Conduit artery structure and function in lowlanders and native highlanders: relationships with oxidative stress and role of sympathoexcitation

Impact of moderate altitude exposure on cardiovascular risk

Acute exposure to extreme altitude increases arterial stiffness by activation of the sympathetic and endothelin system as well as hypoxia-induced reactive oxygen species production. Beyond a certain individual threshold, these physiological adaptations represent a relevant cardiovascular risk factor. In this pilot study we investigated to what extent temporary exposure to moderate altitude, as present during hiking, skiing or in aeroplanes, leads to changes in vascular tone. Pulse wave parameters of 8 healthy individuals were assessed with a BR-102 plus pulse wave analyser (Schiller, Germany) at baseline (521 m) and after 24 h exposure to moderate altitude (2650 m). We identified a significant increase in heart rate (61 vs. 68/min, p = 0.021) as well as changes in central (35.6 vs. 41.4 mmHg, p = 0.024) and peripheral pulse pressure (44.7 vs. 52.6 mmHg, p = 0.006). Amplitudes of forward (21.6 vs. 25.4 mmHg, p = 0.012) and backward pulse waves (15.3 vs. 17.6 mmHg, p = 0.043) were significantly elevated. Pulse wave velocity showed no significant change from 5.8 m/s at baseline to 6.1 m/s at moderate altitude (p = 0.056). We show that temporary exposure to moderate altitude leads to mild changes in vascular tone reflected by pulse pressure and pulse wave amplitude in healthy adults. Although the observed effects were mild in our study, it indicates that adaptation capacity is of crucial importance and any restrictions by disease or simply with the process of ageing demand increased awareness, even in moderate altitude.

The cardiovascular system at high altitude: A bibliometric and visualization analysis

BACKGROUND

When exposed to high-altitude environments, the cardiovascular system undergoes various changes, the performance and mechanisms of which remain controversial.

AIM

To summarize the latest research advancements and hot research points in the cardiovascular system at high altitude by conducting a bibliometric and visualization analysis.

METHODS

The literature was systematically retrieved and filtered using the Web of Science Core Collection of Science Citation Index Expanded. A visualization analysis of the identified publications was conducted employing CiteSpace and VOSviewer.

RESULTS

A total of 1674 publications were included in the study, with an observed annual increase in the number of publications spanning from 1990 to 2022. The United States of America emerged as the predominant contributor, while Universidad Peruana Cayetano Heredia stood out as the institution with the highest publication output. Notably, Jean-Paul Richalet demonstrated the highest productivity among researchers focusing on the cardiovascular system at high altitude. Furthermore, Peter Bärtsch emerged as the author with the highest number of cited articles. Keyword analysis identified hypoxia, exercise, acclimatization, acute and chronic mountain sickness, pulmonary hypertension, metabolism, and echocardiography as the primary research hot research points and emerging directions in the study of the cardiovascular system at high altitude.

CONCLUSION

Over the past 32 years, research on the cardiovascular system in high-altitude regions has been steadily increasing. Future research in this field may focus on areas such as hypoxia adaptation, metabolism, and cardiopulmonary exercise. Strengthening interdisciplinary and multi-team collaborations will facilitate further exploration of the pathophysiological mechanisms underlying cardiovascular changes in high-altitude environments and provide a theoretical basis for standardized disease diagnosis and treatment.

Recent insights into mechanisms of hypoxia-induced vasodilatation in the human brain

The cerebral vasculature manages oxygen delivery by adjusting arterial blood in-flow in the face of reductions in oxygen availability. Hypoxic cerebral vasodilatation, and the associated hypoxic cerebral blood flow reactivity, involve many vascular, erythrocytic and cerebral tissue mechanisms that mediate elevations in cerebral blood flow via micro- and macrovascular dilatation. This contemporary review focuses on in vivo human work - with reference to seminal preclinical work where necessary - on hypoxic cerebrovascular reactivity, particularly where recent advancements have been made. We provide updates with the following information: in humans, hypoxic cerebral vasodilatation is partially mediated via a - likely non-obligatory - combination of: (1) nitric oxide synthases, (2) deoxygenation-coupled S-nitrosothiols, (3) potassium channel-related vascular smooth muscle hyperpolarization, and (4) prostaglandin mechanisms with some contribution from an interrelationship with reactive oxygen species. And finally, we discuss the fact that, due to the engagement of deoxyhaemoglobin-related mechanisms, reductions in O2 content via haemoglobin per se seem to account for ∼50% of that seen with hypoxic cerebral vasodilatation during hypoxaemia. We further highlight the issue that methodological impediments challenge the complete elucidation of hypoxic cerebral reactivity mechanisms in vivo in healthy humans. Future research is needed to confirm recent advancements and to reconcile human and animal findings. Further investigations are also required to extend these findings to address questions of sex-, heredity-, age-, and disease-related differences. The final step is to then ultimately translate understanding of these mechanisms into actionable, targetable pathways for the prevention and treatment of cerebral vascular dysfunction and cerebral hypoxic brain injury.

Combining hypoxia with thermal stimuli in humans: physiological responses and potential sex differences

Increasing prevalence of native lowlanders sojourning to high altitudes (>2,500 m) for recreational, occupational, military, and competitive reasons has generated increased interest in physiological responses to multistressor environments. Exposure to hypoxia poses recognized physiological challenges that are amplified during exercise and further complicated by environments that might include combinations of heat, cold, and high altitude. There is a sparsity of data examining integrated responses in varied combinations of environmental conditions, with even less known about potential sex differences. How this translates into performance, occupational, and health outcomes requires further investigation. Acute hypoxic exposure decreases arterial oxygen saturation, resulting in a reflex hypoxic ventilatory response and sympathoexcitation causing an increase in heart rate, myocardial contractility, and arterial blood pressure, to compensate for the decreased arterial oxygen saturation. Acute altitude exposure impairs exercise performance, for example, reduced time to exhaustion and slower time trials, largely owing to impairments in pulmonary gas exchange and peripheral delivery resulting in reduced V̇o2max. This exacerbates with increasing altitude, as does the risk of developing acute mountain sickness and more serious altitude-related illnesses, but modulation of those risks with additional stressors is unclear. This review aims to summarize and evaluate current literature regarding cardiovascular, autonomic, and thermoregulatory responses to acute hypoxia, and how these may be affected by simultaneous thermal environmental challenges. There is minimal available information regarding sex as a biological variable in integrative responses to hypoxia or multistressor environments; we highlight these areas as current knowledge gaps and the need for future research.

GSI CTA evaluation of the vertebrobasilar artery in normal adults at high altitude

Objective

Vascular geometry is influenced by several factors during its growth and development. Here, we compared the differences in vertebrobasilar geometry among residents of a plateau region at different altitudes and investigated the relationship between vascular geometry and altitude.

Methods

Data of some adults in the plateau region who experienced vertigo and headache as the main symptoms but had no evident abnormalities found during imaging examination were collected. They were divided into three groups based on an altitude gradient: group A (1,800-2,500 masl), group B (2,500-3,500 masl), and group C (over 3,500 masl). They underwent head-neck energy-spectrum computed tomography angiography with a gemstone spectral imaging scanning protocol. The following indices were observed: (1) vertebrobasilar geometric configurations (walking, tuning fork, lambda, and no confluence), (2) vertebral artery (VA) hypoplasia, (3) the bending number of bilateral VA intracranial segment, (4) length and tortuosity of the basilar artery (BA), and (5) anteroposterior (AP)-mid-BA angle, BA-VA angle, lateral-mid-BA angle, and VA-VA angle.

Results

Of the 222 subjects, 84 of them were included in group A, 76 in group B, and 62 in group C. The number of participants in walking, tuning fork, lambda, and no confluence geometries was 93, 71, 50, and 8, respectively. As altitude increased, the tortuosity of the BA also increased (1.05 ± 0.06 vs. 1.06 ± 0.08 vs. 1.10 ± 0.13, P = 0.005), as did the lateral-mid-BA angle (23.18° ± 9.53° vs. 26.05° ± 10.10° vs. 31.07° ± 15.12°, P = 0.007) and the BA-VA angle (32.98° ± 17.85° vs. 34.51° ± 17.96° vs. 41.51° ± 19.22°, P = 0.024). There was a relatively weak positive correlation between the altitude and the tortuosity of the BA (r_s= 0.190, P = 0.005), the lateral-mid-BA angle (r_s= 0.201, P = 0.003), and the BA-VA angle (r_s= 0.183, P = 0.006) which showed a significant difference. Compared with groups A and B, there were more multibending groups and fewer oligo-bending groups in group C (P < 0.001). There was no difference found in the vertebral artery hypoplasia, actual length of the BA, VA-VA angle, and AP-mid-BA angle among the three groups.

Conclusion

As the altitude increased, the tortuosity of the BA and the sagittal angle of the vertebrobasilar arterial system also increased. The increase in altitude can lead to changes in vertebrobasilar geometry.

Impact of Oxygen Supplementation on Brachial Artery Hemodynamics and Vascular Function During Ascent to 5,050 m

Vizcardo-Galindo, Gustavo A., Connor A. Howe, Ryan L. Hoiland, Howard H. Carter, Christopher K. Willie, Philip N. Ainslie, and Joshua C. Tremblay. Impact of oxygen supplementation on brachial artery hemodynamics and vascular function during ascent to 5,050 m. High Alt Med Biol. 24:27-36, 2023.-High-altitude trekking alters upper limb hemodynamics and reduces brachial artery vascular function in lowlanders. Whether these changes are reversible with the removal of hypoxia is unknown. We investigated the impact of 20 minutes of oxygen supplementation (O₂) on brachial artery hemodynamics, reactive hyperemia (RH; microvascular function), and flow-mediated dilation (FMD; endothelial function). Participants (aged 21-42 years) were examined before and with O₂ at 3,440 m (n = 7), 4,371 m (n = 7), and 5,050 m (n = 12) using Duplex ultrasound (days 4, 7, and 10 respectively). At 3,440 m, O₂ decreased brachial artery diameter (-5% ± 5%; p = 0.04), baseline blood flow (-44% ± 15%; p < 0.001), oxygen delivery (-39 ± 16; p < 0.001), and peak RH (-8% ± 8%; p = 0.02), but not RH normalized for baseline blood flow. Elevated FMD (p = 0.04) with O₂ at 3,440 m was attributed to the reduction in baseline diameter. At 5,050 m, a reduction in brachial artery blood flow (-17% ± 22%; p = 0.03), but not oxygen delivery, diameter, RH, or FMD occurred with O₂. These findings suggest that during early trekking at high altitude, O₂ causes vasoconstriction in the upper limb along the arterial tree (conduit and resistance arteries). With incremental high-altitude exposure, O₂ reduces blood flow without compromising oxygen delivery, RH, or FMD, suggesting a differential impact on vascular function modulated by the duration and severity of high-altitude exposure.

Nocturnal hypoxemia, blood pressure, vascular status and chronic mountain sickness in the highest city in the world

INTRODUCTION

Chronic mountain sickness (CMS) is a condition characterized by excessive erythrocytosis in response to chronic hypobaric hypoxia. CMS frequently triggers cardiorespiratory diseases such as pulmonary hypertension and right or left heart failure. Ambient hypoxia might be further amplified night-time by intermittent hypoxia related to sleep-disordered breathing (SDB) so that sleep disturbance may be an important feature of CMS. Our aim was to characterize in a cross-sectional study nocturnal hypoxaemia, SDB, blood pressure (BP), arterial stiffness and carotid intima-media thickness (CIMT) in highlanders living at extreme altitude.

METHODS

Men aged 18 to 55 years were prospectively recruited. Home sleep apnoea test, questionnaires (short-form health survey; Montreal cognitive assessment; Pittsburgh Sleep Questionnaire Index and the Insomnia severity index), 24-h ambulatory BP monitoring, CIMT and arterial stiffness were evaluated in 3 groups: i) Andean lowlanders (sea-level); ii) highlanders living at 3,800 m and iii) highlanders living at 5,100 m. Analyses were conducted in sub-groups according to 1) CMS severity 2) healthy subjects living at the three different altitude.

RESULTS

Ninety-two males were evaluated at their living altitudes. Among the 54 highlanders living at 5,100 m, subjects with CMS showed lower mean nocturnal oxygen saturation (SpO₂), SpO₂ nadir, lower pulse wave velocity and higher nocturnal BP variability than those with no-CMS. Lower nocturnal SpO₂ nadir was associated with higher CMS severity (ß= -0.14, p=.009). Among the 55 healthy subjects, healthy highlanders at 5,100 m were characterized by lower scores on quality of life and sleep quality scales and lower mean SpO₂ compared to lowlanders.

CONCLUSIONS

Lower nocturnal SpO₂ and higher nocturnal BP variability are associated with CMS severity in individuals living permanently at high altitude. The role of lower SpO₂ and higher nocturnal BP variability in the cardiovascular progression of CMS and in the overall prognosis of the disease need to be evaluated in further studies.

Brachial artery responses to acute hypercapnia: The roles of shear stress and adrenergic tone

NEW FINDINGS

What is the central question of this study? What are the contributions of shear stress and adrenergic tone to brachial artery vasodilatation during hypercapnia? What is the main finding and its importance? In healthy young adults, shear-mediated vasodilatation does not occur in the brachial artery during hypercapnia, as elevated α₁-adrenergic activity typically maintains vascular tone and offsets distal vasodilatation controlling flow.

ABSTRACT

We aimed to assess the shear stress dependency of brachial artery (BA) responses to hypercapnia, and the α₁-adrenergic restraint of these responses. We hypothesized that elevated shear stress during hypercapnia would cause BA vasodilatation, but where shear stress was prohibited (via arterial compression), the BA would not vasodilate (study 1); and, in the absence of α₁-adrenergic activity, blood flow, shear stress and BA vasodilatation would increase (study 2). In study 1, 14 healthy adults (7/7 male/female, 27 ± 4 years) underwent bilateral BA duplex ultrasound during hypercapnia (partial pressure of end-tidal carbon dioxide, +10.2 ± 0.3 mmHg above baseline, 12 min) via dynamic end-tidal forcing, and shear stress was reduced in one BA using manual compression (compression vs. control arm). Neither diameter nor blood flow was different between baseline and the last minute of hypercapnia (P = 0.423, P = 0.363, respectively) in either arm. The change values from baseline to the last minute, in diameter (%; P = 0.201), flow (ml/min; P = 0.234) and conductance (ml/min/mmHg; P = 0.503) were not different between arms. In study 2, 12 healthy adults (9/3 male/female, 26 ± 4 years) underwent the same design with and without α₁-adrenergic receptor blockade (prazosin; 0.05 mg/kg) in a placebo-controlled, double-blind and randomized design. BA flow, conductance and shear rate increased during hypercapnia in the prazosin control arm (interaction, P < 0.001), but in neither arm during placebo. Even in the absence of α₁-adrenergic restraint, downstream vasodilatation in the microvasculature during hypercapnia is insufficient to cause shear-mediated vasodilatation in the BA.

Effects of continuous hypoxia on flow-mediated dilation in the cerebral and systemic circulation: on the regulatory significance of shear rate phenotype

Emergent evidence suggests that cyclic intermittent hypoxia increases cerebral arterial shear rate and endothelial function, whereas continuous exposure decreases anterior cerebral oxygen (O₂) delivery. To examine to what extent continuous hypoxia impacts cerebral shear rate, cerebral endothelial function, and consequent cerebral O₂ delivery (CDO₂), eight healthy males were randomly assigned single-blind to 7 h passive exposure to both normoxia (21% O₂) and hypoxia (12% O₂). Blood flow in the brachial and internal carotid arteries were determined using Duplex ultrasound and included the combined assessment of systemic and cerebral endothelium-dependent flow-mediated dilatation. Systemic (brachial artery) flow-mediated dilatation was consistently lower during hypoxia (P = 0.013 vs. normoxia), whereas cerebral flow-mediated dilation remained preserved (P = 0.927 vs. normoxia) despite a reduction in internal carotid artery antegrade shear rate (P = 0.002 vs. normoxia) and CDO₂ (P < 0.001 vs. normoxia). Collectively, these findings indicate that the reduction in CDO₂ appears to be independent of cerebral endothelial function and contrasts with that observed during cyclic intermittent hypoxia, highlighting the regulatory importance of (hypoxia) dose duration and flow/shear rate phenotype.

Haemodynamic Adaptive Mechanisms at High Altitude: Comparison between European Lowlanders and Nepalese Highlanders

Background: Exposure to high altitudes determines several adaptive mechanisms affecting in a complex way the whole cardiovascular, respiratory, endocrine systems because of the hypobaric hypoxic condition. The aim of our study was to evaluate the circulatory adaptive mechanisms at high altitudes, during a scientific expedition in the Himalayas. Methods: Arterial distensibility was assessed measuring carotid-radial and carotid-femoral pulse wave velocity. Tests were carried out at several altitudes, from 1350 to 5050 m above sea level, on 8 lowlander European researchers and 11 highlander Nepalese porters. Results: In Europeans, systolic blood pressure and pulse pressure increased slightly but significantly with altitude (p < 0.05 and p < 0.001, respectively). Norepinephrine showed a significant increase after the lowlanders had spent some time at high altitude (p < 0.001). With increasing altitude, a progressive increase in carotid-radial and carotid-femoral pulse wave velocity values was observed in lowlanders, showing a particularly significant increase (p < 0.001) after staying at high altitude (carotid-radial pulse wave velocity, median value (interquartile range) from 9.2 (7.9−10.0) to 11.2 (10.9−11.8) m/s and carotid-femoral pulse wave velocity from 8.5 (7.9−9.0) to 11.3 (10.9−11.8) m/s). At high altitudes (3400 and 5050 m above sea level), no significant differences were observed between highlanders and lowlanders in hemodynamic parameters (blood pressure, carotid-radial and carotid-femoral pulse wave velocity). Conclusions: The progressive arterial stiffening with altitude observed in European lowlanders could explain the increase in systolic and pulse pressure values observed at high altitudes in this ethnic group. Further studies are needed to evaluate the role of aortic stiffening in the pathogenesis of acute mountain sickness.

EPR spectroscopic evidence of iron-catalysed free radical formation in chronic mountain sickness: Dietary causes and vascular consequences

Chronic mountain sickness (CMS) is a high-altitude (HA) maladaptation syndrome characterised by elevated systemic oxidative-nitrosative stress (OXNOS) due to a free radical-mediated reduction in vascular nitric oxide (NO) bioavailability. To better define underlying mechanisms and vascular consequences, this study compared healthy male lowlanders (80 m, n = 10) against age/sex-matched highlanders born and bred in La Paz, Bolivia (3600 m) with (CMS+, n = 10) and without (CMS-, n = 10) CMS. Cephalic venous blood was assayed using electron paramagnetic resonance spectroscopy and reductive ozone-based chemiluminescence. Nutritional intake was assessed via dietary recall. Systemic vascular function and structure were assessed via flow-mediated dilatation, aortic pulse wave velocity and carotid intima-media thickness using duplex ultrasound and applanation tonometry. Basal systemic OXNOS was permanently elevated in highlanders (P = <0.001 vs. lowlanders) and further exaggerated in CMS+, reflected by increased hydroxyl radical spin adduct formation (P = <0.001 vs. CMS-) subsequent to liberation of free 'catalytic' iron consistent with a Fenton and/or nucleophilic addition mechanism(s). This was accompanied by elevated global protein carbonylation (P = 0.046 vs. CMS-) and corresponding reduction in plasma nitrite (P = <0.001 vs. lowlanders). Dietary intake of vitamins C and E, carotene, magnesium and retinol were lower in highlanders and especially deficient in CMS + due to reduced consumption of fruit and vegetables (P = <0.001 to 0.028 vs. lowlanders/CMS-). Systemic vascular function and structure were also impaired in highlanders (P = <0.001 to 0.040 vs. lowlanders) with more marked dysfunction observed in CMS+ (P = 0.035 to 0.043 vs. CMS-) in direct proportion to systemic OXNOS (r = -0.692 to 0.595, P = <0.001 to 0.045). Collectively, these findings suggest that lifelong exposure to iron-catalysed systemic OXNOS, compounded by a dietary deficiency of antioxidant micronutrients, likely contributes to the systemic vascular complications and increased morbidity/mortality in CMS+. TRIAL REGISTRY: ClinicalTrials.gov; No: NCT01182792; URL: www.clinicaltrials.gov.

GLOBAL REACH 2018: intra-arterial vitamin C improves endothelial-dependent vasodilatory function in humans at high altitude

High altitude-induced hypoxaemia is often associated with peripheral vascular dysfunction. However, the basic mechanism(s) underlying high-altitude vascular impairments remains unclear. This study tested the hypothesis that oxidative stress contributes to the impairments in endothelial function during early acclimatization to high altitude. Ten young healthy lowlanders were tested at sea level (344 m) and following 4-6 days at high altitude (4300 m). Vascular endothelial function was determined using the isolated perfused forearm technique with forearm blood flow (FBF) measured by strain-gauge venous occlusion plethysmography. FBF was quantified in response to acetylcholine (ACh), sodium nitroprusside (SNP) and a co-infusion of ACh with the antioxidant vitamin C (ACh+VitC). The total FBF response to ACh (area under the curve) was ∼30% lower at high altitude than at sea level (P = 0.048). There was no difference in the response to SNP at high altitude (P = 0.860). At sea level, the co-infusion of ACh+VitC had no influence on the FBF dose response (P = 0.268); however, at high altitude ACh+VitC resulted in an average increase in the FBF dose response by ∼20% (P = 0.019). At high altitude, the decreased FBF response to ACh, and the increase in FBF in response to ACh+VitC, were associated with the magnitude of arterial hypoxaemia (R² = 0.60, P = 0.008 and R² = 0.63, P = 0.006, respectively). Collectively, these data support the hypothesis that impairments in vascular endothelial function at high altitude are in part attributable to oxidative stress, a consequence of the magnitude of hypoxaemia. These data extend our basic understanding of vascular (mal)adaptation to high-altitude sojourns, with important implications for understanding the aetiology of high altitude-related vascular dysfunction. KEY POINTS: Vascular dysfunction has been demonstrated in lowlanders at high altitude (>4000 m). However, the extent of impairment and the delineation of contributing mechanisms have remained unclear. Using the gold-standard isolated perfused forearm model, we determined the extent of vasodilatory dysfunction and oxidative stress as a contributing mechanism in healthy lowlanders before and 4-6 days after rapid ascent to 4300 m. The total forearm blood flow response to acetylcholine at high altitude was decreased by ∼30%. Co-infusion of acetylcholine with the antioxidant vitamin C partially restored the total forearm blood flow by ∼20%. The magnitude of forearm blood flow reduction, as well as the impact of oxidative stress, was positively associated with the individual severity of hypoxaemia. These data extend our basic understanding of vascular (mal)adaptation to high-altitude sojourns, with important implications for understanding the aetiology of high altitude-related changes in endothelial-mediated vasodilatory function.

Global Reach 2018: Nitric oxide-mediated cutaneous vasodilation is reduced in chronic, but not acute, hypoxia independently of enzymatic superoxide formation

We tested the hypotheses that 1) cutaneous microvascular function is impaired by acute normobaric and chronic hypobaric hypoxia and 2) that the superoxide free radical (via NADPH oxidase or xanthine oxidase) contributes to this impairment via nitric oxide (NO) scavenging. Local heating-induced cutaneous hyperemia (39 °C) was measured in the forearm of 11 male lowlanders at sea level (SL) and following 14-18 days at high altitude (HA; 4340 m in Cerro de Pasco, Peru), and compared to 11 highlanders residing permanently at this elevation. Cutaneous vascular conductance (CVC; laser-Doppler flux/mean arterial pressure) was not different during 39 °C [control site: 73 (19) vs. 71 (18)%max; P = 0.68] between normoxia and acute normobaric hypoxia (F_IO₂ = 0.125; equivalent to HA), respectively. At HA, CVC was reduced during 39 °C in lowlanders compared to SL [control site: 54 (14) vs. 73 (19)%max; P < 0.01] and was lower in Andean highlanders compared to lowlanders at HA [control site: 50 (24) vs. 54 (14)%max; P = 0.02]. The NO contribution to vasodilation during 39 °C (i.e., effect of NO synthase inhibition) was reduced in lowlanders at HA compared to SL [control site: 41 (11) vs 49 (10)%max; P = 0.04] and in Andean highlanders compared to lowlanders at HA [control site: 32 (21) vs. 41 (11)%max; P = 0.01]. Intradermal administration (cutaneous microdialysis) of the superoxide mimetic Tempol, inhibition of xanthine oxidase (via allopurinol), or NADPH oxidase (via apocynin) had no influence on cutaneous endothelium-dependent dilation during any of the conditions (all main effects of drug P > 0.05). These results suggest that time at HA impairs NO-mediated cutaneous vasodilation independent of enzymatic superoxide formation.

The deleterious effects of acute hypoxia on microvascular and large vessel endothelial function

NEW FINDINGS

What is the central question of this study? The aim was primarily to determine the effect of hypoxia on microvascular function and secondarily whether superior cardiorespiratory fitness is protective against hypoxia-induced impairment in vascular function. What is the main finding and its importance? Hypoxia reduced endothelium-dependent but not endothelium-independent microvascular function. The extent of impairment was twofold higher in the microcirculation compared with the large blood vessels. This study suggests that individuals with superior cardiorespiratory fitness might preserve microvascular function in hypoxia. These findings highlight the sensitivity of the microvascular circulation to hypoxia.

ABSTRACT

Hypoxia is associated with diminished bioavailability of the endothelium-derived vasodilator, nitric oxide (NO). Diminished NO bioavailability can have deleterious effects on endothelial function. The endothelium is a heterogeneous tissue; therefore, a comprehensive assessment of endothelial function is crucial to understand the significance of hypoxia-induced endothelial dysfunction. We hypothesized that acute hypoxia would have a deleterious effect on microvascular and large vessel endothelial function. Twenty-nine healthy adults [24 (SD = 4 ) years of age] completed normoxic and hypoxic [inspired O₂  fraction = 0.209] trials in this double-blinded, counterbalanced crossover study. After 30 min, we assessed the laser Doppler imaging-determined perfusion response to iontophoresis of ACh as a measure of endothelium-dependent microvascular function and iontophoresis of sodium nitroprusside as a measure of endothelium-independent microvascular function. After 60 min, we assessed brachial flow-mediated dilatation as a measure of large vessel endothelial function. Thirty minutes of hypoxia reduced endothelium-dependent microvascular function determined by the perfusion response to ACh (median difference (x̃∆) = -109% {interquartile range: 542.7}, P < 0.05), but not endothelium-independent microvascular function determined by the perfusion response to sodium nitroprusside (x̃∆ = 69% {interquartile range: 453.7}, P = 0.6). In addition, 60 min of hypoxia reduced allometrically scaled flow-mediated dilatation compared with normoxia ( x ¯ Δ = - 1.19 [95% CI = -1.80, -0.58 (Confidence Intervals)]%, P < 0.001). The decrease in microvascular endothelial function was associated with cardiorespiratory fitness (r  = 0.45, P = 0.02). In conclusion, acute exposure to normobaric hypoxia significantly reduced endothelium-dependent vasodilatory capacity in small and large vessels. Collectively, these findings highlight the sensitivity of the microvascular circulation to hypoxic insult, particularly in those with poor cardiorespiratory fitness.

Long-Term High-Altitude Exposure Does Not Increase the Incidence of Atrial Fibrillation Associated with Organic Heart Diseases

Wang, Man, Mengxue Liu, Jia Huang, Dan Fan, Shengzhong Liu, Tao Yu, Keli Huang, Xinchuan Wei, and Qian Lei. Long-term high-altitude exposure does not increase the incidence of atrial fibrillation associated with organic heart diseases. High Alt Med Biol. 00:000-000, 2021.- Background: Atrial fibrillation (AF) is one of the most common arrhythmias and is associated with several complications following cardiac surgery. However, the differences in the incidence of AF associated with organic heart diseases between highland and lowland populations have not been comprehensively studied. Methods: In this retrospective study, a total of 2,316 highland and lowland patients who underwent cardiac surgery between January 2013 and December 2018 in a single center were enrolled. According to the altitude of residence, patients were divided into high-altitude (>1,500 m) and low-altitude (<1,500 m) groups. A propensity score matching analysis was performed to estimate the association of lifetime high-altitude exposure with AF. Results: Among the enrolled patients, 239 (10.9%) were from a high-altitude plateau, while 1,946 (89.1%) were from a low-altitude area. There were statistical differences in age, gender, European System for Cardiac Operative Risk Evaluation, and other factors, between the two groups (p < 0.05). According to the propensity score, 237 patients in the high-altitude group were successfully matched to 237 patients in the low-altitude group without significant difference in baseline data (p > 0.05). Among the matched patients, 125 patients (26.4%) suffered from AF, with 66 (27.8%) in the high-altitude group and 59 (24.9%) in the low-altitude group. The incidence of AF was statistically similar between the two groups and not significantly influenced by long-term high-altitude exposure (odds ratio 1.07; 95% confidence interval 0.71-1.60, p > 0.05). Conclusion: Long-term high-altitude exposure did not significantly increase the occurrence of AF in patients with organic heart diseases. Clinical Trial No. ChiCTR1900028612.

Normoxic low-altitude simulation (at 714 mmHg) improves limb blood perfusion in mice with hindlimb ischemia

Humans have fewer cardiovascular events and improved outcomes after cardiovascular events when living at low and moderate altitudes (<3000 m) above sea level. We have previously shown that low‐altitude simulation using reductions in barometric pressure enhances vasodilation ex vivo in arterial segments and reduces systemic vascular resistance in vivo and can also improve left ventricular function after a myocardial infarction. We hypothesize that low‐altitude simulation could also improve hindlimb ischemia, a model of peripheral artery disease in humans. We performed femoral artery ligation to generate hindlimb ischemia in 3‐month‐old C57BL6 mice. Control group mice (n = 10) recovered at 754 mmHg (control) for 14 days. Treatment group mice (n = 15) were placed in a low‐altitude simulation chamber (at 714 mmHg) to recover from surgery for 3‐hours daily for 14 days. Hindlimb perfusion imaging using a laser Doppler line scanner was performed for all mice prior to the surgery, and then on days 1, 3, 7, and 14 post‐surgery. At 2 weeks, ischemic reserve was significantly higher in the treatment group mice (0.50 ± 0.13 vs. 0.20 ± 0.06; p = 0.01). Treatment mice had higher functional scores and were able to walk better at two weeks. There was approximately three times less HIF1α found via western blotting and a small but statistically significant improvement of lectin perfusion in calf tissue of treatment mice. We conclude that low‐altitude simulation improves blood perfusion in murine hindlimb ischemia. This approach may have therapeutic implications for humans with peripheral artery disease.

A sympathetic view of blood pressure control at high altitude: new insights from microneurographic studies

NEW FINDINGS

What is the topic of the review? Sympathoexcitation and sympathetic control of blood pressure at high altitude. What advances does it highlight? Sustained sympathoexcitation is fundamental to integrative control of blood pressure in humans exposed to chronic hypoxia. The largest gaps in current knowledge are in understanding the complex mechanisms by which central sympathetic outflow is regulated at high altitude.

ABSTRACT

High altitude (HA) hypoxia is a potent activator of the sympathetic nervous system, eliciting increases in sympathetic vasomotor activity. Microneurographic evidence of HA sympathoexcitation dates back to the late 20th century, yet only recently have the characteristics and underpinning mechanisms been explored in detail. This review summarises recent findings and highlights the importance of HA sympathoexcitation for the regulation of blood pressure in lowlanders and indigenous highlanders. In addition, this review identifies gaps in our knowledge and corresponding avenues for future study.

Endothelial function and shear stress in hypobaric hypoxia: time course and impact of plasma volume expansion in men

High-altitude exposure typically reduces endothelial function, and this is modulated by hemoconcentration resulting from plasma volume contraction. However, the specific impact of hypobaric hypoxia independent of external factors (e.g., cold, varying altitudes, exercise, diet, and dehydration) on endothelial function is unknown. We examined the temporal changes in blood viscosity, shear stress, and endothelial function and the impact of plasma volume expansion (PVX) during exposure to hypobaric hypoxia while controlling for external factors. Eleven healthy men (25 ± 4 yr, mean ± SD) completed two 4-day chamber visits [normoxia (NX) and hypobaric hypoxia (HH; equivalent altitude, 3,500 m)] in a crossover design. Endothelial function was assessed via flow-mediated dilation in response to transient (reactive hyperemia; RH-FMD) and sustained (progressive handgrip exercise; SS-FMD) increases in shear stress before entering and after 1, 6, 12, 48, and 96 h in the chamber. During HH, endothelial function was also measured on the last day after PVX to preexposure levels (1,140 ± 320 mL balanced crystalloid solution). Blood viscosity and arterial shear stress increased on the first day during HH compared with NX and remained elevated at 48 and 96 h (P < 0.005). RH-FMD did not differ during HH compared with NX and was unaffected by PVX despite reductions in blood viscosity (P < 0.05). The stimulus-response slope of increases in shear stress to vasodilation during SS-FMD was preserved in HH and increased by 44 ± 73% following PVX (P = 0.023). These findings suggest that endothelial function is maintained in HH when other stressors are absent and that PVX improves endothelial function in a shear-stress stimulus-specific manner.NEW & NOTEWORTHY Using a normoxic crossover study design, we examined the impact of hypobaric hypoxia (4 days; altitude equivalent, 3,500 m) and hemoconcentration on blood viscosity, shear stress, and endothelial function. Blood viscosity increased during the hypoxic exposure and was accompanied by elevated resting and exercising arterial shear stress. Flow-mediated dilation stimulated by reactive hyperemia and handgrip exercise was preserved throughout the hypoxic exposure. Plasma volume expansion reversed the hypoxia-associated hemoconcentration and selectively increased handgrip exercise flow-mediated dilation.

Are the tribal highlanders protected from hypertension? A meta-analysis on prevalence of hypertension among high altitude tribal population of India

Background

The so-called protected tribal population are also facing burden of noncommunicable diseases. The high altitude tribes are thought to be genetically and environmentally protected from hypertension (HTN) like diseases.

Objective

The objective is to estimate the prevalence of HTN among tribes residing at high altitudes (>6000 feet above sea level) of India.

Methods

The meta-analysis was undertaken during March to August 2018. National Library of Medicine's PubMed database, and Google scholar were comprehensively searched including search terms such as "blood pressure, hypertension and prevalence" combined with "tribal, tribes, high altitude, India." Articles on the prevalence of HTN were searched first and then were segregated on the basis of high altitude (>6000 ft).

Results

Initially, 69 references and a total of 16 abstracts were screened. After applying the inclusion and exclusion criteria on 16, eight studies were included. Meta-analysis of the prevalence of selected studies resulted in a pooled estimate mean prevalence of HTN among tribal population to be 11.43% (95% confidence interval: 6.72%-17.21%).

Conclusion

The role of epidemiological transition needs to be read in the context of social anthropology to identify factors preventing HTN among high altitude tribes.

Influence of altitude on hypertension phenotypes and responses to antihypertensive therapy: Review of the literature and design of the INTERVENCION trial

Systemic arterial hypertension constitutes the leading cause of mortality worldwide, and affects people living at different altitudes above sea level (AASL). AASL has a major impact on cardiovascular function and various biologic pathways that regulate blood pressure-related phenotypes, but whether it affects the clinical response to antihypertensive therapy is unknown. The hemodynamic adaptations observed among lowlanders acutely exposed to high altitude (HA) is distinct from those observed among HA dwellers. However, the phenotypic patterns of hypertension and the response to standard antihypertensive agents among adults chronically exposed to different AASL are poorly understood. The authors describe the protocol for the INTERVENCION trial, a randomized clinical trial designed to assess the effects of three first-line antihypertensive monotherapies (a thiazide diuretic, an angiotensin receptor blocker, and a calcium channel blocker) on peripheral and central blood pressure, in-office blood pressure, and ambulatory blood pressure hemodynamics of hypertensive patients living at different AASL (low altitude, intermediate altitude, and high altitude). The primary end point is the reduction in 24-hour brachial systolic blood pressure. The INTERVENCION trial will provide the first clinical trial data regarding the influence of AASL on the response to antihypertensive monotherapy, as well as the hemodynamic characteristics of arterial hypertension at different AASL.

Global REACH 2018: The carotid artery diameter response to the cold pressor test is governed by arterial blood pressure during normoxic but not hypoxic conditions in healthy lowlanders and Andean highlanders

NEW FINDINGS

What is the central question of this study? What is the impact of oxygen on the circulatory responses to an isocapnic cold pressor test (CPT) in lowlanders and Andean highlanders? What is the main finding and its importance? Overall, the circulatory responses to an isocapnic CPT were largely unaltered with acute normobaric hypoxia and chronic hypobaric hypoxia exposure in lowlanders. However, the relationship between mean arterial pressure and common carotid artery diameter was dampened in hypoxic conditions. Furthermore, there were no differences in the circulatory responses to the CPT between lowlanders and Andean highlanders with lifelong exposure to high altitude.

ABSTRACT

The impact of oxygen on the circulatory responses to a cold pressor test (CPT) in lowlanders and Andean highlanders remains unknown. Our hypotheses were as follows: (i) in lowlanders, acute normobaric and hypobaric hypoxia would attenuate the common carotid artery (CCA) diameter response to the CPT compared with normobaric normoxia; (ii) Andean highlanders would exhibit a greater CCA diameter response compared with lowlanders; and (iii) a positive relationship between CCA diameter and blood pressure in response to the CPT would be present in both lowlanders and highlanders. Healthy lowlanders (n = 13) and Andean highlanders (n = 8) were recruited and conducted an isocapnic CPT, which consisted of a 3 min foot immersion into water at 0-1°C. Blood pressure (finger photoplethysmography) and CCA diameter and blood flow (Duplex ultrasound) were recorded continuously. The CPT was conducted in lowlanders at sea level in isocapnic normoxic and hypoxic conditions and after 10 days of acclimatization to 4300 m (Cerro de Pasco, Peru) in hypoxic and hyperoxic conditions. Andean highlanders were tested at rest at high altitude. The main findings were as follows: (i) in lowlanders, normobaric but not hypobaric hypoxia elevated CCA reactivity to the CPT; (ii) no differences in response to the CPT were observed between lowlanders and highlanders; and (iii) although hypobaric hypoxaemia reduced the relationship between CCA diameter and blood pressure compared with normobaric normoxia (P = 0.132), hypobaric hyperoxia improved this relationship (P = 0.012), and no relationship was observed in Andean highlanders (P = 0.261). These data demonstrate that the circulatory responses to a CPT were modified by oxygen in lowlanders, but were unaltered with lifelong hypoxic exposure.

Hematocrit, hemoglobin and red blood cells are associated with vascular function and vascular structure in men

High and low hematocrit (Hct) and hemoglobin (Hb) levels are associated with the risk of cardiovascular disease. The purpose of this study was to determine the relationships of Hct, Hb and red blood cells (RBCs) with vascular function and structure. We measured flow-mediated vasodilation (FMD), nitroglycerin-induced vasodilation (NID), brachial intima media thickness (IMT), and brachial-ankle pulse wave velocity (baPWV) in 807 men. The subjects were divided into six groups according to the levels of Hct, Hb and RBCs. NID was highest in the 46.0–48.9% Hct group among the six groups according to Hct levels. Brachial IMT was lowest in the 46.0–48.9% Hct group among the six groups. There were no significant differences in FMD and baPWV among the six groups. We used 46.0–48.9% Hct as a reference to define the lower tertile. The adjusted odds ratio of being in the low tertile of NID was significantly higher in the < 42.9% and ≥ 49.0% Hct groups. Adjusted odds ratio of being in the low tertile of brachial IMT was significantly lower in the < 39.9% Hct groups. Similar results were obtained for Hb and RBCs. Low and high levels of Hct, Hb and RBCs were associated with vascular smooth muscle dysfunction, and low Hct levels were associated with abnormal vascular structure. Increases in the levels of Hct, Hb and RBCs within normal ranges may have beneficial effects on the vasculature.

The 2018 Global Research Expedition on Altitude Related Chronic Health (Global REACH) to Cerro de Pasco, Peru: an Experimental Overview

NEW FINDINGS

What is the central question of this study? Herein, a methodological overview of our research team's (Global REACH) latest high altitude research expedition to Peru is provided. What is the main finding and its importance? The experimental objectives, expedition organization, measurements and key cohort data are discussed. The select data presented in this manuscript demonstrate the haematological differences between lowlanders and Andeans with and without excessive erythrocytosis. The data also demonstrate that exercise capacity was similar between study groups at high altitude. The forthcoming findings from our research expedition will contribute to our understanding of lowlander and indigenous highlander high altitude adaptation.

ABSTRACT

In 2016, the international research team Global Research Expedition on Altitude Related Chronic Health (Global REACH) was established and executed a high altitude research expedition to Nepal. The team consists of ∼45 students, principal investigators and physicians with the common objective of conducting experiments focused on high altitude adaptation in lowlanders and in highlanders with lifelong exposure to high altitude. In 2018, Global REACH travelled to Peru, where we performed a series of experiments in the Andean highlanders. The experimental objectives, organization and characteristics, and key cohort data from Global REACH's latest research expedition are outlined herein. Fifteen major studies are described that aimed to elucidate the physiological differences in high altitude acclimatization between lowlanders (n = 30) and Andean-born highlanders with (n = 22) and without (n = 45) excessive erythrocytosis. After baseline testing in Kelowna, BC, Canada (344 m), Global REACH travelled to Lima, Peru (∼80 m) and then ascended by automobile to Cerro de Pasco, Peru (∼4300 m), where experiments were conducted over 25 days. The core studies focused on elucidating the mechanism(s) governing cerebral and peripheral vascular function, cardiopulmonary regulation, exercise performance and autonomic control. Despite encountering serious logistical challenges, each of the proposed studies was completed at both sea level and high altitude, amounting to ∼780 study sessions and >3000 h of experimental testing. Participant demographics and data relating to acid-base balance and exercise capacity are presented. The collective findings will contribute to our understanding of how lowlanders and Andean highlanders have adapted under high altitude stress.

Global Reach 2018 Heightened α-Adrenergic Signaling Impairs Endothelial Function During Chronic Exposure to Hypobaric Hypoxia

RATIONALE

Chronic exposure to hypoxia is associated with elevated sympathetic nervous activity and reduced vascular function in lowlanders, and Andean highlanders suffering from excessive erythrocytosis (EE); however, the mechanistic link between chronically elevated sympathetic nervous activity and hypoxia-induced vascular dysfunction has not been determined.

OBJECTIVE

To determine the impact of heightened sympathetic nervous activity on resistance artery endothelial-dependent dilation (EDD), and endothelial-independent dilation, in lowlanders and Andean highlanders with and without EE.

METHODS AND RESULTS

We tested healthy lowlanders (n=9) at sea level (344 m) and following 14 to 21 days at high altitude (4300 m), and permanent Andean highlanders with (n=6) and without (n=9) EE at high altitude. Vascular function was assessed using intraarterial infusions (3 progressive doses) of acetylcholine (ACh; EDD) and sodium nitroprusside (endothelial-independent dilation) before and after local α+β adrenergic receptor blockade (phentolamine and propranolol). Intraarterial blood pressure, heart rate, and simultaneous brachial artery diameter and blood velocity were recorded at rest and during drug infusion. Changes in forearm vascular conductance were calculated. The main findings were (1) chronic hypoxia reduced EDD in lowlanders (changes in forearm vascular conductance from sea level: ACh1: -52.7±19.6%, ACh2: -25.4±38.7%, ACh3: -35.1±34.7%, all P≤0.02); and in Andeans with EE compared with non-EE (changes in forearm vascular conductance at ACh3: -36.4%, P=0.007). Adrenergic blockade fully restored EDD in lowlanders at high altitude, and normalized EDD between EE and non-EE Andeans. (2) Chronic hypoxia had no effect on endothelial-independent dilation in lowlanders, and no differences were detected between EE and non-EE Andeans; however, EID was increased in the non-EE Andeans after adrenergic blockade (P=0.012), but this effect was not observed in the EE Andeans.

CONCLUSIONS

These data indicate that chronic hypoxia reduces EDD via heightened α-adrenergic signaling in lowlanders and in Andeans with EE. These vascular mechanisms have important implications for understanding the physiological consequences of acute and chronic high altitude adaptation.

The impact of hypoxaemia on vascular function in lowlanders and high altitude indigenous populations

Exposure to hypoxia elicits widespread physiological responses that are critical for successful acclimatization; however, these responses may induce apparent maladaptive consequences. For example, recent studies conducted in both the laboratory and the field (e.g. at high altitude) have demonstrated that endothelial function is reduced in hypoxia. Herein, we review the several proposed mechanism(s) pertaining to the observed reduction in endothelial function in hypoxia including: (i) changes in blood flow patterns (i.e. shear stress), (ii) increased inflammation and production of reactive oxygen species (i.e. oxidative stress), (iii) heightened sympathetic nerve activity, and (iv) increased red blood cell concentration and mass leading to elevated nitric oxide scavenging. Although some of these mechanism(s) have been examined in lowlanders, less in known about endothelial function in indigenous populations that have chronically adapted to environmental hypoxia for millennia (e.g. the Peruvian, Tibetan and Ethiopian highlanders). There is some evidence indicating that healthy Tibetan and Peruvian (i.e. Andean) highlanders have preserved endothelial function at high altitude, but less is known about the Ethiopian highlanders. However, Andean highlanders suffering from chronic mountain sickness, which is characterized by an excessive production of red blood cells, have markedly reduced endothelial function. This review will provide a framework and mechanistic model for vascular endothelial adaptation to hypoxia in lowlanders and highlanders. Elucidating the pathways responsible for vascular adaption/maladaptation to hypoxia has potential clinical implications for disease featuring low oxygen delivery (e.g. heart failure, pulmonary disease). In addition, a greater understanding of vascular function at high altitude will clinically benefit the global estimated 85 million high altitude residents.

Evaluation of forearm vascular resistance during orthostatic stress: Velocity is proportional to flow and size doesn't matter

Background

The upright posture imposes a significant challenge to blood pressure regulation that is compensated through baroreflex-mediated increases in heart rate and vascular resistance. Orthostatic cardiac responses are easily inferred from heart rate, but vascular resistance responses are harder to elucidate. One approach is to determine vascular resistance as arterial pressure/blood flow, where blood flow is inferred from ultrasound-based measurements of brachial blood velocity. This relies on the as yet unvalidated assumption that brachial artery diameter does not change during orthostatic stress, and so velocity is proportional to flow. It is also unknown whether the orthostatic vascular resistance response is related to initial blood vessel diameter.

Methods

We determined beat-to-beat heart rate (ECG), blood pressure (Portapres) and vascular resistance (Doppler ultrasound) during a combined orthostatic stress test (head-upright tilting and lower body negative pressure) continued until presyncope. Participants were 16 men (aged 38.4±2.3 years) who lived permanently at high altitude (4450m).

Results

The supine brachial diameter ranged from 2.9–5.6mm. Brachial diameter did not change during orthostatic stress (supine: 4.19±0.2mm; tilt: 4.20±0.2mm; -20mmHg lower body negative pressure: 4.19±0.2mm, p = 0.811). There was no significant correlation between supine brachial artery diameter and the maximum vascular resistance response (r = 0.323; p = 0.29). Forearm vascular resistance responses evaluated using brachial arterial flow and velocity were strongly correlated (r = 0.989, p<0.00001) and demonstrated high equivalency with minimal bias (-6.34±24.4%).

Discussion

During severe orthostatic stress the diameter of the brachial artery remains constant, supporting use of brachial velocity for accurate continuous non-invasive orthostatic vascular resistance responses. The magnitude of the orthostatic forearm vascular resistance response was unrelated to the baseline brachial arterial diameter, suggesting that upstream vessel size does not matter in the ability to mount a vasoconstrictor response to orthostasis.

Population History and Altitude-Related Adaptation in the Sherpa

The first ascent of Mount Everest by Tenzing Norgay and Sir Edmund Hillary in 1953 brought global attention to the Sherpa people and human performance at altitude. The Sherpa inhabit the Khumbu Valley of Nepal, and are descendants of a population that has resided continuously on the Tibetan plateau for the past ∼25,000 to 40,000 years. The long exposure of the Sherpa to an inhospitable environment has driven genetic selection and produced distinct adaptive phenotypes. This review summarizes the population history of the Sherpa and their physiological and genetic adaptation to hypoxia. Genomic studies have identified robust signals of positive selection across EPAS1, EGLN1, and PPARA, that are associated with hemoglobin levels, which likely protect the Sherpa from altitude sickness. However, the biological underpinnings of other adaptive phenotypes such as birth weight and the increased reproductive success of Sherpa women are unknown. Further studies are required to identify additional signatures of selection and refine existing Sherpa-specific adaptive phenotypes to understand how genetic factors have underpinned adaptation in this population. By correlating known and emerging signals of genetic selection with adaptive phenotypes, we can further reveal hypoxia-related biological mechanisms of adaptation. Ultimately this work could provide valuable information regarding treatments of hypoxia-related illnesses including stroke, heart failure, lung disease and cancer.

Baroreflex control of sympathetic vasomotor activity and resting arterial pressure at high altitude: insight from Lowlanders and Sherpa

KEY POINTS

Hypoxia, a potent activator of the sympathetic nervous system, is known to increase muscle sympathetic nerve activity (MSNA) to the peripheral vasculature of native Lowlanders during sustained high altitude (HA) exposure. We show that the arterial baroreflex control of MSNA functions normally in healthy Lowlanders at HA, and that upward baroreflex resetting permits chronic activation of basal sympathetic vasomotor activity under this condition. The baroreflex MSNA operating point and resting sympathetic vasomotor outflow both are lower for highland Sherpa compared to acclimatizing Lowlanders; these lower levels may represent beneficial hypoxic adaptation in Sherpa. Acute hyperoxia at HA had minimal effect on baroreflex control of MSNA in Lowlanders and Sherpa, raising the possibility that mechanisms other than peripheral chemoreflex activation contribute to vascular sympathetic baroreflex resetting and sympathoexcitation. These findings provide a better understanding of sympathetic nervous system activation and the control of blood pressure during the physiological stress of sustained HA hypoxia.

ABSTRACT

Exposure to high altitude (HA) is characterized by heightened muscle sympathetic neural activity (MSNA); however, the effect on arterial baroreflex control of MSNA is unknown. Furthermore, arterial baroreflex control at HA may be influenced by genotypic and phenotypic differences between lowland and highland natives. Fourteen Lowlanders (12 male) and nine male Sherpa underwent haemodynamic and sympathetic neural assessment at low altitude (Lowlanders, low altitude; 344 m, Sherpa, Kathmandu; 1400 m) and following gradual ascent to 5050 m. Beat-by-beat haemodynamics (photoplethysmography) and MSNA (microneurography) were recorded lying supine. Indices of vascular sympathetic baroreflex function were determined from the relationship of diastolic blood pressure (DBP) and corresponding MSNA at rest (i.e. DBP 'operating pressure' and MSNA 'operating point'), as well as during a modified Oxford baroreflex test (i.e. 'gain'). Operating pressure and gain were unchanged for Lowlanders during HA exposure; however, the operating point was reset upwards (48 ± 16 vs. 22 ± 12 bursts 100 HB^-1 , P = 0.001). Compared to Lowlanders at 5050 m, Sherpa had similar gain and operating pressure, although the operating point was lower (30 ± 13 bursts 100 HB^-1 , P = 0.02); MSNA burst frequency was lower for Sherpa (22 ± 11 vs. 30 ± 9 bursts min^-1 P = 0.03). Breathing 100% oxygen did not alter vascular sympathetic baroreflex function for either group at HA. For Lowlanders, upward baroreflex resetting promotes heightened sympathetic vasoconstrictor activity and maintains blood pressure stability, at least during early HA exposure; mechanisms other than peripheral chemoreflex activation could be involved.  Sherpa adaptation appears to favour a lower sympathetic vasoconstrictor activity compared to Lowlanders for blood pressure homeostasis.

Supraspinal Fatigue and Neural-evoked Responses in Lowlanders and Sherpa at 5050 m

PURPOSE

At high altitude, Lowlanders exhibit exacerbated fatigue and impaired performance. Conversely, Sherpa (native Highlanders) are known for their outstanding performance at altitude. Presently, there are no reports comparing neuromuscular fatigue and its etiology between Lowlanders and Sherpa at altitude.

METHODS

At 5050 m, nine age-matched Lowlanders and Sherpa (31 ± 10 vs 30 ± 12 yr, respectively) completed a 4-min sustained isometric elbow flexion at 25% maximal voluntary contraction (MVC) torque. Mid-minute, stimuli were applied to the motor cortex and brachial plexus to elicit a motor-evoked potential and maximal compound muscle action potential (Mmax), respectively. Supraspinal fatigue was assessed as the reduction in cortical voluntary activation (cVA) from prefatigue to postfatigue. Cerebral hemoglobin concentrations and tissue oxygenation index (TOI) were measured over the prefrontal cortex by near-infrared spectroscopy.

RESULTS

Prefatigue, MVC torque, and cVA were significantly greater for Lowlanders than Sherpa (79.5 ± 3.6 vs 50.1 ± 11.3 N·m, and 95.4% ± 2.7% vs 88.2% ± 6.6%, respectively). With fatigue, MVC torque and cVA declined similarly for both groups (~24%-26% and ~5%-7%, respectively). During the task, motor-evoked potential area increased more and sooner for Lowlanders (1.5 min) than Sherpa (3.5 min). The Mmax area was lower than baseline throughout fatigue for Lowlanders but unchanged for Sherpa. TOI increased earlier for Lowlanders (2 min) than Sherpa (4 min). Total hemoglobin increased only for Lowlanders (2 min). Mmax was lower, whereas TOI and total hemoglobin were higher for Lowlanders than Sherpa during the second half of the protocol.

CONCLUSIONS

Although neither MVC torque loss nor development of supraspinal fatigue was different between groups, neural-evoked responses and cerebral oxygenation indices were less perturbed in Sherpa. This represents an advantage for maintenance of homeostasis, presumably due to bequeathed genotype and long-term altitude adaptations.

UBC-Nepal Expedition: An experimental overview of the 2016 University of British Columbia Scientific Expedition to Nepal Himalaya

The University of British Columbia Nepal Expedition took place over several months in the fall of 2016 and was comprised of an international team of 37 researchers. This paper describes the objectives, study characteristics, organization and management of this expedition, and presents novel blood gas data during acclimatization in both lowlanders and Sherpa. An overview and framework for the forthcoming publications is provided. The expedition conducted 17 major studies with two principal goals—to identify physiological differences in: 1) acclimatization; and 2) responses to sustained high-altitude exposure between lowland natives and people of Tibetan descent. We performed observational cohort studies of human responses to progressive hypobaric hypoxia (during ascent), and to sustained exposure to 5050 m over 3 weeks comparing lowlander adults (n = 30) with Sherpa adults (n = 24). Sherpa were tested both with (n = 12) and without (n = 12) descent to Kathmandu. Data collected from lowlander children (n = 30) in Canada were compared with those collected from Sherpa children (n = 57; 3400–3900m). Studies were conducted in Canada (344m) and the following locations in Nepal: Kathmandu (1400m), Namche Bazaar (3440m), Kunde Hospital (3480m), Pheriche (4371m) and the Ev-K2-CNR Research Pyramid Laboratory (5050m). The core studies focused on the mechanisms of cerebral blood flow regulation, the role of iron in cardiopulmonary regulation, pulmonary pressures, intra-ocular pressures, cardiac function, neuromuscular fatigue and function, blood volume regulation, autonomic control, and micro and macro vascular function. A total of 335 study sessions were conducted over three weeks at 5050m. In addition to an overview of this expedition and arterial blood gas data from Sherpa, suggestions for scientists aiming to perform field-based altitude research are also presented. Together, these findings will contribute to our understanding of human acclimatization and adaptation to the stress of residence at high-altitude.

UBC-Nepal expedition: peripheral fatigue recovers faster in Sherpa than lowlanders at high altitude

KEY POINTS

The reduced oxygen tension of high altitude compromises performance in lowlanders. In this environment, Sherpa display superior performance, but little is known on this issue. Sherpa present unique genotypic and phenotypic characteristics at the muscular level, which may enhance resistance to peripheral fatigue at high altitude compared to lowlanders. We studied the impact of gradual ascent and exposure to high altitude (5050 m) on peripheral fatigue in age-matched lowlanders and Sherpa, using intermittent electrically-evoked contractions of the knee extensors. Peripheral fatigue (force loss) was lower in Sherpa during the first part of the protocol. Post-protocol, the rate of force development and contractile impulse recovered faster in Sherpa than in lowlanders. At any time, indices of muscle oxygenation were not different between groups. Muscle contractile properties in Sherpa, independent of muscle oxygenation, were less perturbed by non-volitional fatigue. Hence, elements within the contractile machinery contribute to the superior physical performance of Sherpa at high altitude.

ABSTRACT

Altitude-related acclimatisation is characterised by marked muscular adaptations. Lowlanders and Sherpa differ in their muscular genotypic and phenotypic characteristics, which may influence peripheral fatigability at altitude. After gradual ascent to 5050 m, 12 lowlanders and 10 age-matched Sherpa (32 ± 10 vs. 31 ± 11 years, respectively) underwent three bouts (separated by 15 s rest) of 75 intermittent electrically-evoked contractions (12 pulses at 15 Hz, 1.6 s between train onsets) of the dominant leg quadriceps, at the intensity which initially evoked 30% of maximal voluntary force. Trains were also delivered at minutes 1, 2 and 3 after the protocol to measure recovery. Tissue oxygenation index (TOI) and total haemoglobin (tHb) were quantified by a near-infrared spectroscopy probe secured over rectus femoris. Superficial femoral artery blood flow was recorded using ultrasonography, and delivery of oxygen was estimated (eDO₂ ). At the end of bout 1, peak force was greater in Sherpa than in lowlanders (91.5% vs. 84.5% baseline, respectively; P < 0.05). Peak rate of force development (pRFD), the first 200 ms of the contractile impulse (CI₂₀₀ ), and half-relaxation time (HRT) recovered faster in Sherpa than in lowlanders (percentage of baseline at 1 min: pRFD: 89% vs. 74%; CI₂₀₀ : 91% vs. 80%; HRT: 113% vs. 123%, respectively; P < 0.05). Vascular measures were pooled for lowlanders and Sherpa as they did not differ during fatigue or recovery (P < 0.05). Mid bout 3, TOI was decreased (90% baseline) whereas tHb was increased (109% baseline). After bout 3, eDO₂ was markedly increased (1266% baseline). The skeletal muscle of Sherpa seemingly favours repeated force production at altitude for similar oxygen delivery compared to lowlanders.

UBC-Nepal expedition: upper and lower limb conduit artery shear stress and flow-mediated dilation on ascent to 5,050 m in lowlanders and Sherpa

The study of conduit artery endothelial adaptation to hypoxia has been restricted to the brachial artery, and comparisons with highlanders have been confounded by differences in altitude exposure, exercise, and unknown levels of blood viscosity. To address these gaps, we tested the hypothesis that lowlanders, but not Sherpa, would demonstrate decreased mean shear stress and increased retrograde shear stress and subsequently reduced flow-mediated dilation (FMD) in the upper and lower limb conduit arteries on ascent to 5,050 m. Healthy lowlanders (means ± SD, n = 22, 28 ± 6 yr) and Sherpa ( n = 12, 34 ± 11 yr) ascended over 10 days, with measurements taken on nontrekking days at 1,400 m (baseline), 3,440 m ( day 4), 4,371 m ( day 7), and 5,050 m ( day 10). Arterial blood gases, blood viscosity, shear stress, and FMD [duplex ultrasound of the brachial and superficial femoral arteries (BA and SFA, respectively)] were acquired at each time point. Ascent decreased mean and increased retrograde shear stress in the upper and lower limb of lowlanders and Sherpa. Although BA FMD decreased in lowlanders from 7.1 ± 3.9% to 3.8 ± 2.8% at 5,050 versus 1,400 m ( P < 0.001), SFA FMD was preserved. In Sherpa, neither BA nor SFA FMD were changed upon ascent to 5,050 m. In lowlanders, the ascent-related exercise may favorably influence endothelial function in the active limb (SFA); selective impairment in FMD in the BA in lowlanders is likely mediated via the low mean or high oscillatory baseline shear stress. In contrast, Sherpa presented protected endothelial function, suggesting a potential vascular aspect of high-altitude acclimatization/adaptation. NEW & NOTEWORTHY Upper and lower limb arterial shear stress and flow-mediated dilation (FMD) were assessed on matched ascent from 1,400 to 5,050 m in lowlanders and Sherpa. A shear stress pattern associated with vascular dysfunction/risk manifested in both limbs of lowlanders and Sherpa. FMD was impaired only in the upper limb of lowlanders. The findings indicate a limb-specific impact of high-altitude trekking on FMD and a vascular basis to acclimatization wherein endothelial function is protected in Sherpa on ascent.

UBC-Nepal Expedition: imposed oscillatory shear stress does not further attenuate flow-mediated dilation during acute and sustained hypoxia

Experimentally induced oscillatory shear stress (OSS) and hypoxia reduce endothelial function in humans. Acute and sustained hypoxia may cause increases in resting OSS; however, whether this influences endothelial susceptibility to further increases in OSS is unknown. Healthy lowlanders ( n = 15, 30 ± 6 yr; means ± SD) participated in three OSS interventions: two interventions at sea level [normoxia and after 20 min of normobaric hypoxia (acute hypoxia, 11% O₂)] and one intervention 5-7 days after a 9-day ascent to 5,050 m (sustained hypoxia). OSS was provoked in the brachial artery using a 30-min distal cuff inflation (75 mmHg). Endothelial function was assessed before and after each intervention by reactive hyperemia flow-mediated dilation (FMD). Shear stress magnitude and patterns were obtained via Duplex ultrasound. Baseline retrograde shear stress and OSS were greater in acute hypoxia versus normoxia ( P < 0.001), and OSS was elevated in sustained hypoxia versus normoxia ( P = 0.011). The intervention further augmented OSS during each condition. Preintervention FMD was decreased by 29 ± 48% in acute hypoxia and by 25 ± 31% in sustained hypoxia compared with normoxia ( P = 0.001 and 0.026); these changes correlated with changes in baseline mean and antegrade shear stress. After the intervention, FMD decreased during normoxia (-41 ± 26%, P < 0.001) and was unaltered during acute or sustained hypoxia. Therefore, a 30-min exposure to OSS reduced FMD during normoxia, a condition with an unchallenged, healthy endothelium; however, imposed OSS did not appear to worsen endothelial function during acute or sustained hypoxia. Exposure to an altered magnitude and pattern of shear stress at baseline in hypoxia may contribute to the insensitivity to further acute augmentation of OSS. NEW & NOTEWORTHY We investigated whether the endothelium remains sensitive to experimental increases in oscillatory shear stress in acute (11% O₂) and sustained (2 wk at 5,050 m) hypoxia. Hypoxia altered baseline shear stress and decreased endothelial function (flow-mediated dilation); however, exposure to experimentally induced oscillatory shear stress only impaired flow-mediated dilation in normoxia.

Disturbed blood flow worsens endothelial dysfunction in moderate-severe chronic obstructive pulmonary disease

The aims of this study were: (1) to test whether oscillatory shear stress further exacerbates endothelial dysfunction in patients with moderate-severe COPD, and (2) to test whether low flow oxygen administration improves endothelial function and is protective against oscillatory shear stress-induced endothelial dysfunction in patients with moderate-severe COPD. In 17 patients and 10 age-matched non-smoking control subjects we examined brachial artery flow-mediated dilation (FMD) and circulating microparticles before and after 20 minutes of experimentally-induced oscillatory shear stress. COPD patients performed this intervention a second time following a 20-minute wash in period of low flow supplemental oxygen to normalize arterial oxygen saturation. COPD patients had ~six-fold greater baseline retrograde shear rate (P < 0.05) and lower FMD (P < 0.05). The oscillatory shear stress intervention induced significant decreases in brachial artery FMD of all groups (P < 0.05). Oscillatory shear stress elevated circulating markers of endothelial cell apoptosis (CD31+/CD41b- microparticles) in COPD patients, but not age-matched controls. Supplemental oxygen administration abrogated the oscillatory shear stress-induced increase in CD31+/CD41b- microparticles, and improved FMD after accounting for the shear stress stimulus. We have demonstrated that acutely disturbed blood flow with increased retrograde shear stress further deteriorates the already impaired endothelial function with attendant endothelial apoptosis in patients with moderate-severe COPD.

Endothelial dysfunction assessment by flow-mediated dilation in a high-altitude population

Introduction

Endothelial function at high altitude has been measured only in populations that are genetically adapted to chronic hypoxia. The objective of this study was to evaluate endothelial dysfunction (ED) in a nongenetically adapted high-altitude population of the Andes mountains, in Huancayo, Peru (3,250 meters above sea level).

Methods

Participants included 61 patients: 28 cases and 33 controls. The cases were subjects with hypertension, diabetes mellitus, obesity, or a history of stroke or coronary artery disease. Flow-mediated vasodilation (FMD) of the brachial artery was measured in the supine position, at noon, after 5 minutes of resting. The brachial artery was identified above the elbow. Its basal diameter was measured during diastole, and FMD was tested after 5 minutes of forearm ischemia. Intima–media complex in the right carotid artery was also determined. An increase in the artery’s baseline diameter <10% indicated a positive test. Endothelium-independent vasodilation was evaluated with sublingual nitrate administration. The intima–media complex in the right carotid artery was also measured.

Results

100% of diabetics had ED; ED was also found in 68.8% of obese individuals, 55% of hypertensive patients, and 46.5% of controls. Age, height, body mass index, and waist diameter were higher in the cases as compared with the controls. A total of 57.9% (n=11) of the cases and 45.2% (n=19) of the controls presented ED. Patients without ED had a mean increase in brachial artery diameter of 23.16%, while in those with ED it was only 3.84%. Individuals with diabetes or hypertension had a greater thickness of the carotid artery intima media layer (1.092 versus 0.664 cm) (p=0.037). A positive test for ED was associated with a greater basal diameter of the brachial artery (4.66±0.62 versus 4.23±0.59 cm) (p=0.02). A total of 7 patients presented paradoxical response, developing posthyperemia vasoconstriction.

Discussion

The proportion of ED was high among controls and among patients with risk factors. Controls showed better FMD profiles than subjects studied in Tibet and the Himalayas.

Discordance of metabolic syndrome and abdominal obesity prevalence according to different criteria in Andean highlanders: A community-based study

AIMS

There is no consensus as to which Metabolic Syndrome (MetS) definition to use for South-American populations. The aim of this study is to compare the prevalence of MetS and abdominal obesity using different criteria in Andean adults aged 40 and older living permanently at high altitude.

METHODS

We conducted a cross-sectional study in Chivay (Andean highlands). 237 participants were included. Anthropometric measurements, glucose and lipid assessments were done in all subjects. Adult Treatment Panel III (ATPIII) and International Diabetes Federation (IDF) criteria for MetS were used. Abdominal obesity prevalence was determined using the American Diabetes Association (ADA), IDF, and PREVENCION Study criteria. Cohen's Kappa coefficient (κ) was analyzed to assess agreement level between different criteria. Multiple regression analyses were performed to find predictors for waist circumference.

RESULTS

MetS was identified in 28.7% (95%CI=23.8-33.5) using ATPIII criteria, and 37.9% (95%CI=32.7-43.0) using IDF criteria, with higher prevalence in women. The κ statistics for agreement between both criteria was 0.775 (95%CI=0.690-0.859). Abdominal obesity prevalence according to ADA, IDF, and PREVENCION criteria was 35.9% (95%CI=29.7-42.0), 75.9% (95%CI=70.5-81.4), and 42.6% (95%CI=36.3-49.0), respectively. Agreement between ADA and PREVENCION criteria was highest (κ=0.859, 95%CI=0.792-0.925). The strongest predictors for higher waist circumference values were triglycerides and BMI in women, and systolic blood pressure, triglycerides, fasting plasma glucose, and HDL-cholesterol in men.

CONCLUSIONS

MetS according to ATP III and IDF criteria was highly prevalent. IDF criteria identified a larger number of subjects with MetS. Different abdominal obesity criteria tended to show variation when applied to our sample population.

Acute hypoxaemia and vascular function in healthy humans

NEW FINDINGS

What is the central question of this study? Endothelium-dependent flow-mediated dilatation (FMD) is impaired during acute (60 min) exposure to moderate hypoxia. We examined whether FMD is impaired to the same degree during exposure to milder hypoxia. Additionally, we assessed whether smooth muscle vasodilatory capacity [glyceryl trinitrate (GTN)-induced dilatation] is impaired during acute hypoxic exposure. What is the main finding and its importance? A graded impairment in FMD and GTN-induced dilatation was evident during acute (≤60 min) exposure to mild and moderate hypoxia. This study is the first to document these graded impairments, and provides rationale to examine the relationship between graded increases in sympathetic nerve activity with hypoxia on FMD and GTN-induced dilatation. Endothelium-dependent flow-mediated dilatation (FMD) and endothelium-independent dilatation [induced with glyceryl trinitrate (GTN)] are impaired at high altitude (5050 m), and FMD is impaired after acute exposure (<60 min) to normobaric hypoxia equivalent to ∼5050 m (inspired oxygen fraction ∼0.11). Whether GTN-induced dilatation is impaired acutely and whether FMD is impaired during milder hypoxia are unknown. Therefore, we assessed brachial FMD at baseline and after 30 min of mild (end-tidal PO2 74 ± 2 mmHg) and moderate (end-tidal PO2 50 ± 3 mmHg) normobaric hypoxia (n = 12) or normoxia (time-control trial; n = 10). We also assessed GTN-induced dilatation after the hypoxic FMD tests and in normoxia on a separate control day (n = 8). Compared with the normoxic baseline, reductions during mild and moderate hypoxic exposure were evident in FMD (mild versus moderate, -1.2 ± 1.1 versus -3.1 ± 1.7%; P = 0.01) and GTN-induced dilatation (-2.1 ± 1.0 versus -4.2 ± 2.0%; P = 0.01); the declines in FMD and GTN-induced dilatation were greater during moderate hypoxia (P < 0.01). When allometrically corrected for baseline diameter and FMD shear rate under the curve, FMD was attenuated in both conditions (mild versus moderate, 0.6 ± 0.9 versus 0.8 ± 0.7%; P ≤ 0.01). After 30 min of normoxic time control, FMD was reduced (-0.6 ± 0.3%; P = 0.02). In summary, there was a graded impairment in FMD during mild and moderate hypoxic exposure, which appears to be influenced by shear patterns and incremental decline in smooth muscle vasodilator capacity (impaired GTN-induced dilatation). Our findings from the normoxic control study suggest the decline in FMD in acute hypoxia also appears to be influenced by 30 min of supine rest/inactivity.

UBC-Nepal Expedition: acute alterations in sympathetic nervous activity do not influence brachial artery endothelial function at sea level and high altitude

Evidence indicates that increases in sympathetic nervous activity (SNA), and acclimatization to high altitude (HA), may reduce endothelial function as assessed by brachial artery flow-mediated dilatation (FMD); however, it is unclear whether such changes in FMD are due to direct vascular constraint, or consequential altered hemodynamics (e.g., shear stress) associated with increased SNA as a consequence of exposure to HA. We hypothesized that 1) at rest, SNA would be elevated and FMD would be reduced at HA compared with sea-level (SL); and 2) at SL and HA, FMD would be reduced when SNA was acutely increased, and elevated when SNA was acutely decreased. Using a novel, randomized experimental design, brachial artery FMD was assessed at SL (344 m) and HA (5,050 m) in 14 participants during mild lower-body negative pressure (LBNP; -10 mmHg) and lower-body positive pressure (LBPP; +10 mmHg). Blood pressure (finger photoplethysmography), heart rate (electrocardiogram), oxygen saturation (pulse oximetry), and brachial artery blood flow and shear rate (Duplex ultrasound) were recorded during LBNP, control, and LBPP trials. Muscle SNA was recorded (via microneurography) in a subset of participants (n = 5). Our findings were 1) at rest, SNA was elevated (P < 0.01), and absolute FMD was reduced (P = 0.024), but relative FMD remained unaltered (P = 0.061), at HA compared with SL; and 2) despite significantly altering SNA with LBNP (+60.3 ± 25.5%) and LBPP (-37.2 ± 12.7%) (P < 0.01), FMD was unaltered at SL (P = 0.448) and HA (P = 0.537). These data indicate that acute and mild changes in SNA do not directly influence brachial artery FMD at SL or HA.NEW & NOTEWORTHY The role of the sympathetic nervous system on endothelial function remains unclear. We used lower-body negative and positive pressure to manipulate sympathetic nervous activity at sea level and high altitude and measured brachial endothelial function via flow-mediated dilation. We found that acutely altering sympathetic nervous activity had no effect on endothelial function.

Measuring high-altitude adaptation

High altitudes (>8,000 ft or 2,500 m) provide an experiment of nature for measuring adaptation and the physiological processes involved. Studies conducted over the past ~25 years in Andeans, Tibetans, and, less often, Ethiopians show varied but distinct O₂ transport traits from those of acclimatized newcomers, providing indirect evidence for genetic adaptation to high altitude. Short-term (acclimatization, developmental) and long-term (genetic) responses to high altitude exhibit a temporal gradient such that, although all influence O₂ content, the latter also improve O₂ delivery and metabolism. Much has been learned concerning the underlying physiological processes, but additional studies are needed on the regulation of blood flow and O₂ utilization. Direct evidence of genetic adaptation comes from single-nucleotide polymorphism (SNP)-based genome scans and whole genome sequencing studies that have identified gene regions acted upon by natural selection. Efforts have begun to understand the connections between the two with Andean studies on the genetic factors raising uterine blood flow, fetal growth, and susceptibility to Chronic Mountain Sickness and Tibetan studies on genes serving to lower hemoglobin and pulmonary arterial pressure. Critical for future studies will be the selection of phenotypes with demonstrable effects on reproductive success, the calculation of actual fitness costs, and greater inclusion of women among the subjects being studied. The well-characterized nature of the O₂ transport system, the presence of multiple long-resident populations, and relevance for understanding hypoxic disorders in all persons underscore the importance of understanding how evolutionary adaptation to high altitude has occurred.NEW & NOTEWORTHY Variation in O₂ transport characteristics among Andean, Tibetan, and, when available, Ethiopian high-altitude residents supports the existence of genetic adaptations that improve the distribution of blood flow to vital organs and the efficiency of O₂ utilization. Genome scans and whole genome sequencing studies implicate a broad range of gene regions. Future studies are needed using phenotypes of clear relevance for reproductive success for determining the mechanisms by which naturally selected genes are acting.

One session of remote ischemic preconditioning does not improve vascular function in acute normobaric and chronic hypobaric hypoxia

NEW FINDINGS

What is the central question of this study? It is suggested that remote ischemic preconditioning (RIPC) might offer protection against ischaemia-reperfusion injuries, but the utility of RIPC in high-altitude settings remains unclear. What is the main finding and its importance? We found that RIPC offers no vascular protection relative to pulmonary artery pressure or peripheral endothelial function during acute, normobaric hypoxia and at high altitude in young, healthy adults. However, peripheral chemosensitivity was heightened 24 h after RIPC at high altitude. Application of repeated short-duration bouts of ischaemia to the limbs, termed remote ischemic preconditioning (RIPC), is a novel technique that might have protective effects on vascular function during hypoxic exposures. In separate parallel-design studies, at sea level (SL; n = 16) and after 8-12 days at high altitude (HA; n = 12; White Mountain, 3800 m), participants underwent either a sham protocol or one session of four bouts of 5 min of dual-thigh-cuff occlusion with 5 min recovery. Brachial artery flow-mediated dilatation (FMD; ultrasound), pulmonary artery systolic pressure (PASP; echocardiography) and internal carotid artery (ICA) flow (ultrasound) were measured at SL in normoxia and isocapnic hypoxia (end-tidal PO2 maintained at 50 mmHg) and during normal breathing at HA. The hypoxic ventilatory response (HVR) was measured at each location. All measures at SL and HA were obtained at baseline (BL) and at 1, 24 and 48 h post-RIPC or sham. At SL, RIPC produced no changes in FMD, PASP, ICA flow, end-tidal gases or HVR in normoxia or hypoxia. At HA, although HVR increased 24 h post-RIPC compared with BL [2.05 ± 1.4 versus 3.21 ± 1.2 l min^-1  (% arterial O₂ saturation)^-1 , P < 0.01], there were no significant differences in FMD, PASP, ICA flow and resting end-tidal gases. Accordingly, a single session of RIPC is insufficient to evoke changes in peripheral, pulmonary and cerebral vascular function in healthy adults. Although chemosensitivity might increase after RIPC at HA, this did not confer any vascular changes. The utility of a single RIPC session seems unremarkable during acute and chronic hypoxia.

Renin–Angiotensin–Aldosterone System Is Not Involved in the Arterial Stiffening Induced by Acute and Prolonged Exposure to High Altitude

This randomized, double-blind, placebo-controlled study was designed to explore the effects of exposure to very high altitude hypoxia on vascular wall properties and to clarify the role of renin–angiotensin–aldosterone system inhibition on these vascular changes. Forty-seven healthy subjects were included in this study: 22 randomized to telmisartan (age, 40.3±10.8 years; 7 women) and 25 to placebo (age, 39.3±9.8 years; 7 women). Tests were performed at sea level, pre- and post-treatment, during acute exposure to 3400 and 5400-m altitude (Mt. Everest Base Camp), and after 2 weeks, at 5400 m. The effects of hypobaric hypoxia on mechanical properties of large arteries were assessed by applanation tonometry, measuring carotid–femoral pulse wave velocity, analyzing arterial pulse waveforms, and evaluating subendocardial oxygen supply/demand index. No differences in hemodynamic changes during acute and prolonged exposure to 5400-m altitude were found between telmisartan and placebo groups. Aortic pulse wave velocity significantly increased with altitude ( P <0.001) from 7.41±1.25 m/s at sea level to 7.70±1.13 m/s at 3400 m and to 8.52±1.59 m/s at arrival at 5400 m ( P <0.0001), remaining elevated during prolonged exposure to this altitude (8.41±1.12 m/s; P <0.0001). Subendocardial oxygen supply/demand index significantly decreased with acute exposure to 3400 m: from 1.72±0.30 m/s at sea level to 1.41±0.27 m/s at 3400 m ( P <0.001), remaining significantly although slightly less reduced after reaching 5400 m (1.52±0.33) and after prolonged exposure to this altitude (1.53±0.25; P <0.001). In conclusion, the acute exposure to hypobaric hypoxia induces aortic stiffening and reduction in subendocardial oxygen supply/demand index. Renin–angiotensin–aldosterone system does not seem to play any significant role in these hemodynamic changes.

Clinical Trial Registration—

URL: https://www.clinicaltrialsregister.eu/ . Unique identifier: 2008-000540-14.

Reduced blood flow through intrapulmonary arteriovenous anastomoses during exercise in lowlanders acclimatizing to high altitude

NEW FINDINGS

What is the central question of this study? The aim was to determine, using the technique of agitated saline contrast echocardiography, whether exercise after 4-7 days at 5050 m would affect blood flow through intrapulmonary arteriovenous anastomoses (Q̇IPAVA) compared with exercise at sea level. What is the main finding and its importance? Despite a significant increase in both cardiac output and pulmonary pressure during exercise at high altitude, there is very little Q̇IPAVA at rest or during exercise after 4-7 days of acclimatization. Mathematical modelling suggests that bubble instability at high altitude is an unlikely explanation for the reduced Q̇IPAVA. Blood flow through intrapulmonary arteriovenous anastomoses (Q̇IPAVA) is elevated during exercise at sea level (SL) and at rest in acute normobaric hypoxia. After high altitude (HA) acclimatization, resting Q̇IPAVA is similar to that at SL, but it is unknown whether this is true during exercise at HA. We reasoned that exercise at HA (5050 m) would exacerbate Q̇IPAVA as a result of heightened pulmonary arterial pressure. Using a supine cycle ergometer, seven healthy adults free from intracardiac shunts underwent an incremental exercise test at SL [25, 50 and 75% of SL peak oxygen consumption (V̇O2 peak )] and at HA (25 and 50% of SL V̇O2 peak ). Echocardiography was used to determine cardiac output (Q̇) and pulmonary artery systolic pressure (PASP), and agitated saline contrast was used to determine Q̇IPAVA (bubble score; 0-5). The principal findings were as follows: (i) Q̇ was similar at SL rest (3.9 ± 0.47 l min^-1 ) compared with HA rest (4.5 ± 0.49 l min^-1 ; P = 0.382), but increased from rest during both SL and HA exercise (P < 0.001); (ii) PASP increased from SL rest (19.2 ± 0.7 mmHg) to HA rest (33.7 ± 2.8 mmHg; P = 0.001) and, compared with SL, PASP was further elevated during HA exercise (P = 0.003); (iii) Q̇IPAVA was increased from SL rest (0) to HA rest (median = 1; P = 0.04) and increased from resting values during SL exercise (P < 0.05), but was unchanged during HA exercise (P = 0.91), despite significant increases in Q̇ and PASP. Theoretical modelling of microbubble dissolution suggests that the lack of Q̇IPAVA in response to exercise at HA is unlikely to be caused by saline contrast instability.

Hypoxia-Induced Oxidative Stress Modulation with Physical Activity

Increased oxidative stress, defined as an imbalance between prooxidants and antioxidants, resulting in molecular damage and disruption of redox signaling, is associated with numerous pathophysiological processes and known to exacerbate chronic diseases. Prolonged systemic hypoxia, induced either by exposure to terrestrial altitude or a reduction in ambient O2 availability is known to elicit oxidative stress and thereby alter redox balance in healthy humans. The redox balance modulation is also highly dependent on the level of physical activity. For example, both high-intensity exercise and inactivity, representing the two ends of the physical activity spectrum, are known to promote oxidative stress. Numerous to-date studies indicate that hypoxia and exercise can exert additive influence upon redox balance alterations. However, recent evidence suggests that moderate physical activity can attenuate altitude/hypoxia-induced oxidative stress during long-term hypoxic exposure. The purpose of this review is to summarize recent findings on hypoxia-related oxidative stress modulation by different activity levels during prolonged hypoxic exposures and examine the potential mechanisms underlying the observed redox balance changes. The paper also explores the applicability of moderate activity as a strategy for attenuating hypoxia-related oxidative stress. Moreover, the potential of such moderate intensity activities used to counteract inactivity-related oxidative stress, often encountered in pathological, elderly and obese populations is also discussed. Finally, future research directions for investigating interactive effects of altitude/hypoxia and exercise on oxidative stress are proposed.

The effect of α1 -adrenergic blockade on post-exercise brachial artery flow-mediated dilatation at sea level and high altitude

KEY POINTS

Our objective was to quantify endothelial function (via brachial artery flow-mediated dilatation) at sea level (344 m) and high altitude (3800 m) at rest and following both maximal exercise and 30 min of moderate-intensity cycling exercise with and without administration of an α₁ -adrenergic blockade. Brachial endothelial function did not differ between sea level and high altitude at rest, nor following maximal exercise. At sea level, endothelial function decreased following 30 min of moderate-intensity exercise, and this decrease was abolished with α₁ -adrenergic blockade. At high altitude, endothelial function did not decrease immediately after 30 min of moderate-intensity exercise, and administration of α₁ -adrenergic blockade resulted in an increase in flow-mediated dilatation. Our data indicate that post-exercise endothelial function is modified at high altitude (i.e. prolonged hypoxaemia). The current study helps to elucidate the physiological mechanisms associated with high-altitude acclimatization, and provides insight into the relationship between sympathetic nervous activity and vascular endothelial function.

ABSTRACT

We examined the hypotheses that (1) at rest, endothelial function would be impaired at high altitude compared to sea level, (2) endothelial function would be reduced to a greater extent at sea level compared to high altitude after maximal exercise, and (3) reductions in endothelial function following moderate-intensity exercise at both sea level and high altitude are mediated via an α₁ -adrenergic pathway. In a double-blinded, counterbalanced, randomized and placebo-controlled design, nine healthy participants performed a maximal-exercise test, and two 30 min sessions of semi-recumbent cycling exercise at 50% peak output following either placebo or α₁ -adrenergic blockade (prazosin; 0.05 mg kg ^-1 ). These experiments were completed at both sea-level (344 m) and high altitude (3800 m). Blood pressure (finger photoplethysmography), heart rate (electrocardiogram), oxygen saturation (pulse oximetry), and brachial artery blood flow and shear rate (ultrasound) were recorded before, during and following exercise. Endothelial function assessed by brachial artery flow-mediated dilatation (FMD) was measured before, immediately following and 60 min after exercise. Our findings were: (1) at rest, FMD remained unchanged between sea level and high altitude (placebo P = 0.287; prazosin: P = 0.110); (2) FMD remained unchanged after maximal exercise at sea level and high altitude (P = 0.244); and (3) the 2.9 ± 0.8% (P = 0.043) reduction in FMD immediately after moderate-intensity exercise at sea level was abolished via α₁ -adrenergic blockade. Conversely, at high altitude, FMD was unaltered following moderate-intensity exercise, and administration of α₁ -adrenergic blockade elevated FMD (P = 0.032). Our results suggest endothelial function is differentially affected by exercise when exposed to hypobaric hypoxia. These findings have implications for understanding the chronic impacts of hypoxaemia on exercise, and the interactions between the α₁ -adrenergic pathway and endothelial function.