In uncomplicated surgical incision wounds possessing adequate nutritive flow, the optimal oxygen tension for healing is probably passed when the oxygen treatment is extended to hyperbaric conditions. This is supported by the finding that the rate of gain in the tensile strength of healing skin incision wounds was significantly lower in rats treated intermittently for two hours twice daily with hyperbaric oxygen at 2 ATA than in rats breathing air.
Hyperbarically oxygenated normal wounds contained less collagen hydroxyproline as well as less DNA and RNA than control wounds.12
The healing of open wounds involves contraction and epithelization, processes of relatively little significance in the healing of incisional wounds. In a study of rats, long-term intermittent hyperbaric oxygenation for two hours twice daily at 2 ATA had no effect on the healing rate of open skin wounds in which the circulation was left intact. When wound edges were devascularized, however, hyperbaric oxygen enhanced the rate of wound closure in the final stages of healing, thus counteracting the delay caused by disturbed blood supply.29
HBO in wound healing has resulted in increased growth factor production, particularly VEGF, but also PDGF-receptor when combined with PDGF treatment.6 The additional angiogenic effect of HBO is likely due in part to an enhancement of the normal VEGF response to wounding.30 Combined treatment of ischemic rabbit ear ulcers with hyperbaric oxygen and recombinant PDGF-BB increased PDGF-a receptor content of the treated tissue.31 This positive effect seemed to be due to the effect of oxygen as signal transducer via reactive oxygen species. Furthermore, recombinant growth factors (r-TGF-^1, r-PDGF-BB) enhanced wound healing in ischemic rabbit ear ulcers. When these growth factors were applied together with hyperbaric oxygen, the negative effect of ischemia on wound healing was completely reversed.32
In clinical patients with problem wounds HBO has been used as an adjuvant to surgical debridement, tissue grafting and antibiotics. The central question is whether there is an adequately perfused capillary bed in the wound area to allow oxygen delivery even at high arterial oxygen tensions possible with hyperbaric oxygen. Although hyperbaric oxygen can be a powerful adjuvant in the management of selected soft tissue wounds, it is only a part of co-ordinated medical-surgical approach to such patients. Careful attention to underlying diseases, effective treatment of infection, and meticulous wound care with necessary surgical debridement and grafting remain the cornerstones of treatment. Hyperbaric oxygen is used to improve results.26
A number of studies have shown the importance of enhancing tissue perfusion and oxygenation in obtaining healing in chronic wounds. Transcutaneous PO2 in the skin adjacent to chronic wounds is remarkably predictive to whether wounds will heal or not with hyperbaric oxygenation.26'33-35 The technique can be used in the evaluation of healing potential, selection of amputation level, and patient selection for HBO. Furthermore, the efficacy of HBO can be estimated by repeated, continuous recording of the response of the peri-wound TcPO2 in the chamber during hyperbaric oxygenation. Transcutaneous oxymetry has become a routine method in many hyperbaric centers. However, as with any measurement method, its application requires strict calibration and checking observations to provide clinicians with values reliable to be interpreted.36
With the exception of TcPO2 measurements, none of the techniques for measuring perfusion and oxygenation have been fully applicable for routine use during HBO. Any technique must be safe and measurements inside the chamber must be possible without electrical equipment to avoid the risk of fire. From this viewpoint the implanted Silastic tube tonometer and capillary sampling technique give an excellent opportunity to monitor tissue oxygen and carbon dioxide tensions, even under hyperbaric oxygen.37-39 Korhonen40 used these techniques to measure the response of peri-wound subcutaneous PO2 to HBO in patients with necrotizing fasciitis. For comparison, the rise of subcutaneous PO2 levels were simultaneously measured in intact distant tissue in these patients as well as in healthy controls under hyperbaric oxygen. The response of subcutaneous tissue PO2 to HBO seemed to be regularly higher in the vicinity of the debrided infected area than in healthy or distant tissues. It was suggested that hyperbaric oxygenation created a specific "superoxygenated" zone around the infected area thus forming an oxygen barrier against the spreading of infective organisms.
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