An Historical Perspective on Negative Pressure in Wound Care

  Modern wound care has increased substantially, both in volume and complexity. To help address the spike in demand, there has been a concomitant advancement in both products and devices that are aimed at improving wound treatment. Negative pressure wound therapy (NPWT) has enjoyed a significant increase in use, and the devices employed have become more refined over time. There has been substantial debate over the efficacy of these devices, with publications often finding the data inconclusive.¹ Recent meta-analyses, however, have shown NPWT to be an effective treatment for chronic wounds.1 In addition to its impact on wound healing, control of exudate, ease of wound care, maintenance of appropriate wound moisture, and device portability have all added to the desirability of NPWT in the wound clinic. As wound care itself continues to evolve as a science, new devices and dressings are arriving that offer additional benefits to patient care. Mechanically powered NPWT represents a fresh take on NPWT, with its own unique advantages. This article reflects on the progress we have seen in NPWT in general and compares the usage of electrical and mechanical NPWT, which should be considered as yet another tool within the wound care practitioner’s armamentarium.

NPWT: A Continuous Improvement

  NPWT has undergone a significant maturation since inception. Surgically closed suction drains represent the earliest forms of NPWT. These drains close “dead space” and remove exudate, but are crude approaches to negative pressure. As experience increased, there were makeshift attempts at developing NPWT by using occlusive dressings and wall suction that was often delivered by surgical drains (Figure 1). This marked an improvement on simple, closed-suction drains by offering wound coverage, moisture control, exudate management, and “24-hour wound care” versus intermittent dressing changes.   As formal negative pressure devices came into the wound care market, treatment enjoyed an upgrade with measurable, constant pressure and improved dressings. The mechanism of action of these devices relies on simple physics. There is no true “negative pressure,” as everything around us exerts pressure — even atmospheric air. Atmospheric air is approximately 15 psi at sea level. Increases in altitude decrease this pressure (as there is less atmospheric air contributing to the pressure) and submersion in depths of water increases the pressure (due to the added pressure of the overlying water). Thus, there is only “relatively” negative pressure in that the pressure lower than atmospheric pressure is referred to as “negative.” Successful NPWT manages to provide adequate negative pressure in the setting of a changing wound environment.

Mechanical Vs. Electrical NPWT

   “Pressure” is defined as the density of air (or matter) in a given space. Powered NPWT removes air from the wound, creating a “negative” pressure environment (Figure 2). Alternatively, the space can be increased while maintaining the same amount of air, therefore decreasing the density of the air and the pressure in the space (Figure 3). This is the mechanism of action of mechanically powered NPWT, which utilizes a constant-force spring that applies a continuous force to the wound space and maintains a constant pressure. This allows the device to adjust to exudates and air leaks as needed to maintain an appropriate level of negative pressure (Figure 4). Both electrically powered negative pressure and mechanically powered negative pressure devices reduce air density and apply constant pressure. Mechanically powered NPWT devices, however, offer advantages such as smaller size of equipment, portability, shelf storage/availability, and no need for an electrical charge. Electrically powered NPWT may be better suited for larger wounds or those with excessive exudate, as mechanical products are constrained by limited canister size. One multicenter, randomized controlled trial directly compared these two approaches to NPWT among 132 patients living with diabetic wounds and venous ulcers. According to the findings, no difference in healing rates occurred when considering the use of mechanically powered versus electrically powered NPWT.² Additionally, both forms of NPWT have been introduced for closed-incision use. Both also aid in wound-tension relief, control of exudate, and maintenance of a sterile environment. Wound care of the closed surgical incision with devices such as these will hopefully prevent future need for chronic wound care. Lee J. Goldstein is assistant professor of surgery in the division of vascular surgery at University of Miami Miller School of Medicine’s DeWitt Daughtry Family Department of Surgery. He may be reached at lgoldstein@med.miami.edu.

References

1. Suissa D, Danino A, Nikolis A. Negative-pressure therapy versus standard wound care: A meta-analysis of randomized trials. Plast Reconstr Surg. 2011;128(5):498e–503e. 2. Armstrong DG, Marston WA, Reyzelman AM, Kirsner RS. Comparative effectiveness of mechanically and electrically powered negative pressure wound therapy devices: A multicenter randomized controlled trial. Wound Repair Regen. 2012;20(3):332–41.