The use of the six-minute walk test (6-MWT) to screen for symptoms of acute mountain sickness and predict susceptibility during a long-term exposure in normobaric hypoxia within young to middle-aged adults

Introduction:
Individuals travelling to altitudes of >2000-2500m are at increased risk of acute mountain sickness (AMS) and may benefit from undergoing a standardised screening assessment prior to departure. As such, a simple and time-efficient tool for predicting human physiological responses to altitude exposure (both simulated and real), is the 6-minute walking test (6-MWT). The 6-MWT also demonstrates acceptable construct validity when monitoring acclimatisation at terrestrial altitude of ~3400m, however, it is unknown if this test can predict symptoms of AMS during a prolonged exposure to normobaric hypoxia. Therefore, the aim of this study was to: 1) compare the performance outcome and humans responses of the 6-MWT between normoxia and normobaric hypoxia; and 2) investigate if the 6-MWT in normobaric hypoxia can predict symptoms of AMS during an 8-hour normobaric hypoxia exposure.

Methods:
Forty-four healthy participants (25 males, 19 females, 31±10 years, 73.9±14.6 kg 174.1±8.0 cm) visited the laboratories on 3 occasions to complete a normoxic and normobaric hypoxic 6-MWT, and then an 8-hour exposure to normobaric hypoxia at rest. Participants underwent a familiarisation trial and 30-mins rest in each condition before starting the 6-MWT, which was completed in a randomised order. Normobaric hypoxia occurred within an environment simulating an altitude of 4500 m (FiO2 12.5%).

Results:
A significantly impaired performance (668±83 vs. 725±90m, p<0.05), lower pulse oxygen saturation (SpO2: 75±8 vs. 97±2%, p<0.05) and higher AMS scores via the Lake Louise Questionnaire (1±1 vs. 0±0 arbitrary units, p<0.05) were observed following the 6-MWT in normobaric hypoxia compared to normoxia. During the 8-hour normobaric hypoxic exposure, a reduction in resting SpO2 (mean: 81±4%, end: 82±6%, pre-to-post change:-16±6%) and an increase in AMS scores were observed (mean: +1±4, end: +2±2 pre-to-post change: +2±2 arbitrary units). Low correlation coefficient values were found for post-test and pre-to-post test changes in SpO2 (r = 0.3 and 0.0) and AMS scores (r = 0.3 and 0.2) between the 6-MWT and 8-hour prolonged exposure in normobaric hypoxia, respectively.

Conclusion:
The 6-MWT demonstrates impairments to performance, increased physiological strain and the occurrence of AMS symptoms in response to normobaric hypoxia compared to normoxia. However, it appears the 6-MWT is unable to predict AMS susceptibility, nor impairment to SpO2 within a prolonged exposure. Whilst the 6-MWT is useful for monitoring acclimatisation progress and provides a good tool for educational purposes to improve individuals’ knowledge on the effect hypoxia has on performance and health outcomes prior to departing for an altitude sojourn, it is currently not possible to be used in screening for or, predicting AMS susceptibility.