Negative pressure wound therapy: an important adjunct to wound care.

The advent of negative pressure wound therapy (NPWT) is a major development in wound care. It has now given physicians and primary care providers another option in dealing with complex wounds. Since Argenta first reported his series of patients who utilized NPWT in 1997, (1) there has been an explosion in its use.

Experimental studies have shown that negative pressure therapy enhances the formation of granulation tissue and angiogenesis. Micromechanical forces exert distraction forces, which draw the wound edges together. The suction force decreases the wound exudate and has been shown to alter bacterial counts. However, we still do not know the precise mechanism by which negative pressure brings about wound healing. The growth factors and cytokines responsible for initiating the process of cell migration and angiogenesis have yet to be elucidated. Although models of negative pressure therapy have been created, it has yet to be demonstrated in vitro or vivo that traction forces on the wound surface are related to the biologic changes within the wound environment. Moreover, biomechanics surrounding the ideal negative pressure settings and the mode of delivery (intermittent/continuous) has not yet been investigated. These gaps in our knowledge of the biologic mechanism of negative pressure therapy provide opportunities for future research.

One trial investigating chronic wounds reported a significant difference in the percentage change in wound volume at 6 weeks in favor of NPWT (78% versus 30%). (2) Another trial involving diabetic patients showed a difference in the number of days to satisfactory healing in favor of NPWT (22.8 [+ or -] 17.4 versus 42.8 [+ or -] 32.5 d), although no statistical analysis was performed. (3) Definitive closure was achieved by delayed primary closure in 4 of 5 NPWT patients and in 2 of 5 control patients. One NPWT patient and three control patients ultimately achieved healing by secondary intention. In addition, they reported a difference in the percentage change in surface area at 2 weeks in favor of topical negative pressure, although no statistical analysis was performed.

Negative pressure therapy has been shown to decrease the need for free tissue transfer in acute traumatic wounds, with exposed bone/hardware and tendon. It prepares the wound bed for definitive skin grafting or local muscle/ fasciocutaneous flap. Although it may not abolish the need for free tissue transfer in massive wounds with exposed bone/hardware where tissue bulk is needed, further studies are needed in this regard. Studies are also needed in the long-term outcome following reconstruction regarding tissue bulk, wound breakdown, functional outcome and quality of life.

Negative pressure therapy has been shown to enhance chronic wound closure with decreased time to healing, decreased exudate and lower bacterial counts. The need for less radical surgery has been shown in patients undergoing negative pressure therapy and the incidence of readmissions post healing has also been reduced. However, for patients with concomitant diabetes mellitus and peripheral vascular disease, there is a tendency for a higher failure rate of negative pressure therapy. There is still a paucity of randomized controlled trials for the use of negative pressure therapy on chronic wounds. The two trials reported (2,3) had small sample sizes, which makes accurate data comparison unreliable. Future randomized controlled studies of negative pressure therapy must involve double blinding, an objective method of assessing wound dimensions and flawless methodology.

NPWT aids in the debridement of pressure wounds. However, in the presence of osteomyelitis, unless the bone and necrotic tissue are debrided, poor results will be obtained. It is still unclear if negative pressure therapy leads to quicker wound healing. Moreover, it is unknown why negative pressure therapy is most effective in the first 2 weeks and subsequently tends toward a plateau phase. The gaps in the published literature exist in the long-term outcome of pressure wounds healed by NPWT therapy. There are no studies investigating the recurrence rate of pressure wounds post healing.

Negative pressure therapy has been shown to aid in the healing of skin grafts. The reasons for this are still unclear, although it is proposed that negative pressure results in increased apposition of the graft with the wound bed, suction of hematoma and decreased shearing effect of the graft. Further research is needed to determine the role of the wound bed (example, cavity wounds, flat wounds) and the anatomic site in the take of skin grafts with negative therapy.

Certain complications of the VAC technique are documented, such as overgrowth of granulation tissue into the sponge with bleeding at dressing change, recurrent infections and maceration of adjacent skin. The exact extent of these problems is still unknown, with only isolated reports of complications.

Concern has been expressed in certain quarters on the excessive and unjustified usage of NPWT. One study reported a few cases of prolonged application of NPWT at the expense of early surgical reconstruction, which compromised the outcome in those cases. (4) They also noted an expansion of the list of indications for its use since 1997. While granulation tissue is encouraging, it is not the panacea to wound closure. Too much emphasis may be placed on waiting for granulation at the expense of timely and expeditious surgical reconstruction.

In conclusion, I believe that negative pressure wound therapy is here to stay. However, it should only be used with a clear management plan within a defined time frame. Should any unfavorable response be seen with this therapy, early referral to the plastic and reconstructive surgeon is warranted.

References

1. Argenta LC, Morykwas MJ. Vacuum-assisted closure: a new method for wound control and treatment: clinical experience. Ann Plast Surg 1997;38:563-577.

2. Joseph E, Hamori CA, Bergman S, et al. A prospective randomised trial of vacuum assisted closure versus standard therapy of chronic non-healing wounds. Wounds 2000;12:60-67.

3. McCallon SK, Knight CA, Valiulus JP, et al. Vacuum-assisted closure versus saline-moistened gauze in the healing of postoperative diabetic foot wounds. Ostomy Wound Manage 2000;46:28-34.

4. Dieu T, Leung M, Leong J, et al. Too much vacuum-assisted closure. ANZ J Surg 2003;73:1057-1060.

We are here to add what we can to life, not to get what we can from it.
--William Osler

Derick Amith Mendonca, MBBS, MSC, MRCS

From the Department of Burns, Plastic and Reconstructive Surgery, Selly Oak Hospital, Birmingham, United Kingdom.

Reprint requests to Derick Amith Mendonca, MBBS, MSc, MRCS, 3, St. Chads Court, Cross Keys, Lichfield WS13 6EA, United Kingdom. Email: [e-mail]

Accepted February 14, 2006.