A Guide To Current And Emerging NPWT Modalities

Adam Isaac, DPM, and David G. Armstrong, DPM, PhD, MD

In addition to assessing the mechanisms and impact of negative pressure wound therapy (NPWT) in lower extremity wound care, these authors discuss the advent of fluid installation therapy in thwarting complications, recent literature on the use of the modality for closed incision sites and wounds secondary to traumatic fractures, and the potential of portable NPWT for small wounds.

In 2011, an estimated 366 million people worldwide had diabetes and this number is expected to increase to 552 million by 2030.1 In the United States, diabetes affects 25.8 million people (8.3 percent of the population) with more than 60 percent of non-traumatic lower limb amputations occurring in patients with diabetes.2

   Of these amputations, 85 percent are preceded by a foot ulcer and the risk of developing a foot ulcer during the lifetime of a person with diabetes can be as high as 25 percent.3 The management of these foot wounds, with the ultimate goal of amputation prevention, remains one of the great challenges faced by the multidisciplinary foot care team today.

   Over the past 20 years, negative pressure wound therapy (NPWT) has proven to be a safe, effective modality for the treatment of diabetic foot ulcers. Most commonly delivered by a Vacuum Assisted Closure device (VAC therapy, KCI), the NPWT system consists of a reticulated, open cell foam, covered with a semi-permeable adhesive drape, and connected to a negative pressure therapy unit via evacuation tubing.

   The foam, engineered to apply a uniform pressure throughout the entire wound bed, comes in several varieties including: polyurethane ether; polyurethane ether with microbonded metallic silver impregnated into the foam; and white, pre-moistened, hydrophilic polyvinyl alcohol foam. The negative pressure unit is typically at 125 mmHg on a continuous or intermittent setting.4,5

   In 1997, Morykwas and colleagues reported a fourfold increase in blood flow to the subcutaneous tissue and muscle of Chester pigs when applying VAC therapy at 125 mmHg over 15-minute intervals.6 During the same interval, however, the blood flow values decreased to below baseline at pressures greater than 400 mmHg. The authors also found that the increase in local blood flow subsided after a period of five to seven minutes but an “off” interval of two minutes was sufficient for re-establishing the increase in blood flow. This represents the concept behind intermittent pressure.

   Timmers and co-authors applied VAC therapy to the healthy, intact forearm skin of 10 healthy volunteers and measured the response of cutaneous blood flow to different values of negative pressure between 25 and 500 mmHg.7 The authors reported a significant increase in cutaneous blood flow at a negative pressure of 300 mmHg, more than double the pressure used in previous studies. In fact, even at negative pressures approaching 500 mmHg, they found no decrease in the baseline blood flow.

   In 2005, Armstrong and Lavery reported on the results of a multicenter, randomized clinical trial comparing the proportion and rate of wound healing in 162 patients with diabetic wounds secondary to partial foot amputation and evidence of adequate perfusion.8 The patients were randomly assigned to two groups, one of which received NPWT while the other group had standard moist wound therapy. The study, which took place over a period of 16 weeks in 18 study sites, found that treatment with NPWT resulted in a higher number of healed wounds (56 percent versus 39 percent) as well as a faster rate of wound healing. Furthermore, the group receiving NPWT demonstrated an increase in the rate of granulation tissue formation as well as a potential trend toward a reduced risk of re-amputation.

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