Negative pressure wound therapy (NPWT), also known as topical negative pressure therapy (TNP),^1,2 and negative pressure wound care (NPWC),³ were first documented as an adjunct treatment for open wounds in the early 1990s.² By 2004 this active therapy was reported as standard of care for a variety of diagnoses in modern wound management.¹ Through the application of a closed wound dressing and attached suction, NPWT applies controlled, subatmospheric pressure to open wounds and has been shown to provide the following benefits for wound healing:

• 
  

  Promotion of moist wound healing⁴

  
  
• 
  

  Reduction of edema and interstitial fluid^1,4,5

  
  
• 
  

  Increased local perfusion^6–8

  
  
• 
  

  Approximation of wound edges^4,9

  
  
• 
  

  Stimulation of granulation tissue formation^9,10

  
  
• 
  

  Reduction in bacterial load^9,11

  
  
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  Reduction in the frequency of dressing changes¹²

Since its acceptance into evidence-based wound management, NPWT has been utilized to treat a wide variety of acute and chronic wounds¹³ including acute traumatic and surgical^2,14 wounds healing by primary and secondary intention; burns^15,16; and chronic wounds associated with venous insufficiency,^3,13 diabetes,^3,10,17,18 and pressure.¹⁹ Events such as the devastating 2010 earthquake in Haiti¹⁵ and war in the Middle East^15,20,21 have also shown how the benefits of NPWT can positively influence limb salvage rates and decrease morbidity and mortality in mass casualty and high-energy injury situations.

A NPWT system consists of a pump unit that provides suction, a wound filler that transfers negative pressure to the wound bed and allows flow of fluids from the wound, a transparent occlusive sheet that covers the wound filler and creates an airtight seal, and flexible tubing that delivers suction and serves as a conduit for removal of drainage and wound debris.²² Wound fluids are collected in a disposable container that is attached to the pump (FIGURE 15-1). TABLE 15-1 further describes NPWT components.

The first commercially available NPWT device marketed in the United States as a wound healing device²⁵ was the Vacuum Assisted Closure or V.A.C. (Kinetic Concepts, Inc (KCI), San Antonio, TX). The initial V.A.C. device was a somewhat bulky, electrically powered unit designed primarily for inpatient use.²⁵ A smaller, portable, battery-operated unit was designed later for use in the outpatient setting.^3,16 Over the past few years, an explosion in NPWT interest and development has occurred and now multiple vendors offer relatively silent NPWT devices in various sizes that are reusable, recyclable, or disposable; some units are specially designed for specific types of wounds (see TABLE 15-2 for partial vendor list). Increased healing, portability, and ease of management can result in decreased length of hospital stay, faster return to function,²⁶ cost savings,²⁷ and improved quality of life.

NPWT promotes wound healing primarily through removal of wound fluid, mechanical stimulation of cells, facilitation of wound contraction, and occlusion of the wound from environmental contaminants. As fluid is suctioned from the wound, interstitial edema is reduced. Inflammatory mediators and bacteria are removed along with tissue fluids, thereby reducing wound inflammation and facilitating progression to the proliferative healing phase. Mechanical stimulation at both the tissue (macrostrain) and cellular (microstrain) levels²⁸ encourages increased cellular proliferation, granulation tissue formation, and wound contraction.^14,29 The occlusive covering maintains a moist wound healing environment and an airtight seal that is vital in sustaining negative pressure.^1,5,14,30 Despite differences in size and shape, all NPWT devices basically offer the same benefits, and selection of a device may depend on the patient’s medical coverage. Most devices offer several options for “tailoring”²² NPWT to each patient need so that optimal care can usually be achieved regardless of specific vendor. CASE STUDY

INTRODUCTION

LS is a 59-year-old male referred for wound care of the left hand. He was admitted to the hospital with an infected bite on the dorsal hand that required extensive surgical debridement, which was performed the day before the initial consultation for wound care (FIGURE 15-2).

FIGURE 15-2

Case study wound for NPWT Left-hand wound after surgical debridement of infected tissue. There are exposed tendons and muscle, and Penrose drains (at the arrow) have been placed to allow drainage from the deeper tissue.

DISCUSSION QUESTIONS

1. 
   

   What medical information would you need from the patient’s chart to determine if NPWT is indicated?

   
   
2. 
   

   What wound characteristics would determine if NPWT is indicated?

NPWT has several documented effects at the cellular and tissue levels. Due to the close, interrelated nature of these effects, cellular and tissue changes associated with NPWT are discussed together and presented in the following main categories: occlusion, cellular deformation, fluid removal, circulation, contraction, and bioburden (TABLE 15-3).

Occlusion

The benefits of moist wound healing have been well established.^1,5,14,30 Since NPWT requires an airtight seal in order to maintain suction, all devices require that an occlusive dressing be applied over the open wound site. As long as the seal is maintained, a moist wound environment is created. NPWT dressings are changed every 48–72 hours; thus, exposure of the wound to air (which can decrease wound bed moisture and temperature, and subsequently slow down the healing process) is reduced and the moist wound environment preserved.

Cellular Deformation

The suction force associated with NPWT causes mechanical deformation of cells.^2,14,33 Referred to as microstrain^13,15,28 or simply strain,³³ the stretching force creates changes within the cells that result in increased release of growth factors and cytokines associated with cellular proliferation.^9,13 Increased cellular division and migration of macrophages, lymphocytes, and fibroblasts are also facilitated.^13,14,33 Mechanical deformation caused by negative pressure results in increased angiogenesis and granulation tissue formation and is one of the most significant effects of NPWT.^1,33 DeFranzo et al.³⁷ demonstrated that in healthy, noninfected wounds, newly formed granulation tissue is routinely present within 48 hours after NPWT is initiated.

Fluid Removal

The removal of interstitial wound fluid is a significant factor associated with NPWT.^1,5,14 Removal of edema associated with acute injury and trauma facilitates oxygen and nutrient diffusion³⁴ as well as decreased inflammation as inflammatory mediators are removed with wound fluid.^1,2 It has been well documented that chronic wound fluid is detrimental to wound healing^1,35,36,41 and due to its unique method of fluid removal, NPWT is especially beneficial in the treatment of chronic wounds.³⁷ Studies conducted by Labanaris⁴² and Dini et al.¹³ show that NPWT induces proliferation of lymph vessels and suggest that this aids in quick reduction of wound fluid especially in the first 4 days of treatment. Dini et al.¹³ also demonstrated that NPWT encouraged opening of collapsed lymph vessel lumens in chronic venous insufficiency wounds. In many cases, tissue edema can be significantly resolved within 3–5 days, thereby facilitating reduction of wound surface area fairly quickly.³⁷

Circulation

Local circulation is improved with NPWT^{6,7,8,14,28,33} through the combined benefits of cellular deformation and fluid removal. In an animal study, Borgquist⁸ demonstrated improved blood flow at 1.0 and 2.5 cm from the wound edge with the application of negative pressure. Conversely, it has been shown that due to compression of tissues, blood flow is actually decreased at the superficial wound edge and may extend as far as 0.5 cm from the wound edge indicating that NPWT may create a gradient of blood flow changes.^6,7,34 Researchers believe hypoperfusion at the wound edge acts as a stimulus for angiogenesis and is therefore seen as a beneficial effect of NPWT.^6,34 NPWT-induced hypoperfusion may not, however, be beneficial in situations where blood flow is already significantly compromised; further decreases in blood flow could result in ischemia and tissue necrosis. In this case, the use of lower negative pressures would be appropriate so that compression of tissue at the wound edge is reduced.⁸ Also, when wound edge blood flow is a concern, intermittent NPWT may be more appropriate than continuous therapy since the on/off cycles provide decompression or rest periods that may decrease the risk of ischemia.⁷ (Intermittent and continuous therapy are discussed in more detail in the section on Parameters.) In all cases, since the amount of wound edge compression is pressure dependent, it is recommended that the lowest effective pressure be used in order to decrease potential complications with hypoperfusion.²²

Contraction

Studies support that the centripetal¹ pulling or suction force of NPWT assists in approximating wound edges and with wound contraction.^14,32 This action, referred to as “macrostrain,”^9,15,28 combined with reductions in localized tissue edema via fluid removal, effectively reduces the wound surface area. Additionally, wound edge macrostrain has been shown to mechanically stimulate larger cytoskeleton structures within granulation tissue, resulting in increased cellular proliferation.^1,14,28

Bioburden

The exact method of how NPWT affects bacterial load remains unclear.^{9,15,20,22,32} A study conducted by Weed et al.³⁹ showed wound closure with NPWT even in the presence of 10⁶ bacteria. Other studies have demonstrated improved healing although bacteria levels actually increased during NPWT use.³² It appears that NPWT positively affects wound bioburden through all of the mechanisms discussed above.⁴³ Some of the newer NPWT units provide for the instillation of topical solutions as another mechanism of combating bacteria. Overall, NPWT is just one component in a treatment strategy for highly contaminated or infected wounds^14,17,33 and should be combined with other standard therapies, including sharp/surgical debridement, systemic antibiotics, irrigation with pulsed lavage with suction (see Chapter 17, Pulsed Lavage with Suction), and topical antibacterials.³² In 2009, a multidisciplinary expert panel stressed this point by identifying prerequisite recommendations regarding the use of NPWT on infected wounds. These recommendations include the following: the patient is free of systemic signs of infection, necrotic tissue is debrided and abscesses are drained, and perfusion is adequate for healing (TABLE 15-4 and FIGURES 15-3 and 15-4).⁴⁴

TABLE 15-5 provides a list of wound etiologies for which NPWT is indicated (FIGURES 15-5 to 15-12). The cellular and tissue effects need to be considered for each indication in order to obtain optimal patient outcomes and for future problem solving.

Acute Wounds

Evidence supports the use of NPWT for most acute surgical wounds¹⁴ and for traumatic^2,37 wounds (especially those with exposed bone, tendon,^1,27,45 or hardware¹) that require increased granulation prior to surgical closure or grafting. One exception to acute surgical wounds is an abdominal wound with thin tissue protecting any part of the gastric system in which case the negative pressure may increase the risk of enterocutaneous fistula formation. Due to the cellular and tissue effects of NPWT, placement over a new split-thickness skin graft (STSG) has been shown to increase graft “take” through edema reduction and graft stabilization^2,32 and is indeed considered by some as the standard of care for high-risk STSGs.² When treatment is initiated and the wound filler compresses, NPWT can assist with stabilization or splinting of the wound site. DeFranzo and colleagues³⁷ also found that NPWT utilized in complicated traumatic lower extremity, ankle, and foot injuries allowed patients to be more mobile sooner than with other dressings due to the increased stabilization. In mass casualty or battlefield situations, NPWT is used to stabilize and protect large open wounds until patients can be transported to advanced facilities for appropriate care.^15,20,32

NPWT application over suture lines improves wound integrity by maintaining wound edge approximation and decreasing periwound tension, thereby decreasing the risk of dehiscence in high-risk patients.^48,49,54 Disposable, 7-day NPWT units, such as Prevena (Prevena Incision Management System, KCI USA, an Acelity Company, San Antonio, TX) are available specifically for use with surgical incisions (FIGURE 15-13). NPWT may also be indicated for full-thickness burns after debridement of nonviable tissue has been performed.^9,50 However, guidelines developed by an expert panel in 2010 recommended that the decision to use NPWT on full-thickness burns should only be made by a burn specialist (refer to Chapter 10 for more discussion about treatment of burns).⁵⁰ Application of NPWT to deep partial thickness burns may prevent progression of tissue damage due to improved circulation and edema reduction.⁵⁰

Chronic Wounds

Successful use of NPWT has been documented for ulcers associated with pressure,^1,37,53 venous insufficiency,¹⁴ and diabetes¹ as well as chronic wounds related to reconstructive surgery²⁹ and exposure to radiation.¹⁹ NPWT can also be effective in palliative care of terminal patients with highly exudating malignant wounds by reducing odor, controlling drainage, and decreasing dressing changes (FIGURE 15-14).¹ NPWT in the presence of arterial insufficiency requires caution due to wound and periwound hypoperfusion as previously discussed.^6,8,34 CLINICAL CONSIDERATION

Toe and forefoot amputation sites after vascular reconstruction need careful monitoring to ensure that perfusion is adequate for healing, otherwise the pressure may occlude superficial capillaries and result in tissue necrosis.

Special Populations

NPWT has been used on patients of all ages including neonates and the elderly.²⁶ Evidence documenting NPWT use on the very young stresses the need for special consideration regarding patient size and weight as neonates, infants, and children are more susceptible to dehydration.²⁶ In pediatrics, pressure settings range from 50 to 125 mmHg and are dependent on age, wound etiology, and location.²⁶ In 2009, Baharestani and expert colleagues²⁶ published detailed best practice guidelines for the use of NPWT in pediatric patients (TABLES 15-6 to 15-8) and providers engaged in pediatric wound management are advised to follow these guidelines. Dehydration can also be an issue for elderly patients and fluid levels should be monitored closely in patients with high-output wounds. Physicians should be notified if two collection canisters are filled within a 24-hour period.¹⁹

Precautions and contraindications for NPWT are listed in TABLE 15-9. Wounds with low-level vascular compromise may benefit from NPWT and in this situation, research supports the use of intermittent mode therapy at lower pressure levels to reduce the risk of ischemia and ischemic pain.^{6,7,8,22,34,44} In 2004, Banwell and Musgrave¹ reported that NPWT showed no benefit for wounds with significant ischemia. This was reported again in 2012 by McCallon⁵⁵ who stated that NPWT was ineffective on wounds with significant proximal arterial occlusion. NPWT has the potential to increase wound ischemia leading to further tissue necrosis and ischemic pain,²⁷ therefore initiation of treatment is deferred until blood flow is restored by revascularization surgery or endovascular procedures. If blood supply remains marginal, treatment is deferred until signs of granulation appear at the wound edges indicating there is sufficient oxygenation to support angiogenesis. Signs of oxygenation are especially important if there is necrotic tissue in the wound that needs to be debrided before initiation of NPWT.

A study by Jung et al. reinforced the caution that is necessary with patients who have compromised vascularity, especially those with diabetes. TcPO₂ measurements were taken on the dorsal foot of patients with diabetes before, during, and after application of −125 mmHg negative pressure with polyurethane foam. A significant decrease in surface TcPO₂ occurred during treatment (84%) and after discontinuation of therapy (40.3%). The rationale for this decrease was the presence of vasculopathy that affects both the macro- and microcirculation associated with diabetes. Their results suggest that minimal compression of the foam is recommended when applying NPWT to the diabetic foot ulcer, and as stated above, NPWT should be applied after successful revascularization.⁵⁹