Comparison of the Effects of Different Negative Pressure Wound Therapy Modes-Continuous, Noncontinuous, and With Instillation-on Porcine Excisional Wounds

METHODS

All animal procedures were performed under a protocol approved by the Institutional Animal Care and Use Committee at the test facility. Five female domestic swine (weight range, 65-75 kg) were fully anesthetized and physiological parameters including heart rate, blood pressure, body temperature, and respiratory rate were monitored during the surgical procedure and postsurgical recovery. Each animal received 10 paraspinal (5 per side) dorsal full-thickness 5-cm diameter excisional wounds to the muscle fascia, with the epidermal, dermal, subdermal fat, and subcutaneous fat layers removed. Light pressure with saline moistened gauze was applied to the wound to stop bleeding post excision, as needed. Adjacent or contralateral wounds, as appropriate, were dressed and bridged together as pairs. Bridged wound pairs were assigned to one of the following treatment groups (Fig 1), with each pig having one of every treatment group:

Figure 1. Schematics of negative pressure profiles evaluated in this study: (a) continuous NPWT at -125 mm Hg; (b) intermittent NPWT with cycles of 5 minutes at -125 mm Hg followed by 2 minutes of 0 mm Hg; (c) dynamic NPWT with a 3-minute rise to -125 mm Hg followed by a 3-minute fall to -25 mm Hg; and (d) NPWTi with each cycle consisting of a short instillation phase, following by a 5-minute soak phase, followed by a 2.5-h negative pressure phase.

RESULTS

Wound images

A representative time course of images of a continuous NPWT-treated wound is shown in Figure 2. The 5-mm full-thickness excisional wound immediately following creation has clearly defined edges with the smooth fascia of the underlying muscle intact. At the first dressing change (after 2 days of therapy), the wound appears more pink and with some texture present, but no granulation tissue has formed. By day 5 and continuing to day 7, the wound bed is covered with beefy red granulation tissue.

Figure 2. Progression of a single representative wound at (a) day 0 creation, (b) day 2 dressing change, (c) day 5 dressing change, and (d) day 7 termination. Note the paucity of granulation tissue at day 2, suggesting the wound has not fully left the inflammatory phase of healing. However, the wound enters the granulation phase of healing by day 5, and a robust granulation layer is present by day 7. (Wound shown was treated with continuous NPWT, scale bar = 2 cm.)

Histomorphometry and histopathology

A representative photomicrograph of a continuous NPWT-treated wound is shown in Figure 3, with incremental granulation tissue thickness measurements indicated. The average granulation tissue thickness at day 7 for each treatment group is shown in Figure 4. There was no statistical difference between the 3 NPWT treatment groups (continuous NPWT [3.29 ± 0.33 mm], intermittent NPWT [3.03 ± 0.47 mm], or dynamic NPWT [3.40 ± 0.34 mm]). However, the NPWTi with saline treatment group (4.75 ± 0.54 mm) was significantly greater than all of these (P < .05). The granulation tissue in the NPWTi with saline treatment group was 44% thicker than in continuous NPWT, 57% thicker than in intermittent NPWT, and 40% thicker than in dynamic NPWT.

Figure 3. Representative photomicrograph of a wound treated with continuous NPWT. The top image shows the histological section stained with hematoxylin and eosin. The bottom image shows the same section with the bottom (black tracing) and top (green tracing) of the granulation tissue marked. The vertical blue lines are the incremental granulation tissue thickness measurements, spaced every 2 mm. These increments are averaged to determine the average granulation tissue thickness for this wound. (Scale bar = 5 mm)

Figure 4. Average granulation tissue thickness measured in histology specimens at day 7. Data are shown as mean ± standard error of the mean. (n = 10 wounds per group; *P < .05 for NPWTi compared to all NPWT groups)

Three-dimensional wound reconstruction measurements

Changes in wound perimeter and wound surface area from day 0 to day 7 are shown in Figure 5. These figures show a decrease in wound perimeter and wound surface area with statistically significant smaller mean values at day 7 for wounds treated with NPWTi with saline compared to wounds treated with continuous, intermittent, or dynamic NPWT modalities (P < .05).

Figure 5. Changes in wound perimeter (top) and surface area (bottom) calculated from 3D wound reconstructions. Data are shown as mean ± standard error of the mean. (n = 10 wounds per group, *P < .05 for NPWTi compared to all NPWT groups.)

The rates of wound volume (percent fill per day) from day 5 to day 7 were: continuous NPWT, 18.6% ± 3.0% per day; intermittent NPWT, 20.9% ± 4.1% per day; dynamic NPWT, 15.8% ± 2.7% per day; and NPWTi with saline, 26.1% ± 1.8% per day (Fig 6). The rate of wound fill with NPWTi with saline was statistically faster (P < .05) than with continuous NPWT (40% faster) or dynamic NPWT (65% faster); however, the difference in wound fill rate between NPWTi with saline and intermittent NPWT treatment group was not statistically significant (25%, P > .05).

Figure 6. The rate of change of wound volume, calculated as percent fill per day. Data are shown as mean ± standard error of the mean. (n = 10 wounds per group, *P < .05 for NPWTi compared to continuous NPWT and dynamic NPWT.)

DISCUSSION

The first documented uses of NPWTi have suggested the therapy to be important in the management of contaminated wounds with measurable bioburden.^13,19 However, several preclinical studies have suggested that the cleansing provided by NPWTi may also benefit noncontaminated wounds. In 2010, Leung et al published a study on pigs showing that NPWTi with saline elicited a faster rate of wound filling with granulation and increased collagen content compared to continuous NPWT.¹⁵ In subsequent work by Lessing et al,¹⁴ the differences in granulation were more pronounced: the granulation tissue in porcine wounds treated with NPWTi with saline was 43% thicker than in wounds treated with continuous NPWT after 7 days of therapy.¹⁴ However, as mentioned previously, these studies were interpreted with caution, because it was unknown whether the increase in granulation tissue was due to wound washing and cleansing or due to the noncontinuous nature of NPWTi.

This study compares, for the first time, continuous and noncontinuous (intermittent and dynamic) NPWT profiles to NPWTi. Despite prior reports by Morykwas et al⁵ and Malmsjo et al¹⁷ associating noncontinuous negative pressure with more granulation tissue than continuous negative pressure, no significant differences were observed between the continuous and noncontinuous NPWT groups for all evaluated outcomes in this study.

A limitation of the NPWT systems used in the studies of Morykwas et al⁵ and Malmsjo et al¹⁷ could explain this difference: less sophisticated NPWT systems measure the pressure at or near the pump, but not at the wound bed. Pressure measurement at the wound is critical to the pump's adjustment to challenges that develop in the tubing during the course of therapy, such as changes in exudate volume and viscosity, the vertical distance between the pump and the wound (referred to as head height), leaks, and blockages. Studies have shown that, when faced with these challenges, negative pressure systems or other vacuum sources that did not measure the pressure at the wound failed to deliver the set negative pressure level to the wound bed.^20,21 As stated by Ahearn¹⁸ in 2009 review, “suboptimal strain in wounds may result in suboptimal healing.” The therapy units used in the present study measured pressure at the wound bed, and review of the therapy and alarm logs indicate that there were no persistent therapy interruptions or deviations.

Continuous NPWT and NPWTi have been shown to create an environment that supports wound healing by promoting granulation tissue formation, so a subset of the analyses were designed to determine the rate of new tissue formation during this granulation phase. The canonical understanding of skin wound healing states that healing occurs in discrete, but overlapping, phases following injury: hemostasis (complete in the first minutes to hours postinjury), inflammation (beginning at injury but continuing for several days), proliferation and granulation (beginning 3-5 days postinjury), and remodeling (beginning several weeks postinjury).²² Thus, the day 0 and day 2 time points were excluded from the evaluation of the rate of granulation tissue formation. No statistically significant differences in the rate of change of wound volume (percent fill increase) were observed between day 5 and day 7 in the continuous and noncontinuous NPWT groups (Fig 6); however, NPWTi-treated wounds exhibited a faster increase in wound volume fill compared to the continuous and noncontinuous NPWT groups. Furthermore, wounds treated with NPWTi showed larger reductions in wound perimeter and surface area than with either continuous or noncontinuous NPWT, suggesting faster wound size reductions in NPWTi-treated wounds.

These data support the hypothesis that the mechanism for the increased granulation response with NPWTi is extended, deliberate wound cleansing beyond the initial debridement rather than the intermittent nature of the therapy. The layer of exudate on a wound has evolved to serve as a barrier to infection, and its components play important roles in wound healing: for example, cytokines attract host cells including immune and inflammatory cells, reactive oxygen species target microorganisms, and proteolytic enzymes including matrix-metalloproteinases break down devitalized tissue. However, when uncontrolled, these exudate components contribute to wound chronicity. Consequently, exudate management is a pillar of modern wound bed preparation, along with debridement and bioburden management.²³ NPWTi dilutes and removes excess wound exudates and infectious materials, and it can be considered as an extension of the initial wound debridement and cleansing. A clean wound environment may allow the limited cellular metabolic and energetic resources in the wound to be dedicated to healing pathways, including cell proliferation and matrix production, as opposed to immune and inflammatory responses.

After NPWTi is prescribed, a topical wound treatment solution must be selected and a protocol developed. Considerations for selection of a solution should include: wound type and overall status, known or suspected bioburden (level and type), goals of therapy, patient allergies, and recommendations of the solution manufacturer related to solution soak time and application frequency. Given the complexity and number of permutations of these factors, each wound should be evaluated individually and its NPWTi treatment program developed and revised as the wound progresses.

Compatibility of the solution with the NPWTi system and its components (including the tubing, dressing, drape, and adhesive) should also be considered, as reactive topical wound solutions could have adverse interactions with the components. A list of compatible solutions can be obtained from the manufacturer.

Recent clinical publications report the use of NPWTi-compatible topical wound treatment solutions, including the following.

• Polyhexanide on osteomyelitis of the pelvis or lower extremity,²⁴ skin and soft tissue wounds,²⁵ chronic or acute orthopedic wounds,¹² and infected and surgically debrided wounds²⁶

• Saline or sterile water on complex wounds and wounds where conventional NPWT was ineffective²⁷

• Hypochlorite-based solutions on venous stasis ulcers²⁸ and various other wounds¹¹

• Silver nitrate on complex wounds.²⁹

Other NPWTi-compatible solutions include mafenide acetate, octenidine dihydrochloride, acetic acid (ethanoic acid), and benzalkonium chloride; anecdotal reports of their use on individual patients are not yet available in the peer-reviewed literature. It is unknown how solutions other than saline may affect granulation tissue formation clinically.

Noncontinuous pressure modes will continue to play an important role in NPWT. At the subcellular level, the cytoskeleton of many cell types can adapt rather quickly to microdeformational changes.^30,31 As such, intermittent NPWT is, anecdotally, used to jump-start stalled wounds that have quit responding to continuous NPWT. However, intermittent NPWT is not commonly used at the beginning of a wound-treatment program because of the potential of patient discomfort associated with the contraction and expansion of dressings that occurs as negative pressure transitions between the therapeutic set point and 0 mm Hg. Dynamic NPWT has been developed to increase patient comfort during noncontinuous NPWT—the rate of pressure change is controlled, and the pressure does not drop below -25 mm Hg, preventing expansion of the dressing. Additional studies are needed to assess the effectiveness of intermittent and dynamic NPWT modes and their role as a clinical option for chronic and stalled wounds.

As with any preclinical evaluation of a medical device, the relevance of the model is open for discussion. The porcine excisional wound model is an acute injury in a young, healthy animal, and these wounds will generally heal without requiring advanced therapies; however, the porcine model is the accepted preclinical model for wound healing.³² Even with the limitations of the model, demonstration of a difference between advanced treatments is noteworthy; each treatment group was represented in duplicate in each pig, so internal controls were present. This study would be challenging to repeat in humans, because the heterogeneity of clinical wounds and lack of internal controls would require much larger sample sizes. Also, determination of granulation tissue thickness is only possible as a destructive measurement (ie, tissue must be collected as a biopsy or other excisional method to determine the thickness).

Finally, as dressing choice is often a topic of discussion in the literature, it should also be mentioned here that new ROCF dressings have been specially designed for use with NPWTi.¹⁴ These dressings have both increased mechanical properties to reduce the likelihood of tearing and fragmentation and reduced hydrophobicity, which improves their ability to distribute fluids within the wound bed.

Together these preclinical data suggest that wounds treated with NPWTi with saline instillation may exhibit faster granulation rates than wounds treated with either continuous or noncontinuous NPWT. The granulation response of NPWT may be tied to many factors, including wound microstrain, increased perfusion, edema reduction, and removal of exudate and debris that may impair wound healing. NPWTi provides these factors as well as automated delivery and removal of topical wound treatment solutions and suspensions with a controlled soak time. The soak phase provided by NPWTi also allows for controlled solution exposure times and may help to loosen soluble debris and exudate while increasing solution coverage of the wound bed, providing a cleaner environment for healing. Ultimately, further investigations to understand mechanisms of action of NPWTi and noncontinuous NPWT are warranted, and the significance of these findings must be confirmed in clinical studies.