Accuracy and resolution power of aerosol-derived airway morphometry in a simple lung model

Regional Lung Deposition: In Vivo Data

The method section of this chapter on in vivo regional lung deposition highlights a nonradioactive method to measure regional deposition, which uses a photometer to quantify inhaled and exhaled particles and in that way is able to estimate the lung region from which the particles are exhaled and to what amount. The radioactive methods cover the measurement of clearance of the deposited particles as well as different imaging techniques to determine regional deposition. The result section reviews in vivo trials in human subjects. It also addresses different parameters that influence the regional deposition in the lungs: particle size, inhalation maneuver, carrier gas, disease, and inhalation device. All of these factors can affect regional deposition significantly. By choosing specific values of these parameters, it should be feasible to target different regions of the lungs for the therapy of different diseases.

Charting the human respiratory tract with airborne nanoparticles: evaluation of the Airspace Dimension Assessment technique

Airspace Dimension Assessment (AiDA) is a technique to assess lung morphology by measuring lung deposition of inhaled nanoparticles. Nanoparticles deposit in the lungs predominately by diffusion, and average diffusion distances, corresponding to effective airspace radii ( rAiDA), can be inferred from measurements of particle recovery after varied breath holds. Also, particle recovery after a 0-s breath hold ( R0) may hold information about the small conducting airways. This study investigates rAiDA at different volumetric sample depths in the lungs of healthy subjects. Measurements were performed with 50-nm polystyrene nanospheres on 19 healthy subjects aged 17–67 yr. Volumetric sample depths ranged from 200 to 5,000 ml and breath-hold times from 5 to 20 s. At the examined volumetric sample depths, rAiDA values ranged from ~200–600 μm, which correspond to dimensions of the bronchiolar and the gas-exchanging regions of the lungs. R0 decreased with volumetric sample depth and showed more intersubject variation than rAiDA. Correlations were found between the AiDA parameters, anthropometry, and lung function tests, but not between rAiDA and R0. For repeated measurements on 3 subjects over an 18-mo period, rAiDA varied on average within ± 7 μm (± 2.4%). The results indicate that AiDA has potential as an efficient new in vivo technique to assess individual lung properties. The information obtained by such measurements may be of value for lung diagnostics, especially for the distal lungs, which are challenging to examine directly by other means.

NEW & NOTEWORTHY This is the first study to measure effective airspace radii ( rAiDA) at volumetric sample depths 200–5,000 ml in healthy subjects by Airspace Dimension Assessment (AiDA). Observed rAiDA were 200–600 μm, which corresponds to airspaces for the bronchiolar and the gas-exchanging regions around airway generation 14–17. rAiDA correlated with lung function tests and anthropometry. Measurements of rAiDA on 3 subjects over 11–18 mo were within ± 7 μm.

Development of an infant complete-airway in vitro model for evaluating aerosol deposition

A complete-airway in vitro model would be very useful for toxicological dosimetry testing and for developing targeted inhaled medications in cases where conducting in vivo experiments are exceedingly difficult, as with infants. The objective of this study was to determine whether packed bed in vitro models, which contain spheres as the primary repeating unit, provide a realistic representation of aerosol deposition in the tracheobronchial region of infant lungs based on computational fluid dynamics (CFD) predictions. The packed bed (PB) CFD model contained an inlet consistent with airway bifurcation B3 (∼lobar bronchi) leading to a spherical array with voids between the spheres forming a divided flow pathway. The hydrodynamic diameter of the voids was approximately matched to the diameter of bifurcations in various lung regions. For comparison, a CFD stochastic individual pathway (SIP) geometry with realistic bifurcations extending from B4-B15 (terminal bronchioles) was selected as an anatomically accurate model. The CFD-SIP model predictions were benchmarked with existing algebraic correlations for aerosol deposition in the lungs and found to be reasonable. Unfortunately, the CFD-PB model did not provide a good representation of aerosol deposition in the tracheobronchial region of human lungs. Through careful selection of the PB sphere size and inlet conditions, total deposition in the CFD-PB model matched CFD-SIP deposition within 10% absolute error across a range of relevant aerosol sizes. However, regional deposition within the CFD-PB model was very different from the CFD-SIP case. Therefore, the PB approach cannot be recommended for determining spatial or temporal distribution of aerosol transport and impaction deposition through the lungs.

Do nanoparticles provide a new opportunity for diagnosis of distal airspace disease?

There is a need for efficient techniques to assess abnormalities in the peripheral regions of the lungs, for example, for diagnosis of pulmonary emphysema. Considerable scientific efforts have been directed toward measuring lung morphology by studying recovery of inhaled micron-sized aerosol particles (0.4-1.5 µm). In contrast, it is suggested that the recovery of inhaled airborne nanoparticles may be more useful for diagnosis. The objective of this work is to provide a theoretical background for the use of nanoparticles in measuring lung morphology and to assess their applicability based on a review of the literature. Using nanoparticles for studying distal airspace dimensions is shown to have several advantages over other aerosol-based methods. 1) Nanoparticles deposit almost exclusively by diffusion, which allows a simpler breathing maneuver with minor artifacts from particle losses in the oropharyngeal and upper airways. 2) A higher breathing flow rate can be utilized, making it possible to rapidly inhale from residual volume to total lung capacity (TLC), thereby eliminating the need to determine the TLC before measurement. 3) Recent studies indicate better penetration of nanoparticles than micron-sized particles into poorly ventilated and diseased regions of the lungs; thus, a stronger signal from the abnormal parts is expected. 4) Changes in airspace dimensions have a larger impact on the recovery of nanoparticles. Compared to current diagnostic techniques with high specificity for morphometric changes of the lungs, computed tomography and magnetic resonance imaging with hyperpolarized gases, an aerosol-based method is likely to be less time consuming, considerably cheaper, simpler to use, and easier to interpret (providing a single value rather than an image that has to be analyzed). Compared to diagnosis by carbon monoxide (DL,CO), the uptake of nanoparticles in the lung is not affected by blood flow, hemoglobin concentration or alterations of the alveolar membranes, but relies only on lung morphology.

A new method for measuring lung deposition efficiency of airborne nanoparticles in a single breath

Assessment of respiratory tract deposition of nanoparticles is a key link to understanding their health impacts. An instrument was developed to measure respiratory tract deposition of nanoparticles in a single breath. Monodisperse nanoparticles are generated, inhaled and sampled from a determined volumetric lung depth after a controlled residence time in the lung. The instrument was characterized for sensitivity to inter-subject variability, particle size (22, 50, 75 and 100 nm) and breath-holding time (3–20 s) in a group of seven healthy subjects. The measured particle recovery had an inter-subject variability 26–50 times larger than the measurement uncertainty and the results for various particle sizes and breath-holding times were in accordance with the theory for Brownian diffusion and values calculated from the Multiple-Path Particle Dosimetry model. The recovery was found to be determined by residence time and particle size, while respiratory flow-rate had minor importance in the studied range 1–10 L/s. The instrument will be used to investigate deposition of nanoparticles in patients with respiratory disease. The fast and precise measurement allows for both diagnostic applications, where the disease may be identified based on particle recovery, and for studies with controlled delivery of aerosol-based nanomedicine to specific regions of the lungs.

Can aerosol-derived airway morphometry detect early, asymptomatical lung emphysema?

The aerosol-derived airway morphometry technique (ADAM) can be used to assess non-invasively peripheral airspace dimensions. It has been shown that this technique can identify permanent peripheral airspace enlargement in patients with lung emphysema, but it is yet unknown if early stages of emphysema can be detected. In this study, 89 aluminum welders were investigated. Although all (except two subjects) showed normal spirometry, in 29% of the subjects visual signs of early emphysema were observed with high-resolution computed tomography (HRCT) in a previous study. Using the ADAM technique, 28% of the subjects showed increased peripheral airspace dimensions. However, both groups with positive findings overlapped only in about half of the cases. Peripheral airspace dimensions correlated significantly with the mean lung density calculated from the HRCT scans, and lung density was significantly decreased in the group with increased airspace dimensions. The poor overlap of the positive findings observed with both techniques can be explained if it is considered that the visual HRCT technique and ADAM focus on different aspects of emphysematous changes in the lungs. Whereas visual HRCT is a powerful tool to identify focal changes in lung density but cannot detect mild homogeneous emphysema, ADAM delivers a measure for homogeneously distributed emphysema but cannot detect focal emphysema or regions with emphysema which are badly ventilated. Since ADAM is easy to perform, non-invasive, and can be repeatedly applied to human subjects without radiological concerns, this technique might become a useful tool for the detection and monitoring of lung emphysema in occupational medicine, epidemiology, and pharmaceutics.

Aerosol-derived airway morphometry and aerosol bolus dispersion in patients with lung fibrosis and lung emphysema

OBJECTIVE

Patients with lung emphysema show increased aerosol-derived dimensions of peripheral airspaces and increased aerosol bolus dispersion (AD). To apply these tests in epidemiologic studies, the objective of this pilot study was to investigate whether morphometric changes caused by lung fibrosis can be distinguished from those caused by emphysema.

DESIGN

This study was designed as a cross-sectional study in which airspace dimensions and AD in patients with emphysema and in patients with fibrosis were compared. Forty patients participated in the study: 20 patients had high-resolution CT (HRCT)-proved lung emphysema and 20 patients had HRCT-proved lung fibrosis. All patients underwent conventional lung function tests, aerosol-derived airway morphometry (ADAM), and AD measurements.

RESULTS

Patients with lung emphysema showed normal dimensions of small airways but enlarged airspace dimensions in the lung periphery. Patients with fibrosis showed in all lung depths increased airspace dimensions. AD was increased in patients with emphysema but was normal in patients with fibrosis.

CONCLUSIONS

These results show that when using ADAM and AD, morphometric changes caused by emphysema can be distinguished from those caused by fibrosis with high sensitivity and specificity.