Doi:10.1016/j.medengphy.2006.10.013

ARTICLE IN PRESS
Medical Engineering & Physics xxx (2006) xxx-xxx Photoacoustic monitoring of the absorption of F.L. Dumas , F.R. Marciano , L.V.F. Oliveira , P.R. Barja , D. Acosta-Avalos a Instituto de Pesquisa e Desenvolvimento (IP&D), Universidade do Vale do Paraiba, Av. Shishima Hifumi 2911, CEP 12244-000, S˜ao Jos'e dos Campos, SP, Brazil b Centro Brasileiro de Pesquisas F'isicas (CBPF), Rua Xavier Sigaud 150, CEP 22290-180, Rio de Janeiro, RJ, Brazil Received 9 February 2006; received in revised form 5 August 2006; accepted 27 October 2006 Abstract
Viscosity and elasticity are the fundamental rheologic properties of respiratory mucus, and are important determinants of transportability of mucus in the mucociliary system. One technique that permits to monitor indirectly the rheologic properties of any sample is the photoacoustictechnique. Using that technique, the absorption of isotonic saline solution by human mucus was monitored. The mucus was obtained from 11volunteers, divided into two groups: five volunteers presenting pneumology symptoms (group I) and six healthy volunteers (group II). Thephotoacoustic signal of the mucus absorbing the saline solution was monitored as function of time, with measurements being performed each10 min, up to 120 min. The resulting curves were fitted to sigmoidal curves to simulate the evolution on time of the photoacoustic signal.
A characteristic time for the half saturation of the absorption process was obtained. For group I the time obtained was 23.3 ± 5.3 min andfor group II the time obtained was 55.0 ± 7.7 min, both means being significantly different (Student t-test, p < 0.05). This result supportsthe empirical practice of treating individuals presenting symptoms of airway obstruction with about 30 min of inhalations of isotonic salinesolution vapor for the clearance of the airways.
2006 IPEM. Published by Elsevier Ltd. All rights reserved.
Keywords: Photoacoustic techniques; Isotonic saline solution; Human mucus; Airway clearance 1. Introduction
It has been observed that the rheologic properties of mucusare often abnormal in patients with airway disease. In chronic The mucociliary apparatus of the tracheobronchial tree is upper and lower respiratory diseases, both the viscosity and one defense mechanism to protect the lung. It incorporates the elasticity of respiratory mucus are much higher than several defense strategies, for example, the surface liquids the optimal values for mucociliary transport. Such abnormal covering the airway epithelium trap impacted particles and rheologic properties could be a cause of decelerated mucocil- ciliary action clears them from the tracheobronchial tree.
iary clearance in upper and lower respiratory diseases In this way, mucociliary dysfunction can be defined as any Because mucus transportability is best in a certain viscos- defect in the ciliary and secretory component of the mucocil- ity range, many drugs believed to optimize the rheologic iary apparatus that disrupts the normal physical, chemical, properties of mucus have been evaluated for their benefi- and biological defense functions of the airway epithelium cial effects on mucociliary clearance o it is important to have experimental techniques that permit the monitoring of Viscosity and elasticity are the fundamental rheologic the rheologic properties of mucus during the interaction with properties of respiratory mucus, and are important determi- the analyzed drug. Typically the viscoelasticity of the nasal nants of transportability of mucus in the mucociliary system.
mucus is determined with an oscillating sphere magneticrheometer technique that indirectly could mon- ∗ Corresponding author. Tel.: +55 21 21417167.
itor the rheologic properties of mucus is the photoacoustic E-mail address: (D. Acosta-Avalos).
1350-4533/$ - see front matter 2006 IPEM. Published by Elsevier Ltd. All rights reserved.
doi: Please cite this article in press as: Dumas FL, et al., Photoacoustic monitoring of the absorption of isotonic saline solution by humanmucus, Med Eng Phys (2006), doi: ARTICLE IN PRESS
F.L. Dumas et al. / Medical Engineering & Physics xxx (2006) xxx-xxx The photoacoustic (PA) signal results from the conversion local hospital. The idea behind this therapy is that changes in of amplitude modulated electromagnetic energy to modulated the rheologic properties of the mucus must facilitate mucus thermal energy, through non-radiative decays in the absorbing elimination, promoting clearance of the airways.
material sample t depends on the thermal properties ofthe sample and of the base on which the sample rests, as theRosencwaig-Gersho model shows optically 2. Experiment
opaque and thermally thin samples, the PA signal has thefollowing expression Initially, mucus from 20 healthy volunteers and 15 sick volunteers was obtained. The study was approved by the Ethics Review Committee of the Instituto de Pesquisa e Desenvolvimento at Universidade do Vale do Paraiba, S˜ao Jos'e dos Campos, S˜ao Paulo, Brazil, and the proper informed g is the thermal diffusion coefficient of the gas inside the PA chamber, ω = 2␲f, f the chopping frequency consent was obtained in writing from each volunteer before the mucus be collected. However, many samples were dis- and thermal conductivity of the base, respectively, and Y is carded due to the exclusion criteria adopted: excessive a constant factor. In the PA chamber, the base corresponds presence of saliva, presence of blood, higher dilution state to the material behind the absorber of the chopped light.
and low quantity of mucus. After selection, the samples of Another thermal property is the thermal effusivity, defined mucus selected for analysis were obtained from 11 volun- as e = k/ α. The thermal effusivity essentially measures teers, being four females and seven males, with ages around the thermal impedance of the sample, or the sample's ability 30 ± 5 years. From this group, five presented symptoms of to exchange heat with the environment Using the defi- pneumonia and were recovering at the Santa Casa Hospital nition of the thermal effusivity, Eq. be rewritten as in S˜ao Jos'e dos Campos, S˜ao Paulo, Brazil. The other six volunteers were healthy adults from the Instituto de Pesquisa e Desenvolvimento at Universidade do Vale do Para'iba. For analysis, volunteers were distributed into two groups: group I was formed by the five volunteers with pneumonia symptomsand group II was formed by the six healthy volunteers. To Eq. that, in thermally thin and optically opaque collect the mucus, we followed the protocol stated by Bossi materials, the PA signal is sensible to the thermal proper- The collected mucus was stored in Eppendorf tubes ties of the base. In this way, if the thermal properties of the filled with vaseline oil to avoid dehydration. The samples base change, then the signal in the PA chamber also changes.
were stored at −45 ◦C until experimental use. After unfreez- This enables the PA technique to indirectly monitor differ- ing, each mucus sample was briefly submersed in petroleum ent processes that change the structure, and consequently the to eliminate the vaseline oil The PA setup comprised a thermal properties, of materials in the base position. Exam- Tungsten arc lamp (150 W), a mechanical chopper, lenses, a ples of this use of the PA measurements are the study of PA cell and a two-phase lock-in amplifier. Light was chopped chemical reactions, as photopolymerization of dental resins at 23 Hz. As ws, the PA cell had two faces, one of also blood sedimentation drop evaporation them closed with a glass window and the other closed with an transport of drugs through skin Different stud- aluminum foil (25 ␮m thick). The PA chamber had a diame- ies have established a relation between the thermal diffusivityand the viscosity of different systems, as corn masa flour or wood pulp owever, the relation established amongboth parameters is not unique and depends on the studiedsystem, being for example a direct relation in reference or an inverse relation in reference Nevertheless, as thephotoacoustic signal depends on the thermal parameters ofthe mucus, its temporal evolution must reflect the temporalchanges in its rheologic properties.
In this report, this technique is applied to study the absorp- tion of isotonic saline solution (ISS) by human mucus. Themain goal of this study is to monitor the ISS absorptionprocess as a function of time, to estimate the absorption char-acteristic time in this process. Its importance is based oncommon therapies where patients with airways obstructed Fig. 1. Diagram of the photoacoustic cell used in this study. Chopped light by mucus are indicated to breathe ISS vapor, in order to impinges first in the glass window that closes one face of the cell and later fluidize and expel the mucus more easily. Usually, prescrip- in the aluminum foil that closes the other face. The mucus sample rests over tion requires a 30 min-period of this inhalation therapy in a Please cite this article in press as: Dumas FL, et al., Photoacoustic monitoring of the absorption of isotonic saline solution by humanmucus, Med Eng Phys (2006), ARTICLE IN PRESS
F.L. Dumas et al. / Medical Engineering & Physics xxx (2006) xxx-xxx ter of 0.8 cm, with a depth of 1 cm, accounting for a volume 3. Results and discussion
of 0.5 cm3. The light was incident from the front, impingingfirst on the glass window. The power of the incident light ws an example of the obtained PA signal as a was 100 mW, distributed homogeneously in the whole sur- function of time for a mucus sample. This curve corresponds face of the aluminum foil (about 0.5 cm2). The aluminum with a mucus from group II, but by no means it is represen- foil absorbed the white light generating the PA signal. As tative of this group. As it can be seen, the signal follows a the absorber used in this experiment (Al foil) is optically monotonic growth with time. The experimental data were fit opaque and thermally thin at 23 Hz, the corresponding PA signal will depend on the thermal properties of the material over the aluminum foil; if the thermal properties of this base change, then the amplitude and phase of the signal will also 1 + e(t−t0/ t) change. An electret microphone detected the sound gener- where A1 is the basal level, A2 the saturation level, t0 the ated in the PA chamber, and its voltage was directed to the time to reach the maximum rate of change in the process, lock-in amplifier, that was interfaced to a computer to collect t is the time interval during which the saturation of the experimental data. A program controlled the chopper and the ISS absorption process takes place. For the study done here, the most interesting parameters are t0 and Measurements were done as follows: about 0.1 mL of they characterize the absorption time of the isotonic saline mucus was put on the aluminum base and the PA signal solution by the mucus. In this process, t0 is related to the was monitored for 1 min. This measurement was repeated half-saturation time of the solubilization process. Parameters after 10 min, and 10 min later too. These initial measurements A1 and A2 are related to the physical characteristics of the were done to give information about the level of the PA sig- mucus, such as quantity and density, because these variables nal before the interaction with isotonic saline solution. After determine the intensity of the signal. As the aim of the present 20 min, the mucus was taken from the aluminum base and study is to analyze the kinetics of ISS absorption process, the introduced into an Eppendorf tube filled with 1.5 mL of ISS, parameters A1 and A2 were ignored. ws the results where it was kept for 10 min. After this time another measure- for the two parameters t0 and t for the studied groups. It can ment was done, putting the mucus sample on the PA chamber be seen that samples from group I show lower values for these and being put inside the ISS Eppendorf again after the mea- parameters than those presented by group II. A Student's t- surement. This procedure was repeated 10 times, for up to test was carried out to evaluate if the mean values of t0 and t are significantly different between groups I and II. For At every measurement time 50 repetitions were done at both parameters, average values were significantly different intervals of 3 s, lasting 2.5 min for the measurement of each (p < 0.05, The power of the Student's t-test done experimental data. This was done in this way to monitor the stability of the system. So, the error showed in probability β of committing a type II error was lower than sponds with the standard deviation of the 50 measurements 0.01. This result shows that the difference observed among the calculated mean values can be sustained with the number The experimental data obtained were analyzed using the of measurements done in each group. ws the analysis tools of the software Microcal Origin 6.0.
versus t0 values for volunteers in each group. As can be seen,each group is well clustered in different regions of the plot of t versus t0. This means that each group of mucus sample has different absorption kinetics for the isotonic saline solution.
In other words, the absorption kinetics must be related to thepathology associated with the mucus. Also, it can be observedthat the adjusted data in every group show a high dispersion,and for this reason the is not representative for thegroup II. In the can see that the mucus Table 1Mean values and standard deviations for parameters t0 and t in both studiedgroups Fig. 2. PA signal as a function of time of ISS absorption by the mucus. Thesquare represent the mean value and the bar in every square represents the N corresponds with the number of individuals. The t-Student test between standard deviation of 50 measurements. Continuous line represents the data parameters in both groups showed that mean values are significantly different Please cite this article in press as: Dumas FL, et al., Photoacoustic monitoring of the absorption of isotonic saline solution by humanmucus, Med Eng Phys (2006), doi: ARTICLE IN PRESS
F.L. Dumas et al. / Medical Engineering & Physics xxx (2006) xxx-xxx Acknowledgment
F. R. Marciano acknowledge CNPq by a PIBIC-INPE References
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