Acclimatisation, de-acclimatisation and re-acclimatisation to hypoxia

World-wide, increasing numbers of individuals repeatedly alternate between low and high altitude for work and play. There is a general impression that acquired acclimatisation status persists for some time following return to sea level and that subsequent altitude tolerance is improved by previous hypoxic experience. However, it is unknown whether previous exposure to high altitude fundamentally alters the process of hypoxic re-acclimatisation (RA). My Doctoral research employed a number of approaches to investigate potential differences between the processes of initial acclimatisation (IA) and RA. The time course and mechanisms of hypoxic de-acclimatisation (DA) were explored to determine the time domains across which the retention of previous acclimatisation status might facilitate RA. Cross-sectional and longitudinal field studies were conducted to compare functional outcomes, cardiorespiratory function in rest and exercise and haematological responses throughout IA and RA in high-altitude trekkers. Results indicated that clinical outcomes and trekking performance were improved in RA but with limited physiological evidence of underlying improvements in hypoxic compensation. The haematological response to hypoxia was slightly greater in RA than IA, prompting further investigation of haematological RA in an animal model. Three paradigms of RA were used to examine the effect of IA and DA duration on the process of haematological RA in mice exposed to normobaric hypoxia. Despite altered erythropoietic control in RA treatments, the resulting haematological responses were generally consistent between IA and RA with no evidence of improved responses in RA. In fact, haematological acclimation was impaired in one RA treatment, possibly due to reduced availability of nutrients required for haemoglobin synthesis following an extended period of IA and upregulated erythropoiesis. Given the lack of physiological explanation for improved functional outcomes in RA, non-physiological mechanisms were pursued. Interviews with altitude-experienced individuals identified a perception that prior altitude experience leads to reduced altitude-induced anxiety and improved psychological tolerance of sensations associated with altitude exposure. Although physiological aspects of hypoxic re-acclimatisation merit further investigation, it is possible that improved psychological tolerance of high altitude contributes to the improved functional outcomes in RA that are reported here and elsewhere.Education, Faculty ofKinesiology, School ofGraduat