Physiological Responses to Two Hypoxic Conditioning Strategies in Healthy Subjects

Intermittent Hypoxic-Hyperoxic Training During Inpatient Rehabilitation Improves Exercise Capacity and Functional Outcome in Patients With Long Covid: Results of a Controlled Clinical Pilot Trial

INTRODUCTION

Long COVID-19 illness is a severely disabling disease with shortness of breath, weakness and fatigue as leading symptoms, resulting in poor quality of life and substantial delay in return to work. No specific respiratory therapy has been validated for patients with long COVID. The intermittent hypoxia-hyperoxia training (IHHT) is a respiratory therapeutic modality to improve exercise performance via controlled respiratory conditioning. The purpose of the present study is to investigate the therapeutic effect of IHHT on functional and symptomatic recovery of patients with long COVID syndrome.

METHODS

A prospective, controlled, open-treatment interventional study was conducted in patients with long COVID who were admitted to an inpatient rehabilitation programme. Patients were assigned nonrandomized to receive IHHT in addition to the standardized rehabilitation programme (IHHT group) or standard rehabilitation alone (control group). The IHHT group received supervised sessions of intermittent hypoxic (10-12% O2) and hyperoxic (30-35% O2) breathing three times per week throughout the rehabilitation period. Primary endpoint was improved walking distance in a 6-min walk test (6MWT) between study groups. Secondary endpoints were change in stair climbing power, dyspnoea (Borg dyspnoea Scale), fatigue assessment scale (FAS) and change in health-related quality of life (HRQoL) assessed by patient global assessment (PGA), EQ-5D analogue scale and the MEDIAN Corona Recovery Score (MCRS). Further assessments included maximum handgrip strength, nine hole peg test, timed up-and-go, respiratory function and functional ambulation category (FAC), serum analyses and safety of the intervention.

RESULTS

A total of 145 patients were included in the study (74% female, mean age 53 ± 12 years) and assigned to IHHT (n = 70) or standard care (n = 75). The 6MWT distance improved 2.8-fold in the IHHT group compared to the control group (91.7 ± 50.1 m vs. 32.6 ± 54.2 m, ANCOVA p < 0.001). Stair climbing power improved 3.7-fold in the IHHT group compared to controls (-1.91 ± 2.23 s vs. -0.51 ± 1.93 s, p < 0.001). Secondary endpoints on dyspnoea, fatigue and HRQoL (PGA, EQ-5D and MCRS) improved significantly in the IHHT group compared to controls. The IHHT group exhibited a significant decrease in blood pressure, heart rate and increase in haemoglobin levels that was not observed in the control group. No adverse events were observed.

CONCLUSION

Respiratory treatment with IHHT in addition to a multidisciplinary rehabilitation programme improves functional capacity, symptomatic status and quality of life in patients with disabling long COVID. IHHT has been demonstrated to be safe, well tolerated and feasible to be integrated in an inpatient rehabilitation programme to improve outcome in long COVID.

Acute psycho-physiological responses to submaximal constant-load cycling under intermittent hypoxia-hyperoxia vs. hypoxia-normoxia in young males

Background

Hypoxia and hyperoxia can affect the acute psycho-physiological response to exercise. Recording various perceptual responses to exercise is of particular importance for investigating behavioral changes to physical activity, given that the perception of exercise-induced pain, discomfort or unpleasure, and a low level of exercise enjoyment are commonly associated with a low adherence to physical activity. Therefore, this study aimed to compare the acute perceptual and physiological responses to aerobic exercise under intermittent hypoxia-hyperoxia (IHHT), hypoxia-normoxia (IHT), and sustained normoxia (NOR) in young, recreational active, healthy males.

Methods

Using a randomized, single-blinded, crossover design, 15 males (age: 24.5 ± 4.2 yrs) performed 40 min of submaximal constant-load cycling (at 60% peak oxygen uptake, 80 rpm) under IHHT (5 × 4 min hypoxia and hyperoxia), IHT (5 × 4 min hypoxia and normoxia), and NOR. Inspiratory fraction of oxygen during hypoxia and hyperoxia was set to 14% and 30%, respectively. Heart rate (HR), total hemoglobin (tHb) and muscle oxygen saturation (SmO2) of the right vastus lateralis muscle were continuously recorded during cycling. Participants' peripheral oxygen saturation (SpO2) and perceptual responses (i.e., perceived motor fatigue, effort perception, perceived physical strain, affective valence, arousal, motivation to exercise, and conflict to continue exercise) were surveyed prior, during (every 4 min), and after cycling. Prior to and after exercise, peripheral blood lactate concentration (BLC) was determined. Exercise enjoyment was ascertained after cycling. For statistical analysis, repeated measures analyses of variance were conducted.

Results

No differences in the acute perceptual responses were found between conditions (p ≥ 0.059, ηp 2 ≤ 0.18), while the physiological responses differed. Accordingly, SpO2 was higher during the hyperoxic periods during the IHHT compared to the normoxic periods during the IHT (p < 0.001, ηp 2 = 0.91). Moreover, HR (p = 0.005, ηp 2 = 0.33) and BLC (p = 0.033, ηp 2 = 0.28) were higher during IHT compared to NOR. No differences between conditions were found for changes in tHb (p = 0.684, ηp 2 = 0.03) and SmO2 (p = 0.093, ηp 2 = 0.16).

Conclusion

IHT was associated with a higher physiological response and metabolic stress, while IHHT did not lead to an increase in HR and BLC compared to NOR. In addition, compared to IHT, IHHT seems to improve reoxygenation indicated by a higher SpO2 during the hyperoxic periods. However, there were no differences in perceptual responses and ratings of exercise enjoyment between conditions. These results suggest that replacing normoxic by hyperoxic reoxygenation-periods during submaximal constant-load cycling under intermittent hypoxia reduced the exercise-related physiological stress but had no effect on perceptual responses and perceived exercise enjoyment in young recreational active healthy males.

The Hypoxia Response Pathway: A Potential Intervention Target in Parkinson's Disease?

Parkinson's disease (PD) is a progressive neurodegenerative disorder for which only symptomatic treatments are available. Both preclinical and clinical studies suggest that moderate hypoxia induces evolutionarily conserved adaptive mechanisms that enhance neuronal viability and survival. Therefore, targeting the hypoxia response pathway might provide neuroprotection by ameliorating the deleterious effects of mitochondrial dysfunction and oxidative stress, which underlie neurodegeneration in PD. Here, we review experimental studies regarding the link between PD pathophysiology and neurophysiological adaptations to hypoxia. We highlight the mechanistic differences between the rescuing effects of chronic hypoxia in neurodegeneration and short-term moderate hypoxia to improve neuronal resilience, termed "hypoxic conditioning". Moreover, we interpret these preclinical observations regarding the pharmacological targeting of the hypoxia response pathway. Finally, we discuss controversies with respect to the differential effects of hypoxia response pathway activation across the PD spectrum, as well as intervention dosing in hypoxic conditioning and potential harmful effects of such interventions. We recommend that initial clinical studies in PD should focus on the safety, physiological responses, and mechanisms of hypoxic conditioning, as well as on repurposing of existing pharmacological compounds. © 2023 International Parkinson and Movement Disorder Society.

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.

Cardiac Autonomic Modulation and Response to Sub-Maximal Exercise in Chilean Hypertensive Miners

Cardiac autonomic modulation in workers exposed to chronic intermittent hypoxia (CIH) has been poorly studied, especially considering hypertensive ones. Heart rate variability (HRV) has been proven as valuable tool to assess cardiac autonomic modulation under different conditions. The aim of this study is to investigate the cardiac autonomic response related to submaximal exercise (i.e., six-minute walk test, 6MWT) in hypertensive (HT, n = 9) and non-hypertensive (NT, n = 10) workers exposed for > 2 years to CIH. Participants worked on 7-on 7-off days shift between high altitude (HA: > 4.200 m asl) and sea level (SL: < 500 m asl). Data were recorded with electrocardiography (ECG) at morning upon awakening (10 min supine, baseline), then at rest before and after (5 min sitting, pre and post) the 6MWT, performed respectively on the first day of their work shift at HA, and after the second day of SL sojourn. Heart rate was higher at HA in both groups for each measurement (p < 0.01). Parasympathetic indices of HRV were lower in both groups at HA, either in time domain (RMSSD, p < 0.01) and in frequency domain (log HF, p < 0.01), independently from measurement's time. HRV indices in non-linear domain supported the decrease of vagal tone at HA and showed a reduced signal's complexity. ECG derived respiration frequency (EDR) was higher at HA in both groups (p < 0.01) with interaction group x altitude (p = 0.012), i.e., higher EDR in HT with respect to NT. No significant difference was found in 6MWT distance regarding altitude for both groups, whereas HT covered a shorter 6MWT distance compared to NT (p < 0.05), both at HA and SL. Besides, conventional arm-cuff blood pressure and oxygen blood saturation values (recorded before, at the end and after 5-min recovery from 6MWT), reported differences related to HA only. HA is the main factor affecting cardiac autonomic modulation, independently from hypertension. However, presence of hypertension was associated with a reduced physical performance independently from altitude, and with higher respiratory frequency at HA.

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.

No Decrease in Blood Pressure After an Acute Bout of Intermittent Hyperpnea and Hypoxia in Prehypertensive Elderly

Prevalence of hypertension, subjective sleep complaints and snoring increases with age. Worse sleep and snoring, in turn, are independent risk factors to develop hypertension. Both respiratory muscle training (RMT) and intermittent hypoxia (IH) are suggested to have positive effects on these physiological and behavioral variables. This study therefore aimed to test the acute effects of a single bout of RMT, with and without IH, on resting blood pressure (BP) and sleep. Fourteen prehypertensive elderly performed a 60-min session of (a) intermittent voluntary normocapnic hyperpnea (HYP) alone, (b) HYP in combination with IH (HYP&IH) and (c) a sham intervention in randomized order. BP, hemodynamics, heart rate variability (HRV), cardiac baroreflex sensitivity (BRS) and pulse wave velocity (PWV) were assessed before and 15, 30 and 45 min after each intervention. Variables of sleep were assessed with actigraphy, pulse oximetry and with questionnaires during and after the night following each intervention. Neither HYP nor HYP&IH resulted in a decrease in BP. Repeated measures ANOVA revealed no significant interaction effect for systolic BP (p = 0.090), diastolic BP (p = 0.151), HRV parameters, BRS and PWV (all p > 0.095). Fragmentation index was lower after both HYP (−6.5 units) and HYP&IH (−8.4 units) compared to sham, p(ANOVA) = 0.046, although pairwise comparisons reveal no significant differences. There were no other significant effects for the remaining sleep variables. We conclude that one bout of intermittent hyperpnea, alone or in combination with IH, is not effective in lowering blood pressure or improving sleep in prehypertensive elderly.

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.

Comparison of the Effectiveness of High-Intensity Interval Training in Hypoxia and Normoxia in Healthy Male Volunteers: A Pilot Study

Aims

The study investigated the effect of high-intensity interval training in hypoxia and normoxia on serum concentrations of proangiogenic factors, nitric oxide, and inflammatory responses in healthy male volunteers.

Methods

Twelve physically active male subjects completed a high-intensity interval training (HIIT) in normoxia (NorTr) and in normobaric hypoxia (HypTr) (FiO₂ = 15.2%). The effects of HIIT in hypoxia and normoxia on maximal oxygen uptake, hypoxia-inducible factor-1-alpha, vascular endothelial growth factor, nitric oxide, and cytokines were analyzed.

Results

HIIT in hypoxia significantly increases maximal oxygen uptake (p=0.01) levels compared to pretraining levels. Serum hypoxia-inducible factor-1 (p=0.01) and nitric oxide levels (p=0.05), vascular endothelial growth factor (p=0.04), and transforming growth factor-β (p=0.01) levels were increased in response to exercise test after hypoxic training. There was no effect of training conditions for serum baseline angiogenic factors and cytokines (p > 0.05) with higher HIF-1α and NO levels after hypoxic training compared to normoxic training (F = 9.1; p < 0.01 and F = 5.7; p < 0.05, respectively).

Conclusions

High-intensity interval training in hypoxia seems to induce beneficial adaptations to exercise mediated via a significant increase in the serum concentrations of proangiogenic factors and serum nitric oxide levels compared to the same training regimen in normoxia.

Dose-response relationship of intermittent normobaric hypoxia to stimulate erythropoietin in the context of health promotion in young and old people

PURPOSE

Erythropoietin (EPO) has multifactorial positive effects on health and can be increased by intermittent normobaric hypoxia (IH). Recommendations about the intensity and duration of IH to increase EPO exist, but only for young people. Therefore, the aim of the study was to investigate the dose-response relationship regarding the duration of hypoxia until an EPO expression and the amount of EPO expression in old vs. young cohorts.

METHODS

56 young and 67 old people were assigned to two separate investigations with identical study designs (3-h hypoxic exposure) but with different approaches to adjust the intensity of hypoxia: (i) the fraction of inspired oxygen (FiO₂) was 13.5%; (ii) the FiO₂ was individually adjusted to an oxygen saturation of the blood of 80%. Age groups were randomly assigned to a hypoxia or control group (normoxic exposure). EPO was assessed before, during (90 and 180 min), and 30 min after the hypoxia.

RESULTS

EPO increased significantly after 180 min in both cohorts and in both investigations [old: (i) + 16%, p = 0.007 and (ii) + 14%, p < 0.001; young: (i) + 27%, p < 0.001 and (ii) + 45%, p = 0.007]. In investigation (i), EPO expression was significantly higher in young than in old people after 180 min of hypoxic exposure (p = 0.024) and 30 min afterwards (p = 0.001).

CONCLUSION

The results indicate that after a normobaric hypoxia of 180 min, EPO increases significantly in both age cohorts. The amount of EPO expression is significantly higher in young people during the same internal intensity of hypoxia than in old people.

Physiological responses to hypoxic constant-load and high-intensity interval exercise sessions in healthy subjects

PURPOSE

The aim of this study was to assess the acute cardiorespiratory as well as muscle and cerebral tissue oxygenation responses to submaximal constant-load (CL) and high-intensity interval (HII) cycling exercise performed in normoxia and in hypoxia at similar intensity, reproducing whole-body endurance exercise training sessions as performed in sedentary and clinical populations.

METHODS

Healthy subjects performed two CL (30 min, 75% of maximal heart rate, n = 12) and two HII (15 times 1-min high-intensity exercise-1-min passive recovery, n = 12) cycling exercise sessions in normoxia and in hypoxia [mean arterial oxygen saturation 76 ± 1% (clamped) during CL and 77 ± 5% (inspiratory oxygen fraction 0.135) during HII]. Cardiorespiratory and near-infrared spectroscopy parameters as well as the rate of perceived exertion were continuously recorded.

RESULTS

Power output was 21 ± 11% and 15% (according to protocol design) lower in hypoxia compared to normoxia during CL and HII exercise sessions, respectively. Heart rate did not differ between normoxic and hypoxic exercise sessions, while minute ventilation was higher in hypoxia during HII exercise only (+ 13 ± 29%, p < 0.05). Quadriceps tissue saturation index did not differ significantly between normoxia and hypoxia (CL 60 ± 8% versus 59 ± 5%; HII 59 ± 10% versus 56 ± 9%; p > 0.05), while prefrontal cortex deoxygenation was significantly greater in hypoxia during both CL (66 ± 4% versus 56 ± 6%) and HII (58 ± 5% versus 55 ± 5%; p < 0.05) sessions. The rate of perceived exertion did not differ between normoxic and hypoxic CL (2.4 ± 1.7 versus 2.9 ± 1.8) and HII (6.9 ± 1.4 versus 7.5 ± 0.8) sessions (p > 0.05).

CONCLUSION

This study indicates that at identical heart rate, reducing arterial oxygen saturation near 75% does not accentuate muscle deoxygenation during both CL and HII exercise sessions compared to normoxia. Hence, within these conditions, larger muscle hypoxic stress should not be expected.

Sex Differences in Autonomic Cardiac Control and Oxygen Saturation Response to Short-Term Normobaric Hypoxia and Following Recovery: Effect of Aerobic Fitness

Introduction: The main aims of this study were to investigate autonomic nervous system (ANS) and arterial oxygen saturation (SpO₂) responses to simulated altitude in males and females, and to determine the association between maximal oxygen uptake (VO₂max) and these responses. Materials and Methods: Heart rate variability (HRV) and SpO₂ were monitored in a resting supine position during Preliminary (6 min normoxia), Hypoxia (10 min, fraction of inspired oxygen (FiO₂) of 9.6%, simulated altitude ~6,200 m) and Recovery (6 min normoxia) phases in 28 males (age 23.7 ± 1.7 years, normoxic VO₂max 59.0 ± 7.8 ml.kg^-1.min^-1, body mass index (BMI) 24.2 ± 2.1 kg.m^-2) and 30 females (age 23.8 ± 1.8 years, VO₂max 45.1 ± 8.7 ml.kg^-1.min^-1, BMI 21.8 ± 3.0 kg.m^-2). Spectral analysis of HRV quantified the ANS activity by means of low frequency (LF, 0.05-0.15 Hz) and high frequency (HF, 0.15-0.50 Hz) power, transformed by natural logarithm (Ln). Time domain analysis incorporated the square root of the mean of the squares of the successive differences (rMSSD). Results: There were no significant differences in SpO₂ level during hypoxia between the males (71.9 ± 7.5%) and females (70.8 ± 7.1%). Vagally-related HRV variables (Ln HF and Ln rMSSD) exhibited no significant differences between sexes across each phase. However, while the sexes demonstrated similar Ln LF/HF values during the Preliminary phase, the males (0.5 ± 1.3) had a relatively higher (p = 0.001) sympathetic activity compared to females (-0.6 ± 1.4) during the Hypoxia phase. Oxygen desaturation during resting hypoxia was significantly correlated with VO₂max in males (r = -0.45, p = 0.017) but not in females (r = 0.01, p = 0.952) and difference between regression lines were significant (p = 0.024). Conclusions: Despite similar oxygen desaturation levels, males exhibited a relatively higher sympathetic responses to hypoxia exposure compared with females. In addition, the SpO₂ response to resting hypoxia exposure was related to maximal aerobic capacity in males but not females.

Hypoxic Air Inhalation and Ischemia Interventions Both Elicit Preconditioning Which Attenuate Subsequent Cellular Stress In vivo Following Blood Flow Occlusion and Reperfusion

Ischemic preconditioning (IPC) is valid technique which elicits reductions in femoral blood flow occlusion mediated reperfusion stress (oxidative stress, Hsp gene transcripts) within the systemic blood circulation and/or skeletal muscle. It is unknown whether systemic hypoxia, evoked by hypoxic preconditioning (HPC) has efficacy in priming the heat shock protein (Hsp) system thus reducing reperfusion stress following blood flow occlusion, in the same manner as IPC. The comparison between IPC and HPC being relevant as a preconditioning strategy prior to orthopedic surgery. In an independent group design, 18 healthy men were exposed to 40 min of (1) passive whole-body HPC (FiO₂ = 0.143; no ischemia. N = 6), (2) IPC (FiO₂ = 0.209; four bouts of 5 min ischemia and 5 min reperfusion. n = 6), or (3) rest (FiO₂ = 0.209; no ischemia. n = 6). The interventions were administered 1 h prior to 30 min of tourniquet derived femoral blood flow occlusion and were followed by 2 h subsequent reperfusion. Systemic blood samples were taken pre- and post-intervention. Systemic blood and gastrocnemius skeletal muscle samples were obtained pre-, 15 min post- (15PoT) and 120 min (120PoT) post-tourniquet deflation. To determine the cellular stress response gastrocnemius and leukocyte Hsp72 mRNA and Hsp32 mRNA gene transcripts were determined by RT-qPCR. The plasma oxidative stress response (protein carbonyl, reduced glutathione/oxidized glutathione ratio) was measured utilizing commercially available kits. In comparison to control, at 15PoT a significant difference in gastrocnemius Hsp72 mRNA was seen in HPC (−1.93-fold; p = 0.007) and IPC (−1.97-fold; p = 0.006). No significant differences were observed in gastrocnemius Hsp32 and Hsp72 mRNA, leukocyte Hsp72 and Hsp32 mRNA, or oxidative stress markers (p > 0.05) between HPC and IPC. HPC provided near identical amelioration of blood flow occlusion mediated gastrocnemius stress response (Hsp72 mRNA), compared to an established IPC protocol. This was seen independent of changes in systemic oxidative stress, which likely explains the absence of change in Hsp32 mRNA transcripts within leukocytes and the gastrocnemius. Both the established IPC and novel HPC interventions facilitate a priming of the skeletal muscle, but not leukocyte, Hsp system prior to femoral blood flow occlusion. This response demonstrates a localized tissue specific adaptation which may ameliorate reperfusion stress.

Hypoxic conditioning and the central nervous system: A new therapeutic opportunity for brain and spinal cord injuries?

Central nervous system diseases are among the most disabling in the world. Neuroprotection and brain recovery from either acute or chronic neurodegeneration still represent a challenge in neurology and neurorehabilitation as pharmacology treatments are often insufficiently effective. Conditioning the central nervous system has been proposed as a potential non-pharmacological neuro-therapeutic. Conditioning refers to a procedure by which a potentially deleterious stimulus is applied near to but below the threshold of damage to the organism to increase resistance to the same or even different noxious stimuli given above the threshold of damage. Hypoxic conditioning has been investigated in several cellular and preclinical models and is now recognized as inducing endogenous mechanisms of neuroprotection. Ischemic, traumatic, or chronic neurodegenerative diseases can benefit from hypoxic conditioning strategies aiming at preventing the deleterious consequences or reducing the severity of the pathological condition (preconditioning) or aiming at inducing neuroplasticity and recovery (postconditioning) following central nervous system injury. Hypoxic conditioning can consist in single (sustained) or cyclical (intermittent, interspersed by short period of normoxia) hypoxia stimuli which duration range from few minutes to several hours and that can be repeated over several days or weeks. This mini-review addresses the existing evidence regarding the use of hypoxic conditioning as a potential innovating neuro-therapeutic modality to induce neuroprotection, neuroplasticity and brain recovery. This mini-review also emphasizes issues which remain to be clarified and future researches to be performed in the field. Impact statement Neuroprotection and brain recovery from either acute or chronic neurodegeneration still represent a challenge in neurology and neurorehabilitation. Hypoxic conditioning may represent a harmless and efficient non-pharmacological new therapeutic modality in the field of neuroprotection and neuroplasticity, as supported by many preclinical data. Animal studies provide clear evidence for neuroprotection and neuroplasticity induced by hypoxic conditioning in several models of neurological disorders. These studies show improved functional outcomes when hypoxic conditioning is applied and provides important information to translate this intervention to clinical practice. Some studies in humans provide encouraging data regarding the tolerance and therapeutic effects of hypoxic conditioning strategies. The main issues to address in future research include the definition of the appropriate hypoxic dose and pattern of exposure, the determination of relevant physiological biomarkers to assess the effects of the treatment and the evaluation of combined strategies involving hypoxic conditioning and other pharmacological or non-pharmacological treatments.