Principles of Hyperbaric Oxygen

Normally, 97% of the oxygen delivered to body tissues is bound to hemoglobin, while only 3% is dissolved in the plasma. At sea level, the barometric pressure is 1 ATA or 1 atmosphere, or 760 mm Hg, and the partial pressure of oxygen in the arterial blood (Pa O2) is approximately 100 mm Hg. At rest, the tissues of the body consume about 5 ml of O2 per 100 ml of blood.

During Hyperbaric Oxygen Treatments, barometric pressures are usually limited to 3 ATA or lower. The oxygen content of inspired air in the chamber is typically 95% to 100%. The combination of the increased pressure, (up to 3 ATA) and the increased oxygen concentration (100%) dissolves enough oxygen in the plasma alone to sustain life in a resting state. Under hyperbaric conditions, oxygen content in the plasma is increased from 0.3 to 6.6 mL of blood. Hyperbaric oxygen at 3ATA increases oxygen delivery to the tissues from 20.0 to 26.7 mL of O2 per 100mL of blood.

Proposed Healing Mechanisms 

Increased oxygen delivery to the tissues is believed to facilitate healing through a number of mechanisms.

Vascoconstriction. High tissue oxygen concentrations cause blood vessels to consrtict, which can lead to a 20% decrease in regional blood flow. In normoxic environments, tissue hypoxia may develop; however, this is not the case with hyperbaric oxygen. The decrease in regional blood flow is more than compensated for by the increased plasma oxygen that reaches the tissue. The net effect is decreased tissue inflamation without hypoxia, a mechanism by which hyperbaric oxygen therapy is believed to improve crush injuries, thermal burns, and compartment syndrome.

Neovascularization and epitheliazation. High tissue oxygen concentrations accelerate the development of new blood vessels. This can be induced in both acute and chronic injuries. Regenerating epithelial cells also function more effectively in a high-oxygen environment. These effects have proven effective in treating tissue ulcers and skin grafts.

Stimulation of fibroblasts and osteoclasts.  In a low oxygen, or hypoxic environment, fibroblasts are unable to synthesize collagen, and osteoclasts are unable to lay down new bone. Collagen deposition, wound strength, and the rate of wound healing are affected by the amount of oxygen available. Ischemic areas of wounds benefit most from the increased delivery of oxygen. Hyperbaric oxygen increases tissue levels of oxygen, allowing for fibroblasts and osteoclasts to function appropriately. This mechanism may play a role in aiding the treatment of osteomyelitis and slowly healing fractures.

Immune response. When tissue oxygen tensions fall below 30mm Hg, the host responses to infection and ischemia are compromised. Studies have shown that the local tissue resistance to infection is directly related to the level of oxygen found in the tissue. High oxygen concentrations may prevent the production of certain bacterial toxins and may kill certain anaerobic organisms such as clostridium perfringens. More important however, oxygen aids polymorphonuclear leukocytes PMN. Oxygen is converted within the PMN into toxic substrates (superoxides, peroxides, and hydroxyl radicals) that are lethal to bacteria. These effects on the immune system allow hyperbaric oxygen to aid in the healing of soft tissue infections and osteomyelitis. Hyperbaric oxygen has also been found to inhibit PMN adherence to postcapillary venules. Although this may seem paradoxic, this effect is beneficial bacause it helps limit reperfusion injury after crush injury and compartment syndrome.

Maintaining high-energy phosphate bonds. When circulation to a wound is compromised, resultant ischemia lowers the concentration of adenosine triphosphate (ATP) and increases lactic acid levels. ATP is necessary for ion and molecular transport across cell membranes and maintainance of cellular viability. Increased oxygen delivery to the tissue with hyperbaric oxygen may prevent tissue damage by decreasing the tissue lactic acid level and helping maintain the ATP level. This may help prevent tissue damage in ischemic and reperfusion injuries.

Successful

Already, clinical experience has shown that Hyperbaric oxygen can be successfully applied to many of the same conditions in the horse as is applied in human medicine. Equine medical therapy is often a composite of multiple medical treatments used to promote healing and recovery. Hyperbaric oxygen therapy gives the veteriarian one more important weapon in their arsenal of treatment options.