Cath Lab Digest

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The least intense staining was observed in both the foam and gauze control wounds that were sealed as for NPWT but not subjected to negative pressure (see Figures 1 and 2).

  Tissue morphology. Both CGRP and substance P staining was observed in the wound edge; the staining was most intense in the superficial tissue (75 ± 5 au for CGRP and 78 ± 6 au for substance P) that had been exposed to the forces of the negative pressure and in direct contact with the wound filler and less intense in deeper tissue (23 ± 2 au for CGRP and 31 ± 3 au for substance P). CGRP and substance P staining was especially intense in the dermis, but was also present in the subcutaneous and muscle tissue. Slides stained for CGRP and substance P were compared to adjacent slides stained with hematoxylin-eosin. Staining for both CGRP and substance P were primarily observed located to nerves and leukocytes (see Figures 3 and 4).

Discussion

  The results of the present study show that substance P and CGRP are more abundant in the wound edge of porcine wounds after the removal of foam than the removal of gauze after NPWT. This is in line with clinical observations that dressing changes are more painful when NPWT has been carried out using foam than when using gauze.

  A literature review by Krasner et al¹² addressed the application and removal of foam as a source of procedural pain. It was suggested that patients who experienced pain during dressing changes be given pain relief 30 to 60 minutes before the procedure, following the World Health Organization’s three-step analgesic ladder, proceeding from nonopioids to opioids. In many cases, it was necessary to administer opioids systemically before changing dressings to ensure an adequate and acceptable (to the patient) level of pain relief. Attempts also have been made to reduce pain by instilling a 1% lidocaine solution through the drain to reach the surface of the wound; the effects were evaluated in a clinical, double blind, prospective, randomized study.¹⁴ However, we believe the administration of local anesthetics is undesirable because they may be cytotoxic, as shown in in vitro studies.¹⁵

  Previously, foam was used as a wound filler in almost all patients treated with NPWT. Today, NPWT that utilizes gauze-type dressings are widely available.

  The effects of gauze and foam on the wound bed have been shown to differ substantially in porcine wound models.^2,3 The granulation tissue may adhere to the foam or actually grow into the pores of the foam, but in vivo studies of porcine wounds treated with NPWT show no ingrowth into gauze.^2,13 On a cellular level, ingrowth has been examined in histological sections of the wound bed treated with overlying gauze or foam wound filler.² Preclinical in vivo studies² show that more force is needed to remove foam than to remove gauze after NPWT.

  The pain during NPWT dressing changes may be due to the disruption of the granulation tissue in the wound bed resulting from foam removal.^2,13 Per clinician observation, it has been suggested that dressing changes when using gauze as the wound filler result in less pain than when using foam. Another strategy may be to use foam of a smaller pore size, such as the polyvinyl alcohol (PVA) white foam dressing, but data examining tissue ingrowth for this type of foam have not been published.