Abstract
T
New approaches to acute mountain sickness (AMS) prevention such as intermittent normobaric hypoxia preacclimatization are gaining attention in high-altitude (HA) research. At the same time, intermittent normobaric hypoxia has been used with the intention to increase the likelihood of summit success and accelerate the ascent profile on commercial expeditions to extreme altitude, creating an incentive to critically review the current evidence regarding safety and effectiveness and discuss future directions.
Initial research regarding the effectiveness of intermittent normobaric hypoxia to prevent AMS was inconclusive or negative, whereas recent studies have demonstrated a reduction in AMS incidence during HA exposure after various intermittent normobaric hypoxia protocols (Wille et al., 2012; Dehnert et al., 2014). But, intermittent normobaric hypoxia seems to be effective only if the hypoxic exposure amounts to >1 to several 100 hours distributed across several weeks to months (Faulhaber et al., 2016). These findings do not directly predict the effect of intermittent normobaric hypoxia on the outcome or timeline of complex ventures such as expeditions to extreme altitudes.
The most frequently reported adverse effects of preacclimatization protocols were symptoms of AMS, whereas more severe conditions such as HA pulmonary or cerebral edemas have not been detected (Mairer et al., 2012). Beyond indications for positive effects on decision-making, the scope of the current literature does not allow to draw conclusions about positive or negative safety-related issues associated with an accelerated ascent profile after intermittent normobaric hypoxia. As a consequence, several research questions remain open.
First, is preacclimatization safe for the subjects? It is likely that at the present many climbers acclimatize in hypoxic tents at home without any medical advice and monitoring. Increasing hematocrit and hemoglobin levels at sea level may induce side effects such as thromboembolic events in high-risk subjects, particularly after a long travel from Western countries to Nepal. Subjects with unknown pre-existing medical conditions such as coronary artery disease may not well tolerate hypoxia during nighttime.
Second, it is still unknown what is the optimal intermittent normobaric hypoxia protocol. And third, is intermittent normobaric hypoxia effective in all categories of climbers? Commercial expeditions lead, among others, climbers with limited training and experience in HA conditions. Whether such protocols will improve the chances of reaching the summit in elite climbers with no supplementary oxygen remains to be investigated.
From preliminary studies, case reports, and personal communications, it can be assumed that intermittent normobaric hypoxia decreases AMS symptoms and may increase the chances to succeed in HA expeditions. In contrast, robust evidence is lacking and particular attention should be paid to safety issues. HA research should focus on how to select candidates, safety operating procedures, and optimal ascent profiles for intermittent normobaric hypoxia preacclimatization. The results will help to give advice on preacclimatization to HA expedition agencies. Finally, apart from safety and effectiveness concerns, the ethical question regarding preacclimatization remains to be answered by each mountaineer on her or his own.
Footnotes
Acknowledgments
The authors would like to thank the UIAA members who attended the meeting for their contributions: Giancelso Agazzi, Corrado Angelini, Martin Burtscher, Eckhart van Delft, David Hillebrandt, Lenka Horakova, Norihiro Kamikomaki, Hidenori Kanazawa, Thomas Küppel, Kaste Mateikaite, Giulio Sergio Roi, Alison Rosier, Rianne van der Spek, Marieke van Vessem.