Extension of oxygen tolerance in man: philosophy and significance.

The respiration of oxygen over a range of partial pressures higher than in the natural environment has expanding usefulness in health and disease. It provides for denitrogenation of the astronaut to prevent aerospace decompression sickness, it facilitates diving of many forms and improves safety in decompression after diving, and it is the key to therapy of diving and iatrogenic gas embolic diseases. In the continuum of general and hyperbaric medicine, it is essential for sustaining viability of damaged or diseased tissues not adequately oxygenated at natural oxygen pressures. Over the entire range of its clinical and operational usefulness, the pressure and duration of tolerable exposure to oxygen is limited by adverse effects on multiple chemical targets, cells, tissues, and organ functions. The rates of development and the qualitative expressions of these effects are different at different respired oxygen pressures. Successful extension of oxygen tolerance, as by slowing the rate of development of adverse effects, will further expand the medical and operational usefulness and safety of oxygen in normal, hypobaric, and hyperbaric environments. A prerequisite baseline for overall extension of oxygen tolerance is the quantitative investigation of early stages of toxic oxygen effects upon specific chemical and composite functions of multiple organ systems, including rates of development and rates of recovery. The practicality of limited oxygen tolerance extension by systematic interruption of oxygen exposure has been demonstrated and the procedure widely used. Broad present goals are both to establish oxygen tolerance for specific tissues and to optimize tolerance extension over the full range of useful oxygen pressures.