Basic principles and considerations
As for external vacuum therapy, the negative pressure is transferred to the wound area via specific connecting materials such as porous sponges or customized films. The material is attached to the distal end of a pressure-resistant plastic tube, which is routed externally and connected to a pump generating negative pressure. Unlike for applications in external wounds, which require the treated area to be sealed with an airtight film, this is not necessary with EVT, as the surrounding tissue collapses around the connecting material and automatically isolates the area of interest from the atmosphere, creating a contained negative pressure environment.
Polyurethane foam-based drains
EVT is usually performed with polyurethane sponges . One must bear in mind that commercially available PU sponges have variable mechanical properties in terms of porosity and density. Generally, macroporous, low-density sponges are preferred because of the greater debriding capacity and the stronger contraction under negative pressure, which leads to a more pronounced shrinkage (macro-deformation) of the wound cavity. On the other hand, low-density macroporous sponges are more difficult to put in place due to their size. They also grow into the granulation tissue more easily and are therefore more difficult to remove.
PU sponge drains for EVT can be self-manufactured by combining readily available PU foams from vacuum therapy for external wounds with conventional naso-gastric tubes. Since 2014, the EsoSPONGE® (B. Braun Melsungen AG, Melsungen, Germany) is approved as a medical device and commercially available in Europe. The EsoSPONGE® features a low-density macroporous PU foam fitted to a pressure-resistant plastic tube. It is supplied with a specifically designed insertion set using an overtube (Fig. 2). In our experience, this insertion set represents a groundbreaking advantage as it both facilitates and standardizes the procedure.
Open-pore film drains
For certain indications, permeable films have significant advantages as a connection material compared with PU foam-based drains. So-called “open-pore film drains,” in which the perforated area of the drain is directly wrapped with an open-pored film, are easier to put into place due to their smaller diameter and have the advantage of less adherence to the wound cavity, which also facilitates removal (Fig. 3a,b). As an alternative, the open-pored films can also be used to coat the PU foam in order to reduce tissue ingrowth [28, 38] (Fig. 4). Open-pore film drains for EVT were popularized by G. Loske, who developed a range of different drain types for various indications at the gastrointestinal tract . Based on those publications and suggestions, we use a double-layered open-pored film (Suprasorb® CNP Drainage Film, Lohmann & Rauscher International GmbH & Co, Rengsdorf, Germany), which has been specifically designed and approved for vacuum therapy in open abdominal wounds (Fig. 5a,b) [39, 40]. The Suprasorb® CNP drainage film has a very high permeability and allows for superior fluid transport compared with other commercially available products as shown in a recent randomized trial .
Stent-over-Sponge (SOS) procedure and VacStent®
The stent-over-sponge (SOS) procedure is a technical variant of EVT that combines PU sponges with covered self-expanding metal stents (SEMS). The SOS treatment was introduced at the Zurich University Hospital in 2013 , and our experience in patients with upper gastrointestinal leaks was recently published . Compared with EVT using PU sponges only, SOS has several advantages: First, the SEMS keeps the gastrointestinal lumen open after sponge insertion, allowing oral fluid or food intake. Second, the SEMS both seals the sponge and secures its position, which optimizes the direction and the effect of the negative pressure. Third, covered stents isolate the PU foam from saliva and other intestinal secretions, which otherwise may clog its pores with subsequent loss of function. Finally, the negative pressure between the PU foam and the SEMS might also reduce the risk of stent migration. In our experience, the SOS technique is particularly helpful in patients with relevant extraluminal cavities. In this situation, the sponge can be placed directly in the abscess cavity, which is then sealed with the SEMS. Obviously, a major limitation of the SOS technique is the relatively high cost of the procedure, particularly if multiple device changes are required.
The VacStent® (VAC Stent Medtec AG, Steinhausen, Switzerland) is a new device for EVT. Like in the SOS procedure, the VacStent® combines PU foam with a covered SEMS, however in a pre-manufactured setup (Fig. 6a,b). The available evidence for this new system is still very limited with two recent case series [44, 45]. Unlike the SOS approach, which allows extra- and intraluminal placement of PU sponges, the VacStent® is only suitable for intraluminal EVT due to the cylindrical shape of the PU foam.
Electronic pumps and negative system pressure
Preclinical basic research focusing on the underlying mechanisms of EVT is still very limited. In particular, there are no accepted standards for ideal treatment duration and optimal negative system pressure, and these parameters must be selected empirically. Various electronically controlled pumps for vacuum therapy that generate variable negative pressures as low as − 200 mmHg are currently commercially available. However, most devices on the market are specifically designed for external vacuum therapy and are equipped with double-lumen connection systems for pressure monitoring and leakage control that are not compatible with the single-lumen tubing of an EVT device.
In our hands, the electronic Thopaz® vacuum pump from Medela (Medela Healthcare, Baar, Switzerland) has proven a reliable and effective solution. Originally designed for use with chest tubes, the pump produces a maximum negative pressure of − 75 mmHg and tolerates air leaks of up to 2000 ml/min. The system comes with a single-lumen, pressure-resistant tubing that conveniently connects with all types of EVT devices. All relevant information such as negative pressure, leakage, and the amount of liquid collected is continuously monitored and can be accessed via an USB port. In our hands, the relatively low maximum negative pressure of − 75 mmHg has proven sufficient to promote macroscopically visible tissue granulation (micro-deformation), and removal of the sponge without residua is usually unproblematic.