Cardiac output, O2 delivery and (V) over dotO2 kinetics during step exercise in acute normobaric hypoxia

We hypothesised that phase II time constant (tau(2)) of alveolar O-2 uptake ((V) over dotO(2A)) is longer in hypoxia than in normoxia as a consequence of a parallel deceleration of the kinetics of O-2 delivery ((Q) over dotaO(2)). To test this hypothesis, breath-by-breath (V) over dotO(2A) and beat-by-beat (Q) over dotaO(2) were measured in eight male subjects (25.4 +/- 3.4 yy, 1.81 +/- 0.05 m, 78.8 +/- 5.7 kg) at the onset of cycling exercise (100W) in normoxia and acute hypoxia (FIO2; = 11%). Blood lactate ([La](b)) accumulation during the exercise transient was also measured. The tau(2) for (Q) over dotO(2) was shorter than that for (V) over dotO(2A) in normoxia (8.3 +/- 6.8 s versus 17.8 +/- 3.1 s), but not in hypoxia (31.5 +/- 21.7 s versus 28.4 +/- 5.4 5.4s). [La](b), was increased in the exercise transient in hypoxia (3.0 +/- 0.5 mM at exercise versus 1.7 +/- 0.2 mM at rest), but not in normoxia. We conclude that the slowing down of the (Q) over dotO(2) kinetics generated the longer tau(2) for (V) over dotO(2A) in hypoxia, with consequent contribution of anaerobic lactic metabolism to the energy balance in exercise transient, witnessed by the increase in [La](b). (C) 2013 Elsevier B.V. All rights reserved.