High-altitude physiology and pathophysiology: implications and relevance for intensive care medicine

Acute environmental hypoxia induces LC3 lipidation in a genotype-dependent manner

Hypoxia-induced muscle wasting is a phenomenon often described with prolonged stays at high altitude, which has been attributed to altered protein metabolism. We hypothesized that acute normobaric hypoxia would induce a negative net protein balance by repressing anabolic and activating proteolytic signaling pathways at rest and postexercise and that those changes could be partially genetically determined. Eleven monozygotic twins participated in an experimental trial in normoxia and hypoxia (10.7% O2). Muscle biopsy samples were obtained before and after a 20-min moderate cycling exercise. In hypoxia at rest, autophagic flux was increased, as indicated by an increased microtubule-associated protein 1 light chain 3 type II/I (LC3-II/I) ratio (+25%) and LC3-II expression (+60%) and decreased p62/SQSTM1 expression (-25%; P<0.05), whereas exercise reversed those changes to a level similar to that with normoxia except for p62/SQSTM1, which was further decreased (P<0.05). Hypoxia also increased Bnip3 (+34%) and MAFbx (+18%) mRNA levels as well as REDD1 expression (+439%) and AMP-activated protein kinase phosphorylation (+22%; P<0.05). Among the molecular responses to hypoxia and/or exercise, high monozygotic similarity was found for REDD1, LC3-II, and LC3-II/I (P<0.05). Our results indicate that environmental hypoxia modulates protein metabolism at rest and after moderate exercise by primarily increasing markers of protein breakdown and, more specifically, markers of the autophagy-lysosomal system, with a modest genetic contribution.

Xtreme Everest 2: unlocking the secrets of the Sherpa phenotype?

Xtreme Everest 2 (XE2) was part of an ongoing programme of field, laboratory and clinical research focused on human responses to hypoxaemia that was conducted by the Caudwell Xtreme Everest Hypoxia Research Consortium. The aim of XE2 was to characterise acclimatisation to environmental hypoxia during a standardised ascent to high altitude in order to identify biomarkers of adaptation and maladaptation. Ultimately, this may lead to novel diagnostic and treatment strategies for the pathophysiological hypoxaemia and cellular hypoxia observed in critically ill patients. XE2 was unique in comparing participants drawn from two distinct populations: native ancestral high-altitude dwellers (Sherpas) and native lowlanders. Experiments to study the microcirculation, mitochondrial function and the effect that nitric oxide metabolism may exert upon them were focal to the scientific profile. In addition, the genetic and epigenetic (methylation and histone modification) basis of observed differences in phenotype was explored. The biological samples and phenotypic metadata already collected during XE2 will be analysed as an independent study. Data generated will also contribute to (and be compared with) the bioresource obtained from our previous observational high-altitude study, Caudwell Xtreme Everest (2007).

Design and conduct of 'Xtreme Alps': a double-blind, randomised controlled study of the effects of dietary nitrate supplementation on acclimatisation to high altitude

The study of healthy human volunteers ascending to high altitude provides a robust model of the complex physiological interplay that emulates human adaptation to hypoxaemia in clinical conditions. Nitric oxide (NO) metabolism may play an important role in both adaptation to high altitude and response to hypoxaemia during critical illness at sea level. Circulating nitrate and nitrite concentrations can be augmented by dietary supplementation and this is associated with improved exercise performance and mitochondrial efficiency. We hypothesised that the administration of a dietary substance (beetroot juice) rich in nitrate would improve oxygen efficiency during exercise at high altitude by enhancing tissue microcirculatory blood flow and oxygenation. Furthermore, nitrate supplementation would lead to measurable increases in NO bioactivity throughout the body. This methodological manuscript describes the design and conduct of the 'Xtreme Alps' expedition, a double-blind randomised controlled trial investigating the effects of dietary nitrate supplementation on acclimatisation to hypobaric hypoxia at high altitude in healthy human volunteers. The primary outcome measure was the change in oxygen efficiency during exercise at high altitude between participants allocated to receive nitrate supplementation and those receiving a placebo. A number of secondary measures were recorded, including exercise capacity, peripheral and microcirculatory blood flow and tissue oxygenation. Results from this study will further elucidate the role of NO in adaption to hypoxaemia and guide clinical trials in critically ill patients. Improved understanding of hypoxaemia in critical illness may provide new therapeutic avenues for interventions that will improve survival in critically ill patients.

Effect of acute environmental hypoxia on protein metabolism in human skeletal muscle

Hypoxia-induced muscle wasting has been observed in several environmental and pathological conditions. However, the molecular mechanisms behind this loss of muscle mass are far from being completely elucidated, certainly in vivo. When studying the regulation of muscle mass by environmental hypoxia, many confounding factors have to be taken into account, such as decreased protein ingestion, sleep deprivation or reduced physical activity, which make difficult to know whether hypoxia per se causes a reduction in muscle mass.

AIM

We hypothesized that acute exposure to normobaric hypoxia (11% O2 ) would repress the activation of the mTOR pathway usually observed after a meal and would activate the proteolytic pathways in skeletal muscle.

METHODS

Fifteen subjects were exposed passively for 4 h to normoxic and hypoxic conditions in a random order after consumption of a light breakfast. A muscle biopsy and a blood sample were taken before, after 1 and 4 h of exposure.

RESULTS

After 4 h, plasma insulin concentration and the phosphorylation state of PKB and S6K1 in skeletal muscle were higher in hypoxia than in normoxia (P < 0.05). At the same time, Redd1 mRNA level was upregulated (P < 0.05), whilst MAFbx mRNA decreased (P < 0.05) in hypoxia compared with normoxia. Proteasome, cathepsin L and calpain activities were not altered by environmental hypoxia.

CONCLUSION

Contrary to our hypothesis and despite an increase in the mRNA level of Redd1, an inhibitor of the mTORC1 pathway, short-term acute environmental hypoxia induced a higher response of PKB and S6K1 to a meal, which may be due to increased plasma insulin concentration.

Everest 60 years on: what next?

On 29 May 1953, Sherpa Tenzing Norgay and Edmund Hilary stood on the 8,848 m (29,029 ft) summit of Mount Everest, finally demonstrating that humans could overcome the physical and mental challenges required to conquer the world's highest peak. The 60th anniversary of this event is sadly the first with no member of the original expedition alive, since the death of George Lowe on 20 March 2013 at the age of 89 The successful 1953 expedition followed seven British expeditions to the north side of Everest during the 1920s and 30s. Although unsuccessful, these early expeditions achieved impressive altitudes. On several occasions, climbers exceeded 8,000 m (26,246 ft) both with supplemental oxygen (1922, 8,320 m/27,300 ft) and without (1924, 8,570 m/28120 ft; 1938, 8,230 m/27,000 ft).

Oxygen therapy in critical illness: precise control of arterial oxygenation and permissive hypoxemia

OBJECTIVE

The management of hypoxemia in critically ill patients is challenging. Whilst the harms of tissue hypoxia are well recognized, the possibility of harm from excess oxygen administration, or other interventions targeted at mitigating hypoxemia, may be inadequately appreciated. The benefits of attempting to fully reverse arterial hypoxemia may be outweighed by the harms associated with high concentrations of supplemental oxygen and invasive mechanical ventilation strategies. We propose two novel related strategies for the management of hypoxemia in critically ill patients. First, we describe precise control of arterial oxygenation involving the specific targeting of arterial partial pressure of oxygen or arterial hemoglobin oxygen saturation to individualized target values, with the avoidance of significant variation from these levels. The aim of precise control of arterial oxygenation is to avoid the harms associated with inadvertent hyperoxia or hypoxia through careful and precise control of arterial oxygen levels. Secondly, we describe permissive hypoxemia: the acceptance of levels of arterial oxygenation lower than is conventionally tolerated in patients. The aim of permissive hypoxemia is to minimize the possible harms caused by restoration of normoxemia while avoiding tissue hypoxia. This review sets out to discuss the strengths and limitations of precise control of arterial oxygenation and permissive hypoxemia as candidate management strategies in hypoxemic critically ill patients.

DESIGN

We searched PubMed for references to "permissive hypoxemia/hypoxaemia" and "precise control of arterial oxygenation" as well as reference to "profound hypoxemia/hypoxaemia/hypoxia," "severe hypoxemia/hypoxaemia/hypoxia." We searched personal reference libraries in the areas of critical illness and high altitude physiology and medicine. We also identified large clinical studies in patients with critical illness characterized by hypoxemia such as acute respiratory distress syndrome.

SUBJECTS

Studies were selected that explored the physiology of hypoxemia in healthy volunteers or critically ill patients.

SETTING

The data were subjectively assessed and combined to generate the narrative.

RESULTS

Inadequate tissue oxygenation and excessive oxygen administration can be detrimental to outcome but safety thresholds lack definition in critically ill patients. Precise control of arterial oxygenation provides a rational approach to the management of arterial oxygenation that reflects recent clinical developments in other settings. Permissive hypoxemia is a concept that is untested clinically and requires robust investigation prior to consideration of implementation. Both strategies will require accurate monitoring of oxygen administration and arterial oxygenation. Effective, reliable measurement of tissue oxygenation along with the use of selected biomarkers to identify suitable candidates and monitor harm will aid the development of permissive hypoxemia as viable clinical strategy.

CONCLUSIONS

Implementation of precise control of arterial oxygenation may avoid the harms associated with excessive and inadequate oxygenation. However, at present there is no direct evidence to support the immediate implementation of permissive hypoxemia and a comprehensive evaluation of its value in critically ill patients should be a high research priority.

Attenuating systemic inflammatory markers in simulated high-altitude exposure by heat shock protein 70-mediated hypobaric hypoxia preconditioning in rats

BACKGROUND/PURPOSE

The primary goal of this study was to test whether high-altitude exposure (HAE: 0.9% O(2) at 0.47 ATA for 24 hours) was capable of increasing the systemic inflammatory markers as well as the toxic organ injury indicators in rats, with a secondary goal to test whether preinduction of heat shock protein (HSP) 70 by hypobaric hypoxia preconditioning (HHP: 18.3% O(2) at 0.66 ATA for 5 h/day on 5 days consecutively for 2 weeks) attenuated the proposed increased serum levels of both the systemic inflammatory markers and the toxic organ injury indicators.

METHODS

Rats were assigned to: (1) non-HHP (21% O(2) at 1.0 ATA)+non-HAE (21% O(2) at 1.0 ATA) group; (2) non-HHP+HAE group; (3) HHP+non-HAE group; (4) HHP+HAE group; and (5) HHP+HSP70 antibodies (Ab)+HAE group. For the HSP70Ab group, a neutralizing HSP70Ab was injected intravenously at 24 hours prior to HAE. All the physiological and biochemical parameters were obtained at the end of HAE or the equivalent time period of non-HAE. Blood samples were obtained for determination of both the systemic inflammatory markers (e.g., serum tumor necrosis factor-α, interleukin-1β, E-selectin, intercellular adhesion molecule-1, and liver myeloperoxidase activity) and the toxic organ injury indicators (e.g., nitric oxide metabolites, 2,3-dihydroxybenzoic acid, and lactate dehydrogenase).

RESULTS

HHP, in addition to inducing overexpression of tissue HSP70, significantly attenuated the HAE-induced hypotension, bradycardia, hypoxia, acidosis, and increased tissue levels of both the systemic inflammatory markers and the toxic organ injury indicators. The beneficial effects of HHP in inducing tissue overexpression of HSP70 as well as in preventing the HAE-induced increased levels of the systemic inflammatory markers and the toxic organ injury indicators could be significantly reduced by HSP70Ab preconditioning.

CONCLUSION

These results suggest that HHP may downgrade both the systemic inflammatory markers and the toxic organ injury indicators in HAE by upregulating tissue HSP70.

Can Dietary Nitrate Supplements Improve Tolerance to Hypoxia?

This article is a review of the rationale and methodology of a translational study conducted at altitude investigating the potential role of nitrate supplements to improve tolerance to hypoxia, and a discussion of the applicability of the findings to intensive care medicine.

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.

The role of nitrogen oxides in human adaptation to hypoxia

Lowland residents adapt to the reduced oxygen availability at high altitude through a process known as acclimatisation, but the molecular changes underpinning these functional alterations are not well understood. Using an integrated biochemical/whole-body physiology approach we here show that plasma biomarkers of NO production (nitrite, nitrate) and activity (cGMP) are elevated on acclimatisation to high altitude while S-nitrosothiols are initially consumed, suggesting multiple nitrogen oxides contribute to improve hypoxia tolerance by enhancing NO availability. Unexpectedly, oxygen cost of exercise and mechanical efficiency remain unchanged with ascent while microvascular blood flow correlates inversely with nitrite. Our results suggest that NO is an integral part of the human physiological response to hypoxia. These findings may be of relevance not only to healthy subjects exposed to high altitude but also to patients in whom oxygen availability is limited through disease affecting the heart, lung or vasculature, and to the field of developmental biology.

Traumatic brain injury and aeromedical evacuation: when is the brain fit to fly?

BACKGROUND

To review the inflammatory sequelae of traumatic brain injury (TBI) and altitude exposure and discuss the potential impact of aeromedical evacuation (AE) on this process.

METHODS

Literature review and expert opinion regarding the inflammatory effects of TBI and AE.

RESULTS

Traumatic brain injury has been called the signature injury of the current military conflict. As a result of the increasing incidence of blast injury, TBI is responsible for significant mortality and enduring morbidity in injured soldiers. Common secondary insults resulting from post-traumatic cerebral inflammation are recognized to adversely impact outcome. AE utilizing Critical Care Air Transport Teams has become a standard of care practice following battlefield injury, to quickly and safely transport critically injured soldiers to more sophisticated echelons of care. Exposure to the hypobaric conditions of the AE process may impose an additional physiologic risk on the TBI patient as well as a "second hit" inflammatory stimulus.

CONCLUSIONS

We review the known inflammatory effects of TBI and altitude exposure and propose that optimizing the post-traumatic inflammatory profile may assist in determining an ideal time to fly for head-injured soldiers.

Caudwell xtreme Everest expedition

The Caudwell Xtreme Everest (CXE) expedition involved the detailed study of 222 subjects ascending to 5300 m or higher during the first half of 2007. Following baseline measurements at sea level, 198 trekker-subjects trekked to Everest Base Camp (EBC) following an identical ascent profile. An additional group of 24 investigator-subjects followed a similar ascent to EBC and remained there for the duration of the expedition, with a subgroup of 14 collecting data higher on Everest. This article focuses on published data obtained by the investigator-subjects at extreme altitude (>5500 m). Unique measurements of peak oxygen consumption, middle cerebral artery diameter and blood velocity, and microcirculatory blood flow were made on the South Col (7950 m). Unique arterial blood gas values were obtained from 4 subjects at 8400 m during descent from the summit of Everest. Arterial blood gas and microcirculatory blood flow data are discussed in detail.

Design and conduct of Caudwell Xtreme Everest: an observational cohort study of variation in human adaptation to progressive environmental hypoxia

BACKGROUND

The physiological responses to hypoxaemia and cellular hypoxia are poorly understood, and inter-individual differences in performance at altitude and outcome in critical illness remain unexplained. We propose a model for exploring adaptation to hypoxia in the critically ill: the study of healthy humans, progressively exposed to environmental hypobaric hypoxia (EHH). The aim of this study was to describe the spectrum of adaptive responses in humans exposed to graded EHH and identify factors (physiological and genetic) associated with inter-individual variation in these responses.

METHODS

DESIGN

Observational cohort study of progressive incremental exposure to EHH.

SETTING

University human physiology laboratory in London, UK (75 m) and 7 field laboratories in Nepal at 1300 m, 3500 m, 4250 m, 5300 m, 6400 m, 7950 m and 8400 m.

PARTICIPANTS

198 healthy volunteers and 24 investigators trekking to Everest Base Camp (EBC) (5300 m). A subgroup of 14 investigators studied at altitudes up to 8400 m on Everest.

MAIN OUTCOME MEASURES

Exercise capacity, exercise efficiency and economy, brain and muscle Near Infrared Spectroscopy, plasma biomarkers (including markers of inflammation), allele frequencies of known or suspected hypoxia responsive genes, spirometry, neurocognitive testing, retinal imaging, pupilometry. In nested subgroups: microcirculatory imaging, muscle biopsies with proteomic and transcriptomic tissue analysis, continuous cardiac output measurement, arterial blood gas measurement, trans-cranial Doppler, gastrointestinal tonometry, thromboelastography and ocular saccadometry.

RESULTS

Of 198 healthy volunteers leaving Kathmandu, 190 reached EBC (5300 m). All 24 investigators reached EBC. The completion rate for planned testing was more than 99% in the investigator group and more than 95% in the trekkers. Unique measurements were safely performed at extreme altitude, including the highest (altitude) field measurements of exercise capacity, cerebral blood flow velocity and microvascular blood flow at 7950 m and arterial blood gas measurement at 8400 m.

CONCLUSIONS

This study demonstrates the feasibility and safety of conducting a large healthy volunteer cohort study of human adaptation to hypoxia in this difficult environment. Systematic measurements of a large set of variables were achieved in 222 subjects and at altitudes up to 8400 m. The resulting dataset is a unique resource for the study of genotype:phenotype interactions in relation to hypoxic adaptation.

Concepts in hypoxia reborn

The human fetus develops in a profoundly hypoxic environment. Thus, the foundations of our physiology are built in the most hypoxic conditions that we are ever likely to experience: the womb. This magnitude of exposure to hypoxia in utero is rarely experienced in adult life, with few exceptions, including severe pathophysiology in critical illness and environmental hypobaric hypoxia at high altitude. Indeed, the lowest recorded levels of arterial oxygen in adult humans are similar to those of a fetus and were recorded just below the highest attainable elevation on the Earth's surface: the summit of Mount Everest. We propose that the hypoxic intrauterine environment exerts a profound effect on human tolerance to hypoxia. Cellular mechanisms that facilitate fetal well-being may be amenable to manipulation in adults to promote survival advantage in severe hypoxemic stress. Many of these mechanisms act to modify the process of oxygen consumption rather than oxygen delivery in order to maintain adequate tissue oxygenation. The successful activation of such processes may provide a new chapter in the clinical management of hypoxemia. Thus, strategies employed to endure the relative hypoxia in utero may provide insights for the management of severe hypoxemia in adult life and ventures to high altitude may yield clues to the means by which to investigate those strategies.

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.

Metabolic adaptation of skeletal muscle to high altitude hypoxia: how new technologies could resolve the controversies

In most tissues of the body, cellular ATP production predominantly occurs via mitochondrial oxidative phosphorylation of reduced intermediates, which are in turn derived from substrates such as glucose and fatty acids. In order to maintain ATP homeostasis, and therefore cellular function, the mitochondria require a constant supply of fuels and oxygen. In many disease states, or in healthy individuals at altitude, tissue oxygen levels fall and the cell must meet this hypoxic challenge to maintain energetics and limit oxidative stress. In humans at altitude and patients with respiratory disease, loss of skeletal muscle mitochondrial density is a consistent finding. Recent studies that have used cultured cells and genetic mouse models have elucidated a number of elegant adaptations that allow cells with a diminished mitochondrial population to function effectively in hypoxia. This article reviews these findings alongside studies of hypoxic human skeletal muscle, putting them into the context of whole-body physiology and acclimatization to high-altitude hypoxia. A number of current controversies are highlighted, which may eventually be resolved by a systems physiology approach that considers the time-or tissue-dependent nature of some adaptive responses. Future studies using high-throughput metabolomic, transcriptomic, and proteomic technologies to investigate hypoxic skeletal muscle in humans and animal models could resolve many of these controversies, and a case is therefore made for the integration of resulting data into computational models that account for factors such as duration and extent of hypoxic exposure, subjects' backgrounds, and whether data have been acquired from active or sedentary individuals. An integrated and more quantitative understanding of the body's metabolic response to hypoxia and the conditions under which adaptive processes occur could reveal much about the ways that tissues function in the very many disease states where hypoxia is a critical factor.

Changes in skeletal muscle oxygenation during exercise measured by near-infrared spectroscopy on ascent to altitude

INTRODUCTION

We sought to quantify changes in skeletal muscle oxygenation during exercise using near-infrared spectroscopy (NIRS) in healthy volunteers ascending to high altitude.

METHODS

Using NIRS, skeletal muscle tissue oxygen saturation (StO2) was measured in the vastus lateralis of 24 subjects. Measurements were performed at sea level (SL; 75 m), at 3,500 m, on arrival at 5,300 m (5,300 m-a; days 15 to 17) and at 5,300 m again (5,300 m-b; days 69 to 71). Amongst the subjects, nine remained at 5,300 m whilst 14 climbed to a maximum of 8,848 m. Exercise was 3 minutes of unloaded cycling followed by an incremental ramp protocol to exhaustion. The absolute StO2 at different stages of exercise along with the difference between StO2 at stages and the rate of change in StO2 were compared between altitudes. Resting peripheral oxygen saturation was recorded.

RESULTS

NIRS data achieving predefined quality criteria were available for 18 subjects at 75 m, 16 subjects at 3,500 m, 16 subjects on arrival at 5,300 m and 16 subjects on departure from 5,300 m. At SL, mean StO2 declined from 74.4% at rest to 36.4% at maximal oxygen consumption (P < 0.0001) and then rose to 82.3% (P < 0.0001) 60 seconds after exercise had ceased. At 3,500 m-a and 5,300 m-b, the pattern was similar to SL but absolute values were approximately 15% lower at all stages. At 5,300 m-a, the resting StO2 was similar to SL and the change in StO2 at each exercise stage less marked. At 5,300 m-b, the rate of decline in StO2 during exercise was more rapid than SL (P = 0.008); here the climbers had a smaller decline in StO2 during exercise (41.0%) and a slower rate of desaturation (0.086%/second) than those who had remained at 5,300 m (62.9% and 0.127%/second) (P = 0.031 and P = 0.047, respectively).

CONCLUSIONS

In most individuals, NIRS can be used to measure exercising skeletal muscle oxygenation in the field. During exercise the patterns of absolute oxygenation are broadly similar at altitude and SL. Following prolonged adaptation to altitude, the rate of muscle desaturation is more rapid than observed at SL but less so in those exposed to extreme hypoxia above 5,300 m.

Increased cerebral output of free radicals during hypoxia: implications for acute mountain sickness?

This study examined whether hypoxia causes free radical-mediated disruption of the blood-brain barrier (BBB) and impaired cerebral oxidative metabolism and whether this has any bearing on neurological symptoms ascribed to acute mountain sickness (AMS). Ten men provided internal jugular vein and radial artery blood samples during normoxia and 9-h passive exposure to hypoxia (12.9% O(2)). Cerebral blood flow was determined by the Kety-Schmidt technique with net exchange calculated by the Fick principle. AMS and headache were determined with clinically validated questionnaires. Electron paramagnetic resonance spectroscopy and ozone-based chemiluminescence were employed for direct detection of spin-trapped free radicals and nitric oxide metabolites. Neuron-specific enolase (NSE), S100beta, and 3-nitrotyrosine (3-NT) were determined by ELISA. Hypoxia increased the arterio-jugular venous concentration difference (a-v(D)) and net cerebral output of lipid-derived alkoxyl-alkyl free radicals and lipid hydroperoxides (P < 0.05 vs. normoxia) that correlated with the increase in AMS/headache scores (r = -0.50 to -0.90, P < 0.05). This was associated with a reduction in a-v(D) and hence net cerebral uptake of plasma nitrite and increased cerebral output of 3-NT (P < 0.05 vs. normoxia) that also correlated against AMS/headache scores (r = 0.74-0.87, P < 0.05). In contrast, hypoxia did not alter the cerebral exchange of S100beta and both global cerebral oxidative metabolism (cerebral metabolic rate of oxygen) and neuronal integrity (NSE) were preserved (P > 0.05 vs. normoxia). These findings indicate that hypoxia stimulates cerebral oxidative-nitrative stress, which has broader implications for other clinical models of human disease characterized by hypoxemia. This may prove a risk factor for AMS by a mechanism that appears independent of impaired BBB function and cerebral oxidative metabolism.

The cerebral effects of ascent to high altitudes

Cellular hypoxia is the common final pathway of brain injury that occurs not just after asphyxia, but also when cerebral perfusion is impaired directly (eg, embolic stroke) or indirectly (eg, raised intracranial pressure after head injury). We Review recent advances in the understanding of neurological clinical syndromes that occur on exposure to high altitudes, including high altitude headache (HAH), acute mountain sickness (AMS), and high altitude cerebral oedema (HACE), and the genetics, molecular mechanisms, and physiology that underpin them. We also present the vasogenic and cytotoxic bases for HACE and explore venous hypertension as a possible contributory factor. Although the factors that control susceptibility to HACE are poorly understood, the effects of exposure to altitude (and thus hypobaric hypoxia) might provide a reproducible model for the study of cerebral cellular hypoxia in healthy individuals. The effects of hypobaric hypoxia might also provide new insights into the understanding of hypoxia in the clinical setting.

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.

Can Altitude Research Benefit Critically ill Patients?

The Caudwell Xtreme Everest (CXE) expedition in the spring of 2007 was a large field-study investigating adaptive responses to hypoxia, and was supported by a grant from the Intensive Care Foundation. This article sets out to explain why the CXE team went to Everest, what they hope to learn, and discusses some of the key studies.

Caudwell Xtreme Everest: a field study of human adaptation to hypoxia

Caudwell Xtreme Everest (CXE) is a large healthy volunteer field study investigating human adaptation to environmental hypoxia. More than 200 individuals were studied at sea-level and in four laboratories on the trek to Everest Base Camp (5,300 m). Fifteen physicians climbed high on Everest and continued the studies as they ascended; eight of these individuals reached the summit of Everest and succeeded in sampling arterial blood at 8,400 m on their descent. Core measurements included cardiopulmonary exercise testing, neuropsychological assessment, near infra-red spectroscopy of brain and exercising muscle, blood markers and daily recording of simple physiological variables. The goal of CXE is to further our understanding of human adaptation to cellular hypoxia, a fundamental mechanism of injury in critical illness, with the aim of improving the care of critically ill patients.