUSIONS

Increasing the dose delivered to the lower TB region by a factor of 20-30×or achieving 90% delivery to the alveolar airways was considered effective aerosol targeting compared with conventional devices. The trend of higher flow rates resulting in better alveolar delivery of aerosols is unique to EEG and may be used to design highly efficient dry powder inhalers.

Multiscale image-based modeling and simulation of gas flow and particle transport in the human lungs

Improved understanding of structure and function relationships in the human lungs in individuals and subpopulations is fundamentally important to the future of pulmonary medicine. Image-based measures of the lungs can provide sensitive indicators of localized features, however to provide a better prediction of lung response to disease, treatment, and environment, it is desirable to integrate quantifiable regional features from imaging with associated value-added high-level modeling. With this objective in mind, recent advances in computational fluid dynamics (CFD) of the bronchial airways-from a single bifurcation symmetric model to a multiscale image-based subject-specific lung model-will be reviewed. The interaction of CFD models with local parenchymal tissue expansion-assessed by image registration-allows new understanding of the interplay between environment, hot spots where inhaled aerosols could accumulate, and inflammation. To bridge ventilation function with image-derived central airway structure in CFD, an airway geometrical modeling method that spans from the model 'entrance' to the terminal bronchioles will be introduced. Finally, the effects of turbulent flows and CFD turbulence models on aerosol transport and deposition will be discussed.

Comparing MDI and DPI aerosol deposition using in vitro experiments and a new stochastic individual path (SIP) model of the conducting airways

PURPOSE

Deposition characteristics of MDI and DPI aerosols were compared throughout the conducting airways for the first time using a combination of in vitro experiments and a newly developed stochastic individual path (SIP) model for different inhalation profiles.

METHODS

In vitro experiments were used to determine initial particle distribution profiles and to validate computational fluid dynamics (CFD) model results for a MDI and DPI delivering the same dose of drug in a geometry of the mouth-throat and tracheobronchial airways. The validated CFD model was then used to predict the transport and deposition of the drug using correct and incorrect inhalation profiles for each inhaler.

RESULTS

The MDI delivered approximately two times more drug to the tracheobronchial region compared with the DPI for both correct and incorrect inhalation profiles. Errors in inhalation reduced the deposited tracheobronchial dose by approximately 30% for both inhalers. The DPI delivered the largest dose to the mouth-throat (~70%) and the MDI delivered the largest dose to the alveolar airways (~50%).

CONCLUSIONS

The developed in silico model provides new insights into the lung delivery of pharmaceutical aerosols and can be applied in future studies in combination with pharmacokinetic analysis to establish bioequivalence between devices.

Performance of combination drug and hygroscopic excipient submicrometer particles from a softmist inhaler in a characteristic model of the airways

Excipient enhanced growth (EEG) of inhaled submicrometer pharmaceutical aerosols is a recently proposed method intended to significantly reduce extrathoracic deposition and improve lung delivery. The objective of this study was to evaluate the size increase of combination drug and hygroscopic excipient particles in a characteristic model of the airways during inhalation using both in vitro experiments and computational fluid dynamic (CFD) simulations. The airway model included a characteristic mouth-throat (MT) and upper tracheobronchial (TB) region through the third bifurcation and was enclosed in a chamber geometry used to simulate the thermodynamic conditions of the lungs. Both in vitro results and CFD simulations were in close agreement and indicated that EEG delivery of combination submicrometer particles could nearly eliminate MT deposition for inhaled pharmaceutical aerosols. Compared with current inhalers, the proposed delivery approach represents a 1-2 order of magnitude reduction in MT deposition. Transient inhalation was found to influence the final size of the aerosol based on changes in residence times and relative humidity values. Aerosol sizes following EEG when exiting the chamber (2.75-4.61 μm) for all cases of initial submicrometer combination particles were equivalent to or larger than many conventional pharmaceutical aerosols that frequently have MMADs in the range of 2-3 μm.

Characterization of respiratory drug delivery with enhanced condensational growth using an individual path model of the entire tracheobronchial airways

The objective of this study was to evaluate the delivery of inhaled pharmaceutical aerosols using an enhanced condensational growth (ECG) approach in an airway model extending from the oral cavity to the end of the tracheobronchial (TB) region. The geometry consisted of an elliptical mouth-throat (MT) model, the upper TB airways extending to bifurcation B3, and a subsequent individual path model entering the right lower lobe of the lung. Submicrometer monodisperse aerosols with diameters of 560 and 900 nm were delivered to the mouth inlet under control (25 °C with subsaturated air) or ECG (39 or 42 °C with saturated air) conditions. Flow fields and droplet characteristics were simulated using a computational fluid dynamics model that was previously demonstrated to accurately predict aerosol size growth and deposition. Results indicated that both the control and ECG delivery cases produced very little deposition in the MT and upper TB model (approximately 1%). Under ECG delivery conditions, large size increases of the aerosol droplets were observed resulting in mass median aerodynamic diameters of 2.4-3.3 μm exiting B5. This increase in aerosol size produced an order of magnitude increase in aerosol deposition within the TB airways compared with the controls, with TB deposition efficiencies of approximately 32-46% for ECG conditions. Estimates of downstream pulmonary deposition indicted near full lung retention of the aerosol during ECG delivery. Furthermore, targeting the region of TB deposition by controlling the inlet temperature conditions and initial aerosol size also appeared possible.

Kinetics of satratoxin g tissue distribution and excretion following intranasal exposure in the mouse

Intranasal exposure of mice to satratoxin G (SG), a macrocyclic trichothecene produced by the indoor air mold Stachybotrys chartarum, selectively induces apoptosis in olfactory sensory neurons (OSNs) of the nose and brain. The purpose of this study was to measure the kinetics of distribution and clearance of SG in the mouse. Following intranasal instillation of female C57B16 mice with SG (500 microg/kg bw), the toxin was detectable from 5 to 60 min in blood and plasma, with the highest concentrations, 30 and 19 ng/ml, respectively, being observed at 5 min. SG clearance from plasma was rapid and followed single-compartment kinetics (t(1/2) = 20 min) and differed markedly from that of other tissues. SG concentrations were maximal at 15-30 min in nasal turbinates (480 ng/g), kidney (280 ng/g), lung (250 ng/g), spleen (200 ng/g), liver (140 ng/g), thymus (90 ng/g), heart (70 ng/g), olfactory bulb (14 ng/g), and brain (3 ng/g). The half-lives of SG in the nasal turbinate and thymus were 7.6 and 10.1 h, respectively, whereas in other organs, these ranged from 2.3 to 4.4 h. SG was detectable in feces and urine, but cumulative excretion over 5 days via these routes accounted for less than 0.3% of the total dose administered. Taken together, SG was rapidly taken up from the nose, distributed to tissues involved in respiratory, immune, and neuronal function, and subsequently cleared. However, a significant amount of the toxin was retained in the nasal turbinate, which might contribute to SG's capacity to evoke OSN death.

Effects of generation time on spray aerosol transport and deposition in models of the mouth-throat geometry

BACKGROUND

For most newly developed spray aerosol inhalers, the generation time is a potentially important variable that can be fully controlled. The objective of this study was to determine the effects of spray aerosol generation time on transport and deposition in a standard induction port (IP) and more realistic mouth-throat (MT) geometry.

METHODS

Capillary aerosol generation (CAG) was selected as a representative system in which spray momentum was expected to significantly impact deposition. Sectional and total depositions in the IP and MT geometries were assessed at a constant CAG flow rate of 25 mg/sec for aerosol generation times of 1, 2, and 4 sec using both in vitro experiments and a previously developed computational fluid dynamics (CFD) model.

RESULTS AND CONCLUSIONS

Both the in vitro and numerical results indicated that extending the generation time of the spray aerosol, delivered at a constant mass flow rate, significantly reduced deposition in the IP and more realistic MT geometry. Specifically, increasing the generation time of the CAG system from 1 to 4 sec reduced the deposition fraction in the IP and MT geometries by approximately 60 and 33%, respectively. Furthermore, the CFD predictions of deposition fraction were found to be in good agreement with the in vitro results for all times considered in both the IP and MT geometries. The numerical results indicated that the reduction in deposition fraction over time was associated with temporal dissipation of what was termed the spray aerosol "burst effect." Based on these results, increasing the spray aerosol generation time, at a constant mass flow rate, may be an effective strategy for reducing deposition in the standard IP and in more realistic MT geometries.

Evaluation of the Respimat Soft Mist Inhaler using a concurrent CFD and in vitro approach

BACKGROUND

The Respimat Soft Mist Inhaler is reported to generate an aerosol with low spray momentum and a small droplet size. However, the transport characteristics of the Respimat aerosol are not well understood. The objective of this study was to characterize the transport and deposition of an aerosol emitted from the Respimat inhaler using a combination of computational fluid dynamics (CFD) modeling and in vitro experiments.

METHODS

Deposition of the Respimat aerosol was assessed in the inhaler mouthpiece (MP), a standard induction port (IP), and a more realistic mouth-throat (MT) geometry at an inhalation flow rate of 30 L/min. Aerosols were generated using an albuterol sulfate (0.6%) solution, and the drug deposition was quantified using both in vitro experiments and a CFD model of the Respimat inhaler. Laser diffraction experiments were used to determine the initial polydisperse aerosol size distribution.

RESULTS AND CONCLUSIONS

It was found that the aerosol generated from the highly complex process of jet collision and breakup could be approximated in the model using effective spray conditions. Computational predictions of deposition fractions agreed well with in vitro results for both the IP (within 20% error) and MT (within 10% error) geometries. The experimental results indicated that the deposition fraction of drug in the MP ranged from 27 to 29% and accounted for a majority of total drug loss. Based on the CFD solution, high MP deposition was due to a recirculating flow pattern that surrounded the aerosol spray and entrained a significant number of small droplets. In contrast, deposition of the Respimat aerosol in both the IP (4.2%) and MT (7.4%) geometries was relatively low. Results of this study indicate that modifications to the current Respimat MP and control of specific patient variables may significantly reduce deposition in the device and may decrease high oropharyngeal drug loss observed in vivo.

Transport and Uptake of MTBE and Ethanol Vapors in a Human Upper Airway Model

Potential human exposure to vapors of methyl tertiary-butyl ether (MTBE) and ethanol is of increasing concern because these materials are widely used as gasoline additives. In this study we analyzed numerically the transport and deposition of MTBE and ethanol vapors in a model of the human upper respiratory airway, consisting of an oral airway and the first four generations of the tracheobronchial tree. Airflow characteristics and mass transfer processes were analyzed at different inspiratory flow conditions using a three-dimensional computational fluid and particle dynamics method. The deposition data were analyzed in terms of regional deposition fractions (DF = regional uptake/mouth concentration) and deposition enhancement factors (DEF = local DF/average DF) at local micro surface areas. Results show that DF in the entire upper airway model is 21.9%, 12.4%, and 6.9% for MTBE and 67.5%, 51.5%, and 38.5% for ethanol at a flow rate of 15, 30, and 60 L/min, respectively. Of the total DF, 65-70% is deposited in the oral airway for both vapors. Deposition is localized at various sites within the upper airway structure, with a maximum DEF of 1.5 for MTBE and 7.8 for ethanol. Local deposition patterns did not change with inhalation conditions, but DF and the maximum DEF increased with diffusivity, solubility, and the degree of airway wall absorption of vapors, as shown by a greater deposition of ethanol than MTBE. The vapor deposition efficiency as expressed by the dimensionless mass transfer coefficient correlated well with a product of Reynolds (Re) and Schmidt (Sc) numbers. In conclusion, MTBE and ethanol vapors deposit substantially in the upper airway structure with a marked enhancement of dose at local sites, and the deposition dose may be reasonably estimated by a functional relationship with dimensionless fluid flow and diffusion parameters.

Airflow and nanoparticle deposition in a 16-generation tracheobronchial airway model

In order to achieve both manageable simulation and local accuracy of airflow and nanoparticle deposition in a representative human tracheobronchial (TB) region, the complex airway network was decomposed into adjustable triple-bifurcation units, spreading axially and laterally. Given Q(in) = 15 and 30 L/min and a realistic inlet velocity profile, the experimentally validated computer simulation model provided some interesting 3-D airflow patterns, i.e., for each TB-unit they depend on the upstream condition, local geometry and local Reynolds number. Directly coupled to the local airflow fields are the convective-diffusive transport and deposition of nanoparticles, i.e., 1 nm < or = d(p) < or = 100 nm. The CFD modeling predictions were compared to experimental observations as well as analytical modeling results. The CFD-simulated TB deposition values agree astonishingly well with analytical modeling results. However, measurable differences can be observed for bifurcation-by-bifurcation deposition fractions obtained with these two different approaches due to the effects of more realistic inlet conditions and geometric features incorporated in the CFD model. Specifically, while the difference between the total TB deposition fraction (DF) is less than 16%, it may be up to 70% for bifurcation-by-bifurcation DFs. In addition, it was found that fully developed flow and uniform nanoparticle concentrations can be assumed beyond generation G12. For nanoparticles with d(p) > 10 nm, the geometric effects, including daughter-branch rotation, are minor. Furthermore, the deposition efficiencies at each individual bifurcation in the TB region can be well correlated as a function of an effective diffusion parameter.

Modeling airflow and particle transport/deposition in pulmonary airways

A review of research papers is presented, pertinent to computer modeling of airflow as well as nano- and micron-size particle deposition in pulmonary airway replicas. The key modeling steps are outlined, including construction of suitable airway geometries, mathematical description of the air-particle transport phenomena and computer simulation of micron and nanoparticle depositions. Specifically, diffusion-dominated nanomaterial deposits on airway surfaces much more uniformly than micron particles of the same material. This may imply different toxicity effects. Due to impaction and secondary flows, micron particles tend to accumulate around the carinal ridges and to form "hot spots", i.e., locally high concentrations which may lead to tumor developments. Inhaled particles in the size range of 20nm< or =dp< or =3microm may readily reach the deeper lung region. Concerning inhaled therapeutic particles, optimal parameters for mechanical drug-aerosol targeting of predetermined lung areas can be computed, given representative pulmonary airways.

Effects of Oral Airway Geometry Characteristics on the Diffusional Deposition of Inhaled Nanoparticles

The deposition of ultrafine aerosols in the respiratory tract presents a significant health risk due to the increased cellular-level response that these particles may invoke. However, the effects of geometric simplifications on local and regional nanoparticle depositions remain unknown for the oral airway and throughout the respiratory tract. The objective of this study is to assess the effects of geometric simplifications on diffusional transport and deposition characteristics of inhaled ultrafine aerosols in models of the extrathoracic oral airway. A realistic model of the oral airway with the nasopharynx (NP) included has been constructed based on computed tomography scans of a healthy adult in conjunction with measurements reported in the literature. Three other geometries with descending degrees of physical realism were then constructed with successive geometric simplifications of the realistic model. A validated low Reynolds number k-ω turbulence model was employed to simulate laminar, transitional, and fully turbulent flow regimes for the transport of 1–200 nm particles. Results of this study indicate that the geometric simplifications considered did not significantly affect the total deposition efficiency or maximum local deposition enhancement of nanoparticles. However, particle transport dynamics and the underlying flow characteristics such as separation, turbulence intensity, and secondary motions did show an observable sensitivity to the geometric complexity. The orientation of the upper trachea was shown to be a major factor determining local deposition downstream of the glottis and should be retained in future models of the respiratory tract. In contrast, retaining the NP produced negligible variations in airway dynamics and could be excluded for predominantly oral breathing conditions. Results of this study corroborate the use of existing diffusion correlations based on a circular oral airway model. In comparison to previous studies, an improved correlation for the deposition of nanoparticles was developed based on a wider range of particle sizes and flow rates, which captures the dependence of the Sherwood number on both Reynolds and Schmidt numbers.

Stachybotrys chartarum, trichothecene mycotoxins, and damp building-related illness: new insights into a public health enigma

Damp building-related illnesses (DBRI) include a myriad of respiratory, immunologic, and neurologic symptoms that are sometimes etiologically linked to aberrant indoor growth of the toxic black mold, Stachybotrys chartarum. Although supportive evidence for such linkages is limited, there are exciting new findings about this enigmatic organism relative to its environmental dissemination, novel bioactive components, unique cellular targets, and molecular mechanisms of action which provide insight into the S. chartarum's potential to evoke allergic sensitization, inflammation, and cytotoxicity in the upper and lower respiratory tracts. Macrocyclic trichothecene mycotoxins, produced by one chemotype of this fungus, are potent translational inhibitors and stress kinase activators that appear to be a critical underlying cause for a number of adverse effects. Notably, these toxins form covalent protein adducts in vitro and in vivo and, furthermore, cause neurotoxicity and inflammation in the nose and brain of the mouse. A second S. chartarum chemotype has recently been shown to produce atranones-mycotoxins that can induce pulmonary inflammation. Other biologically active products of this fungus that might contribute to pathophysiologic effects include proteinases, hemolysins, beta-glucan, and spirocyclic drimanes. Solving the enigma of whether Stachybotrys inhalation indeed contributes to DBRI will require studies of the pathophysiologic effects of low dose chronic exposure to well-characterized, standardized preparations of S. chartarum spores and mycelial fragments, and, coexposures with other environmental cofactors. Such studies must be linked to improved assessments of human exposure to this fungus and its bioactive constituents in indoor air using both state-of-the-art sampling/analytical methods and relevant biomarkers.

Computational analyses of a pressurized metered dose inhaler and a new drug-aerosol targeting methodology

The popular pressurized metered dose inhaler (pMDI), especially for asthma treatment, has undergone various changes in terms of propellant use and valve design. Most significant are the choice of hydrofluoroalkane-134a (HFA-134a) as a new propellant (rather than chlorofluorocarbon, CFC), a smaller exit nozzle diameter and attachment of a spacer in order to reduce ultimately droplet size and spray inhalation speed, both contributing to higher deposition efficiencies and hence better asthma therapy. Although asthma medicine is rather inexpensive, the specter of systemic side effects triggered by inefficient pMDI performance and the increasing use of such devices as well as new targeted drug-aerosol delivery for various lung and other diseases make detailed performance analyses imperative. For the first time, experimentally validated computational fluid-particle dynamics technique has been applied to simulate airflow, droplet spray transport and aerosol deposition in a pMDI attached to a human upper airway model, considering different device propellants, nozzle diameters, and spacer use. The results indicate that the use of HFA (replacing CFC), smaller valve orifices (0.25 mm instead of 0.5 mm) and spacers (ID = 4.2 cm) leads to best performance mainly because of smaller droplets generated, which penetrate more readily into the bronchial airways. Experimentally validated computer simulations predict that 46.6% of the inhaled droplets may reach the lung for an HFA-pMDI and 23.2% for a CFC-pMDI, both with a nozzle-exit diameter of 0.25 mm. Commonly used inhalers are nondirectional, and at best only regional drug-aerosol deposition can be achieved. However, when inhaling expensive and aggressive medicine, or critical lung areas have to be reached, locally targeted drug-aerosol delivery is imperative. For that reason the underlying principle of a future line of "smart inhalers" is introduced. Specifically, by generating a controlled air-particle stream, most of the inhaled drug aerosols reach predetermined lung sites, which are associated with specific diseases and/or treatments. Using the same human upper airway model, experimentally confirmed computer predictions of controlled particle transport from mouth to generation 3 are provided.

In vivo-in vitro comparison of deposition in three mouth-throat models with Qvar and Turbuhaler inhalers

In vitro polydisperse aerosol deposition in three mouth-throat models, namely, the USP (United States Pharmacopeia) mouth-throat (induction port), idealized mouth-throat, and highly idealized mouth-throat, was investigated experimentally. Aerosol particles emitted from two commercial inhalers, Qvar (pMDI) and Turbuhaler (DPI), were used. The in vitro deposition results in these three mouth-throat models were compared with in vivo data available from the literature. For the DPI, mouth-throat deposition was 57.3 +/- 4.5% for the USP mouth-throat, 67.8 +/- 2.2% for the idealized mouth-throat, and 69.3 +/- 1.1% for the highly idealized mouth-throat, which are all relatively close to the in vivo value of 65.8 +/- 10.1%. In contrast, for the pMDI, aerosol deposition in the idealized mouth-throat (25.8 +/- 4.2%) and the highly idealized mouth-throat (24.9 +/- 2.8%) agrees with the in vivo data (29.0 +/- 18.0%) reported in the literature better than that for the USP mouth-throat (12.2 +/- 2.7%). In both cases, the USP mouth-throat gives the lowest deposition among the three mouth-throat models studied. In summary, both the idealized mouth-throat and highly idealized mouth-throat improve the accuracy of predicted mean in vivo deposition in the mouth-throat region. This result hints at the potential applicability of either the idealized mouth-throat or highly idealized mouth-throat as a future USP mouth-throat standard to provide mean value prediction of in vivo mouth-throat deposition.

Transport and Deposition of Micro-Aerosols in Realistic and Simplified Models of the Oral Airway

A number of in vivo, in vitro and numerical studies have considered flow field characteristics and micro-particle deposition in the oral airway extending from the mouth through the larynx. These studies have highlighted the effects of flow rates, turbulence and particle characteristics on deposition values in realistic and simplified geometries. However, the effect of geometry simplifications on regional and local deposition patterns remains largely un-quantified for the oral airway and throughout the respiratory tract. The objective of this study is to assess the effects of geometry simplifications on regionally averaged and local micro-aerosol deposition characteristics in models of the extrathoracic oral airway. To achieve this objective, a realistic model of the oral airway has been constructed based on CT scans of a healthy adult in conjunction with measurements reported in the literature. Three other geometries with descending degrees of physical realism were constructed based on successive geometric simplifications of the realistic model. A validated low Reynolds number (LRN) k-omega turbulence model was employed to simulate laminar, transitional and fully turbulent flow regimes for 1-31 microm particles. Geometric simplifications were found to have a significant effect on aerosol dynamics, hot spot formations and cellular-level deposition values in the extrathoracic airway models considered. For all models, regional deposition efficiency results were found to be approximately within one standard deviation of available experimental data when plotted as a function of Stokes number. The realistic geometry provided the best predictions of regional deposition in comparison to experimental data as a function of particle diameter. Considering localized deposition, maximum deposition enhancement factors, which represent the ratio of local to total deposition, were one to two orders of magnitude higher for the realistic model. Geometric factors that significantly contributed to enhanced particle localization in the realistic model include a triangular-shaped glottis and a dorsal-sloped trachea. Therefore, highly realistic models of the oral airway geometry may be necessary to evaluate localized deposition patterns and hot spot formations, which are critical for accurately predicting cellular-level dose.