Effect of hyperoxia and hypoxia on leg blood flow and pulmonary and leg oxygen uptake at the onset of kicking exercise

The purpose of this study was to examine the interactions of adaptations in O2 transport and utilization under conditions of altered arterial O2 content (CaO2), during rest to exercise transitions. Simultaneous measures of alveolar (VO2alv) and leg (VO2mus) oxygen uptake and leg blood flow (LBF) responses were obtained in normoxic (FiO2 (inspired fraction of O2) = 0.21), hypoxic (FiO2 = 0.14), and hyperoxic (FiO2 = 0.70) gas breathing conditions. Six healthy subjects performed transitions in leg kicking exercise from rest to 48 +/- 3 W. LBF was measured continuously with pulsed and echo Doppler ultrasound methods, VO2alv was measured breath-by-breath at the mouth and VO2mus was determined from LBF and radial artery and femoral vein blood samples. Even though hypoxia reduced CaO2 to 175.9 +/- 5.0 from 193.2 +/- 5.0 mL/L in normoxia, and hyperoxia increased CaO2 to 205.5 +/- 4.1 mL/L, there were no differences in the absolute values of VO2alv or VO2mus across gas conditions at any of the rest or exercise time points. A reduction in leg O2 delivery in hypoxia at the onset of exercise was compensated by a nonsignificant increase in O2 extraction and later by small increases in LBF to maintain VO2mus. The dynamic response of VO2alv was slower in the hypoxic condition; however, hyperoxia did not affect the responses of oxygen delivery or uptake at the onset of moderate intensity leg kicking exercise. The finding of similar VO2mus responses at the onset of exercise for all gas conditions demonstrated that physiological adaptations in LBF and O2 extraction were possible, to counter significant alterations in CaO2. These results show the importance of the interplay between O2 supply and O2 utilization mechanisms in meeting the challenge provided by small alterations in O2 content at the onset of this submaximal exercise task.