Negative Pressure Wound Therapy. Therapy Settings and Biological Effects in Peripheral Wounds

Negative pressure wound therapy (NPWT) promotes wound healing through several mechanisms, e.g., altered periwound blood flow, mechanical deformation of the wound edge tissue, and drainage of excess fluid and debris. The general aim of this thesis was to study the impact of different levels of negative pressure, different wound filling materials (foam or gauze), and different ways of applying the negative pressure (continuously, intermittently or variably) on the biological effects of NPWT in peripheral wounds. The intention was to provide a scientific basis for the choice of these parameters in order to be able to optimize the healing of NPWT-treated wounds in the future. Studies were carried out on peripheral wounds created on the backs of pigs. The effect of NPWT on periwound blood flow was investigated using invasive and transcutaneous laser Doppler flowmetry, as well as thermodiffusion. Blood flow was found to decrease 0.5 cm laterally from the wound edge and increase 2.5 cm from the wound edge; a transition zone being seen 1 cm from the wound edge. Blood flow changed gradually with increasing levels of negative pressure, reaching half maximal effect at approximately –45 mmHg and maximum effect at about –80 mmHg. The blood flow response was found to depend on the measurement technique. Applying intermittent and variable pressure resulted in concomitant increases and decreases in periwound blood flow. The combination of hypo- and hyperperfusion may be beneficial in the process of wound healing. The mechanical effects of NPWT were studied with regards to macro- and microdeformation. It was found that the degree of macrodeformation, i.e., wound contraction, increased gradually with increasing negative pressure level, reaching half maximal effect at about –45 mmHg and near-maximal effect at –75 mmHg. The degree of wound contraction was the same regardless of whether foam or gauze was used as wound filler. The effects of NPWT on microdeformation, i.e., the microscopic interaction between the wound filler and the newly formed granulation tissue, were examined histologically using stained sections of the wound bed. Both foam- and gauze-based NPWT were shown to induce microdeformation of the wound bed tissue. The effect of NPWT on fluid evacuation from the wound cavity was measured gravimetrically. The amount of evacuated fluid increased gradually with increasing level of negative pressure, reaching a near-maximum at –125 mmHg. It may thus be beneficial to treat wounds containing large volumes of exudate with a high negative pressure initially (e.g., –125 mmHg), and then reduce the pressure to a level more appropriate for the wound edge tissue. In conclusion, the biological effects of NPWT were influenced by the negative pressure level, the wound filling material and the way in which NPWT was applied (continuously, intermittently or variably). Hopefully, the results of these studies may provide a scientific basis for the choice of NPWT parameters in the treatment of wounds. Further clinical studies are needed to corroborate our findings before recommendations can be made regarding the NPWT settings for treatment of different wound types and tissues in order to improve wound healing.