Oxygen toxicity: an introduction

Although oxygen has been known to be toxic for more than 200 years, the clinical importance of oxygen toxicity was not appreciated until an epidemic of retrolental fibroplasia occurred in the early 1950s. Oxygen at high partial pressures is toxic to the respiratory, cardiovascular, nervous, and gastrointestinal systems. Toxicity results from the formation of oxygen-free radicals. These arise within mitochondria as oxygen is reduced to water, as byproducts of prostaglandin and thromboxane synthesis, and by the xanthine oxidase catalyzed reduction of xanthine or hypoxanthine. They are also produced by activated macrophages as part of the immune response. Superoxide anion is the radical most commonly produced. It dismutes to hydrogen peroxide, which is able to diffuse through lipid membranes. Hydrogen peroxide reacts with transition metals to produce the highly reactive hydroxyl radical which can initiate chain reactions of lipid peroxidation leading to cell rupture. Oxygen radical scavengers such as superoxide dismutase and catalase protect the body against normal levels of oxygen-free radicals. Oxygen toxicity can result from either reperfusion of ischemic tissue or prolonged exposure to high concentrations of oxygen. Limiting hyperoxia to maintain arterial oxygen percent saturation (SaO2) greater than or equal to 90% is recommended.