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Sound Evidence: Combination of Negative Pressure Wound Therapy and Acoustic Pressure Wound Therapy for Treatment of Infected Surgical Wounds: A Case Series
Infected wounds are difficult to treat and often require “advanced” wound healing modalities to supplement conventional wound care using moist dressings and topical agents.1 In these wounds, the presence of devitalized tissue and bacterial infection slows both the granulation process and the progression of wound closure.2 Two advanced wound-healing modalities apply forms of mechanical pressure to the wound tissue that is intended to promote healing by stimulating cellular proliferation. Negative pressure wound therapy (NPWT) employs an open-cell, foam dressing with adhesive drape connected to a vacuum pump that applies either intermittent or continuous subatmospheric pressure to the wound tissues. Acoustic pressure wound therapy (APWT) delivers ultrasound-generated acoustic pressure to wound tissues via a fine mist of sterile saline. Whether these two therapies stimulate the cellular activity necessary for wound healing is not definitively known. Regardless, published studies1,3 of NPWT and APWT have reported noteworthy improvements compared to conventional wound care in outcomes such as time to wound closure, proportion of wounds closed, and volume or area reduction. It should be noted that the preponderance of evidence supporting NPWT and APWT comes from case series reports and small randomized trials.1,3 Negative pressure wound therapy is specifically associated with reduced drainage from wounds with large amounts of exudate but is not considered a debridement modality.4 Conversely, APWT is not known for reducing drainage per se but is indicated for cleansing and maintenance debridement by removing yellow slough, fibrin, tissue exudates, and bacteria.
Using a combination of NPWT and APWT in a series of infected wounds post surgery or surgical debridement was evaluated at a 60-bed, acute rehabilitation hospital.
Case Series
Six patients with large, infected wounds of surgical origin with serosanguineous exudate were treated with a combination of NPWT (V.A.C.® Kinetic Concepts Inc., San Antonio, TX) and APWT (MIST Therapy® System, Celleration, Inc. Eden Prairie, MN). A summary of patient and wound characteristics, medical histories, and wound treatments is provided in Table 1. Infections were confirmed by swab culture. Six wounds were in need of therapy to remove devitalized tissue and microbial infection, as well as promote granulation tissue formation and, ultimately, wound closure.
After unsuccessful attempts at debridement using wet-to-dry dressings daily over a period of 1 to 3 weeks, a combined approach using NPWT and APWT was pursued. Before initiating NPWT and APWT, the wounds had been present and nonhealing for anywhere from 11 days to 8.5 weeks. In accordance with the manufacturer’s clinical guidelines, NPWT dressings were applied with continuous pressure at 125 mm Hg and dressings were changed three times per week. Acoustic pressure wound therapy was administered three times per week at the same visits as the NPWT dressing changes; APWT treatment duration (minutes per session) was based on wound area per the manufacturer’s recommended treatment algorithm (ie, longer times for larger wounds). No other wound care modalities were administered during the study period.
As shown in Table 1, NPWT and APWT were administered concurrently over treatment periods ranging from 4 to 12 weeks; all but one patient received 8.5 weeks of treatment or less. Treatment involved between 4 and 20 minutes per session, with the majority needing 9 minutes or more. Patients received a total of 14 to 26 APWT treatments each.
Also shown in Table 1, wound volume was reduced by 99% to 100% in all wounds, except for Patient 5 in whom depth at end of treatment was not measurable due to hypergranulation in most of the wound bed (NPWT was continued in this wound until very small areas of bony presence were covered.) Surface area of the wounds was reduced by 82% to 100%, with the exception of the hypergranular wound, which was reduced by 60%. Serosanguineous wound drainage was reduced from copious/large to minimal/small in three wounds, copious to moderate in one wound, and remained unchanged in two wounds (one large, one moderate). Wound healing in patient 2 is shown in Figure 1a-c. 
Discussion
Optimal wound therapy for infected wounds post surgery or for surgical debridement has not been established.4-6 The patients in this series were at increased risk for slow healing of their postsurgical wounds due to multiple medical comorbidities (listed in Table 1). Understanding the reported clinical benefits of NPWT and APWT individually, we hypothesized that enhanced benefit might be attained from using the two therapies simultaneously in an effort to hasten healing in these medically complex patients.
Negative pressure wound therapy is preferred for wounds with the substantial drainage seen in this case series. With the exception of one randomized prospective study of NPWT versus saline wet-to-moist dressing,7 the great majority of studies on NPWT have been consecutive case series in open wounds healing by secondary intention.1 This modality has been shown in these studies to reduce drainage in such wounds and some evidence suggests it may hasten healing.1,7 Additionally, preliminary data from a randomized, controlled study of NPWT for intact surgical wounds indicates significant drainage reduction with NPWT than with standard postoperative dressings (3.1 days versus 1.6 days; P = 0.03).4 The published studies of APWT to date have evaluated this noncontact, nonthermal, low-frequency ultrasound therapy in chronic wounds primarily of the lower extremities, including two randomized, controlled trials (one in diabetic foot ulcers8 and another in patients with chronic critical limb ischemia9). Again, these trials and published reports of consecutive case series suggest that APWT may speed healing of chronic wounds.3
The current paucity of evidence from high-quality trials is not limited to NPWT and APWT. Current evidence supporting “modern” dressings (ie, foam, bead, alginate, or hydrocolloid) over traditional moistened gauze dressings in the treatment of postoperative wounds healing by secondary intention is also limited,6 with the exception of hydrocolloid occlusive dressings, which appear to be superior to conventional gauze dressings.10 Although pain, nursing time, and costs appear to be lower with the modern dressings (particularly foam compared with gauze), evidence of improved healing with modern dressings is inconsistent.5,6
Conclusion
Six patients were treated with a combination of NPWT and APWT in an attempt to hasten healing of large, infected surgical wounds. Given the lack of a clear best practice for the care of such wounds, this small case series offers some insight into the potential healing outcomes that may be expected with combination NPWT-APWT therapy. Prospective, randomized trials are needed to evaluate this combination therapy in an evidence-based manner.
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10. Singh A, Halder S, Menon GR, et al. Meta-analysis of randomized controlled trials on hydrocolloid occlusive dressing versus conventional gauze dressing in the healing of chronic wounds. Asian J Surg. 2004;27(4):326-332.
