Negative Pressure Wound Therapy - Mechanisms of Action and Protecting Exposed Blood Vessels in the Wound Bed

University dissertation from Thoracic Surgery

Abstract: NPWT has recently been associated with severe complications and bleeding when used in wounds with exposed blood vessels. The aims of this work were to investigate the mechanisms of action of NPWT and to explore the possibility of using thin plastic discs to protect exposed blood vessels in the wound bed during NPWT. Three different kinds of wounds were created in pigs: 6 cm and 10 cm diameter circular defect wounds on the back and 6 cm incision wounds in the groin, exposing the femoral artery. Microvascular blood flow was studied with transcutaneous laser Doppler flowmetry (LDF), invasive LDF, and thermodiffusion. Femoral artery blood flow was studied with invasive LDF. Pressure in the wound edge tissue, in the wound cavity and periarterial pressure was measured with pressure transducers. Wound contraction and wound fluid removal were also studied. Tissue pressure 0.1 cm from the wound edge decreased while an increase was found further (0.5 cm) from the wound edge. Increased tissue pressure is believed to be the result of wound contraction and wound edge tissue deformation. The use of a small foam wound filler allowed significant wound contraction, which may result in considerable mechanical stress. In contrast, gauze or a large foam filler led to less wound contraction, which may be more appropriate when NPWT causes pain. Furthermore, NPWT induced a decrease in blood flow 0.5 cm, and an increase 2.5 cm from the wound edge, with a transition zone at 1 cm. This combination of hypo- and hyperperfusion may facilitate both oxygenation and stimulate angiogenesis. However, NPWT should be used with caution in tissues with compromised vascularity due to the risk of ischemia. Thin plastic discs of different designs were placed in the wound bed during NPWT. Femoral artery blood flow and wound bed tissue blood flow decreased when NPWT was applied, but was restored when a disc was inserted. The key mechanisms of NPWT – i.e., pressure transmission to the wound cavity, wound contraction, and wound fluid removal – were not impaired by the discs. Further development and studies on the possible protective effects of thin plastic discs used during NPWT are needed before these can be implemented in clinical practice.