The main finding of this study was that the diaphragm-based EIT-belt position by ultrasound was feasible. We found that tidal variation with the diaphragm-based EIT-belt position was higher than that with the conventional EIT-belt position. The optimized effect of EIT image by ultrasound is observed in the patients with higher BMI.

To objectively compare the effects of various electrode planes, we defined the “optimal” belt position was the one achieved the highest TV. Someone might argue that such criterion was arbitrary. In a previous study, the best plane of EIT-belt was defined as the highest correlation of VT and TV [6]. Since the healthy subjects in the present study were breathing spontaneously, it was not possible to track the volume changes continuously to calculate the correlation. Nevertheless, the diaphragm-based EIT-belt plane of the highest TV was a reasonable criterion: (1) the healthy subjects were asked to perform stable relaxing tidal breathing, highest TV results in highest TV/VT ratio, which implied that the EIT measurement plane covered the largest lung tissues. (2) The proposed value 6 cm over the diaphragm dome, as the diaphragm-based EIT-belt plane, within the conventional position (4th–6th ICS) in the health volunteer. (3) When the electrode belt was lower than the 6th ICS, negative TV was observed (purple area on the image). The negative TV corresponded to the opposite phase-shift movement caused from the abdominal cavity. When targeting the highest TV, the proposed method of diaphragm-based EIT-belt plane could minimize the effect of diaphragm on EIT ventilation image. Since lower lung always located at L-2 cm level, the ventilation was mainly distributed at dorsal region. And, upper lung was at L-10 cm level, the ventilation was mainly distributed at ventral region. Hence, it is easy to understand ventral distribution was increasing and dorsal distribution was decreasing when belt position was raised (Fig. 2).

Clinical studies had found the influence of various diaphragm positions on EIT lung ventilation image [5, 8]. With the aim to reduce impact of diaphragm position, the EIT-belt plane was placed at the second to fourth ICS in major upper abdominal surgery and laparoscopic gastroplasty of the obese patients [10, 11]. To resolve the impact of diaphragm-based variation of diaphragm position on EIT ventilation image, we introduced a novel method for EIT-belt location that have a fixed distance from diaphragm. Hence, the diaphragm-based EIT-belt plane by ultrasound might have a comparable tomography of the same lung than a fixed body surface position(4th–6th ICS) in the critically ill patients with various diaphragm positions. In the present study, the patients with higher BMI also had a higher diaphragm-based EIT-belt plane determined by ultrasound than the conventional plane. The obese patients always have a high location of diaphragm, so it is reasonable that a high EIT-belt location for the high BMI patients. Moreover, a higher TV and TV/VT ratio were achieved by diaphragm-based EIT-belt plane in the disagree group with higher BMI. It is supported that the novel method has potential to optimize the EIT-belt plane in the obese patients. Importantly, more than 50% patients have a different EIT-location by ultrasound than traditional EIT-belt location. In other words, traditional EIT-belt location, which might be improper, had room to improve in more than 50% patients. Since the diaphragm position is one of the main cause of inconsistent ventilation–impedance ratio, the great interest of the present study was that this innovative method might provide more consistent and precise measurements than the standard setting (4–6th ICS). These findings could be beneficial for clinical practice, which requires further investigation.

Ultrasound allows for a non-invasive, easy, accurate, reproducible, low-cost, and safe assessment of the anatomy and function of the diaphragm in mechanically ventilated patients [12, 13]. Moreover, ultrasound and EIT have much common in the respiratory failure management such as regional ventilation assessment, guide weaning of ventilator, guide PEEP titration and lung recruitment, etc. [1, 14,15,16]. Moreover, it is relatively easy for intensivists to learn the skill of how to find the diaphragm location. Combined ultrasound and EIT could enhance the ability for the respiratory management [17].

There are serval limitations of this study. (1) Since all healthy volunteers were male, the gender could cause the selection bias. (2) Since the lung image of diaphragm-based EIT-belt plane could be improved in the disagree group, other EIT-related parameters such as global inhomogeneity index, center of ventilation, regional ventilation delay and pendelluft parameters might be impacted [4, 18]. Further study is required to validate the impact degree of diaphragm-based EIT-belt other EIT-related parameters. (3) The best EIT-belt position varies and controversial in different aims condition. Electrode placement at 5th intercostal space might not be ideal for every diaphragm-based during EIT measurement during PEEP titration [9]. The mentioned diaphragm-based EIT-belt position by ultrasound requires validation in the titrations of PEEP and tidal volume. (4) Since EIT measurement plane is covering part of the lungs, not the entire lungs, we hypothesized that the larger the voxel EIT measurement covers, the better the electrode placement is. Nevertheless, the proposed method, defined by the highest tidal impedance variation in healthy volunteers, has inherent limitations in its reliability. Hence, the hypothesis requires further study. (5) Most of the enrolled patients was not with severe respiratory failure. Hence, this novel way of EIT electrodes placement based on diaphragm position better reflect the ventilation for in critically patients with mild respiratory failure. Interestingly, this improvement was mainly in the patients with high BMI who always had a high diaphragm position in the present study. Hence, this similar effect by novel method might be also present in patients with severe respiratory failure patients who always had a small lung and high diaphragm position. Further study is required to validate the impact of the novel method on the management of severe respiratory failure based on the lung EIT image.