Immediate Impact of Negative Pressure Wound Therapy Using Foam-based Dressing on Subjacent Tissue: In vivo Evaluation of Tissue Shape and Edema

Introduction:

While negative pressure wound therapy (NPWT) is becoming more commonplace, questions regarding mechanisms of action (MOA) remain. Some researchers contend that NPWT-induced subjacent tissue-compression is a critical component of MOA,1 despite reports of increased blood flow.2,3 The purpose of this in vivo magnetic resonance (MR) imaging study was to evaluate the immediate impact of foam-based NPWT on subjacent tissue-shape and edema.

Methods:

A 5cm-diameter full-thickness wound was created in 5 micro-Yucatan pigs. In sequence, foam-based dressing, a custom surface MR-coil and 2 vials of reference liquids (water and alcohol) were placed on the wound. T2-weighted MR images were obtained prior to initiating negative pressure (NP), upon NP-initiation, and after NP-cessation. Outcome measures were determined from axial MR slices: wound-bed shape, distance of wound-bed from reference markers, T2-signal intensity [measure of fluid presence]. Least Means Square test with Bonferroni adjustment was utilized for statistical analyses.

Results and Discussion:

Prior to NP-initiation, the top view of the wound-bed was concave. Upon NP-initiation, tissue beneath the dressing lifted, abrogating the concavity, and tissue-edges were brought closer. Distance of wound-bed from markers decreased 6.5mm (from 18.8mm±0.97mm to 12.3mm±1.4mm; p=0.0167) with NP-initiation and increased slightly upon NP-cessation (15.7mm±1.1mm; p=0.0341), acknowledging contributions from foam-buckling and -de-buckling. T2-signal intensity (fluid presence) at the tissue-dressing interface increased from 0.18±0.02 to 0.24±0.02 (p=0.0007) with NP-initiation and did not change immediately after NP-cessation (0.22±0.01; p=0.4541). NPWT-induced tissue distension may reduce resistance to interstitial fluid movement, which together with suction/NP would more easily direct fluid towards the dressing as observed in this study. This distension could also explain, in part, the increased blood flow and lymphatic uptake described in several studies.1-4

Conclusion: Application of NPWT in an accepted animal model immediately stretched underlying tissue in the direction of the dressing. This contradicts the supposition of tissue compression, suggested by others.1