Differences in fluid removal of different open-pore elements for endoscopic negative pressure therapy in the upper gastrointestinal tract

Endoscopic negative pressure therapy is an effective treatment strategy for various defects of the gastrointestinal tract. The functional principle is based on an open-pore element, which is placed around a perforated drainage tube and connected to a vacuum source. The resulting open-pore suction device can undergo endoluminal or intracavitary placement. Different open-pore suction devices are used for endoscopic negative pressure therapy of upper gastrointestinal tract defects. Comparative analyses for features and properties of these devices are still lacking. Eight different (six hand-made devices and two commercial devices) open-pore suction devices for endoscopic negative pressure therapy of the upper gastrointestinal tract were used, amount fluid removed was evaluated. The evaluation parameters included the time to reach the target pressure, the time required to remove 100 ml of water, and the material resistance of the device. All open-pore suction devices are able to aspirate the target volume of fluids. The time to reach the target volume varied considerably. Target negative pressure was not achieved with all open-pore suction devices during the aspiration of fluids; however, there was no negative effect on suction efficiency. Of the measurement data, material resistance could be calculated for six open-pore elements. We present a simple experimental, nonphysiologically setup for open-pore suction devices used for endoscopic negative pressure therapy. The expected quantity of fluids secreted into the treated organs should affect open-pore suction device for endoscopic negative pressure therapy.

. Due to the evident safety and efficacy of this treatment mode, ENPT was used to treat other perforating defects of the gastrointestinal tract. Its applications for the UGI were reported since 2007 [2][3][4][5] . ENPT improves local perfusion, resolves interstitial wound oedema, removes fluids, and debrides the wound base. Vital granulation tissue is formed after wound cleaning 6 . ENPT is also named Endovac therapy and endoscopic vacuum Therapy (EVAC or EVT).
The basic principle of ENPT is an open-pore element sheathing the perforated distal end of a tube, and resulting product is called open-pore suction device (OPSD). From 2000 until 2015, PU sponges were exclusively used for ENPT. Then, the CNP drainage-film was introduced by Loske to establish ENPT in the urinary and gastrointestinal tracts 7,8 . The CNP film is an open-pore drainage film that can be used to create a thin ENPT device through wrapping on gastrointestinal or Redon tubes. Simultaneous ENPT and tube feeding of patients is possible using two-or three-lumen feeding tubes. In the article "Tips and tricks for endoscopic negative pressure therapy" Loske introduced different OPSDs based on the open-pore film and PU sponge 9 . In 2018, Heiss introduced a new device for ENPT of the UGI that combines the benefits of self-expandable metal stent-therapy and ENPT: the VAC Stent 10 . The advantages of this stent are the possibility of oral food intake in combined with continuous wound cleaning by EPNT.
We have implemented ENPT as first-line treatment strategy for various defects in the UGI since 2017 11,12 . The OPSD used for different locations and indications is chosen by the endoscopist according to his or her expertise and preferences. An analysis of the characteristics of OPSDs are still lacking.

Results
Achievement of the target negative pressure. The target negative pressure was achieved in 18 of 24 measurements. In three tests with prototype gastric tubes with CNP film (a), a target vacuum was achieved. In tests with the Trelumina drainage with CNP-film wrapped gastric tube (d), the target negative pressure was achieved in two of three measurements before 100 ml water was removed.
Time to achievement of the target vacuum level. The two commercially available OPSDs (Eso-Sponge g = 27.2 s; VACS h = 28.3 s) and the enteral feeding tubes wrapped with a PU-sponge and CNP-film (f = 13.2 s) allow rapid achievement of 125 mmHg or 16,665.25 Pa suction.
Removal of 100 ml. One hundred millilitres of water were removed for every measurement with every OPSD. The mean evacuation time for all tests was 115.94 s. The suction time differed considerably between the different OPSDs. Fast evacuation of 100 ml water was achieved by three prototype OPSDs (a = mean 26.09 s; b = mean 29.43 s; d = mean 26.43 s). Of the commercially available ENPT products, the evacuation times of the target amount of water were similar (g = mean 57.4 s; h = mean 53.45 s). The prototype enteral feeding tube with PU-sponge and CNP-film (f = mean 327.5 s) achieved the longest time for removal of 100 ml water.
Calculation of material resistance. Material resistance was calculated with the following formula: R = U/I, with U is ∆ pressure and I is the flow rate (volume/time). Material resistance was determined in 7 of 8 OPSDs. The calculation of resistance was possible only in cases where the given pressure of 125 mmHg was reached. Results of the measurements are summarized in Table 1.

Discussion
We present the results of a simple experimental setup for testing different OPSDs used for ENPT in the UGI. This nonphysiologically test can be used to analyse the characteristics and competencies of fluid removal. Of course, a container filled with water does not simulate the environment of the UGI, and water is not equivalent to gastrointestinal fluids. This simple test was performed to analyse modes of action of the presented OPSDs. In Table 1. Results of measurements according to the tubes and devices for endoscopic negative pressure therapy of the upper gastrointestinal tract. www.nature.com/scientificreports/ particular, for OPSDs placed into the duodenum for perforation therapy, a relevant quantity of fluids must be moved through the devices. To our knowledge, this is the first description of an experimental comparison of the characteristics of different OPSDs for use as ENPT devices. We are aware of the simplicity of the experimental design presented. The results for the negative pressure achievement and material resistance calculation shows the following. For tubes (16Ch) wrapped with CNP-film or PU-sponge material (probes a, d, e) rapid fluid removal is possible, but the target negative pressure is not achieved, and it is impossible to calculate the material resistance of probes a and e. The commercially available products (g and h) show mostly consistent results with similar findings in the calculation of material resistance. The self-made devices using the 16Ch nasogastric tube, intestinal tube and PU-sponge with (f) or without (b) wrapping with CNP-film resulted in the highest material resistance values and similar times to achievement of the target pressure. Time it took to remove 100 ml fluids was up to 7 min in these probes. What do these tests tell us? The choice of OPSD for ENPT in the UGI should be made depend on the expected amount of fluids in the treated organ. Higher quantities of fluids are expected for ENPT of duodenal leakages or insufficiencies. The considerable differences between the OPSDs suggest that different treatment goals of the ENPT could be reached. A "one fits all" approach is also unlikely in the ENPT subject area.
The commercially available products for ENPT produced uniform results in the tests performed. Both analysed products are approved for ENPT in the oesophagus. Self-made OPSDs with more than two components showed variable outcomes, with increasing material resistance depending on the number of materials used.
Literature on ENPT of defects of the UGI consists of case reports and monocentric case series [13][14][15] , comparisons of negative pressure and stent-based therapy [16][17][18] , and descriptions of new opportunities of use 19,20 . We need more data about features and properties of different OPSDs used for ENPT. Points of interests are the grade of granulation induction, the optimal pressure level and the optimal treatment time in patients with defects of the UGI. To answer these key questions, we need animal models and standardised protocols. Prospective multicenter studies are desirable.

Conclusion.
Knowledge about properties and features of OPSD for ENPT is still limited. The expected quantity of fluid drainage from the treated organ, the expertise of the endoscopist, and the wound conditions should dedicate the choice of OPSD for ENPT.

Material and methods
Experimental setup. OPSDs were individually placed into a container filled with water and were connected to an electric vacuum pump with pressure monitoring (KCI V.A.C. Ultra Therapy Unit, KCI USA Inc., San Antonio, Texas, United States, see Fig. 1). The pressure of the target vacuum was 125 mmHg in all measurements according to the negative pressure usually used for ENPT of the gastrointestinal tract 9,12,16 . The measured values were as follows: the achievement of the target negative pressure, the time taken to reach the negative pressure and the time taken to aspirate 100 ml water.
Eight open-pore suction devices were tested (Fig. 2). Every probe was tested three times. Differences and advantages of the different prototype and commercial OPSDs are summarised in Table 2.
For the Trelumina probe (d) the ventilation tube was closed with plasters. The intestinal tubes were closed by clamps during the tests.

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.