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

Denison et al. reported an initial transient increase in reaction time on a spatial transformation task while exercising at 27 W at an altitude of 2438 m (8000 ft). This increase was attributed to ‘task novelty’. However, subsequent experiments using different performance tests have failed to demonstrate a reliable effect at this altitude. In experiment 1 the spatial transformation task was performed at 27 W with SaO2 (percentage arterial oxyhaemoglobin saturation) held at the equivalent of 2438 m while breathing a low-oxygen mixture. Initial reaction time did not increase. In experiment 2, SaO2 was first 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. These results lead to the conclusion that the minimum altitude at which hypoxic performance decrements can be detected is greater than 2438 m and they raise doubts about the ‘task novelty’ hypothesis. The decreased SaO2 in experiment 2 can be explained by a combination of hypoxia, exercise and hypoventilation due to breathing resistance. It is argued that this combination may have been a factor in the increased reaction time found by Denison et al.