Electrical impedance tomography for chest imaging in acute respiratory failure

To the Editor:

Electrical impedance tomography (EIT) is a functional imaging method that can continuously monitor respiratory function at the bedside. Different measures can be generated from EIT patient examinations allowing the assessment of ventilation distribution, regional lung volume changes and respiratory mechanics, as well as lung perfusion, stroke volume or regional oxygen uptake. In a clinical setting, EIT has mainly been applied in patients undergoing mechanical ventilation as summarised in a systematic review published in 2016 [1] and the most recent international consensus statement on EIT chest imaging published in 2017 [2]. EIT has been applied: to detect regional lung overinflation, alveolar collapse and tidal recruitment, and air-leak; to assess patients' responses to changes in ventilator settings and mode, different recruitment manoeuvres or suctioning; and for setting optimum positive end-expiratory pressure. The use of EIT in critical care medicine is increasing.

We were therefore pleased to note that EIT has been selected as one of five imaging methods (alongside chest radiography, computed tomography, lung ultrasound and positron emission tomography) in a European Respiratory Society (ERS) statement on chest imaging in acute respiratory failure (ARF) [3]. The aim of this report was to “provide physicians with a comprehensive clinical review of chest imaging techniques for the assessment of patients with acute respiratory failure, based on the scientific evidence as identified by systematic searches”. The authors of the document searched the literature on each method from inception to September 2018. Therefore, we expected this report to reflect the previous achievements of EIT already described in detail in the two abovementioned comprehensive reviews [1, 2] extended by the publications from about three further years. To our surprise, the Results section of the statement contains only three sentences on EIT in one short paragraph with an additional sentence, stating that “based on our criteria selection, no articles have been found on this topic”.

The search criteria used for the review are included as supplemental material. Unfortunately, the accepted version of the ERS statement [3] does not provide the search terms for EIT (only for the other four imaging methods). This could explain, but not justify that all EIT articles on the topic are missing. The constraints for correspondence articles preclude us from providing all EIT articles meeting the general search criteria specified in the Methods section of the statement (original studies in articles written in English, adult humans aged >18 years, minimum of 10 study participants), since only ten references are permitted. Nonetheless, we would like to give here a few examples of EIT studies carried out in adult patients with disease entities selected by the task force panel: acute respiratory distress syndrome [4, 5] and acute lung injury [6], COPD (acutely exacerbated) [7], pneumonia [8], acute heart failure [9] and pleural effusion [10].

Being experts on EIT, we are primarily qualified to judge the quality of information regarding this technique and not on other imaging methods. The presented results on the other methods are larger than on EIT and they are based on a total of 135 studies. However, some uncertainty exists on the quality of the literature search for these methods as well because tables 1–4 in the statement (outlining the studies included in the preparation of the document) list two chest X-ray, five lung ultrasound, 10 computed tomography and one positron emission tomography studies that do not fulfil the criterion of minimum study participants defined in the Methods. It is also not clear whether studies on healthy subjects and patients not suffering from ARF should have been included.

An update on the use of imaging methods in patients with ARF is topical and relevant. A document reliably summarising the current status of knowledge on all available methods in form of a statement article is of considerable value to clinicians and researchers alike. As little as a decade ago such a review would have been limited to chest radiography and the potential of CT. The pace of change, and potential, in this field is exciting. Thus, it is disappointing that important knowledge in the field of chest EIT has not been included.

We would like to finish our correspondence with one positive statement: regarding EIT, we agree with the authors of the task force report that “EIT is gaining a more clinical role after having represented a research tool for many years” and that “EIT will become more frequently used in patients with acute respiratory failure”, as written in the Conclusions. But we recommend that readers who are interested in this method should perform their own literature search and refer to other EIT reviews instead of relying solely on the ERS statement [3].

• Received July 27, 2019.
• Accepted July 30, 2019.

• Copyright ©ERS 2019