Mechanisms of Action
During day-to-day living we breathe air, composed of (essentially) 21% oxygen and 78% nitrogen. A patient in a hyperbaric chamber breathes 100% oxygen, under controlled, increased pressure. This combination of pressure and 100% oxygen increases the number of oxygen molecules that are dissolved in the blood plasma. At 3.0 atmospheres absolute, an arterial P02 of nearly 2,200 mmHg may be achieved and up to 6.9 volume percent of oxygen may be forced into solution, a quantity sufficient to maintain life in the absence of hemoglobin. The goal of HBOT is to increase the amount and pressure of dissolved oxygen that is delivered to the body tissues.
During the late 1930's, oxygen at pressure was proposed as a treatment of decompression sickness (the bends). In the early 1960's Dutch investigators showed the efficacy of hyperbaric oxygen in the treatment of gas gangrene. Subsequent studies have shown the importance of oxygen in the treatment of wounds, enhancement of white cell killing ability, preservation of compromised tissue, and angiogenesis. The clinical indications for the use of adjunctive hyperbaric oxygen therapy continue to be expanded and defined.
Fibroblast Proliferation: Nonhealing tissues are hypoxic with tissue oxygen tensions frequently in the range of 5 to 15 mmHg. Although abnormally low tissue oxygen tensions stimulate fibroblast activity, tissue oxygen tensions of a least 30 to 40 mmHg (optimal tissue oxygen tensions are probably in the 90-100 mmHg range) are necessary for fibroblast turnover, collagen synthesis, and the development of a collagen matrix to support capillary budding into avascular areas. Raising the tissue oxygen tension in hypoxic wounds can readily be accomplished through the intact circulation with hyperbaric oxygen therapy.
Angiogenesis: Following hyperbaric oxygen sessions, restoration of abnormally low P02 to physiologic levels will result in capillary proliferation. The previously stimulated fibroblasts provide a scaffolding and infrastructure for new capillary ingrowth.
Enhancement of White Blood Cells: Polymorphonuclear (WBC) cells in low oxygen tensions (eg., 5 to 15 mmHg) show diminished ability to kill organisms through the peroxidase system (an oxygen-dependent mechanism). The killing ability of white blood cells can be greatly enhanced in an almost linear fashion as oxygen tensions increase.
Vasoconstriction: Exposure to oxygen at pressure causes a 20-50% reduction in blood flow resulting in less diapedesis and bleeding in areas of capillary damage. Theis effect might seem to be undesirable in ischemic conditions, but the tenfold increase in oxygen content of the plasma and the resultant increase in the diffusion distance of oxygen more than compensates for decreased arterial flow.
Enhancement of Antibiotic Activity: Recent evidence shows that certain antibiotics may be more readily incorporated into the bacterial cell wall in the presence of elevated oxygen tensions.