High-altitude population neonatal and maternal phenotypes associated with birthweight protection.

BACKGROUND: States which reduce foetal oxygen delivery are associated with impaired intrauterine growth. Hypoxia results when barometric pressure falls with ascent to altitude, and with it the partial pressure of inspired oxygen ('hypobaric hypoxia'). birthweight is reduced when native lowlanders gestate at such high altitude (HA)-an effect mitigated in native (millennia) HA populations. Studying HA populations offer a route to explore the mechanisms by which hypoxia impacts foetal growth. METHODS: Between February 2017 and January 2019, we prospectively studied 316 pregnant women, in Leh, Ladakh (altitude 3524 m, where oxygen partial pressure is reduced by 1/3) and 101 pregnant women living in Delhi (low altitude, 216 m above sea level). RESULTS: Of Ladakhi HA newborns, 14% were small for gestational age (<10th birthweight centile) vs 19% of newborn at low altitude. At HA, increased maternal body mass index, age, and uterine artery (UtA) diameter were positively associated with growth >10th weight centile. CONCLUSIONS: This study showed that Ladakhi offspring birthweight is relatively spared from the expected adverse HA effects. Furthermore, maternal body composition and greater UtA size may be physiological HA adaptations and warrant further study, as they offer potential mechanisms to overcome hypoxia-related growth issues. IMPACT: Reduced foetal oxygen delivery seen in native lowlanders who gestate at HA causes foetal growth restriction-an effect thought to be mitigated in native HA populations. We found that greater maternal body mass and UtA diameter were associated with increased offspring birthweight in a (Ladakh) HA population. This supports a role for them as physiological mediators of adaptation and provides insights into potential mechanisms that may treat hypoxia-related growth issues.

A carbon monoxide 'single breath' method to measure total haemoglobin mass: a feasibility study.

NEW FINDINGS: What is the central question of this study? Is it possible to modify the CO-rebreathing method to acquire reliable measurements of haemoglobin mass in ventilated patients? What is the main finding and its importance? A 'single breath' of CO with a subsequent 30 s breath hold provides almost as exact a measure of haemoglobin mass as the established optimized CO-rebreathing method when applied to healthy subjects. The modified method has now to be checked in ventilated patients before it can be used to quantify the contributions of blood loss and of dilution to the severity of anaemia. ABSTRACT: Anaemia is defined by the concentration of haemoglobin (Hb). However, this value is dependent upon both the total circulating haemoglobin mass (tHb-mass) and the plasma volume (PV) - neither of which is routinely measured. Carbon monoxide (CO)-rebreathing methods have been successfully used to determine both PV and tHb-mass in various populations. However, these methods are not yet suitable for ventilated patients. This study aimed to modify the CO-rebreathing procedure such that a single inhalation of a CO bolus would enable its use in ventilated patients. Eleven healthy volunteers performed four CO-rebreathing tests in a randomized order, inhaling an identical CO volume. In two tests, CO was rebreathed for 2 min (optimized CO rebreathing; oCOR), and in the other two tests, a single inhalation of a CO bolus was conducted with a subsequent breath hold of 15 s (Procnew 15s) or 30 s (Procnew 30s). Subsequently, the CO volume in the exhaled air was continuously determined for 20 min. The amount of CO exhaled after 7 and 20 min was respectively 3.1 ± 0.3 and 5.9 ± 1.1 ml for oCOR, 8.7 ± 3.6 and 12.0 ± 4.4 ml for Procnew 15s and 5.1 ± 2.0 and 8.4 ±2.6 ml for Procnew 30s. tHb-mass was 843 ± 293 g determined by oCOR, 821 ± 288 g determined by Procnew 15s (difference: P < 0.05) and 849 ± 311 g determined by Procnew 30s. Bland-Altman plots demonstrated slightly lower tHb-mass values for Procnew 15s compared with oCOR (-21.8 ± 15.3 g) and similar values for Procnew 30s. In healthy volunteers, a single inhalation of a CO bolus, preferably followed by a 30 s breath hold, can be used to determine tHb-mass. These results must now be validated for ventilated patients.

Metabolic basis to Sherpa altitude adaptation.

The Himalayan Sherpas, a human population of Tibetan descent, are highly adapted to life in the hypobaric hypoxia of high altitude. Mechanisms involving enhanced tissue oxygen delivery in comparison to Lowlander populations have been postulated to play a role in such adaptation. Whether differences in tissue oxygen utilization (i.e., metabolic adaptation) underpin this adaptation is not known, however. We sought to address this issue, applying parallel molecular, biochemical, physiological, and genetic approaches to the study of Sherpas and native Lowlanders, studied before and during exposure to hypobaric hypoxia on a gradual ascent to Mount Everest Base Camp (5,300 m). Compared with Lowlanders, Sherpas demonstrated a lower capacity for fatty acid oxidation in skeletal muscle biopsies, along with enhanced efficiency of oxygen utilization, improved muscle energetics, and protection against oxidative stress. This adaptation appeared to be related, in part, to a putatively advantageous allele for the peroxisome proliferator-activated receptor A (PPARA) gene, which was enriched in the Sherpas compared with the Lowlanders. Our findings suggest that metabolic adaptations underpin human evolution to life at high altitude, and could have an impact upon our understanding of human diseases in which hypoxia is a feature.

Metabolic phenotype of skeletal muscle in early critical illness.

OBJECTIVES: To characterise the sketetal muscle metabolic phenotype during early critical illness. METHODS: Vastus lateralis muscle biopsies and serum samples (days 1 and 7) were obtained from 63 intensive care patients (59% male, 54.7±18.0 years, Acute Physiology and Chronic Health Evaluation II score 23.5±6.5). MEASUREMENTS AND MAIN RESULTS: From day 1 to 7, there was a reduction in mitochondrial beta-oxidation enzyme concentrations, mitochondrial biogenesis markers (PGC1α messenger mRNA expression (-27.4CN (95% CI -123.9 to 14.3); n=23; p=0.025) and mitochondrial DNA copy number (-1859CN (IQR -5557-1325); n=35; p=0.032). Intramuscular ATP content was reduced compared tocompared with controls on day 1 (17.7mmol/kg /dry weight (dw) (95% CI 15.3 to 20.0) vs. 21.7 mmol/kg /dw (95% CI 20.4 to 22.9); p<0.001) and decreased over 7 days (-4.8 mmol/kg dw (IQR -8.0-1.2); n=33; p=0.001). In addition, the ratio of phosphorylated:total AMP-K (the bioenergetic sensor) increased (0.52 (IQR -0.09-2.6); n=31; p<0.001). There was an increase in intramuscular phosphocholine (847.2AU (IQR 232.5-1672); n=15; p=0.022), intramuscular tumour necrosis factor receptor 1 (0.66 µg (IQR -0.44-3.33); n=29; p=0.041) and IL-10 (13.6 ng (IQR 3.4-39.0); n=29; p=0.004). Serum adiponectin (10.3 µg (95% CI 6.8 to 13.7); p<0.001) and ghrelin (16.0 ng/mL (IQR -7-100); p=0.028) increased. Network analysis revealed a close and direct relationship between bioenergetic impairment and reduction in muscle mass and between intramuscular inflammation and impaired anabolic signaling. ATP content and muscle mass were unrelated to lipids delivered. CONCLUSIONS: Decreased mitochondrial biogenesis and dysregulated lipid oxidation contribute to compromised skeletal muscle bioenergetic status. In addition, intramuscular inflammation was associated with impaired anabolic recovery with lipid delivery observed as bioenergetically inert. Future clinical work will focus on these key areas to ameliorate acute skeletal muscle wasting. TRIAL REGISTRATION NUMBER: NCT01106300.

Metabolic adjustment to high-altitude hypoxia: from genetic signals to physiological implications.

Ascent to high altitude is associated with physiological responses that counter the stress of hypobaric hypoxia by increasing oxygen delivery and by altering tissue oxygen utilisation via metabolic modulation. At the cellular level, the transcriptional response to hypoxia is mediated by the hypoxia-inducible factor (HIF) pathway and results in promotion of glycolytic capacity and suppression of oxidative metabolism. In Tibetan highlanders, gene variants encoding components of the HIF pathway have undergone selection and are associated with adaptive phenotypic changes, including suppression of erythropoiesis and increased blood lactate levels. In some highland populations, there has also been a selection of variants in PPARA, encoding peroxisome proliferator-activated receptor alpha (PPARα), a transcriptional regulator of fatty acid metabolism. In one such population, the Sherpas, lower muscle PPARA expression is associated with a decreased capacity for fatty acid oxidation, potentially improving the efficiency of oxygen utilisation. In lowlanders ascending to altitude, a similar suppression of fatty acid oxidation occurs, although the underlying molecular mechanism appears to differ along with the consequences. Unlike lowlanders, Sherpas appear to be protected against oxidative stress and the accumulation of intramuscular lipid intermediates at altitude. Moreover, Sherpas are able to defend muscle ATP and phosphocreatine levels in the face of decreased oxygen delivery, possibly due to suppression of ATP demand pathways. The molecular mechanisms allowing Sherpas to successfully live, work and reproduce at altitude may hold the key to novel therapeutic strategies for the treatment of diseases to which hypoxia is a fundamental contributor.

Mitochondrial uncoupling proteins regulate angiotensin-converting enzyme expression: crosstalk between cellular and endocrine metabolic regulators suggested by RNA interference and genetic studies.

Uncoupling proteins (UCPs) regulate mitochondrial function, and thus cellular metabolism. Angiotensin-converting enzyme (ACE) is the central component of endocrine and local tissue renin-angiotensin systems (RAS), which also regulate diverse aspects of whole-body metabolism and mitochondrial function (partly through altering mitochondrial UCP expression). We show that ACE expression also appears to be regulated by mitochondrial UCPs. In genetic analysis of two unrelated populations (healthy young UK men and Scandinavian diabetic patients) serum ACE (sACE) activity was significantly higher amongst UCP3-55C (rather than T) and UCP2 I (rather than D) allele carriers. RNA interference against UCP2 in human umbilical vein endothelial cells reduced UCP2 mRNA sixfold (P < 0·01) whilst increasing ACE expression within a physiological range (<1·8-fold at 48 h; P < 0·01). Our findings suggest novel hypotheses. Firstly, cellular feedback regulation may occur between UCPs and ACE. Secondly, cellular UCP regulation of sACE suggests a novel means of crosstalk between (and mutual regulation of) cellular and endocrine metabolism. This might partly explain the reduced risk of developing diabetes and metabolic syndrome with RAS antagonists and offer insight into the origins of cardiovascular disease in which UCPs and ACE both play a role.

Hemoglobin concentration, total hemoglobin mass and plasma volume in patients: implications for anemia.

In practice, clinicians generally consider anemia (circulating hemoglobin concentration < 120 g.l-1 in non-pregnant females and < 130 g.l-1 in males) as due to impaired hemoglobin synthesis or increased erythrocyte loss or destruction. Rarely is a rise in plasma volume relative to circulating total hemoglobin mass considered as a cause. But does this matter? We explored this issue in patients, measuring hemoglobin concentration, total hemoglobin mass (optimized carbon monoxide rebreathing method) and thereby calculating plasma volume in healthy volunteers, surgical patients, and those with inflammatory bowel disease, chronic liver disease or heart failure. We studied 109 participants. Hemoglobin mass correlated well with its concentration in the healthy, surgical and inflammatory bowel disease groups (r=0.687-0.871, P<0.001). However, they were poorly related in liver disease (r=0.410, P=0.11) and heart failure patients (r=0.312, P=0.16). Here, hemoglobin mass explained little of the variance in its concentration (adjusted R2=0.109 and 0.052; P=0.11 and 0.16), whilst plasma volume did (R2 change 0.724 and 0.805 in heart and liver disease respectively, P<0.0001). Exemplar patients with identical (normal or raised) total hemoglobin masses were diagnosed as profoundly anemic (or not) depending on differences in plasma volume that had not been measured or even considered as a cause. The traditional inference that anemia generally reflects hemoglobin deficiency may be misleading, potentially resulting in inappropriate tests and therapeutic interventions to address 'hemoglobin deficiency' not 'plasma volume excess'. Measurement of total hemoglobin mass and plasma volume is now simple, cheap and safe, and its more routine use is advocated.

An Exploratory Study of Long-Term Outcome Measures in Critical Illness Survivors: Construct Validity of Physical Activity, Frailty, and Health-Related Quality of Life Measures.

OBJECTIVE: Functional capacity is commonly impaired after critical illness. We sought to clarify the relationship between objective measures of physical activity, self-reported measures of health-related quality of life, and clinician reported global functioning capacity (frailty) in such patients, as well as the impact of prior chronic disease status on these functional outcomes. DESIGN: Prospective outcome study of critical illness survivors. SETTING: Community-based follow-up. PATIENTS: Participants of the Musculoskeletal Ultrasound Study in Critical Care: Longitudinal Evaluation Study (NCT01106300), invasively ventilated for more than 48 hours and on the ICU greater than 7 days. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Physical activity levels (health-related quality of life [36-item short-form health survey] and daily step counts [accelerometry]) were compared to norm-based or healthy control scores, respectively. Controls for frailty (Clinical Frailty Score) were non-morbid, age- and gender-matched to survivors. Ninety-one patients were recruited on ICU admission: 41 were contacted for post-discharge assessment, and data were collected from 30 (14 female; mean age, 55.3 yr [95% CI, 48.3-62.3]; mean post-discharge, 576 d [95% CI, 539-614]). Patients' mean daily step count (5,803; 95% CI, 4,792-6,813) was lower than that in controls (11,735; 95% CI, 10,928-12,542; p < 0.001), and lower in those with preexisting chronic disease than without (2,989 [95% CI, 776-5,201] vs 7,737 [95% CI, 4,907-10,567]; p = 0.013). Physical activity measures (accelerometry, health-related quality of life, and frailty) demonstrated good construct validity across all three tools. Step variability (from SD) was highly correlated with daily steps (r = 0.67; p < 0.01) demonstrating a potential boundary constraint. CONCLUSIONS: Subjective and objective measures of physical activity are all informative in ICU survivors. They are all reduced 18 months post-discharge in ICU survivors, and worse in those with pre-admission chronic disease states. Investigating interventions to improve functional capacity in ICU survivors will require stratification based on the presence of premorbidity.

How wasting is saving: weight loss at altitude might result from an evolutionary adaptation.

At extreme altitude (>5,000 - 5,500 m), sustained hypoxia threatens human function and survival, and is associated with marked involuntary weight loss (cachexia). This seems to be a coordinated response: appetite and protein synthesis are suppressed, and muscle catabolism promoted. We hypothesise that, rather than simply being pathophysiological dysregulation, this cachexia is protective. Ketone bodies, synthesised during relative starvation, protect tissues such as the brain from reduced oxygen availability by mechanisms including the reduced generation of reactive oxygen species, improved mitochondrial efficiency and activation of the ATP-sensitive potassium (KATP ) channel. Amino acids released from skeletal muscle also protect cells from hypoxia, and may interact synergistically with ketones to offer added protection. We thus propose that weight loss in hypoxia is an adaptive response: the amino acids and ketone bodies made available act not only as metabolic substrates, but as metabolic modulators, protecting cells from the hypoxic challenge.

The effect of renin angiotensin system genetic variants in acute pancreatitis.

OBJECTIVES: We sought association of genetic variants in the renin-angiotensin system (RAS) and vitamin D system with acute pancreatitis (AP) development and severity. BACKGROUND: The endocrine RAS is involved in circulatory homeostasis through the pressor action of angiotensin II at its AT1 receptor. However, local RAS regulate growth and inflammation in diverse cells and tissues, and their activity may be suppressed by vitamin D. Intrapancreatic angiotensin II generation has been implicated in the development of AP. METHODS: Five hundred forty-four white patients with AP from 3 countries (United Kingdom, 22; Germany, 136; and The Netherlands 386) and 8487 control subjects (United Kingdom 7833, The Netherlands 717) were genotyped for 8 polymorphisms of the RAS/vitamin D systems, chosen on the basis of likely functionality. RESULTS: The angiotensin-converting enzyme I (rather than D) allele was significantly associated with alcohol-related AP when all cohorts were combined (P = 0.03). The renin rs5707 G (rather than A) allele was associated with AP (P = 0.002), infected necrosis (P = 0.025) and mortality (P = 0.046). CONCLUSIONS: The association of 2 RAS polymorphisms with AP suggests the need for further detailed analysis of the role of RAS/vitamin D in the genesis or severity of AP, particularly given the ready potential for pharmacological manipulation of this system using existing marketed agents. However, further replication studies will be required before any such association is considered robust, particularly given the significant heterogeneity of AP causation and clinical course.

Qualitative Ultrasound in Acute Critical Illness Muscle Wasting.

OBJECTIVES: A rapid and early loss of skeletal muscle mass underlies the physical disability common amongst survivors of critical illness. However, skeletal muscle function depends not only on its quantity but its quality, which may be adversely affected. We set out to characterise the changes in macroscopic muscle echogenicity and fascial characteristics that occur early in critical illness, and to relate these to microscopic histologically defined myofibre necrosis and fascial pathology. DESIGN AND SETTING: Prospective two center observational study. PATIENTS: Thirty subjects comprising a subgroup of patients recruited to the Musculoskeletal Ultrasound in Critical Illness: Longitudinal Evaluation (MUSCLE) study. MEASUREMENTS AND MAIN RESULTS: Comparisons were made between sequential Vastus Lateralis histological specimens and ultrasound assessment of Rectus Femoris echogenicity. Change in muscle echogenicity was greater in patients who developed muscle necrosis (n = 15) than in those who did not (8.2% [95% CI, -5.3 to 21.7] vs -15.0% [95% CI, -28.9 to -1.09]; p = 0.016). The area under receiver operator curve for ultrasound echogenicity's prediction of myofiber necrosis was 0.74 (95% CI, 0.565 to 0.919; p = 0.024) increasing to 0.85 (95% CI, 0.703 to -0.995; p = 0.003) with the removal of those with potential iatrogenic muscle damage. Fasciitis was observed in 18 of 30 biopsies (60%). CONCLUSIONS: Myofiber necrosis and fascial inflammation can be detected noninvasively using ultrasound in the critically ill. Fasciitis precedes and frequently accompanies muscle necrosis. These findings may have functional implications for survivors of critical illness.

A pilot study of change in fracture risk in patients with acute respiratory distress syndrome.

INTRODUCTION: Acute skeletal muscle wasting is a major contributor to post critical illness physical impairment. However, the bone response remains uncharacterized. We prospectively investigated the early changes in bone mineral density (BMD) and fracture risk in critical illness. METHODS: Patients were prospectively recruited ≤24 hours following intensive care unit (ICU) admission to a university teaching or a community hospital (August 2009 to April 2011). All were aged >18 years and expected to be intubated for >48 hours, spend >7 days in critical care and survive ICU admission. Forty-six patients were studied (55.3% male), with a mean age of 54.4 years (95% confidence interval (CI): 49.1 to 59.6) and an APACHE II score of 23.9 (95% CI: 22.4 to 25.5). Calcaneal dual X-ray absorptiometry (DXA) assessment of BMD was performed on day 1 and 10. Increase in fracture risk was calculated from the change in T-score. RESULTS: BMD did not change between day 1 and 10 in the cohort overall (0.434 (95% CI: 0.405 to 0.463) versus 0.425 g/cm(2) (95% CI: 0.399 to 0.450), P = 0.58). Multivariable logistical regression revealed admission corrected calcium (odds ratio (OR): 1.980 (95% CI: 1.089 to 3.609), P = 0.026) and admission PaO2-to-FiO2 ratio (OR: 0.916 (95% CI: 0.833 to 0.998), P = 0.044) to be associated with >2% loss of BMD. Patients with acute respiratory distress syndrome had a greater loss in BMD than those without (-2.81% (95% CI: -5.73 to 0.118%), n = 34 versus 2.40% (95% CI: 0.204 to 4.586%), n = 12, P = 0.029). In the 34 patients with acute respiratory distress syndrome, fracture risk increased by 19.4% (95% CI: 13.9 to 25.0%). CONCLUSIONS: Patients with acute respiratory distress syndrome demonstrated early and rapid bone demineralisation with associated increase in fracture risk.

Genetic signatures reveal high-altitude adaptation in a set of ethiopian populations.

The Tibetan and Andean Plateaus and Ethiopian highlands are the largest regions to have long-term high-altitude residents. Such populations are exposed to lower barometric pressures and hence atmospheric partial pressures of oxygen. Such "hypobaric hypoxia" may limit physical functional capacity, reproductive health, and even survival. As such, selection of genetic variants advantageous to hypoxic adaptation is likely to have occurred. Identifying signatures of such selection is likely to help understanding of hypoxic adaptive processes. Here, we seek evidence of such positive selection using five Ethiopian populations, three of which are from high-altitude areas in Ethiopia. As these populations may have been recipients of Eurasian gene flow, we correct for this admixture. Using single-nucleotide polymorphism genotype data from multiple populations, we find the strongest signal of selection in BHLHE41 (also known as DEC2 or SHARP1). Remarkably, a major role of this gene is regulation of the same hypoxia response pathway on which selection has most strikingly been observed in both Tibetan and Andean populations. Because it is also an important player in the circadian rhythm pathway, BHLHE41 might also provide insights into the mechanisms underlying the recognized impacts of hypoxia on the circadian clock. These results support the view that Ethiopian, Andean, and Tibetan populations living at high altitude have adapted to hypoxia differently, with convergent evolution affecting different genes from the same pathway.

Correction: Impact of maintaining serum potassium concentration ≥ 3.6mEq/L versus ≥ 4.5mEq/L for 120 hours after isolated coronary artery bypass graft surgery on incidence of new onset atrial fibrillation: Protocol for a randomized non-inferiority trial.

[This corrects the article DOI: 10.1371/journal.pone.0296525.].

Impact of maintaining serum potassium concentration ≥ 3.6mEq/L versus ≥ 4.5mEq/L for 120 hours after isolated coronary artery bypass graft surgery on incidence of new onset atrial fibrillation: Protocol for a randomized non-inferiority trial.

BACKGROUND: Atrial Fibrillation After Cardiac Surgery (AFACS) occurs in about one in three patients following Coronary Artery Bypass Grafting (CABG). It is associated with increased short- and long-term morbidity, mortality and costs. To reduce AFACS incidence, efforts are often made to maintain serum potassium in the high-normal range (≥ 4.5mEq/L). However, there is no evidence that this strategy is efficacious. Furthermore, the approach is costly, often unpleasant for patients, and risks causing harm. We describe the protocol of a planned randomized non-inferiority trial to investigate the impact of intervening to maintain serum potassium ≥ 3.6 mEq/L vs ≥ 4.5 mEq/L on incidence of new-onset AFACS after isolated elective CABG. METHODS: Patients undergoing isolated CABG at sites in the UK and Germany will be recruited, randomized 1:1 and stratified by site to protocols maintaining serum potassium at either ≥ 3.6 mEq/L or ≥ 4.5 mEq/L. Participants will not be blind to treatment allocation. The primary endpoint is AFACS, defined as an episode of atrial fibrillation, flutter or tachycardia lasting ≥ 30 seconds until hour 120 after surgery, which is both clinically detected and electrocardiographically confirmed. Assuming a 35% incidence of AFACS in the 'tight control group', and allowing for a 10% loss to follow-up, 1684 participants are required to provide 90% certainty that the upper limit of a one-sided 97.5% confidence interval (CI) will exclude a > 10% difference in favour of tight potassium control. Secondary endpoints include mortality, use of hospital resources and incidence of dysrhythmias not meeting the primary endpoint (detected using continuous heart rhythm monitoring). DISCUSSION: The Tight K Trial will assess whether a protocol to maintain serum potassium ≥ 3.6 mEq/L is non inferior to maintaining serum potassium ≥ 4.5 mEq/L in preventing new-onset AFACS after isolated CABG. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT04053816. Registered on 13 August 2019. Last update 7 January 2021.

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.

Natural selection on EPAS1 (HIF2alpha) associated with low hemoglobin concentration in Tibetan highlanders.

By impairing both function and survival, the severe reduction in oxygen availability associated with high-altitude environments is likely to act as an agent of natural selection. We used genomic and candidate gene approaches to search for evidence of such genetic selection. First, a genome-wide allelic differentiation scan (GWADS) comparing indigenous highlanders of the Tibetan Plateau (3,200-3,500 m) with closely related lowland Han revealed a genome-wide significant divergence across eight SNPs located near EPAS1. This gene encodes the transcription factor HIF2alpha, which stimulates production of red blood cells and thus increases the concentration of hemoglobin in blood. Second, in a separate cohort of Tibetans residing at 4,200 m, we identified 31 EPAS1 SNPs in high linkage disequilibrium that correlated significantly with hemoglobin concentration. The sex-adjusted hemoglobin concentration was, on average, 0.8 g/dL lower in the major allele homozygotes compared with the heterozygotes. These findings were replicated in a third cohort of Tibetans residing at 4,300 m. The alleles associating with lower hemoglobin concentrations were correlated with the signal from the GWADS study and were observed at greatly elevated frequencies in the Tibetan cohorts compared with the Han. High hemoglobin concentrations are a cardinal feature of chronic mountain sickness offering one plausible mechanism for selection. Alternatively, as EPAS1 is pleiotropic in its effects, selection may have operated on some other aspect of the phenotype. Whichever of these explanations is correct, the evidence for genetic selection at the EPAS1 locus from the GWADS study is supported by the replicated studies associating function with the allelic variants.

Acute skeletal muscle wasting in critical illness.

IMPORTANCE: Survivors of critical illness demonstrate skeletal muscle wasting with associated functional impairment. OBJECTIVE: To perform a comprehensive prospective characterization of skeletal muscle wasting, defining the pathogenic roles of altered protein synthesis and breakdown. DESIGN, SETTING, AND PARTICIPANTS: Sixty-three critically ill patients (59% male; mean age: 54.7 years [95% CI, 50.0-59.6 years]) with an Acute Physiology and Chronic Health Evaluation II score of 23.5 (95% CI, 21.9-25.2) were prospectively recruited within 24 hours following intensive care unit (ICU) admission from August 2009 to April 2011 at a university teaching and a community hospital in England. Patients were recruited if older than 18 years and were anticipated to be intubated for longer than 48 hours, to spend more than 7 days in critical care, and to survive ICU stay. MAIN OUTCOMES AND MEASURES: Muscle loss was determined through serial ultrasound measurement of the rectus femoris cross-sectional area (CSA) on days 1, 3, 7, and 10. In a subset of patients, the fiber CSA area was quantified along with the ratio of protein to DNA on days 1 and 7. Histopathological analysis was performed. In addition, muscle protein synthesis, breakdown rates, and respective signaling pathways were characterized. RESULTS: There were significant reductions in the rectus femoris CSA observed at day 10 (−17.7% [95% CI, −25.9% to 8.1%]; P < .001). In the 28 patients assessed by all 3 measurement methods on days 1 and 7, the rectus femoris CSA decreased by 10.3% (95% CI, 6.1% to 14.5%), the fiber CSA by 17.5% (95% CI, 5.8% to 29.3%), and the ratio of protein to DNA by 29.5% (95% CI, 13.4% to 45.6%). Decrease in the rectus femoris CSA was greater in patients who experienced multiorgan failure by day 7 (−15.7%; 95% CI, −27.7% to 11.4%) compared with single organ failure (−3.0%; 95% CI, −5.3% to 2.1%) (P < .001), even by day 3 (−8.7% [95% CI, −59.3% to 50.6%] vs −1.8% [95% CI, −12.3% to 10.5%], respectively; P = .03). Myofiber necrosis occurred in 20 of 37 patients (54.1%). Protein synthesis measured by the muscle protein fractional synthetic rate was depressed in patients on day 1 (0.035%/hour; 95% CI, 0.023% to 0.047%/hour) compared with rates observed in fasted healthy controls (0.039%/hour; 95% CI, 0.029% to 0.048%/hour) (P = .57) and increased by day 7 (0.076% [95% CI, 0.032%-0.120%/hour]; P = .03) to rates associated with fed controls (0.065%/hour [95% CI, 0.049% to 0.080%/hour]; P = .30), independent of nutritional load. Leg protein breakdown remained elevated throughout the study (8.5 [95% CI, 4.7 to 12.3] to 10.6 [95% CI, 6.8 to 14.4] µmol of phenylalanine/min/ideal body weight × 100; P = .40). The pattern of intracellular signaling supported increased breakdown (n = 9, r = −0.83, P = .005) and decreased synthesis (n = 9, r = −0.69, P = .04). CONCLUSIONS AND RELEVANCE: Among these critically ill patients, muscle wasting occurred early and rapidly during the first week of critical illness and was more severe among those with multiorgan failure compared with single organ failure. These findings may provide insights into skeletal muscle wasting in critical illness.

Cardiopulmonary exercise testing before and after intravenous iron in preoperative patients: a prospective clinical study.

BACKGROUND: Anemia is associated with impaired physical performance and adverse perioperative outcomes. Iron-deficiency anemia is increasingly treated with intravenous iron before elective surgery. We explored the relationship between exercise capacity, anemia, and total hemoglobin mass (tHb-mass) and the response to intravenous iron in anemic patients prior to surgery. METHODS: A prospective clinical study was undertaken in patients having routine cardiopulmonary exercise testing (CPET) with a hemoglobin concentration ([Hb]) < 130 g.l-1 and iron deficiency/depletion. Patients underwent CPET and tHb-mass measurements before and a minimum of 14 days after receiving intravenous (i.v.) Ferric derisomaltose (Monofer®) at the baseline visit. Comparative analysis of hematological and CPET variables was performed pre and post-iron treatment. RESULTS: Twenty-six subjects were recruited, of whom 6 withdrew prior to study completion. The remaining 20 (9 [45%] male; mean ± SD age 68 ± 10 years) were assessed 25 ± 7 days between baseline and the final visit. Following i.v. iron, increases were seen in [Hb] (mean ± SD) from 109 ± 14 to 116 ± 12 g l-1 (mean rise 6.4% or 7.3 g l-1, p = < 0.0001, 95% CI 4.5-10.1); tHb-mass from 497 ± 134 to 546 ± 139 g (mean rise 9.3% or 49 g, p = < 0.0001, 95% CI 29.4-69.2). Oxygen consumption at anerobic threshold ([Formula: see text] O2 AT) did not change (9.1 ± 1.7 to 9.8 ± 2.5 ml kg-1 min-1, p = 0.09, 95% CI - 0.13 - 1.3). Peak oxygen consumption ([Formula: see text] O2 peak) increased from 15.2 ± 4.1 to 16 ± 4.4 ml.kg.-1 min-1, p = 0.02, 95% CI 0.2-1.8) and peak work rate increased from 93 [67-112] watts to 96 [68-122] watts (p = 0.02, 95% CI 1.3-10.8). CONCLUSION: Preoperative administration of intravenous iron to iron-deficient/deplete anemic patients is associated with increases in [Hb], tHb-mass, peak oxygen consumption, and peak work rate. Further appropriately powered prospective studies are required to ascertain whether improvements in tHb-mass and performance in turn lead to reductions in perioperative morbidity. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT 033 46213.