Mechanical effects of negative pressure wound therapy on abdominal wounds – effects of different pressures and wound fillers

Abstract

The mechanical deformation of the wound edge resulting from negative pressure wound therapy (NPWT) at the standard setting of around −120 mmHg has positive effects in promoting wound healing. However, it may cause pain to the patient during treatment. It is therefore important to study the mechanical effects of the wound edges using lower pressure and different wound fillers. Abdominal wounds were created on eight pigs. The wounds were sealed for NPWT using foam or gauze. Negative pressures between −20 and −160 mmHg were applied, and the decrease in wound diameter and the force with which the edges of the wound were drawn together (wound edge force) were measured. Increasing levels of negative pressure resulted in a gradual decrease in wound diameter and increase in wound edge force and reached a maximum at −120 mmHg, which is the pressure commonly used in clinical practice. Both the decrease in wound diameter and the increase in wound edge force was greater with foam than with gauze. A pressure of −80 mmHg has only 15% less effect than −120 mmHg, while a lower pressure (−40 mmHg) diminished the effects on diameter and force markedly. The NPWT‐induced decrease in wound diameter and increase in wound edge force are greater at higher levels of negative pressure and when using foam than when using gauze as a wound filler. It may be possible to tailor the type of wound filler and level of negative pressure to obtain the best balance between wound healing and patient comfort.

Introduction

Negative pressure wound therapy (NPWT) has improved the treatment of patients with both open and closed abdominal wounds 1, 2. The mechanisms by which negative pressure promotes wound healing include the creation of a moist environment 3, the removal of exudate 4, 5, 6, reduction of tissue oedema 7, contraction of the wound edges 4, 5, 6, mechanical stimulation of the wound bed 8, 9, 10, altered blood flow in the wound edges 5, 11, 12, the stimulation of angiogenesis 13, 14 and the formation of granulation tissue 5, 7. The biological effects of NPWT on the wound bed depend on the type of wound filler and the negative pressure applied 15.

It is well known that patients commonly experience pain during NPWT 16, 17, 18, 19. This pain is both associated with dressing changes 16, 20, 21 and is often the result of the ingrowth of tissue into the wound filler 22 and during therapy 16, 17, 18 due to the forces created when the wound edges are drawn together as NPWT is applied 8. These mechanical forces cannot be completely eliminated as they are a part of the mechanisms by which NPWT is known to promote wound healing 8, 9, 10. However, it has been suggested that the mechanical forces, and subsequently the pain, can be influenced by the choice of wound filler and negative pressure level. Interestingly, in peripheral and sternotomy wounds, it was shown that the decrease in wound diameter also depends on the type of wound filler 23, 24, 25. The pain experienced during the treatment of peripheral wounds was reduced by lowering the amount of negative pressure applied 17, 26. To the best of our knowledge, no such study has yet been performed for abdominal wounds.

In the present study, a porcine abdominal wound model was used to analyse the decrease in wound diameter and increase in mechanical forces on the wound edge (wound edge force) resulting from NPWT. The effect of foam and gauze as wound fillers was compared during NPWT at different negative pressure levels from −20 to −160 mmHg.

Materials and methods

Ethics

The experimental protocol for this study was approved by the Ethics Committee for Animal Research at Lund University, Sweden. All animals received humane care in compliance with the European Convention on Animal Care. The pigs were also used for other experiments.

Animals and anaesthesia

Eight healthy, domestic, female pigs, with a mean body weight of 70 kg, were used in the study. The animals were fasted overnight with free access to water. The pigs were pre‐medicated with an intramuscular injection of xylazine (Rompun® vet. 20 mg/ml; Bayer AG, Leverkusen, Germany; 2 mg/kg) in combination with ketamine (Ketaminol® vet. 100 mg/ml; Farmaceutici Gellini SpA, Aprilia, Italy; 20 mg/kg). Intravenous catheters were inserted into the auricular veins of both ears. Anaesthesia was induced with intravenous sodium thiopental (Pentothal®; Abbott Scandinavia, Stockholm, Sweden; 10–12 mg/kg) and maintained with a continuous infusion of fentanyl (Leptanal®; Lilly, France; ∼3·5 µg/kg/h) in Ringer's acetate or buffered 2·5% glucose (250–500 ml/h) in combination with sodium thiopental (∼6 mg/kg/h), delivered through an infusion pump (Compact Perfusor, Braun, Melsungen, Germany). After orotracheal intubation using a 7·0‐mm diameter cuffed endotracheal tube, the pig was connected to a rebreathing circuit (Servo 900C; Siemens‐Elema AB, Solna, Sweden) and its lungs ventilated mechanically in the volume‐controlled mode (65% N₂O, 35% O₂). The ventilatory settings were: respiratory rate, 15 breaths/minute and minute ventilation, 10 l/minute. A positive end‐expiratory pressure of 5 cmH₂O was applied. A 12 Ch Foley catheter was inserted into the urinary bladder through the urethra. The animals remained anaesthetised throughout the experiment, and a lethal dose of potassium chloride was administered intravenously upon completion of the experiments.

Experimental procedure

Abdominal incisional wounds were created. Gauze or open pore polyurethane foam of a constant size in order to fit the wound was used as wound filler. The drainage tube was connected to a vacuum source, and the wound was sealed with a transparent adhesive drape that overlapped the wound margins by 5–10 cm. The vacuum source was set to deliver a continuous negative pressure, ranging from −20 to −160 mmHg with increments of 20 mmHg. Marks were made on each edge of the wound, and the distance between the wound edges was measured using a slide caliper before (baseline) and after the application of negative pressure (Figure 1). The results are presented as percentage change when negative pressure was applied compared with the baseline values.

Figure 1.

Photograph of porcine abdominal wounds before (0 mmHg) and after the application of negative pressure wound therapy (−80 mmHg), using foam (left) and gauze (right) as a wound filler. When negative pressure is applied, the wound filler is compressed and the wound decreases in size. Note that the decrease in wound diameter is greater when using foam than when using gauze, as illustrated by the red scale bar.

The force with which the edges of the wound were drawn together (wound edge force) when NPWT was applied was measured by a force gauge (AFG 25 N, MecMesin, UK) and connected to a computer. The wound edges were fixed by suturing lines (Figure 2). The force was recorded (MecMesin DataPlot, 1994–1999, Oriel Systems Limited, Version 1.05a 2/2/99) before (baseline) and after the application of the different negative pressures. The data presented are the total force generated in the experimental setup in which half of the force will be generated on one side of the wound and half on the other side of the wound, with a small friction loss in the pulleys.

Figure 2.

Photograph of the experimental setup used to measure the force on the wound edges following the application of negative pressure. A porcine abdominal wound has been filled with foam. An advanced force gauge, four pulleys and a line were used to construct a force measurement device. The force gauge was connected to a computer, and the ends of the lines were sutured to the middle of the wound edges. The drainage tube (blue) was connected to a vacuum source set to deliver a continuous negative pressure ranging from −20 to −160 mmHg in 20 mmHg increments. The inset shows how the ends of the lines were sutured to the middle part of the wound edges and connected to the lines attached to the force gauge. The red arrows show the direction of the force generated by the negative pressure drawing the wound edges together.

Calculations and statistics

Calculations and statistical analysis were performed using GraphPad Prism. Statistical analysis was performed using the Wilcoxon matched‐pairs signed rank test. Results are presented as the median values (range) obtained from eight experiments. All differences referred to in the text were statistically significant (P < 0·05).

Results

Increasing levels of negative pressure resulted in a gradual decrease in wound diameter and, at the same time, a gradual increase in wound edge force that reached a maximum at −120 mmHg and then stabilised. These results are in line with clinical practice where the most commonly used pressure level in NPWT is around −120 mmHg, and we have therefore chosen to present the results of this study in relation to this pressure.

Figure 3 shows the results from all eight animals for the measured wound diameter as a function of the negative pressure applied. At −80 mmHg, the median wound diameter decreased to 89%, and at −40 mmHg, it dropped to 78% of the diameter achieved at −120 mmHg, when using foam as wound filler. When using gauze as a wound filler, the median diameter decreased to 86% at −80 mmHg and to 63% at −40 mmHg.

Figure 3.

Measured decrease in wound diameter as a function of negative pressure in a porcine abdominal wound model. Wounds were treated with continuous negative pressures of −20 to −160 mmHg, in 20 mmHg increments, using foam (×) and gauze (•) as wound fillers. The decrease in wound diameter was calculated as the percentage change upon negative pressure application relative to the baseline values. Results are presented as a scatter plot with median values (n = 8). It can be seen that the decrease in wound diameter was greater when using foam than when using gauze.

As the wound diameter decreased, the wound edge force simultaneously increased. Figure 4 shows the results from all eight animals for the measured wound edge force as a function of the negative pressure applied. At −80 mmHg, the median wound force was 89%, and at −40 mmHg, it was 67% of what was achieved at −120 mmHg when using foam as wound filler. When using gauze as a wound filler, the median wound edge force was 82% at −80 mmHg and 67% at −40 mmHg.

Figure 4.

Measured wound edge force as a function of negative pressure in a porcine abdominal wound. Wounds were treated with continuous negative pressures of −20 to −160 mmHg, in 20 mmHg increments, using foam (×) and gauze (•) as wound fillers. Results are presented as a scatter plot with median values (n = 8). It can be seen that the wound edge force is greater when using foam than when using gauze.

The median decrease in wound diameter was greater when using foam than when using gauze [e.g. 17% (range 13–22) for foam and 6% (range 3–7) for gauze at −80 mmHg, P < 0·01]. Likewise, the median force generated in the wound edge was greater when using foam than when using gauze [e.g. 0·7 N (range 0·1–1·3) for foam and 0·1 N (range 0·0–0·6) for gauze at −80 mmHg, P < 0·01].

Discussion

Wound edge macrodeformation and high negative pressure levels have been associated with pain, and it has been shown that the pain experienced by the patient can be alleviated if the negative pressure is lowered 27. Therefore, it may be beneficial to the patient to use the smallest negative pressure required to achieve a specific decrease in wound diameter. The results show that most of the decreases in wound diameter take place at low levels of negative pressure, with approximately 85% of the maximum effect already achieved at negative pressures up to −80 mmHg. This is consistent with findings in our previous study on sternotomy wounds, where maximal decrease in wound diameter was achieved at −75 mmHg 28. This may imply that many of the beneficial effects of NPWT, such as mechanical stimulation of the wound bed 8, 9, 10, altered blood flow in the wound edges 5, 11, 12, stimulation of angiogenesis 13, 14 and the formation of granulation tissue 5, 7, would be seen at relatively low negative pressure levels.

Indeed, in a retrospective study on a series of cases, McCord et al. observed beneficial wound‐healing effects with NPWT using pressure levels between −50 and −125 mmHg when treating a variety of wounds in 68 children (neonates to 18 years) 29. No correlation was observed between the rate of granulation tissue formation and the pressure level, and the outcomes were generally good. Furthermore, Nease et al. reported that negative pressure levels below −80 mmHg resulted in excellent wound‐healing results 17.

Multiple strategies are often required to manage pain adequately during NPWT treatment. The most obvious method is to provide the patient with analgesics. However, it is difficult to manage pain adequately in this way 18. It is better to consider the cause of the pain, and we believe that wound edge force is a major factor in causing pain during NPWT. At all the pressure levels applied in the present study, the wound edge force was lower when using gauze than when using foam. Therefore, the pain associated with NPWT could possibly be reduced by using gauze as a wound filler. In addition, clinical studies have shown that dressing changes are more painful when using foam than when using gauze 16, 20, 21. The reason for this is believed to be ingrowth of tissue in the foam, requiring greater force to remove it, while no ingrowth is seen when using gauze 22.

The present study also shows that a greater decrease in wound diameter and a greater increase in wound edge force are achieved in NPWT‐treated abdominal wounds when using foam instead of gauze, which is consistent with the findings in a previous study on sternotomy wounds 24. The reason for the difference between these materials may be that foam has a porous structure, allowing greater compression and volume reduction. Gauze, on the other hand, is a denser, woven material that is not as compressible. However, several studies on NPWT in peripheral porcine wounds revealed no differences in wound contraction between foam and gauze 8, 9. Those studies were carried out on smaller, circular wounds (5 cm in diameter). The wounds in the current study were larger, allowing a greater decrease in wound diameter.

Another clinical strategy for minimising pain during NPWT treatment is to reduce the negative pressure level 27, although this has not been studied scientifically. The most commonly used pressure level in clinical practice is −125 mmHg, which is known to cause pain in many cases 17, 26. It is feared that less pressure may have adverse effects on the wound‐healing properties of the therapy. In vivo studies carried out by our group in peripheral wounds have shown that a pressure of −80 mmHg is sufficient to achieve the maximal biological effects on the wound edges in terms of wound contraction 28, regional blood flow 30 and the formation of granulation tissue 22. In addition, clinical studies have shown that a negative pressure level of −75 mmHg also resulted in excellent wound healing 17. The results of these studies conclude that it may be safe to reduce the pressure level to −75 mmHg without adverse effects on wound healing. However, as the mechanical effects of NPWT on the wound edges (i.e. macrodeformation) are believed to be of the utmost importance for the wound‐healing process, it may be necessary to set the pressure level to ensure adequate wound healing while minimising pain in the patient. In the present study, neither maximal wound edge force nor maximal decrease in wound diameter could be achieved with the pressure levels used. We therefore suggest that many of the beneficial effects of NPWT could be obtained at smaller negative pressures than those used in clinical practice today. However, further studies are needed to demonstrate this.

In conclusion, NPWT causes mechanical deformation of the wound edges through the force drawing the wound together. Mechanical stimulation of the wound edges accelerates wound healing but may also cause pain. In the present study, the abdominal wound edge force and the decrease in wound diameter due to NPWT changed gradually with increasing negative pressure. About 85% of the maximal effect was already reached at −80 mmHg. Mechanical stimulation of the wound edge, and thus pain, may be reduced by applying a smaller pressure or by using gauze instead of foam as a wound filler. The level of negative pressure applied and the type of wound filler used may be chosen to balance the stimulatory effects on wound healing for those in pain.