Two-week normobaric intermittent hypoxia exposures enhance oxyhemoglobin equilibrium and cardiac responses during hypoxemia

Similar endothelium-dependent vascular responses to intermittent hypoxia in young and older adults

Aging is associated with vascular endothelial dysfunction observed through a progressive loss of flow-mediated dilation caused partly by a decreased nitric oxide bioavailability. Intermittent hypoxia, consisting of alternating short bouts of breathing hypoxic and normoxic air, was reported to either maintain or improve vascular function in young adults. The aim of this study was to determine the impact of age on the vascular response to intermittent hypoxia. Twelve young adults and 11 older adults visited the laboratory on two occasions. Plasma nitrate concentrations and brachial artery flow-mediated dilation were assessed before and after exposure to either intermittent hypoxia or a sham protocol. Intermittent hypoxia consisted of eight 4-min hypoxic cycles at a targeted oxygen saturation of 80% interspersed with breathing room air to resaturation, and the sham protocol consisted of eight 4-min normoxic cycles interspersed with breathing room air. Vascular responses were assessed during intermittent hypoxia and the sham protocol. Intermittent hypoxia elicited a brachial artery vasodilation but did not change brachial artery shear rate in both young and older adults. Plasma nitrate concentrations were not significantly affected by intermittent hypoxia compared with the sham protocol in both groups. Brachial artery flow-mediated dilation was not acutely affected by intermittent hypoxia or the sham protocol in either young or older adults. In conclusion, the brachial artery vasodilatory response to intermittent hypoxia was not influenced by age. Intermittent hypoxia increased brachial artery diameter but did not acutely affect endothelium-dependent vasodilation in young or older adults.NEW & NOTEWORTHY The objective of this study was to determine the impact of age on the vascular response to intermittent hypoxia. Eight 4-min bouts of hypoxia at a targeted oxygen saturation of 80% induced a brachial artery vasodilation in both young and older adults, indicating that age does not influence the vasodilatory response to intermittent hypoxia. Intermittent hypoxia did not acutely affect brachial artery flow-mediated dilation in young or older adults.

Impaired erythropoietin response to hypoxia in type 2 diabetes

AIMS

Patients with type 2 diabetes have a 20% lower total blood volume than age- and weight-matched healthy adults, suggesting a reduced capacity to transport oxygen in this population. Intermittent hypoxia, consisting of alternating short bouts of breathing hypoxic and normoxic air, increases erythropoietin levels, the hormone regulating red blood cell production, in young and older adults. The objective of this study was to determine the effect of a single session of intermittent hypoxia on erythropoietin levels and hemoglobin mass, the absolute mass of hemoglobin contained in red blood cells, in patients with type 2 diabetes.

METHODS

Ten patients with type 2 diabetes were exposed to an intermittent hypoxia protocol consisting of eight 4-min cycles at a targeted oxygen saturation of 80% interspersed with normoxic cycles to resaturation. Erythropoietin and hemoglobin mass responses to intermittent hypoxia in patients with type 2 diabetes were compared to previously published data from an identical intermittent hypoxia protocol performed in age-matched older adults.

RESULTS

Intermittent hypoxia increased erythropoietin levels in older adults but did not induce any change in erythropoietin levels in patients with type 2 diabetes (3.2 ± 2.2 vs. 0.2 ± 2.7 mU/ml, p = 0.01). Hemoglobin mass indexed to body weight was 21% lower in patients with type 2 diabetes than in older adults (8.1 ± 1.7 vs. 10.2 ± 2.1 g/kg, p < 0.01).

CONCLUSIONS

These findings suggest an impaired erythropoietin response to decreased oxygen levels in patients with type 2 diabetes, which may contribute to the reduced oxygen transport capacity observed in this population.

Intermittent hypoxia preconditioning can attenuate acute hypoxic injury after a sustained normobaric hypoxic exposure: A randomized clinical trial

BACKGROUND

Intermittent hypoxia (IH) is emerging as a cost-effective nonpharmacological method for vital organ protection. We aimed to assess the effects of a short-term moderate intermittent hypoxia preconditioning protocol (four cycles of 13% hypoxia lasting for 10 min with 5-min normoxia intervals) on acute hypoxic injury induced by sustained hypoxic exposure (oxygen concentration of 11.8% for 6 h).

METHODS

One hundred healthy volunteers were recruited and randomized to the IH group and the control group to receive IH or sham-IH preconditioning for 5 days, respectively, and then were sent to a hypoxic chamber for simulated acute high-altitude exposure (4500 m).

RESULTS

The overall incidence of acute mountain sickness was 27% (27/100), with 14% (7/50) in the IH group and 40% (20/50) in the control group (p = 0.003). After 6-h simulated high-altitude exposure, the mean Lake Louise Score was lower in the IH group as compared to controls (1.30 ± 1.27 vs. 2.04 ± 1.89, p = 0.024). Mean peripheral oxygen saturations (SpO2 ) and intracranial pressure (ICP) measures after acute hypoxic exposure exhibited significant differences, with the IH group showing significantly greater SpO2 values (85.47 ± 5.14 vs. 83.10 ± 5.15%, p = 0.026) and lower ICP levels than the control group (115.59 ± 32.15 vs. 130.36 ± 33.83 mmH2 O, p = 0.028). IH preconditioning also showed greater effects on serum protein gene product 9.5 (3.89 vs. 29.16 pg/mL; p = 0.048) and C-reactive protein (-0.28 vs. 0.41 mg/L; p = 0.023).

CONCLUSION

The short-term moderate IH improved the tolerance to hypoxia and exerted protection against acute hypoxic injury induced by exposure to sustained normobaric hypoxia, which provided a novel method and randomized controlled trial evidence to develop treatments for hypoxia-related disease.

Impact of Age on Cognitive Testing Practice Effects and Cardiorespiratory Responses

Objective: This study tested the hypothesis that healthy aging attenuates cognitive practice effects and, consequently, limits the familiarity-associated reductions in heart rate (HR) and breathing frequency (BF) responses during retesting. Methods: Twenty-one cognitively normal older and younger adults (65 ± 2 vs. 26 ± 1 years old) participated in the study. Mini-Mental State Examination (MMSE), Digit-Span-Test (DST), Trail Making Test (TMT-B), and California Verbal Learning Test (CVLT-II) were administered twice at 3-week intervals, while HR and BF were monitored by electrocardiography and plethysmography, respectively. Results: Cognitive performances were not affected by the age factor, and the retest factor only affected CVLT-II. HR and BF increased only in the younger adults (p < .01) during cognitive tests; retesting attenuated these responses (retest factor p < .01). Long-delay free-recall in CVLT-II was unchanged in cognitively normal older versus younger adults. Healthy aging did not diminish short-term memory assessed by DST and CVLT-II short-delay or long-delay free-recalls. Conclusions: Only CVLT-II, but not MMSE, DST or TMT-B, demonstrated cognitive retesting practice effects in the younger and older adults. Cognitive testing at 3-week intervals in cognitively normal older and younger subjects revealed divergent cardiorespiratory responses to MMSE, DST, and TMT-B cognitive testing, particularly HR, which increased only in younger adults and to a lesser extent during retesting despite the absence of practice effects.

The forearm vascular response to sympathetic activation is attenuated in female, but not male, participants following acute intermittent hypoxia

Acute exposure to hypoxia promotes both an increase in sympathetic nervous system activity (SNA) and local vasodilation. In rodents, intermittent hypoxia (IH)-mediated increases in SNA are associated with an increase in blood pressure in males but not females; notably, the protective effect of female sex is lost following ovariectomy. These data suggest the vascular response to hypoxia and/or SNA following IH may be sex- and/or hormone specific-although mechanisms are unclear. We hypothesized that hypoxia-mediated vasodilation and SNA-mediated vasoconstriction would be unchanged following acute IH in male adults. We further hypothesized that hypoxic vasodilation would be augmented and SNA-mediated vasoconstriction would be attenuated in female adults following acute IH, with the greatest effect when endogenous estradiol was high. Twelve male (25 ± 1 yr) and 10 female (25 ± 1 yr) participants underwent 30 min of IH. Females were studied in a low (early follicular) and high (late follicular) estradiol state. Preceding and following IH, participants completed two trials [steady-state hypoxia and cold pressor test (CPT)], where forearm blood flow and blood pressure were measured and used to determine forearm vascular conductance (FVC). The FVC response to hypoxia (P = 0.67) and sympathetic activation (P = 0.73) were unchanged following IH in males. There was no effect of IH on hypoxic vasodilation in females, regardless of estradiol state (P = 0.75). In contrast, the vascular response to sympathetic activation was attenuated in females following IH (P = 0.02), independent of estradiol state (P = 0.65). Present data highlight sex-related differences in neurovascular responsiveness following acute IH.NEW & NOTEWORTHY We examined the effects of acute intermittent hypoxia (AIH) on the vascular response to sympathetic activation and acute hypoxia. Present findings show, despite no effect of AIH on the vascular response to hypoxia, the forearm vasoconstrictor response to acute sympathetic activation is attenuated in females following AIH, independent of estradiol state. These data provide mechanistic understanding of potential benefits of AIH, as well as the impact of biological sex.

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.

Effects of Hypoxia on Cerebral Microvascular Angiogenesis: Benefits or Damages?

Cerebrovascular microcirculation is essential for maintaining the physiological functions of the brain. The brain can be protected from stress injury by remodeling the microcirculation network. Angiogenesis is a type of cerebral vascular remodeling. It is an effective approach to improve the blood flow of the cerebral microcirculation, which is necessary for preventing and treating various neurological disorders. Hypoxia is one of the most important regulators of angiogenesis, affecting the sprouting, proliferation, and maturation stages of angiogenesis. Moreover, hypoxia negatively affects cerebral vascular tissue by impairing the structural and functional integrity of the blood-brain barrier and vascular-nerve decoupling. Therefore, hypoxia has a dual effect on blood vessels and is affected by confounding factors including oxygen concentration, hypoxia duration, and hypoxia frequency and extent. Establishing an optimal model that promotes cerebral microvasculogenesis without causing vascular injury is essential. In this review, we first elaborate on the effects of hypoxia on blood vessels from two different perspectives: (1) the promotion of angiogenesis and (2) cerebral microcirculation damage. We further discuss the factors influencing the dual role of hypoxia and emphasize the benefits of moderate hypoxic irritation and its potential application as an easy, safe, and effective treatment for multiple nervous system disorders.

Intermittent Hypoxia Training Prevents Deficient Learning-Memory Behavior in Mice Modeling Alzheimer's Disease: A Pilot Study

In mouse models of Alzheimer's disease (AD), normobaric intermittent hypoxia training (IHT) can preserve neurobehavioral function when applied before deficits develop, but IHT's effectiveness after onset of amyloid-β (Aβ) accumulation is unclear. This study tested the hypothesis that IHT improves learning-memory behavior, diminishes Aβ accumulation in cerebral cortex and hippocampus, and enhances cerebrocortical contents of the neuroprotective trophic factors erythropoietin and brain-derived neurotrophic factor (BDNF) in mice manifesting AD traits. Twelve-month-old female 3xTg-AD mice were assigned to untreated 3xTg-AD (n = 6), AD+IHT (n = 6), and AD+sham-IHT (n = 6) groups; 8 untreated wild-type (WT) mice also were studied. AD+IHT mice alternately breathed 10% O2 for 6 min and room air for 4 min, 10 cycles/day for 21 days; AD+sham-IHT mice breathed room air. Spatial learning-memory was assessed by Morris water maze. Cerebrocortical and hippocampal Aβ40 and Aβ42 contents were determined by ELISA, and cerebrocortical erythropoietin and BDNF were analyzed by immunoblotting and ELISA. The significance of time (12 vs. 12 months + 21 days) and treatment (IHT vs. sham-IHT) was evaluated by two-factor ANOVA. The change in swimming distance to find the water maze platform after 21 d IHT (-1.6 ± 1.8 m) differed from that after sham-IHT (+5.8 ± 2.6 m). Cerebrocortical and hippocampal Aβ42 contents were greater in 3xTg-AD than WT mice, but neither time nor treatment significantly affected Aβ40 or Aβ42 contents in the 3xTg-AD mice. Cerebrocortical erythropoietin and BDNF contents increased appreciably after IHT as compared to untreated 3xTg-AD and AD+sham-IHT mice. In conclusion, moderate, normobaric IHT prevented spatial learning-memory decline and restored cerebrocortical erythropoietin and BDNF contents despite ongoing Aβ accumulation in 3xTg-AD mice.

Short exposure to intermittent hypoxia increases erythropoietin levels in healthy individuals

Few minutes of hypoxic exposure stabilizes hypoxia-inducible factor-1α, resulting in erythropoietin (EPO) gene transcription and production. The objective of this study was to identify the shortest intermittent hypoxia protocol necessary to increase serum EPO levels in healthy individuals. In a first experiment, spontaneous EPO changes under normoxia (NORM) and the EPO response to five 4-min cycles of intermittent hypoxia (IH5) were determined in six individuals. In a second experiment, the EPO response to eight 4-min cycles of intermittent hypoxia (IH8) and 120 min of continuous hypoxia (CONT) was determined in six individuals. All hypoxic protocols were performed at a targeted arterial oxygen saturation of 80%. There was no significant change in EPO levels in response to normoxia or in response to five cycles of intermittent hypoxia (NORM: 9.5 ± 1.8 to 10.5 ± 1.8, IH5: 11.4 ± 2.3 to 13.4 ± 2.1 mU/mL, main effect for time P = 0.35). There was an increase in EPO levels in response to eight cycles of intermittent hypoxia and 120 min of continuous hypoxia, with peak levels observed 4.5 h after the onset of hypoxia (IH8: 11.2 ± 2.0 to 16.7 ± 2.2, CONT: 11.1 ± 3.8 to 19.4 ± 3.8 mU/mL, main effect for time P < 0.01). Eight cycles of intermittent hypoxia increased EPO levels to a similar extent as 120 min of continuous hypoxia (main effect for condition P = 0.36). Eight 4-min cycles of intermittent hypoxia represent the shortest protocol to increase serum EPO levels in healthy individuals.NEW & NOTEWORTHY The objective of this study was to identify the shortest intermittent hypoxia protocol necessary to increase serum erythropoietin levels in healthy individuals. Eight 4-min bouts of intermittent hypoxia, representing a hypoxic duration of 32 min at an arterial oxygen saturation of 80%, significantly increased erythropoietin levels in healthy individuals. These findings suggest that a short session of intermittent hypoxia has the potential to increase oxygen-carrying capacity.

Hypoxic preconditioning attenuates ischemia-reperfusion injury in young healthy adults

Ischemia-reperfusion injury induced by restoration of blood flow following occlusion impairs flow-mediated dilation, a marker of endothelium-dependent vasodilation. In young healthy adults, exposure to intermittent hypoxia, consisting of alternating short bouts of breathing hypoxic and normoxic air, before an ischemia-reperfusion injury significantly attenuated the reduction in flow-mediated dilation. Thus, hypoxic preconditioning represents a potential strategy to mitigate the effect of ischemia-reperfusion injury associated with ischemic events.

Effect of a Single Session of Intermittent Hypoxia on Erythropoietin and Oxygen-Carrying Capacity

Intermittent hypoxia, defined as alternating bouts of breathing hypoxic and normoxic air, has the potential to improve oxygen-carrying capacity through an erythropoietin-mediated increase in hemoglobin mass. The purpose of this study was to determine the effect of a single session of intermittent hypoxia on erythropoietin levels and hemoglobin mass in young healthy individuals. Nineteen participants were randomly assigned to an intermittent hypoxia group (Hyp, n = 10) or an intermittent normoxia group (Norm, n = 9). Intermittent hypoxia consisted of five 4-min hypoxic cycles at a targeted arterial oxygen saturation of 90% interspersed with 4-min normoxic cycles. Erythropoietin levels were measured before and two hours following completion of the protocol. Hemoglobin mass was assessed the day before and seven days after exposure to intermittent hypoxia or normoxia. As expected, the intermittent hypoxia group had a lower arterial oxygen saturation than the intermittent normoxia group during the intervention (Hyp: 89 ± 1 vs. Norm: 99 ± 1%, p < 0.01). Erythropoietin levels did not significantly increase following exposure to intermittent hypoxia (Hyp: 8.2 ± 4.5 to 9.0 ± 4.8, Norm: 8.9 ± 1.7 to 11.1 ± 2.1 mU·mL⁻¹, p = 0.15). Hemoglobin mass did not change following exposure to intermittent hypoxia. This single session of intermittent hypoxia was not sufficient to elicit a significant rise in erythropoietin levels or hemoglobin mass in young healthy individuals.

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.

Cardiovascular and metabolic responses to passive hypoxic conditioning in overweight and mildly obese individuals

Although severe intermittent hypoxia (IH) is well known to induce deleterious cardiometabolic consequences, moderate IH may induce positive effects in obese individuals. The present study aimed to evaluate the effect of two hypoxic conditioning programs on cardiovascular and metabolic health status of overweight or obese individuals. In this randomized single-blind controlled study, 35 subjects (54 ± 9.3 yr, 31.7 ± 3.5 kg/m²) were randomized into three 8-wk interventions (three 1-h sessions per week): sustained hypoxia (SH), arterial oxygen saturation ([Formula: see text]) = 75%; IH, 5 min [Formula: see text] = 75% - 3 min normoxia; normoxia. Ventilation, heart rate, blood pressure, and tissue oxygenation were measured during the first and last hypoxic conditioning sessions. Vascular function, blood glucose and insulin, lipid profile, nitric oxide metabolites, and oxidative stress were evaluated before and after the interventions. Both SH and IH increased ventilation in hypoxia (+1.8 ± 2.1 and +2.3 ± 3.6 L/min, respectively; P < 0.05) and reduced normoxic diastolic blood pressure (-12 ± 15 and -13 ± 10 mmHg, respectively; P < 0.05), whereas changes in normoxic systolic blood pressure were not significant (+3 ± 9 and -6 ± 13 mmHg, respectively; P > 0.05). IH only reduced heart rate variability (e.g., root-mean-square difference of successive normal R-R intervals in normoxia -21 ± 35%; P < 0.05). Both SH and IH induced no significant change in body mass index, vascular function, blood glucose, insulin and lipid profile, nitric oxide metabolites, or oxidative stress, except for an increase in superoxide dismutase activity following SH. This study indicates that passive hypoxic conditioning in obese individuals induces some positive cardiovascular and respiratory improvements despite no change in anthropometric data and even a reduction in heart rate variability during IH exposure.

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.

Circulating exosomes in obstructive sleep apnea as phenotypic biomarkers and mechanistic messengers of end-organ morbidity

Obstructive sleep apnea (OSA), the most severe form of sleep disordered breathing, is characterized by intermittent hypoxia during sleep (IH), sleep fragmentation, and episodic hypercapnia. OSA is associated with increased risk for morbidity and mortality affecting cardiovascular, metabolic, and neurocognitive systems, and more recently with non-alcoholic fatty liver disease (NAFLD) and cancer-related deaths. Substantial variability in OSA outcomes suggests that genetically-determined and environmental and lifestyle factors affect the phenotypic susceptibility to OSA. Furthermore, OSA and obesity often co-exist and manifest activation of shared molecular end-organ injury mechanisms that if properly identified may represent potential therapeutic targets. A challenge in the development of non-invasive diagnostic assays in body fluids is the ability to identify clinically relevant biomarkers. Circulating extracellular vesicles (EVs) include a heterogeneous population of vesicular structures including exosomes, prostasomes, microvesicles (MVs), ectosomes and oncosomes, and are classified based on their size, shape and membrane surface composition. Of these, exosomes (30-100nm) are very small membrane vesicles derived from multi-vesicular bodies or from the plasma membrane and play important roles in mediating cell-cell communication via cargo that includes lipids, proteins, mRNAs, miRNAs and DNA. We have recently identified a unique cluster of exosomal miRNAs in both humans and rodents exposed to intermittent hypoxia as well as in patients with OSA with divergent morbid phenotypes. Here we summarize such recent findings, and will focus on exosomal miRNAs in both adult and children which mediate intercellular communication relevant to OSA and endothelial dysfunction, and their potential value as diagnostic and prognostic biomarkers.

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.

δ-Opioid receptors: Pivotal role in intermittent hypoxia-augmentation of cardiac parasympathetic control and plasticity

BACKGROUND

Intermittent hypoxia training (IHT) produces robust myocardial protection against ischemia-reperfusion induced infarction and arrhythmias. Blockade of this cardioprotection by antagonism of either β1-adrenergic or δ-opioid receptors (δ-OR) suggests autonomic and/or opioidergic adaptations.

PURPOSE

To test the hypothesis that IHT shifts cardiac autonomic balance toward greater cholinergic and opioidergic influence.

METHODS

Mongrel dogs completed 20d IHT, non-hypoxic sham training, or IHT with the δ-OR antagonist naltrindole (200μg/kgsc). The vagolytic effect of the δ-OR agonist met-enkephalin-arg-phe delivered by sinoatrial microdialysis was evaluated following IHT. Sinoatrial, atrial and left ventricular biopsies were analyzed for changes in δ-OR, the neurotrophic monosialoganglioside, GM-1, and cholinergic and adrenergic markers.

RESULTS

IHT enhanced vagal bradycardia vs. sham dogs (P<0.05), and blunted the δ2-OR mediated vagolytic effect of met-enkephalin-arg-phe. The GM-1 labeled fibers overlapped strongly with cholinergic markers, and IHT increased the intensity of both signals (P<0.05). IHT increased low and high intensity vesicular acetylcholine transporter labeling of sinoatrial nodal fibers (P<0.05) suggesting an increase in parasympathetic arborization. IHT reduced select δ-OR labeled fibers in both the atria and sinoatrial node (P<0.05) consistent with moderation of the vagolytic δ2-OR signaling described above. Furthermore, blockade of δ-OR signaling with naltrindole during IHT increased the protein content of δ-OR (atria and ventricle) and vesicular acetylcholine transporter (atria) vs. sham and untreated IHT groups. IHT also reduced the sympathetic marker, tyrosine hydroxylase in ventricle (P<0.05).

SUMMARY

IHT shifts cardiac autonomic balance in favor of parasympathetic control via adaptations in opioidergic, ganglioside, and adrenergic systems.

Intermittent hypoxia training protects cerebrovascular function in Alzheimer's disease

Alzheimer's disease (AD) is a leading cause of death and disability among older adults. Modifiable vascular risk factors for AD (VRF) include obesity, hypertension, type 2 diabetes mellitus, sleep apnea, and metabolic syndrome. Here, interactions between cerebrovascular function and development of AD are reviewed, as are interventions to improve cerebral blood flow and reduce VRF. Atherosclerosis and small vessel cerebral disease impair metabolic regulation of cerebral blood flow and, along with microvascular rarefaction and altered trans-capillary exchange, create conditions favoring AD development. Although currently there are no definitive therapies for treatment or prevention of AD, reduction of VRFs lowers the risk for cognitive decline. There is increasing evidence that brief repeated exposures to moderate hypoxia, i.e. intermittent hypoxic training (IHT), improve cerebral vascular function and reduce VRFs including systemic hypertension, cardiac arrhythmias, and mental stress. In experimental AD, IHT nearly prevented endothelial dysfunction of both cerebral and extra-cerebral blood vessels, rarefaction of the brain vascular network, and the loss of neurons in the brain cortex. Associated with these vasoprotective effects, IHT improved memory and lessened AD pathology. IHT increases endothelial production of nitric oxide (NO), thereby increasing regional cerebral blood flow and augmenting the vaso- and neuroprotective effects of endothelial NO. On the other hand, in AD excessive production of NO in microglia, astrocytes, and cortical neurons generates neurotoxic peroxynitrite. IHT enhances storage of excessive NO in the form of S-nitrosothiols and dinitrosyl iron complexes. Oxidative stress plays a pivotal role in the pathogenesis of AD, and IHT reduces oxidative stress in a number of experimental pathologies. Beneficial effects of IHT in experimental neuropathologies other than AD, including dyscirculatory encephalopathy, ischemic stroke injury, audiogenic epilepsy, spinal cord injury, and alcohol withdrawal stress have also been reported. Further research on the potential benefits of IHT in AD and other brain pathologies is warranted.

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

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