Abstract: Negative pressure wound therapy (NPWT) is a topical treatment used to promote healing in acute and chronic wounds by applying negative pressure to the wound bed. The most widely used NPWT product in the United States and Canada is Vacuum-Assisted Closure® (V.A.C.® Therapy™, KCI Inc., San Antonio, Tex). This article briefly reviews the literature on V.A.C. pressure levels, discusses the conventional pressure settings that are commonly used with the V.A.C., and explores new literature that challenges commonly recommended pressure settings associated with use of the V.A.C.

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egative pressure wound therapy (NPWT) is a topical treatment used to promote healing in acute and chronic wounds by applying negative pressure to the wound bed. Providing vacuum pressure to the wound facilitates the drainage of excessive fluid and debris. The most widely used NPWT product in the United States and Canada is Vacuum-Assisted Closure® (V.A.C.^® Therapy™, KCI Inc., San Antonio, Tex). This article briefly reviews the literature on V.A.C. pressure levels and discusses the conventional pressure settings that are commonly used with the V.A.C. A new article by Wackenfors et al.¹ explores microvascular blood flow to an inguinal wound in pigs during V.A.C. therapy and challenges commonly recommended pressure settings associated with use of the V.A.C. The recommendations of Wackenfors et al. support early Russian literature and a new NPWT product, the Versatile 1™ (BlueSky Medical, LaCosta, Calif), which uses lower pressure levels than the levels recommended by the V.A.C. manufacturer for use with its device. The Wackenfors et al. article is an important step forward in understanding how NPWT works and lays the foundation for ways to better define treatment protocols, such as pressure intensity, duration of treatment, and treatment intervals. The complexity of wound healing necessitates that these unresolved issues be addressed through further research and clinical correlation if clinicians are to optimize the negative pressure modality.

Literature Review

       The literature supports use of NPWT on many wound types, such as pressure ulcers, abdominal wounds, skin grafts, sternal wounds, enterocutaneous fistulas, spinal wounds, leg ulcers, traumatic wounds, and fasciotomy wounds for compartment syndrome.^2–10
       Negative pressure wound therapy promotes wound healing through exudate removal, which helps establish fluid balance.11 It provides a moist wound environment,¹² removes slough,¹² potentially decreases wound bacterial burden,¹³ reduces edema and third space fluids, increases blood flow to the wound,^12–15 increases growth factors, and promotes white cells and fibroblasts within the wound.¹⁶
       Although NPWT has appeared in the literature for approximately 50 years,¹7 the physiological and molecular biological mechanisms by which NPWT accelerates wound healing are largely unknown. It is generally believed that blood perfusion and oxygenation are crucial to ensure proper healing.¹⁸ Granulation tissue formation, which is limited by the available vascular supply, has been shown to increase during NPWT.^2,14,19 The increased vascular supply may be the result of microcirculation around the wound during removal of excessive interstitial fluid, which decompresses small blood vessels and restores perfusion.¹
       Since little is known about microvascular blood flow activity in NPWT, clinicians lack defined practice parameters regarding pressure intensity, duration of treatment, and interval between treatments to provide the most efficient therapy. Current protocols are based on a few articles and have focused on a single product, the V.A.C. The V.A.C. has demonstrated healing efficacy in many different types of wounds. A consensus group, Canadian Vacuum Assisted Closure^®, was convened in 2004 to create “best practice” statements that serve to guide V.A.C. treatment approaches and stimulate further study.¹³ As noted, this group limited their scope to a single product entity, did not consider other treatment options, and based their statements on literature published after 1997.
       The V.A.C. was largely developed based on the work of Wake Forest University researchers Morykwas and Argenta.^2,14,20 In 1997, Morykwas and Argenta published 3 articles where subatmospheric pressure^2,14,20 was applied through a closed system to an open wound for periods of 48 hours. Subatmospheric pressure was applied at the wound surface through a reticulated, hydrophobic polyurethane foam, which allows for distribution of the negative pressure in either a continuous or intermittent mode based on the clinical experience of the physician.
       In 1997, Morykwas et al.²⁰ studied blood flow and showed that microvascular blood flow increases 4 times the baseline values with negative pressures of 125 mmHg, while blood flow was inhibited at negative pressures greater than or equal to 400 mmHg. Based on this result, and subsequent experience-based reports with the V.A.C., negative pressures of 125 mmHg became the most common setting when using the V.A.C. for NPWT.
       However, 10 years prior to the Morykwas and Argenta work, physicians in Russia were exploring NPWT. In 1987, Usupov and Yepifanov¹² utilized a rabbit model to identify a negative pressure range of 75–80 mmHg as optimal for wound healing. Usupov and Yepifanov also demonstrated new tissue hemorrhage of previously coagulated vessels with negative pressures below –120 to –125 mmHg.¹²
       A new product consistent with this early Russian work is now available. The Versatile 1 Wound Vacuum System™ (Versatile 1 WVS, Blue Sky Medical, La Costa, Calif) uses the lower vacuum levels consistent with these early Russian articles. With the Versatile 1 WVS, a single layer of gauze or other nonadherent porous contact layer is placed on the wound surface. A flat silicone hemovac drain is placed on the gauze to cover the maximum dimensions of the wound. A second piece of gauze is placed over the drain, creating a gauze sandwich around the drain. The entire gauze drain is covered with a clear semipermeable dressing cut to fit the dimensions of the wound and overlaps 3–4 cm onto healthy skin edges. The semipermeable dressing creates a seal over the wound. The drain is connected to tubing, which is then connected to the vacuum pump. Edema fluid is removed through the drain.
       In addition to discrepancies in pressure levels, there were differences in pressure intervals between the Morykwas and Argenta literature and the Russian experience. Morykwas and Argenta^2,20 stated that optimal results were obtained when NPWT was applied continuously for the first 48 hours and then applied in a intermittent manner (5 minutes on and 2 minutes off) after the initial 48 hour treatment. In 1986, Davydov et al.21 successfully applied NPWT twice daily for 2 1/2 to 3 hours.
       These confounding results indicate the need for further research. In a recent article, Miller and Lowery²² identified discrepancies between the findings of earlier Russian studies and those illustrated by Morykwas and Argenta. The disparity in findings caused the authors to call for further research to define the parameters for pressure intensity, duration of treatment, interval between treatments, mode of application, and timing of application to provide the most efficient and cost-effective therapy.
       Coincidentally, a new article on this subject appeared. Wackenfors et al.¹ explored the microvascular blood flow to an inguinal wound in pigs during V.A.C. therapy at negative pressures ranging from 50 to 200 mmHg. The authors used laser Doppler to measure blood flow in which the sum of all red blood cell motion was quantified in a volume of 1 mm³, making it feasible to reliably perform measurements in small, closely spaced skin areas.^23,24 The goal of the study was to examine how V.A.C. therapy affects microvascular blood flow, with consideration of tissue type and the distance of blood flow from the wound edge. Previous work by Morkywas and Argenta did not consider tissue type and blood flow distance.
       Wackenfors et al.¹ found that V.A.C. therapy induced an increase in microvascular blood flow a few centimeters from the wound edge, which may accelerate granulation tissue formation and the healing process. Conversely, closer to the wound edge, the V.A.C. therapy induced hypoperfusion that increases with increasing subatmospheric pressure and may result in ischemic tissue damage. Additionally, the type of tissue made a difference. The increase in blood flow occurred closer to the wound edge in muscular as compared to subcutaneous tissue (1.5 cm and 3 cm at negative pressures of 75 mmHg).¹
       Wackenfors et al.¹ suggested a balance is necessary when selecting the amount of negative pressure for NPWT treatments. The vacuum needs to be strong enough to drain the wounds and tamponade superficial bleeding yet, at the same time, not cause a large ischemic zone. Based on their findings, the authors concluded that when treating stiff tissue, such as muscle, a negative pressure of 100 mmHg may be reasonable, thereby limiting the extent of the hypoperfused zone to 1 cm from the wound edge. When treating softer tissue, such as subcutaneous tissue and fat that is more vulnerable to hypoperfusion, the application of a lower negative pressure, such as 75 mmHg, may be more beneficial.¹ Although these pressure recommendations are contradictory to common V.A.C. practices, the Russian literature supports the use of these pressures during treatment and validates the vacuum levels provided by the Versatile 1 WVS.
       Table 1 compares 3 NPWT systems.

Table 1

Conclusion

       Negative pressure wound therapy is beneficial in wound healing. Numerous case studies and articles clearly document its efficacy. However, little is known about the physiological and molecular biological mechanism by which NPWT accelerates wound healing. Additionally, questions remain in determining the optimum mechanism and protocols for deploying NPWT. Wackenfors et al.¹ used laser Doppler to measure microvascular blood flow to an inguinal wound in pigs during V.A.C. therapy. Their findings suggested setting the negative pressure at 100 mmHg for wounds in muscular tissue and using negative pressure set at 75 mmHg for wounds in softer tissue or fat. The authors’ pressure recommendations performed on V.A.C. equipment countered standard V.A.C. protocols, which typically use a negative pressure of 125 mmHg. However, the authors’ pressure recommendations support early Russian literature and coincide with pressure recommendations for the Versatile 1 WVS, which uses lower pressure levels.
       The Wackenfors et al. article¹ is an important step toward understanding how NPWT works and better defining treatment protocols, such as pressure intensity, duration of treatment, and interval between treatments. The complexity of wound healing necessitates that these unresolved issues be addressed through further research and clinical correlation if clinicians are to optimize NPWT.