Effects of hypoxic hypoxia on O2uptake and heart rate kinetics during heavy exercise

Engelen, Marielle, Janos Porszasz, Marshall Riley, Karlman Wasserman, Kazuhira Maehara, and Thomas J. Barstow. Effects of hypoxic hypoxia on O 2 uptake and heart rate kinetics during heavy exercise. J. Appl. Physiol. 81(6): 2500–2508, 1996.—It is unclear whether hypoxia alters the kinetics of O 2 uptake (V˙o 2 ) 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 inV˙o 2 (period of first 2–3 min of exercise) and had no significant effect on either the time constant or the amplitude of the slowV˙o 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 forV˙o 2 and an additional anaerobic contribution. However, the sum total of the processes representing the slow component ofV˙o 2 is unaffected.