Oxygen And Wound Infection

Oxygen is essential to immune mechanisms in wounds, because oxygen radicals derived from molecular oxygen are important agents in bacterial killing. Normal leukocytes contain a NADPH-linked oxygenase that is activated during phagocytosis by assembly of its components into the phagosome membrane. This enzyme is the first step in a cycle in which various oxidants are produced from ambient oxygen. After activation, a "respiratory" or "oxidative burst" follows during which molecular oxygen is reduced in large quantities to superoxide radicals. These radicals are then sequestered in the phagosomes where they and other oxidants derived from them are produced and kill bacteria by oxidizing cell membranes. Two molecules of superoxide are subsequently reduced to one molecule of oxygen and one of hydrogen peroxide by superoxide dismutase. Myeloperoxidase then combines hydrogen peroxide with chloride or iodide to form hypochlorite or hypoiodite. Excess hydrogen peroxide is reduced intracellulary to oxygen by catalase. If iron is present and the reaction occurs extracellulary, hydrogen peroxide can be reduced to OH which is a particularly harmful oxygen radical. While hydroxyl radicals kill bacteria quite effectively, they also injure surrounding cells.1,27

In wounds, the activity of the NADPH-linked enzyme can be a limiting factor and superoxide production is reduced. The Km for this reaction has been variously estimated as 15-12 mmHg and about 75 mmHg. If the latter estimate is accurate, full resistance to infection is reached only when intraleukocytic PO2 rises as high as 750 mmHg. This is clearly possible only in hyperbaric circumstances. The kinetics curve, however, is hyperbolic, and the greatest portion of the effect is exerted within the first 200 mmHg. This also gives a target for HBO.1,27

In a study of patients having colorectal operations a direct correlation was found between the subcutaneous tissue PO2 and postoperative wound infection.28 If an increase in oxygen concentration in the inspired gas did not result in an increased subcutaneous PO2, 45% of the patients developed a postoperative infection. If, however, the tissue perfusion was sufficient to result in an increase of PO2 in subcutaneous tissue to 90 mmHg or more, no patient developed a wound infection.

0 0

Post a comment