The most well known mechanism of action of HBOT is to provide free molecular oxygen to hypoxic tissues. During HBOT, the amount of oxygen available to hypoxic tissues can be increased up to 15 times the normal level found in the body.
Oxygen pressure is increased and causes a rise in plasma oxygen content. The amount of pressure and the length of time under pressure are determined by the specific condition(s) being treated. Pressures are usually between 1.5 and 3 times atmospheric and may last from one to two hours at pressure.
The main mechanisms of action of HBOT are:
1. The mechanical effect of increased pressure
Any free gas trapped in the body will decrease in volume as pressure exerted on it increases (Boyle’s Law). Reduction in bubble size may allow it to pass through the circulatory system, or at least travel into a smaller vessel which will reduce the size of any resulting infarction. This effect is useful in the management of gas embolism and decompression sickness.
2. Mass action of gases
Flooding the body with oxygen forces the rapid elimination of other gases, and in this way reduces damage caused by toxic gases such as carbon monoxide. The elevated pressures used during hyperbaric oxygen therapy further speeds up this elimination process.
HBO acts as an alpha-adrenergic drug. Adrenaline produces vasoconstriction. It is the critical chemical that constricts the vascular system so that, should wounding happen, there will be lessened blood loss. Vasoconstriction can result in reduction of edema following burns or crush injuries. Even with a reduction in blood flow, enough extra oxygen is carried by the blood plasma so an increase in oxygen is delivered to the tissues.
4. Anti-bacterial effect
Anaerobic bacteria don’t have the defenses to protect them from the superoxides, peroxides and other compounds formed in the presence of high oxygen levels. Many of the body’s bacterial defense mechanisms are oxygen-dependent. When tissue O2 drops too much, effective ingestion and killing by phagocytic leukocytes is retarded. Re-oxygenation of those tissues allows phagocytosis and other host defense mechanisms to come back into play.
5. Anti-ischemic effect
Hyperbaric oxygen physically dissolves extra oxygen into the plasma (Henry’s Law). The quantity of oxygen carried and transferred to ischemic tissue by the blood plasma is increased. Relieving the ischemia with increased oxygenation promotes osteoclastic and osteoblastic activity, collagen matrix formation and the breakdown of many toxins. The extra oxygen also helps the ischemic tissue meet the increased metabolic need required by the healing process.