Concepts in hypoxia reborn

Searching for molecular hypoxia sensors among oxygen-dependent enzymes

The ability to sense and respond to changes in cellular oxygen levels is critical for aerobic organisms and requires a molecular oxygen sensor. The prototypical sensor is the oxygen-dependent enzyme PHD: hypoxia inhibits its ability to hydroxylate the transcription factor HIF, causing HIF to accumulate and trigger the classic HIF-dependent hypoxia response. A small handful of other oxygen sensors are known, all of which are oxygen-dependent enzymes. However, hundreds of oxygen-dependent enzymes exist among aerobic organisms, raising the possibility that additional sensors remain to be discovered. This review summarizes known and potential hypoxia sensors among human O2-dependent enzymes and highlights their possible roles in hypoxia-related adaptation and diseases.

Hypoxia Induces Early Neurogenesis in Human Fetal Neural Stem Cells by Activating the WNT Pathway

Fetal neural stem cells (FNSCs) present in the human fetal brain differentiate into cells of neuronal and glial lineages. The developing fetus is exposed to lower oxygen concentrations than adults, and this physiological hypoxia may influence the growth and differentiation of the FNSCs. This study aimed to evaluate the effect of hypoxia on the differentiation potential of human FNSCs isolated from the subventricular zone of aborted fetal brains (n = 5). FNSCs were isolated, expanded, and characterized by Nestin and Sox2 expression using immunocytochemistry and flow cytometry, respectively. These FNSCs were exposed to 20% oxygen (normoxia) and 0.2% oxygen (hypoxia) concentrations for 48 h, and hypoxia exposure (n = 5) was validated. Whole transcriptome analyses (Genespring GX13) of FNSCs exposed to hypoxia (Agilent 4 × 44 K human array slides) highlighted that genes associated with neurogenesis were enriched upon exposure to hypoxia. The pathway analysis of these enriched genes (using Metacore) showed the involvement of the WNT signaling pathway. Microarray analyses were validated using neuronal and glial lineage commitment markers, namely, NEUROG1, NEUROG2, ASCL1, DCX, GFAP, OLIG2, and NKX2.2, using qPCR (n = 9). DCX, ASCL1, NGN1, and GFAP protein expression was analyzed by Western blotting (n = 3). This demonstrated upregulation of the neuronal commitment markers upon hypoxia exposure, while no change was observed in astrocytic and oligodendrocyte lineage commitment markers. Increased expression of downstream targets of the WNT signaling pathway, TCF4 and ID2, by qPCR (n = 9) and increased protein expression of CTNNB1 (β-catenin) and ID2 by Western blot (n = 3) indicated its involvement in mediating neuronal differentiation upon exposure to hypoxia.

Umbilical cord pH, blood gases, and lactate at birth: normal values, interpretation, and clinical utility

Normal birth is a eustress reaction, a beneficial hedonic stress with extremely high catecholamines that protects us from intrauterine hypoxia and assists in the rapid shift to extrauterine life. Occasionally the cellular O₂ requirement becomes critical and an O₂ deficit in blood (hypoxemia) may evolve to a tissue deficit (hypoxia) and finally a risk of organ damage (asphyxia). An increase in H⁺ concentration is reflected in a decrease in pH, which together with increased base deficit is a proxy for the level of fetal O₂ deficit. Base deficit (or its negative value, base excess) was introduced to reflect the metabolic component of a low pH and to distinguish from the respiratory cause of a low pH, which is a high CO₂ concentration. Base deficit is a theoretical estimate and not a measured parameter, calculated by the blood gas analyzer from values of pH, the partial pressure of CO₂, and hemoglobin. Different brands of analyzers use different calculation equations, and base deficit values can thus differ by multiples. This could influence the diagnosis of metabolic acidosis, which is commonly defined as a pH <7.00 combined with a base deficit ≥12.0 mmol/L in umbilical cord arterial blood. Base deficit can be calculated as base deficit in blood (or actual base deficit) or base deficit in extracellular fluid (or standard base deficit). The extracellular fluid compartment represents the blood volume diluted with the interstitial fluid. Base deficit in extracellular fluid is advocated for fetal blood because a high partial pressure of CO₂ (hypercapnia) is common in newborns without concomitant hypoxia, and hypercapnia has a strong influence on the pH value, then termed respiratory acidosis. An increase in partial pressure of CO₂ causes less increase in base deficit in extracellular fluid than in base deficit in blood, thus base deficit in extracellular fluid better represents the metabolic component of acidosis. The different types of base deficit for defining metabolic acidosis in cord blood have unfortunately not been noticed by many obstetrical experts and organizations. In addition to an increase in H⁺ concentration, the lactate production is accelerated during hypoxia and anaerobic metabolism. There is no global consensus on definitions of normal cord blood gases and lactate, and different cutoff values for abnormality are used. At a pH <7.20, 7% to 9% of newborns are deemed academic; at <7.10, 1% to 3%; and at <7.00, 0.26% to 1.3%. From numerous studies of different eras and sizes, it can firmly be concluded that in the cord artery, the statistically defined lower pH limit (mean -2 standard deviations) is 7.10. Given that the pH for optimal enzyme activity differs between different cell types and organs, it seems difficult to establish a general biologically critical pH limit. The blood gases and lactate in cord blood change with the progression of pregnancy toward a mixed metabolic and respiratory acidemia because of increased metabolism and CO₂ production in the growing fetus. Gestational age-adjusted normal reference values have accordingly been published for pH and lactate, and they associate with Apgar score slightly better than stationary cutoffs, but they are not widely used in clinical practice. On the basis of good-quality data, it is reasonable to set a cord artery lactate cutoff (mean +2 standard deviations) at 10 mmol/L at 39 to 40 weeks' gestation. For base deficit, it is not possible to establish statistically defined reference values because base deficit is calculated with different equations, and there is no consensus on which to use. Arterial cord blood represents the fetus better than venous blood, and samples from both vessels are needed to validate the arterial origin. A venoarterial pH gradient of <0.02 is commonly used to differentiate arterial from venous samples. Reference values for pH in cord venous blood have been determined, but venous blood comes from the placenta after clearance of a surplus of arterial CO₂, and base deficit in venous blood then overestimates the metabolic component of fetal acidosis. The ambition to increase neonatal hemoglobin and iron depots by delaying cord clamping after birth results in falsely acidic blood gas and lactate values if the blood sampling is also delayed. Within seconds after birth, sour metabolites accumulated in peripheral tissues and organs will flood into the central circulation and further to the cord arteries when the newborn starts to breathe, move, and cry. This influence of "hidden acidosis" can be avoided by needle puncture of unclamped cord vessels and blood collection immediately after birth. Because of a continuing anaerobic glycolysis in the collected blood, it should be analyzed within 5 minutes to not result in a falsely high lactate value. If the syringe is placed in ice slurry, the time limit is 20 minutes. For pH, it is reasonable to wait no longer than 15 minutes if not in ice. Routine analyses of cord blood gases enable perinatal audits to gain the wisdom of hindsight, to maintain quality assurance at a maternity unit over years by following the rate of neonatal acidosis, to compare results between hospitals on regional or national bases, and to obtain an objective outcome measure in clinical research. Given that the intrapartum cardiotocogram is an uncertain proxy for fetal hypoxia, and there is no strong correlation between pathologic cardiotocograms and fetal acidosis, a cord artery pH may help rather than hurt a staff person subjected to a malpractice suit based on undesirable cardiotocogram patterns. Contrary to common beliefs and assumptions, up to 90% of cases of cerebral palsy do not originate from intrapartum events. Future research will elucidate whether cell injury markers with point-of-care analysis will become valuable in improving the dating of perinatal injuries and differentiating hypoxic from nonhypoxic injuries.

Hypoxia-induced upregulation of matrix metalloproteinase 9 increases basement membrane degradation by downregulating collagen type IV alpha 1 chain

Hypoxia can cause basement membrane (BM) degradation in tissues. Matrix metalloproteinase 9 (MMP-9) is involved in various human cancers as well as BM degradation by downregulating type IV collagen (COL4). This study investigated the role of MMP-9 in hypoxia-mediated BM degradation in rat bone marrow based on its regulation of collagen type IV alpha 1 chain (COL4A1). Eighty male rats were randomly divided into four groups based on exposure to hypoxic conditions at a simulated altitude of 7,000 m, control (normoxia) and 3, 7, and 10 days of hypoxia exposure. BM degradation in bone marrow was determined by transmission electron microscopy. MMP-9 levels were assessed by western blot and real-time PCR, and COL4A1 levels were assessed by western blot and immunohistochemistry. Microvessels BMs in bone marrow exposed to acute hypoxia were observed by electron microscopy. MMP-9 expression increased, COL4A1 protein expression decreased, and BM degradation occurred in the 10-, 7-, and 3-day hypoxia groups compared with that in the control group (all P < 0.05). Hypoxia increased MMP-9 levels, which in turn downregulated COL4A1, thereby increasing BM degradation. MMP-9 upregulation significantly promoted BM degradation and COL4A1 downregulation. Our results suggest that MMP-9 is related to acute hypoxia-induced BM degradation in bone marrow by regulating COL4A1.

Hypoxia-induced upregulation of matrix metalloproteinase 9 increases basement membrane degradation by downregulating collagen type IV alpha 1 chain

Hypoxia can cause basement membrane (BM) degradation in tissues. Matrix metalloproteinase 9 (MMP-9) is involved in various human cancers as well as BM degradation by downregulating type IV collagen (COL4). This study investigated the role of MMP-9 in hypoxia-mediated BM degradation in rat bone marrow based on its regulation of collagen type IV alpha 1 chain (COL4A1). Eighty male rats were randomly divided into four groups based on exposure to hypoxic conditions at a simulated altitude of 7,000 m, control (normoxia) and 3, 7, and 10 days of hypoxia exposure. BM degradation in bone marrow was determined by transmission electron microscopy. MMP-9 levels were assessed by western blot and real-time PCR, and COL4A1 levels were assessed by western blot and immunohistochemistry. Microvessels BMs in bone marrow exposed to acute hypoxia were observed by electron microscopy. MMP-9 expression increased, COL4A1 protein expression decreased, and BM degradation occurred in the 10-, 7-, and 3-day hypoxia groups compared with that in the control group (all P < 0.05). Hypoxia increased MMP-9 levels, which in turn downregulated COL4A1, thereby increasing BM degradation. MMP-9 upregulation significantly promoted BM degradation and COL4A1 downregulation. Our results suggest that MMP-9 is related to acute hypoxia-induced BM degradation in bone marrow by regulating COL4A1.

Mitochondrial Regulation of the Hypoxia-Inducible Factor in the Development of Pulmonary Hypertension

Pulmonary hypertension (PH) is a severe progressive lung disorder characterized by pulmonary vasoconstriction and vascular remodeling, culminating in right-sided heart failure and increased mortality. Data from animal models and human subjects demonstrated that hypoxia-inducible factor (HIF)-related signaling is essential in the progression of PH. This review summarizes the regulatory pathways and mechanisms of HIF-mediated signaling, emphasizing the role of mitochondria in HIF regulation and PH pathogenesis. We also try to determine the potential to therapeutically target the components of the HIF system for the management of PH.

Association between haemoglobin A1c and cerebral microbleeds in community-based stroke-free individuals: A cross-sectional study

AIMS

The association between haemoglobin A1c (HbA1c) and cerebral microbleeds (CMBs) remains unclear. We aimed to investigate the association between HbA1c and CMBs in community-based individuals without stroke or transient ischaemic attack (TIA) and whether the association differs between individuals with and without diabetes mellitus (DM).

MATERIALS AND METHODS

All individuals were recruited from a community in Beijing, China, from January 2015 to September 2019. All individuals completed a questionnaire and underwent blood tests and brain magnetic resonance imaging. A susceptibility-weighted imaging sequence was acquired to detect CMBs, which were defined as small, round and low-signal lesions with <10 mm diameter. The association between HbA1c and CMBs was analysed using multivariable logistic regression adjusted for demographics, medical history and blood sample test results. Subgroup analyses stratified by history of DM were performed.

RESULTS

Of 544 recruited individuals, 119 (21.88%) had CMBs. HbA1c was independently associated with CMBs (odds ratio [OR], 1.51; 95% confidence interval [CI], 1.03-2.22). In 87 individuals with DM, multivariable logistic analysis showed that HbA1c was significantly associated with CMBs (OR, 1.67; 95% CI, 1.04-2.69), whereas in individuals without DM, no significant association was observed between HbA1c and CMBs (OR, 1.07; 95% CI, 0.50-2.30).

CONCLUSIONS

HbA1c was associated with CMBs in individuals without stroke or TIA, particularly in individuals with DM, suggesting that the status of glycaemic control warrants attention for the prevention of CMBs. It would be beneficial to manage HbA1c specifically to control the risk of CMBs, especially in individuals with DM.

Anterior Circulation Acute Ischemic Stroke in the Plateau of China: Risk Factors and Clinical Characteristics

Background and Purpose

Acute ischemic stroke has a high incidence in the plateau of China. It has unique characteristics compared to the plains, and the specific relationship with altitude has not yet been appreciated. This study aimed to investigate the specificity of the plateau's anterior circulation acute ischemic stroke in China.

Methods

To retrospectively collect clinical data of patients with first-episode acute ischemic stroke in the anterior circulation in Tianjin and Xining city. The differences in clinical presentation, laboratory, and imaging examinations were compared.

Results

Patients at high altitudes showed a significant trend toward lower age (61.0 ± 10.2 vs. 64.8 ± 8.1, P = 0.010) and had a history of dyslipidemia, higher levels of inflammatory markers, erythrocytosis, and alcohol abuse. The main manifestations were higher diastolic blood pressure (85.5 ± 14.0 mmHg vs. 76.8 ± 11.6 mmHg, P < 0.001), triglycerides [2.0 (1.8) mmol/L vs. 1.3 (0.9) mmol/L, P < 0.001], CRP [4.7 (4.4) mg/L vs. 2.1 (1.9) mg/L, P < 0.001], homocysteine levels [14.5 (11.7) μmol/L vs. 11.2 (5.2) μmol/L, P < 0.001]; larger infarct volume [3.5 (4.8) cm³ vs. 9.0 (6.9) cm³, P < 0.001] and worse prognosis. Patients at high altitudes had higher atherosclerotic indexes in cIMT and plaque than those in plains.

Conclusions

The natural habituation and genetic adaptation of people to the particular geo-climatic environment of the plateau have resulted in significant differences in disease characteristics. Patients with the anterior circulation acute ischemic stroke in the plateau show more unfavorable clinical manifestations and prognosis. This study provides a preliminary interpretation of the effects of altitude and suggests developing preventive and therapeutic protocol measures that are more appropriate for the plateau of China.

Comparison of Chest-to-Back Skin-to-Skin Contact and Chest-to-Chest Skin-to-Skin Contact on the Risk of Oxygen Desaturation and Change in Heart Rate in Low Birth Weight and/or Premature Babies: A Randomized Controlled Clinical Trial

Chest-to-chest (CC) skin-to-skin contact (SSC) is a widely used method of SSC to prevent low birth weight (LBW) and/or premature babies with the risk of hypothermia. However, very recently, a study has also shown that the chest-to-back (CB) SSC is also useful for such a purpose. It is also evident that CC SSC enhances the cardiorespiratory performance of LBW and/or premature babies from the risk of cold stress. However, whether babies kept in CB SSC have the risk of clinically relevant decreases of oxygen saturation or critical changes of the baby heart rate comparing the two SSC methods has been studied hardly. Thus, we assessed the risk of oxygen desaturation and changes in babies' heart rate among LBW and/or premature babies kept in CB SSC compared to the standard. In this study, we enrolled 46 LBW and/or premature babies born between 32 and 37 completed weeks of gestation. We used a parallel-group randomized controlled clinical trial. Peripheral arterial blood oxygen saturation (SpO2) and heart rate (HR) were measured using an OxiMaxN-600X Pulse Oximeter. We transformed these measurements into stability of the cardiorespiratory system in premature infant (SCRIP) scores. We applied a generalized estimating equation model to analyze the data. No statistically significant difference was observed between babies kept in CB SSC compared to babies kept in CC SSC in either blood oxygen saturation or heart rate (P > 0.05). Thus, the CB SSC can be used as one possible way to care for LBW and preterm babies in the kangaroo mother care. We suggest more studies before scaling up the approach in routine care.

Effects of Normoxia, Hyperoxia, and Mild Hypoxia on Macro-Hemodynamics and the Skeletal Muscle Microcirculation in Anesthetised Rats

Objectives: Excessive oxygen (O₂) administration may have a negative impact on tissue perfusion by inducing vasoconstriction and oxidative stress. We aimed to evaluate the effects of different inhaled oxygen fractions (FiO₂) on macro-hemodynamics and microvascular perfusion in a rat model.

Methods: Isoflurane-anesthetised spontaneously breathing male Wistar rats were equipped with arterial (carotid artery) and venous (jugular vein) catheters and tracheotomy, and randomized into three groups: normoxia (FiO₂ 21%, n = 6), hyperoxia (FiO₂ 100%, n = 6) and mild hypoxia (FiO₂ 15%, n = 6). Euvolemia was maintained by infusing Lactate Ringer solution at 10 ml/kg/h. At hourly intervals for 4 h we collected measurements of: mean arterial pressure (MAP); stroke volume index (SVI), heart rate (HR), respiratory rate (by means of echocardiography); arterial and venous blood gases; microvascular density, and flow quality (by means of sidestream dark field videomicroscopy on the hindlimb skeletal muscle).

Results: MAP and systemic vascular resistance index increased with hyperoxia and decreased with mild hypoxia (p < 0.001 in both cases, two-way analysis of variance). Hyperoxia induced a reduction in SVI, while this was increased in mild hypoxia (p = 0.002). The HR increased under hyperoxia (p < 0.05 vs. normoxia at 3 h). Cardiax index, as well as systemic O₂ delivery, did not significantly vary in the three groups (p = 0.546 and p = 0.691, respectively). At 4 h, microvascular vessel surface (i.e., the percentage of tissue surface occupied by vessels) decreased by 29 ± 4% in the hyperoxia group and increased by 19 ± 7 % in mild hypoxia group (p < 0.001). Total vessel density and perfused vessel density showed similar tendencies (p = 0.003 and p = 0.005, respectively). Parameters of flow quality (microvascular flow index, percentage of perfused vessels, and flow heterogeneity index) remained stable and similar in the three groups.

Conclusions: Hyperoxia induces vasoconstriction and reduction in skeletal muscle microvascular density, while mild hypoxia has an opposite effect.

Birth signalling hormones and the developmental consequences of caesarean delivery

Rates of delivery by caesarean section (CS) are increasing around the globe and, although several epidemiological associations have already been observed between CS and health outcomes in later life, more are sure to be discovered as this practice continues to gain popularity. The components of vaginal delivery that protect offspring from the negative consequences of CS delivery in later life are currently unknown, although much attention to date has focused on differences in microbial colonisation. Here, we present the case that differing hormonal experiences at birth may also contribute to the neurodevelopmental consequences of CS delivery. Levels of each of the 'birth signalling hormones' (oxytocin, arginine vasopressin, epinephrine, norepinephrine and the glucocorticoids) are lower following CS compared to vaginal delivery, and there is substantial evidence for each that manipulations in early life results in long-term neurodevelopmental consequences. We draw from the research traditions of neuroendocrinology and developmental psychobiology to suggest that the perinatal period is a sensitive period, during which hormones achieve organisational effects. Furthermore, there is much to be learned from research on developmental programming by early-life stress that may inform research on CS, as a result of shared neuroendocrine mechanisms at work. We compare and contrast the effects of early-life stress with those of CS delivery and propose new avenues of research based on the links between the two bodies of literature. The research conducted to date suggests that the differences in hormone signalling seen in CS neonates may produce long-term neurodevelopmental consequences.

Endurance Athletes Are at Increased Risk for Early Acute Mountain Sickness at 3450 m

INTRODUCTION

Acute mountain sickness (AMS) may develop in nonacclimatized individuals after exposure to altitudes ≥2500 m. Anecdotal reports suggest that endurance-trained (ET) athletes with a high maximal oxygen uptake (V˙O2max) may be at increased risk for AMS. Possible underlying mechanisms include a training-induced increase in resting parasympathetic activity, higher resting metabolic rate (RMR), and lower hypoxic ventilatory response (HVR).

METHODS

In 38 healthy, nonacclimatized men (19 ET and 19 untrained controls [UT], V˙O2max 66 ± 6 mL·min·kg vs 45 ± 7 mL·min·kg; P < 0.001) peripheral oxygen saturation (SpO2), heart rate variability, RMR, and poikilocapnic HVR were assessed at 424 m and during 48 h at 3450 m after passive ascent by train (~2 h). Acute mountain sickness was evaluated by AMS cerebral (AMS-C) score.

RESULTS

On day 1 at altitude, ET presented with a higher AMS incidence (42% vs 11%; P < 0.05) and severity (AMS-C score: ET, 0.48 ± 0.5 vs UT, 0.21 ± 0.2; P = 0.03), but no group difference was found on days 2 and 3. SpO2 decreased upon arrival at altitude (ET: 82% ± 6% vs UT: 83% ± 4%; ptime <0.001) with a significantly different time course between ET and UT (ptime × group = 0.045). Parasympathetic activity decreased at altitude (P < 0.001) but was always higher in ET (P < 0.05). At altitude RMR increased (P < 0.001) and was higher in ET (P < 0.001). Hypoxic ventilatory response increased only in ET (P < 0.05) and was greater than in UT after 24 and 48 h (P < 0.05).

CONCLUSIONS

Endurance-trained athletes are at higher risk for developing AMS on the first day after passive and rapid ascent to 3450 m, possibly due to an increased parasympathetic activity and an increased RMR, while HVR appeared to be of minor importance. Differences in AMS time course and physiological responses should be taken into consideration when ET are planning high-altitude sojourns.

The oxygen cascade in patients treated with hemodialysis and native high-altitude dwellers: lessons from extreme physiology to benefit patients with end-stage renal disease

Patients treated with hemodialysis (HD) repeatedly undergo intradialytic low arterial oxygen saturation and low central venous oxygen saturation, reflecting an imbalance between upper body systemic oxygen supply and demand, which are associated with increased mortality. Abnormalities along the entire oxygen cascade, with impaired diffusive and convective oxygen transport, contribute to the reduced tissue oxygen supply. HD treatment impairs pulmonary gas exchange and reduces ventilatory drive, whereas ultrafiltration can reduce tissue perfusion due to a decline in cardiac output. In addition to these factors, capillary rarefaction and reduced mitochondrial efficacy can further affect the balance between cellular oxygen supply and demand. Whereas it has been convincingly demonstrated that a reduced perfusion of heart and brain during HD contributes to organ damage, the significance of systemic hypoxia remains uncertain, although it may contribute to oxidative stress, systemic inflammation, and accelerated senescence. These abnormalities along the oxygen cascade of patients treated with HD appear to be diametrically opposite to the situation in Tibetan highlanders and Sherpa, whose physiology adapted to the inescapable hypobaric hypoxia of their living environment over many generations. Their adaptation includes pulmonary, vascular, and metabolic alterations with enhanced capillary density, nitric oxide production, and mitochondrial efficacy without oxidative stress. Improving the tissue oxygen supply in patients treated with HD depends primarily on preventing hemodynamic instability by increasing dialysis time/frequency or prescribing cool dialysis. Whether dietary or pharmacological interventions, such as the administration of L-arginine, fermented food, nitrate, nuclear factor erythroid 2-related factor 2 agonists, or prolyl hydroxylase 2 inhibitors, improve clinical outcome in patients treated with HD warrants future research.

The Hypoglycemic and Renal Protection Properties of Crocin via Oxidative Stress-Regulated NF-κB Signaling in db/db Mice

Background

As the main ingredient of Crocus sativus L. (Iridaceae) extract, crocin- I (CR) has been reported to show various pharmacological activities. The aim of this study was to investigate the hypoglycemic and renal protection properties of CR in db/db mice.

Methods

Eight-week-old db/db mice were treated with metformin (Met) (100 mg/kg) and CR (50 mg/kg) for eight weeks.

Results

CR treatment showed hypoglycemic functions indicated by reduced bodyweight, food and water intake, plasma glucose, and serum levels of glycated hemoglobin A1c. Additionally, the CR group showed increased serum levels of insulin and pyruvate kinase, hypolipidemic functions indicated by the suppressed levels of total cholesterol and triglyceride, and enhanced levels of high-density lipoprotein cholesterol, which are also indicators of hypoglycemic functions. The renal protection function of CR was demonstrated by its protection of renal structures and its regulation of potential indicators of nephropathy. The anti-oxidation and anti-inflammation effects of CR were verified by enzyme-linked immunosorbent assay. In the kidneys of db/db mice, CR decreased the expression of phospho-IκBα and phospho-nuclear factor-κB (NF-κB), whereas it enhanced the expression of nuclear respiratory factor 2, manganese superoxide dismutase 1, heme oxygenase-1, and catalase.

Conclusions

The anti-diabetic and anti-diabetic nephritic effects of CR were related to its modulation of oxidative stress-mediated NF-κB signaling.

Acute mountain sickness: Do different time courses point to different pathophysiological mechanisms?

Acute mountain sickness (AMS) is a syndrome of nonspecific symptoms (i.e., headache, anorexia, nausea, vomiting, dizziness, and fatigue) that may develop in nonacclimatized individuals after rapid exposure to altitudes ≥2,500 m. In field studies, mean AMS scores usually peak after the first night at a new altitude. Analyses of the individual time courses of AMS in four studies performed at 3,450 m and 4,559 m revealed that three different patterns are hidden in the above-described overall picture. In 41% of those who developed AMS (i.e., AMS-C score >0.70), symptoms peaked on day 1, in 39%, symptoms were most prominent on day 2, and in 20%, symptoms were most prominent on day 3. We suggest to name the different time courses of AMS type I, type II, and type III, respectively. Here, we hypothesize that the variation of time courses of AMS are caused by different pathophysiological mechanisms. This assumption could explain why no consistent correlations between an overall assessment of AMS and single pathophysiological factors have been found in a large number of studies over the past 50 yr. In this paper, we will briefly review the fundamental mechanisms implicated in the pathophysiology of AMS and discuss how they might contribute to the three different AMS time courses.

Anti-Diabetic Nephropathy Activities of Polysaccharides Obtained from Termitornyces albuminosus via Regulation of NF-κB Signaling in db/db Mice

Termitornyces albuminosus is a kind of traditional Chinese edible fungus rich in nutrients and medicinal ingredients, and it has anti-oxidative, analgesic and anti-inflammatory effects. However, the hypoglycemic and nephroprotective effects of polysaccharides separated from T. albuminosus (PTA) have not been reported. The properties of PTA were analyzed in a BKS.Cg-Dock7^m +/+ Lepr^db/JNju (db/db) mouse model of diabetes. After the administration of PTA for eight weeks, the hypoglycemic and hypolipidemic activities of PTA in the db/db mice were assessed. The results of a cytokine array combined with an enzyme-linked immunosorbent assay confirmed the anti-oxidative and anti-inflammatory activities of PTA. An eight-week administration of PTA caused hypoglycemic and hypolipidemic functioning, as indicated by suppressed plasma glucose levels, as well as the modulation of several cytokines related to glycometabolism, in the sera and kidneys of the mice. PTA treatment also had a protective effect on renal function, restoring renal structures and regulating potential indicators of nephropathy. In the kidneys of the db/db mice, PTA treatment reduced the activation of protein kinase B, the inhibitor of κB kinase alpha and beta, and the inhibitor of κB alpha and nuclear factor-κB (NF-κB). We establish the hypoglycemic, hypolipidemic, and anti-diabetic nephropathy effects of PTA, and we find that the renal protection effects of PTA may be related to anti-inflammatory activity via the regulation of NF-κB signaling.

Hypobaric birth room may prevent intraventricular hemorrhage in extremely low birth weights infants

In the early postnatal period, intraventricular hemorrhage may develop in infants with extremely low birth weights due to hemodynamic instability. One of the most significant factors in intraventricular hemorrhage development is fluctuations in the cerebral blood flow due to left-to-right shunting as a result of patent ductus arteriosus, and such cases most frequently develop intraventricular hemorrhage within the first 72 h. The frequency of intraventricular hemorrhage may be reduced through the prevention of fluctuations in the cerebral blood flow in this time frame. Based on our hypothesis, we recommend that extremely low birth weight infants should be delivered and monitored in hypobaric rooms for the first three days after birth, as this may reduce left-to-right shunting as a result of patent ductus arteriosus by preventing the rapid drops seen in pulmonary pressure after birth. A more stable hemodynamic status may be achieved by increasing the cerebral blood flow during an acute term in a hypobaric environment. Gradual transition to the normobaric status at the end of the third day may prevent the long-term negative effects of hypobaric conditions.

Extreme pregnancy: maternal physical activity at Everest Base Camp

High-altitude natives employ numerous physiological strategies to survive and reproduce. However, the concomitant influence of altitude and physical activity during pregnancy has not been studied above 3,700 m. We report a case of physical activity, sleep behavior, and physiological measurements on a 28-yr-old third-trimester pregnant native highlander (Sherpa) during ascent from 3,440 m to Everest Base Camp (~5,300 m) over 8 days in the Nepal Himalaya and again ~10 mo postpartum during a similar ascent profile. The participant engaged in 250-300 min of moderate to vigorous physical activity per day during ascent to altitude while pregnant, with similar volumes of moderate to vigorous physical activity while postpartum. There were no apparent maternal, fetal, or neonatal complications related to the superimposition of the large volumes of physical activity at altitude. This report demonstrates a rare description of physical activity and ascent to high altitude during pregnancy and points to novel questions regarding the superimposition of pregnancy, altitude, and physical activity in high-altitude natives.

Oxygen saturation targeting by pulse oximetry in the extremely low gestational age neonate: a quixotic quest

PURPOSE OF REVIEW

A collaboration of comparative effectiveness research trials of pulse oximeter saturation (SpO2) targeting in extremely low-gestational-age neonates have begun to report their aggregate results. We examine the results of those trials, collectively referred to as the Neonatal Oxygenation Prospective Meta-analysis or NeOProM. We also discuss the uncertainties that remain and the clinical challenges that lie ahead.

RECENT FINDINGS

The primary outcome from NeOProM was a composite of death or disability at 18-24 months corrected age. In 2016 the last of these reports was published. Although there were no differences in the primary outcome overall, analyses of secondary outcomes and data subsets following a pulse oximeter revision show significant treatment differences between targeting a lower compared with a higher SpO2.

SUMMARY

NeOProM represents the largest collaborative clinical research study of SpO2 targets in extremely low-gestational-age neonates. Although aggregate results give us some insight into the feasibility and efficacy of SpO2 targeting in this population, many questions remain. A patient-level analysis, tracking individual outcomes based on actual SpO2 experienced, may shed some light on these questions. However, finding a single optimal SpO2 range seems unlikely.

Age Related Bioenergetics Profiles in Isolated Rat Cardiomyocytes Using Extracellular Flux Analyses

Mitochondrial dysfunction is increasingly recognized and studied as a mediator of heart disease. Extracellular flux analysis (XF) has emerged as a powerful tool to investigate cellular bioenergetics in the context of cardiac health and disease, however its use and interpretation requires improved understanding of the normal metabolic differences in cardiomyocytes (CM) at various stages of maturation. This study standardized XF analyses methods (mitochondrial stress test, glycolytic stress test and palmitate oxidation test) and established age related differences in bioenergetics profiles of healthy CMs at newborn (NB1), weaning (3WK), adult (10WK) and aged (12–18MO) time points. Findings show that immature CMs demonstrate a more robust and sustained glycolytic capacity and a relative inability to oxidize fatty acids when compared to older CMs. The study also highlights the need to recognize the contribution of CO₂ from the Krebs cycle as well as lactate from anaerobic glycolysis to the proton production rate before interpreting glycolytic capacity in CMs. Overall, this study demonstrates that caution should be taken to assure that translatable developmental time points are used to investigate mitochondrial dysfunction as a cause of cardiac disease. Specifically, XF analysis of newborn CMs should be reserved to study fetal/neonatal disease and older CMs (≥10 weeks) should be used to investigate adult disease pathogenesis. Knowledge gained will aid in improved investigation of developmentally programmed heart disease and stress the importance of discerning maturational differences in bioenergetics when developing mitochondrial targeted preventative and therapeutic strategies for cardiac disease.

Alteration of SLP2-like immunolabeling in mitochondria signifies early cellular damage in developing and adult mouse brain

Mitochondria play a critical role in various pathways of regulated cell death. Here we propose a novel method for detection of initial derangement of mitochondria in degenerating and dying neuronal cells. The method is based on our recent finding that antibodies directed against the cannabinoid type 1 receptor (CB1) also bind the mitochondrial stomatin-like protein 2 (SLP2) that belongs to an inner mitochondrial membrane protein complex. It is well established that SLP2 regulates mitochondrial biogenesis and respiratory functions. We now show that anti-CB1 antibodies recognize conformational epitopes but not the linear amino acid sequence of SLP2. In addition we found that anti-CB1 serum mostly labels swollen mitochondria with early or advanced stages of pathology in mouse brain while other proteins of the complex may mask epitopes of SLP2 in the normal mitochondria. Although neurons and endothelial cells in healthy brains contain occasional immunopositive mitochondria detectable with anti-CB1 serum, their numbers increase significantly after hypoxic insults in parallel with signs of cellular damage. Moreover, use of electron microscopy suggests relocation of SLP2 from its normal functional position in the inner mitochondrial membrane into the mitochondrial matrix in pathological cells. Thus, SLP2-like immunolabeling serves as an in situ histochemical target detecting early derangement of mitochondria. Anti-CB1 serum is crucial for this purpose because available anti-SLP2 antibodies do not provide selective labeling of mitochondria in the fixed tissue. This new method of detecting mitochondrial dysfunction can benefit the in vitro research of human diseases and developmental disorders by enabling analysis in live animal models.

Placental oxygen transport estimated by the hyperoxic placental BOLD MRI response

Estimating placental oxygen transport capacity is highly desirable, as impaired placental function is associated with fetal growth restriction (FGR) and poor neonatal outcome. In clinical obstetrics, a noninvasive method to estimate the placental oxygen transport is not available, and the current methods focus on fetal well-being rather than on direct assessment of placental function. In this article, we aim to estimate the placental oxygen transport using the hyperoxic placental blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) response. In 21 normal pregnancies and in four cases of severe early onset FGR, placental BOLD MRI was performed in a 1.5 Tesla MRI system (TR:8000 msec, TE:50 msec, Flip angle:90). Placental histological examination was performed in the FGR cases. In normal pregnancies, the average hyperoxic placental BOLD response was 12.6 ± 5.4% (mean ± SD). In the FGR cases, the hyperoxic BOLD response was abnormal only in cases with histological signs of maternal hypoperfusion of the placenta. The hyperoxic placental BOLD response is mainly derived from an increase in the saturation of maternal venous blood. In the normal placenta, the pO₂ of the umbilical vein is closely related to the pO₂ of the uterine vein. Therefore, the hyperoxic placental BOLD response may reflect the placental oxygen supply to the fetus. In early onset FGR, the placental oxygen transport is reduced mainly because of the maternal hypoperfusion, and in these cases the placental BOLD response might be altered. Thus, the placental BOLD MRI might provide direct noninvasive assessment of placental oxygen transport.

Molecular mechanisms of hypoxia-inducible factor-induced pulmonary arterial smooth muscle cell alterations in pulmonary hypertension

Oxygen (O2) is essential for the viability and function of most metazoan organisms and thus is closely monitored at both the organismal and the cellular levels. However, alveoli often encounter decreased O2 levels (hypoxia), leading to activation of physiological or pathophysiological responses in the pulmonary arteries. Such changes are achieved by activation of transcription factors. The hypoxia-inducible factors (HIFs) are the most prominent hypoxia-regulated transcription factors in this regard. HIFs bind to hypoxia-response elements (HREs) in the promoter region of target genes, whose expression and translation allows the organism, amongst other factors, to cope with decreased environmental O2 partial pressure (pO2). However, prolonged HIF activation can contribute to major structural alterations, especially in the lung, resulting in the development of pulmonary hypertension (PH). PH is characterized by a rise in pulmonary arterial pressure associated with pulmonary arterial remodelling, concomitant with a reduced intravascular lumen area. Patients with PH develop right heart hypertrophy and eventually die from right heart failure. Thus, understanding the molecular mechanisms of HIF regulation in PH is critical for the identification of novel therapeutic strategies. This review addresses the relationship of hypoxia and the HIF system with pulmonary arterial dysfunction in PH. We particularly focus on the cellular and molecular mechanisms underlying the HIF-driven pathophysiological processes.

Epigenetics in the perioperative period

The perioperative period is characterized by profound changes in the body's homoeostatic processes. This review seeks to address whether epigenetic mechanisms may influence an individual's reaction to surgery and anaesthesia. Evidence from animal and human studies suggests that epigenetic mechanisms can explain many facets of susceptibility to acute and chronic pain, making them potential therapeutic targets. Modern pain management is still based upon opiates, and both the developmental expression of opioid receptors and opioid-induced hyperalgesia have been linked to epigenetic mechanisms. In general, opiates seem to increase global DNA methylation levels. This is in contrast to local anaesthetics, which have been ascribed a global demethylating effect. Even though no direct investigations have been carried out, the potential influence of epigenetics on the inflammatory response that follows surgery seems a promising area for research. There is a considerable body of evidence that supports the involvement of epigenetics in the complex process of wound healing. Epigenetics is an important emerging research topic in perioperative medicine, with a huge potential to positively influence patient outcome.

Intermittent hypoxia in childhood: the harmful consequences versus potential benefits of therapeutic uses

Intermittent hypoxia (IH) often occurs in early infancy in both preterm and term infants and especially at 36-44 weeks postmenstrual age. These episodes of IH could result from sleep-disordered breathing or may be temporally unrelated to apnea or bradycardia events. There are numerous reports indicating adverse effects of IH on development, behavior, academic achievement, and cognition in children with sleep apnea syndrome. It remains uncertain about the exact causative relationship between the neurocognitive and behavioral morbidities and IH and/or its associated sleep fragmentation. On the other hand, well-controlled and moderate IH conditioning/training has been used in sick children for treating their various forms of bronchial asthma, allergic dermatoses, autoimmune thyroiditis, cerebral palsy, and obesity. This review article provides an updated and impartial analysis on the currently available evidence in supporting either side of the seemingly contradictory scenarios. We wish to stimulate a comprehensive understanding of such a complex physiological phenomenon as intermittent hypoxia, which may be accompanied by other confounding factors (e.g., hypercapnia, polycythemia), in order to prevent or reduce its harmful consequences, while maximizing its potential utility as an effective therapeutic tool in pediatric patients.

Suppression of erythropoiesis by dietary nitrate

In mammals, hypoxia-triggered erythropoietin release increases red blood cell mass to meet tissue oxygen demands. Using male Wistar rats, we unmask a previously unrecognized regulatory pathway of erythropoiesis involving suppressor control by the NO metabolite and ubiquitous dietary component nitrate. We find that circulating hemoglobin levels are modulated by nitrate at concentrations achievable by dietary intervention under normoxic and hypoxic conditions; a moderate dose of nitrate administered via the drinking water (7 mg NaNO₃/kg body weight/d) lowered hemoglobin concentration and hematocrit after 6 d compared with nonsupplemented/NaCl-supplemented controls. The underlying mechanism is suppression of hepatic erythropoietin expression associated with the downregulation of tissue hypoxia markers, suggesting increased pO₂. At higher nitrate doses, however, a partial reversal of this effect occurred; this was accompanied by increased renal erythropoietin expression and stabilization of hypoxia-inducible factors, likely brought about by the relative anemia. Thus, hepatic and renal hypoxia-sensing pathways act in concert to modulate hemoglobin in response to nitrate, converging at an optimal minimal hemoglobin concentration appropriate to the environmental/physiologic situation. Suppression of hepatic erythropoietin expression by nitrate may thus act to decrease blood viscosity while matching oxygen supply to demand, whereas renal oxygen sensing could act as a brake, averting a potentially detrimental fall in hematocrit.—Ashmore, T., Fernandez, B. O., Evans, C. E., Huang, Y., Branco-Price, C., Griffin, J. L., Johnson, R. S., Feelisch, M., Murray, A. J. Suppression of erythropoiesis by dietary nitrate.

The epigenetic side of human adaptation: hypotheses, evidences and theories

CONTEXT

Epigenetics represents a still unexplored research field in the understanding of micro- and macro-evolutionary mechanisms, as epigenetic changes create phenotypic diversity within both individuals and populations.

OBJECTIVE

The purpose of this review is to dissect the landscape of studies focused on DNA methylation, one of the most described epigenetic mechanisms, emphasizing the aspects that could be relevant in human adaptations.

METHODS

Theories and results here considered were collected from the most recent papers published.

RESULTS

The matter of DNA methylation inheritance is here described as well as the recent evolutionary theories regarding the role of DNA methylation-and epigenetics in a broader sense-in human evolution. The complex relation between (1) DNA methylation and genetic variability and (2) DNA methylation and the environmental stimuli crucial in shaping genetic and phenotypic variability through the human lineage-such as diet, climate and pathogens exposure-are described. Papers about population epigenetics are also illustrated due to their high relevance in this context.

CONCLUSION

Genetic, epigenetic and phenotypic variations of the species, together with cultural ones, are considerably shaped by a vast range of environmental stimuli, thus representing the foundation of all human bio-cultural adaptations.

Hypothesis of mitochondrial oncogenesis as the trigger of normal cells to cancer cells

The Warburg Effect showed that energy metabolism of cancer cells was similar to prokaryotic cells, which were different from normal eucaryotic cells. The Endosymbiotic Theory offered a plausible explanation that the eucaryotic cells were evolved from prokaryotic cells, by which host cells (ancient prokaryotic cells) had ingested mitochondria (ancient aerobic bacteria), which depended on oxidative phosphorylation rather than glycolysis for generating energy. The alteration of energy metabolism might mean that the survival style of cancer cells were the re-evolution from eucaryotic cells to prokaryotic cells. But how this alteration happened was still unknown. This hypothesis tries to explain how mitochondria take part in the re-evolution from normal cell to cancer cell.

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.

Advancing critical care: time to kiss the right frog

The greatest advances in critical care over the past two decades have been achieved through doing less to the patient. We have learnt through salutary experience that our burgeoning Master-of-the-Universe capabilities and the oh-so-obvious stratagems instilled in us from youth were often ineffective or even deleterious. This re-education process, however, is far from complete. We are now rightly agonizing over the need for better characterization of pathophysiology, earlier identification of disease processes and a more directed approach to therapeutic intervention. We need to delineate the point at which intrinsic and protective adaptation ends and true harmful pathology begins, and how our iatrogenic meddling either helps or hinders. We need to improve trial design in the heterogeneous populations we treat, and to move away from syndromic fixations that, while offering convenience, have generally proved counterproductive. Importantly, we need to discover a far more holistic approach to patient care, evolving from the prevailing overmedicalized, number-crunching perspective towards a true multidisciplinary effort that embraces psychological as well as physiological well-being, with appropriate pharmacological minimization or supplementation. Complacency, with an unfair apportion of blame on the patient for not getting better, is the biggest threat to continued improvement.

Our favorite unproven ideas for future critical care

The future of critical care medicine will be shaped not only by the evidence-validated foundations of science, but also by innovations based on unproven and, in many cases, untested concepts and thoughtful visions of scientists and clinicians familiar with the complex problems actually faced in clinical practice. Clinical investigations and trials often lag behind collective experience and impressions, in a well-intentioned and necessary quest to determine the fallacy or validity of ongoing practice. Progress made in this way can be painfully slow, and imperfect theory may prove difficult to challenge. On occasion, an innovative paradigm shift fostered by a novel approach can reorient the forces of academic investigation toward generating an evidence base upon which such concepts and interpretations can find scientific justification. This discussion presents a selected set of ideas to improve the future practice of critical care - each having a defensible rationale, but unconfirmed validity.

Genetics and epigenetics in perioperative medicine

PURPOSE OF REVIEW

To summarize is to review recent progress in 'genomic' science and how this may be applied to the perioperative environment. Although investigations that relate genetic variation to perioperative outcomes continue, it is increasingly apparent that epigenetic mechanisms may contribute to much of the observed variation in complex outcomes not otherwise explained by differences in genetic sequence.

RECENT FINDINGS

Examples of recent findings relating to the role of epigenetic modifications in complex disease and outcomes are derived from research into type 1 diabetes, pain, and the hypoxic response. These studies provide models for future cohort study design, potential perioperative drug targets, and hypothesis development. Genetic and epigenetic factors combine to alter both gene expression and drug responses at both pharmacokinetic and pharmacodynamic levels. These factors impact on the efficacy and safety of multiple drug classes used in perioperative medicine.

SUMMARY

Enhancing our understanding of the way in which patients as genomic organisms interact with the perioperative environment requires a more sophisticated appreciation of the factors governing gene expression than has been the case to date. Epigenetic mechanisms are sure to play a pivotal role in what is essentially an acquired phenotype.

Carotid chemoreceptor "resetting" revisited

Carotid body (CB) chemoreceptors transduce low arterial O(2) tension into increased action potential activity on the carotid sinus nerves, which contributes to resting ventilatory drive, increased ventilatory drive in response to hypoxia, arousal responses to hypoxia during sleep, upper airway muscle activity, blood pressure control and sympathetic tone. Their sensitivity to O(2) is low in the newborn and increases during the days or weeks after birth to reach adult levels. This postnatal functional maturation of the CB O(2) response has been termed "resetting" and it occurs in every mammalian species studied to date. The O(2) environment appears to play a key role; the fetus develops in a low O(2) environment throughout gestation and initiation of CB "resetting" after birth is modulated by the large increase in arterial oxygen tension occurring at birth. Although numerous studies have reported age-related changes in various components of the O(2) transduction cascade, how the O(2) environment shapes normal CB prenatal development and postnatal "resetting" remains unknown. Viewing CB "resetting" as environment-driven (developmental) phenotypic plasticity raises important mechanistic questions that have received little attention. This review examines what is known (and not known) about mechanisms of CB functional maturation, with a focus on the role of the O(2) environment.

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.

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.

Is enough oxygen too much?

Human cells require O₂ for their energy supply, and critical illness can threaten the efficient delivery of O₂ in accordance with tissue metabolic needs. In the accompanying article, Martin and colleagues point out that hypoxia is a normal and well-tolerated stress during embryonic development. A better understanding of how fetal cells survive these conditions and how adult cells adapt to high altitude exposure may provide insight into how these mechanisms might be engaged in the treatment of hypoxemic patients. They suggest that 'permissive hypoxia' represents a therapeutic possibility. But before we turn down the inspired O₂ levels we should consider the broader effects of hypoxia on tissue repair in critical illness.