Interval hypoxic training improves autonomic cardiovascular and respiratory control in patients with mild chronic obstructive pulmonary disease

Effects of Exercise Training Under Hypoxia Versus Normoxia on Cognitive Function in Clinical and Non-Clinical Populations: A Systematic Review and Meta-analysis

OBJECTIVE

To compare the effects of exercise training under hypoxia versus normoxia on cognitive function in clinical and non-clinical populations.

DATA SOURCES

From inception to June 13th, 2022, a systematic search was performed on PubMed, Web of Science, Embase, Scopus, and Cochrane Central Register of Controlled Trials.

STUDY SELECTION

Randomized controlled trials comparing the effects of exercise under hypoxic vs normoxic on cognition in clinical and non-clinical populations were included. The systematic search generated 14,894 relevant studies, of which 12 were finally included.

DATA EXTRACTION

Two reviewers independently extracted data from included studies. Results were expressed as standardized mean difference (SMD). Each included study was assessed using the Cochrane Risk of Bias 1.0 (RoB1.0) tool. Finally, the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) system was used to rate the certainty of evidence for each outcome.

DATA SYNTHESIS

Overall, 12 studies with a total of 338 participants met the inclusion criteria. The pooled results suggested that hypoxia exercise had a small but not statistically significant positive effect on overall cognitive function (SMD=0.064, 95% confidence interval (CI): -0.156-0.284, P=.567, very low-certainty evidence), when compared with normoxic exercise. Regarding the domain-specific cognitive functions, there was a medium and significant positive effect on memory (SMD=0.594, 95% CI: 0.068 to 1.120, P=.027, very low-certainty evidence), while effects on visuospatial function (SMD=0.490, 95% CI: -0.030 to 1.010, P=.065, very low-certainty evidence), attention (SMD=0.037, 95% CI: -0.340 to 0.414, P=.847, very low-certainty evidence), executive function (SMD=0.096, 95% CI: -0.268 to 0.460, P=.605, very low-certainty evidence), and processing speed (SMD=-0.145, 95% CI: -0.528 to 0.239, P=.459, very low-certainty evidence) were not statistically significant.

CONCLUSIONS

The current pooled results revealed that hypoxic exercise was related to improved cognitive performance. Nevertheless, exercise under hypoxia did not have a significant advantage in cognitive promotion when compared with exercise under normoxia.

Effects of hypoxic compound exercise to promote HIF-1α expression on cardiac pumping function, sleep activity behavior, and exercise capacity in Drosophila

Alteration of HIF-1α expression levels under hypoxic conditions affects the sequence of its downstream target genes thereby producing different effects. In order to investigate whether the effect of hypoxic compound exercise (HE) on HIF-1α expression alters cardiac pumping function, myocardial structure, and exercise capacity, we developed a suitable model of hypoxic exercise using Drosophila, a model organism, and additionally investigated the effect of hypoxic compound exercise on nocturnal sleep and activity behavior. The results showed that hypoxic compound exercise at 6% oxygen concentration for five consecutive days, lasting 1 h per day, significantly improved the cardiac stress resistance of Drosophila. The hypoxic complex exercise promoted the whole-body HIF-1α expression in Drosophila, and improved the jumping ability, climbing ability, moving speed, and moving distance. The expression of HIF-1α in the heart was increased after hypoxic exercise, which made a closer arrangement of myofilaments, an increase in the diameter of cardiac tubules, and an increase in the pumping function of the heart. The hypoxic compound exercise improved the sleep quality of Drosophila by increasing its nocturnal sleep time, the number of deep sleeps, and decreasing its nocturnal awakenings and activities. Therefore, we conclude that hypoxic compound exercise promoted the expression of HIF-1α to enhance the exercise capacity and heart pumping function of Drosophila, and improved the quality of sleep.

Combined low-carbohydrate diet and long-term exercise in hypoxia in type 2 diabetes: A randomized controlled trial protocol to assess glycemic control, cardiovascular risk factors and body composition

Background: Cardiovascular disease is the leading cause of mortality associated with diabetes, which is characterized by chronic hyperglycemia. Low-carbohydrate diet has gained popularity as an intervention in patients with type 2 diabetes mellitus, acting to improve glycemic profile and serum lipids. In its turn, exercise in hypoxia induces specific adaptations, mostly modulated via hypoxia-induced transcription factor signaling cascade, which increases with exposure to altitude, and promotes angiogenesis, glycogen supply, glucose tolerance, and raises GLUT-4 expression. Aim: Given that hyperglycemia decreases HIF-1α and it is better controlled when following a low-carbohydrate diet, this study aims to examine the hypothesis that a combination of both low-carbohydrate diet and chronic exercise in hypoxia in type 2 diabetes mellitus is associated with improved glycemic control and cardiovascular parameters, whose protocol is described. Methods: Patients with type 2 diabetes mellitus (n = 48) will be recruited and randomized into one of the three groups: (a) Control group: Control diet (low-fat and moderate-carbohydrate diet) + exercise in normoxia; (2) exercise in hypoxia group: Control diet + exercise in hypoxia; (3) intervention group: Low-carbohydrate diet (low-carbohydrate and high-fat diet) + exercise in hypoxia. Before and after 8 weeks of interventions, cardiopulmonary tests (Bruce protocol), body composition and blood pressure will be evaluated. Blood samples will be collected to measure hypoxia-induced transcription factor, C-reactive protein, glycemic and lipid profiles. Summary: This will be the first trial to examine the isolated and combined effect of chronic exercise in hypoxia and low-carbohydrate diet in type 2 diabetes mellitus. This trial will help to fill a significant research gap, guide future research and contribute to the combined nutrition and exercise approach to type 2 diabetes mellitus.

Hypoxia Sensing and Responses in Parkinson's Disease

Parkinson's disease (PD) is associated with various deficits in sensing and responding to reductions in oxygen availability (hypoxia). Here we summarize the evidence pointing to a central role of hypoxia in PD, discuss the relation of hypoxia and oxygen dependence with pathological hallmarks of PD, including mitochondrial dysfunction, dopaminergic vulnerability, and alpha-synuclein-related pathology, and highlight the link with cellular and systemic oxygen sensing. We describe cases suggesting that hypoxia may trigger Parkinsonian symptoms but also emphasize that the endogenous systems that protect from hypoxia can be harnessed to protect from PD. Finally, we provide examples of preclinical and clinical research substantiating this potential.

Mechanisms underlying the health benefits of intermittent hypoxia conditioning

Intermittent hypoxia (IH) is commonly associated with pathological conditions, particularly obstructive sleep apnoea. However, IH is also increasingly used to enhance health and performance and is emerging as a potent non-pharmacological intervention against numerous diseases. Whether IH is detrimental or beneficial for health is largely determined by the intensity, duration, number and frequency of the hypoxic exposures and by the specific responses they engender. Adaptive responses to hypoxia protect from future hypoxic or ischaemic insults, improve cellular resilience and functions, and boost mental and physical performance. The cellular and systemic mechanisms producing these benefits are highly complex, and the failure of different components can shift long-term adaptation to maladaptation and the development of pathologies. Rather than discussing in detail the well-characterized individual responses and adaptations to IH, we here aim to summarize and integrate hypoxia-activated mechanisms into a holistic picture of the body's adaptive responses to hypoxia and specifically IH, and demonstrate how these mechanisms might be mobilized for their health benefits while minimizing the risks of hypoxia exposure.

Comparative efficacy of various hypoxic training paradigms on maximal oxygen consumption: A systematic review and network meta-analysis

Background

Enhancement in maximal oxygen consumption (VO2max) induced by hypoxic training is important for both athletes and non-athletes. However, the lack of comparison of multiple paradigms and the exploration of related modulating factors leads to the inability to recommend the optimal regimen in different situations. This study aimed to investigate the efficacy of seven common hypoxic training paradigms on VO2max and associated moderators.

Methods

Electronic (i.e., five databases) and manual searches were performed, and 42 studies involving 1246 healthy adults were included. Pairwise meta-analyses were conducted to compare different hypoxic training paradigms and hypoxic training and control conditions. The Bayesian network meta-analysis model was applied to calculate the standardised mean differences (SMDs) of pre-post VO2max alteration among hypoxic training paradigms in overall, athlete, and non-athlete populations, while meta-regression analyses were employed to explore the relationships between covariates and SMDs.

Results

All seven hypoxic training paradigms were effective to varying degrees, with SMDs ranging from 1.45 to 7.10. Intermittent hypoxia interval training (IHIT) had the highest probability of being the most efficient hypoxic training paradigm in the overall population and athlete subgroup (42%, 44%), whereas intermittent hypoxic training (IHT) was the most promising hypoxic training paradigm among non-athletes (66%). Meta-regression analysis revealed that saturation hours (coefficient, 0.004; P = 0.038; 95% CI [0.0002, 0.0085]) accounted for variations of VO2max improvement induced by IHT.

Conclusion

Efficient hypoxic training paradigms for VO2max gains differed between athletes and non-athletes, with IHIT ranking best for athletes and IHT for non-athletes. The practicability of saturation hours is confirmed with respect to dose-response issues in the future hypoxic training and associated scientific research.

Registration

This study was registered in the PROSPERO international prospective register of systematic reviews (CRD42022333548).

Environmental and behavioral regulation of HIF-mitochondria crosstalk

Reduced oxygen availability (hypoxia) can lead to cell and organ damage. Therefore, aerobic species depend on efficient mechanisms to counteract detrimental consequences of hypoxia. Hypoxia inducible factors (HIFs) and mitochondria are integral components of the cellular response to hypoxia and coordinate both distinct and highly intertwined adaptations. This leads to reduced dependence on oxygen, improved oxygen supply, maintained energy provision by metabolic remodeling and tapping into alternative pathways and increased resilience to hypoxic injuries. On one hand, many pathologies are associated with hypoxia and hypoxia can drive disease progression, for example in many cancer and neurological diseases. But on the other hand, controlled induction of hypoxia responses via HIFs and mitochondria can elicit profound health benefits and increase resilience. To tackle pathological hypoxia conditions or to apply health-promoting hypoxia exposures efficiently, cellular and systemic responses to hypoxia need to be well understood. Here we first summarize the well-established link between HIFs and mitochondria in orchestrating hypoxia-induced adaptations and then outline major environmental and behavioral modulators of their interaction that remain poorly understood.

Oxidative Stress Response Kinetics after 60 Minutes at Different Levels (10% or 15%) of Normobaric Hypoxia Exposure

In this study, the metabolic responses of hypoxic breathing for 1 h to inspired fractions of 10% and 15% oxygen were investigated. To this end, 14 healthy nonsmoking subjects (6 females and 8 males, age: 32.2 ± 13.3 years old (mean ± SD), height: 169.1 ± 9.9 cm, and weight: 61.6 ± 16.2 kg) volunteered for the study. Blood samples were taken before, and at 30 min, 2 h, 8 h, 24 h, and 48 h after a 1 h hypoxic exposure. The level of oxidative stress was evaluated by considering reactive oxygen species (ROS), nitric oxide metabolites (NOx), lipid peroxidation, and immune-inflammation by interleukin-6 (IL-6) and neopterin, while antioxidant systems were observed in terms of the total antioxidant capacity (TAC) and urates. Hypoxia abruptly and rapidly increased ROS, while TAC showed a U-shape pattern, with a nadir between 30 min and 2 h. The regulation of ROS and NOx could be explained by the antioxidant action of uric acid and creatinine. The kinetics of ROS allowed for the stimulation of the immune system translated by an increase in neopterin, IL-6, and NOx. This study provides insights into the mechanisms through which acute hypoxia affects various bodily functions and how the body sets up the protective mechanisms to maintain redox homeostasis in response to oxidative stress.

The association between blood oxygen saturation and baroreflex sensitivity in adults with type 1 diabetes with and without albuminuria

BACKGROUND

Low baroreflex sensitivity is an indicator of early cardiovascular autonomic neuropathy. We explored the association between baroreflex sensivity and blood oxygen saturation (SpO₂) in type 1 diabetes and various degrees of microvascular disease.

METHODS

In this Danish-Finnish cross-sectional multicentre study, baroreflex sensivity and SpO₂ (pulse oximetry) were examined in persons with type 1 diabetes and normoalbuminuria (n = 98), microalbuminuria (n = 28), or macroalbuminuria (n = 43), and in non-diabetic controls (n = 54). Associations and differences between groups were analysed using regression models and adjustment included age, sex, smoking, HbA_1c, blood haemoglobin, urine albumin creatinine ratio, body mass index, and estimated glomerular filtration rate.

RESULTS

In type 1 diabetes, higher baroreflex sensitivity was associated with higher SpO₂ before adjustment (% increase per one % increase in SpO₂ = 20 % (95%CI: 11-30); p < 0.001) and the association remained significant after adjustment (p = 0.02). Baroreflex sensitivity was not different between non-diabetic controls and persons with type 1 diabetes and normoalbuminuria (p = 0.052). Compared with type 1 diabetes and normoalbuminuria, baroreflex sensitivity was lower in micro- (p < 0.001) and macroalbuminuria (p < 0.001). SpO₂ was lower in persons with type 1 diabetes and normoalbuminuria compared with non-diabetic controls (p < 0.01). Within the participants with type 1 diabetes, SpO₂ was not different in micro- or macroalbuminuria compared with normoalbuminuria (p-values > 0.05), but lower in macro-compared with microalbuminuria (p < 0.01).

CONCLUSIONS

Lower baroreflex sensitivity was associated with lower SpO₂ in type 1 diabetes. The present study support the hypothesis that hypoxia could be a therapeutic target in persons with type 1 diabetes.

Effectiveness of Intermittent Hypoxia-Hyperoxia Therapy in Different Pathologies with Possible Metabolic Implications

Intermittent oxygen therapy (IHT), initially used in the hypoxic administration variant, has been shown to be effective in various pathologies studied, from cardiopulmonary to vascular and metabolic pathologies and more. IHT used to prevent and treat various diseases has thus gained more and more attention as the years have passed. The mechanisms underlying the beneficial effects have been investigated at multiple biological levels, from systemic physiological reactions to genomic regulation. In the last decade, a new method of intermittent oxygen therapy has been developed that combines hypoxic and hyperoxic periods. They can be applied both at rest and during physical exercise, hence the specific indications in sports medicine. It has been hypothesized that replacing normoxia with moderate hyperoxia may increase the adaptive response to the intermittent hypoxic stimulus by upregulating reactive oxygen species and hypoxia-inducible genes. This systematic literature review is based on the "Preferred Reporting Items for Systematic Reviews and Meta-Analysis"-"PRISMA"-methodology, the widely internationally accepted method.

Four weeks of repetitive acute hypoxic preconditioning did not alleviate allergen-induced airway dysfunction in rats

Clinical case series suggest beneficial effects of low-dose intermittent hypoxia in asthma. We tested cardiopulmonary effects of repetitive acute hypoxic preconditioning (RAHP) during allergic inflammation. Brown Norway rats were sensitized to house dust mites (HDM) and exposed to 4-week RAHP or normoxia (SHAM), concurrent with weekly HDM or saline (SAL) challenges. We assessed methacholine responses and lung HIF-1α expression at endpoint, and weekly blood pressure (BP). RAHP relative to SHAM: 1) in HDM-challenged rats, showed no protection against HDM-induced airway dysfunction and did not significantly impact BP (week 4 mean BP difference = 10.51 mmHg, p = 0.09) or HIF-1α expression; 2) in SAL-challenged rats, attenuated airway responses to methacholine, reduced BP (week 4 mean BP average difference = -8.72 mmHg, p = 0.04) and amplified HIF-1α expression (p = 0.0086). Four weeks of RAHP did not mitigate the allergen-induced lower airway dysfunction and may detrimentally affect BP. However, it elicited beneficial cardiopulmonary responses in SAL-challenged rats, concurrent with increased HIF-1α expression.

Divergent Ventilatory and Blood Pressure Responses are Evident Following Repeated Daily Exposure to Mild Intermittent Hypoxia in Males with OSA and Hypertension

Introduction: Resting minute ventilation and ventilation during and following hypoxia may be enhanced following daily exposure to mild intermittent hypoxia (MIH). In contrast, resting systolic blood pressure (SBP) is reduced following daily exposure to MIH. However, it is presently unknown if the reduction in resting SBP following daily exposure, is coupled with reduced SBP responses during and after acute exposure to MIH.

Methods: Participants with obstructive sleep apnea (OSA) and hypertension (n = 10) were exposed to twelve 2-min bouts of MIH (oxygen saturation—87%)/day for 15 days. A control group (n = 6) was exposed to a sham protocol during which compressed air (i.e., F_IO₂ = 0.21) was inspired in place of MIH.

Results: The hypoxic ventilatory response (HVR) and hypoxic systolic blood pressure response (HSBP) increased from the first to the last hypoxic episode on the initial (HVR: 0.08 ± 0.02 vs. 0.13 ± 0.02 L/min/mmHg, p = 0.03; HSBP: 0.13 ± 0.04 vs. 0.37 ± 0.06 mmHg/mmHg, p < 0.001) and final (HVR: 0.10 ± 0.01 vs. 0.15 ± 0.03 L/min/mmHg, p = 0.03; HSBP: 0.16 ± 0.03 vs. 0.41 ± 0.34 mmHg/mmHg, p < 0.001) day. The magnitude of the increase was not different between days (p ≥ 0.83). Following exposure to MIH, minute ventilation and SBP was elevated compared to baseline on the initial (MV: 16.70 ± 1.10 vs. 14.20 ± 0.28 L/min, p = 0.01; SBP: 167.26 ± 4.43 vs. 151.13 ± 4.56 mmHg, p < 0.001) and final (MV: 17.90 ± 1.25 vs. 15.40 ± 0.77 L/min, p = 0.01; SBP: 156.24 ± 3.42 vs. 137.18 ± 4.17 mmHg, p < 0.001) day. The magnitude of the increases was similar on both days (MV: 3.68 ± 1.69 vs. 3.22 ± 1.27 L/min, SBP: 14.83 ± 2.64 vs. 14.28 ± 1.66 mmHg, p ≥ 0.414). Despite these similarities, blood pressure at baseline and at other time points during the MIH protocol was reduced on the final compared to the initial day (p ≤ 0.005).

Conclusion: The ventilatory and blood pressure responses during and following acute MIH were similar on the initial and final day of exposure. Alternatively, blood pressure was down regulated, while ventilation was similar at all time points (i.e., baseline, during and following MIH) after daily exposure to MIH.

Effects of Intermittent Hypoxia-Hyperoxia on Performance- and Health-Related Outcomes in Humans: A Systematic Review

BACKGROUND

Intermittent hypoxia applied at rest or in combination with exercise promotes multiple beneficial adaptations with regard to performance and health in humans. It was hypothesized that replacing normoxia by moderate hyperoxia can increase the adaptive response to the intermittent hypoxic stimulus.

OBJECTIVE

Our objective was to systematically review the current state of the literature on the effects of chronic intermittent hypoxia-hyperoxia (IHH) on performance- and health-related outcomes in humans.

METHODS

PubMed, Web of Science™, Scopus, and Cochrane Library databases were searched in accordance with PRISMA guidelines (January 2000 to September 2021) using the following inclusion criteria: (1) original research articles involving humans, (2) investigation of the chronic effect of IHH, (3) inclusion of a control group being not exposed to IHH, and (4) articles published in peer-reviewed journals written in English.

RESULTS

Of 1085 articles initially found, eight studies were included. IHH was solely performed at rest in different populations including geriatric patients (n = 1), older patients with cardiovascular (n = 3) and metabolic disease (n = 2) or cognitive impairment (n = 1), and young athletes with overtraining syndrome (n = 1). The included studies confirmed the beneficial effects of chronic exposure to IHH, showing improvements in exercise tolerance, peak oxygen uptake, and global cognitive functions, as well as lowered blood glucose levels. A trend was discernible that chronic exposure to IHH can trigger a reduction in systolic and diastolic blood pressure. The evidence of whether IHH exerts beneficial effects on blood lipid levels and haematological parameters is currently inconclusive. A meta-analysis was not possible because the reviewed studies had a considerable heterogeneity concerning the investigated populations and outcome parameters.

CONCLUSION

Based on the published literature, it can be suggested that chronic exposure to IHH might be a promising non-pharmacological intervention strategy for improving peak oxygen consumption, exercise tolerance, and cognitive performance as well as reducing blood glucose levels, and systolic and diastolic blood pressure in older patients with cardiovascular and metabolic diseases or cognitive impairment. However, further randomized controlled trials with adequate sample sizes are needed to confirm and extend the evidence. This systematic review was registered on the international prospective register of systematic reviews (PROSPERO-ID: CRD42021281248) ( https://www.crd.york.ac.uk/prospero/ ).

Hypoxic breathing produces more intense hypoxemia in elderly women than in elderly men

Background: Brief hypoxic exposures are increasingly applied as interventions for aging-related conditions. To optimize the therapeutic impact of hypoxia, knowledge of the sex-related differences in physiological responses to hypoxia is essential. This study compared hypoxia-induced hypoxemic responses in elderly men and women. Methods: Seven elderly men (70.3 ± 6.0 years old) and nine women (69.4 ± 5.5 years old) breathed 10% O₂ for 5 min while arterial (SaO₂; transcutaneous photoplethysmography) and cerebral tissue O₂ saturation (ScO₂; near-infrared spectroscopy), ventilatory frequency, tidal volume, minute-ventilation, and partial pressures of end-tidal O₂ (P_ETO₂) and CO₂ (mass spectrometry) were continuously monitored. Cerebral tissue oxygen extraction fraction (OEF) equaled (SaO₂-ScO₂)/SaO₂. Results: During 5 min hypoxia SaO₂ fell from 97.0 ± 0.8% to 80.6 ± 4.6% in the men and from 96.3 ± 1.4% to 72.6 ± 4.0% in the women. The slope ΔSaO₂/min was steeper in the women than the men (-4.71 ± 0.96 vs. -3.24 ± 0.76%/min; p = 0.005). Although SaO₂ fell twice as sharply per unit decrease in P_ETO₂ in the women than the men (-1.13 ± 0.11 vs. -0.54 ± 0.06%/mmHg; p = 0.003), minute-ventilation per unit hypoxemia increased less appreciably in the women (-0.092 ± 0.014 vs. -0.160 ± 0.021 L/min/%; p = 0.023). OEF fell with hypoxia duration in the women, but remained stable in the men. Conclusion: During 5 min hypoxic breathing, elderly women experience more intense hypoxemia and reduced chemoreflex sensitivity vs. their male counterparts, which may lower OEF stability in women despite augmented O₂ dissociation from hemoglobin during hypoxia. These sex-related differences merit attention when implementing brief hypoxic exposures for therapeutic purposes.

Intermittent hypoxia treatment alleviates memory impairment in the 6-month-old APPswe/PS1dE9 mice and reduces amyloid beta accumulation and inflammation in the brain

Background

Alzheimer’s disease (AD) is a progressive, degenerative, and terminal disease without cure. There is an urgent need for a new strategy to treat AD. The aim of this study was to investigate the effects of intermittent hypoxic treatment (IHT) on cognitive functions in a mouse model of AD and unravel the mechanism of action of IHT.

Methods

Six-month-old APPswe/PS1dE9 (APP/PS1) male mice were exposed to hypoxic environment (14.3% O₂) 4 h/day for 14 days or 28 days. Cognitive functions were measured by Morris water maze test after either 14 days or 42 days of interval. Thereafter the distribution of amyloid plaque and microglial activation were determined by mouse brain immunohistochemistry, while the amyloid beta (Aβ) and inflammatory cytokines were measured by ELISA and Western Blot. Microarray was used for studying gene expressions in the hippocampus.

Results

IHT for 14 days or 28 days significantly improved the spatial memory ability of the 6-month-old APP/PS1 mice. The memory improvement by 14 days IHT lasted to 14 days, but not to 42 days. The level of Aβ plaques and neurofilament accumulations was reduced markedly after the IHT exposure. IHT reduced the pro-inflammatory cytokines IL-1β, IL-6 levels, and β-secretase cleavage of APP processing which implies reduced Aβ production. Microarray analysis revealed a large number of genes in the hippocampus were significantly altered which are known to be metabolism-regulated genes.

Conclusions

This study provides evidence of the beneficial effect of IHT on the progression of AD by alleviating memory impairment, reducing Aβ accumulation and inflammation in the brain. IHT can be developed as a novel measure to relieve the progression of AD by targeting multiple pathways in the AD pathogenesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-021-00935-z.

Hypoxia and brain aging: Neurodegeneration or neuroprotection?

The absolute reliance of the mammalian brain on oxygen to generate ATP renders it acutely vulnerable to hypoxia, whether at high altitude or in clinical settings of anemia or pulmonary disease. Hypoxia is pivotal to the pathogeneses of myriad neurological disorders, including Alzheimer's, Parkinson's and other age-related neurodegenerative diseases. Conversely, reduced environmental oxygen, e.g. sojourns or residing at high altitudes, may impart favorable effects on aging and mortality. Moreover, controlled hypoxia exposure may represent a treatment strategy for age-related neurological disorders. This review discusses evidence of hypoxia's beneficial vs. detrimental impacts on the aging brain and the molecular mechanisms that mediate these divergent effects. It draws upon an extensive literature search on the effects of hypoxia/altitude on brain aging, and detailed analysis of all identified studies directly comparing brain responses to hypoxia in young vs. aged humans or rodents. Special attention is directed toward the risks vs. benefits of hypoxia exposure to the elderly, and potential therapeutic applications of hypoxia for neurodegenerative diseases. Finally, important questions for future research are discussed.

A comprehensive review of respiratory, autonomic and cardiovascular responses to intermittent hypoxia in humans

This review explores forms of respiratory and autonomic plasticity, and associated outcome measures, that are initiated by exposure to intermittent hypoxia. The review focuses primarily on studies that have been completed in humans and primarily explores the impact of mild intermittent hypoxia on outcome measures. Studies that have explored two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of ventilation and upper airway muscle activity, are initially reviewed. The role these forms of plasticity might have in sleep disordered breathing are also explored. Thereafter, the role of intermittent hypoxia in the initiation of autonomic plasticity is reviewed and the role this form of plasticity has in cardiovascular and hemodynamic responses during and following intermittent hypoxia is addressed. The role of these responses in individuals with sleep disordered breathing and spinal cord injury are subsequently addressed. Ultimately an integrated picture of the respiratory, autonomic and cardiovascular responses to intermittent hypoxia is presented. The goal of the integrated picture is to address the types of responses that one might expect in humans exposed to one-time and repeated daily exposure to mild intermittent hypoxia. This form of intermittent hypoxia is highlighted because of its potential therapeutic impact in promoting functional improvement and recovery in several physiological systems.

[Into thin air - Altitude training and hypoxic conditioning: From athlete to patient]

INTRODUCTION

Hypoxic exposure should be considered as a continuum, the effects of which depend on the dose and individual response to hypoxia. Hypoxic conditioning (HC) represents an innovative and promising strategy, ranging from improved human performance to therapeutic applications.

STATE OF THE ART

With the aim of improving sports performance, the effectiveness of hypoxic exposure, whether natural or simulated, is difficult to demonstrate because of the large variability of the protocols used. In therapeutics, the benefits of HC are described in many pathological conditions such as obesity or cardiovascular pathologies. If the HC benefits from a strong preclinical rationale, its application to humans remains limited.

PERSPECTIVES

Advances in training and acclimation will require greater personalization and precise periodization of hypoxic exposures. For patients, the harmonization of HC protocols, the identification of biomarkers and the development and subsequent validation of devices allowing a precise control of the hypoxic stimulus are necessary steps for the development of HC.

CONCLUSIONS

From the athlete to the patient, HC represents an innovative and promising field of research, ranging from the improvement of human performance to the prevention and treatment of certain pathologies.

Altitude and COVID-19: Friend or foe? A narrative review

Recent reports suggest that high-altitude residence may be beneficial in the novel coronavirus disease (COVID-19) implicating that traveling to high places or using hypoxic conditioning thus could be favorable as well. Physiological high-altitude characteristics and symptoms of altitude illnesses furthermore seem similar to several pathologies associated with COVID-19. As a consequence, high altitude and hypoxia research and related clinical practices are discussed for potential applications in COVID-19 prevention and treatment. We summarize the currently available evidence on the relationship between altitude/hypoxia conditions and COVID-19 epidemiology and pathophysiology. The potential for treatment strategies used for altitude illnesses is evaluated. Symptomatic overlaps in the pathophysiology of COVID-19 induced ARDS and high altitude illnesses (i.e., hypoxemia, dyspnea…) have been reported but are also common to other pathologies (i.e., heart failure, pulmonary embolism, COPD…). Most treatments of altitude illnesses have limited value and may even be detrimental in COVID-19. Some may be efficient, potentially the corticosteroid dexamethasone. Physiological adaptations to altitude/hypoxia can exert diverse effects, depending on the constitution of the target individual and the hypoxic dose. In healthy individuals, they may optimize oxygen supply and increase mitochondrial, antioxidant, and immune system function. It is highly debated if these physiological responses to hypoxia overlap in many instances with SARS-CoV-2 infection and may exert preventive effects under very specific conditions. The temporal overlap of SARS-CoV-2 infection and exposure to altitude/hypoxia may be detrimental. No evidence-based knowledge is presently available on whether and how altitude/hypoxia may prevent, treat or aggravate COVID-19. The reported lower incidence and mortality of COVID-19 in high-altitude places remain to be confirmed. High-altitude illnesses and COVID-19 pathologies exhibit clear pathophysiological differences. While potentially effective as a prophylactic measure, altitude/hypoxia is likely associated with elevated risks for patients with COVID-19. Altogether, the different points discussed in this review are of possibly some relevance for individuals who aim to reach high-altitude areas. However, due to the ever-changing state of understanding of COVID-19, all points discussed in this review may be out of date at the time of its publication.

Hypoxia, HIF-1α, and COVID-19: from pathogenic factors to potential therapeutic targets

The pandemic of coronavirus disease 2019 (COVID-19) and its pathogen, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have become the greatest current threat to global public health. The highly infectious SARS-CoV-2 virus primarily attacks pulmonary tissues and impairs gas exchange leading to acute respiratory distress syndrome (ARDS) and systemic hypoxia. The current pharmacotherapies for COVID-19 largely rely on supportive and anti-thrombi treatment and the repurposing of antimalarial and antiviral drugs such as hydroxychloroquine and remdesivir. For a better mechanistic understanding of COVID-19, our present review focuses on its primary pathophysiologic features: hypoxia and cytokine storm, which are a prelude to multiple organ failure and lethality. We discussed a possible link between the activation of hypoxia inducible factor 1α (HIF-1α) and cell entry of SARS-CoV-2, since HIF-1α is shown to suppress the angiotensin-converting enzyme 2 (ACE2) receptor and transmembrane protease serine 2 (TMPRSS2) and upregulate disintegrin and metalloproteinase domain-containing protein 17 (ADAM17). In addition, the protein targets of HIF-1α are involved with the activation of pro-inflammatory cytokine expression and the subsequent inflammatory process. Furthermore, we hypothesized a potential utility of so-called "hypoxic conditioning" to activate HIF-1α-induced cytoprotective signaling for reduction of illness severity and improvement of vital organ function in patients with COVID-19. Taken together, we would propose further investigations into the hypoxia-related molecular mechanisms, from which novel targeted therapies can be developed for the improved management of COVID-19.

Intermittent hypoxia enhances shear-mediated dilation of the internal carotid artery in young adults

We explored the effects of cyclic intermittent hypoxia (IH) on shear-mediated dilation of the internal carotid artery (ICA), a potential index of cerebral endothelial function, in young adults. Cyclic IH increased blood flow and shear rate in the ICA and, as a result, increased shear-mediated dilation of the ICA. These data suggest that cyclic IH could potentially be applied as a nonpharmacological therapy to optimize cerebral vascular health.

Acute intermittent hypoxia as a potential adjuvant to improve walking following spinal cord injury: evidence, challenges, and future directions

Purpose of Review

The reacquisition and preservation of walking ability are highly valued goals in spinal cord injury (SCI) rehabilitation. Recurrent episodes of breathing low oxygen (i.e., acute intermittent hypoxia, AIH) is a potential therapy to promote walking recovery after incomplete SCI via endogenous mechanisms of neuroplasticity. Here, we report on the progress of AIH, alone or paired with other treatments, on walking recovery in persons with incomplete SCI. We evaluate the evidence of AIH as a therapy ready for clinical and home use and the real and perceived challenges that may interfere with this possibility.

Recent Findings

Repetitive AIH is a safe and an efficacious treatment to enhance strength, walking speed and endurance, as well as, dynamic balance in persons with chronic, incomplete SCI.

Summary

The potential for AIH as a treatment for SCI remains high, but further research is necessary to understand treatment targets and effectiveness in a large cohort of persons with SCI.

Intermittent Hypoxia Training for Treating Mild Cognitive Impairment: A Pilot Study

Although intermittent hypoxia training (IHT) has proven effective against various clinical disorders, its impact on mild cognitive impairment (MCI) is unknown. This pilot study examined IHT's safety and therapeutic efficacy in elderly patients with amnestic MCI (aMCI). Seven patients with aMCI (age 69 ± 3 years) alternately breathed 10% O2 and room-air, each 5 minutes, for 8 cycles/session, 3 sessions/wk for 8 weeks. The patients' resting arterial pressures fell by 5 to 7 mm Hg (P < .05) and cerebral tissue oxygenation increased (P < .05) following IHT. Intermittent hypoxia training enhanced hypoxemia-induced cerebral vasodilation (P < .05) and improved mini-mental state examination and digit span scores from 25.7 ± 0.4 to 27.7 ± 0.6 (P = .038) and from 24.7 ± 1.2 to 26.1 ± 1.3 (P = .047), respectively. California verbal learning test score tended to increase (P = .102), but trail making test-B and controlled oral word association test scores were unchanged. Adaptation to moderate IHT may enhance cerebral oxygenation and hypoxia-induced cerebrovasodilation while improving short-term memory and attention in elderly patients with aMCI.

Intermittent Hypoxia-Hyperoxia Training Improves Cognitive Function and Decreases Circulating Biomarkers of Alzheimer's Disease in Patients with Mild Cognitive Impairment: A Pilot Study

Alzheimer's disease (AD) affects not only the central nervous system, but also peripheral blood cells including neutrophils and platelets, which actively participate in pathogenesis of AD through a vicious cycle between platelets aggregation and production of excessive amyloid beta (Aβ). Platelets adhesion on amyloid plaques also increases the risk of cerebral microcirculation disorders. Moreover, activated platelets release soluble adhesion molecules that cause migration, adhesion/activation of neutrophils and formation of neutrophil extracellular traps (NETs), which may damage blood brain barrier and destroy brain parenchyma. The present study examined the effects of intermittent hypoxic-hyperoxic training (IHHT) on elderly patients with mild cognitive impairment (MCI), a precursor of AD. Twenty-one participants (age 51-74 years) were divided into three groups: Healthy Control (n = 7), MCI+Sham (n = 6), and MCI+IHHT (n = 8). IHHT was carried out five times per week for three weeks (total 15 sessions). Each IHHT session consisted of four cycles of 5-min hypoxia (12% FIO2) and 3-min hyperoxia (33% FIO2). Cognitive parameters, Aβ and amyloid precursor protein (APP) expression, microRNA 29, and long non-coding RNA in isolated platelets as well as NETs in peripheral blood were investigated. We found an initial decline in cognitive function indices in both MCI+Sham and MCI+IHHT groups and significant correlations between cognitive test scores and the levels of circulating biomarkers of AD. Whereas sham training led to no change in these parameters, IHHT resulted in the improvement in cognitive test scores, along with significant increase in APP ratio and decrease in Aβ expression and NETs formation one day after the end of three-week IHHT. Such effects on Aβ expression and NETs formation remained more pronounced one month after IHHT. In conclusion, our results from this pilot study suggested a potential utility of IHHT as a new non-pharmacological therapy to improve cognitive function in pre-AD patients and slow down the development of AD.

The Effect of Natural or Simulated Altitude Training on High-Intensity Intermittent Running Performance in Team-Sport Athletes: A Meta-Analysis

BACKGROUND

While adaptation to hypoxia at natural or simulated altitude has long been used with endurance athletes, it has only recently gained popularity for team-sport athletes.

OBJECTIVE

To analyse the effect of hypoxic interventions on high-intensity intermittent running performance in team-sport athletes.

METHODS

A systematic literature search of five journal databases was performed. Percent change in performance (distance covered) in the Yo-Yo intermittent recovery test (level 1 and level 2 were used without differentiation) in hypoxic (natural or simulated altitude) and control (sea level or normoxic placebo) groups was meta-analyzed with a mixed model. The modifying effects of study characteristics (type and dose of hypoxic exposure, training duration, post-altitude duration) were estimated with fixed effects, random effects allowed for repeated measurement within studies and residual real differences between studies, and the standard-error weighting factors were derived or imputed via standard deviations of change scores. Effects and their uncertainty were assessed with magnitude-based inference, with a smallest important improvement of 4% estimated via between-athlete standard deviations of performance at baseline.

RESULTS

Ten studies qualified for inclusion, but two were excluded owing to small sample size and risk of publication bias. Hypoxic interventions occurred over a period of 7-28 days, and the range of total hypoxic exposure (in effective altitude-hours) was 4.5-33 km h in the intermittent-hypoxia studies and 180-710 km h in the live-high studies. There were 11 control and 15 experimental study-estimates in the final meta-analysis. Training effects were moderate and very likely beneficial in the control groups at 1 week (20 ± 14%, percent estimate, ± 90% confidence limits) and 4-week post-intervention (25 ± 23%). The intermittent and live-high hypoxic groups experienced additional likely beneficial gains at 1 week (13 ± 16%; 13 ± 15%) and 4-week post-intervention (19 ± 20%; 18 ± 19%). The difference in performance between intermittent and live-high interventions was unclear, as were the dose of hypoxia and inclusion of training in hypoxia.

CONCLUSIONS

Hypoxic intervention appears to be a worthwhile training strategy for improvement in high-intensity running performance in team-sport athletes, with enhanced performance over control groups persisting for at least 4 weeks post-intervention. Pending further research on the type of hypoxia, dose of hypoxia and training in hypoxia, coaches have considerable scope for customising hypoxic training methods to best suit their team's training schedule.

Moderate and severe hypoxia elicit divergent effects on cardiovascular function and physiological rhythms

KEY POINTS

In the present study, we provide evidence for divergent physiological responses to moderate compared to severe hypoxia, addressing an important knowledge gap related to severity, duration and after-effects of hypoxia encountered in cardiopulmonary situations. The physiological responses to moderate and severe hypoxia were not proportional, linear or concurrent with the time-of-day. Hypoxia elicited severity-dependent physiological responses that either persisted or fluctuated throughout normoxic recovery. The physiological basis for these distinct cardiovascular responses implicates a shift in the sympathovagal set point and probably not molecular changes at the artery resulting from hypoxic stress.

ABSTRACT

Hypoxia is both a consequence and cause of many acute and chronic diseases. Severe hypoxia causes hypertension with cardiovascular sequelae; however, the rare studies using moderate severities of hypoxia indicate that it can be beneficial, suggesting that hypoxia may not always be detrimental. Comparisons between studies are difficult because of the varied classifications of hypoxic severities, methods of delivery and use of anaesthetics. Thus, to investigate the long-term effects of moderate hypoxia on cardiovascular health, radiotelemetry was used to obtain in vivo physiological measurements in unanaesthetized mice during 24 h of either moderate (FIO2=0.15) or severe (FIO2=0.09) hypoxia, followed by 72 h of normoxic recovery. Systolic blood pressure was decreased during recovery following moderate hypoxia but increased following severe hypoxia. Moderate and severe hypoxia increased haeme oxygenase-1 expression during recovery, suggesting parity in hypoxic stress at the level of the artery. Severe but not moderate hypoxia increased the low/high frequency ratio of heart rate variability 72 h post-hypoxia, indicating a shift in sympathovagal balance. Moderate hypoxia dampened the amplitude of circadian rhythm, whereas severe disrupted rhythm during the entire insult, with perturbations persisting throughout normoxic recovery. Thus, hypoxic severity differentially regulates circadian blood pressure.

Enhanced cerebral perfusion during brief exposures to cyclic intermittent hypoxemia

Cerebral vasodilation and increased cerebral oxygen extraction help maintain cerebral oxygen uptake in the face of hypoxemia. This study examined cerebrovascular responses to intermittent hypoxemia in eight healthy men breathing 10% O2 for 5 cycles, each 6 min, interspersed with 4 min of room air breathing. Hypoxia exposures raised heart rate ( P < 0.01) without altering arterial pressure, and increased ventilation ( P < 0.01) by expanding tidal volume. Arterial oxygen saturation ([Formula: see text]) and cerebral tissue oxygenation ([Formula: see text]) fell ( P < 0.01) less appreciably in the first bout (from 97.0 ± 0.3% and 72.8 ± 1.6% to 75.5 ± 0.9% and 54.5 ± 0.9%, respectively) than the fifth bout (from 94.9 ± 0.4% and 70.8 ± 1.0% to 66.7 ± 2.3% and 49.2 ± 1.5%, respectively). Flow velocity in the middle cerebral artery ( VMCA) and cerebrovascular conductance increased in a sigmoid fashion with decreases in [Formula: see text] and [Formula: see text]. These stimulus-response curves shifted leftward and upward from the first to the fifth hypoxia bouts; thus, the centering points fell from 79.2 ± 1.4 to 74.6 ± 1.1% ( P = 0.01) and from 59.8 ± 1.0 to 56.6 ± 0.3% ( P = 0.002), and the minimum VMCA increased from 54.0 ± 0.5 to 57.2 ± 0.5 cm/s ( P = 0.0001) and from 53.9 ± 0.5 to 57.1 ± 0.3 cm/s ( P = 0.0001) for the [Formula: see text]- VMCA and [Formula: see text]- VMCA curves, respectively. Cerebral oxygen extraction increased from prehypoxia 0.22 ± 0.01 to 0.25 ± 0.02 in minute 6 of the first hypoxia bout, and remained elevated between 0.25 ± 0.01 and 0.27 ± 0.01 throughout the fifth hypoxia bout. These results demonstrate that cerebral vasodilation combined with enhanced cerebral oxygen extraction fully compensated for decreased oxygen content during acute, cyclic hypoxemia.

NEW & NOTEWORTHY Five bouts of 6-min intermittent hypoxia (IH) exposures to 10% O2 progressively reduce arterial oxygen saturation ([Formula: see text]) to 67% without causing discomfort or distress. Cerebrovascular responses to hypoxemia are dynamically reset over the course of a single IH session, such that threshold and saturation for cerebral vasodilations occurred at lower [Formula: see text] and cerebral tissue oxygenation ([Formula: see text]) during the fifth vs. first hypoxia bouts. Cerebral oxygen extraction is augmented during acute hypoxemia, which compensates for decreased arterial O2 content.

The Use of Simulated Altitude Techniques for Beneficial Cardiovascular Health Outcomes in Nonathletic, Sedentary, and Clinical Populations: A Literature Review

Lizamore, Catherine A., and Michael J. Hamlin. The use of simulated altitude techniques for beneficial cardiovascular health outcomes in nonathletic, sedentary, and clinical populations: A literature review. High Alt Med Biol 18:305-321, 2017.

BACKGROUND

The reportedly beneficial improvements in an athlete's physical performance following altitude training may have merit for individuals struggling to meet physical activity guidelines.

AIM

To review the effectiveness of simulated altitude training methodologies at improving cardiovascular health in sedentary and clinical cohorts.

METHODS

Articles were selected from Science Direct, PubMed, and Google Scholar databases using a combination of the following search terms anywhere in the article: "intermittent hypoxia," "intermittent hypoxic," "normobaric hypoxia," or "altitude," and a participant descriptor including the following: "sedentary," "untrained," or "inactive."

RESULTS

1015 articles were returned, of which 26 studies were accepted (4 clinical cohorts, 22 studies used sedentary participants). Simulated altitude methodologies included prolonged hypoxic exposure (PHE: continuous hypoxic interval), intermittent hypoxic exposure (IHE: 5-10 minutes hypoxic:normoxic intervals), and intermittent hypoxic training (IHT: exercising in hypoxia).

CONCLUSIONS

In a clinical cohort, PHE for 3-4 hours at 2700-4200 m for 2-3 weeks may improve blood lipid profile, myocardial perfusion, and exercise capacity, while 3 weeks of IHE treatment may improve baroreflex sensitivity and heart rate variability. In the sedentary population, IHE was most likely to improve submaximal exercise tolerance, time to exhaustion, and heart rate variability. Hematological adaptations were unclear. Typically, a 4-week intervention of 1-hour-long PHE intervals 5 days a week, at a fraction of inspired oxygen (F_IO₂) of 0.15, was beneficial for pulmonary ventilation, submaximal exercise, and maximum oxygen consumption ([Formula: see text]O_2max), but an F_IO₂ of 0.12 reduced hyperemic response and antioxidative capacity. While IHT may be beneficial for increased lipid metabolism in the short term, it is unlikely to confer any additional advantage over normoxic exercise over the long term. IHT may improve vascular health and autonomic balance.

Effect of respiratory rehabilitation techniques on the autonomic function in patients with chronic obstructive pulmonary disease: A systematic review

Patients with chronic obstructive pulmonary disease (COPD) show several extrapulmonary abnormalities such as impairment in the autonomic function (AF). Similarly, the use of respiratory training techniques such as controlled breathing techniques, noninvasive mechanical ventilation (NIMV), and oxygen supplementation for AF modulation in patients with COPD is popular in existing literature. However, the evidence to support their use is nonexistent. A systematic search of studies reporting on the effect of controlled breathing techniques, NIMV, and/or oxygen supplementation techniques on AF outcome parameters was conducted in three online databases: PubMed, Embase, and Web of Science. Following the Preferred Reporting Items for Systematic reviews and Meta-Analyses statement, relevant studies were retained and qualitatively analyzed for evidence synthesis. The methodological quality in these studies was evaluated using the evidence based guideline development (EBRO) checklists per designs provided by the Dutch Cochrane Centre. Eighteen studies met the inclusion criteria of the review and were included and discussed. The evidence synthesis revealed that a strong and moderate level evidence supported oxygen supplementation and slow breathing techniques, respectively, in significantly enhancing the baroreceptor sensitivity (BRS) values in patients with COPD. The effect of the examined techniques on the heart rate variability and muscle sympathetic nerve activity was of a limited or inconsistent evidence. The findings from this review suggest that oxygen supplementation and controlled breathing techniques have profound positive influence on the BRS in patients with COPD. However, it is not fully clear whether these influence translates to any therapeutic benefit on the general AF of patients with COPD in the long term.

Integrated cardiovascular/respiratory control in type 1 diabetes evidences functional imbalance: Possible role of hypoxia

BACKGROUND

Cardiovascular (baroreflex) and respiratory (chemoreflex) control mechanisms were studied separately in diabetes, but their reciprocal interaction (well known for diseases like heart failure) had never been comprehensively assessed. We hypothesized that prevalent autonomic neuropathy would depress both reflexes, whereas prevalent autonomic imbalance through sympathetic activation would depress the baroreflex but enhance the chemoreflexes.

METHODS

In 46 type-1 diabetic subjects (7.0±0.9year duration) and 103 age-matched controls we measured the baroreflex (average of 7 methods), and the chemoreflexes, (hypercapnic: ventilation/carbon dioxide slope during hyperoxic progressive hypercapnia; hypoxic: ventilation/oxygen saturation slope during normocapnic progressive hypoxia). Autonomic dysfunction was evaluated by cardiovascular reflex tests.

RESULTS

Resting oxygen saturation and baroreflex sensitivity were reduced in the diabetic group, whereas the hypercapnic chemoreflex was significantly increased in the entire diabetic group. Despite lower oxygen saturation the hypoxic chemoreflex showed a trend toward a depression in the diabetic group.

CONCLUSION

Cardio-respiratory control imbalance is a common finding in early type 1 diabetes. A reduced sensitivity to hypoxia seems a primary factor leading to reflex sympathetic activation (enhanced hypercapnic chemoreflex and baroreflex depression), hence suggesting a functional origin of cardio-respiratory control imbalance in initial diabetes.

Erythropoietin: Endogenous Protection of Ischemic Brain

The human brain requires uninterrupted delivery of blood-borne oxygen and nutrients to sustain its function. Focal ischemia, particularly, ischemic stroke, and global ischemia imposed by cardiac arrest disrupt the brain's fuel supply. The resultant ATP depletion initiates a complex injury cascade encompassing intracellular Ca²⁺ overload, glutamate excitotoxicity, oxido-nitrosative stress, extracellular matrix degradation, and inflammation, culminating in neuronal and astroglial necrosis and apoptosis, neurocognitive deficits, and even death. Unfortunately, brain ischemia has proven refractory to pharmacological intervention. Many promising treatments afforded brain protection in animal models of focal and global ischemia, but failed to improve survival and neurocognitive recovery of stroke and cardiac arrest patients in randomized clinical trials. The culprits are the blood-brain barrier (BBB) that limits transferral of medications to the brain parenchyma, and the sheer complexity of the injury cascade, which presents a daunting array of targets unlikely to respond to monotherapies. Erythropoietin is a powerful neuroprotectant capable of interrupting multiple aspects of the brain injury cascade. Preclinical research demonstrates erythropoietin's ability to suppress glutamate excitotoxicity and intracellular Ca²⁺ overload, dampen oxidative stress and inflammation, interrupt the apoptotic cascade, and preserve BBB integrity. However, the erythropoietin dosages required to traverse the BBB and achieve therapeutically effective concentrations in the brain parenchyma impose untoward side effects. Recent discoveries that hypoxia induces erythropoietin production within the brain and that neurons, astroglia, and cerebrovascular endothelium harbor membrane erythropoietin receptors, raise the exciting prospect of harnessing endogenous erythropoietin to protect the brain from the ravages of ischemia-reperfusion.

Intermittent hypoxia as a means to improve aerobic capacity in type 2 diabetes

Physical inactivity and a low maximal aerobic capacity (VO_2max) strongly predict morbidity and mortality in patients with type 2 diabetes (T2D). Patients with T2D have a reduced VO_2max when compared with healthy individuals of similar age, weight, and physical activity levels, and this lower aerobic capacity is usually attributed to a reduced oxygen delivery to the working muscles. The oxygen carrying capacity of the blood, as well as increases in cardiac output and blood flow, contribute to the delivery of oxygen to the active muscles during exercise. Hemoglobin mass (Hb mass), a key determinant of oxygen carrying capacity, is suggested to be reduced in patients with T2D following the observation of a lower blood volume (BV) in combination with normal hematocrit levels in this population. Therefore, a lower Hb mass, in addition to a reported lower BV and impaired cardiovascular response to exercise, likely contributes to the reduced oxygen delivery and VO_2max in patients with T2D. While exercise training increases Hb mass, BV, and consequently VO_2max, the majority of patients with T2D are not physically active, highlighting the need for alternative methods to improve VO_2max in this population. Exposure to hypoxia triggers the release of erythropoietin, the hormone regulating red blood cell production, which increases Hb mass and consequently BV. Exposure to mild intermittent hypoxia (IH), characterized by few and short episodes of hypoxia at a fraction of inspired oxygen ranging between 10 and 14% interspersed with cycles of normoxia, increased red blood cell volume, Hb mass, and plasma volume in patients with coronary artery disease or chronic obstructive pulmonary disease, which resulted in an improved VO_2max in both populations. We hypothesize that 12 exposures to mild IH over a period of 4weeks will increase Hb mass, BV, cardiac function, and VO_2max in patients with T2D. Therefore, exposures to mild IH may increase oxygen delivery and VO_2max without the need to perform exercise in patients with T2D.

Physiological Responses to Two Hypoxic Conditioning Strategies in Healthy Subjects

Objective: Hypoxic exposure can be used as a therapeutic tool by inducing various cardiovascular, neuromuscular, and metabolic adaptations. Hypoxic conditioning strategies have been evaluated in patients with chronic diseases using either sustained (SH) or intermittent (IH) hypoxic sessions. Whether hypoxic conditioning via SH or IH may induce different physiological responses remains to be elucidated.

Methods: Fourteen healthy active subjects (7 females, age 25 ± 8 years, body mass index 21.5 ± 2.5 kg·m⁻²) performed two interventions in a single blind, randomized cross-over design, starting with either 3 x SH (48 h apart), or 3 x IH (48 h apart), separated by a 2 week washout period. SH sessions consisted of breathing a gas mixture with reduced inspiratory oxygen fraction (FiO₂), continuously adjusted to reach arterial oxygen saturations (SpO₂) of 70–80% for 1 h. IH sessions consisted of 5 min with reduced FiO₂ (SpO₂ = 70–80%), followed by 3-min normoxia, repeated seven times. During the first (S1) and third (S3) sessions of each hypoxic intervention, cardiorespiratory parameters, and muscle and pre-frontal cortex oxygenation (near infrared spectroscopy) were assessed continuously.

Results: Minute ventilation increased significantly during IH sessions (+2 ± 2 L·min⁻¹) while heart rate increased during both SH (+11 ± 4 bpm) and IH (+13 ± 5 bpm) sessions. Arterial blood pressure increased during all hypoxic sessions, although baseline normoxic systolic blood pressure was reduced from S1 to S3 in IH only (−8 ± 11 mmHg). Muscle oxygenation decreased significantly during S3 but not S1, for both hypoxic interventions (S3: SH −6 ± 5%, IH −3 ± 4%); pre-frontal oxygenation decreased in S1 and S3, and to a greater extent in SH vs. IH (−13 ± 3% vs. −6 ± 6%). Heart rate variability indices indicated a significantly larger increase in sympathetic activity in SH vs. IH (lower SDNN, PNN50, and RMSSD values in SH). From S1 to S3, further reduction in heart rate variability was observed in SH (SDNN, PNN50, and RMSSD reduction) while heart rate variability increased in IH (SDNN and RMSSD increase).

Conclusions: These results showed significant differences in heart rate variability, blood pressure, and tissue oxygenation changes during short-term SH vs. IH conditioning interventions. Heart rate variability may provide useful information about the early adaptations induced by such intervention.

Integrated Cardio-Respiratory Control: Insight in Diabetes

Autonomic dysfunction is a frequent and relevant complication of diabetes mellitus, as it is associated with increased morbidity and mortality. In addition, it is today considered as predictive of the most severe diabetic complications, like nephropathy and retinopathy. The classical methods of screening are the cardiovascular reflex tests and were originally interpreted as evidence of nerve damage. A more modern approach, based on the integrated control of cardiovascular and respiratory function, reveals that these abnormalities are to a great extent functional, at least in the early stage of the disease, thus suggesting new potential interventions. Therefore, this review aims to go further investigating how the imbalance of the autonomic nervous system is altered and can be influenced in many chronic pathologies through a global view of cardio-respiratory and metabolic interactions and how the same mechanisms are applicable to diabetes.

Intermittent hypoxia initiated plasticity in humans: A multipronged therapeutic approach to treat sleep apnea and overlapping co-morbidities

Over the past three decades exposure to intermittent hypoxia (IH) has generally been considered a stimulus associated with a number of detrimental outcomes. However, there is sufficient evidence to link IH to many beneficial outcomes but they have largely been ignored, particularly in the field of sleep medicine in the United States. Recent reviews have postulated that this apparent contradiction is related to the severity and duration of exposure to IH; mild forms of IH initiate beneficial outcomes while severe forms of IH are coupled to detrimental consequences. In the present review we explore the role that IH has in initiating respiratory plasticity and the potential this form of plasticity has to mitigate obstructive sleep apnea (OSA) in humans. In taking this approach, we address the possibility that IH could serve as an adjunct therapy coupled with continuous positive airway pressure (CPAP) to treat OSA. Our working hypothesis is that exposure to mild IH leads to respiratory plasticity that manifests in increased stability of the upper airway, which could ultimately reduce the CPAP required to treat OSA. In turn, this reduction could increase CPAP compliance and extend the length of treatment each night, which might improve the magnitude of outcome measures. Improved treatment compliance coupled with the direct effect that IH has on numerous overlapping conditions (i.e. asthma, chronic obstructive pulmonary disease, spinal cord injury) may well lead to substantial improvements that exceed outcomes following treatment with CPAP alone. Overall, this review will consider evidence from the published literature which suggests that IH could serve as an effective multipronged therapeutic approach to treat sleep apnea and its overlapping co-morbidities.

δ-Opioid receptor (DOR) signaling and reactive oxygen species (ROS) mediate intermittent hypoxia induced protection of canine myocardium

Intermittent, normobaric hypoxia confers robust cardioprotection against ischemia-induced myocardial infarction and lethal ventricular arrhythmias. δ-Opioid receptor (DOR) signaling and reactive oxygen species (ROS) have been implicated in cardioprotective phenomena, but their roles in intermittent hypoxia are unknown. This study examined the contributions of DOR and ROS in mediating intermittent hypoxia-induced cardioprotection. Mongrel dogs completed a 20 day program consisting of 5-8 daily, 5-10 min cycles of moderate, normobaric hypoxia (FIO2 0.095-0.10), with intervening 4 min room air exposures. Subsets of dogs received the DOR antagonist naltrindole (200 μg/kg, sc) or antioxidant N-acetylcysteine (250 mg/kg, po) before each hypoxia session. Twenty-four hours after the last session, the left anterior descending coronary artery was occluded for 60 min and then reperfused for 5 h. Arrhythmias detected by electrocardiography were scored according to the Lambeth II conventions. Left ventricles were sectioned and stained with 2,3,5-triphenyl-tetrazolium-chloride, and infarct sizes were expressed as percentages of the area at risk (IS/AAR). Intermittent hypoxia sharply decreased IS/AAR from 41 ± 5 % (n = 12) to 1.8 ± 0.9 % (n = 9; P < 0.001) and arrhythmia score from 4.1 ± 0.3 to 0.7 ± 0.2 (P < 0.001) vs. non-hypoxic controls. Naltrindole (n = 6) abrogated the cardioprotection with IS/AAR 35 ± 5 % and arrhythmia score 3.7 ± 0.7 (P < 0.001 vs. untreated intermittent hypoxia). N-acetylcysteine (n = 6) interfered to a similar degree, with IS/AAR 42 ± 3 % and arrhythmia score 4.7 ± 0.3 (P < 0.001 vs. untreated intermittent hypoxia). Without the intervening reoxygenations, hypoxia (n = 4) was not cardioprotective (IS/AAR 50 ± 8 %; arrhythmia score 4.5 ± 0.5; P < 0.001 vs. intermittent hypoxia). Thus DOR, ROS and cyclic reoxygenation were obligatory participants in the gradually evolving cardioprotection produced by intermittent hypoxia.

Physiological impairment in mild COPD

Chronic obstructive pulmonary disease (COPD) is a common and often progressive inflammatory disease of the airways, alveoli and microvasculature that is both preventable and treatable. It is well established that smokers with mild airway obstruction, as spirometrically defined, represent the vast majority of patients with COPD, yet this population has not been extensively studied. An insidious preclinical course means that mild COPD is both underdiagnosed and undertreated. In this context, recent studies have confirmed that even patients with mild COPD can have extensive physiological impairment, which contributes to poor perceived health status compared with non-smoking healthy controls. This review describes the heterogeneous pathophysiology that can exist in COPD patients with only mild airway obstruction on spirometry. It exposes the compensatory adaptations that develop in such patients to ensure that the respiratory system fulfils its primary task of maintaining adequate pulmonary gas exchange for the prevailing metabolic demand. It demonstrates that adaptations such as increased inspiratory neural drive to the diaphragm due to combined effects of increased mechanical loading and chemostimulation underscore the increased dyspnoea and exercise intolerance in this population. Finally, based on available evidence, we present what we believe is a sound physiological rationale for earlier diagnosis in this population.

Heart rate and blood pressure responses during hypoxic cycles of a 3-week intermittent hypoxia breathing program in patients at risk for or with mild COPD

The aim of this study was to provide information on heart rate and blood pressure responses during a 3-week intermittent hypoxia breathing program in COPD patients. Sixteen participants with COPD symptoms were randomly assigned to a hypoxia or control group and completed a 3-week intermittent hypoxia breathing program (five sessions per week, each consisting of three to five breathing cycles, each cycle lasting 3–5 minutes with 3-minute breaks between cycles). During the breathing cycles, the hypoxia group received hypoxic air (inspired fraction of oxygen 15%–12%), whereas the control group received normal air (sham hypoxia). During the breaks, all participants breathed normoxic room air. Arterial oxygen saturation, systolic and diastolic blood pressure, and heart rate were measured during the normoxic and hypoxic/sham hypoxic periods. For each breathing cycle, changes from normoxia to hypoxia/sham hypoxia were calculated, and changes were averaged for each of the 15 sessions and for each week. Changes in arterial oxygen saturation were significantly different between groups in the course of the 3 weeks (two-way analysis of variance for repeated measures), with post hoc differences in weeks 1, 2, and 3. During the course of the intermittent hypoxia application, no between-group differences were detected for blood pressure or rate pressure product values. Changes in heart rate were significantly different between groups in the course of the 3 weeks (two-way analysis of variance for repeated measures), with post hoc differences only in week 3. Averages over all 15 sessions were significantly higher in the hypoxia group for heart rate and rate pressure product, and tended to be increased for systolic blood pressure. The applied intermittent hypoxia breathing program resulted in specific and moderate heart rate and blood pressure responses, and did not provoke a progressive increase in blood pressure during the hypoxic cycles in the course of the application.

Hypoxic Conditioning as a New Therapeutic Modality

Preconditioning refers to a procedure by which a single noxious stimulus below the threshold of damage is applied to the tissue in order to increase resistance to the same or even different noxious stimuli given above the threshold of damage. Hypoxic preconditioning relies on complex and active defenses that organisms have developed to counter the adverse consequences of oxygen deprivation. The protection it confers against ischemic attack for instance as well as the underlying biological mechanisms have been extensively investigated in animal models. Based on these data, hypoxic conditioning (consisting in recurrent exposure to hypoxia) has been suggested a potential non-pharmacological therapeutic intervention to enhance some physiological functions in individuals in whom acute or chronic pathological events are anticipated or existing. In addition to healthy subjects, some benefits have been reported in patients with cardiovascular and pulmonary diseases as well as in overweight and obese individuals. Hypoxic conditioning consisting in sessions of intermittent exposure to moderate hypoxia repeated over several weeks may induce hematological, vascular, metabolic, and neurological effects. This review addresses the existing evidence regarding the use of hypoxic conditioning as a potential therapeutic modality, and emphasizes on many remaining issues to clarify and future researches to be performed in the field.

Therapeutic potential of intermittent hypoxia: a matter of dose

Intermittent hypoxia (IH) has been the subject of considerable research in recent years, and triggers a bewildering array of both detrimental and beneficial effects in multiple physiological systems. Here, we review the extensive literature concerning IH and its impact on the respiratory, cardiovascular, immune, metabolic, bone, and nervous systems. One major goal is to define relevant IH characteristics leading to safe, protective, and/or therapeutic effects vs. pathogenesis. To understand the impact of IH, it is essential to define critical characteristics of the IH protocol under investigation, including potentially the severity of hypoxia within episodes, the duration of hypoxic episodes, the number of hypoxic episodes per day, the pattern of presentation across time (e.g., within vs. consecutive vs. alternating days), and the cumulative time of exposure. Not surprisingly, severe/chronic IH protocols tend to be pathogenic, whereas any beneficial effects are more likely to arise from modest/acute IH exposures. Features of the IH protocol most highly associated with beneficial vs. pathogenic outcomes include the level of hypoxemia within episodes and the number of episodes per day. Modest hypoxia (9-16% inspired O2) and low cycle numbers (3-15 episodes per day) most often lead to beneficial effects without pathology, whereas severe hypoxia (2-8% inspired O2) and more episodes per day (48-2,400 episodes/day) elicit progressively greater pathology. Accumulating evidence suggests that "low dose" IH (modest hypoxia, few episodes) may be a simple, safe, and effective treatment with considerable therapeutic potential for multiple clinical disorders.

Meditation research, past, present, and future: perspectives from the Nalanda contemplative science tradition

This article offers an overview of meditation research: its history, recent developments, and future directions. As the number and scope of studies grow, the field has converged with cognitive and affective neuroscience, and spawned many clinical applications. Recent work has shed light on the mechanisms and effects of diverse practices, and is entering a new phase where consensus and coherent paradigms are within reach. This article suggests an unusual path for future advancement: complementing conventional research with rigorous dialogue with the contemplative traditions that train expert meditators and best know the techniques. It explores the Nalanda tradition developed in India and preserved in Tibet, because its cumulative approach to contemplative methods produced a comprehensive framework that may help interpret data and guide research, and because its naturalistic theories and empirical methods may help bridge the gulf between science and other contemplative traditions. Examining recent findings and models in light of this framework, the article introduces the Indic map of the central nervous system and presents three testable predictions based on it. Finally, it reviews two studies that suggest that the multimodal Nalanda approach to contemplative learning is as well received as more familiar approaches, while showing promise of being more effective.

Chronic obstructive pulmonary disease: clinical integrative physiology

Peripheral airway dysfunction, inhomogeneous ventilation distribution, gas trapping, and impaired pulmonary gas exchange are variably present in all stages of chronic obstructive pulmonary disease (COPD). This article provides a cogent physiologic explanation for the relentless progression of activity-related dyspnea and exercise intolerance that all too commonly characterizes COPD. The spectrum of physiologic derangements that exist in smokers with mild airway obstruction and a history compatible with COPD is examined. Also explored are the perceptual and physiologic consequences of progressive erosion of the resting inspiratory capacity. Finally, emerging information on the role of cardiocirculatory impairment in contributing to exercise intolerance in patients with varying degrees of airway obstruction is reviewed.

Effects of a single bout of interval hypoxia on cardiorespiratory control in patients with type 1 diabetes

Hypoxemia is common in diabetes, and reflex responses to hypoxia are blunted. These abnormalities could lead to cardiovascular/renal complications. Interval hypoxia (IH) (5-6 short periods of hypoxia each day over 1-3 weeks) was successfully used to improve the adaptation to hypoxia in patients with chronic obstructive pulmonary disease. We tested whether IH over 1 day could initiate a long-lasting response potentially leading to better adaptation to hypoxia. In 15 patients with type 1 diabetes, we measured hypoxic and hypercapnic ventilatory responses (HCVRs), ventilatory recruitment threshold (VRT-CO2), baroreflex sensitivity (BRS), blood pressure, and blood lactate before and after 0, 3, and 6 h of a 1-h single bout of IH. All measurements were repeated on a placebo day (single-blind protocol, randomized sequence). After IH (immediately and after 3 h), hypoxic and HCVR increased, whereas the VRT-CO2 dropped. No such changes were observed on the placebo day. Systolic and diastolic blood pressure increased, whereas blood lactate decreased after IH. Despite exposure to hypoxia, BRS remained unchanged. Repeated exposures to hypoxia over 1 day induced an initial adaptation to hypoxia, with improvement in respiratory reflexes. Prolonging the exposure to IH (>2 weeks) in type 1 diabetic patients will be a matter for further studies.