Ventilatory chemosensitive adaptations to intermittent hypoxic exposure with endurance training and detraining

The normal distribution of the hypoxic ventilatory response and methodological impacts: a meta-analysis and computational investigation

The hypoxic ventilatory response (HVR) is the increase in breathing in response to reduced arterial oxygen pressure. Over several decades, studies have revealed substantial population-level differences in the magnitude of the HVR as well as significant inter-individual variation. In particular, low HVRs occur frequently in Andean high-altitude native populations. However, our group conducted hundreds of HVR measures over several years and commonly observed low responses in sea-level populations as well. As a result, we aimed to determine the normal HVR distribution, whether low responses were common, and to what extent variation in study protocols influence these findings. We conducted a comprehensive search of the literature and examined the distributions of HVR values across 78 studies that utilized step-down/steady-state or progressive hypoxia methods in untreated, healthy human subjects. Several studies included multiple datasets across different populations or experimental conditions. In the final analysis, 72 datasets reported mean HVR values and 60 datasets provided raw HVR datasets. Of the 60 datasets reporting raw HVR values, 35 (58.3%) were at least moderately positively skewed (skew > 0.5), and 21 (35%) were significantly positively skewed (skew > 1), indicating that lower HVR values are common. The skewness of HVR distributions does not appear to be an artifact of methodology or the unit with which the HVR is reported. Further analysis demonstrated that the use of step-down hypoxia versus progressive hypoxia methods did not have a significant impact on average HVR values, but that isocapnic protocols produced higher HVRs than poikilocapnic protocols. This work provides a reference for expected HVR values and illustrates substantial inter-individual variation in this key reflex. Finally, the prevalence of low HVRs in the general population provides insight into our understanding of blunted HVRs in high-altitude adapted groups. KEY POINTS: The hypoxic ventilatory response (HVR) plays a crucial role in determining an individual's predisposition to hypoxia-related pathologies. There is notable variability in HVR sensitivity across individuals as well as significant population-level differences. We report that the normal distribution of the HVR is positively skewed, with a significant prevalence of low HVR values amongst the general healthy population. We also find no significant impact of the experimental protocol used to induce hypoxia, although HVR is greater with isocapnic versus poikilocapnic methods. These results provide insight into the normal distribution of the HVR, which could be useful in clinical decisions of diseases related to hypoxaemia. Additionally, the low HVR values found within the general population provide insight into the genetic adaptations found in populations residing in high altitudes.

Peripheral hypercapnic chemosensitivity in trained and untrained females and males during exercise

Hypercapnic chemosensitivity is the response to the increased partial pressure of carbon dioxide and results from central and peripheral chemosensor stimulation. The hypercapnic chemosensitivity of the peripheral chemoreceptors is potentially impacted by acute exercise, aerobic fitness, and sex. We sought to determine the peripheral chemoresponse to transient hypercapnia at rest and during exercise in males and females of various fitness. We hypothesized that 1) higher fitness participants would have lower hypercapnic chemosensitivity compared with those with lower fitness and 2) males would have a higher chemoresponse than females. Forty healthy participants (20 females) participated in one test day involving transient hypercapnic chemosensitivity testing and a maximal exercise test. Chemosensitivity testing involved two breaths of 10% CO₂ repeated five times (45 s to 1 min between repeats) at rest and the first two stages of a maximal exercise test. There was no significant difference between higher and lower aerobic fitness groups, (mean difference 0.23 ± 0.22 rest; -0.07 ± 0.04 stage 1; 0.11 ± 0.17 stage 2 L/mmHg·min) during each stage (P = 0.472). However, we saw a significant increase in the hypercapnic response during stage 1 (0.98 ± 0.4 L/mmHg·min) compared with rest (0.79 ± 0.5 L/mmHg·min; P = 0.01). Finally, at 80 W, males had a higher chemoresponse compared with females, which persisted following body surface area correction (0.56 ± 0.2 vs. 0.42 ± 0.2 L/mmHg·min·m², for females and males respectively (P = 0.038). Our findings suggest that sex, unlike aerobic fitness, influences peripheral hypercapnic chemosensitivity and that context (i.e., rest vs. exercise) is an important consideration.NEW & NOTEWORTHY The hypercapnic chemoresponse to transient CO₂ showed an increase during acute physical activity; however, this response did not persist with further increases in intensity and was not different between participants of different aerobic fitness. Males and females show a differing response to CO₂ during exercise when compared with an iso-V̇co₂. Our results suggest that adaptations that lead to increased aerobic fitness do not impact the hypercapnic ventilatory response but there is an effect of sex.

Breath Tools: A Synthesis of Evidence-Based Breathing Strategies to Enhance Human Running

Running is among the most popular sporting hobbies and often chosen specifically for intrinsic psychological benefits. However, up to 40% of runners may experience exercise-induced dyspnoea as a result of cascading physiological phenomena, possibly causing negative psychological states or barriers to participation. Breathing techniques such as slow, deep breathing have proven benefits at rest, but it is unclear if they can be used during exercise to address respiratory limitations or improve performance. While direct experimental evidence is limited, diverse findings from exercise physiology and sports science combined with anecdotal knowledge from Yoga, meditation, and breathwork suggest that many aspects of breathing could be improved via purposeful strategies. Hence, we sought to synthesize these disparate sources to create a new theoretical framework called “Breath Tools” proposing breathing strategies for use during running to improve tolerance, performance, and lower barriers to long-term enjoyment.

Higher cardiovascular fitness level is associated with lower cerebrovascular reactivity and perfusion in healthy older adults

Aging is accompanied by vascular and structural changes in the brain, which include decreased grey matter volume (GMV), cerebral blood flow (CBF), and cerebrovascular reactivity (CVR). Enhanced fitness in aging has been related to preservation of GMV and CBF, and in some cases CVR, although there are contradictory relationships reported between CVR and fitness. To gain a better understanding of the complex interplay between fitness and GMV, CBF and CVR, the present study assessed these factors concurrently. Data from 50 participants, aged 55 to 72, were used to derive GMV, CBF, CVR and VO₂peak. Results revealed that lower CVR was associated with higher VO₂peak throughout large areas of the cerebral cortex. Within these regions lower fitness was associated with higher CBF and a faster hemodynamic response to hypercapnia. Overall, our results indicate that the relationships between age, fitness, cerebral health and cerebral hemodynamics are complex, likely involving changes in chemosensitivity and autoregulation in addition to changes in arterial stiffness. Future studies should collect other physiological outcomes in parallel with quantitative imaging, such as measures of chemosensitivity and autoregulation, to further understand the intricate effects of fitness on the aging brain, and how this may bias quantitative measures of cerebral health.

Assessment of the impact of 10-day intermittent hypoxia on the autonomic control measured by heart rate variability

Purpose

The purpose of this study is to establish the alterations in the activity of the autonomic nervous system (ANS) via heart rate variability (HRV) in subjects exposed to 1 h of exogenous hypoxia for 10 consecutive days.

Methods

Twelve healthy non-smoker males at mean age of 29.8 ± 7.4 (mean ± SD) breathed hypoxic air delivered through hypoxicator (FiО2 = 12.3% ± 1.5%) for 1 h in 10 consecutive days. Pulse oximetry and electrocardiography were monitored during the visit and HRV was calculated for the entire 1-h hypoxic period.

Results

Comparing the last hypoxic visit to the first, subjects had higher standard deviation of normal-to-normal interbeat intervals (SDNNs) (65.7 ± 32.5 vs. 81.1 ± 32.0 ms, p = 0.013) and root mean square of successive R–R interval difference (RMSSD) (58.1 ± 30.9 vs. 76.5 ± 34.6 ms, p = 0.029) as well as higher lnTotal power (8.1 ± 1.1 vs. 8.5 ± 0.9 ms2, p = 0.015) and high frequency (lnHF) (6.8 ± 1.3 vs. 7.5 ± 1.2 ms2, p = 0.05) and lower LF/HF (2.4 ± 1.4 vs. 1.5 ± 1.0, p = 0.026). Changes in saturation (87.0 ± 7.1 vs. 90.8 ± 5.0%, p = 0.039) and heart rate (67.1 ± 8.9 vs. 62.5 ± 6.0 beats/min, p = 0.040) were also observed.

Conclusions

Intermittent hypoxic training consisting of 1-h hypoxic exposure for 10 consecutive days could diminish the effects of acute exogenous hypoxia on the ANS characterized by an increased autonomic control (SDNN and total power) with augmentation of the parasympathetic nervous system activity (increased RMSSD and HF and decreased LF/HF). Therefore, it could be applied as a pre-acclimatization technique aiming at an increase in the autonomic control and oxygen saturation in subjects with upcoming sojourn to high altitude.

Minute-Ventilation Variability during Cardiopulmonary Exercise Test is Higher in Sedentary Men Than in Athletes

BACKGROUND

The occurrence of minute-ventilation oscillations during exercise, named periodic breathing, exhibits important prognostic information in heart failure. Considering that exercise training could influence the fluctuation of ventilatory components during exercise, we hypothesized that ventilatory variability during exercise would be greater in sedentary men than athletes.

OBJECTIVE

To compare time-domain variability of ventilatory components of sedentary healthy men and athletes during a progressive maximal exercise test, evaluating their relationship to other variables usually obtained during a cardiopulmonary exercise test.

METHODS

Analysis of time-domain variability (SD/n and RMSSD/n) of minute-ventilation (Ve), respiratory rate (RR) and tidal volume (Vt) during a maximal cardiopulmonary exercise test of 9 athletes and 9 sedentary men was performed. Data was compared by two-tailed Student T test and Pearson´s correlations test.

RESULTS

Sedentary men exhibited greater Vt (SD/n: 1.6 ± 0.3 vs. 0.9 ± 0.3 mL/breaths; p < 0.001) and Ve (SD/n: 97.5 ± 23.1 vs. 71.6 ± 4.8 mL/min x breaths; p = 0.038) variabilities than athletes. VE/VCO2 correlated to Vt variability (RMSSD/n) in both groups.

CONCLUSIONS

Time-domain variability of Vt and Ve during exercise is greater in sedentary than athletes, with a positive relationship between VE/VCO2 pointing to a possible influence of ventilation-perfusion ratio on ventilatory variability during exercise in healthy volunteers.

Enhanced muscular oxygen extraction in athletes exaggerates hypoxemia during exercise in hypoxia

High rate of muscular oxygen utilization facilitates the development of hypoxemia during exercise at altitude. Because endurance training stimulates oxygen extraction capacity, we investigated whether endurance athletes are at higher risk to developing hypoxemia and thereby acute mountain sickness symptoms during exercise at simulated high altitude. Elite athletes (ATL; n = 8) and fit controls (CON; n = 7) cycled for 20 min at 100 W (EX100W), as well as performed an incremental maximal oxygen consumption test (EXMAX) in normobaric hypoxia (0.107 inspired O2 fraction) or normoxia (0.209 inspired O2 fraction). Cardiorespiratory responses, arterial Po2 (PaO2), and oxygenation status in m. vastus lateralis [tissue oxygenation index (TOIM)] and frontal cortex (TOIC) by near-infrared spectroscopy, were measured. Muscle O2 uptake rate was estimated from change in oxyhemoglobin concentration during a 10-min arterial occlusion in m. gastrocnemius. Maximal oxygen consumption in normoxia was 70 ± 2 ml·min(-1·)kg(-1) in ATL vs. 43 ± 2 ml·min(-1·)kg(-1) in CON, and in hypoxia decreased more in ATL (-41%) than in CON (-25%, P < 0.05). Both in normoxia at PaO2 of ∼95 Torr, and in hypoxia at PaO2 of ∼35 Torr, muscle O2 uptake was twofold higher in ATL than in CON (0.12 vs. 0.06 ml·min(-1)·100 g(-1); P < 0.05). During EX100W in hypoxia, PaO2 dropped to lower (P < 0.05) values in ATL (27.6 ± 0.7 Torr) than in CON (33.5 ± 1.0 Torr). During EXMAX, but not during EX100W, TOIM was ∼15% lower in ATL than in CON (P < 0.05). TOIC was similar between the groups at any time. This study shows that maintenance of high muscular oxygen extraction rate at very low circulating PaO2 stimulates the development of hypoxemia during submaximal exercise in hypoxia in endurance-trained individuals. This effect may predispose to premature development of acute mountain sickness symptoms during exercise at altitude.

Advancing hypoxic training in team sports: from intermittent hypoxic training to repeated sprint training in hypoxia

Over the past two decades, intermittent hypoxic training (IHT), that is, a method where athletes live at or near sea level but train under hypoxic conditions, has gained unprecedented popularity. By adding the stress of hypoxia during 'aerobic' or 'anaerobic' interval training, it is believed that IHT would potentiate greater performance improvements compared to similar training at sea level. A thorough analysis of studies including IHT, however, leads to strikingly poor benefits for sea-level performance improvement, compared to the same training method performed in normoxia. Despite the positive molecular adaptations observed after various IHT modalities, the characteristics of optimal training stimulus in hypoxia are still unclear and their functional translation in terms of whole-body performance enhancement is minimal. To overcome some of the inherent limitations of IHT (lower training stimulus due to hypoxia), recent studies have successfully investigated a new training method based on the repetition of short (<30 s) 'all-out' sprints with incomplete recoveries in hypoxia, the so-called repeated sprint training in hypoxia (RSH). The aims of the present review are therefore threefold: first, to summarise the main mechanisms for interval training and repeated sprint training in normoxia. Second, to critically analyse the results of the studies involving high-intensity exercises performed in hypoxia for sea-level performance enhancement by differentiating IHT and RSH. Third, to discuss the potential mechanisms underpinning the effectiveness of those methods, and their inherent limitations, along with the new research avenues surrounding this topic.

Exercise training effects on hypoxic and hypercapnic ventilatory responses in mice selected for increased voluntary wheel running

NEW FINDINGS

What is the central question of this study? We used experimental evolution to determine how selective breeding for high voluntary wheel running and exercise training (7-11 weeks) affect ventilatory chemoreflexes of laboratory mice at rest. What is the main finding and its importance? Selective breeding, although significantly affecting some traits, did not systematically alter ventilation across gas concentrations. As with most human studies, our findings support the idea that endurance training attenuates resting ventilation. However, little evidence was found for a correlation between ventilatory chemoreflexes and the amount of individual voluntary wheel running. We conclude that exercise 'training' alters respiratory behaviours, but these changes may not be necessary to achieve high levels of wheel running. Ventilatory control is affected by genetics, the environment and gene-environment and gene-gene interactions. Here, we used an experimental evolution approach to test whether 37 generations of selective breeding for high voluntary wheel running (genetic effects) and/or long-term (7-11 weeks) wheel access (training effects) alter acute respiratory behaviour of mice resting in normoxic, hypoxic and hypercapnic conditions. As the four replicate high-runner (HR) lines run much more than the four non-selected control (C) lines, we also examined whether the amount of exercise among individual mice was a quantitative predictor of ventilatory chemoreflexes at rest. Selective breeding and/or wheel access significantly affected several traits. In normoxia, HR mice tended to have lower mass-adjusted rates of oxygen consumption and carbon dioxide production. Chronic wheel access increased oxygen consumption and carbon dioxide production in both HR and C mice during hypercapnia. Breathing frequency and minute ventilation were significantly reduced by chronic wheel access in both HR and C mice during hypoxia. Selection history, while significantly affecting some traits, did not systematically alter ventilation across all gas concentrations. As with most human studies, our findings support the idea that endurance training (access to wheel running) attenuates resting ventilation. However, little evidence was found for a correlation at the level of the individual variation between ventilatory chemoreflexes and performance (amount of individual voluntary wheel running). We tentatively conclude that exercise 'training' alters respiratory behaviours, but these changes may not be necessary to achieve high levels of wheel running.

Substrate utilization during exercise and recovery at moderate altitude

Recent studies have shown that exercise training at moderate altitude or in moderate hypoxia improved glycemic parameters. From these data, it has been supposed that endurance exercise in moderate hypoxia affects substrate utilization and that exposure to moderate hypoxia in combination with exercise may be utilized as part of metabolic or diabetes prevention program. However, the influence of exercise at moderate hypoxia on circulating metabolites and hormones in terms of substrate utilization is unclear. The purpose of this study was to elucidate the influence of exercise in moderate hypoxia on substrate utilization. We determined cardiorespiratory, metabolic, and hormonal parameters during exercise and postexercise recovery at a simulated moderate altitude of 2000 m, and then we compared these variables with values obtained at sea level. Seven men participated in this study; subjects reported to the laboratory on 4 occasions. Two maximal exercise tests were performed to estimate peak oxygen uptake at the simulated 2000-m altitude and sea level on different days. Afterward, submaximal exercise tests were carried out at a simulated altitude of 2000 m or sea level, separated by 1 week. Subjects performed submaximal exercise at the same relative exercise intensity (50% peak oxygen uptake) at a simulated altitude of 2000 m and at sea level for 30 minutes. The tests were performed in random order, and subjects were blinded to the respective altitudes. Venous blood samples and expired gases were obtained before, during exercise (15 and 30 minutes), and during postexercise recovery periods (15, 30, 45, and 60 minutes). The respiratory exchange ratio during exercise and recovery at moderate altitude was greater than at sea level. The epinephrine and norepinephrine concentrations during exercise and recovery were higher (P < .05) at moderate altitude than at sea level. Free fatty acids and glycerol concentrations during recovery were lower (P < .05) at moderate altitude than at sea level. These results suggest that carbohydrate utilization is increased during exercise and postexercise recovery period in moderate hypoxia as compared with normoxia. It is also suggested that moderate hypoxia influences the changes in circulating metabolites and hormones in terms of substrate metabolism during exercise and the recovery.

Is hypoxia training good for muscles and exercise performance?

Altitude training has become very popular among athletes as a means to further increase exercise performance at sea level or to acclimatize to competition at altitude. Several approaches have evolved during the last few decades, with "live high-train low" and "live low-train high" being the most popular. This review focuses on functional, muscular, and practical aspects derived from extensive research on the "live low-train high" approach. According to this, subjects train in hypoxia but remain under normoxia for the rest of the time. It has been reasoned that exercising in hypoxia could increase the training stimulus. Hypoxia training studies published in the past have varied considerably in altitude (2300-5700 m) and training duration (10 days to 8 weeks) and the fitness of the subjects. The evidence from muscle structural, biochemical, and molecular findings point to a specific role of hypoxia in endurance training. However, based on the available performance capacity data such as maximal oxygen uptake (Vo(2)max) and (maximal) power output, hypoxia as a supplement to training is not consistently found to be advantageous for performance at sea level. Stronger evidence exists for benefits of hypoxic training on performance at altitude. "Live low-train high" may thus be considered when altitude acclimatization is not an option. In addition, the complex pattern of gene expression adaptations induced by supplemental training in hypoxia, but not normoxia, suggest that muscle tissue specifically responds to hypoxia. Whether and to what degree these gene expression changes translate into significant changes in protein concentrations that are ultimately responsible for observable structural or functional phenotypes remains open. It is conceivable that the global functional markers such as Vo(2)max and (maximal) power output are too coarse to detect more subtle changes that might still be functionally relevant, at least to high-level athletes.

Training in hypoxia and its effects on skeletal muscle tissue

It is well established that local muscle tissue hypoxia is an important consequence and possibly a relevant adaptive signal of endurance exercise training in humans. It has been reasoned that it might be advantageous to increase this exercise stimulus by working in hypoxia. However, as long-term exposure to severe hypoxia has been shown to be detrimental to muscle tissue, experimental protocols were developed that expose subjects to hypoxia only for the duration of the exercise session and allow recovery in normoxia (live low-train high or hypoxic training). This overview reports data from 27 controlled studies using some implementation of hypoxic training paradigms. Hypoxia exposure varied between 2300 and 5700 m and training duration ranged from 10 days to 8 weeks. A similar number of studies was carried out on untrained and on trained subjects. Muscle structural, biochemical and molecular findings point to a specific role of hypoxia in endurance training. However, based on the available data on global estimates of performance capacity such as maximal oxygen uptake (VO2max) and maximal power output (Pmax), hypoxia as a supplement to training is not consistently found to be of advantage for performance at sea level. There is some evidence mainly from studies on untrained subjects for an advantage of hypoxic training for performance at altitude. Live low-train high may be considered when altitude acclimatization is not an option.

Control of ventilation in humans following intermittent hypoxia

Exposure to chronic or intermittent hypoxia produces alterations in the ventilatory response to hypoxia. These adaptations can differ depending on the severity of the hypoxic stimulus, its duration, its pattern, and the presence or absence of other chemical stimuli. As such, there are significant differences between the responses to intermittent versus continuous hypoxia. Intermittent hypoxia (IH) has been shown to elicit significant changes in the peripheral chemoresponse, but the functional implications of these changes for resting and exercise ventilation are not clear. We summarize the impact of IH on resting chemosensitivity and discuss the use of IH to better understand ventilatory control during exercise. We also suggest future directions for this relatively young field, including potential clinical applications of IH research.

Effect of hypoxic "dose" on physiological responses and sea-level performance

Live high-train low (LH+TL) altitude training was developed in the early 1990s in response to potential training limitations imposed on endurance athletes by traditional live high-train high (LH+TH) altitude training. The essence of LH+TL is that it allows athletes to "live high" for the purpose of facilitating altitude acclimatization, as manifest by a profound and sustained increase in endogenous erythropoietin (EPO) and ultimately an augmented erythrocyte volume, while simultaneously allowing athletes to "train low" for the purpose of replicating sea-level training intensity and oxygen flux, thereby inducing beneficial metabolic and neuromuscular adaptations. In addition to "natural/terrestrial" LH+TL, several simulated LH+TL devices have been developed to conveniently bring the mountain to the athlete, including nitrogen apartments, hypoxic tents, and hypoxicator devices. One of the key questions regarding the practical application of LH+TL is, what is the optimal hypoxic dose needed to facilitate altitude acclimatization and produce the expected beneficial physiological responses and sea-level performance effects? The purpose of this paper is to objectively answer that question, on the basis of an extensive body of research by our group in LH+TL altitude training. We will address three key questions: 1) What is the optimal altitude at which to live? 2) How many days are required at altitude? and 3) How many hours per day are required? On the basis of consistent findings from our research group, we recommend that for athletes to derive the physiological benefits of LH+TL, they need to live at a natural elevation of 2000-2500 m for >or=4 wk for >or=22 h.d(-1).

Two patterns of daily hypoxic exposure and their effects on measures of chemosensitivity in humans

The purpose of this study was to compare chemoresponses following two different intermittent hypoxia (IH) protocols in humans. Ten men underwent two 7-day courses of poikilocapnic IH. The long-duration IH (LDIH) protocol consisted of daily 60-min exposures to normobaric 12% O2. The short-duration IH (SDIH) protocol comprised twelve 5-min bouts of 12% O2, separated by 5-min bouts of room air, daily. Isocapnic hypoxic ventilatory response (HVR) was measured daily during the protocol and 1 and 7 days following. Hypercapnic ventilatory response (HCVR) and CO2 threshold and sensitivity (by the modified Read rebreathing technique) were measured on days 1, 8, and 14. Following 7 days of IH, the mean HVR was significantly increased from 0.47 ± 0.07 and 0.47 ± 0.08 to 0.70 ± 0.06 and 0.79 ± 0.06 l·min−1·%SaO2−1 (LDIH and SDIH, respectively), where %SaO2 is percent arterial oxygen saturation. The increase in HVR reached a plateau after the third day. One week post-IH, HVR values were unchanged from baseline. HCVR increased from 3.0 ± 0.4 to 4.0 ± 0.5 l·min−1·mmHg−1. In both the hyperoxic and hypoxic modified Read rebreathing tests, the slope of the CO2/ventilation plot was unchanged by either intervention, but the CO2/ventilation curve shifted to the left following IH. There were no correlations between the changes in response to hypoxia and hypercapnia. There were no significant differences between the two IH protocols for any measures, indicating that comparable changes in chemoreflex control occur with either protocol. These results also suggest that the two methods of measuring CO2 response are not completely concordant and that the changes in CO2 control do not correlate with the increase in the HVR.

Intermittent hypoxia reduces cerebrovascular sensitivity to isocapnic hypoxia in humans

The purpose of this study was to determine the changes in human cerebrovascular function associated with intermittent poikilocapnic hypoxia (IH). Healthy men (n=8; 24+/-1 years) were exposed to IH for 10 days (12% O(2) for 5min followed by 5min of normoxia for 1h). During the hypoxic exposures, oxyhemoglobin saturation (SaO(2)) was 85% and the end-tidal partial pressure of CO(2) was permitted to fall as a result of hypoxic hyperventilation. Pre- and post-IH intervention subjects underwent a progressive isocapnic hypoxic test where ventilation, blood pressure, heart rate, and cerebral blood flow velocity (middle cerebral artery, transcranial Doppler) were measured to determine the ventilatory, cardiovascular and cerebrovascular sensitivities to isocapnic hypoxia. When compared to the pre-IH trial, cerebrovascular sensitivity to hypoxia significantly decreased (pre-IH=0.28+/-0.15; post-IH=0.16+/-0.14cms(-1)%SaO(2)(-1); P<0.05). No changes in ventilatory, blood pressure or heart rate sensitivity were observed (P>0.05). We have previously shown that the ability to oxygenate cerebral tissue measured using spatially resolved near infrared spectroscopy is significantly reduced following IH in healthy humans. Our collective findings indicate that intermittent hypoxia can blunt cerebrovascular regulation. Thus, it appears that intermittent hypoxia has direct cerebrovascular effects that can occur in the absence of changes to the ventilatory and neurovascular control systems.

Military Applications of Hypoxic Training for High-Altitude Operations

Rapid deployment of unacclimatized soldiers to high mountainous environments causes debilitating effects on operational capabilities (physical work performance), and force health (altitude sickness). Most of these altitude-induced debilitations can be prevented or ameliorated by a wide range of physiological responses collectively referred to as altitude acclimatization. Acclimatization to a target altitude can be induced by slow progressive ascents or continuous sojourns at intermediate altitudes. However, this "altitude residency" requirement reduces their utilization in rapid response military missions that exploit the air mobility capability of modern military forces to quickly deploy to an area of operations on short notice. A more recent approach to induce altitude acclimatization is the use of daily intermittent hypoxic exposures (IHE) in lieu of continuous residence at high altitudes. IHE treatments consist of three elements: 1) IHE simulated altitude (inspired oxygen partial pressure: PIO2), 2) IHE session duration, and 3) total number of IHE sessions over the treatment period. This paper reviews and summarizes the results of 25 published IHE studies. This review finds that an IHE altitude>or=4000 m, and daily exposure duration of at least 1.5 h repeated over a week or more are required to have a high probability of developing altitude acclimatization. The efficacy of shorter duration (<1.5 h) hypoxic exposures at >or=4000 m simulated altitudes, and longer exposures (>4 h) at moderate altitudes (2500-3500 m) is not well documented. The predominate IHE-induced altitude acclimatization response appears to be increased arterial oxygen content through ventilatory acclimatization. Thus, IHE is a promising approach to provide the benefits of altitude acclimatization to low-altitude-based soldiers before their deployment to high mountainous regions.

Live High + Train Low: Thinking in Terms of an Optimal Hypoxic Dose

“Live high-train low” (LH+TL) altitude training allows athletes to “live high” for the purpose of facilitating altitude acclimatization, as characterized by a significant and sustained increase in endogenous erythropoietin and subsequent increase in erythrocyte volume, while simultaneously enabling them to “train low” for the purpose of replicating sea-level training intensity and oxygen flux, thereby inducing beneficial metabolic and neuromuscular adaptations. In addition to natural/terrestrial LH+TL, several simulated LH+TL devices have been developed including nitrogen apartments, hypoxic tents, and hypoxicator devices. One of the key issues regarding the practical application of LH+TL is what the optimal hypoxic dose is that is needed to facilitate altitude acclimatization and produce the expected beneficial physiological responses and sea-level performance effects. The purpose of this review is to examine this issue from a research-based and applied perspective by addressing the following questions: What is the optimal altitude at which to live, how many days are required at altitude, and how many hours per day are required? It appears that for athletes to derive the hematological benefits of LH+TL while using natural/terrestrial altitude, they need to live at an elevation of 2000 to 2500 m for >4 wk for >22 h/d. For athletes using LH+TL in a simulated altitude environment, fewer hours (12-16 h) of hypoxic exposure might be necessary, but a higher elevation (2500 to 3000 m) is required to achieve similar physiological responses.

Application of Altitude/Hypoxic Training by Elite Athletes

At the Olympic level, differences in performance are typically less than 0.5%. This helps explain why many contemporary elite endurance athletes in summer and winter sport incorporate some form of altitude/hypoxic training within their year-round training plan, believing that it will provide the "competitive edge" to succeed at the Olympic level. The purpose of this paper is to describe the practical application of altitude/hypoxic training as used by elite athletes. Within the general framework of the paper, both anecdotal and scientific evidence will be presented relative to the efficacy of several contemporary altitude/hypoxic training models and devices currently used by Olympic-level athletes for the purpose of legally enhancing performance. These include the three primary altitude/hypoxic training models: 1) live high+train high (LH+TH), 2) live high+train low (LH+TL), and 3) live low+train high (LL+TH). The LH+TL model will be examined in detail and will include its various modifications: natural/terrestrial altitude, simulated altitude via nitrogen dilution or oxygen filtration, and hypobaric normoxia via supplemental oxygen. A somewhat opposite approach to LH+TL is the altitude/hypoxic training strategy of LL+TH, and data regarding its efficacy will be presented. Recently, several of these altitude/hypoxic training strategies and devices underwent critical review by the World Anti-Doping Agency (WADA) for the purpose of potentially banning them as illegal performance-enhancing substances/methods. This paper will conclude with an update on the most recent statement from WADA regarding the use of simulated altitude devices.

Effects of intermittent hypoxic training on cycling performance in well-trained athletes

This study aimed to investigate the effects of a short-term period of intermittent hypoxic training (IHT) on cycling performance in athletes. Nineteen participants were randomly assigned to two groups: normoxic (NT, n = 9) and intermittent hypoxic training group (IHT, n = 10). A 3-week training program (5 x 1 h-1 h 30 min per week) was completed. Training sessions were performed in normoxia (approximately 30 m) or hypoxia (simulated altitude of 3,000 m) for NT and IHT group, respectively. Each subject performed before (W0) and after (W4) the training program, three cycling tests including an incremental test to exhaustion in normoxia and hypoxia for determination of maximal aerobic power (VO2max) and peak power output (PPO) as well as a 10-min cycle time trial in normoxia (TT) to measure the average power output (P(aver)). No significant difference in VO2max was observed between the two training groups before or after the training period. When measured in normoxia, the PPO significantly increased (P < 0.05) by 7.2 and 6.6% in NT and IHT groups, respectively. However, only the IHT group significantly improved (11.3%; P < 0.05) PPO when measured in hypoxia. The NT group improved (P < 0.05) P(aver) in TT by 8.1%, whereas IHT group did not show any significant difference. Intermittent training performed in hypoxia was less efficient for improving endurance performance at sea level than similar training performed in normoxia. However, IHT has the potential to assist athletes in preparation for competition at altitude.

Human ventilatory responsiveness to hypoxia is unrelated to maximal aerobic capacity

Ventilatory responsiveness to hypoxia (HVR) has been reported to be different between highly trained endurance athletes and healthy sedentary controls. However, a linkage between aerobic capacity and HVR has not been a universal finding. The purpose of this study was to examine the relationship between HVR and maximal oxygen consumption (V̇o2 max) in healthy men with a wide range of aerobic capacities. Subjects performed a HVR test followed by an incremental cycle test to exhaustion. Participants were classified according to their maximal aerobic capacity. Those with a V̇o2 max of ≥60 ml·kg−1·min−1 were considered highly trained ( n = 13); those with a V̇o2 max of 50–60 ml·kg−1·min−1 were considered moderately-trained ( n = 18); and those with a V̇o2 max of <50 ml·kg−1·min−1 were considered untrained ( n = 24). No statistical differences were detected between the three groups for HVR ( P > 0.05), and the HVR values were variable within each group (range: untrained = 0.28–1.61, moderately trained = 0.23–2.39, and highly trained = 0.08–1.73 l·min·%arterial O2 saturation−1). The relationship between HVR and V̇o2 max was not statistically significant ( r = −0.1723; P > 0.05). HVR was also unrelated to maximal minute ventilation and ventilatory equivalents for O2 and CO2. We found that a spectrum of hypoxic ventilatory control is present in well-trained endurance athletes and moderately and untrained men. We interpret these observations to mean that other factors are more important in determining hypoxic ventilatory control than physical conditioning per se.

Effects of enhanced human chemosensitivity on ventilatory responses to exercise

It is not clear what the effects of different types of intermittent hypoxia have on human exercise ventilation. The purpose of this study was to determine whether short-duration intermittent hypoxia, and the subsequent augmentation of the hypoxic ventilatory response (HVR), would lead to an increase in ventilatory responses during exercise at sea level. It was hypothesized that subjects exposed to short-duration intermittent hypoxia would have a greater increase in the ventilatory response to exercise compared to those exposed to long-duration intermittent hypoxia. Subjects (n = 17, male) were randomly assigned to short-duration intermittent hypoxia (SDIH: 5 min of 12% O2 separated by 5 min of normoxia for 1 h) or long-duration intermittent hypoxia (LDIH: 30 min of 12% O2). Both groups had 10 exposures over a 12 day period. The HVR was measured on days 1 and 12. Maximal oxygen consumption (VO2max) was determined using a ramped cycle exercise test. Maximal exercise data were not different (P > 0.05) between SDIH and LDIH groups or following intermittent hypoxia. Minute ventilation, tidal volume and respiratory frequency were compared at 20, 40, 60, 80 and 100% of VO2max . There was no difference in the ventilatory responses at any intensity of exercise following the intermittent hypoxia period. The HVR was significantly increased following the intermittent hypoxia intervention (P < 0.05) but was not different between SDIH and LDIH (P > 0.05). The relationships between HVR and VO2max were non-significant on day 1 (r = 0.30) and day 12 (r = 0.47; P > 0.05). Our findings point to a lack of functional significance of increasing HVR via intermittent hypoxia on ventilatory responses to exercise at sea level.

Living high-training low: tolerance and acclimatization in elite endurance athletes

The "living high-training low" (LHTL) model is frequently used to enhance aerobic performance. However, the clinical tolerance and acclimatization process to this intermittent exposure needs to be examined. Forty one athletes from three federations (cross-country skiers, n=11; swimmers, n=18; runners, n=12) separately performed a 13 to 18-day training at the altitude of 1,200 m, by sleeping either at 1,200 m (CON) or in hypoxic rooms (HYP), with an O2 fraction corresponding to 2,500 m (5 nights for swimmers and 6 for skiers and runners), 3,000 m (6 nights for skiers, 8 for swimmers and 12 for runners) and 3,500 m (6 nights for skiers). Measurements performed before, 1 or 15 days after training were ventilatory response (HVRe) and desaturation (deltaSaO2e) during hypoxic exercise, an evaluation of cardiac function by echocardiography, and leukocyte count. Lake Louise AMS score and arterial O2 saturation during sleep were measured daily for HYP. Subjects did not develop symptoms of AMS. Mean nocturnal SaO2 decreased with altitude down to 90% at 3,500 m and increased with acclimatization (except at 3,500 m). Leukocyte count was not affected except at 3,500 m. The heart function was not affected by LHTL. Signs of ventilatory acclimatization were present immediately after training (increased HVRe and decreased deltaSaO2e) and had disappeared 15 days later. In conclusion, LHTL was well tolerated and compatible with aerobic training. Comparison of the three patterns of training suggests that a LHTL session should not exceed 3,000 m, for at least 18 days, with a minimum of 12 h day(-1) of exposure.

Effects of two protocols of intermittent hypoxia on human ventilatory, cardiovascular and cerebral responses to hypoxia

We determined the ventilatory, cardiovascular and cerebral tissue oxygen response to two protocols of normobaric, isocapnic, intermittent hypoxia. Subjects (n = 18, male) were randomly assigned to short-duration intermittent hypoxia (SDIH, 12% O2 separated by 5 min of normoxia for 1 h) or long-duration intermittent hypoxia (LDIH, 30 min of 12% O2). Both groups had 10 exposures over a 12 day period. The hypoxic ventilatory response (HVR) was measured before each daily intermittent hypoxia exposure on days 1, 3, 5, 8, 10 and 12. The HVR was measured again 3 and 5 days after the end of intermittent hypoxia. During all procedures, ventilation, blood pressure, heart rate, arterial oxyhaemoglobin saturation and cerebral tissue oxygen saturation were measured. The HVR increased throughout intermittent hypoxia exposure regardless of protocol, and returned to baseline by day 17 (day 1, 0.84 +/- 0.50; day 12, 1.20 +/- 1.01; day 17, 0.95 +/- 0.58 l min(-1) %S(aO2)(-1); P < 0.01). The change in systolic blood pressure sensitivity (r = +0.68; P < 0.05) and the change in diastolic blood pressure sensitivity (r = +0.73; P < 0.05) were related to the change in HVR, while the change in heart rate sensitivity was not (r = +0.32; NS). The change in cerebral tissue oxygen saturation sensitivity to hypoxia was less on day 12, and returned to baseline by day 17 (day 1, -0.51 +/- 0.13; day 12, -0.64 +/- 0.18; day 17, -0.51 +/- 0.13; P < 0.001). Acute exposure to SDIH increased mean arterial pressure (+5 mmHg; P < 0.01), but LDIH did not (P > 0.05). SDIH and LDIH had similar effects on the ventilatory and cardiovascular response to acute progressive hypoxia and hindered cerebral oxygenation. Our findings indicate that the vascular processes required to control blood flow and oxygen supply to cerebral tissue in a healthy human are hindered following exposure to 12 days of isocapnic intermittent hypoxia.

Effects of concurrent inspiratory and expiratory muscle training on respiratory and exercise performance in competitive swimmers

The efficiency of the respiratory system presents significant limitations on the body's ability to perform exercise due to the effects of the increased work of breathing, respiratory muscle fatigue, and dyspnoea. Respiratory muscle training is an intervention that may be able to address these limitations, but the impact of respiratory muscle training on exercise performance remains controversial. Therefore, in this study we evaluated the effects of a 12-week (10 sessions week(-1)) concurrent inspiratory and expiratory muscle training (CRMT) program in 34 adolescent competitive swimmers. The CRMT program consisted of 6 weeks during which the experimental group (E, n = 17) performed CRMT and the sham group (S, n = 17) performed sham CRMT, followed by 6 weeks when the E and S groups performed CRMT of differing intensities. CRMT training resulted in a significant improvement in forced inspiratory volume in 1 s (FIV1.0) (P = 0.050) and forced expiratory volume in 1 s (FEV1.0) (P = 0.045) in the E group, which exceeded the S group's results. Significant improvements in pulmonary function, breathing power, and chemoreflex ventilation threshold were observed in both groups, and there was a trend toward an improvement in swimming critical speed after 12 weeks of training (P = 0.08). We concluded that although swim training results in attenuation of the ventilatory response to hypercapnia and in improvements in pulmonary function and sustainable breathing power, supplemental respiratory muscle training has no additional effect except on dynamic pulmonary function variables.

Changes in ventilatory responses to hypercapnia and hypoxia after intermittent hypoxia in humans

The purpose of this study was to clarify the changes in hypercapnic and hypoxic ventilatory responses (HCVR and HVR) after intermittent hypoxia and following the cessation of hypoxic exposure. Twenty-nine males were assigned to one of four groups, i.e., a hypoxic (EX1-H, n=7) or a control (EX1-C, n=7) group in Experiment 1, and a hypoxic (EX2-H, n=8) or a control (EX2-C, n=7) group in Experiment 2. In each experiment, the hypoxic tent system was utilized for intermittent hypoxia, and the oxygen levels in the tent were maintained at 12.3+/-0.2%. In Experiment 1, the EX1-H group spent 3 h/day in the hypoxic tent for 1 week. HCVR and HVR were determined before and after 1 week of intermittent hypoxia, and again 1 and 2 week after the cessation of hypoxic exposure. In Experiment 2, the subjects in the EX2-H group performed 3 h/day for 2 weeks in intermittent hypoxia. HCVR and HVR tests were carried out before and after intermittent hypoxia, and were repeated again after 2 weeks of the cessation of hypoxic exposure. The slope of the HCVR in the EX1-H group did not show a significant increase after 1 week of intermittent hypoxia, while HCVR in the EX2-H group increased significantly after 2 weeks of intermittent hypoxia. The HCVR intercept was unchanged following 1 or 2 weeks of intermittent hypoxia. There was a significant increase in the slope of the HVR after 1 and 2 weeks of intermittent hypoxia. The increased HCVR and HVR returned to pre-hypoxic levels after 2 weeks of the cessation of hypoxia. These results suggest that 3 h/day for 2 weeks of intermittent hypoxia leads to an increase in central hypercapnic ventilatory chemosensitivity, which is not accompanied by a re-setting of the central chemoreceptors, and that the increased hypercapnic and hypoxic chemosensitivities are restored within 2 weeks after the cessation of hypoxia.

Repeated measurement of hypoxic ventilatory response as an intermittent hypoxic stimulus

Measurement of hypoxic ventilatory response (HVR) involves an exposure to hypoxia which, if repeated over several days might act as an intermittent hypoxic stimulus. The purpose of this study was to measure HVR repeatedly over 5 days to determine whether it was affected by repeated measurement. Nine healthy male subjects completed an isocapnic HVR test, on one occasion, followed 5 days later by one measurement each day for 5 days. Each test lasted approximately 5-8 min with inspired oxygen concentration declining to as a low as 5-6%. No systematic trend was observed in HVR over the 5-day period (p>0.05). There were no significant differences in HVR between any of the test days. Regression failed to show any trend in HVR over the five sequential days. The calculated mean coefficient of variation for HVR for each subject was 27%. There is no evidence that the short exposure to hypoxia as part of HVR measurement is a co-intervention when measured repeatedly over 5 days in physiological studies.

Effects of five consecutive nocturnal hypoxic exposures on the cerebrovascular responses to acute hypoxia and hypercapnia in humans

The effects of discontinuous hypoxia on cerebrovascular regulation in humans are unknown. We hypothesized that five nocturnal hypoxic exposures (8 h/day) at a simulated altitude of 4,300 m (inspired O2 fraction = ∼13.8%) would elicit cerebrovascular responses that are similar to those that have been reported during chronic altitude exposures. Twelve male subjects (26.6 ± 4.1 yr, mean ± SD) volunteered for this study. The technique of end-tidal forcing was used to examine cerebral blood flow (CBF) and regional cerebral O2 saturation (SrO2) responses to acute variations in O2 and CO2 twice before, immediately after, and 5 days after the overnight hypoxic exposures. Transcranial Doppler ultrasound was used to assess CBF, and near-infrared spectroscopy was used to assess SrO2. Throughout the nocturnal hypoxic exposures, end-tidal Pco2 decreased ( P < 0.001) whereas arterial O2 saturation increased ( P < 0.001) compared with overnight normoxic control measurements. Symptoms associated with altitude illness were significantly greater than control values on the first night ( P < 0.001) and second night ( P < 0.01) of nocturnal hypoxia. Immediately after the nocturnal hypoxic intervention, the sensitivity of CBF to acute variations in O2 and CO2 increased 116% ( P < 0.01) and 33% ( P < 0.05), respectively, compared with control values. SrO2 was highly correlated with arterial O2 saturation ( R2 = 0.94 ± 0.04). These results show that discontinuous hypoxia elicits increases in the sensitivity of CBF to acute variations in O2 and CO2, which are similar to those observed during chronic hypoxia.

Hyperpnea training attenuates peripheral chemosensitivity and improves cycling endurance

Well-trained endurance athletes frequently have a lower peripheral chemoreceptor (pRc) sensitivity and a lower minute ventilation(V̇E) during exercise compared to untrained individuals. We speculated that the decreased pRcresponse may be specifically associated with repeated exposure to the high rates of ventilation occurring during exercise training. We therefore examined the effect of respiratory muscle training (RMT; 20× 30 min sessions of voluntary normocapnic hyperpnea) on the pRc sensitivity during exercise and on cycling performance. RMT was chosen to achieve a high V̇E, similar to that of heavy exercise,while avoiding the other accompanying effects of whole body exercise. 20 trained male cyclists were randomized into RMT (N=10) or control(N=10) groups. Subjects' pRc response was assessed by a modified Dejours O2 test (10-12 breaths of 100% O2,repeated 4-6 times) during cycling exercise at 40% of the maximal work capacity (Ẇmax). Cycling performance was measured during a cycling test to exhaustion (85%Ẇmax). The RMT group exhibited a significantly reduced pRc sensitivity (mean ±S.D.) compared to the control group (-5.8±6.0% versus0.1±4.6%, P&lt;0.5). Cycling endurance improved significantly after RMT in comparison to the control group (+3.26±4.98 versus -1.46±3.67 min, P&lt;0.05). However, these changes in pRc response were not significantly correlated with exercise ventilation or cycling endurance time. We conclude that the high levels of ventilation achieved during exercise, as simulated by RMT in this study, appear to be accompanied by a reduction in pRc sensitivity;however, the role of the pRc in the control of ventilation during exercise seems to be minor.

Living high-training low increases hypoxic ventilatory response of well-trained endurance athletes

This study determined whether "living high-training low" (LHTL)-simulated altitude exposure increased the hypoxic ventilatory response (HVR) in well-trained endurance athletes. Thirty-three cyclists/triathletes were divided into three groups: 20 consecutive nights of hypoxic exposure (LHTLc, n = 12), 20 nights of intermittent hypoxic exposure (four 5-night blocks of hypoxia, each interspersed with 2 nights of normoxia, LHTLi, n = 10), or control (Con, n = 11). LHTLc and LHTLi slept 8-10 h/day overnight in normobaric hypoxia (approximately 2,650 m); Con slept under ambient conditions (600 m). Resting, isocapnic HVR (DeltaVE/DeltaSp(O(2)), where VE is minute ventilation and Sp(O(2)) is blood O(2) saturation) was measured in normoxia before hypoxia (Pre), after 1, 3, 10, and 15 nights of exposure (N1, N3, N10, and N15, respectively), and 2 nights after the exposure night 20 (Post). Before each HVR test, end-tidal PCO(2) (PET(CO(2))) and VE were measured during room air breathing at rest. HVR (l. min(-1). %(-1)) was higher (P < 0.05) in LHTLc than in Con at N1 (0.56 +/- 0.32 vs. 0.28 +/- 0.16), N3 (0.69 +/- 0.30 vs. 0.36 +/- 0.24), N10 (0.79 +/- 0.36 vs. 0.34 +/- 0.14), N15 (1.00 +/- 0.38 vs. 0.36 +/- 0.23), and Post (0.79 +/- 0.37 vs. 0.36 +/- 0.26). HVR at N15 was higher (P < 0.05) in LHTLi (0.67 +/- 0.33) than in Con and in LHTLc than in LHTLi. PET(CO(2)) was depressed in LHTLc and LHTLi compared with Con at all points after hypoxia (P < 0.05). No significant differences were observed for VE at any point. We conclude that LHTL increases HVR in endurance athletes in a time-dependent manner and decreases PET(CO(2)) in normoxia, without change in VE. Thus endurance athletes sleeping in mild hypoxia may experience changes to the respiratory control system.

Chilean miners commuting from sea level to 4500 m: a prospective study

The development of mining activities in North Chile involves a great number of workers intermittently exposed to high altitude for a long period of time (chronic intermittent hypoxia, CIH). A 2(1/2)-year prospective study aimed to characterize this model of exposure to CIH and to know whether this condition may progressively lead to a chronic pattern. Twenty-nine miners, aged 25 +/- 5 yr, working 7 days at HA (3800 to 4600 m) and resting 7 days at sea level (SL) were studied. Subjects underwent a physical examination, EKG, hematological status, maximal exercise test, ventilatory and cardiac response to hypoxia (F(iO2) = 0.114) at rest and exercise, pulmonary vascular response to hypoxia by echocardiography, and 24-h monitoring of EKG and arterial pressure. Basal evaluations were performed at SL before the first exposure to hypoxia. HA measurements were daily AMS score, sleep status, and 24-h monitoring of EKG and arterial pressure. All these measurements were repeated after a mean period of 12, 19, and 31 months. Hematocrit increased but reached values lower than those observed in chronic permanent exposure. Systemic and pulmonary arterial pressures measured at SL did not change, but were higher in hypoxia. Right ventricle showed a slight dilatation. Exercise performance at SL declined with exposure to CIH to reach a 12.3% decrease after 31 months of CIH, associated with a 6.8% decrease in maximal heart rate. Signs of ventilatory acclimatization were observed after 12 months. Symptoms of AMS and sleep disturbances were still seen on the first 2 days at HA, whatever the time of exposure to CIH. In conclusion, CIH induced a clear acclimatization process. Subjects did not reach a health status comparable to that seen in permanent residents at HA and remained at risk of acute altitude-induced illnesses.

Interval hypoxic training

Interval hypoxic training (IHT) is a technique developed in the former Soviet Union, that consists of repeated exposures to 5-7 minutes of steady or progressive hypoxia, interrupted by equal periods of recovery. It has been proposed for training in sports, to acclimatize to high altitude, and to treat a variety of clinical conditions, spanning from coronary heart disease to Cesarean delivery. Some of these results may originate by the different effects of continuous vs. intermittent hypoxia (IH), which can be obtained by manipulating the repetition rate, the duration and the intensity of the hypoxic stimulus. The present article will attempt to examine some of the effects of IH, and, whenever possible, compare them to those of typical IHT. IH can modify oxygen transport and energy utilization, alter respiratory and blood pressure control mechanisms, induce permanent modifications in the cardiovascular system. IHT increases the hypoxic ventilatory response, increase red blood cell count and increase aerobic capacity. Some of these effects might be potentially beneficial in specific physiologic or pathologic conditions. At this stage, this technique appears interesting for its possible applications, but still largely to be explored for its mechanisms, potentials and limitations.

Invited review: Adaptive responses of skeletal muscle to intermittent hypoxia: the known and the unknown

Intermittent hypoxia (IH) describes conditions of repeated, transient reductions in O2 that may trigger unique adaptations. Rest periods during IH may avoid potentially detrimental effects of long-term O2 deprivation. For skeletal muscle, IH can occur in conditions of obstructive sleep apnea, transient altitude exposures (with or without exercise), intermittent claudication, cardiopulmonary resuscitation, neonatal blood flow obstruction, and diving responses of marine animals. Although it is likely that adaptations in these conditions vary, some patterns emerge. Low levels of hypoxia shift metabolic enzyme activity toward greater aerobic poise; extreme hypoxia shifts metabolism toward greater anaerobic potential. Some conditions of IH may also inhibit lactate release during exercise. Many related cellular phenomena could be involved in the response, including activation of specific O2 sensors, reactive oxygen and nitrogen species, preconditioning, hypoxia-induced transcription factors, regulation of ion channels, and influences of paracrine/hormonal stimuli. The net effect of a variety of adaptive programs to IH may be to preserve contractile function and cell integrity in hypoxia or anoxia, a response that does not always translate into improvements in exercise performance.

Oxygen sensing during intermittent hypoxia: cellular and molecular mechanisms

To the majority of the population, recurrent episodes of hypoxia are more likely encountered in life than sustained hypoxia. Until recently, much of the information on the long-term effects of intermittent hypoxia has come from studies on human subjects experiencing chronic recurrent apneas. Recent development of animal models of intermittent hypoxia and techniques for exposing cell cultures to alternating cycles of hypoxia have led to new information on the effects of episodic hypoxia on oxygen-sensing mechanisms in the carotid body chemoreceptors and regulation of gene expression. The purpose of this review is to highlight some recent studies on the effects of intermittent hypoxia on oxygen sensing at the carotid bodies and regulation of gene expression. In a rodent model, chronic intermittent hypoxia selectively enhances hypoxic sensitivity of the carotid body chemoreceptors. More interestingly, chronic intermittent hypoxia also induces a novel form of plasticity in the carotid body, leading to long-term facilitation in the sensory discharge. Studies on cell cultures reveal that intermittent hypoxia is more potent in activating activator protein-1 and hypoxia-inducible factor-1 transcription factors than sustained hypoxia. Moreover, some evidence suggests that intermittent hypoxia utilizes intracellular signaling pathways distinct from sustained hypoxia. Reactive oxygen species generated during the reoxygenation phase of intermittent hypoxia might play a key role in the effects of intermittent hypoxia on carotid body function and gene expression. Global gene profile analysis in cell cultures suggests that certain genes are selectively affected by intermittent hypoxia, some upregulated and some downregulated. It is suggested that, in intact animals, coordinated gene regulation of gene expression might be critical for eliciting phenotypic changes in the cardiorespiratory systems in response to intermittent hypoxia. It is hoped that future studies will unravel new mechanisms that are unique to intermittent hypoxia that may lead to a better understanding of the changes in the cardiorespiratory systems and new therapies for diseases associated with chronic recurrent episodes of hypoxia.

Physiological effects of intermittent hypoxia

Intermittent hypoxia (IH), or periodic exposure to hypoxia interrupted by return to normoxia or less hypoxic conditions, occurs in many circumstances. In high altitude mountaineering, IH is used to optimize acclimatization although laboratory studies have not generally revealed physiologically significant benefits. IH enhances athletic performance at sea level if blood oxygen capacity increases and the usual level of training is not decreased significantly. IH for high altitude workers who commute from low altitude homes is of considerable practical interest and the ideal commuting schedule for physical and mental performance is being studied. The effect of oxygen enrichment at altitude (i.e., intermittent normoxia on a background of chronic hypoxia) on human performance is under study also. Physiological mechanisms of IH, and specifically the differences between effects of IH and acute or chronic continuous hypoxia remains to be determined. Biomedical researchers are defining the molecular and cellular mechanisms for effects of hypoxia on the body in health and disease. A comparative approach may provide additional insight about the biological significance of these effects.

Intermittent hypoxia increases ventilation and Sa(O2) during hypoxic exercise and hypoxic chemosensitivity

The purpose of this study was 1) to test the hypothesis that ventilation and arterial oxygen saturation (Sa(O2)) during acute hypoxia may increase during intermittent hypoxia and remain elevated for a week without hypoxic exposure and 2) to clarify whether the changes in ventilation and Sa(O2) during hypoxic exercise are correlated with the change in hypoxic chemosensitivity. Six subjects were exposed to a simulated altitude of 4,500 m altitude for 7 days (1 h/day). Oxygen uptake (VO2), expired minute ventilation (VE), and Sa(O2) were measured during maximal and submaximal exercise at 432 Torr before (Pre), after intermittent hypoxia (Post), and again after a week at sea level (De). Hypoxic ventilatory response (HVR) was also determined. At both Post and De, significant increases from Pre were found in HVR at rest and in ventilatory equivalent for O2 (VE/VO2) and Sa(O2) during submaximal exercise. There were significant correlations among the changes in HVR at rest and in VE/VO2 and Sa(O2) during hypoxic exercise during intermittent hypoxia. We conclude that 1 wk of daily exposure to 1 h of hypoxia significantly improved oxygenation in exercise during subsequent acute hypoxic exposures up to 1 wk after the conditioning, presumably caused by the enhanced hypoxic ventilatory chemosensitivity.

Intermittent vs continuous hypoxia: effects on ventilation and erythropoiesis in humans

OBJECTIVE

Recently, we showed that 5 days of normobaric intermittent hypoxia at rest (IH; 2 hours daily at 3,800 m simulated altitude; partial pressure of inspired oxygen 90 torr) can induce an increase in the isocapnic hypoxic ventilatory response (HVR) and blood reticulocyte count. The purpose of the present study was to compare these data with continuous exposure to the same hypoxic level.

METHODS

Four of the same subjects were exposed, a year later, to 2 days of continuous hypoxia (CH), and 4 different subjects were exposed to 8 weeks of CH, both at the White Mountain Research Station (3,800 m altitude, barometric pressure approximately 489 torr). Inspired minute ventilation (VI), end-tidal partial pressure of carbon dioxide, arterial oxygen saturation (SaO2[sat]), hematocrit, and hemoglobin concentration were measured at different times during the continuous exposures. The HVR was expressed as the increase in V1 per 1% decrease in SaO2.

RESULTS

The HVR showed no significant difference in the control values 1 year apart (IH, 0.06 +/- 0.03; CH2d (2 days' continuous hypoxia), 0.19 +/- 0.07 L x min(-1) x %sat(-1); means +/- SE), and the HVR values were similar after 2 days of IH compared to CH (0.42 +/- 0.26 and 0.51 +/- 0.22 L x min(-1) x %sat(-1), respectively). On the new subjects after 2 weeks of CH, the HVR showed a maximum increase, similar to the increase observed after only 5 days of IH, hemoglobin concentrations and hematocrit were significantly increased (45.0 +/- 2.7% vs 51.5 +/- 3.0% and 14.5 +/- 0.7 vs 17.2 +/- 1.0 g x dL(-1), respectively). The HVR did not change significantly from week 2 to 8 of CH, whereas hematological data were still increasing at the end of the 8 weeks.

CONCLUSION

Changes in ventilatory oxygen sensitivity induced by IH and CH are similar in magnitude but occur with different time courses. The effects of IH on erythropoiesis are significant but fewer than on CH.

Effects of intermittent hypoxia on the isocapnic hypoxic ventilatory response and erythropoiesis in humans

Isocapnic hypoxic ventilatory response (HVR) and hematological variables were measured in nine adult males (age: 29.3+/-3.4) exposed to normobaric intermittent hypoxia (IH, 2 h daily at FI(O(2))=0.13, equivalent to 3800 m altitude) for 12 days. Mean HVR significantly increased during IH, however, after reaching a peak on Day 5 (0.79+/-0.12 vs. 0.27+/-0.11 L.min(-1).%(-1) on Day 1, P<0.05), it progressively decreased toward a lower value (0.46+/-0.16 L min(-1) x %(-1) on Day 12). In contrast, the subjects showed no changes in the ventilatory data and arterial O(2)-saturation in normoxia or poikilocapnic hypoxia (PET(CO(2)) uncontrolled). Hematocrit and hemoglobin concentration did not change, but the reticulocyte count increased by Day 5 (P<0.01). Our results suggest that moderate intermittent hypoxia induces changes in ventilatory O(2)-sensitivity and triggers the hematological acclimatization by increasing the percentage of reticulocytes in the blood. Normal ventilatory acclimatization to hypoxia was, however, not observed and the mechanisms involved in the biphasic changes in HVR we observed remain to be determined.

Cardiovascular response to hypoxia after endurance training at altitude and sea level and after detraining

The purpose of this study was to elucidate 1) the effects of endurance exercise training during hypoxia or normoxia and of detraining on ventilatory and cardiovascular responses to progressive isocapnic hypoxia and 2) whether the change in the cardiovascular response to hypoxia is correlated to changes in the hypoxic ventilatory response (HVR) after training and detraining. Seven men (altitude group) performed endurance training using a cycle ergometer in a hypobaric chamber of simulated 4,500 m, whereas the other seven men (sea-level group) trained at sea level (K. Katayama, Y. Sato, Y. Morotome, N. Shima, K. Ishida, S. Mori, and M. Miyamura. J. Appl. Physiol. 86: 1805-1811, 1999). The HVR, systolic and diastolic blood pressure responses (DeltaSBP/DeltaSa(O(2)), DeltaDBP/DeltaSa(O(2))), and heart rate response (DeltaHR/DeltaSa(O(2)); Sa(O(2)) is arterial oxygen saturation) to progressive isocapnic hypoxia were measured before and after training and during detraining. DeltaSBP/DeltaSa(O(2)) increased significantly in the altitude group and decreased significantly in the sea-level group after training. The changed DeltaSBP/DeltaSa(O(2)) in both groups was restored during 2 wk of detraining, as were the changes in HVR, whereas there were no changes in the DeltaDBP/DeltaSa(O(2)) and DeltaHR/DeltaSa(O(2)) throughout the experimental period. The changes in DeltaSBP/DeltaSa(O(2)) after training and detraining were significantly correlated with those in HVR. These results suggest that DeltaSBP/DeltaSa(O(2)) to progressive isocapnic hypoxia is variable after endurance training during hypoxia and normoxia and after detraining, as is HVR, but DeltaDBP/DeltaSa(O(2)) and DeltaHR/DeltaSa(O(2)) are not. It also suggests that there is an interaction between the changes in DeltaSBP/DeltaSa(O(2)) and HVR after endurance training or detraining.