A re-evaluation of the minimum altitude at which hypoxic performance decrements can be detected

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Abstract
An initial transient increase was reported in reaction time on a spatial transformation task while exercising at 27 W at an altitude at 2438 m (8000 ft). This increase was attributed to task novelty. Subsequent experiments using different performance tests failed to demonstrate a reliable effect as this altitude. In experiment 1 the spatial transformation task was performed at 27 W with SaO2 (percentage arterial HbO2 saturation) held at the equivalent of 2438 m while breathing a low O2 mixture. Initial reaction time did not increase. In experiment 2, SaO2 was 1st stabilized at 2438 m, then the 27 W workload was applied while SaO2 was allowed to vary freely and the spatial transformation task performed. There was an increase in reaction time which was attributed to an accompanying decrease in SaO2. The minimum altitude at which hypoxic performance decrements can be detected was greater than 2438 m and doubts about the task novelty hypothesis were raised. The decreased SaO2 in experiment in 2 was explained by a combination of hypoxia, exercise and hypoventilation due to breathing resistance.