RTO MP-062 / HFM-050 - FTP Directory Listing - Nato

KN2-2
Starling forces in favor of interstitial fluid absorption. The clinical implications are important when
dealing with injured tissue where interstitial edema produces a diffusion barrier and compresses the
microcirculation. HBO has the capacity to support injured edematous tissue while simultaneously
reducing excess interstitial fluid. In more acute situations HBO may prevent the evolution of edema
altogether.
‰ NEOVASCULARIZATION - Nonhealing or “problem” wounds are usually due to a multiplicity of
factors. For most patients local hypoxia is a significant part of the problem. The use of
transcutaneous oximetry allows for a more accurate identification of those wounds in which the cells
are too hypoxic to respond to the reparative call of local growth factors. Under the influence of HBO,
new blood vessels bud into these pathologically hypoxic wounds. In time, the pO2 levels within the
wound may reach a level where the healing process becomes self-sustaining without the support
HBO. Until recently little was known about the physiology of oxygen induced angiogenesis. 5,6 At
the cellular level there is evidence that oxygen behaves as a cell signal to trigger wound healing as
manifest by the sustained effect messenger ribonucleic acid (mRNA) production following a single
dose of HBO. Work from several different investigators describe the effects of HBO more as a
growth factor than a metabolite demonstrating that a single dose of oxygen stimulates mRNA
transcription for 24-72 hrs. 7,8 Cells in a wound may sense oxygen pressure as a trigger which signals
that appropriate conditions exist to proceed with healing. 9 Regular subsequent exposure to the
threshold level reinforces the signal and supplies an important factor for repair.
‰ ANTIBACTERIAL EFFECTS - Despite the very high arterial pO2 achieved with HBO, the tissue levels
in the heart of necrotic wounds may only achieve one to two hundred mm Hg. Although such levels
are not bactericidal to anaerobes, it is bacteriostatic. As such, oxygen behaves like many bacteriostatic
antibiotics by containing the infection and allowing normal host defense mechanisms to eradicate the
infection. In addition, HBO has been shown to produce a direct synergistic effect with numerous
antibiotics, such as aminoglycosides, cephalosporins, penicillin, and amphotericin B. 10,11,12 HBO also
exerts an indirect synergism by converting anaerobic wounds w it h a low pH to aerobic wounds
characterized by a normal pH. This pH shift is important, as many antibiotics do not work well at an
acid pH.
‰ LEUKOCYTE OXIDATIVE KILLING - Although phagocytosis is relatively unhindered by an anaerobic
environment, the killing capacity of neutrophils is markedly reduced. It is important to understand
that phagocytosis per se does not kill bacteria. 13 The actual destruction of bacteria is a complex
interdependence of neutrophil enzymes with neutrophil generated oxygen radicals. 14,15 By providing
supplemental oxygen, HBO “turbocharges” the destructive capacity of the neutrophil. 16
‰ ATTENUATION OF REPERFUSION INJURY - Much of the damage associated with reperfusion is
mediated by the inappropriate activation of leukocytes. Following an ischemic interval the total
injury pattern is the result of two components: a direct and irreversible injury component from
hypoxia, and an indirect injury, which results from leukocyte activation. Hyperbaric oxygen
diminishes the indirect component of injury by reducing the inappropriate activation of leukocytes. 17
The net effect is the preservation of marginal tissue that would otherwise be lost to reperfusion injury.
The clinical ramifications of this mechanism are self evident for those scenarios involving significant
interruptions of blood flow e.g. plastic surgery, vascular surgery, and fluid resuscitation of the
microcirculation.
Selected Indications
CATEGORY I - CRUSH INJURIES AND THEIR SEQUELAE
Crush injuries are often associated with both immediate and delayed problems. Immediately following a
crush or blast type injury, tissue can be categorized as viable, nonviable, or transitional. Viable tissue