Factors affecting the development of hyperbaric oxygen toxicity in the awake rat brain

Hyperbaric oxygen toxicity (HPO), under various environmental and physiological conditions, was identified by measuring the electrical, hemodynamic, and metabolic activity of the awake brain in rats. The electrical activity was evaluated by the electrocorticogram; the hemodynamic and metabolic activity were measured by use of the surface fluorometry/reflectometry technique. The oxidation-reduction state of NADH (metabolic activity) was measured by using the quartz fiber-optic system to excite the tissue and transmit the fluorescence-emitted light from the brain. The hemodynamic responses were evaluated from the reflectance signal. In three sets of experiments we tested the effects of pressure level, age, and protection agents (pentobarbital and diazepam) on the various parameters measured from the brain. The results can be summarized as follows. 1) The effect of the pressure level of the toxicity process is significant; the higher the pressure, the greater the toxicity. 2) The 60-psi level of pressure is a turning point in the toxicity process, and the kinetic of the process is different below and above this point. 3) The 14-day-old rats showed resistance to the HPO exposure as compared to the older aged rats. 4) The pentobarbital and diazepam protect against the toxicity at 60 and 90 psi, as evaluated by various parameters measured.