Edema reduction and removal of wound fluid

Removal of exudate containing toxic cytokines, bacteria, and matrix metalloproteinases and reduction of interstitial edema is a primary benefit of NPWT (Ubbink et al, 2008). By reducing tissue edema, the compressive effects of edema upon the periwound vasculature are decreased, thereby improving tissue perfusion, delivery of nutrients, and uptake of oxygen culminating in the wound’s ability to resist bacterial proliferation and microbial penetration (Orgill et al, 2009). A decrease in wound bacterial load was first described in the porcine model by Morykwas et al (1997). Three mechanisms have been theorized to address the reduced wound bioburden associated with NPWT: (1) removal of stagnant wound fluid, (2) direct removal of bacteria from the wound bed, and (3) increased perfusion and thus increased immunity and oxygenation (Orgill et al, 2009). A porcine model study with daily biopsies revealed a reduction of Staphylococcus aureus from 10⁸ to 10⁵ by days 4 and 5 when the wound was treated with NPWT. In comparison, the moist saline dressing wound did not experience a decrease in Staphylococcus until day 11 (Banwell and Téot, 2003). Additional inconsistent reductions in bioburden have been reported. For example, NPWT has created a significant decrease in nonfermentative, gram-negative bacilli, but an increase in S. aureus was found in the NPWT wound (Moues et al, 2004). Removal and collection of wound exudate allow closed system containment that significantly reduces the frequency of dressing changes and exposure to external contaminants (Pham et al, 2006).

Macro-deformation and wound contraction

Intact skin and soft tissue have a natural tension that, when disrupted, causes skin and soft tissues to typically pull apart (separate). Distractive tissue forces may act to keep a wound open, with vectors that oppose contraction, thereby delaying healing. Negative pressure dressings can oppose these distractive soft tissue vectors (Franz et al, 2008). The application of suction allows contraction or pulling together of the wound edges, simulating the application of an abdominal binder. The pulling together of the wound edges (contraction) has allowed earlier healing, with delayed primary or secondary intention. This contraction process occurs three dimensionally through mechanical forces, thus allowing decreased wound volume symmetrically and facilitating faster wound closure (Orgill et al, 2009; Ubbink et al, 2008).

Micro-deformation and mechanical stretch

Histologic wound sections have demonstrated micro-deformations of the wound surface present with NPWT that is not present underneath occlusive dressings alone (Saxena et al, 2004). Mechanically, deformation results in fluid flow within the matrix and strain where the matrix cells are anchored. Thus, cells are subjected to both mechanical stretch mediated by their attachments to the matrix as well as shear stresses due to fluid flow. Fluid shear stresses are known to regulate cellular proliferation (Ubbink et al, 2008).

Wound surface micro-deformations seem to stimulate increased vascular growth. The stretching of the wound surface is hypothesized to stimulate cell division, proliferation, and angiogenesis (Orgill et al, 2004). In vivo models have demonstrated that cells respond to tension with directional growth and specific gene release (vascular endothelial growth factor) when mechanical tension has been applied (Orgill et al, 2004; Saxena et al, 2004). Saxena et al (2004) hypothesized that application of mechanical force initiates cellular response inducing a cascade of cellular proliferation, angiogenesis, and thus promotion of wound healing. The term mechanotransduction has been adopted to denote this cellular and physiologic response to mechanical strain (Orgill et al, 2009). In terms of the effect of NPWT on surrounding skin perfusion, it is hypothesized that increased perfusion occurs with the treatment, although the testing mechanism (laser Doppler) was criticized as being an indirect measure of blood flow (Morykwas et al, 2006; Orgill et al, 2009).

Morykwas et al (1997) reported an increase of more than 60% in granulation tissue formation using NPWT in comparison to moist gauze in porcine wounds. Numerous human studies have found increased and/or faster closure rates of wounds when treated with NPWT versus standard practice. Comparison of NPWT to standard compression therapy in primarily venous lower leg ulcerations revealed faster wound bed preparation (7 vs 17 days) and faster healing rates (29 vs 45 days) (Vuerstack et al, 2006). Faster granulation tissue coverage over diabetic foot wounds resulted in accelerated wound closure when NPWT was used (Blume et al, 2008).