Barometric pressures at extreme altitudes on Mt. Everest: physiological significance

A revision of maximal oxygen consumption and exercise capacity at altitude 70 years after the first climb of Mount Everest

On the 70th anniversary of the first climb of Mount Everest by Edmund Hillary and Tensing Norgay, we discuss the physiological bases of climbing Everest with or without supplementary oxygen. After summarizing the data of the 1953 expedition and the effects of oxygen administration, we analyse the reasons why Reinhold Messner and Peter Habeler succeeded without supplementary oxygen in 1978. The consequences of this climb for physiology are briefly discussed. An overall analysis of maximal oxygen consumption (V ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ ) at altitude follows. In this section, we discuss the reasons for the non-linear fall ofV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ at altitude, we support the statement that it is a mirror image of the oxygen equilibrium curve, and we propose an analogue of Hill's model of the oxygen equilibrium curve to analyse theV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ fall. In the following section, we discuss the role of the ventilatory and pulmonary resistances to oxygen flow in limitingV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ , which becomes progressively greater while moving toward higher altitudes. On top of Everest, these resistances provide most of theV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ limitation, and the oxygen equilibrium curve and the respiratory system provide linear responses. This phenomenon is more accentuated in athletes with elevatedV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ , due to exercise-induced arterial hypoxaemia. The large differences inV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ that we observe at sea level disappear at altitude. There is no need for a very highV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ at sea level to climb the highest peaks on Earth.

The relationship between altitude and BMI varies across low- and middle-income countries

OBJECTIVES

Studies suggest that living at high altitude decreases obesity risk, but this research is limited to single-country analyses. We examine the relationship between altitude and body mass index (BMI) among women living in a diverse sample of low- and middle-income countries.

MATERIALS AND METHODS

Using Demographic and Health Survey data from 1 583 456 reproductive age women (20-49 years) in 54 countries, we fit regression models predicting BMI and obesity by altitude controlling for a range of demographic factors-age, parity, breastfeeding status, wealth, and education.

RESULTS

A mixed-effects model with country-level random intercepts and slopes predicts an overall -0.162 kg/m2 (95% CI -0.220, -0.104) reduction in BMI and lower odds of obesity (OR 0.90, 95% CI 0.87, 0.95) for every 200 m increase in altitude. However, countries vary dramatically in whether they exhibit a negative or positive association between altitude and BMI (34 countries negative, 20 positive). Mixed findings also arise when examining odds of obesity.

DISCUSSION

We show that past findings of declining obesity risk with altitude are not universal. Increasing altitude predicts slightly lower BMIs at the global level, but the relationship within individual countries varies in both strength and direction.

High Altitude

With ascent to high altitude, barometric pressure declines, leading to a reduction in the partial pressure of oxygen at every point along the oxygen transport chain from the ambient air to tissue mitochondria. This leads, in turn, to a series of changes over varying time frames across multiple organ systems that serve to maintain tissue oxygen delivery at levels sufficient to prevent acute altitude illness and preserve cognitive and locomotor function. This review focuses primarily on the physiological adjustments and acclimatization processes that occur in the lungs of healthy individuals, including alterations in control of breathing, ventilation, gas exchange, lung mechanics and dynamics, and pulmonary vascular physiology. Because other organ systems, including the cardiovascular, hematologic and renal systems, contribute to acclimatization, the responses seen in these systems, as well as changes in common activities such as sleep and exercise, are also addressed. While the pattern of the responses highlighted in this review are similar across individuals, the magnitude of such responses often demonstrates significant interindividual variability which accounts for subsequent differences in tolerance of the low oxygen conditions in this environment.

Oxygen saturation thresholds for bronchiolitis at high altitudes: a cost-effectiveness analysis

BACKGROUND

There is evidence suggesting that exaggerated reliance on pulse oximetry (SpO2) and the use of arbitrary/inadequate thresholds of SpO2 might drive unnecessary hospitalizations for viral bronchiolitis, especially among high-altitude residents. The aim of the present study was to compare the cost-effectiveness of two oxygen SpO2 thresholds for deciding whether infants with viral bronchiolitis living at high altitudes need hospital admission or can be discharged home.

METHODS

A cost-effectiveness study was performed to compare the cost and clinical outcomes of two oxygen SpO2 thresholds, adjusted or not, to an altitude above the sea level of Bogota, Colombia (2640 m), for deciding whether infants with viral bronchiolitis need hospitalization or can be discharged home. The principal outcome was avoidance of hospital admission.

RESULTS

Compared to the use of an SpO2 threshold of 90%, using an SpO2 threshold of 85% in infants with viral bronchiolitis was associated with lower overall costs (US$130.4 vs. US$194.0 average cost per patient) and a higher probability of hospitalization avoided (0.7500 vs. 0.5900), thus leading to dominance.

CONCLUSIONS

The use of an SpO2 threshold below 90% for deciding on hospitalization in infants with viral bronchiolitis living at high altitudes appears to be logical, secure, and cost-effective.

Comparison of two oxygen saturation targets to decide on hospital discharge of infants with viral bronchiolitis living at high altitudes: a cost-effectiveness analysis

OBJECTIVES

The objective of the current study was to evaluate the cost-effectiveness of two pulse oximetry (SpO2) thresholds to decide on hospital discharge when all other discharge criteria are met, in infants with viral bronchiolitis living at high altitudes.

METHODS

A decision analysis model was developed to estimate the cost-effectiveness of the use of an SpO2 threshold of 90% versus one of 85% for deciding whether infants hospitalized for viral bronchiolitis can be safely discharged to home, from a third-party payer's perspective. The main outcome was discharge to home at day 4 of the initial hospitalization. The time horizon was 28 days after discharge from hospital. We performed deterministic sensitivity analyses and probabilistic sensitivity analyses.

RESULTS

Compared to the use of an SpO2 threshold of 90%, treating infants with viral bronchiolitis with the use of an SpO2 threshold of 85% resulted in lower total costs (US$119.39 vs. US$188.357 mean cost per patient) and a greater probability of discharge to home at day 4 of the initial hospitalization (0.8400 vs. 0.7600), therefore being a dominant strategy. Sensitivity analyses were in line with base case results.

CONCLUSIONS

In Bogota, a high-altitude city, in infants admitted for viral bronchiolitis, the use of an SpO2 threshold of 85% to decide on hospital discharge when all other discharge criteria are met is dominant because it entails a greater probability of discharge to home at day 4 of the initial hospitalization and generates fewer costs than the use of an SpO2 threshold of 90%.

Mechanobiological Implications of Cancer Progression in Space

The human body is normally adapted to maintain homeostasis in a terrestrial environment. The novel conditions of a space environment introduce challenges that changes the cellular response to its surroundings. Such an alteration causes physical changes in the extracellular microenvironment, inducing the secretion of cytokines such as interleukin-6 (IL-6) and tumor growth factor-β (TGF-β) from cancer cells to enhance cancer malignancy. Cancer is one of the most prominent cell types to be affected by mechanical cues via active interaction with the tumor microenvironment. However, the mechanism by which cancer cells mechanotransduce in the space environment, as well as the influence of this process on human health, have not been fully elucidated. Due to the growing interest in space biology, this article reviews cancer cell responses to the representative conditions altered in space: microgravity, decompression, and irradiation. Interestingly, cytokine and gene expression that assist in tumor survival, invasive phenotypic transformation, and cancer cell proliferation are upregulated when exposed to both simulated and actual space conditions. The necessity of further research on space mechanobiology such as simulating more complex in vivo experiments or finding other mechanical cues that may be encountered during spaceflight are emphasized.

Heat Balance When Climbing Mount Everest

Background: Mountaineers must control and regulate their thermal comfort and heat balance to survive the rigors of high altitude environment. High altitudes feature low air pressure and temperatures, strong winds and intense solar radiation, key factors affecting an expedition’s success. All these climatic elements stress human heat balance and survival. We assess components of human heat balance while climbing Mt. Everest.

Materials and Methods: We calculated climbers’ heat balance using the Man-ENvironment heat EXchange model (MENEX-2005) and derived meteorological data from the National Geographic Expedition’s in situ dataset. Three weather stations sited between 3810 and 7945 m a.s.l. provided data with hourly resolution. We used data for summer (1 May–15 August 2019) and winter (16 October 2019–6 January 2020) seasons to analyze heat balance elements of convection, evaporation, respiration and radiation (solar and thermal).

Results: Meteorological and other factors affecting physiology—such as clothing insulation of 3.5–5.5 clo and activity levels of 3–5 MET—regulate human heat balance. Elevation above sea level is the main element affecting heat balance. In summer two to three times more solar radiation can be absorbed at the summit of the mountain than at the foot. Low air pressure reduces air density, which reduces convective heat loss at high altitude by up to half of the loss at lower locations with the same wind speed and air temperature.

Conclusion: 1. Alpinists face little risk of overheating or overcooling while actively climbing Mt. Everest, despite the potential risk of overcooling at extreme altitudes on Mt. Everest in winter. 2. Convection and evaporation are responsible for most of the heat lost at altitude. 3. Levels of physical activity and clothing insulation play the greatest role in counteracting heat loss at high altitude.

A Breathtaking Lift: Sex and Body Mass Index Differences in Cardiopulmonary Response in a Large Cohort of Unselected Subjects with Acute Exposure to High Altitude

Vignati, Carlo, Massimo Mapelli, Benedetta Nusca, Alice Bonomi, Elisabetta Salvioni, Irene Mattavelli, Susanna Sciomer, Andrea Faini, Gianfranco Parati, and Piergiuseppe Agostoni. A breathtaking lift: sex and body mass index differences in cardiopulmonary response in a large cohort of unselected subjects with acute exposure to high altitude. High Alt Med Biol 00:000-000, 2021. Background: Every year, thousands of people travel to high altitude and experience hypoxemia. At high altitude, the partial pressure of oxygen decreases. The aim of this observational study was to determine if there is a relationship between anthropometric features and basic cardiorespiratory variables, including oxygen saturation (SpO₂), heart rate (HR), and blood pressure (BP), following acute exposure to high altitude. Materials and Methods: At the 3,466 m top of a cableway station, we installed an automated system for measuring peripheral SpO₂, HR, BP, height, weight, and body mass index (BMI). Results: Between January and October 2020, out of 4,874 volunteers (age 39.9 ± 15.4 years, male 54.4%), 3,267 provided complete data (1,808 cases during winter and 1,459 during summer). SpO₂ was 86.8% ± 6.8%. At multivariable analysis, SpO₂ was significantly associated with age, sex, season, BMI, and HR but not with BP. We identified 391 (12%) subjects with SpO₂ ≤80%: they were older, with a higher BMI and HR but without sex or BP differences. Finally, winter season was associated with greater frequency of SpO₂ ≤80% (13.3% vs. 10.3%, p = 0.008). Conclusion: Our data show that high BMI, older age, and male sex were associated with greater degrees of hypoxemia following exposure to high altitude, particularly during the winter.

Death Zone Weather Extremes Mountaineers Have Experienced in Successful Ascents

Background

Few data are available on mountaineers’ survival prospects in extreme weather above 8000 m (the Death Zone). We aimed to assess Death Zone weather extremes experienced in climbing-season ascents of Everest and K2, all winter ascents of 8000 m peaks (8K) in the Himalayas and Karakoram, environmental records of human survival, and weather extremes experienced with and without oxygen support.

Materials and Methods

We analyzed 528 ascents of 8K peaks: 423 non-winter ascents without supplemental oxygen (Everest–210, K2–213), 76 ascents in winter without oxygen, and 29 in winter with oxygen. We assessed environmental conditions using the ERA5 dataset (1978–2021): barometric pressure (BP), temperature (Temp), wind speed (Wind), wind chill equivalent temperature (WCT), and facial frostbite time (FFT).

Results

The most extreme conditions that climbers have experienced with and without supplemental oxygen were: BP 320 hPa (winter Everest) vs. 329 hPa (non-winter Everest); Temp –41°C (winter Everest) vs. –45°C (winter Nanga Parbat); Wind 46 m⋅s^–1 (winter Everest) vs. 48 m⋅s^–1 (winter Kangchenjunga). The most extreme combined conditions of BP ≤ 333 hPa, Temp ≤ −30°C, Wind ≥ 25 m⋅s^–1, WCT ≤ −54°C and FFT ≤ 3 min were encountered in 14 ascents of Everest, two without oxygen (late autumn and winter) and 12 oxygen-supported in winter. The average extreme conditions experienced in ascents with and without oxygen were: BP 326 ± 3 hPa (winter Everest) vs. 335 ± 2 hPa (non-winter Everest); Temp −40 ± 0°C (winter K2) vs. −38 ± 5°C (winter low Karakoram 8K peaks); Wind 36 ± 7 m⋅s^–1 (winter Everest) vs. 41 ± 9 m⋅s^–1 (winter high Himalayan 8K peaks).

Conclusions

Comparison of Environmental Conditions on Summits of Mount Everest and K2 in Climbing and Midwinter Seasons

(1) Background: Today’s elite alpinists target K2 and Everest in midwinter. This study aimed to asses and compare weather at the summits of both peaks in the climbing season (Everest, May; K2, July) and the midwinter season (January and February). (2) Methods: We assessed environmental conditions using the ERA5 dataset (1979–2019). Analyses examined barometric pressure (BP), temperature (Temp), wind speed (Wind), perceived altitude (Alt), maximal oxygen uptake (VO₂max), vertical climbing speed (Speed), wind chill equivalent temperature (WCT), and facial frostbite time (FFT). (3) Results: Most climbing-season parameters were found to be more severe (p < 0.05) on Everest than on K2: BP (333 ± 1 vs. 347 ± 1 hPa), Alt (8925 ± 20 vs. 8640 ± 20 m), VO₂max (16.2 ± 0.1 vs. 17.8 ± 0.1 ml·kg⁻¹·min⁻¹), Speed (190 ± 2 vs. 223 ± 2 m·h⁻¹), Temp (−26 ± 1 vs. −21 ± 1°C), WCT (−45 ± 2 vs. −37 ± 2 °C), and FFT (6 ± 1 vs. 11 ± 2 min). Wind was found to be similar (16 ± 3 vs. 15 ± 3 m·s⁻¹). Most midwinter parameters were found to be worse (p < 0.05) on Everest vs. K2: BP (324 ± 2 vs. 326 ± 2 hPa), Alt (9134 ± 40 vs. 9095 ± 48 m), VO₂max (15.1 ± 0.2 vs. 15.3 ± 0.3 ml·kg⁻¹·min⁻¹), Speed (165 ± 5 vs. 170 ± 6 m·h⁻¹), Wind (41 ± 6 vs. 27 ± 4 m·s⁻¹), and FFT (<1 min vs. 1 min). Everest’s Temp of −36 ± 2 °C and WCT −66 ± 3 °C were found to be less extreme than K2’s Temp of −45 ± 1 °C and WCT −76 ± 2 °C. (4) Conclusions: Everest presents more extreme conditions in the climbing and midwinter seasons than K2. K2’s 8° higher latitude makes its midwinter BP similar and Temp lower than Everest’s. K2’s midwinter conditions are more severe than Everest’s in the climbing season.

Into Thick(er) Air? Oxygen Availability at Humans' Physiological Frontier on Mount Everest

Global audiences are captivated by climbers pushing themselves to the limits in the hypoxic environment of Mount Everest. However, air pressure sets oxygen abundance, meaning it varies with the weather and climate warming. This presents safety issues for mountaineers but also an opportunity for public engagement around climate change. Here we blend new observations from Everest with ERA5 reanalysis (1979-2019) and climate model results to address both perspectives. We find that plausible warming could generate subtle but physiologically relevant changes in summit oxygen availability, including an almost 5% increase in annual minimum VO₂ max for 2°C warming since pre-industrial. In the current climate we find evidence of swings in pressure sufficient to change Everest's apparent elevation by almost 750 m. Winter pressures can also plunge lower than previously reported, highlighting the importance of air pressure forecasts for the safety of those trying to push the physiological frontier on Mt. Everest.

Comparative morphometric analysis of lungs of the semifossorial giant pouched rat (Cricetomys gambianus) and the subterranean Nigerian mole rat (Cryptomys foxi)

Lungs of the rodent species, the African giant pouched rat (Cricetomys gambianus) and the Nigerian mole rat (Cryptomys foxi) were investigated. Significant morphometric differences exist between the two species. The volume of the lung per unit body mass was 2.7 times larger; the respiratory surface area 3.4 times greater; the volume of the pulmonary capillary blood 2 times more; the harmonic mean thickness of the blood-gas (tissue) barrier (τht) ~29% thinner and; the total pulmonary morphometric diffusing capacity (DLo2) for O2 2.3 times more in C. foxi. C. gambianus occupies open burrows that are ventilated with air while C. foxi lives in closed burrows. The less morphometrically specialized lungs of C. gambianus may be attributed to its much larger body mass (~6 times more) and possibly lower metabolic rate and its semifossorial life whereas the 'superior' lungs of C. foxi may largely be ascribed to the subterranean hypoxic and hypercapnic environment it occupies. Compared to other rodents species that have been investigated hitherto, the τht was mostly smaller in the lungs of the subterranean species and C. foxi has the highest mass-specific DLo2. The fossorial- and the subterranean rodents have acquired various pulmonary structural specializations that relate to habitats occupied.

Enzymatic Analysis of the Gingival Crevicular Fluid in Hypoxia of High Altitude (Everest)

The aim of this study is to determine the qualitative and quantitative changes of alkaline phosphatase (ALP) that occur in the Gingival Crevicular Fluid (GCF) in hypobaric-hypoxic conditions (high altitude). Hypoxia affects systemic adaptation responses in different organs. We examined 17 Caucasians subjects, of whom 13 were mountain climbers (1 female and 12 males), and 4 Tibetans (2 females and 2 males) following exposure to the hypoxia environment of high altitude. The study was conducted at different altitudes (0 m control, 1000 m, 5200 m above sea level) on Mount Everest. The protocol consisted of withdrawing crevicular fluid through the use of cones made of endodontic paper size 30 sectioned to 15 mm from the apex, inserted for 30 seconds in the gingival sulcus (about 2 mm). The analyzed sites were the mesial and distal, buccal and palatal of tooth 1.1 and 2.1. Blood exams were performed on the subjects using I-Stat, furnishing analysis in real time (about 2 mins). In agreement with other results reported in literature, in all the subjects we found an increase in the hematocrit and hemoglobin with a large range of values between them, and with significant differences, as analysed with the Fisher, Scheffe and Bonferroni/Dunn statistical methods. The enzymatic analysis of the GFC showed an increase of the levels of ALP at each altitude studied. With this preliminary study we show that hypoxic environment determines not only the well known cardiovascular systemic responses, but also crevicular fluid adaptation.

A Waist-Worn Inertial Measurement Unit for Long-Term Monitoring of Parkinson's Disease Patients

Inertial measurement units (IMUs) are devices used, among other fields, in health applications, since they are light, small and effective. More concretely, IMUs have been demonstrated to be useful in the monitoring of motor symptoms of Parkinson’s disease (PD). In this sense, most of previous works have attempted to assess PD symptoms in controlled environments or short tests. This paper presents the design of an IMU, called 9 × 3, that aims to assess PD symptoms, enabling the possibility to perform a map of patients’ symptoms at their homes during long periods. The device is able to acquire and store raw inertial data for artificial intelligence algorithmic training purposes. Furthermore, the presented IMU enables the real-time execution of the developed and embedded learning models. Results show the great flexibility of the 9 × 3, storing inertial information and algorithm outputs, sending messages to external devices and being able to detect freezing of gait and bradykinetic gait. Results obtained in 12 patients exhibit a sensitivity and specificity over 80%. Additionally, the system enables working 23 days (at waking hours) with a 1200 mAh battery and a sampling rate of 50 Hz, opening up the possibility to be used for other applications like wellbeing and sports.

Feasibility of pulse oximetry screening for critical congenital heart disease at 2643-foot elevation

To evaluate the feasibility of implementing a pulse oximetry screening protocol at a city of mild elevation with a specific focus on the false-positive screening rate. Pulse oximetry screening was performed according to the proposed guidelines endorsed by the American Academy of Pediatrics at a center in Tucson, AZ, at an elevation of 2,643 ft (806 m). During a 10-month period in 2012, 1069 full-term asymptomatic newborns were screened ≥ 24 h after birth. The mean preductal oxygen saturation was 98.5 ± 1.3 % (range 92-100 %), and the mean postductal oxygen saturation was 98.6 ± 1.3 % (range 94-100 %). Of 1,069 patients screened, 7 were excluded secondary to protocol violations, and 1 screened positive. An echocardiogram was performed on the newborn with the positive screen, and it was normal with the exception of right-to-left shunting across a patent foramen ovale. The false-positive rate was 1/1,062 or 0.094 %. The pulse oximetry screening guidelines recommended by the American Academy of Pediatrics are feasible at an elevation of 2,643 ft (806 m) with a low false-positive rate. Adjustments to the protocol are not required for centers at elevations ≤ 2,643 ft. Future studies at greater elevations are warranted.

Clinical consequences of altered chemoreflex control

Control of ventilation dictates various breathing patterns. The respiratory control system consists of a central pattern generator and several feedback mechanisms that act to maintain ventilation at optimal levels. The concept of loop gain has been employed to describe its stability and variability. Synthesizing all interactions under a general model that could account for every behavior has been challenging. Recent insight into the importance of these feedback systems may unveil therapeutic strategies for common ventilatory disturbances. In this review we will address the major mechanisms that have been proposed as mediators of some of the breathing patterns in health and disease that have raised controversies and discussion on ventilatory control over the years.

Experimental physiology, Everest and oxygen: from the ghastly kitchens to the gasping lung

Often the truth value of a scientific claim is dependent on our faith that laboratory experiments can model nature. When the nature that you are modelling is something as large as the tallest terrestrial mountain on earth, and as mysterious (at least until 1953) as the reaction of the human body to the highest point on the earth's surface, mapping between laboratory and ‘real world’ is a tricky process. The so-called ‘death zone’ of Mount Everest is a liminal space; a change in weather could make the difference between a survivable mountaintop and a site where the human respiratory system cannot maintain basic biological functions. Predicting what would happen to the first human beings to climb that high was therefore literally a matter of life or death – here inaccurate models could kill. Consequently, high-altitude respiratory physiology has prioritized not the laboratory, but the field. A holistic, environmentally situated sort of science used a range of (often non-scientific) expertise to prove the laboratory wrong time after time. In so doing, Everest was constructed paradoxically both as a unique field site which needed to be studied in vivo, and as a ‘natural laboratory’ which could produce generalizable knowledge about the human (male) body.

Career perspective: John B West

I have been fortunate to work in two areas of extreme physiology and medicine: very high altitude and the microgravity of spaceflight. My introduction to high altitude medicine was as a member of Sir Edmund Hillary's Silver Hut Expedition in 1960-1961 when a small group of physiologists spent the winter and spring at an altitude of 5,800 m just south of Mt. Everest. The physiological objective was to obtain a better understanding of the acclimatization process of lowlanders during exposure to a very high altitude for several months. As far as we knew, no one had ever spent so long at such a high altitude before. The success of this expedition prompted me to organize the 1981 American Medical Research Expedition to Everest where the scientific objective was to determine the physiological changes that allow humans to survive in the extreme hypoxia of the highest point on earth. There is good evidence that this altitude is very near the limit of human tolerance to oxygen deprivation. Much novel information was obtained including an extraordinary degree of hyperventilation which reduced the alveolar partial pressure of carbon dioxide (Pco2) to about 8 mmHg (1.1 kPa) on the summit, and this in turn allowed the alveolar partial pressure of oxygen, PO2, to be maintained at a viable level of about 35 mmHg (4.7 kPa). The low Pco2 caused a severe degree of respiratory alkalosis with an arterial pH exceeding 7.7. These were the first physiological measurements to be made on the Everest summit, and essentially, none has been made since. The second extreme environment is microgravity. We carried out an extensive series of measurements on astronauts in the orbiting laboratory known as SpaceLab in the 1990s. Many aspects of pulmonary function are affected by gravity, so it was not surprising that many changes were found. However, overall gas exchange remained efficient. Some of the findings such as an anomalous behavior of inhaled helium and sulfur hexafluoride have still not been explained. Measurements made after astronauts were exposed to 6 months of microgravity in the International Space Station indicate that the function of the lung returns to its preexposure state within a few days.

The paradox of extreme high-altitude migration in bar-headed geese Anser indicus

Bar-headed geese are renowned for migratory flights at extremely high altitudes over the world's tallest mountains, the Himalayas, where partial pressure of oxygen is dramatically reduced while flight costs, in terms of rate of oxygen consumption, are greatly increased. Such a mismatch is paradoxical, and it is not clear why geese might fly higher than is absolutely necessary. In addition, direct empirical measurements of high-altitude flight are lacking. We test whether migrating bar-headed geese actually minimize flight altitude and make use of favourable winds to reduce flight costs. By tracking 91 geese, we show that these birds typically travel through the valleys of the Himalayas and not over the summits. We report maximum flight altitudes of 7290 m and 6540 m for southbound and northbound geese, respectively, but with 95 per cent of locations received from less than 5489 m. Geese travelled along a route that was 112 km longer than the great circle (shortest distance) route, with transit ground speeds suggesting that they rarely profited from tailwinds. Bar-headed geese from these eastern populations generally travel only as high as the terrain beneath them dictates and rarely in profitable winds. Nevertheless, their migration represents an enormous challenge in conditions where humans and other mammals are only able to operate at levels well below their sea-level maxima.

Environmental conditions at the South Col of Mount Everest and their impact on hypoxia and hypothermia experienced by mountaineers

Background

Hypoxia and hypothermia are acknowledged risk factors for those who venture into high-altitude regions. There is, however, little in situ data that can be used to quantify these risks. Here, we use 7 months of continuous meteorological data collected at the South Col of Mount Everest (elevation 7,896 m above sea level) to provide the first in situ characterization of these risks near the summit of Mount Everest.

Methods

This is accomplished through the analysis of barometric pressure, temperature and wind speed data collected by an automatic weather station installed at the South Col. These data were also used as inputs to parameterizations of wind chill equivalent temperature (WCT) and facial frostbite time (FFT).

Results

The meteorological data show clear evidence of seasonality, with evidence of pre-monsoon, monsoon and post-monsoon conditions. Low pressures, cold temperatures and high wind speeds characterize the pre- and post-monsoon periods with significant variability associated with the passage of large-scale weather systems. In contrast, the monsoon period is characterized by higher pressures, warmer temperatures and lower wind speeds with a pronounced reduction in variability. These environmental conditions are reflected in WCTs as low as −50°C and FFTs as short as 2 min during the pre- and post-monsoon periods. During the monsoon, the risk of cold injury is reduced with WCTs of order −20°C and FFTs longer than 60 min. The daily cycle in the various parameters is also investigated in order to assess the changes in conditions that would be experienced during a typical summit day. The post-monsoon period in particular shows a muted daily cycle in most parameters that is proposed to be the result of the random timing of large-scale weather systems.

Conclusions

Our results provide the first in situ characterization of the risk of hypoxia and hypothermia on Mount Everest on daily, weekly and seasonal timescales, and provide additional confirmation as to the extreme environment experienced by those attempting to summit Mount Everest and other high Himalayan mountains.

Pulse oximetry at high altitude

Pulse oximetry is a valuable, noninvasive, diagnostic tool for the evaluation of ill individuals at high altitude and is also being increasingly used to monitor the well-being of individuals traveling on high altitude expeditions. Although the devices are simple to use, data output may be inaccurate or hard to interpret in certain situations, which could lead to inappropriate clinical decisions. The purpose of this review is to consider such issues in greater detail. After examining the operating principles of pulse oximetry, we describe the available devices and the potential uses of oximetry at high altitude. We then consider the pitfalls of pulse oximetry in this environment and provide recommendations about how to deal with these issues. Device users should recognize that oxygen saturation changes rapidly in response to small changes in oxygen tensions at high altitude and that device accuracy declines with arterial oxygen saturations of less than 80%. The normal oxygen saturation at a given elevation may not be known with certainty and should be viewed as a range of values, rather than a specific number. For these reasons, clinical decisions should not be based on small differences in saturation over time or among individuals. Effort should also be made to minimize factors that cause measurement errors, including cold extremities, excess ambient light, and ill-fitting oximeter probes. Attention to these and other issues will help the users of these devices to apply them in appropriate situations and to minimize erroneous clinical decisions.

Acclimatization of skeletal muscle mitochondria to high-altitude hypoxia during an ascent of Everest

Ascent to high altitude is associated with a fall in the partial pressure of inspired oxygen (hypobaric hypoxia). For oxidative tissues such as skeletal muscle, resultant cellular hypoxia necessitates acclimatization to optimize energy metabolism and restrict oxidative stress, with changes in gene and protein expression that alter mitochondrial function. It is known that lowlanders returning from high altitude have decreased muscle mitochondrial densities, yet the underlying transcriptional mechanisms and time course are poorly understood. To explore these, we measured gene and protein expression plus ultrastructure in muscle biopsies of lowlanders at sea level and following exposure to hypobaric hypoxia. Subacute exposure (19 d after initiating ascent to Everest base camp, 5300 m) was not associated with mitochondrial loss. After 66 d at altitude and ascent beyond 6400 m, mitochondrial densities fell by 21%, with loss of 73% of subsarcolemmal mitochondria. Correspondingly, levels of the transcriptional coactivator PGC-1α fell by 35%, suggesting down-regulation of mitochondrial biogenesis. Sustained hypoxia also decreased expression of electron transport chain complexes I and IV and UCP3 levels. We suggest that during subacute hypoxia, mitochondria might be protected from oxidative stress. However, following sustained exposure, mitochondrial biogenesis is deactivated and uncoupling down-regulated, perhaps to improve the efficiency of ATP production.

Lung disease at high altitude

Large numbers of people travel to high altitudes, entering an environment of hypobaric hypoxia. Exposure to low oxygen tension leads to a series of important physiologic responses that allow individuals to tolerate these hypoxic conditions. However, in some cases hypoxia triggers maladaptive responses that lead to various forms of acute and chronic high altitude illness, such as high-altitude pulmonary edema or chronic mountain sickness. Because the respiratory system plays a critical role in these adaptive and maladaptive responses, patients with underlying lung disease may be at increased risk for complications in this environment and warrant careful evaluation before any planned sojourn to higher altitudes. In this review, we describe respiratory disorders that occur with both acute and chronic exposures to high altitudes. These disorders may occur in any individual who ascends to high altitude, regardless of his/her baseline pulmonary status. We then consider the safety of high-altitude travel in patients with various forms of underlying lung disease. The available data regarding how these patients fare in hypoxic conditions are reviewed, and recommendations are provided for management prior to and during the planned sojourn.

The trans-Himalayan flights of bar-headed geese (Anser indicus)

Birds that fly over mountain barriers must be capable of meeting the increased energetic cost of climbing in low-density air, even though less oxygen may be available to support their metabolism. This challenge is magnified by the reduction in maximum sustained climbing rates in large birds. Bar-headed geese (Anser indicus) make one of the highest and most iconic transmountain migrations in the world. We show that those populations of geese that winter at sea level in India are capable of passing over the Himalayas in 1 d, typically climbing between 4,000 and 6,000 m in 7-8 h. Surprisingly, these birds do not rely on the assistance of upslope tailwinds that usually occur during the day and can support minimum climb rates of 0.8-2.2 km·h(-1), even in the relative stillness of the night. They appear to strategically avoid higher speed winds during the afternoon, thus maximizing safety and control during flight. It would seem, therefore, that bar-headed geese are capable of sustained climbing flight over the passes of the Himalaya under their own aerobic power.

Operation Everest II

In October 1985, 25 years ago, 8 subjects and 27 investigators met at the United States Army Research Institute for Environmental Medicine (USARIEM) altitude chambers in Natick, Massachusetts, to study human responses to a simulated 40-day ascent of Mt. Everest, termed Operation Everest II (OE II). Led by Charlie Houston, John Sutton, and Allen Cymerman, these investigators conducted a large number of investigations across several organ systems as the subjects were gradually decompressed over 40 days to the Everest summit equivalent. There the subjects reached a V(O)(2)max of 15.3 mL/kg/min (28% of initial sea-level values) at 100 W and arterial P(O(2)) and P(CO(2)) of approximately 28 and approximately 10 mm Hg, respectively. Cardiac function resisted hypoxia, but the lungs could not: ventilation-perfusion inequality and O(2) diffusion limitation reduced arterial oxygenation considerably. Pulmonary vascular resistance was increased, was not reversible after short-term hyperoxia, but was reduced during exercise. Skeletal muscle atrophy occurred, but muscle structure and function were otherwise remarkably unaffected. Neurological deficits (cognition and memory) persisted after return to sea level, more so in those with high hypoxic ventilatory responsiveness, with motor function essentially spared. Nine percent body weight loss (despite an unrestricted diet) was mainly (67%) from muscle and exceeded the 2% predicted from energy intake-expenditure balance. Some immunological and lipid metabolic changes occurred, of uncertain mechanism or significance. OE II was unique in the diversity and complexity of studies carried out on a single, courageous cohort of subjects. These studies could never have been carried out in the field, and thus complement studies such as the American Medical Research Expedition to Everest (AMREE) that, although more limited in scope, serve as benchmarks and reality checks for chamber studies like OE II.

American medical research expedition to Everest

The primary objective of the American Medical Research Expedition to Everest was to obtain information on human physiology at the highest possible altitude, including the Everest summit. An important data point was the barometric pressure on the summit, because this determines the inspired P(O(2)). The first measurement ever taken was 253.0 mmHg. Because modeling studies had shown that extreme hyperventilation was essential to reach these great altitudes, 34 alveolar gas samples were collected above an altitude of 8000 m, including 4 on the summit. These showed that hyperventilation reduced the alveolar P(CO(2)) to between 7 and 8 mmHg in one climber. An important finding was that alveolar P(O(2)) was defended at a value of about 35 mmHg by the increasing hyperventilation as the climbers ascended higher. Venous blood samples collected on two summiters gave a mean base excess of -7.2 meq.L(-1). Using the alveolar P(CO(2)) value, this gave an arterial pH of over 7.7, indicating an extreme degree of respiratory alkalosis. While climbing at an altitude of 8300 m, one summiter showed a respiratory frequency of 86 breaths.min(-1) and tidal volume of 1.26 L, indicating very rapid shallow breathing. Maximal oxygen consumption for the summit was derived by having well-acclimatized subjects exercise maximally at an altitude of 6300 m while breathing 14% oxygen. The V(O(2)) was just over 1 L.min(-1), which is sufficient to explain how exceptional humans can reach the summit without supplementary oxygen. In addition to the measurements at altitudes over 8000 m, data were obtained at two camps at 5400- and 6300-m altitude. These gave information on the control of ventilation, periodic breathing, blood physiology, cerebral function, and metabolism.

Evaluating the safety of high-altitude travel in patients with adult congenital heart disease

As medical management and surgical techniques continue to improve, patients with congenital heart disease are surviving further into adulthood and seeking to participate in multiple activities. Given the increasing popularity of adventure recreation, it is likely that many of these individuals will express interest in travel to and activities at high altitude. At first glance, the hypoxia associated with acute altitude exposure would appear to pose high risks for patients with underlying cardiopulmonary disease, but few studies have systematically addressed these concerns in the adult congenital heart disease population. In this review, we consider the safety of high-altitude travel in these patients. After reviewing the primary cardiopulmonary responses to acute hypoxia and the risks of high altitude in all individuals regardless of their underlying health status, we consider the risks in adult congenital heart disease patients, in particular. We focus on broad concerns that should be considered in all patients such as whether they have underlying pulmonary hypertension, the adequacy of their ventilatory responses, and their ability to compensate for hypoxemia and right-to-left shunting. We then conclude by providing basic recommendations for pretravel assessment in patients with congenital heart disease of moderate or great complexity.

An analysis of performance in human locomotion

This paper reports an analysis of the principles underlying human performances on the basis of the work initiated by Pietro Enrico di Prampero. Starting from the concept that the maximal speed that can be attained over a given distance with a given locomotion mode is directly proportional to the maximal sustainable power and inversely proportional to the energy cost of locomotion, we discuss the maximal powers (and capacities) of anaerobic (lactic and alactic) and aerobic metabolisms and the factors that limit them, and the factors affecting the energy cost of various locomotion modes. Special attention is given to the role of air resistance and frictional forces. Finally, computation of performance speed is discussed along the approach originally developed by di Prampero.

Changes in sublingual microcirculatory flow index and vessel density on ascent to altitude

We hypothesized that ascent to altitude would result in reduced sublingual microcirculatory flow index (MFI) and increased vessel density. Twenty-four subjects were studied using sidestream dark-field imaging, as they ascended to 5300 m; one cohort remained at this altitude (n = 10), while another ascended higher (maximum 8848 m; n = 14). The MFI, vessel density and grid crossings (GX; an alternative density measure) were calculated. Total study length was 71 days; images were recorded at sea level (SL), Namche Bazaar (3500 m), Everest base camp (5300 m), the Western Cwm (6400 m), South Col (7950 m) and departure from Everest base camp (5300 m; 5300 m-b). Peripheral oxygen saturation (SpO2), heart rate and blood pressure were also recorded. Compared with SL, altitude resulted in reduced sublingual MFI in small (<25 microm; P < 0.0001) and medium vessels (26-50 microm; P = 0.006). The greatest reduction in MFI from SL was seen at 5300 m-b; from 2.8 to 2.5 in small vessels and from 2.9 to 2.4 in medium-sized vessels. The density of vessels <25 microm did not change during ascent, but those >25 microm rose from 1.68 (+/- 0.43) mm mm(-2) at SL to 2.27 (+/- 0.57) mm mm(-2) at 5300 m-b (P = 0.005); GX increased at all altitudes (P < 0.001). The reduction in MFI was greater in climbers than in those who remained at 5300 m in small and medium-sized vessels (P = 0.017 and P = 0.002, respectively). At 7950 m, administration of supplemental oxygen resulted in a further reduction of MFI and increase in vessel density. Thus, MFI was reduced whilst GX increased in the sublingual mucosa with prolonged exposure to hypoxia and was exaggerated in those exposed to extreme altitude.

Hypoxic cutaneous vasodilation is sustained during brief cold stress and is not affected by changes in CO2

Hypoxia decreases core body temperature in animals and humans during cold exposure. In addition, hypoxia increases skin blood flow in thermoneutral conditions, but the impact of hypoxic vasodilation on vasoconstriction during cold exposure is unknown. In this study, skin blood flow was assessed using laser-Doppler flowmetry, and cutaneous vascular conductance (CVC) was calculated as red blood cell flux/mean arterial pressure and normalized to baseline ( n = 7). Subjects were exposed to four different conditions in the steady state (normoxia and poikilocapnic, isocapnic, and hypercapnic hypoxia) and were cooled for 10 min using a water-perfused suit in each condition. CVC increased during all three hypoxic exposures (all P < 0.05 vs. baseline), and the magnitude of these steady-state responses was not affected by changes in end-tidal CO2 levels. During poikilocapnic and hypercapnic hypoxia, cold exposure reduced CVC to the same levels observed during normoxic cooling ( P > 0.05 vs. normoxia), whereas CVC remained elevated throughout cold exposure during isocapnic hypoxia ( P < 0.05 vs. normoxia). The magnitude of vasoconstriction during cold stress was similar in all conditions ( P > 0.05). Thus the magnitude of cutaneous vasodilation during steady-state hypoxia is not affected by CO2 responses. In addition, the magnitude of reflex vasoconstriction is not altered by hypoxia, such that the upward shift in skin blood flow (hypoxic vasodilation) is maintained during whole body cooling.

To what extent does terrestrial life "follow the water"?

Terrestrial life is known to require liquid water, but not all terrestrial water is inhabited. Thus, liquid water is a necessary, but not sufficient, condition for life. To quantify the terrestrial limits on the habitability of water and help identify the factors that make some terrestrial water uninhabited, we present empirical pressure-temperature (P-T) phase diagrams of water, Earth, and terrestrial life. Eighty-eight percent of the volume of Earth where liquid water exists is not known to host life. This potentially uninhabited terrestrial liquid water includes (i) hot and deep regions of Earth where some combination of high temperature (T > 122 degrees C) and restrictions on pore space, nutrients, and energy is the limiting factor and (ii) cold and near-surface regions of Earth, such as brine inclusions and thin films in ice and permafrost (depths less than approximately 1 km), where low temperatures (T < -40 degrees C), low water activity (a(w) < 0.6), or both are the limiting factors. If the known limits of terrestrial life do not change significantly, these limits represent important constraints on our biosphere and, potentially, on others, since approximately 4 billion years of evolution have not allowed life to adapt to a large fraction of the volume of Earth where liquid water exists.

Arterial blood gases and oxygen content in climbers on Mount Everest

BACKGROUND

The level of environmental hypobaric hypoxia that affects climbers at the summit of Mount Everest (8848 m [29,029 ft]) is close to the limit of tolerance by humans. We performed direct field measurements of arterial blood gases in climbers breathing ambient air on Mount Everest.

METHODS

We obtained samples of arterial blood from 10 climbers during their ascent to and descent from the summit of Mount Everest. The partial pressures of arterial oxygen (PaO(2)) and carbon dioxide (PaCO(2)), pH, and hemoglobin and lactate concentrations were measured. The arterial oxygen saturation (SaO(2)), bicarbonate concentration, base excess, and alveolar-arterial oxygen difference were calculated.

RESULTS

PaO(2) fell with increasing altitude, whereas SaO(2) was relatively stable. The hemoglobin concentration increased such that the oxygen content of arterial blood was maintained at or above sea-level values until the climbers reached an elevation of 7100 m (23,294 ft). In four samples taken at 8400 m (27,559 ft)--at which altitude the barometric pressure was 272 mm Hg (36.3 kPa)--the mean PaO(2) in subjects breathing ambient air was 24.6 mm Hg (3.28 kPa), with a range of 19.1 to 29.5 mm Hg (2.55 to 3.93 kPa). The mean PaCO(2) was 13.3 mm Hg (1.77 kPa), with a range of 10.3 to 15.7 mm Hg (1.37 to 2.09 kPa). At 8400 m, the mean arterial oxygen content was 26% lower than it was at 7100 m (145.8 ml per liter as compared with 197.1 ml per liter). The mean calculated alveolar-arterial oxygen difference was 5.4 mm Hg (0.72 kPa).

CONCLUSIONS

The elevated alveolar-arterial oxygen difference that is seen in subjects who are in conditions of extreme hypoxia may represent a degree of subclinical high-altitude pulmonary edema or a functional limitation in pulmonary diffusion.

The influence of barometric pressure changes and standard meteorological variables on the occurrence and clinical features of subarachnoid hemorrhage

BACKGROUND

The purpose of this study was to examine a possible association between standard meteorological variables and their changes and the occurrence and clinical features of SAH.

METHODS

Univariate association between the clinical/radiographic variables of patients with SAH and standard meteorological variables was evaluated. Next, a multivariate analysis was performed to find independent meteorological predictors for the occurrence of SAH by using a binary logistic regression analysis.

RESULTS

Univariate analysis showed significant differences between bleeding days and non-bleeding days for the number of change days (maximal atmospheric difference of the day >10 hPa) (P < .001); for the maximal relative humidity (P < .05); for the maximal difference of vapor pressure of the day 24 hours before the bleeding day (P < .006) and between cluster days and noncluster days for the number of change days (P < .001); for the maximal difference of temperature of the day (P < .035); and for the maximal, minimal, and mean relative humidity (P < .027, P < .018, and P < .03, respectively). In the multivariate models, the variable "change day" (OR, 3.7; 95% CI, 1.2-11.3) and direction of the atmospheric pressure difference of the day (OR, 2.6; 95% CI, 1.8-7.8) were retained as independent predictors for the occurrence of SAH. For the variable cluster day as dependent variable, only change day was maintained in the model (OR, 6.9; 95% CI, 4.7-10.8).

CONCLUSIONS

Atmospheric pressure changes of more than 10 hPa within 24 hours are an independent predictor of clustering of patients with SAH. Hypertension is an independent risk factor for the occurrence of SAH at change day.

Altitude medicine and physiology including heat and cold: A review

With increasing numbers of people travelling to high altitude destinations for recreation or work, there is a need for practitioners of Travel Medicine to be familiar with altitude illnesses and the physiology of altitude. In mountainous areas travellers may also be exposed to problems of heat and cold. This article reviews these topics and gives practical advice on the management of the clinical problems involved, together with a discussion of underlying mechanisms, as far as they are understood at present.

Griffith Pugh, pioneer Everest physiologist

Lewis Griffith Cresswell Evans Pugh (1909-1994), best known as the physiologist on the successful 1953 British Everest Expedition, inspired a generation of scientists in the field of altitude medicine and physiology in the decades after World War II. This paper details his early life, his introduction to exercise physiology during the war, and his crucially important work in preparation for the Everest expedition on Cho Oyu in 1952. Pugh's other great contribution to altitude physiology was as scientific leader of the 1960-1961 Himalayan Scientific and Mountaineering Expedition (the Silver Hut), and the origins and results of this important expedition are discussed. He had a major and continuing interest in the physiology of cold, especially in real-life situations in Antarctica, exposure to cold wet conditions on hills in Britain, and in long distance swimming. He also extended his interest to Olympic athletes at moderate altitude (Mexico City) and to heat stress in athletes. Pugh's strength as a physiologist was his readiness to move from laboratory to fieldwork with ease and his rigor in applying the highest standards in both situations. He led by example in both his willingness to act as a subject for experiments and in his attention to detail. He was not an establishment figure; he was critical of authority and well known for his eccentricity, but he inspired great loyalty in those who worked with him.

Sleep and Breathing at High Altitude

Sleep at high altitude is characterized by poor subjective quality, increased awakenings, frequent brief arousals, marked nocturnal hypoxemia, and periodic breathing. A change in sleep architecture with an increase in light sleep and decreasing slow-wave and REM sleep have been demonstrated. Periodic breathing with central apnea is almost universally seen amongst sojourners to high altitude, although it is far less common in long-standing high altitude dwellers. Hypobaric hypoxia in concert with periodic breathing appears to be the principal cause of sleep disruption at altitude. Increased sleep fragmentation accounts for the poor sleep quality and may account for some of the worsened daytime performance at high altitude. Hypoxic sleep disruption contributes to the symptoms of acute mountain sickness. Hypoxemia at high altitude is most severe during sleep. Acetazolamide improves sleep, AMS symptoms, and hypoxemia at high altitude. Low doses of a short acting benzodiazepine (temazepam) may also be useful in improving sleep in high altitude.

Physiological profile of middle-aged and older climbers who ascended Gasherbrum II, an 8035-m Himalayan peak

BACKGROUND

The purpose of this study was to investigate the physiological characteristics of a group of middle-aged and older Japanese climbers who ascended Gasherbrum II, an 8035-m peak in the Karakoram Range of the Himalayas.

METHODS

Body composition, cardiac structure, and respiratory gas exchange during exercise were estimated in eight climbers with differing levels of experience (seven men and one woman, aged 54 to 63 years) 6 months before their expedition.

RESULTS

Using supplementary O2, the four experienced climbers ascended beyond Camp 4 (7400 m) without showing any health problems and were able to attempt the summit. In contrast, the others, who had minimal experience at extreme altitude, suffered from altitude sickness on the way to Camp 4. Body mass index values were relatively high, but their low percentage of body fat (14.9%-21.4%) was indicative of the climbers' substantial lean body weight. Cardiac structures were generally normal, although three experienced male climbers had borderline hypertension and eccentric hypertrophy of the left ventricle. Peak VO2 ranged from 30.9 to 45.6 ml/kg/min, and no significant relationship between fitness level and the success or failure of the ascent was evident.

CONCLUSIONS

Even sexagenarians are capable of safely climbing 8000-m peaks with supplementary O2. An exceptionally high fitness level, as is seen in elite older athletes, does not appear to be required. What is essential, however, is moderate fitness, good health, and extensive experience.