Altitude Acclimatization Research Articles

Between Individual Variability in Sleep and Awake Pulse Oximetry During Ascent Of Mt. Kilimanjaro

Between individual variability in awake arterial pulse oximetry (aSpO2) is well known and has been proposed for field assessment of altitude acclimatization. Sleep SpO2 (sSpO2) is known to be lower than aSpO2 at high altitude, but the between individual variability in sSpO2 is not as well known. PURPOSE: Compare between individual variability and relationship between sSpO2 and aSpO2 during a 6 day ascent of Mt. Kilimanjaro. METHODS: In 18 (12 men/6 women), age range 18-62 yr, low-altitude residents over 5 of 6 nights sSpO2 was assessed using a finger sensor recording pulse oximeter and compared to a morning aSpO2 measured for 1 minute while the subject was quietly seated. Each individual’s mean SpO2 was calculated for their sleep and awake measurements. Correlations (Pearson product-moment) between sleep and awake SpO2 were evaluated. RESULTS: As expected, both sSpO2 and aSpO2 decreased with increasing altitude. The Coefficient of Variation (CV), of both sSpO2 and aSpO2 increased with increasing altitude. Individual sSpO2 and the next morning aSpO2 were strongly correlated at all altitudes.Table: No title available.CONCLUSION: In low altitude residents climbing to 4830 m over 6 days, sleep and awake SpO2 have similar between subject variability (dispersion) that more than doubles with increasing altitude indicating a broadening of individual ventilation and pulmonary gas exchange responses to the hypoxic environment. Also, individual sSpO2 and aSpO2 are highly correlated to each other. Funding: USAMRMC. The views expressed in this abstract are those of the authors and do not reflect the official policy of the Department of Army, Department of Defense, or the U.S. Government.

Medical Education in the Mountains - The Educational Environment in the Diploma of Mountain Medicine

This article was migrated. The article was not marked as recommended. Aims: The Diploma of Mountain Medicine is a postgraduate blended learning course, combining online and distance learning with fully immersive face-to-face components. Utilising the Dundee Ready Educational Environment Measure (DREEM) to evaluate educational environment, alongside overall global course evaluation using the Postgraduate Taught Experience Survey (PTES), we aimed to further an understanding of the educational environment in this dynamic and increasingly popular method of course design. Methods & Results: All participants completing the Diploma between 2012-2015 were invited complete the online survey. Answers were recorded using a modified Likert scale. Response rate was 57% (n=68). Overall course satisfaction was positive using the PTES, with an overall mean score of 3.77 (out of 5). Only 9.4% of responses were negative. Educational environment using the DREEM was scored as "Excellent" (mean total score of 150.68; range 100-199). When the two tools used were compared, overall scores showed close agreement (r=0.775). The DREEM domain, Perception of Learning was most closely correlated with PTES overall score (r=0.714).Female participants rated the course lower than their male counterparts using both DREEM and PTES tools. Those who had <1 year since their most recent education, and those for whom highest study was at undergraduate level had lower DREEM scores than those with previous postgraduate experience. This may reflect differences in expectations and learning objectives. Conclusions: These multifaceted tools demonstrate a close alignment between educational environment and overall course satisfaction. Managing learner expectations and gender differences are potential challenges to overcome.

Astrocyte‐specific deletion of Kir4.1 increases normoxic ventilation after acclimatization to chronic sustained hypoxia.

Chronic sustained hypoxia (CH) produces ventilatory acclimatization to hypoxia (VAH), which involves time‐dependent increases of ventilation in normoxia and the hypoxic ventilatory response (HVR). Previous results show that glia are activated by CH and contribute to VAH but the contribution of astrocytes involved in central CO2‐sensitivity to VAH is not known. We tested the hypothesis that inwardly rectifying K+ 4.1 channels in astrocytes contribute to VAH using conditional gene deletion in transgenic mice. Conditional knockout (cKO) Kir4.1 mice had loxP sites at both ends of the entire Kir4.1 open reading frame and WT mice did not. WT and cKO Kir4.1 mice expressed tdTomato reporter and Cre‐recombinase/ERT2 receptors in astrocytes under the control of hGFAP promoter. Immunofluorescence against markers of astrocytes and Kir4.1 confirmed that tamoxifen treatment decreased Kir4.1 expression in astrocytes in CO2‐senstivie areas of the medulla of cKO Kir4.1 but not WT mice. After CH (PiO2 = 70 mmHg for 1 week), ventilation measured with barometric pressure plethysmography increased in normoxia, acute hypoxia and acute hypercapnia relative to chronically normoxic control mice. Ventilation was greater in normoxia after CH in Kir4.1cKO than in WT because of greater respiratory frequency and tidal volume. Ventilation in acute hypoxia or hypercapnia after CH was not significantly different between genotypes and metabolic rate was similar between genotypes at all O2 levels. The results support a role in astrocytes in the persistent hyperventilation in normoxia with VAH.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Ventilatory and cerebrovascular regulation and integration at high-altitude.

Ascent to high-altitude elicits compensatory physiological adaptations in order to improve oxygenation throughout the body. The brain is particularly vulnerable to the hypoxemia of terrestrial altitude exposure. Herein we review the ventilatory and cerebrovascular changes at altitude and how they are both implicated in the maintenance of oxygen delivery to the brain. Further, the interdependence of ventilation and cerebral blood flow at altitude is discussed. Following the acute hypoxic ventilatory response, acclimatization leads to progressive increases in ventilation, and a partial mitigation of hypoxemia. Simultaneously, cerebral blood flow increases during initial exposure to altitude when hypoxemia is the greatest. Following ventilatory acclimatization to altitude, and an increase in hemoglobin concentration-which both underscore improvements in arterial oxygen content over time at altitude-cerebral blood flow progressively decreases back to sea-level values. The complimentary nature of these responses (ventilatory, hematological and cerebral) lead to a tightly maintained cerebral oxygen delivery while at altitude. Despite this general maintenance of global cerebral oxygen delivery, the manner in which this occurs reflects integration of these physiological responses. Indeed, ventilation directly influences cerebral blood flow by determining the prevailing blood gas and acid/base stimuli at altitude, but cerebral blood flow may also influence ventilation by altering central chemoreceptor stimulation via central CO2 washout. The causes and consequences of the integration of ventilatory and cerebral blood flow regulation at high altitude are outlined.

The STAR Data Reporting Guidelines for Clinical High Altitude Research.

Brodmann Maeder, Monika, Hermann Brugger, Matiram Pun, Giacomo Strapazzon, Tomas Dal Cappello, Marco Maggiorini, Peter Hackett, Peter Baärtsch, Erik R. Swenson, Ken Zafren (STAR Core Group), and the STAR Delphi Expert Group. The STARdata reporting guidelines for clinical high altitude research. High AltMedBiol. 19:7–14, 2018.Aims: The goal of the STAR (STrengthening Altitude Research) initiative was to produce a uniform set of key elements for research and reporting in clinical high-altitude (HA) medicine. The STAR initiative was inspired by research on treatment of cardiac arrest, in which the establishment of the Utstein Style, a uniform data reporting protocol, substantially contributed to improving data reporting and subsequently the quality of scientific evidence.Materials and Methods: The STAR core group used the Delphi method, in which a group of experts reaches a consensus over multiple rounds using a formal method. We selected experts in the field of clinical HA medicine based on their scientific credentials and identified an initial set of parameters for evaluation by the experts.Results: Of 51 experts in HA research who were identified initially, 21 experts completed both rounds. The experts identified 42 key parameters in 5 categories (setting, individual factors, acute mountain sickness and HA cerebral edema, HA pulmonary edema, and treatment) that were considered essential for research and reporting in clinical HA research. An additional 47 supplemental parameters were identified that should be reported depending on the nature of the research.Conclusions: The STAR initiative, using the Delphi method, identified a set of key parameters essential for research and reporting in clinical HA medicine.

Short-Term Sustained Hypoxia Elevates Basal and Hypoxia-Induced Ventilation but Not the Carotid Body Chemoreceptor Activity in Rats.

Exposure to chronic sustained hypoxia (SH), as experienced in high altitudes, elicits an increase in ventilation, named ventilatory acclimatization to hypoxia (VAH). We previously showed that rats exposed to short-term (24 h) SH exhibit enhanced abdominal expiratory motor activity at rest, accompanied by augmented baseline sympathetic vasoconstrictor activity. In the present study, we investigated whether the respiratory and sympathetic changes elicited by short-term SH are accompanied by carotid body chemoreceptor sensitization. Juvenile male Holtzman rats (60–80 g) were exposed to SH (10% O2 for 24 h) or normoxia (control) to examine basal and hypoxic-induced ventilatory parameters in unanesthetized conditions, as well as the sensory response of carotid body chemoreceptors in artificially perfused in situ preparations. Under resting conditions (normoxia/normocapnia), SH rats (n = 12) exhibited higher baseline respiratory frequency, tidal volume, and minute ventilation compared to controls (n = 11, P < 0.05). SH group also showed greater hypoxia ventilatory response than control group (P < 0.05). The in situ preparations of SH rats (n = 8) exhibited augmented baseline expiratory and sympathetic activities under normocapnia, with additional bursts in abdominal and thoracic sympathetic nerves during late expiratory phase that were not seen in controls (n = 8, P < 0.05). Interestingly, basal and potassium cyanide-induced afferent activity of carotid sinus nerve (CSN) was similar between SH and control rats. Our findings indicate that the maintenance of elevated resting ventilation, baseline sympathetic overactivity, and enhanced ventilatory responses to hypoxia in rats exposed to 24 h of SH are not dependent on increased basal and sensorial activity of carotid body chemoreceptors.

The United States Military Mountain Medicine Course and its relevance to the United Kingdom armed forces

AbstractWorking in mountainous terrain, often at high altitude, presents a unique set of challenges to military medical personnel both in deployed roles and on adventurous training expeditions. Courses such as the increasingly popular Diploma in Mountain Medicine can provide clinicians with the knowledge and skills required to work in this environment. This article describes the current provision of mountain medicine education in the United Kingdom (UK). It also details the United States Military Mountain Medicine course as an example of military specific mountain medicine training and its relevance to UK armed forces medical personnel.

Autophagy Is a Promoter for Aerobic Exercise Performance during High Altitude Training.

High altitude training is one of the effective strategies for improving aerobic exercise performance at sea level via altitude acclimatization, thereby improving oxygen transport and/or utilization. But its underlying molecular mechanisms on physiological functions and exercise performance of athletes are still vague. More recent evidence suggests that the recycling of cellular components by autophagy is an important process of the body involved in the adaptive responses to exercise. Whether high altitude training can activate autophagy or whether high altitude training can improve exercise performance through exercise-induced autophagy is still unclear. In this narrative review article, we will summarize current research advances in the improvement of exercise performance through high altitude training and its reasonable molecular mechanisms associated with autophagy, which will provide a new field to explore the molecular mechanisms of adaptive response to high altitude training.

The potential usefulness of serum biomarkers in high-altitude medicine.

The potential usefulness of serum biomarkers in high-altitude medicine.

Doctor won’t see you now: changing paradigms in mountain medicine

The evolution in communication and digital technologies is revolutionising the practice of medicine. A physician is now able to oversee provision of healthcare at a distance. In this paper, we...

Translating carotid body function into clinical medicine.

The carotid body (CB) is considered the main O2 chemoreceptor, which contributes to cardiorespiratory homeostasis and ventilatory acclimatization. In clinical medicine, the most common pathologies associated with the CB are tumours. However, a growing body of evidence supports the novel idea that an enhanced CB chemosensory discharge contributes to the autonomic dysfunction and pathological consequences in obstructive sleep apnoea (OSA), hypertension, systolic heart failure (HF) and cardiometabolic diseases. Heightened CB chemosensory reactivity elicited by oxidative stress has been involved in sympathetic hyperactivity, cardiorespiratory instability, hypertension and insulin resistance. CB ablation, which reduces sympathetic hyperactivity, decreases hypertension in animal models of OSA and hypertension, eliminates breathing instability and improves animal survival in HF, and restores insulin tolerance in cardiometabolic models. Thus, data obtained from preclinical studies highlight the importance of the CB in the progression of sympathetic-related diseases, supporting the idea that appeasing the enhanced CB chemosensory drive may be useful in improving cardiovascular, respiratory and endocrine alterations. Accordingly, CB ablation has been proposed and used as a treatment for moderating resistant hypertension and HF-induced sympathetic hyperactivity in humans. First-in-human studies have shown that CB ablation reduces sympathetic overactivity, transiently reduces severe hypertension and improves quality of life in HF patients. Thus, CB ablation would be a useful therapy to reverse sympathetic overactivation in HF and severe hypertension, but caution is required before it is widely used due to the crucial physiological function played by the CB. Further studies in preclinical models are required to assess side-effects of CB ablation.

Effects of carbohydrate supplementation during aerobic exercise and altitude acclimatization on exogenous carbohydrate oxidation and performance

Purpose: High Altitude (HA) exposure alters energy substrate metabolism and degrades aerobic exercise performance. Carbohydrate supplementation has not been consistently demonstrated to be ergogenic and the ergogenic potential of carbohydrate supplementation may depend on acclimatization status and the ability to oxidize exogenous carbohydrate. Longitudinal evaluation of exogenous carbohydrate utilization and performance effects of carbohydrate supplementation, before and after acclimatization to high altitude has not been reported.

Knowledge of the Avalanche Victim Resuscitation Checklist and Utility of a Standardized Lecture in Italy

To explore baseline knowledge about avalanche guidelines and the Avalanche Victim Resuscitation Checklist (AVReCh) in Italy and the knowledge acquisition from a standardized lecture. Standardized lecture material discussing AVReCh was presented during 8 mountain medicine courses from November 2014 to April 2016 in different regions of Italy. To determine the knowledge acquisition from the lecture, a pre- and postlecture survey was utilized. A total of 193 surveys were analyzed. More than 50% of the participants had never participated in lectures/courses on avalanche guidelines, and less than 50% of the participants knew about the AVReCh before the lecture. The correct temporal sequence of reportable information in the basic life support section of the AVReCh was selected by 40% of the participants before the lecture and by 75% after the lecture (P<0.001). Within subgroups analysis, most groups saw significant improvement in performance (P<0.05). The selection of the correct burial time increased from 36 to 84% (P<0.05). Health care providers and mountain rescue personnel are not widely aware of avalanche guidelines. The standardized lecture significantly improved knowledge of the principles of avalanche management related to core AVReCh elements. However, the effect that this knowledge acquisition has on avalanche victim survival or adherence to the AVReCh in the field is yet to be determined.

Short-term arrival strategies for endurance exercise performance at moderate altitude.

For sea level-based endurance athletes who compete at moderate and high altitudes, many are not logistically able to arrive at altitude weeks before the event to fully acclimatize. For those who can only arrive at altitude the night before competition, we asked if there is a physiological and performance advantage in reducing altitude exposure time to 2 h before competition. On three separate visits, 10 cyclists completed overnight laboratory exposures of: 1) a 14-h exposure to normobaric hypoxia (16.2% O2, simulating 2,500 m; 14H), 2) a 12-h exposure to normoxia, then a 2-h hypoxic exposure (2H), and 3) a 14-h exposure to normoxia (CON). Immediately following each exposure, subjects completed a 20-km cycle ergometry time trial in normoxia (CON) or 16.2% O2 (14H and 2H). Measures of plasma volume changes, sleep quality, ventilatory acclimatization, perceived exertion, oxygen uptake, and 20-km time were collected. No significant differences were observed in performance measures or perceived exertion between hypoxic trials. Plasma volume loss was significantly greater during 14H than 2H and CON. No differences in ventilatory acclimatization or sleep quality were observed between trials. Although some divergent 20-km performance responses were observed between 14H and 2H, they were not explained by the physiological measures completed. The data suggest that endurance athletes who are logistically restricted from arriving at altitude more than the evening before competition would not gain an advantage by delaying their arrival until a few hours before the competition, although unique individual responses may ultimately influence optimal arrival strategy.NEW & NOTEWORTHY For athletes who cannot arrive at altitude multiple days before an endurance competition to properly acclimatize, this study asked if shortening hypoxic exposure time to 2 h before a competition was more advantageous than arrival at altitude the evening before competition. Our data suggest that athletes who cannot arrive at altitude with adequate time for complete acclimatization can choose the short-term arrival strategy that best fits with the logistics of their travel.

Observation of acclimatization to high altitude among medical personnel

Objective To observe plateau acclimatization process of medical personnel and explore effective coping strategies of high altitude reaction. Methods The blood pressure, heart rate, blood oxygen saturation and altitude sickness symptoms of the 20 medical personnel before entry of plateau and at different time points within three months after entry were compared, and then corresponding countermeasures according to different reactions were taken. Results After entering plateau, all objects emerged varying degrees of elevated blood pressure, increased heart rate, decreased oxygen saturation and discomfort; 11 team members who appeared altitude reaction gradually adapted after symptomatic treatment. Conclusions About 55% of medical personnel from plain to high altitude will appear plateau reaction, but timely symptomatic treatment may effectively alleviate plateau reaction, and finally help Tibet personnel to complete the task of Tibet smoothly. Key words: Plateau; Plateau reaction; Vital signs; Acclimatization

Ibuprofen does not reverse ventilatory acclimatization to chronic hypoxia

Ventilatory acclimatization to hypoxia involves an increase in the acute hypoxic ventilatory response that is blocked by non-steroidal anti-inflammatory drugs administered during sustained hypoxia. We tested the hypothesis that inflammatory signals are necessary to sustain ventilatory acclimatization to hypoxia once it is established. Adult, rats were acclimatized to normoxia or chronic hypoxia (CH, PiO2=70Torr) for 11–12days and treated with ibuprofen or saline for the last 2days of hypoxia. Ventilation, metabolic rate, and arterial blood gas responses to O2 and CO2 were not affected by ibuprofen after acclimatization had been established. Immunohistochemistry and image analysis showed acute (1h) hypoxia activated microglia in a medullary respiratory center (nucleus tractus solitarius, NTS) and this was blocked by ibuprofen administered from the beginning of hypoxic exposure. Microglia returned to the control state after 7days of CH and were not affected by ibuprofen administered for 2 more days of CH. In contrast, NTS astrocytes were activated by CH but not acute hypoxia and activation was not reversed by administering ibuprofen for the last 2days of CH. Hence, ibuprofen cannot reverse ventilatory acclimatization or astrocyte activation after they have been established by sustained hypoxia. The results are consistent with a model for microglia activation or other ibuprofen-sensitive processes being necessary for the induction but not maintenance of ventilatory acclimatization to hypoxia.

Impediments in the Tissue Culture of High Altitude Medicinal Plants

Impediments in the Tissue Culture of High Altitude Medicinal Plants

Is normobaric hypoxia an effective treatment for sustaining previously acquired altitude acclimatization?

This study examined whether normobaric hypoxia (NH) treatment is more efficacious for sustaining high-altitude (HA) acclimatization-induced improvements in ventilatory and hematologic responses, acute mountain sickness (AMS), and cognitive function during reintroduction to altitude (RA) than no treatment at all. Seventeen sea-level (SL) residents (age = 23 ± 6 yr; means ± SE) completed in the following order: 1) 4 days of SL testing; 2) 12 days of HA acclimatization at 4,300 m; 3) 12 days at SL post-HA acclimatization (Post) where each received either NH (n = 9, [Formula: see text] = 0.122) or Sham (n = 8; [Formula: see text] = 0.207) treatment; and 4) 24-h reintroduction to 4,300-m altitude (RA) in a hypobaric chamber (460 Torr). End-tidal carbon dioxide pressure ([Formula: see text]), hematocrit (Hct), and AMS cerebral factor score were assessed at SL, on HA2 and HA11, and after 20 h of RA. Cognitive function was assessed using the SynWin multitask performance test at SL, on HA1 and HA11, and after 4 h of RA. There was no difference between NH and Sham treatment, so data were combined. [Formula: see text] (mmHg) decreased from SL (37.2 ± 0.5) to HA2 (32.2 ± 0.6), decreased further by HA11 (27.1 ± 0.4), and then increased from HA11 during RA (29.3 ± 0.6). Hct (%) increased from SL (42.3 ± 1.1) to HA2 (45.9 ± 1.0), increased again from HA2 to HA11 (48.5 ± 0.8), and then decreased from HA11 during RA (46.4 ± 1.2). AMS prevalence (%) increased from SL (0 ± 0) to HA2 (76 ± 11) and then decreased at HA11 (0 ± 0) and remained depressed during RA (17 ± 10). SynWin scores decreased from SL (1,615 ± 62) to HA1 (1,306 ± 94), improved from HA1 to HA11 (1,770 ± 82), and remained increased during RA (1,707 ± 75). These results demonstrate that HA acclimatization-induced improvements in ventilatory and hematologic responses, AMS, and cognitive function are partially retained during RA after 12 days at SL whether or not NH treatment is utilized.NEW & NOTEWORTHY This study demonstrates that normobaric hypoxia treatment over a 12-day period at sea level was not more effective for sustaining high-altitude (HA) acclimatization during reintroduction to HA than no treatment at all. The noteworthy aspect is that athletes, mountaineers, and military personnel do not have to go to extraordinary means to retain HA acclimatization to an easily accessible and relevant altitude if reexposure occurs within a 2-wk time period.

Evidence from high-altitude acclimatization for an integrated cerebrovascular and ventilatory hypercapnic response but different responses to hypoxia.

Ventilation and cerebral blood flow (CBF) are both sensitive to hypoxia and hypercapnia. To compare chemosensitivity in these two systems, we made simultaneous measurements of ventilatory and cerebrovascular responses to hypoxia and hypercapnia in 35 normal human subjects before and after acclimatization to hypoxia. Ventilation and CBF were measured during stepwise changes in isocapnic hypoxia and iso-oxic hypercapnia. We used MRI to quantify actual cerebral perfusion. Measurements were repeated after 2 days of acclimatization to hypoxia at 3,800 m altitude (partial pressure of inspired O2 = 90 Torr) to compare plasticity in the chemosensitivity of these two systems. Potential effects of hypoxic and hypercapnic responses on acute mountain sickness (AMS) were assessed also. The pattern of CBF and ventilatory responses to hypercapnia were almost identical. CO2 responses were augmented to a similar degree in both systems by concomitant acute hypoxia or acclimatization to sustained hypoxia. Conversely, the pattern of CBF and ventilatory responses to hypoxia were markedly different. Ventilation showed the well-known increase with acute hypoxia and a progressive decline in absolute value over 25 min of sustained hypoxia. With acclimatization to hypoxia for 2 days, the absolute values of ventilation and O2 sensitivity increased. By contrast, O2 sensitivity of CBF or its absolute value did not change during sustained hypoxia for up to 2 days. The results suggest a common or integrated control mechanism for CBF and ventilation by CO2 but different mechanisms of O2 sensitivity and plasticity between the systems. Ventilatory and cerebrovascular responses were the same for all subjects irrespective of AMS symptoms. NEW & NOTEWORTHY Ventilatory and cerebrovascular hypercapnic response patterns show similar plasticity in CO2 sensitivity following hypoxic acclimatization, suggesting an integrated control mechanism. Conversely, ventilatory and cerebrovascular hypoxic responses differ. Ventilation initially increases but adapts with prolonged hypoxia (hypoxic ventilatory decline), and ventilatory sensitivity increases following acclimatization. In contrast, cerebral blood flow hypoxic sensitivity remains constant over a range of hypoxic stimuli, with no cerebrovascular acclimatization to sustained hypoxia, suggesting different mechanisms for O2 sensitivity in the two systems.

Oxygen saturation increases over the course of the night in mountaineers at high altitude (3050-6354 m).

Blood oxygen saturation (SpO 2 ) is frequently measured to determine acclimatization status in high-altitude travellers. However, little is known about nocturnal time course of SpO 2 (SpO 2N ), but alterations in SpO 2N might be practically relevant as well. To this end, we describe the time-course of SpO 2N in mountaineers at high altitude. SpO 2N was continuously measured in ten male mountaineers during a three-week expedition in Peru (3,050-6,354m). Average SpO 2N of the first (SpO 2N1 ) and second half (SpO 2N2 ) of an individual's sleep duration was calculated from 2h intervals of uninterrupted sleep. Heart rate oscillations and sleep dairies were used to exclude periods of wakefulness. SpO 2 was also measured at rest in the morning. SpO 2N significantly increased from SpO 2N1 to SpO 2N2 . The magnitude of this increase (ΔSpO 2 ) was reduced with time spent at altitude. On night 1 (3,050m) SpO 2 increased from 83.4% (N1) to 86.3% (N2). At the same location on night 21, SpO 2 increased from 88.3% to 90.1%, which is a relative change of 4.7% and 2.0%, respectively. This pattern of increase in SpO 2N was perturbed when individual acclimatization was poor or altitude was extreme (5630m). SpO 2N was significantly lower than SpO 2 at rest in the morning. This study is the first to demonstrate an increase of SpO 2 during the night in mountaineers at high altitude (3,050-6,354m) with high consistency between and within subjects. The magnitude of ΔSpO 2N decreased as acclimatization improved, suggesting that these changes in ΔSpO 2 between nights might be a valuable indicator of individual acclimatization. In addition, the failure of any increase in SpO 2N during the night might indicate insufficient acclimatization. Even though underlying mechanisms for the nocturnal increase remain unclear, the timing of SpO 2N measurement is obviously of utmost importance for its interpretation. Finally our study illustrates the detailed effects of ventilatory acclimatization over several weeks.