Rapid ascents of Mt Everest: normobaric hypoxic preacclimatization

Abstract

Background

Acclimatization to high altitude is time consuming. An expedition to Mt Everest (8848 m) requires roughly 8 weeks. Therefore it seems very attractive to reach the summit within 3 weeks from home, which is currently promised by some expedition tour operators. These rapid ascent expeditions are based on two main components, normobaric hypoxic training (NHT) prior to the expedition and the use of high flow supplemental oxygen (HFSO₂). We attempted to assess the relative importance of these two elements.

Methods

We evaluated the effect of NHT on the basis of the available information of these rapid ascent expeditions and our experiences made during an expedition to Manaslu (8163 m) where we used NHT for preacclimatization. To evaluate the effect of an increased O₂ flow rate we calculated its effect at various activity levels at altitudes of 8000 m and above.

Results

So far rapid ascents to Mt Everest have been successful. The participants carried out 8 weeks of NHT, reaching sleeping altitudes = 7100 m and spent at least 300 h in NH. At rest a flow rate of 2 l O₂/min is sufficient to keep the partial pressure of inspired oxygen (PIO₂) close to 50 mm Hg even at the summit. For ativities of ~80% of the maximum rate of oxygen consumption (VO_2max) at the summit 6 l O₂/min are required to maintain a PIO₂ above 50 mm Hg.

Discussion

NHT for preacclimatization seems to be the decisive element of the offered rapid ascent expeditions. An increased O₂ flow rate of 8 l/min is not mandatory for climbing Mt Everest.

Conclusions

Preacclimatization using normobaric hypoxica (NH) is far more important than the use of HFSO₂. We think that NHT will be widely used in the future. The most effective regimen of preacclimatization in NH, the duration of each session and the optimal FIO₂ are still unclear and require further study.

Introduction

The number of tourists travelling to high altitude all over the world is estimated to have reached 100 million a year.¹ Over the past 10 years climbing 8000 m peaks, including Mt Everest (8848 m), has become increasingly popular.^2,3 By the summer of 2019, ~10 000 ascents of Mt Everest had been made even by technically unskilled climbers on commercial expeditions.³ Climbing peaks above 8000 m normally requires time-consuming acclimatization on site.^4,5 On average, an expedition to Mt Everest takes 8 weeks.^6,7 Therefore it seems very attractive to reach the summit within 3 weeks from home. Some commercial expedition companies now offer this possibility.^8,9 The strategy of using normobaric hypoxica (NH) for preacclimatization at home in order to reduce time on the mountain has been used by professional athletes for years.^10–12 Normobaric hypoxic training (NHT) has now found its way into commercial expedition mountaineering. One European company claims that in 2018 and 2019 all of their clients who used NHT and then used high flow supplemental oxygen (HFSO₂) on ascent, reached the summit of Mt Everest via the North Col-Northeast Ridge route.⁸ In 2020 at least eight organizers of commercial expeditions had planned to offer rapid ascents of 8000 m peaks using NHT. These plans were cancelled due to the COVID-19 pandemic.

These rapid ascents were based on two main elements, preacclimatization in NH at home prior to the expedition, with simulated sleeping altitudes comparable to 7100 m, and the use of HFSO₂ on the ascent, with the option of an increased O₂ flow rate up to 8 l O₂/min.¹³ The magnitude of the beneficial effect of an increased O₂ flow rate at extreme altitudes is unclear.¹⁴ We attempted to assess the relative importance of these two elements.

Methods

To study to what extent rapid ascent expeditions have already been scientifically evaluated we searched PubMed with the search terms ‘Flash / expedition’, ‘rapid ascent / expedition’ and ‘preacclimatization / expedition’. In addition we directly contacted the company (Furtenbach Adventures) that claimed 100% summit success in 2018 and 2019 for further information. We checked the claims regarding successful summits using the Himalayan Database [https://www.himalayandatabase.com] and other publicly available data of those two expeditions to Mt Everest in 2018 and 2019.^13,15

In order to estimate the magnitude of the beneficial effect of an increased O₂ flow rate, we calculated the effect of various flow rates on partial alveolar oxygen pressure (PAO₂) at various altitudes at various activity levels on partial alveolar O₂ pressure (SaO₂). We used the following assumptions: estimated For the summit region of Mt Everest, we assumed a respiratory minute volume of 40 l/min at rest,¹⁶ 100 l/min for a strenuous physical exercise¹⁷ and 150 l/min for a maximum physical exercise close to the respiratory limit.¹⁸ We estimated that 50 mm Hg is the critical threshold for PAO₂, which corresponds roughly to an altitude of 7500 m, if no supplemental O₂ is used.⁷ In order to evaluate the effect of HFSO₂ we calculated how a flow rate of 2, 4, 6 and 8 l O₂/min affect the PAO₂ at different activity levels (rest, strenuous physical activity and maximum physical activity) for the altitudes 8000 m, 8500 m and the summit altitude (8848 m).

PAO₂ is calculated as follows: PAO₂ = (P_ATM−pH₂O) × FIO₂ where P_ATM is the barometric pressure according to the standard atmosphere (8000 m: 267 mm Hg; 8500 m: 248 mm Hg; 8848 m: 236 mm Hg); PH₂O is the pressure of water vapor in the lungs (47 mmHg at a body temperature of at 37°C) and FIO₂ is the fraction of inspired oxygen of the inhaled air (Table 1).

FIO₂ is calculated as follows: FIO₂ = (VO_{2 natural} + VO_{2 suppl})/ V_Air where VO_{2 natural} is the volume of inhaled natural O₂ at a given respiratory minute volume (fraction of 0.21 of respiratory minute volume; 40 l/min: 8.4 l O₂; 100 l/min: 20.9 l O₂; 150 l/min: 31.4 l O₂); VO_{2 suppl} is the inhaled supplemental O₂ (2, 4, 6 and 8 l O₂/min) and V_Air is the respiratory minute volume.

Results

The PubMed search produced no relevant references. The company was unwilling to share any information concerning the preacclimatization profile as well as the altitude profile of the ascent to Mt Everest, claiming that all information was proprietary. In the popular press and blog posts , we found reports of two rapid ascent expeditions to Mt. Everest in which the participants carried out at least 300 h in NH during 8 weeks of NHT, until just before departure, during which they reached simulated sleeping altitudes equivalent to 7100 m.^13,19

Physiologic effect of various oxygen flow rates

Depending on the O₂ flow rate the elevation of the summit of Mt Everest can be reduced physiologically to a equivalent altitude between 7400 and 8400 m.¹⁴ For a reduction to 7400 m this requires 2.3 l O₂/min at rest and 6.1 l O₂/min for strenuous activity. For a reduction to 8400 m 0.6 l O₂/min at rest and 1.5 l O₂/min for strenuous activity are sufficient.

At rest, a flow rate of 2 l O₂/min is sufficient for both 8000 m and 8500 m to keep PAO₂ above 50 mm Hg. Even on the summit PAO₂ is 49 mm Hg (Figure 1). Higher flow rates are not necessary at rest, only for the summit 2.3 l O₂/min is required to keep PAO₂ above 50 mm Hg. For activities of ~80% of VO_2max (Figure 2), 2 l O₂/min is just enough at 8000 m and 4 l O₂/min is just enough at 8500 m. Above this altitude a higher flow rate is necessary. On the summit 6 l O₂/min is required to maintain a PAO₂ of 51 mm Hg at this high level of intensity, or one has to climb more slowly in the summit area. With maximal activity in the range of the respiratory limit value (Figure 3), a flow rate of 6 l O₂/min is still sufficient at 8500 m. Close to the summit even with 8 l O₂/min a PAO₂ of 50 mm Hg can no longer achieved.

Discussion

The rapid ascents that have been reported have been based on two main elements: NHT and HFSO₂. The first element is preacclimatization in NH at home prior to the expedition, with simulated sleeping altitudes equivalent to 7100 m. The second element is the liberal use of supplemental O₂ with the option of an increased O₂ flow rate of up to 8 l O₂/min. We evaluated these elements on the basis of the current publications and our own experiences during an expedition to Manaslu (8163 m) in 2016 for which we used NHT for preacclimatization.²⁰ Preacclimatization and maintenance of acclimatization are of great importance for the military.^21–24 One objective of the military training expedition to Manaslu was to investigate whether rapid deployment to base camp altitude (4900 m) is possible after a progressive 10-day NHT protocol.²⁰

Preacclimatization in normobaric hypoxia

NH is induced by lowering the percentage of oxygen in inspired air (FIO₂).²⁵ NH is widely used in fire protection, since at 15% O₂ (at sea level, =2700 m) an open fire is almost impossible and at 13% (at sea level, =3850 m) even explosive substances such as gasoline do not burn.^26–28 Portable NH-generators produce a normobaric hypoxic gas mixture. The oxygen content (PO₂) can be adjusted according to the desired equivalent altitude. Oxygen reduced gas can be inhaled using breathing masks at rest or during physical activity or it can be pumped into lightweight tents for sleeping. NH is simple to use and can be used almost anywhere.^25,29 That is why NH has been used by elite athletes for years in order to optimize performance,^30,31 as well as for preacclimatization for professional mountaineers^10–12 and the military.^21,29,32 For practical reasons NH should be used during the normal everyday working life. This can only be achieved by using NH, because hypobaric hypoxic chambers are expensive, complex to operate and not widespread.^11,20

Preacclimatization is not performed continuously over 24 h per day but as intermittent hypoxic exposure.²⁹ Significant acclimatization effects can be obtained by exposure times of a few hours per day.^33,34 The longer the exposure time the more pronounced is the acclimatization effect.^35,36 In general, the higher the planned altitude, the greater the required hypoxic dose must be. The hypoxic dose of NHT is determined by the degree of hypoxia, the duration of each session, the number of sessions and the timing of the sessions, whether NHT is used during sleep at night, at rest during the day or during exercise.^29,36,37 This results in a large number of possible NHT protocols.^29,32 It is currently unclear which preacclimatization strategy is most effective.^21,29,38

Since acute mountain sickness (AMS) does not play a role in NHT, likely due to its latency of 4–36 h,^39–41 many recommendations for the implementation of NHT may be too conservative.^20,42 Table 2 shows the recommendations of a company that rents NH-devices compared to the NHT regimen we followed to prepare for our expedition to Manaslu. At natural altitudes >2500 m an increase of sleeping altitude should not exceed 300–500 m per day. Below 2500 m there are no restrictions.^4,39–41,43 It is therefore unclear why the company recommends such a slow increase in sleeping altitude. In our opinion, it is not necessary for healthy persons to start with a sleeping altitude below 2500 m. In contrast to natural altitude exposure, NH can be instantly stopped if symptoms occur. Therefore it seems safe to expose healthy persons to pronounced hypoxic conditions.

Pulse oximetry can be used to measure the degree of hypoxemia in order to adjust the simulated altitude.^20,44,45 As arterial oxygen saturation (SaO₂) shows a circadian rhythm and its lowest values occur in the first part of the night, pulse oximetry monitoring should be used during sleep in order to detect distinct desaturation.^46,47 In addition to these diurnal fluctuations, SaO₂ often oscillates considerably at high altitude due to periodic breathing. In a previous study we detected oscillations of up to 17 points within 3 minute periods.⁴⁵ This must be taken into account if a pulse oximeter with alarm function is used as a safety monitoring system, otherwise false alarms will occur frequently and disturb sleep. Based on the results of previous studies we recommend an alarm limit of 70% up to 4000 m and 60% between 4000 and 6000 m.^46,48

Our NHT regimen prior to the Manaslu expedition was applied during normal work routine. Therefore we performed NHT every night and supplemented this with additional NH exposures of 1–1.5 h during the day (Table 2). This regimen involved a greater increase in sleeping altitude than recommended by the rental company. Our regimen induced sufficient acclimatization to Manaslu Base Camp (4900 m), as evidenced by the fact that none of the expedition members suffered from AMS although we reached base camp rapidly by helicopter flight from Kathmandu to 3500 m and a 1-day walk the next day. Eight days after leaving Kathmandu we reached camp III at 6800 m.

The optimum regimen for preacclimatization using NHT is not known. The decisive factor is that NHT can be integrated into the normal working day. In our opinion, the method which can be integrated best into the daily routine will prevail in practice. Most people are likely to use NHT primarily during sleep with additional hypoxic exposures over the day, at rest or during physical activity.

The exact preacclimatization profile of the commercial rapid ascent expeditions to Mt Everest in 2018 and 2019 is considered to by proprietary by the organizers. The duration of 8 weeks was similar to the preacclimatization profile prior to the fastest known ascent to the summit of Mt Everest in 26.5 h from the advanced base camp (6500 m) in Tibet by Kilian Jornet.¹¹ After finishing NHT the rapid ascent expedition in 2018 reached Everest Base Camp (5200 m) on Day 5 and the North Col (7000 m) on Day 10, without using supplemental O₂. The participants reached the summit of Mt Everest on the Day 21 after leaving Europe (Figure 4). For the final summit ascent supplemental O₂ was used, although a flow rate of 8 l O₂/min was never necessary.¹³

Use of supplemental oxygen

The use of supplemental O₂ is standard when climbing Mt Everest,^3,49,50 above 7500 m most climbers use it.⁷ Altitudes above 7500 m are often referred to as the death zone.⁵¹ The very popular Muztagh Ata (7546 m) is frequently climbed, almost always without supplemental O₂.⁵² Many physiologists thought that reaching the summit of Mt Everest without supplemental O₂ was impossible⁵³ until Reinhold Messner and Peter Habeler succeeded on 8 May 1978.^54,55 Relatively few climbers have reached the summit of Mt Everest without supplemental O₂ compared to the number of summiters who used supplemental O₂. At the end of 2017, only 208 climbers, of whom 57 were Nepali high altitude guides (‘Sherpas’), had reached the summit without the use of supplemental O₂.¹³ As mortality is significantly lower with supplemental O₂^3,56 all expedition organizer use supplemental O₂. The organizer of the 2018 and 2019 rapid ascents of Mt Everest promises its customers unlimited oxygen with a maximum possible flow rate of 8 l O₂/min.¹³ This is achieved using cost-intensive support by Sherpas (two climbing Sherpas for each participant) to carry oxygen cylinders and by use of a breathing mask system that allows a high flow rate of 8 l O₂/min.¹³ Usually four to six oxygen cylinders per person and flow rates 2–4 l O₂/min are used.^14,49,50,57 Edmund Hillary and Tenzing Norgay had a flow rate of up to 5 l O₂/min available during their first ascent on 29 May 1953.⁴⁹

For 2018 a comparison of the rapid ascent group to the classic Everest ascent is possible, because two groups from the same organizer climbed Mt Everest on the same route at the same time. The amount of oxygen consumed was the same in both groups. One participant in the rapid ascent group used oxygen only from 8300 m upwards. The rapid ascent group was 1 h faster during the summit ascent than the classic expedition.¹³

Referring to the flow rates we have examined, a flow rate of 2–4 l O₂/min is sufficient most of the time. A flow rate of 6 l O₂/min may be helpful near the summit of Mt Everest. A flow rate of 8 l O₂/min may serve as a safety reserve if it is necessry to climb rapidly for a short time. A flow rate of 8 l O₂/min does not seem to offer an advantage compared to 6 l O₂/min for reaching the summit of Mt Everest, as nobody sprints to the summit. This is supported by the fact that a flow rate of 8 l O₂/min was never needed in the 2018 or the 2019 rapid ascent expedition.¹³

Conclusion

There have been successful rapid ascents of Mt. Everest using NHT. Preacclimatization using NH is far more important than the use of HFSO₂. We think that NHT will be widely used in the future. The most effective regimen of preacclimatization in NH, the duration of each session and the optimal FIO₂ are still unclear and require further study.

Authors’ contribution

Study design, data collection, data interpretation, manuscript preparation and literature research were performed by M Tannheimer, Data interpretation, manuscript preparation and literature research were performed by R Lechner. Statistical analysis and funds collection were not necessary for this article.

Funding

None.

Conflict of interest

None declared.

The study was presented at the seventh Sport Orthopaedic Winter Symposium on the Feldberg Freiburg, Germany on 15 February 2020.

The study was presented as scientific lecture required as part of the habilitation procedure of the University of Ulm, Germany on 22 February 2019.

The opinions expressed in this article are those of the authors and do not necessarily reflect the views of the German Armed Forces.

References