Bacterial Burden and Wound Outcomes as Influenced by Negative Pressure Wound Therapy
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Abstract: Negative pressure wound therapy (NPWT) has consistently shown significant clinical benefits in wound healing, but the mechanisms are not fully elucidated. While a reduction in bacterial burden is one possible contributor, studies have shown mixed results in this regard. The present study used a porcine infected wound model to test the effect of NPWT on bacterial burden. Methods. Infected wounds (Pseudomonas aeruginosa, coagulase negative Staphylococcus, and Bacteroides fragilis) in a porcine model were treated with V.A.C.® therapy with the standard GranuFoam™ dressing, V.A.C. therapy with the GranuFoam Silver® dressing, or moist gauze for a period of 7 days with three dressing changes. Quantitative and semiquantitative bacterial cultures, histological samples, and digital photographs were taken at dressing changes. Results. The wounds continued to show gross and microscopic improvement when treated with standard NPWT and NPWT with silver compared to moist wound care controls. However, the bacterial burden in all wounds continued to increase and broadened to include local skin flora, which had been absent immediately after wounding. These increases in bacteria were not affected by the use of silver dressings. Conclusion. Negative pressure wound therapy with either standard NPWT foam or silver NPWT foam produced significant improvements in local wound appearance. This occurred despite a persistently high level of bacterial infection; thus, the improvement in healing of these infected wounds cannot be explained by a change in the bacterial burden.
Address correspondence to:
John C. Lantis, MD
St Luke’s-Roosevelt Hospital
Vascular/Endovascular Surgery
1111 Amsterdam Ave
MU 208
New York, NY 10025
Email: JLantis@chpnet.org
The benefits of negative pressure wound therapy (NPWT) are now well established. Multiple trials have shown improved wound healing and faster wound closure with NPWT devices.1–3 There are several mechanisms through which NPWT achieves these outcomes, including increased blood flow to the wound bed and removal of surrounding interstitial fluid, which facilitates granulation tissue growth.4 Another mechanism that has been proposed is an increased resistance to infection and improved bacterial clearance in the wound. Early studies in pigs suggested that the vacuum-assisted closure device (V.A.C.® Therapy, KCI, San Antonio, TX) enhanced bacterial clearance in full-thickness wounds that were inoculated with Staphylococcus aureus or Staphylococcus epidermidis.5 Further studies in humans have failed to show this benefit,1,6,7 and it remains unclear whether NPWT exerts an antimicrobial effect.
There are a few theoretical ways in which NPWT could promote bacterial eradication or clearance. Mechanical debridement and removal of exudate could physically clear bacteria and biofilms.8 Increased blood flow could enhance the tissue’s immune response. Increased oxygen delivery may enhance neutrophil oxidative bursts, and the higher oxygen tension may prevent anaerobic growth.3,9 Finally, the use of dressings that incorporate the naturally bactericidal element silver, such as the GranuFoam Silver® (KCI, San Antonio, TX) for the vacuum-assisted closure device may aid in stemming bacterial growth through call wall lysis and poisoning of the intracellular processes of bacteria.10
These effects are theoretical, though, and may not apply to the clinical setting. The aim of the present study was to test the antibacterial effects of the vacuum-assisted closure device in a controlled animal study using the standard foam and the silver foam.
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