The influence of low and high pressure levels during negative pressure wound therapy on wound contraction and fluid evacuation.

BACKGROUND: Negative pressure wound therapy (NPWT) promotes healing by drainage of excessive fluid and debris and also by mechanical deformation of the wound edge tissue. The most commonly used negative pressure is -125 mmHg. However, this pressure may cause pain and ischemia, and the pressure often needs to be reduced. The aim of the present study was to examine wound contraction and fluid removal during low and increasing levels of negative pressures. METHODS: A peripheral wound was created in 70 kg pigs. The immediate effects of NPWT (-10 to -175 mmHg) on wound contraction and fluid removal was studied in eight pigs. The long-term effects on wound contraction were studied in eight additional pigs during 72 hours of NPWT at -75 mmHg. RESULTS: The wound contraction and fluid removal increased gradually with increasing levels of negative pressure until reaching a steady state. Maximum wound contraction was observed at -75 mmHg. When NPWT was discontinued, after 72 hours of therapy, the wound surface area was smaller than before therapy. Maximum wound fluid removal was observed at -125 mmHg. Higher pressures did not further reduce wound surface area or fluid volume. The time required for evacuation of 50% of the maximal fluid drained for a specific pressure level was longer for low negative pressures (∼45 s for pressures below -50 mmHg) than for high negative pressures (∼15-20 s for pressures above -50 mmHg). CONCLUSIONS: NPWT facilitates drainage of wound fluid and exudates and results in mechanical deformation of the wound edge tissue which is known to stimulate granulation tissue formation. Maximum wound contraction is achieved already at -75 mmHg, and this may be a suitable pressure for most wounds. In wounds with large volumes of exudate, higher pressure levels may be needed for the initial treatment period