Effects of hypoxic hypoxia on O-2 uptake and heart rate kinetics during heavy exercise

It is unclear whether hypoxia alters the kinetics of O-2 uptake (V-o2) during heavy exercise [above the lactic acidosis threshold (LAT)] and how these alterations might be linked to the rise in blood lactate. Eight healthy volunteers performed transitions from unloaded cycling to the same absolute heavy work. rate for 8 min while breathing one of three inspired O-2 concentrations: 21% (room air), 15% (mild hypoxia), and 12% (moderate hypoxia). Breathing 12% O-2 slowed the time constant but did not affect the amplitude of the primary rise in Vo(2) (period of first 2-3 min of exercise) and had no significant effect on either the time constant or the amplitude of the slow Vo(2) component (beginning 2-3 min into exercise). Baseline heart rate was elevated in proportion to the severity of the hypoxia, but the amplitude and kinetics of increase during exercise and in recovery were unaffected by level of inspired O-2. We conclude that the predominant effect of hypoxia during heavy exercise is on the early energetics as a slowed time constant for Vo(2) and an additional anaerobic contribution. However, the sum total of the processes representing the slow component of Vo(2) is unaffected.