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 O-2 transport and utilization under conditions of altered arterial O-2 content (CaO2), during rest to exercise transitions. Simultaneous measures of alveolar ((V) over dotO(2)alv) and leg ((V) over dotO(2)mus) oxygen uptake and leg blood flow (LBF) responses were obtained in normoxic (FiO(2) (inspired fraction of O-2) = 0.21), hypoxic (FiO(2) = 0.14), and hyperoxic (FiO(2) = 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, (V) over dotO(2)alv was measured breath-by-breath at the mouth and (V) over dotO(2)mus 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 (V) over dotO(2)alv or (V) over dotO(2)mus across gas conditions at any of the rest or exercise time points. A reduction in leg O-2 delivery in hypoxia at the onset of exercise was compensated by a nonsignificant increase in O-2 extraction and later by small increases in LBF to maintain (V) over dotO(2)mus. The dynamic response of (V) over dotO(2)alv 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 (V) over dotO(2)mus responses at the onset of exercise for all gas conditions demonstrated that physiological adaptations in LBF and O-2 extraction were possible, to counter significant alterations in CaO2. These results show the importance of the interplay between O-2 supply and O-2 utilization mechanisms in meeting the challenge provided by small alterations in O-2 content at the onset of this submaximal exercise task.