Langzeit-EIT-Monitoring als Indikator für die Entstehung eines Lungenödems während maschineller Beatmung im Tierexperiment
Long-term EIT monitoring as an indicator for the development of pulmonary edema during mechanical ventilation in animal experiments
by Julia Niewenhuys
Date of Examination:2024-02-28
Date of issue:2024-02-02
Advisor:Prof. Dr. Onnen Mörer
Referee:Prof. Dr. Onnen Mörer
Referee:Prof. Dr. Stefan Andreas
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EnglishIn the present study, the question of the extent to which long-term EIT monitoring (EIT = electrical impedance tomography) is suitable as an indicator for the development of pulmonary oedema during mechanical VILI ventilation (VILI = ventilator-induced-lung-injury) was investigated in an animal model. In addition, it was examined how a presumably more severe lung damage provoked by VILI ventilation with high PEEP (= positive end-expiratory pressure) is represented in the electrical lung impedance. EIT is a novel, non-invasive imaging procedure without radiation exposure that can continuously visualize regional lung ventilation directly. The impedances of the lungs are measured by electrodes placed in a ring around the thorax and a cross-section of the lungs is recorded. Here, EIT was used for the first time to assess regional lung ventilation as part of a large animal experiment with a test duration of 50 hours (Collino et al. 2019). 40 domestic pigs were ventilated so that lung damage was induced. Lung impedance was measured continuously and computed tomography imaging of the lungs was performed at the end of the experiment. In addition, measurements of hemodynamics, lung mechanics, gas exchange and lung volumes were taken at six-hour intervals. Initially, it was confirmed that the results of the relative EIT, in which the first measured value is used as a reference for all subsequent measurements, and the absolute EIT correspond and that the spatially better resolution of the relative EIT can therefore be used for local assessment of the lung tissue condition even with reference measurements taken 50 hours previously. In order to find out to what extent EIT is suitable for imaging and quantifying pulmonary edema, the results of EIT were compared with PiCCO®, fluid balancing and pathological, histological and radiological parameters of the lung tissue. It was shown that an increase in the lung water averaged over all test animals in the PiCCO® (+ 42 %) leads to a decrease in the mean end-expiratory lung impedance in the EIT (- 15 %). The VT factor [ Ω𝑚/𝑚𝑙] was newly introduced and defined in this work to quantify the pulmonary edema detected with the EIT. The factor is formed via a quotient of the absolute EIT and the tidal volume, so that the impedance change for a specific VT volume (VT = tidal volume) is mapped. Over the test period, the VT factor decreased on average in all test animals (-8.4 - 10-6 ± 6.7 - 10-6 Ωm/ml/h). It was assumed that the most likely cause was a shift in the VT volume of pulmonary edema. The pulmonary edema developing over the experimental period is further reflected in the increase in mean animal weight by 2.4 ± 1.6 kg (approx. + 10 %), the mean lung weight by 110 ± 45 g (approx. + 45 %), the increase in the mean fluid balance of 1606 ± 1289 ml over the experimental period and the histopathological findings of the lungs. To answer the second question of the present study, it can be said that there is no significant difference between the PEEP level and the change in impedance, so that EIT does not appear to be suitable for quantifying ventilator-induced lung injury. When looking at the regional lung proportions, it was noticeable that in the dependent lung regions, compared to the initial situation, a stronger decrease in impedance was seen over the test period than in the rest of the lung. The computed tomography images also showed a higher proportion of less-ventilated lung tissue in the dependent lung. It can therefore be assumed that more pronounced lung pathologies such as atelectasis, pleural effusions, inflammatory processes and thus also pulmonary edema occurred in this area due to gravity. The comparison of the CT and EIT methods with regard to the lung condition at the end of the test clearly shows that lower impedance changes in the EIT are associated with a higher CT number and thus a higher air content, and that a greater drop in impedance in the EIT is associated with a lower CT number and thus a lower air content. In summary, it can be seen that the already established method of CT confirms the tendency of the EIT measurement results for the three lung segments dependent, central and non-dependent. In addition, VTEIT decreased over time, which in turn was also increasingly found in the dependent lung. After excluding other causes, a shift in ventilation to a plane other than the one analyzed can again be assumed, as well as an impedance level altered by lung pathologies, which led to a relative change in impedance despite constant distension. The CoV (=center-of-ventilation) confirms these results, because in the dependent lung there is a threefold greater drop in pulmonary ventilation than in the non-dependent lung. The shift in VT also appeared to be PEEP-dependent. When using the formula of Kuzkov et al. (2010) to calculate the TVICT, which should reflect the pulmonary edema, these results were not repeated. This is presumably due to the fact that the formula for calculating pulmonary edema using CT was used for the entire lung and not for the evaluation of only one lung level. All in all, when considering the results, it should be borne in mind that the measurements performed and analyzed represent only one level of the lungs. It therefore remains questionable whether these results can be used to draw conclusions about the possibly inhomogeneously distributed fluid status of the entire lung. It should also be noted in the regional evaluation of the lungs that the area of the border crossings must be viewed critically for the evaluation of the three lung segments (dependent, central, non-dependent). This is because one of the disadvantages of dividing the lungs into segments is their unequal ability to differentiate between changes that take place within a segment or shift between the segments. For this reason, the calculation of a center, such as the CoV and the CoEEC (center of end expiratory change) of the EIT, is a better measure of the position of a distribution for quantifying local changes (Frerichs et al. 2017; Frerichs et al. 2020; Hahn et al. 2020). The PiCCO® method has limitations in this study, as the final values for EVLW were higher than the final lung weight in a small number of test animals (see Table 14). A particular strength of the present study is the animal experimental setting, which, unlike in comparable studies, reflects clinical reality more realistically. In addition, the novel EIT is compared with a variety of already extensively established methods for recording pulmonary edema. For clinical practice, these results mean that EIT can be used as a valid method for detecting pulmonary edema over a period of at least 50 hours and that its regional significance is of particular clinical relevance. The quantification of pulmonary edema could be investigated in more detail by trials with shorter measurement intervals and a comparison with other methods, such as intermittent lung ultrasound examinations. In addition, an analysis of the impedances of several levels in the thorax would be useful for further recording the characteristics of pulmonary edema. If future trials succeed in improving the quantification of pulmonary oedema using EIT, this could strengthen the importance of this non-invasive method and promote its clinical use.
Keywords: EIT; impedance; lung edema; VILI; ventilation; CT; PiCCO; VT-factor