Human tolerance and physiological responses to exercise while breathing oxygen at 2.0 ATA

Multiple physiological functions were monitored in ten men who performed two 30-min periods of 150-W ergometer exercise during 120-min exposures to O2 at 2.0 ATA. There were no convulsions or electroencephalographic manifestations of increased excitability. Sequential measurements of peripheral visual fields, pulmonary mechanical function, mental performance, and cardiovascular function during the resting recovery after each of the two exercise periods were not detectably altered from pre-exercise control values. Pre- and post-exposure measurements of visual acuity, accommodation, pupil diameter, visual cortical activity, and retinal electrical activity also revealed no significant differences. While CNS symptoms were absent, average arterial PCO2 rose by about 5 mm Hg during both exercise periods. This finding was confirmed in six subjects who performed four 6-min periods of continuous exercise at 50, 100, 150, and 200 W while breathing O2 at 2.0 ATA. Average arterial PCO2 rose nearly linearly from 34.3 mm Hg at rest to 44.0 mm Hg at 200 W. Arterial PCO2-related increments in brain blood flow and PO2 may explain part or all of the known detrimental influence of exercise on CNS O2 tolerance.