The precise mechanism of action of negative pressure therapy is largely unknown, but three common theories prevail. The fluid-based theory proposes that interstitial fluid, which may otherwise compromise the microcirculation and oxygen delivery, and contain substances that negatively impact wound healing, is removed from the wound.1 The second is a mechanically driven process in which cell proliferation is increased in response to mechanical tissue stress. This stimulates angiogenesis and cellular growth and helps to draw in the wound edges.2 The third mechanism is a reduction in bacterial load.3,4 It is not proven that negative pressure therapy will specifically alter collagen ratios.5
Macrostrain refers to the process whereby negative pressure will cause the contact wound dressing to collapse. Negative pressure becomes equally distributed across the wound and force is transferred to the wound edges, bringing them closer together. Microstrain refers to the microdeformation that occurs across a foam dressing in a negative pressure system. This leads to induction of mechanical stress and stimulation of angiogenesis. There has been much research into the mechanical effects of negative pressure on the healing wound with trials of different pressure effects as part of this research. The other terms are not used to describe processes in NPWT.1–3
3. Agha R, Ogawa R, Pietramaggiori G, Orgill DP. A review of the role of mechanical forces in cutaneous wound healing. J Surg Res 2011;171(2):700–708 Black polyurethane foam is hydrophobic and has pore sizes of 400–600 µm compared with a pore size between 60 and 270 µm in polyvinyl alcohol white foam. The theoretical benefit is that tissue growth is maximized. However, there is no clear evidence to show that pore size makes a significant difference in the clinical setting. Clearly, reductions in infection rates, patient pain levels, wound exudate formation, and financial costs are all highly desirable with any given treatment. Indeed, negative pressure dressings have been shown to achieve these goals. However, there does not seem to be strong evidence to suggest that pore size will specifically alter any of these other parameters. Many studies have actually focused more on the comparison between foam and gauze in negative pressure therapy. Foam seems to be better suited for uniform contractible wounds, and gauze for shallow wounds and both remain popular. Reduced pain may be achieved by gauze in some settings.1–6
4. Plikaitis CM, Molnar JA. Subatmospheric pressure wound therapy and the vacuum-assisted closure device: basic science and current clinical successes. Expert Rev Med Devices 2006;3(2):175–184
5. Morykwas MJ, Argenta LC, Shelton-Brown EI, McGuirt W. Vacuum-assisted closure: a new method for wound control and treatment: animal studies and basic foundation. Ann Plast Surg 1997;38(6):553–562
6. Fraccalvieri M, Zingarelli E, Ruka E, et al. Negative pressure wound therapy using gauze and foam: histological, immu- nohistochemical and ultrasonography morphological analysis of the granulation tissue and scar tissue. Preliminary report of a clinical study. Int Wound J 2011;8(4):355–364 CONTACT LAYERS IN NEGATIVE PRESSURE WOUND THERAPY 4. Which one of the following base layers has the largest volume of evidence supporting its use in negative pressure wound therapy (NPWT)? Polyurethane foam was used as the original contact dressing for NPWT and most published evidence relates to this dressing. Polyurethane foam has a pore size designed to maximize tissue growth and is typically a black-colored synthetic with hydrophobic qualities. More recently, different foam types and gauze alternatives have become available as other manufacturers have entered into the market for NPWT devices. These include polyvinyl alcohol “white” foam which is hydrophilic and has a denser pore distribution (60–270 μm). Foam is considered better suited to uniform contractible wounds and gauze for shallow wounds. However, product availability, wound characteristics, patient factors, and surgeon’s choice should be taken into consideration when selecting the base layer.1–6
7. Llanos S, Danilla S, Barraza C, et al. Effectiveness of negative pressure closure in the integration of split thickness skin grafts: a randomized, double-masked, controlled trial. Ann Surg 2006;244(5):700–705 PRESSURE SELECTION IN NEGATIVE PRESSURE WOUND THERAPY 6. You are choosing a setting for a negative pressure dressing. Which one of the following represents the traditional industry standard for continuous pressure? Subatmospheric pressure is used in NPWT. This may be continuous or intermittent. The pressures commonly used range from −50 to −125 mm Hg. The industry standard has traditionally been −125 mm Hg and much of the evidence on pressures has been assessing blood flow, microdeformation, and granulation tissue production in porcine models at varying pressures. For example, Morykwas et al found that −125 mm Hg was most effective at producing granulation tissue in a porcine model and for a long time this has been considered to be the optimal pressure.1 However, in clinical practice, higher pressures can be less well tolerated by patients so reducing the pressure may be helpful. For this reason, studies have been conducted to assess parameters including blood flow at lower pressures, but remaining within the therapeutic range. For example, another animal study compared blood flow at a variety of pressures and found that pressures of −80 mm Hg gave comparable blood flow to −125 mm Hg. However, this paper did not assess granulation production or wound healing.2 In general, a higher negative pressure is considered beneficial in highly exudative wounds, whereas lower pressures should be considered in poorly perfused wounds. An international consensus meeting published guidelines in 2011 having reviewed available evidence at the time. This report suggested that it is increasingly apparent that there may not be one single optimal level of pressure but an effective therapeutic range of negative pressure levels between −40 and −150 mm Hg. More research is still required in this area to clarify the optimal settings.3,4
Although it may seem counterintuitive to use negative pressure therapy on a directly closed wound, incisional NPWT has been universally adopted as a useful treatment modality for some specific wound care settings. Incisional NPWT may be indicated in certain clinical situations where either incision-related or patient-related factors place the wound at an increased risk of healing complications. Patient-related factors include diabetes mellitus, American Society of Anesthesiologists (ASA) 3, advanced age, obesity (body mass index [BMI] 30 kg/ m2), tobacco use, hypoalbuminemia, and corticosteroid use. Incision-related factors include prolonged surgical time, reoperation or re-exploration, edema, extensive undermining, contamination, traumatized soft tissue, and high-tension closure. Incisional NPWT is thought to provide splinting of incisions by reducing edema and tension across the wound while continuously evacuating excessive drainage, thereby avoiding skin irritation and bacterial colonization. It may have a positive effect on local wound blood supply and offloading of pressure. It has been used in a number of settings including internal fixation of lower limb fractures and complex abdominal wall reconstruction.1–4 The technique involves placement of thin strips of adhesive dressing just lateral to the suture or staple line and placement of a nonadherent contact dressing over the incision. Sponge is then placed along the length of the incision over the nonadherent dressing, thereby avoiding contact with the skin. There are also “off the shelf” products designed for this purpose. Although these are neat and user friendly, they do not generally allow absorption of higher volumes of exudate compared with traditional negative pressure dressings and this must be taken into consideration when selecting the appropriate dressing.1–5 NPWT should not be used in the following scenarios: exposed vessels and nerves, malignancy in the wound, untreated osteomyelitis, fresh anastomotic site, or in a site with necrotic tissue with eschar still present.5
Evidence is currently lacking for the use of NPWT as a primary treatment in venous ulcers, although its use as an adjunct to skin grafting in these cases is supported. In contrast there is Level I evidence to support the use of NPWT in the primary management of diabetic foot ulcers both in terms of time to wound closure and decreased incidence of limb amputation.1–4 In large abdominal wall and chest defects, NPWT can be beneficial to respiratory function with decreased ventilator support requirement and shorter duration in high dependency care.5–8 The use of NPWT on split-skin grafts has Level I evidence to support its use and is especially useful in areas such as the perineum and axilla. It not only improves graft appearance, but also improves graft take in some situations and has been shown to reduce subsequent hospital stay. Pressures may be reduced in this setting and continuous settings are preferred. This increased effectiveness would have a major impact on cost savings that would potentially outweigh the NPWT costs.9