Abstract

The use of inspired gas mixtures with an oxygen fraction in excess of 0.3 has been associated with a decrease in the gas exchange ratio (R) during prolonged work in humans. It had been hypothesized that the lower R was due to a plasma catecholamine concentration caused by the hyperoxia. We tested this hypothesis by measuring changes in plasma epinephrine and norepinephrine when the subjects were switched from breathing air to 60 02 (and vice versa) during 40 min of cycle ergometer exercise. The subject breathed one gas mixture for the first 30 min and was switched to the other in the last 10 min. The switch in gas mixtures from air to 60 02 resulted in a significant reduction in R, heart rate, minute ventilation, blood lactate concentration, and plasma epinephrine concentration. The plasma norepinephrine concentration and the plasma free fatty acid concentration were not significantly changed. Although the direction of the epinephrine change was consistent with the change in R, the epinephrine change was quantitatively small (20 pg/ml) and of questionable physiological significance. This suggest a direct effect of P02 on cellular metabolism as one cause of the change in R when the subjects were switched from air to 60 02. Verf.-Referat (gekuerzt)