Role of Topical Oxygen ( TO2) on Wound Tissue pO2

There is a great deal of confusion on the subject of TO2 and tissue oxygenation. Some systemic HBO chamber advocates believe that the only way to increase tissue oxygen levels is to super oxygenate the blood under high pressure (2-3 atmospheres). Increasing blood oxygen level will definitely increase tissue oxygen content as long as the oxygenated blood reaches the tissue cells. It is also possible to increase cellular oxygen levels by bringing oxygen directly in contact with the cells themselves. In order to understand how this occurs, it is helpful to understand certain terms and scientific principles.

Q. What is meant by "partial pressure" or pO2?
A. The term "partial pressure (of oxygen)" is written as pO2. According to Dalton's law of partial pressures, the partial pressure of oxygen, pO2, in room air at one atmosphere (760 mmHg), is approximately 160 mmHg, (21% of 760). Studies have shown that the normal pO2 of arterial blood is approximately 100 mmHg, while wound tissue pO2 can be as low as 5-20 mmHg even when subcutaneous pO2 is 30-50 mmHg. It is generally agreed that normal cell metabolism is impaired at pO2 of under 30 mmHg.

Q. How does gas diffuse through a cell wall?
A. Gas can diffuse through a porous thin membrane such as the cell wall. The rate of "diffusion" is directly proportional to the difference in pressure on either side of the membrane, which is written as Dp. This Dp provides the "driving force" which causes the gas to pass through the cell wall.

Q. Wouldn't pressurized air work as well as oxygen to increase cellular pO2?
A. With pressurized air, which is approximately 79% nitrogen and 21% oxygen, nitrogen would diffuse more rapidly than oxygen through the cell wall according to Graham's law of diffusion. The amount of oxygen passing through would be significantly less than with 100% oxygen.

The theory behind TO2 is this: Normally, since the cell interior pressure is equal to the outside pressure, there is no driving force and gas will not diffuse through the cell wall. However, by replacing nitrogen with oxygen, and then adding external pressure, oxygen will diffuse through the cell wall. Thus, under the conditions of TO2, 100% O2 at 22 mmHg, oxygen will diffuse into the mitochondria, where it is available to support metabolism. The maximum distance, or number of cell layers, that oxygen can diffuse has not been determined but it is believed that the distance does not exceed 5-10 mm. Therefore, periwound measurements of oxygen using oxygen sensors would not be expected to show any increase, nor will blood oxygen levels be elevated. One study, however, reported that capillary pO2 in the wound increased from 76-79 mmHg to 115-120 mmHg after 1-2 hours of exposure to oxygen at 22 mmHg.