(Condition of physiological oxygen-supply dependence)
Webster23 did not find an ergogenic effect on subsequent incremental exercise performance or maximal oxygen consumption after a 1-hr exposure to HBO at 202.6kPa (2.0ata).
Controversial observations and interpretations have been made about the quantity of oxygen uptake in the exercising subject: some authors have found an increase of VO2 max with increased DO2 in human24 and animal25 studies, which has not been observed by others26'27. Nevertheless, most authors have observed a better exercise endurance and a decrease of serum lactate levels, suggesting additional hyperoxia-induced metabolic effects.
Regional effects of hyperoxia on oxygen consumption have been most extensively studied in exercising skeletal muscle in vivo, and in the isolated muscle model, with contradictory results: There are reports indicating considerable increases28'29 in muscle oxygen uptake during hyperoxia and
others - without. Wilson showed that the decrease of local blood flow matched the increase of arterial oxygen content during hyperoxia so that a net increase of VO2 did not occur.
In a rat model, Eynan34 showed that a 24hr exposure to normobaric oxygen reversed the energy-saving (reduced oxygen consumption) effect of training, in comparison with sedentary animals. Under exposure to NBO, the VO2 of the trained rats increased by 17% which was similar to the VO2 of the untrained animals. The authors concluded that prolonged exposure to hyperoxia leads to a reduction of the energy efficiency of the trained rat.
The underlying cause for these opposing observations may be the variety of experimental settings and species, and the modification by autoregulatory mechanisms which are not yet totally defined and predictable.
Hyperoxia interferes with the accuracy of commercially available oxygen consumption measurement devices limiting their use in the hyperbaric environment.
Koch35 devised a set-up for online diving ergospirometry during fin-swimming under realistic diving conditions (Fig. 1.4-2). The divers were tested in a suspended-weights system in normal swimming position where they had to counterbalance increasing weight loads in a ramping protocol. Ergospirometry was performed with the semi-open Douglas-bag method and a tight-fitting full-face mask. In trained subjects there was no significant difference found in total whole-body oxygen uptake between normobaric cycling, cycling at an ambient pressure of 300kPa (20m depth), as well as fin-swimming (300kPa) at comparable workloads.
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