Mj. Macdonald et al., Effect of hyperoxia and hypoxia on leg blood flow and pulmonary and leg oxygen uptake at the onset of kicking exercise, CAN J PHYSL, 78(1), 2000, pp. 67-74
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 con
tent (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) (inspi
red fraction of O-2) = 0.21), hypoxic (FiO(2) = 0.14), and hyperoxic (FiO(2
) = 0.70) gas breathing conditions. Six healthy subjects performed transiti
ons in leg kicking exercise from rest to 48 +/- 3 W. LBF was measured conti
nuously with pulsed and echo Doppler ultrasound methods, (V) over dotO(2)al
v was measured breath-by-breath at the mouth and (V) over dotO(2)mus was de
termined from LBF and radial artery and femoral vein blood samples. Even th
ough hypoxia reduced CaO2 to 175.9 +/- 5.0 from 193.2 +/- 5.0 mL/L in normo
xia, and hyperoxia increased CaO2 to 205.5 +/- 4.1 mL/L, there were no diff
erences in the absolute values of (V) over dotO(2)alv or (V) over dotO(2)mu
s across gas conditions at any of the rest or exercise time points. A reduc
tion in leg O-2 delivery in hypoxia at the onset of exercise was compensate
d by a nonsignificant increase in O-2 extraction and later by small increas
es 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 tha
t physiological adaptations in LBF and O-2 extraction were possible, to cou
nter 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.