Thoracic electrical impedance tomography in Chinese hospitals: a review of clinical research and daily applications

An adequate PEEP is recommended for patients with acute respiratory distress syndrome (ARDS) to keep the alveoli open and prevent atelectasis (Ferguson et al 2012). Insufficient PEEP cannot maintain alveoli open while inappropriately high PEEP may lead to various lung damages (Brower et al 2004). Individualising PEEP is a common consensus but the superior method is still under debate. EIT-guided PEEP titration is one of the well-accepted applications of thoracic EIT (Frerichs et al 2017). Different individualised EIT titration methods have been proposed. In a study from the Netherlands with 12 post-cardiac surgery patients, these EIT-derived measures did not differ from each other (Blankman et al 2014). However in a recent study with 30 ARDS patients, it was found that in about 10% of the patients the EIT-derived measures exhibited high differences when analysing the same patients (Zhao et al 2019d). The patient groups were different between these two studies (post-surgery vs. ARDS), and some of the titration methods and the sample sizes were different. Since the EIT-derived parameters capture various aspects of lung function and ventilation, it was proposed that the existence of differences in the recommended PEEP among the EIT measures might be a good indicator of functional lung status (Zhao et al 2019d). The authors recommended calculating more than one EIT measure at a time to confirm the selected PEEP level.

Retrospective clinical studies and prospective animal experiments demonstrated the use and superiority of EIT for PEEP titration (Meier et al 2008, Wolf et al 2013, Hochhausen et al 2017). The group from the Far Eastern Memorial Hospital conducted the first prospective outcome study using PEEP titration with EIT in ARDS patients (Zhao et al 2019a). As compared with the retrospective group in which PEEP was titrated with quasi-static pressure–volume (PV) curve (lower inflection point plus 2 cmH₂O as selected PEEP), the EIT-based PEEP titration was associated with improved oxygenation, compliance, driving pressure, and weaning success rate (Zhao et al 2019a). In this study, regional respiratory compliance was computed at each PEEP level using the EIT method proposed previously (Costa et al 2009). The PEEP levels (PEEP_max-C) of individual maximum regional compliance were identified during decremental PEEP trials (figure 1). At PEEP levels higher than PEEP_max-C, overdistension percentages were estimated. At PEEP levels lower than PEEP_max-C, the collapse percentages were calculated. The optimal PEEP level selected for the patients was the intercept point of cumulated collapse and overdistension percentage curves, which is postulated to provide the best compromise between collapsed and overdistended lung. If the intercept point occurred between two PEEP levels, the selected PEEP corresponded to the PEEP step toward the lowest global inhomogeneity index, which indicated the degree of homogeneity of ventilation distribution (Zhao et al 2009). The same group had just completed a prospective randomised study comparing the same EIT-based PEEP with maximal hysteresis of the quasi-static PV curve in moderate-to-severe ARDS patients. Preliminary results for 87 randomised patients were presented at the annual EIT conference in 2019 (Hsu et al 2019), suggesting a better survival rate in the EIT group.

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Individual PEEP titration may be needed not only for ARDS patients in the ICU but also for patients under surgery. The current practice is that a fixed PEEP of 5 cmH₂O or no PEEP is applied in surgery patients without lung diseases. Liu et al have conducted a prospective randomised study comparing outcomes of EIT-guided PEEP and a fixed PEEP of 5 cmH₂O in 100 elderly patients undergoing thoracoscopic surgery. The results indicated that individual PEEP settings might improve lung mechanics and oxygenation, but not other outcomes such as lung complications and duration of hospitalisation (Liu et al 2019). Besides the use of PEEP, small tidal volume is recommended for ARDS during mechanical ventilation (Ferguson et al 2012). However, no guideline is given for surgery patients (e.g. for cardiac surgery patients during one-lung ventilation; figure 2). The same research group examined different tidal volumes in nine patients (Wang et al 2018), and examined the combination of various PEEP levels in 24 patients under thoracic surgical procedures (Zhao et al 2018b). The ventilation distribution and the corresponding oxygenation differed significantly among various tidal volume and PEEP steps. One of the findings of these studies was that it was feasible to titrate tidal volume at the bedside by using EIT in combination with PaO₂.

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It is known that not all regions within the lungs are recruitable in patients with ARDS (Gattinoni et al 2006). We might 'guess' whether the lungs are recruitable by considering the oxygenation after the recruitment manoeuvre. However, if the oxygenation is not improved, does it necessarily mean the lungs were not recruited? Should higher recruitment pressure be applied? To answer these questions, Long and his colleagues conducted a study on 20 ARDS patients undergoing a recruitment manoeuvre (Yun et al 2016). It was found that even when remarkable lung tissue reopening was detected (confirmed via EIT), the oxygenation did not necessarily increase. The authors suspected that lung ventilationperfusion mismatch could be significant in such patients. Therefore, a workflow evaluating the effect of recruitment manoeuvre was proposed, which required bedside monitoring of ventilation distribution and oxygenation (figure 3).

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To evaluate lung perfusion, on the other hand, one may use mathematical methods to analyse the cardiac-related signal in the EIT data. However, the accuracy was shown not to be as high as with the method based on hypertonic saline bolus injection (Borges et al 2012). Up to now, no studies have been published on human subjects using EIT with saline bolus injection. Again, Long and his group from the Peking Union Medical College Hospital are conducting a study on lung perfusion detection using EIT and saline bolus injection on various patients (NCT04081142). Preliminary results indicated that with 10 ml 10% saline bolus during end-inspiration or end-expiration hold, lung perfusion could be detected with EIT. No side effects were observed so far. Diagnosis and evaluation of treatment efficacy could be demonstrated in patients with acute pulmonary embolism.

One of the treatment objectives for patients under mechanical ventilation is to resume spontaneous breathing and reduce ventilation support as soon as possible, to minimise the risk of ventilator-associated pneumonia, airway trauma, atrophy and diaphragm dysfunction (Petrof and Hussain 2016). However, spontaneous breathing effort during mechanical ventilation may cause pendelluft, which results in lung damage (Yoshida et al 2013). Therefore, it is important to monitor patients with spontaneous breathing to identify potential risks. Sun et al compared neurally-adjusted ventilator-assist (NAVA) with pressure support ventilation in 15 COPD patients and found that NAVA increased ventilation distribution in the most dependent regions and reduced dead space (2017a). However, to capture the pendelluft during spontaneous breathing, which usually happens during the beginning of inspiration, intratidal gas distribution should be calculated (Lowhagen et al 2010). Zhao et al analysed 30 patients under prolonged mechanical ventilation. Four different intratidal gas distribution patterns were found, corresponding to different weaning success rates (Zhao et al 2017). Following this study, customised software was developed which is now available to end-users to further evaluate whether the method can be used to guide spontaneous breathing trial and to predict weaning outcome (figure 4).

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Anaesthesiologists are interested in ventilation distribution during various types of surgery. Before the era of EIT, no bedside tool could achieve this task. At the moment, several studies are being conducted in this field. For example, the group from the People's Hospital of Quzhou is interested in ventilation distribution during endoscopic submucosal dissection of upper gastrointestinal mucosa (ChiCTR1900025184). The measurement time points during surgery are important, since monopolar or bipolar surgical diathermy may create strong interference to EIT measurement. In the extreme case, EIT hardware could be damaged. Harmonic scalpel use would not influence EIT measurement but it is not widely used compared to diathermy. In another study, the authors have compared post-operative ventilation distribution in cardiac surgical patients after traditional full sternotomy or minimally invasive thoracotomy in 40 patients. While the data are still being analysed, preliminary results show large variations in regional ventilation recovery. The findings indicated that EIT might identify inter-individual differences in postoperative lung function recovery among the patients, enabling personalised therapy and care of patients after extubation.

EIT was also proposed to detect pulmonary oedema (Kunst et al 1999, Trepte et al 2016). The validation of this approach was carried out in a clinical setting on 14 ARDS patients (Zhao et al 2019c). Unfortunately, simple left-to-right and anterior-to-posterior ventilation ratios derived from EIT examinations after postural changes did not reflect total extravascular lung water in the study population. Further advanced measures have to be developed and evaluated to assess the level of pulmonary oedema. In another study, several commonly-used functional EIT (fEIT) images, quantifying tidal ventilation distribution, were evaluated in a clinical setting. The pros and cons of those functional EITs were discussed (Zhao et al 2018c). A more thorough and deeper understanding of the examined approaches to fEIT image generation was provided. It was confirmed that fEIT based on standard deviation calculation is subject to baseline drift and influenced by signals other than respiration. Therefore, it is rarely used in clinical practice. However, in the case of pendelluft, the phase differences in respiratory signals of various regions cannot be captured by other common fEIT images (i.e. using tidal variation and regression) and the ventilation in certain regions might be underestimated (figure 5).

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EIT data acquisition and device maintenance cost money. Because EIT is only a monitoring tool, not a diagnosis or life-support tool, its necessity relative to investment is often under debate. At this moment, only two hospitals in Taiwan have managed to pass the local regulations and charge patients for their usage of EIT. Many hospitals have expressed their interest in equipping their departments with EIT devices but the reimbursement of the device and measurement costs is their biggest concern. We can imagine, once this issue is solved countrywide, that the number of Chinese hospitals equipped with EIT devices would rapidly increase within a short period.

As mentioned at the beginning of the paper, the typical end-users are intensivists, anaesthesiologists, respiratory therapists and rehabilitation physicians. However, these healthcare professionals might potentially have conflicts of interest about other personnel taking care of the patients. In an operation theatre scenario, when an anaesthesiologist wants to add EIT to the routine, this would introduce issues such as additional preparation time or potentially limiting the operation field. Under such circumstances, surgeons might oppose the extra measurement. In an ICU scenario, respiratory therapists are only available in a few ICUs. In most of the ICUs intensivists need to adjust ventilator settings among many other activities. They might be reluctant to add an additional measurement to their routine. ICU nurses could also complain about the EIT measurement procedure, which might be performed during their work on patients. We have met these issues in the early promotion of EIT in the hospital. We consider these to be influenced by a lack of education on EIT technology, which can be eliminated: an embedded EIT educational program in traditional education could be helpful in the long term.

A consensus paper on chest EIT was recently published (Frerichs et al 2017). However, due to a lack of prospective randomised studies at the time of writing that document, specific guidelines for particular applications of exact procedures could not be provided. To use EIT in daily clinical routine, clear guidelines are required on how EIT should be used for predefined applications and how the information should be interpreted. We have summarised terminology and various data analysis methods in the consensus paper (Frerichs et al 2017) but an automatic interpretation of the results and ready-to-use software are still warranted. With more upcoming prospective randomised studies, we will be more confident to formulate our guidelines on the daily use of EIT in the future.

Currently, only one EIT system is available commercially in China, which is PulmoVista 500 from Dräger Medical, Lübeck, Germany. Another major vendor of EIT technology, Timpel, São Paulo, Brazil with the Enlight 1800, plans to introduce the device to the Chinese market in 2021. No further information from other vendors was available at the moment of this review. As of February 2020, no original papers using those devices in Chinese hospitals have been published.

PulmoVista 500 is equipped with electrode belts designed for use in lying patients in intensive care units and operation theatres. Applications for subjects in the sitting position or for women with large breasts are not ideal due to possible bad electrode contact. Certain electrodes with insufficient skin contact would need to be pressed (manually or by leaning on a back rest) or fixed with duct tape. Electrode movement introduces baseline shifts, which could be an issue for long-term monitoring. Besides, ICU patients already have many cables connected to various monitoring devices and the EIT cable may add an extra burden. Wireless electrodes with local analog-to-digital converters could be a solution.

Current online software displays impedance trend, ventilation distribution in different regions of interest, and several indices for PEEP titration. As the clinical application scenarios of EIT dramatically increase, many analyses have to be done offline with customized software for particular applications. Further development of online software should be scenario-based.