Seven intermittent exposures to altitude improves exercise performance at 4300 m

Exercise induced plasma volume expansion lowers cardiovascular strain during 15-km cycling time-trial in acute normobaric hypoxia

The purpose of our study was to assess the influence of a single high-intensity interval exercise (HIIE) bout in normoxia on plasma volume (PV) and consequent cycling performance in normobaric hypoxia (0.15 FiO2, simulating ~2,500 m). Eight males (VO2peak: 48.8 ± 3.4 mL/kg/min, 24.0 ± 1.6 years) completed a hypoxic 15 km cycling time trial (TT), followed by a crossover intervention of either HIIE (8x4 min cycling bouts at 85% of VO2peak) or CON (matched kJ production from HIIE at 50% of VO2peak). 48 hours post intervention, an identical TT was performed. Cardiovascular parameters were measured via impedance cardiography during each TT. Changes in PV was measured 24 and 48 hours post HIIE and CON. HIIE increased PV at 24 (4.1 ± 3.9%, P = 0.031) and 48 (6.7 ± 1.7, P = 0.006) hours post, while no difference was observed following the CON (1.3 ± 1.1% and 0.3 ± 2.8%). The higher PV led to an increased stroke volume (P = 0.03) and cardiac output (P = 0.02) during the hypoxic TT, while heart rate was not changed (P = 0.49). We observed no changes in time to completion (-0.63 ± 0.57 min, P = 0.054) and power output (7.37 ± 7.98 W, P = 0.078) between TTs. In the absence of environmental stress, a single bout of HIIE was an effective strategy to increase PV and reduce the cardiovascular strain during a cycling TT at moderate simulated altitude but did not impact hypoxic exercise performance. Trial registration: Clinical Trials ID: NCT05800808.

Molecular Mechanisms of High-Altitude Acclimatization

High-altitude illnesses (HAIs) result from acute exposure to high altitude/hypoxia. Numerous molecular mechanisms affect appropriate acclimatization to hypobaric and/or normobaric hypoxia and curtail the development of HAIs. The understanding of these mechanisms is essential to optimize hypoxic acclimatization for efficient prophylaxis and treatment of HAIs. This review aims to link outcomes of molecular mechanisms to either adverse effects of acute high-altitude/hypoxia exposure or the developing tolerance with acclimatization. After summarizing systemic physiological responses to acute high-altitude exposure, the associated acclimatization, and the epidemiology and pathophysiology of various HAIs, the article focuses on molecular adjustments and maladjustments during acute exposure and acclimatization to high altitude/hypoxia. Pivotal modifying mechanisms include molecular responses orchestrated by transcription factors, most notably hypoxia inducible factors, and reciprocal effects on mitochondrial functions and REDOX homeostasis. In addition, discussed are genetic factors and the resultant proteomic profiles determining these hypoxia-modifying mechanisms culminating in successful high-altitude acclimatization. Lastly, the article discusses practical considerations related to the molecular aspects of acclimatization and altitude training strategies.

Variations of Time Irreversibility of Heart Rate Variability Under Normobaric Hypoxic Exposure

In the field of biomedicine, time irreversibility is used to describe how imbalanced and asymmetric biological signals are. As an important feature of signals, the direction of time is always ignored. To find out the variation regularity of time irreversibility of heart rate variability (HRV) in the initial stage of hypoxic exposure, the present study implemented 2 h acute normobaric hypoxic exposure on six young subjects who have no plateau or hypoxia experiences; oxygen concentration was set as 12.9%. Electrocardiogram (ECG) signals were recorded in the whole process and RR interval sequences were extracted. Mathematical operations were executed to transform the difference of adjacent RR intervals into proportion and distance with delay time to conduct time irreversibility analysis of HRV. The same calculating method was implemented on six items randomly picked out from the MIT-BIH normal sinus rhythm database as a control group. Results show that variation of time irreversibility of HRV in a hypoxic environment is different from that in a normoxic environment, time irreversibility indices of a hypoxic group decreases continually at a delay time of 1 and 2, and indices curves of time irreversibility gradually tend to be steady and gather with each other at a delay time of 3 or 4. The control group shows no consistent tendency no matter what the delay time is in the range of 1-4. Our study indicates that in short-time hypoxic exposure, as hypoxic time goes by, regulation of the cardiovascular autonomic nervous system weakens; regulation times and intensity of sympathetic and parasympathetic nerves tend to be equal.

Determining the time needed for workers to acclimatize to hypoxia

This study aimed to determine the influence of intermittent hypoxia and the days required for a worker to be acclimatized in high-altitude countries. We conducted an experimental study. Ten nonsmoking male students were randomly recruited from King Saud University. Fourteen days of exposure to intermittent normobaric hypoxia (15%) was the independent variable. Heart rate (HR), respiratory frequency (RF), minute ventilation (VE), respiratory exchange ratio (RER), tidal volume (VT), oxygen uptake (VO₂),VO₂/kg, VO₂/HR, VE/VO₂, and VE/VCO₂ were the dependent variables. Our results showed that 12 days of exposure to intermittent hypoxia were sufficient for workers to acclimatize to hypoxia based on their respiratory responses (i.e., HR, RF, VE). This type of acclimatization session is very important for workers who are suddenly required to work in such an environment, because prolonged exposure to high altitude without acclimatization leads to cell death due to a lack of oxygen, and this, in turn, puts workers' lives at risk.

Effect of 8 days of exercise-heat acclimation on aerobic exercise performance of men in hypobaric hypoxia

Exercise-heat acclimation (EHA) induces adaptations that improve tolerance to heat exposure. Whether adaptations from EHA can also alter responses to hypobaric hypoxia (HH) conditions remains unclear. This study assessed whether EHA can alter time-trial performance and/or incidence of acute mountain sickness (AMS) during HH exposure. Thirteen sea-level (SL) resident men [SL peak oxygen consumption (V̇o_2peak) 3.19 ± 0.43 L/min] completed steady-state exercise, followed by a 15-min cycle time trial and assessment of AMS before (HH1; 3,500 m) and after (HH2) an 8-day EHA protocol [120 min; 5 km/h; 2% incline; 40°C and 40% relative humidity (RH)]. EHA induced lower heart rate (HR) and core temperature and plasma volume expansion. Time-trial performance was not different between HH1 and HH2 after 2 h (106.3 ± 23.8 vs. 101.4 ± 23.0 kJ, P = 0.71) or 24 h (107.3 ± 23.4 vs. 106.3 ± 20.8 kJ, P > 0.9). From HH1 to HH2, HR and oxygen saturation, at the end of steady-state exercise and time-trial tests at 2 h and 24 h, were not different (P > 0.05). Three of 13 volunteers developed AMS during HH1 but not during HH2, whereas a fourth volunteer only developed AMS during HH2. Heat shock protein 70 was not different from HH1 to HH2 at SL [1.9 ± 0.7 vs. 1.8 ± 0.6 normalized integrated intensities (NII), P = 0.97] or after 23 h (1.8 ± 0.4 vs. 1.7 ± 0.5 NII, P = 0.78) at HH. Our results indicate that this EHA protocol had little to no effect-neither beneficial nor detrimental-on exercise performance in HH. EHA may reduce AMS in those who initially developed AMS; however, studies at higher elevations, having higher incidence rates, are needed to confirm our findings.

Preparation for Endurance Competitions at Altitude: Physiological, Psychological, Dietary and Coaching Aspects. A Narrative Review

It was the Summer Olympic Games 1968 held in Mexico City (2,300 m) that required scientists and coaches to cope with the expected decline of performance in endurance athletes and to establish optimal preparation programs for competing at altitude. From that period until now many different recommendations for altitude acclimatization in advance of an altitude competition were proposed, ranging from several hours to several weeks. Those recommendations are mostly based on the separate consideration of the physiology of acclimatization, psychological issues, performance changes, logistical or individual aspects, but there is no review considering all these aspects in their entirety. Therefore, the present work primarily focusses on the period of altitude sojourn prior to the competition at altitude based on physiological and psychological aspects complemented by nutritional and sports practical considerations.

Exercise-induced Fatigue in Severe Hypoxia after an Intermittent Hypoxic Protocol

PURPOSE

Exercise-induced central fatigue is alleviated after acclimatization to high altitude. The adaptations underpinning this effect may also be induced with brief, repeated exposures to severe hypoxia. The purpose of this study was to determine whether (i) exercise tolerance in severe hypoxia would be improved after an intermittent hypoxic (IH) protocol and (ii) exercise-induced central fatigue would be alleviated after an IH protocol.

METHODS

Nineteen recreationally active men were randomized into two groups who completed ten 2-h exposures in severe hypoxia (IH: partial pressure of inspired O2 82 mm Hg; n = 11) or normoxia (control; n = 8). Seven sessions involved cycling for 30 min at 25% peak power (W˙peak) in IH and at a matched heart rate in normoxia. Participants performed baseline constant-power cycling to task failure in severe hypoxia (TTF-Pre). After the intervention, the cycling trial was repeated (TTF-Post). Before and after exercise, responses to transcranial magnetic stimulation and supramaximal femoral nerve stimulation were obtained to assess central and peripheral contributions to neuromuscular fatigue.

RESULTS

From pre- to postexercise in TTF-Pre, maximal voluntary contraction (MVC), cortical voluntary activation (VATMS), and potentiated twitch force (Qtw,pot) decreased in both groups (all P < 0.05). After IH, TTF-Post was improved (535 ± 213 s vs 713 ± 271 s, P < 0.05) and an additional isotime trial was performed. After the IH intervention only, the reduction in MVC and VATMS was attenuated at isotime (P < 0.05). No differences were observed in the control group.

CONCLUSIONS

Whole-body exercise tolerance in severe hypoxia was prolonged after a protocol of IH. This may be related to an alleviation of the central contribution to neuromuscular fatigue.

Cerebrocortical activity during self-paced exercise in temperate, hot and hypoxic conditions

AIM

Heat stress and hypoxia independently influence cerebrocortical activity and impair prolonged exercise performance. This study examined the relationship between electroencephalography (EEG) activity and self-paced exercise performance in control (CON, 18 °C, 40% RH), hot (HOT, 35 °C, 60% RH) and hypoxic (HYP, 18 °C, 40% RH FiO₂ : 0.145) conditions.

METHODS

Eleven well-trained cyclists completed a 750 kJ cycling time trial in each condition on separate days in a counterbalanced order. EEG activity was recorded with α- and β-activity evaluated in the frontal (F3 and F4) and central (C3 and C4) areas. Standardized low-resolution brain electromagnetic tomography (sLORETA) was also utilized to localize changes in cerebrocortical activity.

RESULTS

Both α- and β-activity decreased in the frontal and central areas during exercise in HOT relative to CON (P < 0.05). α-activity was also lower in HYP compared with CON (P < 0.05), whereas β-activity remained similar. β-activity was higher in HYP than in HOT (P < 0.05). sLORETA revealed that α- and β-activity increased at the onset of exercise in the primary somatosensory and motor cortices in CON and HYP, while only β-activity increased in HOT. A decrease in α- and β-activity occurred thereafter in all conditions, with α-activity being lower in the somatosensory and somatosensory association cortices in HOT relative to CON.

CONCLUSION

High-intensity prolonged self-paced exercise induces cerebrocortical activity alterations in areas of the brain associated with the ability to inhibit conflicting attentional processing under hot and hypoxic conditions, along with the capacity to sustain mental readiness and arousal under heat stress.

Cross Acclimation between Heat and Hypoxia: Heat Acclimation Improves Cellular Tolerance and Exercise Performance in Acute Normobaric Hypoxia

BACKGROUND

The potential for cross acclimation between environmental stressors is not well understood. Thus, the aim of this investigation was to determine the effect of fixed-workload heat or hypoxic acclimation on cellular, physiological, and performance responses during post acclimation hypoxic exercise in humans.

METHOD

Twenty-one males (age 22 ± 5 years; stature 1.76 ± 0.07 m; mass 71.8 ± 7.9 kg; [Formula: see text]O2 peak 51 ± 7 mL(.)kg(-1.)min(-1)) completed a cycling hypoxic stress test (HST) and self-paced 16.1 km time trial (TT) before (HST1, TT1), and after (HST2, TT2) a series of 10 daily 60 min training sessions (50% N [Formula: see text]O2 peak) in control (CON, n = 7; 18°C, 35% RH), hypoxic (HYP, n = 7; fraction of inspired oxygen = 0.14, 18°C, 35% RH), or hot (HOT, n = 7; 40°C, 25% RH) conditions.

RESULTS

TT performance in hypoxia was improved following both acclimation treatments, HYP (-3:16 ± 3:10 min:s; p = 0.0006) and HOT (-2:02 ± 1:02 min:s; p = 0.005), but unchanged after CON (+0:31 ± 1:42 min:s). Resting monocyte heat shock protein 72 (mHSP72) increased prior to HST2 in HOT (62 ± 46%) and HYP (58 ± 52%), but was unchanged after CON (9 ± 46%), leading to an attenuated mHSP72 response to hypoxic exercise in HOT and HYP HST2 compared to HST1 (p < 0.01). Changes in extracellular hypoxia-inducible factor 1-α followed a similar pattern to those of mHSP72. Physiological strain index (PSI) was attenuated in HOT (HST1 = 4.12 ± 0.58, HST2 = 3.60 ± 0.42; p = 0.007) as a result of a reduced HR (HST1 = 140 ± 14 b.min(-1); HST2 131 ± 9 b.min(-1) p = 0.0006) and Trectal (HST1 = 37.55 ± 0.18°C; HST2 37.45 ± 0.14°C; p = 0.018) during exercise. Whereas PSI did not change in HYP (HST1 = 4.82 ± 0.64, HST2 4.83 ± 0.63).

CONCLUSION

Heat acclimation improved cellular and systemic physiological tolerance to steady state exercise in moderate hypoxia. Additionally we show, for the first time, that heat acclimation improved cycling time trial performance to a magnitude similar to that achieved by hypoxic acclimation.

Wilderness medicine at high altitude: recent developments in the field

Travel to high altitude is increasingly popular. With this comes an increased incidence of high-altitude illness and therefore an increased need to improve our strategies to prevent and accurately diagnose these. In this review, we provide a summary of recent advances of relevance to practitioners who may be advising travelers to altitude. Although the Lake Louise Score is now widely used as a diagnostic tool for acute mountain sickness (AMS), increasing evidence questions the validity of doing so, and of considering AMS as a single condition. Biomarkers, such as brain natriuretic peptide, are likely correlating with pulmonary artery systolic pressure, thus potential markers of the development of altitude illness. Established drug treatments include acetazolamide, nifedipine, and dexamethasone. Drugs with a potential to reduce the risk of developing AMS include nitrate supplements, propagators of nitric oxide, and supplemental iron. The role of exercise in the development of altitude illness remains hotly debated, and it appears that the intensity of exercise is more important than the exercise itself. Finally, despite copious studies demonstrating the value of preacclimatization in reducing the risk of altitude illness and improving performance, an optimal protocol to preacclimatize an individual remains elusive.

The effect of 10 days of heat acclimation on exercise performance in acute hypobaric hypoxia (4350 m)

To examine the effect (“cross-tolerance”) of heat acclimation (HA) on exercise performance upon exposure to acute hypobaric hypoxia (4350 m). Eight male cyclists residing at 1600 m performed tests of maximal aerobic capacity (VO_2max) at 1600 m and 4350 m, a 16 km time-trial at 4350 m, and a heat tolerance test at 1600 m before and after 10 d HA at 40°C, 20% RH. Resting blood samples were obtained pre-and post- HA to estimate changes in plasma volume (ΔPV). Successful HA was indicated by significantly lower exercise heart rate and rectal temperature on day 10 vs. day 1 of HA and during the heat tolerance tests. Heat acclimation caused a 1.9% ΔPV, however VO_2max was not significantly different at 1600 m or 4350 m. Time-trial cycling performance improved 28 sec after HA (p = 0.07), suggesting a possible benefit for exercise performance at acute altitude and that cross-tolerance between these variables may exist in humans. These findings do not clearly support the use of HA to improve exercise capacity and performance upon acute hypobaric hypoxia, however they do indicate that HA is not detrimental to either exercise capacity or performance.

Day-to-day variability in cardiorespiratory responses to hypoxic cycle exercise

Repeatedly performing exercise in hypoxia could elicit an independent training response and become an unintended co-intervention. The primary purposes of this study were to determine if hypoxic exercise responses changed across repeated testing and to assess the day-to-day variability of commonly used measures of cardiorespiratory and metabolic responses to hypoxic exercise. Healthy young males (aged 23 ± 2 years) with a maximal O2 consumption of 50.7 ± 4.7 mL·kg(-1)·min(-1) performed 5 trials (H1 to H5) over a 2-week period in hypoxia (fraction of inspired oxygen = 0.13). Participants completed 3-min stages at 20%, 40%, 60%, and 10% of individual peak power. With increasing cycle exercise intensity there were increases in minute ventilation, O2 consumption, CO2 production, respiratory exchange ratio, heart rate (HR), blood lactate concentration, and ratings of perceived exertion for legs and respiratory system along with a reduction in oxyhaemoglobin saturation (%SpO2) (all p < 0.001). There were no systematic changes from H1 to H5 (p > 0.05). Most measures were highly repeatable across testing sessions with the coefficient of variation (CV) averaging ≤10% of the mean value in all variables except O2 consumption (17%), CO2 production (11%) and blood lactate concentration (17%). For HR and %SpO2 the CV was <5%. The exercise protocol did not elicit a training response when repeated 5 times during a 2-week period and the variability of exercise responses was low. We conclude that this protocol allows detection of small changes in cardiorespiratory responses to hypoxic exercise that might occur during exposure to hypoxia.

Timing of arrival and pre-acclimatization strategies for the endurance athlete competing at moderate to high altitudes

With the wide array of endurance sport competition offerings at moderate and high altitudes, clinicians are frequently asked about best practice recommendations regarding arrival times prior to the event and acclimatization guidelines. This brief review will offer data and current advice on when to arrive at altitude and various potential sea level-based pre-acclimatization strategies in an effort to maximize performance and minimize the risk of altitude sickness.

A Novel Mechanism for Cross-Adaptation between Heat and Altitude Acclimation: The Role of Heat Shock Protein 90

Heat shock protein 90 (HSP90) is a member of a family of molecular chaperone proteins which can be upregulated by various stressors including heat stress leading to increases in HSP90 protein expression. Its primary functions include (1) renaturing and denaturing of damaged proteins caused by heat stress and (2) interacting with client proteins to induce cell signaling for gene expression. The latter function is of interest because, in cancer cells, HSP90 has been reported to interact with the transcription hypoxic-inducible factor 1α (HIF1α). In a normoxic environment, HIF1α is degraded and therefore has limited physiological function. In contrast, in a hypoxic environment, stabilized HIF1α acts to promote erythropoiesis and angiogenesis. Since HSP90 interacts with HIF1α, and HSP90 can be upregulated from heat acclimation in humans, we present a proposal that heat acclimation can mimic molecular adaptations to those of altitude exposure. Specifically, we propose that heat acclimation increases HSP90 which then stabilizes HIF1α in a normoxic environment. This has many implications since HIF1α regulates red blood cell and vasculature formation. In this paper we will discuss (1) the functional roles of HSP90 and HIF1α, (2) the interaction between HSP90 and other client proteins including HIF1α, and (3) results from in vitro studies that may suggest how the relationship between HSP90 and HIF1α might be applied to individuals preparing to make altitude sojourns.

Effectiveness of preacclimatization strategies for high-altitude exposure

Acute mountain sickness (AMS) and large decrements in endurance exercise performance occur when unacclimatized individuals rapidly ascend to high altitudes. Six altitude and hypoxia preacclimatization strategies were evaluated to determine their effectiveness for minimizing AMS and improving performance during altitude exposures. Strategies using hypobaric chambers or true altitude were much more effective overall than those using normobaric hypoxia (breathing, <20.9% oxygen).

Different duration of high-altitude pre-exposure associated with the incidence of acute mountain sickness on Jade Mountain

OBJECTIVE

The objective of this study is to determine the association between the duration of high-altitude (>3000 m) pre-exposure and acute mountain sickness (AMS) incidence.

METHODS

A prospective observational study was conducted on 2 random days each month from April 2007 to March 2008 at Paiyun Lodge (3402 m), Jade Mountain, Taiwan. Demographic data, prior AMS history, symptoms, and scores and the days and times of high-altitude pre-exposure within the preceding 2 months were obtained from lowland (<1500 m) trekkers.

RESULTS

Totally, 1010 questionnaires were analyzed; 106, 76, and 828 trekkers had pre-exposure lasting at least 3 days (group 1), less than 3 days (group 2), and 0 days (group 3), respectively. Acute mountain sickness incidence was significantly higher in groups 2 and 3 than in group 1 (21.70%, 35.53%, 37.08%, respectively; P = .008). Logistic regression analysis indicated a significantly lower AMS risk in group 1 (group 1, P = .004; odds ratio [OR], 0.479; 95% confidence interval [CI], 0.290-0.791; group 2, P = .226; OR, 0.725; 95% CI, 0.430-1.221). In group 1, 28 and 78 trekkers had single and intermittent multiple pre-exposure, respectively. There was no difference in the incidence or severity of AMS symptoms between single and intermittent multiple pre-exposure (AMS, P = .838; headache, P = .891; dizziness or lightheadedness, P = .414; fatigue and/or weakness, P = .957; gastrointestinal symptoms, P = .257; difficulty sleeping, P = .804; AMS score, P = .796).

CONCLUSIONS

High-altitude pre-exposure lasting at least 3 days within the preceding 2 months was associated with a significant lower AMS incidence during a subsequent ascent among Jade Mountain trekkers.

Metabolic adaptations may counteract ventilatory adaptations of intermittent hypoxic exposure during submaximal exercise at altitudes up to 4000 m

Intermittent hypoxic exposure (IHE) has been shown to induce aspects of altitude acclimatization which affect ventilatory, cardiovascular and metabolic responses during exercise in normoxia and hypoxia. However, knowledge on altitude-dependent effects and possible interactions remains scarce. Therefore, we determined the effects of IHE on cardiorespiratory and metabolic responses at different simulated altitudes in the same healthy subjects. Eight healthy male volunteers participated in the study and were tested before and 1 to 2 days after IHE (7×1 hour at 4500 m). The participants cycled at 2 submaximal workloads (corresponding to 40% and 60% of peak oxygen uptake at low altitude) at simulated altitudes of 2000 m, 3000 m, and 4000 m in a randomized order. Gas analysis was performed and arterial oxygen saturation, blood lactate concentrations, and blood gases were determined during exercise. Additionally baroreflex sensitivity, hypoxic and hypercapnic ventilatory response were determined before and after IHE. Hypoxic ventilatory response was increased after IHE (p<0.05). There were no altitude-dependent changes by IHE in any of the determined parameters. However, blood lactate concentrations and carbon dioxide output were reduced; minute ventilation and arterial oxygen saturation were unchanged, and ventilatory equivalent for carbon dioxide was increased after IHE irrespective of altitude. Changes in hypoxic ventilatory response were associated with changes in blood lactate (r = −0.72, p<0.05). Changes in blood lactate correlated with changes in carbon dioxide output (r = 0.61, p<0.01) and minute ventilation (r = 0.54, p<0.01). Based on the present results it seems that the reductions in blood lactate and carbon dioxide output have counteracted the increased hypoxic ventilatory response. As a result minute ventilation and arterial oxygen saturation did not increase during submaximal exercise at simulated altitudes between 2000 m and 4000 m.

Usefulness of combining intermittent hypoxia and physical exercise in the treatment of obesity

Obesity is an important public health problem worldwide and is a major risk factor for a number of chronic diseases such as type II diabetes, adverse cardiovascular events and metabolic syndrome-related features. Different treatments have been applied to tackle body fat accumulation and its associated clinical manifestations. Often, relevant weight loss is achieved during the first 6 months under different dietary treatments. From this point, a plateau is reached, and a gradual recovery of the lost weight may occur. Therefore, new research approaches are being investigated to assure weight maintenance. Pioneering investigations have reported that oxygen variations in organic systems may produce changes in body composition. Possible applications of intermittent hypoxia to promote health and in various pathophysiological states have been reported. The hypoxic stimulus in addition to diet and exercise can be an interesting approach to lose weight, by inducing higher basal noradrenalin levels and other metabolic changes whose mechanisms are still unclear. Indeed, hypoxic situations increase the diameter of arterioles, produce peripheral vasodilatation and decrease arterial blood pressure. Furthermore, hypoxic training increases the activity of glycolytic enzymes, enhancing the number of mitochondria and glucose transporter GLUT-4 levels as well as improving insulin sensitivity. Moreover, hypoxia increases blood serotonin and decreases leptin levels while appetite is suppressed. These observations allow consideration of the hypothesis that intermittent hypoxia induces fat loss and may ameliorate cardiovascular health, which might be of interest for the treatment of obesity. This new strategy may be useful and practical for clinical applications in obese patients.

Effect of repeated normobaric hypoxia exposures during sleep on acute mountain sickness, exercise performance, and sleep during exposure to terrestrial altitude

There is an expectation that repeated daily exposures to normobaric hypoxia (NH) will induce ventilatory acclimatization and lessen acute mountain sickness (AMS) and the exercise performance decrement during subsequent hypobaric hypoxia (HH) exposure. However, this notion has not been tested objectively. Healthy, unacclimatized sea-level (SL) residents slept for 7.5 h each night for 7 consecutive nights in hypoxia rooms under NH [ n = 14, 24 ± 5 (SD) yr] or “sham” ( n = 9, 25 ± 6 yr) conditions. The ambient percent O2 for the NH group was progressively reduced by 0.3% [150 m equivalent (equiv)] each night from 16.2% (2,200 m equiv) on night 1 to 14.4% (3,100 m equiv) on night 7, while that for the ventilatory- and exercise-matched sham group remained at 20.9%. Beginning at 25 h after sham or NH treatment, all subjects ascended and lived for 5 days at HH (4,300 m). End-tidal Pco2, O2 saturation (SaO2), AMS, and heart rate were measured repeatedly during daytime rest, sleep, or exercise (11.3-km treadmill time trial). From pre- to posttreatment at SL, resting end-tidal Pco2 decreased ( P < 0.01) for the NH (from 39 ± 3 to 35 ± 3 mmHg), but not for the sham (from 39 ± 2 to 38 ± 3 mmHg), group. Throughout HH, only sleep SaO2 was higher (80 ± 1 vs. 76 ± 1%, P < 0.05) and only AMS upon awakening was lower (0.34 ± 0.12 vs. 0.83 ± 0.14, P < 0.02) in the NH than the sham group; no other between-group rest, sleep, or exercise differences were observed at HH. These results indicate that the ventilatory acclimatization induced by NH sleep was primarily expressed during HH sleep. Under HH conditions, the higher sleep SaO2 may have contributed to a lessening of AMS upon awakening but had no impact on AMS or exercise performance for the remainder of each day.

Altitude preexposure recommendations for inducing acclimatization

For many low-altitude (<1500 m) residents, their travel itineraries may cause them to ascend rapidly to high (>2400 m) altitudes without having the time to develop an adequate degree of altitude acclimatization. Prior to departing on these trips, low-altitude residents can induce some degree of altitude acclimatization by ascending to moderate (>1500 m) or high altitudes during either continuous or intermittent altitude preexposures. Generally, the degree of altitude acclimatization developed is proportional to the altitude attained and the duration of exposure. The available evidence suggests that continuous residence at 2200 m or higher for 1 to 2 days or daily 1.5- to 4-h exposures to >4000 m induce ventilatory acclimatization. Six days at 2200 m substantially decreases acute mountain sickness (AMS) and improves work performance after rapid ascent to 4300 m. There is evidence that 5 or more days above 3000 m within the last 2 months will significantly decrease AMS during a subsequent rapid ascent to 4500 m. Exercise training during the altitude preexposures may augment improvement in physical performance. The persistence of altitude acclimatization after return to low altitude appears to be proportional to the degree of acclimatization developed. The subsequent ascent to high altitude should be scheduled as soon as possible after the last altitude preexposure.

Intermittent hypoxic exposure does not improve endurance performance at altitude

PURPOSE

This study examined the effect of 1 wk of normobaric intermittent hypoxic exposure (IHE) combined with exercise training on endurance performance at a 4300-m altitude (HA).

METHODS

Seventeen male lowlanders were divided into an IHE (n = 11) or SHAM (n = 6) group. Each completed cycle endurance testing consisting of two 20-min steady state (SS) exercise bouts (at 40% and 60% V O2peak) followed by a 10-min break and then a 720-kJ cycle time trial at HA before IHE or SHAM treatment (Pre-T). IHE treatment consisted of a 2-h rest at a PO2 of 90 mm Hg followed by two 25-min bouts of exercise at approximately 80% of peak HR at a PO2 of 110 mm Hg for 1 wk in a hypoxia room. SHAM treatment was identical except that the PO2 was 148 mm Hg for both rest and exercise. After IHE or SHAM treatment (Post-T), all completed a second cycle endurance test at HA. HR, arterial oxygen saturation (SaO2), and RPE were obtained from the 10th to the 15th minute during the two SS exercise bouts and every 5 min during the time trial.

RESULTS

Seven volunteers in the IHE group could not finish the 720-kJ time trial either at Pre-T or at Post-T. Time trial analysis was limited, therefore, to the time to reach 360 kJ (halfway point) for all volunteers. From Pre-T to Post-T, there was no improvement in time trial performance (min +/- SE) in the IHE (62.0 +/- 4.8 to 63.7 +/- 5.2) or SHAM (60.9 +/- 6.3 to 54.2 +/- 6.8) group. There was no change from Pre-T to Post-T in HR, SaO2, and RPE during the two SS exercise bouts or time trial in either group.

CONCLUSIONS

One week of IHE combined with exercise training does not improve endurance performance at a 4300-m altitude in male lowlanders.

Intermittent hypoxia does not affect endurance performance at moderate altitude in well-trained athletes

In this study, we examined the effects of a pre-acclimatization programme on endurance performance at moderate altitude using a resting intermittent hypoxia protocol. The time-trial performance of 11 cyclists was determined at low altitude (600 m). Athletes were randomly assigned in a double-blind fashion to the hypoxia or the control group. The pre-acclimatization programme consisted of seven sessions each lasting 1 h in normobaric hypoxia (inspired fraction of oxygen of 12.5%, equivalent to approximately 4500 m) for the hypoxia group (n = 6) and in normoxia (inspired fraction of oxygen of 20.9%) for the control group (n = 5). The time-trials were repeated at moderate altitude (1970 m). Mean power output during the time-trial at moderate altitude was decreased in the hypoxia group (-0.26 +/- 0.11 W x kg(-1)) and in the control group (-0.13 +/- 0.04 W x kg(-1)) compared with at low altitude but did not differ between groups (P = 0.13). Our results suggest that the applied protocol of intermittent hypoxia had no positive effect on endurance performance at moderate altitude. Whether different intermittent hypoxia protocols are advantageous remains to be determined.

Intermittent hypoxia at rest for improvement of athletic performance

Two modalities of applying hypoxia at rest are reviewed in this paper: intermittent hypoxic exposure (IHE), which consists of hypoxic air for 5-6 min alternating with breathing room air for 4-5 min during sessions lasting 60-90 min, or prolonged hypoxic exposure (PHE) to normobaric or hypobaric hypoxia over up to 3 h/day. Hypoxia with IHE is usually in the range of 12-10%, corresponding to an altitude of about 4000-6000 m. Normobaric or hypobaric hypoxia with PHE corresponds to altitudes of 4000-5500 m. Five of six studies applying IHE and all four well-controlled studies using PHE could not show a significant improvement with these modalities of hypoxic exposure for sea level performance after 14-20 sessions of exposure, with the exception of swimmers in whom there might be a slight improvement by PHE in combination with a subsequent tapering. There is no direct or indirect evidence that IHE or PHE induce any significant physiological changes that might be associated with improving athletic performance at sea level. Therefore, IHE and PHE cannot be recommended for preparation of competitions held at sea level.