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On Drying Wet Wounds

November 14, 2010

 It will be a true paradigm shift in medicine when clinicians fully understand the biology of open wounds and their physiologic management.

I’m often told by other health care workers that wounds must be kept moist to keep them from desiccating.  When I challenge them to explain desiccation, I’m told that it means “drying out’ (which, of course I knew).  However, I’m more interested about what is actually drying out.  You see, I think that many clinicians believe that living cells in the exposed wound will dry up and die if left exposed, which is not accurate.

What is known is that cells in culture, such as epithelial cells, need moisture to survive.  However, the cells in an open wound are not the same as isolated cells growing in culture.  The cells in the wound have a blood supply providing them nutrients that diffuse through the salt-based interstitial fluid bathing all cells.  In fact, the serous fluid that normally leaks off an open wound is the manifestation that the wound is perfused and is spontaneously moist.  Hence, they are not in jeopardy of drying out unless their blood supply gets cut off.

Also, we know that open wounds that are otherwise left alone will dry out.  However, what is drying out is a scab, or what we call an eschar.  This consists primarily of coagulated protein from the plasma that has leaked to the wound surface.

It is interesting that one of the original studies that formed the basis for moist dressings on wounds was actually misinterpreted.  Hinman and Maibach published a study in Nature in 1963 in which they compared open versus closed management of split thickness skin wounds in volunteer humans.  The open wounds on one arm were simply left exposed while the others were covered with sterile polyethylene film.  The covered wounds healed much faster, leading them to conclude that the process of drying out impaired healing in the other wounds.

However, the histology presented in the report reveals another story.  The epithelium had to regrow over a longer distance because of the presence of the scab in the open dried out wounds.  Epithelial cells weren’t dying out from desiccation, but were actually trying to grow across the dermis to confluence.  The scab actually prevented contracture that would otherwise occur to flatten out the wound and provide a short course for epithelial resurfacing.

So the problem isn’t that the living tissue will dry out and die.  The problem is that scab formation — which occurs more readily when the wound is dry — delays wound healing.  A more direct approach would be to prevent the scab by frequent cleansing.  In cases where the scab has developed, it is best to remove the scab, which can be done by cutting it away (i.e., surgical debridement) or by using a true wet-to-dry dressing.  These actions leave the wound healing processes of contraction and epithelial resurfacing unimpeded.

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