Negative pressure wound therapy

INTRODUCTION — 

Negative pressure wound therapy (NPWT), also called vacuum-assisted wound closure, refers to wound dressing systems that continuously or intermittently apply subatmospheric pressure to the system, which provides positive pressure to the surface of a wound. NPWT has become a popular treatment modality for the management of many acute and chronic wounds [1].

Subatmospheric pressure has multiple beneficial effects on wound healing in animal models. However, clinical evidence of its superiority over conventional wound dressing techniques for all wound types has not been proven. The available randomized trials have significant heterogeneity in the nature of wounds treated and in primary and secondary endpoints, making rigorous comparisons difficult and limiting the ability to generalize their results.

The general mechanism of action of NPWT, its clinical uses and contraindications, placement and management of the device, and efficacy in specific clinical applications will be reviewed here.

MECHANISM OF ACTION — 

The application of negative pressure to assist in wound healing was first described in the management of soft tissue injury in association with open fracture [2]. The demonstration of beneficial effects in animal models spurred the development of the negative pressure wound systems that are widely available [3,4]. (See 'Device and placement' below.)

NPWT accelerates wound healing, which normally progresses through the following phases: hemostasis, inflammation, proliferation, and remodeling (see "Basic principles of wound healing", section on 'Phases of wound healing'). Animal studies have shown that subatmospheric pressure improves the local wound environment through both direct and indirect effects; these effects accelerate healing and reduce the time to wound closure [3,5-8]. (See 'Direct effects' below and 'Indirect effects' below.)

Both systemic and local wound factors can contribute to delayed wound healing and should be identified and corrected to the extent possible. (See "Risk factors for impaired wound healing and wound complications".)

●Systemic factors include poor nutrition and tissue ischemia, among others.

●Local wound factors include tissue desiccation, edema, excessive exudate, poor tissue apposition (eg, grafts and flaps), and wound infection. Stagnant fluid is associated with cytogenetic factors that impede wound healing [9-15].

Direct effects — The semipermeable dressing of the negative pressure system maintains a moist and warm environment that is stable and more conducive to wound healing. The closed system generates a pressure gradient between the wound and suction canister that promotes fluid transport, first from the wound bed and then from the interstitial space, reducing wound edema. Fluid is removed through the application of pressure, similar to the manner in which compression stockings support fluid removal in venous stasis disease.

The open porous structure of the foam distributes subatmospheric pressure throughout the foam, resulting in a positive pressure on the wound surface [16]. The actual pressure applied varies but is usually in the region of 5 to 10 mmHg. The exact pressure experienced depends upon the negative pressure delivered, the nature of the wound, and its application (whether circumferential, a cavity, or a surface). Clinical judgment should, therefore, dictate the pressure settings applied, as certain patients' circulation may be impaired.

The foam also contracts, drawing the edges of the wound firmly together and bolstering any skin grafts or flaps that are present [17,18]. The degree of tissue deformation depends upon the stiffness of the tissue; scar tissue has less mobility and will have less of a response. Tissue deformation is an important stimulus for tissue remodeling mediated at the cellular level [18,19].

Indirect effects — NPWT is associated with a variety of indirect effects that promote wound healing [5]. These include:

●Alterations in blood flow – The increased wound surface pressure alters the blood flow [3,20-22]. It was initially suggested that NPWT increases perfusion in the wound, thus stimulating healing. However, thermography and other studies have shown that due to the positive pressure applied to the wound surface, perfusion is actually reduced, at least transiently [16,23-27]. This initial relative ischemia stimulates the release of growth factors and other vasoactive agents, resulting in increased granulation routinely seen clinically, which is more prominent with the use of intermittent or variable NPWT settings compared with the continuous setting. Excessive pressures applied to the system (more than -175 mmHg) will decrease blood flow [20].

●Reduced inflammation – Reductions in systemic (eg, interleukins, monocytes) and local mediators of inflammation have been demonstrated in experimental models [28]. In humans, decreased matrix metalloproteinase activity has been documented in patients treated with NPWT [29].

●Reduced bacterial burden – A reduction in bacterial burden was noted during the first clinical experiments evaluating NPWT, but the results were variable in later studies [3,30,31]. In a randomized trial, for example, both increasing and decreasing bacterial loads were seen depending on the initial culture result [31].

●Changes in wound biochemistry – Mechanical deformation alters the local environment through the process of mechanotransduction (conversion of mechanical stimulus into chemical activity). In vitro, stretch increases human fibroblast growth and migration [19]. In an animal wound closure model, the rate of wound closure was significantly higher in the NPWT group compared with controls that used gauze-only dressings at day seven (54 versus 43 percent) [32].

CLINICAL UTILITY

Overall effectiveness — NPWT, also called vacuum-assisted wound closure, has been applied to a wide range of clinical situations, including the open abdomen, following surgical debridement of acute or chronic wounds (eg, orthopedic, necrotizing infection, pressure-induced ulcer), diabetic foot ulcers, and reconstructive surgery (eg, burns, skin graft, muscle flap) [1,33]. It has also been used to prevent surgical wound infection (ie, prophylactic) and as a means of instillation therapy.

Multiple systematic reviews have evaluated the efficacy of NPWT, but none have reached definitive conclusions [33-43]. The most rigorous of these reviews identified eight randomized trials that met three inclusion criteria: randomized, chronic wound, and specific endpoints [36]. NPWT was used to treat diabetic ulcers, pressure-induced (decubitus) ulcers, or chronic lower extremity ulcers. The primary endpoints were one of the following:

●Time to complete healing

●Days to reach 50 percent of initial wound volume

●Percent reduction in wound surface area

●Percent decrease in wound length, width, depth, or volume

●Days until ready for surgery

Because only one study was available for each endpoint, comparisons could not be made. Even if a meta-analysis were able to be performed, there would still be the question of whether chronic (or acute) wounds of differing mechanisms in different patient populations should be compared with each other.

Another approach evaluated the available data (observational and randomized trials) for a specific clinical situation, such as wounds from an acute injury (eg, trauma, burns, surgical debridement) [33,39,40,44], wounds in patients with diabetes (eg, ulcers, postoperative wounds) [37,42], open abdomen [45], and open sternal wounds (eg, following debridement) [41]. Available trials are presented for these individual clinical situations below. (See 'NPWT for open wound management' below.)

Advantages and disadvantages — Compared with traditional wound care modalities, NPWT offers several clinical advantages.

●Advantages

•Traditional therapy for complicated or dirty wounds usually consists of moist dressings that are changed up to three times daily. If too much time elapses between dressing changes, the gauze may become painfully adherent, and its removal may debride desirable granulation tissue as well as devitalized tissue. Much of the pain associated with wound care occurs during dressing changes. In contrast, NPWT dressings are changed less frequently (eg, two to five days), and anticipated pain can be managed preemptively. (See 'Device and placement' below and 'Dressing changes' below.)

•Compared with other wound dressings, NPWT is easier to tailor and maintain in position. Almost every configuration of wound, including circumferential extremity wounds (ie, degloving injuries) and wounds located in proximity to orthopedic fixation frames, can be managed with relative ease [46-48]. As a result, NPWT may allow complex modes of reconstructive surgery.

•Accelerated wound healing with NPWT significantly reduces the time to wound closure in patients with diabetes, returning these patients to baseline more quickly and improving quality of life [49].

●Disadvantages

•From the patient's perspective, the main disadvantage of NPWT is the need to carry the portable pump.

•NPWT systems are more expensive than traditional wound dressings. However, the overall cost of wound care depends upon the frequency of dressing changes, the need for skilled nursing care, and the duration of treatment. Significant clinical reductions in time to wound closure would be needed to offset the increased cost of the device and special supplies for NPWT to be cost-effective, but data are limited.

NPWT for open wound management

Abdominal and pelvic surgery — NPWT for closed abdominal incisions may reduce surgical site infection (SSI) compared with standard therapy, particularly among those at risk for wound complications. The risk of SSI depends upon a number of factors, including the urgency of the procedure. (See "Overview of the evaluation and management of surgical site infection", section on 'Incidence and risk factors'.)

Vascular surgery — Prophylactic NPWT has been used following abdominal aortic surgery (open and endovascular) and following lower extremity revascularization [50-61]. In a meta-analysis of seven trials, NPWT applied to closed groin wounds (open/endovascular aortic repair, lower extremity bypass) significantly reduced the risk of SSI (odds ratio [OR] 0.35, 95% CI 0.24-0.50) [57]. Among trials that reported complications, there were no significant differences, but the number of events was small. In the Cochrane meta-analysis described above [50], prophylactic NPWT significantly reduced the incidence of SSI associated with lower extremity bypass surgery (RR 0.46, 95% CI 0.32-0.66). Longer follow-up is needed to evaluate other outcomes (eg, graft complications) and determine cost-effectiveness in this clinical setting. (See "Overview of lower extremity peripheral artery disease".)

Orthopedic surgery — Prophylactic NPWT has been tried following elective hip or knee arthroplasty, spine surgery, amputation, and fracture fixation [50,62-69]. In the Cochrane meta-analysis described above, SSI was similar for prophylactic NPWT compared with standard dressings for arthroplasty (RR 0.69, 95% CI 0.32-1.49) and lower extremity traumatic injury (RR 1.15, 95% CI 0.61-2.20). In the largest of the included trials (1548 patients), the rate of deep surgical infection was similar at 30 and 90 days for NPWT compared with standard wound dressings (30 days: 5.8 and 6.7 percent, respectively; 90 days: 11.4 and 13.2 percent, respectively) [62]. Other wound complications were not affected by NPWT. Most patients (81 percent) were treated for closed fractures. In an earlier trial that included 249 patients with 263 fractures that were deemed high-risk (calcaneus, tibial pilon, tibial plateau), the incidence of infection was overall reduced in the NPWT group (18.8 versus 9.9 percent) [66]. The differences in rates of SSI between studies may reflect differing fracture sites/types or may reflect other factors (eg, type of NPWT dressing, antimicrobial usage).

Prophylactic use with closed wounds — Evidence suggests there may be a role for prophylactic NPWT. Numerous small trials have been performed in a variety of surgeries. These suggest that rates of SSI for some incision types may be lower if NPWT is used on overlying closed incisions rather than conventional wound dressings. The mechanism of incisional NPWT may involve bolstering, stabilization of the wound, or reduced shear across the incision and any combination of these. Before any recommendation can be made regarding the use of prophylactic NPWT, larger randomized trials with longer follow-ups are needed to assess benefits in specific wound types and patient populations and cost-effectiveness. The following meta-analyses provide examples of outcomes or prophylactic NPWT.

●In a meta-analysis of 31 trials, prophylactic NPWT reduced SSI compared with standard dressings (8.8 versus 13.0 percent; relative risk [RR] 0.66, 95% CI 0.55-0.80) [50]. Among the studies evaluating other outcomes, the rate of deep SSI, hematoma, seroma, blisters, wound dehiscence, pain, reoperation, and death was similar between the groups. Overall, NPWT was not cost-effective.

●The role of prophylactic NPWT following emergency surgery was evaluated in a meta-analysis that included seven studies (two trials, six observational studies) [70]. The rate of SSI (superficial/deep) was lower for prophylactic NPWT compared with standard dressings alone (13.6 versus 25.1 percent; OR 0.43, 95% CI 0.30-0.62), as was the rate of overall wound complications (15.9 versus 30.4 percent; OR 0.41, 95% CI 0.28-0.59). Cost-effectiveness was not evaluated in this study. Abdominal and pelvic incision closure is discussed more fully in separate topic reviews. (See "Principles of abdominal wall closure", section on 'Negative pressure wound therapy' and "Cesarean birth: Overview of issues for patients with obesity", section on 'Negative pressure wound therapy'.)

●Prophylactic NPWT may also be useful for contaminated surgical wounds [71-74]. A trial randomly assigned 69 patients to fascial closure and NPWT versus fascial closure and standard wound care following surgical exploration and management of gastrointestinal perforation [71]. In both groups, delayed wound closure was attempted on postoperative day four. NPWT significantly reduced the incidence of SSI (17.6 versus 61.3 percent) and fascial dehiscence (9 versus 48 percent). Prophylactic NPWT also increased the rate of delayed primary wound closure (91 versus 48 percent) and decreased the median time to wound healing (19 versus 26 days). The results of this small trial are encouraging, and in the absence of contraindications, NPWT can be used to expedite the closure of contaminated surgical wounds.

CONTRAINDICATIONS — 

NPWT should not be used when any of the following are present [75]:

●Exposed vital structures – NPWT, in the presence of exposed organs, blood vessels, or vascular grafts, increases the risk for tissue erosion, which can lead to enteric fistula or hemorrhage [7,76]. NPWT is generally avoided until an intervening granulation layer, or tissue flap or graft provides coverage. Although some clinicians report success using barrier dressings, caution is advised when implementing this practice. (See 'Device and placement' below and 'Complications' below.)

●Presence of malignant tissue – As with normal tissues, the growth of malignant tissue is promoted in the presence of subatmospheric pressure. Malignant tissue is also more friable and prone to bleeding [7,17]. (See 'Bleeding' below.)

Relative contraindications include the following:

●Ischemic wounds – Although not absolutely contraindicated, no benefit has been demonstrated with the use of NPWT in patients with ischemic wounds [5]. The application of negative pressure to these wounds would be expected to worsen tissue ischemia in keeping with the biomechanism of this device.

●Ongoing infection or devitalized tissue – Adequate debridement of devitalized tissue and treatment of infection should generally be undertaken prior to using NPWT [7]. However, instillation therapy has allowed the use of NPWT in the presence of infection or to augment the surgical management of infection. (See 'Overall effectiveness' above and "Acute cellulitis and erysipelas in adults: Treatment" and "Necrotizing soft tissue infections".)

●Fragile skin – Caution should be used when using NPWT in patients with fragile skin due to age, chronic corticosteroid use, or collagen vascular disorder. Shearing forces at the wound margin can lead to skin avulsion and necrosis. NPWT may nevertheless offer considerable benefits in at-risk patients if the perimeter of the wound is adequately protected.

●Adhesive allergy – NPWT requires an adequate seal to maintain the applied suction. The adhesive cover typically overlaps the skin by 4 to 5 cm with a significant amount of adhesive in contact with the patient's skin. Sensitive patients can develop shearing of the skin and bullae formation.

DEVICE AND PLACEMENT — 

NPWT systems consist of an open-pore polyurethane ether foam sponge, a semiocclusive adhesive cover, a fluid collection system, and a suction pump [17]. Commercially available systems for NPWT include the vacuum-assisted closure (VAC therapy) device and the Chariker-Jeter wound sealing kit. VAC therapy is the most widely studied system in randomized trials.

The following steps are involved in placing the device (figure 1):

●The foam sponge is trimmed to fit the size of the open wound and placed into the wound, taking care that the foam does not extend beyond the margin of the wound.

●The foam is then secured beneath an adhesive sheet. A hole is cut in the adhesive, and a suction port (more than one may be used) with tubing is placed; the tubing extends to a disposable collection canister.

●A portable pump is connected to the suction tubing. The pump applies -50 to -175 mmHg of continuous or intermittent suction, which reduces the volume of the foam by up to 80 percent [35]. The porous nature of the polyurethane foam evenly distributes subatmospheric pressure throughout the foam, resulting in a positive pressure to the surface of the wound [16,20], and it also provides a conduit for fluid removal from the wound surface to the collection system. (See 'Mechanism of action' above.)

When fragile structures are present within the wound, they should be protected with an interposition layer placed beneath the foam. The interposition layer should slide easily over the tissue. Examples of materials used as interposition layers include mesh (eg, Vicryl) or petrolatum gauze [17]. Denser foams (eg, white) may also be used for this purpose.

Dressing changes — The dressing and tubing are typically changed every 48 to 120 hours (two to five days), depending upon the clinical situation. The device is turned off, and the semiocclusive dressing is removed. The sponge is carefully removed. If it is adherent to the underlying granulation tissue, the sponge can be soaked with saline and allowed to sit for a few minutes before removal. If pain is excessive during sponge removal, the sponge can be soaked with topical Xylocaine without epinephrine prior to its removal, either directly or via the suction tubing (suction off) [17]. Pain is controlled more effectively when analgesia is administered prior to the dressing change.

Wounds treated with NPWT may become malodorous, and infection may be suspected; however, bacterial counts typically remain low [17]. Irrigation at the time of dressing changes, or the placement of a silver-containing interposition layer (table 1), can help reduce contamination and odor and may reduce healing time [77]. Alternatively, NPWT can be withheld for a day or two. (See "Principles of acute wound management", section on 'Irrigation' and "Overview of treatment of chronic wounds", section on 'Antiseptics and antimicrobial agents'.)

Pain management — Most patients do not complain of significant pain with NPWT between dressing changes. However, NPWT is most often painful when applied circumferentially or used on an intermittent cycle with high intensity or if applied under excessive stretch. Pressures may be adjusted (ie, less negative would cause less force on the wound). Appropriate pain medication should be given as the need arises (eg, acetaminophen with codeine).

Patient training and counseling — The patient should receive training on how to use the device and be comfortable managing the device at home. Patients should be monitored frequently by a trained practitioner in an appropriate care setting. Leaks or blockages in the system are the most common; bleeding is potentially the most significant complication. The patient should be informed of the potential complications associated with the device and of the necessity to seek medical assistance immediately if bleeding is observed. (See 'Complications' below.)

COMPLICATIONS — 

NPWT is generally safe and well tolerated. Complications can include bleeding, infection, pain, organ damage, and possibly death [78]. Such complications are most likely to occur when NPWT is applied to patients whose wounds have devitalized tissue or exposed vital structures (eg, organs, blood vessels, vascular grafts). (See 'Contraindications' above.)

Bleeding — Bleeding is the most serious complication of NPWT and can occur in hospitals, long-term care facilities, and at home [76]. Minor bleeding during dressing changes due to granulation tissue at the base of the wound is common and is best managed with firm pressure on the wound surface. Severe hemorrhage can occur during the removal of foam that has become adherent to the granulation tissue below, especially in patients who are anticoagulated or in patients with exposed vessels or vascular grafts. In patients with severe bleeding, direct pressure should be applied, and emergency services should be contacted. Surgery may be needed to control bleeding.

Infection — Infection related to the use of NPWT is often due to prior wound infection that was inadequately controlled prior to initiating NPWT. When infection is suspected (eg, fever, erythema, cellulitis), the NPWT dressing is discontinued, the wound irrigated and debrided, wound cultures obtained, and empiric antibiotics initiated. (See "Acute cellulitis and erysipelas in adults: Treatment".)

Enterocutaneous fistula — Scattered case reports suggest that NPWT may expedite control and closure of postoperative enterocutaneous fistula. However, NPWT is also more likely to cause enteric fistula formation [79-83]. (See "Management of the open abdomen in adults" and "Enterocutaneous and enteroatmospheric fistulas".)

SOCIETY GUIDELINE LINKS — 

Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Abdominal compartment syndrome" and "Society guideline links: Open wound management".)

SUMMARY AND RECOMMENDATIONS

●Negative pressure wound therapy – Negative pressure wound therapy (NPWT), also called vacuum-assisted closure, is an adjunctive therapy used in the management of open wounds that applies subatmospheric pressure to the wound surface. The wound care system consists of an open-cell foam dressing, a semiocclusive adhesive cover, a fluid collection system, and a suction pump. (See 'Introduction' above and 'Device and placement' above.)

●Mechanism of action – NPWT exerts its effect through direct and indirect effects of subatmospheric pressure. These effects include stabilization of the wound environment, increased blood flow, and deformation of the wound. Deformation is a powerful stimulus for cellular processes that stimulate granulation tissue and accelerate wound healing. (See 'Mechanism of action' above.)

●Advantages and disadvantages for wound healing – NPWT has several advantages over traditional wound management, including simplification of wound care (primarily through a reduced number of dressing changes and ease of tailoring the dressing), accelerated wound healing, and reduction in the complexity of subsequent reconstructive procedures. (See 'Advantages and disadvantages' above.)

●Effectiveness – NPWT has been used in the treatment of acute and chronic wounds. High-quality data supporting the use of NPWT are available only for the management of diabetic foot wounds. NPWT used prophylactically may reduce the incidence of superficial surgical site infection (SSI); however, it has not been shown to be cost-effective. (See 'Clinical utility' above.)

●Contraindications – NPWT should not be directly used on exposed vital structures (organs, blood vessels, or vascular grafts) or on malignant tissues. Perfusion to the wound should be adequate, as NPWT can cause or worsen tissue ischemia in accordance with its mechanism of action. Devitalized tissues should be debrided, and infection should generally be controlled prior to undertaking NPWT; however, instillation therapy has allowed the use of NPWT in the presence of infection or to augment the surgical management of infection. Other relative contraindications include skin or tissue fragility and adhesive allergy. (See 'Contraindications' above.)