5 questions-and answers-about negative pressure wound therapy

Advances in Skin & Wound Care, Sep/Oct 2002 by Koth, Luther C

Q: What is negative pressure wound therapy?

A: Negative pressure wound therapy (NPVM is an adjunctive wound treatment that involves the application of a controlled level of subatmospheric pressure to a wound at 50 mm Hg to 175 mm Hg.1 Generated by a portable programmable pump, the suction effect is applied to the entire interior surface of a clean wound through open-cell polyurethane or polyvinyl alcohol (PVA) foam.2 The polyurethane foam is black and has a pore size of 400 (mu)m to 600 (mu)m, and the PVA foam is white and has a pore size of 200 (mu)m to 300 (mu)m.

The foam dressing must be packed in the wound cavity so that it has contact with the entire base, sidewalls, and undermined and tunneled surfaces. If more than 1 piece of foam is needed, each additional piece must make contact with another piece to ensure that the suction effect is distributed to all wound surfaces. The dressing is then secured in the wound by covering it with a semiocclusive polyurethane film drape supplied with the device. The film drape should also cover at least 4 cm of surrounding intact skin.

The suction effect of the device is exerted on the foam dressing via a polyvinyl tube that is inserted into the foam through a slit made in the vapor-permeable film drape. Care must be taken to create an airtight seal where the tube passes through the slit in the drape by pinching an additional strip of semipermeable film around the tube. The opposite end of the polyvinyl tube is then connected to a tube that leads to a 300 cc canister that is inserted into a recess in the vacuum pump. The disposable canister serves as a collection depot for edema fluid that is suctioned from the wound tissue.

Q: NPWT is indicated for what types of wounds?

A: In 1995, the Food and Drug Administration (FDA) approved NPVVT for the treatment of nonhealing wounds. In January 2000, the FDA expanded the indications for NPWT to include chronic, acute, traumatic, subacute, and dehisced wounds, diabetic ulcers; pressure ulcers; flaps; and grafts. Table I lists examples of patients with wound types for which NPWT may be appropriate.

Q: When would NPWT be contraindicated?

A: NPWT is contraindicated3 when:

* wounds contain necrotic tissue

* osteomyelitis is untreated

* fistulas to body cavities or organs are present

* malignancy is present in the wound

* treatment would place the foam dressing directly over arteries and veins that are exposed in the wound.

NPWT should be used with caution3 on patients when there is active bleeding in the wound, when hemostasis is difficult following debridement, or when anticoagulant therapy is used.

Q: Why is NPWT thought to be effective in managing wounds?

A: Several research studies and clinical trials have demonstrated positive effects of NPWT on wound healing. Three experimental studies-2 on porcine models 4,5 and 1 on humans6have provided evidence that NPWT enhances granulation tissue formation, clears edema fluid and bacteria from wounds, and increases cutaneous perfusion and oxygen tension. Other experimental studies have provided evidence that mechanical stress-in the form of stretching or expansion applied to tissues and cells (which likely occurs with NPWT)-induces a wide variety of cells to proliferate in vivo (nerve, blood vessels, skin) and in vitro (endothelial and vascular smooth muscle cells and fibroblasts).7-11 In addition, 9 studies involving human subjects have reported favorable outcomes following the use of NPWT to promote closure of a variety of wounds. Of these 9 studies, 4 were noncontrolled case series,6,12-14 3 were individual case studies,15-17 and 2 were small randomized controlled trials.17,18 Four of these studies reported significant increases in granulation tissue formation,1,12,14,18 while other studies reported significantly faster progress toward wound closure or improved skin grafting outcomes.6,15,16 One study of 10 diabetic foot wounds reported a clinically significant decrease in the size of wounds treated with NPWT compared with wounds treated with saline-moistened gauze.19

Q: Are different wounds managed differently with NPWT?

A: The NPVVT protocol for management of a wound depends on several factors. Some factors are related to the type of wound and whether it needs removal of edema fluid containing a high bacterial burden, facilitation of granulation tissue growth and/or contraction, or promotion of flap or graft adherence to the wound base. Given the FDA's indications for NPWT, the exclusive manufacturer of the only NPWT device in the United States (Vacuum-Assisted Closure, [V.A.C.]; Kinetic Concepts, Inc [KCI], San Antonio, TX) further delineates recommended guidelines for 7 wound categories to be managed with their NPWT device, including acute/traumatic wounds, surgical wound dehiscence, pressure ulcers, chronic ulcers (diabetic, arterial, vascular), meshed grafts, fresh flaps, and compromised flaps.2 The recommended guidelines encompass the selection of the initial and subsequent operation mode (whether it be continuous or intermittent), selection of the target suction pressure for polyurethane and PVA foams, and the time interval between foam dressing changes.