Lung volume is a determinant of aerosol bolus dispersion

Comparison of Aerosol Emissions during Specific Speech Tasks

OBJECTIVES/HYPOTHESIS

Recent investigations into the behavior of aerosolized emissions from the oral cavity have shown that particulate emissions do indeed occur during speech. To date, there is little information about the relative contribution of different speech sounds in producing particle emissions in a free field. This study compares airborne aerosol generation in participants producing isolated speech sounds: fricative consonants, plosive consonants, and vowel sounds.

STUDY DESIGN

Prospective, reversal experimental design, where each participant served as their own control and all participants were exposed to all stimuli.

METHODS

While participants produced isolated speech tasks, a planar beam of laser light, a high-speed camera, and image software calculated the number of particulates detected over time. This study compared airborne aerosols emitted by human participants at a distance of 2.54 cm between the laser sheet and the mouth.

RESULTS

Statistically significant increases in particulate count over ambient dust distribution for all speech sounds. When collapsed across loudness levels, emitted particles in vowel sounds were statistically greater than consonants, suggesting that mouth opening, as opposed to the place of vocal tract constriction or manner of sound production, might also be influential in the degree to which particulates become aerosolized during speech.

CONCLUSIONS

The results of this research will inform boundary conditions for computational models of aerosolized particulates during speech.

Theoretical diagnosis of emphysema by aerosol bolus inhalation

BACKGROUND

The present contribution deals with the theoretical description of aerosol bolus dispersion in lungs being affected by different manifestations of emphysema. The work constructs the hypothesis that each manifestation of emphysema exhibits specific properties with regard to the dispersion of inhaled and exhaled aerosol boluses as well as the deposition of particles from the aerosol pulse.

METHODS

For an appropriate simulation of single emphysematous manifestations, a previously developed model assuming (I) a random variation of alveolar diameters, (II) an exact localization of diseased structures, and (III) a realistic balance between alveolar air volume and number of air sacs was applied. Dispersion of inhaled and exhaled aerosol boluses was simulated by using the mathematical concept of effective diffusivities. Computations were conducted for an average adult lung (FRC =3,300 mL), symmetric breath-cycles with a length 8 s, and inhalation flow rates of 250 mL/s. Particles used for the model predictions had a uniform diameter of 0.84 µm and a density of 1 g/cm³.

RESULTS

According to the theoretical data obtained from the model highest aerosol bolus dispersion may be observed in lungs affected by panacinar and bullous emphysema, whereas centriacinar and paraseptal emphysema cause a significant reduction of the phenomenon. Also other statistical parameters exhibit partly remarkable differences among the studied manifestations. Particle deposition in lungs affected by bullous emphysema falls below that of lungs impaired by the other types of emphysema by 2%-50%.

CONCLUSIONS

From the hypothetical results presented in this study it may be concluded that aerosol bolus inhalation bears a certain potential for the diagnosis of emphysematous structures and, if applied with sufficient accuracy, also for the distinction of single manifestations of emphysema. For a successful use of the technique, however, all statistical bolus parameters and particle deposition have to be subjected to a detailed evaluation.

Aerosol bolus dispersion in healthy and asthmatic children-theoretical and experimental results

INTRODUCTION

In the past decades, aerosol bolus inhalation increasingly came into the focus of medical interest due to its potential as a non-invasive technique for the diagnosis of lung diseases. The experimental studies were accompanied by the development of theoretical contributions dealing with aerosol bolus behaviour in healthy and diseased lungs. In this study, bolus dispersion in healthy and asthmatic children is subject to a theoretical approach. Model predictions are validated with related experimental findings.

METHODS

Aerosol bolus transport was simulated by using (I) a stochastic model of the human respiratory tract; (II) appropriate scaling procedures for the generation of healthy and asthmatic lungs of children; and (III) the concept of effective diffusivities (Deff) for the prediction of convective mixing processes in the conducting airways and alveoli. The aerosol injected into the inhalative air stream consisted of monodisperse particles with a diameter of 0.4 µm (ρ =1 g∙cm(-3)). Volumetric lung depth, being a measure for the position of the aerosol bolus within the inspired air stream, was varied from 95 mL (shallow bolus) to 540 mL (deep bolus). Half-width of the inhaled bolus was set to 50 mL.

RESULTS

According to the predictions provided by the model, dispersion of the exhaled aerosol bolus increases exponentially with volumetric lung depth in both asthmatic children and healthy controls. Asthmatics tend to develop higher bolus dispersion than healthy subjects, with significant differences between the two groups being noticeable at low volumetric lung depths (<300 mL). Skewness decreases with increasing volumetric lung depth, whereby respective values calculated for asthmatics exceed those values computed for healthy subjects. Theoretical results correspond very well with experimental findings.

DISCUSSION AND CONCLUSIONS

Results of experimental bolus studies may be approximated by theoretical models with high accuracy. Model predictions confirm the assumption that inhalation of aerosol boluses and dispersion measurements have only a limited diagnostic potential.

[Monte-Carlo-Model for the aerosol bolus dispersion in the human lung--part 2: model predictions for the diseased lung]

After a mathematical extension of the existing model for the theoretical description of the aerosol bolus dispersion, the behavior of particle pulses in diseased lung structures was simulated. The geometry usedJbr healthy lungs was modified in two aspects: First, a modelling of possible airway obstructions, which usually occur in patients with chronic bronchitis, chronic asthma or cystic fibrosis, was carried out and, second, a theoretical approximation of the emphysema, being observed in lungs of smokers, but also as an accompanying phenomenon in obstructive diseases, was established. According to the modified model, in lungs with airway obstructions the exhaled bolus exhibited a decreased dispersion with respect to healthy subjects, whereas in emphysematous lungs the respective half-width of the peak was increased. Standard deviation and skewness of the bolus were similarly influenced by the modified lung architecture. A combination of airway obstruction and emphysema caused an extensive compensation of individual dispersion effects, complicating a secure distinction from the healthy lung. According to the model, a special diagnostic value may be assigned to the bolus deposition, showing significant deviations from the normal case for all simulated diseases.