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DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203009 Melanie März*, Sarah Howe, Bernhard Laufer, Knut Moeller and Sabine Krueger-Ziolek for the Digital Clones in Personalized Medicine (DCPM) study team Impact of artificial airway resistances on regional ventilation distribution during airway closure Abstract: Electrical impedance tomography (EIT), a non- range of 0.100 to 0.130 AU. The combination of EIT and the invasive and radiation-free imaging technique can be used in ROcc method provides not only global parameters such as pulmonary function monitoring for determining regional airway resistance under normal breathing conditions, but also ventilation distribution within the lung. Gold standard in results of regional ventilation, which could enable the pulmonary function monitoring is spirometry/body plethys- identification of areas affected by airway obstructions. mography, a method using forced breathing maneuvers to However, the obtained results indicate that EIT might be a obtain global lung function parameters. However, this useful tool in the diagnosis and follow-up of obstructive lung method is heavily dependent on the cooperation of the diseases. patients. Within this observational study, a method under Keywords: Electrical impedance tomography, airway normal breathing was tested with 5 healthy volunteers, which obstructions, artificial airway resistance, regional ventilation provides regional information about ventilation distribution. distribution, body plethysmography. The occlusion method Rocc, a method for determining https://doi.org/10.1515/cdbme-2020-3009 airway resistance, was used to create a short-term airway closure. Regional ventilation during the airway closure was examined with EIT. Simultaneously four different artificial airway resistances were used to simulate airway obstructions. 1 Introduction Results show that EIT in combination with the ROcc method is suitable for the detection of regional differences in Due to the increasing number of chronic obstructive ventilation during airway closure for all four artificial airway pulmonary diseases  new examination methods are resistances. Although the sum of relative impedances at the required. Electrical impedance tomography is a non-invasive, end of the shutter maneuver are smaller (nearly -0.100 AU) radiation-free, functional imaging technique and can be used for the airway resistances Ø 12.5 mm, Ø 10.5 mm and Ø 9.5 in the field of pulmonary function monitoring . This mm than for the smallest one with Ø 30.0 mm (~ -0.070 AU), method is suitable for the detection of ventilation and the changes in impedance from the start to the end of the perfusion  in the lungs of mechanically ventilated patients shutter maneuver differs only slightly between the four in the intensive care unit  as well of spontaneously artificial airway resistances. All impedance changes are in the breathing patients with lung diseases as airway obstructions [5,6]. Due to the high temporal resolutions of EIT even short- term changes in ventilation can be detected . The principle ______ of EIT is based on the injection of a small alternating current *Corresponding author: Melanie März: Institute of Technical (5- 10 mA) in the thorax and the measurement of the Medicine, Furtwangen University, Jakob-Kienzle-Straße 17, Villingen-Schwenningen, Germany, email@example.com resulting voltages on the surface. For image reconstruction, Sarah Howe: Centre for Bio-Engineering, University of measured voltages are converted into relative impedance Canterbury, Christchurch, New Zealand values by mathematical algorithms. This method is therefore Bernhard Laufer, Knut Moeller, Sabine Krueger-Ziolek: suitable for the detection of impedance changes of the lungs Institute of Technical Medicine, Furtwangen University, Villingen- caused by differences in the air content and blood volume Schwenningen, Germany 1 2 3 DCPM study team: Balázs Benyó, Geoff Chase, Thomas Desaive, during respiration . One of the most used methods in Knut Moeller pulmonary function monitoring is the spirometry/body Department of Control Engineering and Information, Budapest plethysmography – a low cost method to determine the University of Technology and Economics, Budapest, Hungary. Centre for Bioengineering, University of Canterbury, Christchurch, airflow . Forced breathing maneuvers are necessary to New Zealand. obtain parameters that indicate the health status of the lungs. GIGA Cardiovascular Science, University of Liege, Liege, Belgium. Institute of Technical Medicine (ITeM), Furtwangen University, Problematic are the forced breathing maneuvers – children or Villingen-Schwenningen, Germany Open Access. © 2020 Melanie März et. al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License. Melanie März et. al., Impact of artificial airway resistances on regional ventilation distribution during airway closure — 2 Figure 1: (a) Relative impedance changes during normal breathing using the occlusion method ROcc (b). Calculation of difference images for each timepoint during expiration (c). patients with severe airway obstructions may not be able to shutter was used for the first time after an even tidal volume perform them. In the case of airway obstructions, areas of the was detectable. The shutter was triggered at least 5 times for lungs can be affected to different extents. These affected 200 ms (at the beginning of expiration) during each areas cannot be identified by spirometry/ body measurement (Figure 1a). After each shutter, the volunteers plethysmography. A combination of EIT with spirometry/ continued with normal breathing. Each measurement was body plethysmography could be a promising approach to performed twice for reproducibility. In parallel, EIT-Data rd obtain global results as well as regional results . were recorded at 3 intercostal space (frame rate 40 Hz) However, forced breathing maneuvers have been used so far using a 16-electrode system (PulmoVista®500, Dräger, for determining the regional ventilation distribution in Germany) (Figure 1b). patients with lung diseases. Therefore, the aim of this study was not only to obtain information about the regional 2.2 EIT data processing ventilation distribution but also to establish a method under normal breathing which is independent of the cooperation of EIT images with a resolution of 32×32 pixels were generated the patients. using the EIT Analysis Tool 6.1 (Dräger, Germany). The following steps were implemented in MATLAB (R2019a, The Mathworks® Inc., Natick, USA). Since the data of the 2 Methods body plethysmograph were recorded with a sample frequency of 200 Hz, the EIT data was interpolated from 40 Hz to 200 Hz. EIT data were filtered with a Butterworth bandstop filter 2.1 Study protocol th -1 -1 (6 order, cut-off frequencies 55 min and 75 min ) to 5 healthy volunteers (2 male, 3 female, 24.8±2.7 years/ exclude impedance values caused by cardiac activity. For 177.0±8.5 cm/ 78.8±12.7 kg) were performing normal further analysis steps the lung region was defined and other breathing in sitting posture in a body plethysmograph tissue such as adipose tissue was excluded. For the definition (PowerCube®Body+, Ganshorn Medizin Electronic, of the lung region, all regional impedance values of the EIT Germany). Obstructions were artificially induced by various images (32×32 pixels) of end-expiration during normal airway resistances (Ø 30.0 mm, Ø 12.5 mm, Ø 10.5 mm, Ø breathing were subtracted from all regional impedance values 9.5 mm) attached between the mouthpiece and the of end-inspiration and a functional EIT image with averaged spirometer. The occlusion method to calculate the airway impedance values was created. The maximum impedance resistance (ROcc), a common method in body value of the functional image was determined and a threshold plethysmography, was used. A shutter can be triggered of 20 % was used. All impedance values greater than 20 % of manually during normal breathing with this method. The the maximum value were added to the lung region. Melanie März et. al., Impact of artificial airway resistances on regional ventilation distribution during airway closure — 3 3 Results The time span of the shutter maneuver can be clearly identified in the sum of relative impedances for all five volunteers (Figure 2). All measurements (Ø 30.0 mm, Ø 12.5 mm, Ø 10.5 mm and Ø 9.5 mm) show a decrease in the sum of the relative impedances as soon as the respiratory flow is interrupted. An increase in the sum of relative impedances can be observed at the end of the shutter maneuver. In the mean of the five volunteers’ slight differences could be observed between the four airway resistances. With a higher airway resistance, the relative impedance at the end of the shutter maneuver decreases from about -0.070 AU (Ø 30.0 mm) to nearly -0.100 AU (Ø 12.5 mm, Ø 10.5 mm and Ø 9.5 mm). In consideration of the changes in impedance, there are Figure 2: Sum of relative impedance during normal breathing almost no differences for the four airway resistances. All with the four airway resistances Ø 30.0 mm, Ø 12.5 mm, impedance changes are in the range of 0.100 to 0.130 AU Ø 10.5 mm and Ø 9.5 mm of the five volunteers. (Figure 2, Table 1). Figure 3 shows exemplarily the ventilation distribution within the differences images during the shutter maneuver (a) as well as the relative impedance 2.3 Data analysis over time for two selected pixel (b) and the sum of relative The bandstop filtered EIT signal was divided into normal impedance in the difference images (c). Ventilation breaths and breaths with shutter. A linear trend was redistribution during the shutter maneuver can be subtracted from all normal breaths and breaths with shutter so demonstrated for all five volunteers with all four airway that each breath begins and ends at an impedance value of resistances. Regional differences between the normal breaths zero. For the following steps only the phase of expiration was and the breaths with shutter can be identified by taking a analyzed, because the shutter was triggered at the beginning closer look at the relative impedances over time of individual of expiration. Since the length of the individual breaths pixels. The breaths with shutter show a characteristic varied, all breaths were interpolated to the time span of the shoulder in the beginning of expiration due to the interruption longest breath. The average of all regional values for the of respiratory flow. With this method, both global parameters normal breaths and breaths with shutter were calculated and scaled to 1. For each timepoint during expiration all regional averaged scaled values of the breaths with shutter were subtracted from the regional values of the normal breaths and difference images (32×32 pixels) were generated (Figure 1c). The sum of relative impedances was calculated in the difference images and normalized to the number of all pixels for each volunteer. Furthermore, the mean value of the sums of relative impedances of all five volunteers was calculated. Table 1: Mean relative impedance values of all 5 volunteers at the start and the end of the shutter as well as the impedance change over the shutter maneuver for the four airway resistances. Airway Shutter Shutter Impedance resistance [mm] start [AU] end [AU] change [AU] Figure 3: (a) Ventilation distribution within the difference images Ø 30.0 0.030 -0.070 0.100 during the shutter maneuver (start, two selected time points Ø 12.5 0.030 -0.100 0.130 in between and end of the shutter maneuver) for the airway resistance with Ø 9.5 mm for volunteer V3. (b) Relative Ø 10.5 0.005 -0.110 0.115 impedance over time is shown for the two pixels 394 Ø 9.5 0.010 -0.100 0.110 (ventral region) and 726 (dorsal region) (c) as well as the normalized sum of relative impedance of all pixels. Melanie März et. al., Impact of artificial airway resistances on regional ventilation distribution during airway closure — 4 (e.g. airway resistance) and knowledge about regional Acknowledgement ventilation can be obtained, which allows a better assessment This work was partially supported by the German Federal of lung health. Ministry of Education and Research (MOVE, Grant 13FH628IX6) and has received funding from EU H2020 R&I programme (MSCA-RISE-2019 call) under grant agreement 4 Discussion #872488 — DCPM. This first study shows that regional differences in ventilation Author Statement distribution during normal breathing with short-term airway Conflict of interest: Authors state no conflict of interest. closures can be obtained using EIT. Different pulmonary Informed consent: Informed consent has been obtained from pressures were induced by using various artificial airway all individuals included in this study. Ethical approval: The resistances, resulting in differences in regional ventilation research related to human use complies with all the relevant distribution. Patients with respiratory diseases have higher national regulations, institutional policies and was performed pulmonary pressures in comparison to healthy persons, so in accordance with the tenets of the Helsinki Declaration, and that this method might be useful for the detection of changes has been approved by the authors' institutional review board in regional ventilation. However, only measurements with or equivalent committee. healthy volunteers were carried out within this study. Airway obstructions were simulated, but this has probably no big References influence on the healthy lung tissue of the volunteers. 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Current Directions in Biomedical Engineering – de Gruyter
Published: Sep 1, 2020
Keywords: Electrical impedance tomography; airway obstructions; artificial airway resistance; regional ventilation distribution; body plethysmography
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