Effect of inspired O2 on cardiopulmonary and metabolic responses to exercise in man.

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Abstract
Measurements of heart rate (HR), cardiac output (Q). ventilation (VE), and blood gases, pH, and lactate (La) were made in 4 subjects during continuous exercise. Severe hypoxia (FIO2, [fractional concentration of oxygen in inspired gas] 0.11) reduced maximum O2 intake (VO2) to 65% of the value obtained at an FIO2 of 0.21, but at any submaximal work load VO2 was independent of FIO2-HR and Q were increased to an extent related to the reduction in arterial 2, content, O2 transport (Q X arterial O2 content) being similar at all F102: maximum HR and Q. were similar at all FIO2. VE was similar at the highest loads for all FIO2 and amounted to 65-90% of the sustained maximum voluntary ventilation. Although the increase in La with work varied between the subjects, the effect of hypoxia on La at any given work load was similar in all subjects. The time course of increase in La at any given work was not affected by hypoxia; a major factor influencing La at all values of inspired O2 appears to be the rate at which cardiovascular adaptations to an increase in work load take place. Although at a given work load La was highest under hypoxic conditions, the highest levels were not found at an FIO2 of 0.11; maximal work of more than 3 min. duration under these conditions does not appear to depend on the maximal use of anaerobic metabolic processes.