Intermittent hypobaric hypoxia exposure does not cause sustained alterations in autonomic control of blood pressure in young athletes

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

Background

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

Methods

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

Results

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

Conclusion

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

Registration

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

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

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

Acute Exercise with Moderate Hypoxia Reduces Arterial Oxygen Saturation and Cerebral Oxygenation without Affecting Hemodynamics in Physically Active Males

Hemodynamic changes during exercise in acute hypoxia (AH) have not been completely elucidated. The present study aimed to investigate hemodynamics during an acute bout of mild, dynamic exercise during moderate normobaric AH. Twenty-two physically active, healthy males (average age; range 23–40 years) completed a cardiopulmonary test on a cycle ergometer to determine their maximum workload (W_max). On separate days, participants performed two randomly assigned exercise tests (three minutes pedaling at 30% of W_max): (1) during normoxia (NORMO), and (2) during normobaric AH at 13.5% inspired oxygen (HYPO). Hemodynamics were assessed with impedance cardiography, and peripheral arterial oxygen saturation (SatO₂) and cerebral oxygenation (Cox) were measured by near-infrared spectroscopy. Hemodynamic responses (heart rate, stroke volume, cardiac output, mean arterial blood pressure, ventricular emptying rate, and ventricular filling rate) were not any different between NORMO and HYPO. However, the HYPO test significantly reduced both SatO₂ (96.6 ± 3.3 vs. 83.0 ± 4.5%) and Cox (71.0 ± 6.6 vs. 62.8 ± 7.4 A.U.) when compared to the NORMO test. We conclude that an acute bout of mild exercise during acute moderate normobaric hypoxia does not induce significant changes in hemodynamics, although it can cause significant reductions in SatO₂ and Cox.

Intermittent Hypoxia Exposure Can Prevent Reductions in Hemoglobin Concentration After Intense Exercise Training in Rats

Intense exercise training can induce low concentrations of hemoglobin, which may be followed by maladaptation. Therefore, it is important for athletes to prevent low concentrations of hemoglobin during intense exercise training. In this study, we explored whether different protocols of intermittent hypoxic exposure (IHE, normobaric hypoxia, 14.5% O₂) could prevent the exercise training-induced reduction in hemoglobin concentration in rats. Six-week-old male Sprague-Dawley rats were subjected to progressive intense treadmill exercise training over three weeks followed by three weeks of training with IHE after exercise. IHE lasted either 1 h, 2 h, or 1 h + 1 h (separated by a 3-h interval) after the exercise sessions. Hematological parameters, including hemoglobin concentration [(Hb)], red blood cells (RBCs), and hematocrit (Hct), and both renal and serum erythropoietin (EPO) were examined. We found that intense exercise training significantly reduced [Hb], RBCs, Hct, food intake and body weight (P < 0.01). Analysis of reticulocyte hemoglobin content (CHr) and reticulocyte counts in the serum of the rats suggested that this reduction was not due to iron deficiency or other cofounding factors. The addition of IHE after the intense exercise training sessions significantly alleviated the reduction in [Hb], RBCs, and Hct (P < 0.05) without an obvious impact on either food intake or body weight (P > 0.05). Increase in reticulocyte count in the rats from the IHE groups (P < 0.05 or P < 0.01) suggests that IHE promotes erythropoiesis to increase the hemoglobin concentration. Furthermore, the addition of IHE after the intense exercise training sessions also significantly increased the concentration of renal EPO (P < 0.05), although the increase of the serum EPO level was statistically insignificant (P > 0.05). The different IHE protocols were similarly effective at increasing renal EPO and preventing the training-induced decreases in [Hb], RBCs, and Hct. Collectively, this study suggests that IHE may be used as a new strategy to prevent intense exercise training-induced reductions in [Hb], and deserves future exploration in athletes.

Effect of normobaric hypoxic exercise on blood pressure in old individuals

Purpose

To test the hypothesis that the combination of endurance training and hypoxia leads to greater improvements in resting and exercise blood pressure in old sedentary individuals compared to endurance training only.

Methods

We randomly assigned 29 old overweight participants (age: 62 ± 6 years, body mass index (BMI): 28.5 ± 0.5 kg/m², 52% men) to single blind 8-week bicycle exercise in hypoxia (fraction of inspired oxygen (F_IO₂) = 0.15) or normoxia (F_IO₂ = 0.21). Brachial blood pressure was measured at rest, during maximal incremental exercise testing, and during a 30 min constant work rate test, at baseline and after the training period.

Results

Work rate, heart rate and perceived exertion during training were similar in both groups, with lower oxygen saturation for participants exercising under hypoxia (88.7 ± 1.5 vs. 96.2 ± 1.2%, t(27) = − 13.04, p < 0.001, |g|= 4.85). Office blood pressure and blood pressure during incremental exercise tests did not change significantly in either group after the training program. Systolic blood pressure during the constant work rate test was reduced after training in hypoxia (160 ± 18 vs. 151 ± 14 mmHg, t(13) = 2.44 p < 0.05, |d|= 0.55) but not normoxia (154 ± 22 vs. 150 ± 16 mmHg, t(14) = 0.75, p = 0.46, |d|= 0.18) with no difference between groups over time (F = 0.08, p = 0.77, η² = 0.01).

Conclusion

In old individuals hypoxia in addition to exercise does not have superior effects on office or exercise blood pressure compared to training in normoxia.

Trial registration number

ClinicalTrials.gov No. NCT02196623 (registered 22 July 2014).

Heart rate variability as a biomarker for anorexia nervosa: A review

OBJECTIVE

Anorexia nervosa (AN) typically begins in early adolescence and other than weight status has few reliable biomarkers. Early diagnosis is a critical prognostic factor, but this can be clinically challenging. Heart rate variability (HRV), the beat-by-beat variance in heart rate (HR), may provide a unique assessment for the presence of AN because it has clinical utility as a biomarker of cardiac autonomic control in various populations (e.g., athletes, the aged, those with cardiovascular diseases, etc.). We present a review of the literature examining HRV in those with AN.

METHOD

Relevant publications were selected from PubMed using the search terms 'anorexia nervosa AND (HR OR HRV)'. Twenty papers were selected and reviewed.

RESULTS

The majority of studies suggest that those with AN have markedly and consistently elevated HRV compared to controls, even greater than among young athletes. However, no studies have explored HRV as a biomarker for AN.

DISCUSSION

Future studies on HRV should elucidate its role as a diagnostic biomarker for AN as well as its responsiveness with serial measurement to track response rates and predict relapse.

Sex differences in integrated neurocardiovascular control of blood pressure following acute intermittent hypercapnic hypoxia

Repetitive hypoxic apneas, similar to those observed in sleep apnea, result in resetting of the sympathetic baroreflex to higher blood pressures (BP). This baroreflex resetting is associated with hypertension in preclinical models of sleep apnea (intermittent hypoxia, IH); however, the majority of understanding comes from males. There are data to suggest that female rats exposed to IH do not develop high BP. Clinical data further support sex differences in the development of hypertension in sleep apnea, but mechanistic data are lacking. Here we examined sex-related differences in the effect of IH on sympathetic control of BP in humans. We hypothesized that after acute IH we would observe a rise in muscle sympathetic nerve activity (MSNA) and arterial BP in young men (n = 30) that would be absent in young women (n = 19). BP and MSNA were measured during normoxic rest before and after 30 min of IH. Baroreflex sensitivity (modified Oxford) was evaluated before and after IH. A rise in mean BP following IH was observed in men (+2.0 ± 0.7 mmHg, P = 0.03), whereas no change was observed in women (-2.7 ± 1.2 mmHg, P = 0.11). The elevation in MSNA following IH was not different between groups (4.7 ± 1.1 vs. 3.8 ± 1.2 bursts/min, P = 0.65). Sympathetic baroreflex sensitivity did not change after IH in either group (P > 0.05). Our results support sex-related differences in the effect of IH on neurovascular control of BP and show that any BP-raising effects of IH are absent in young women. These data enhance our understanding of sex-specific mechanisms that may contribute to BP changes in sleep apnea.

Effects of Two Short-Term, Intermittent Hypoxic Training Protocols on the Finger Temperature Response to Local Cold Stress

The study examined the effects of two short-term, intermittent hypoxic training protocols, namely exercising in hypoxia and living in normoxia (LL-TH; n=8), and exercising in normoxia preceded by a series of brief intermittent hypoxic exposures at rest (IHE+NOR; n=8), on the finger temperature response during a sea-level local cold test. In addition, a normoxic group was assigned as a control group (NOR; n=8). All groups trained on a cycle-ergometer 1 h/day, 5 days/week for 4 weeks at 50% of peak power output. Pre, post, and 11 days after the last training session, subjects immersed their right hand for 30 min in 8°C water. In the NOR group, the average finger temperature was higher in the post (+2.1°C) and 11-day after (+2.6°C) tests than in the pre-test (p≤0.001). Conversely, the fingers were significantly colder immediately after both hypoxic protocols (LL-TH: -1.1°C, IHE+NOR: -1.8°C; p=0.01). The temperature responses returned to the pre-training level 11 days after the hypoxic interventions. Ergo, present findings suggest that short-term intermittent hypoxic training impairs sea-level local cold tolerance; yet, the hypoxic-induced adverse responses seem to be reversible within a period of 11 days.

Humans In Hypoxia: A Conspiracy Of Maladaptation?!

We address adaptive vs. maladaptive responses to hypoxemia in healthy humans and hypoxic-tolerant species during wakefulness, sleep, and exercise. Types of hypoxemia discussed include short-term and life-long residence at high altitudes, the intermittent hypoxemia attending sleep apnea, or training regimens prescribed for endurance athletes. We propose that hypoxia presents an insult to O2 transport, which is poorly tolerated in most humans because of the physiological cost.

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

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

Intermittent hypoxia: a low-risk research tool with therapeutic value in humans

Intermittent hypoxia has generally been perceived as a high-risk stimulus, particularly in the field of sleep medicine, because it is thought to initiate detrimental cardiovascular, respiratory, cognitive, and metabolic outcomes. In contrast, the link between intermittent hypoxia and beneficial outcomes has received less attention, perhaps because it is not universally understood that outcome measures following exposure to intermittent hypoxia may be linked to the administered dose. The present review is designed to emphasize the less recognized beneficial outcomes associated with intermittent hypoxia. The review will consider the role intermittent hypoxia has in cardiovascular and autonomic adaptations, respiratory motor plasticity, and cognitive function. Each section will highlight the literature that contributed to the belief that intermittent hypoxia leads primarily to detrimental outcomes. The second segment of each section will consider the possible risks associated with experimentally rather than naturally induced intermittent hypoxia. Finally, the body of literature indicating that intermittent hypoxia initiates primarily beneficial outcomes will be considered. The overarching theme of the review is that the use of intermittent hypoxia in research investigations, coupled with reasonable safeguards, should be encouraged because of the potential benefits linked to the administration of a variety of low-risk intermittent hypoxia protocols.

Therapeutic potential of intermittent hypoxia: a matter of dose

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

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

Intermittent hypoxia (IH) is extensively applied to challenge cardiovascular and respiratory function, and to induce physiological acclimatization. The purpose of this study was to test the hypothesis that oxyhemoglobin equilibrium and tachycardiac responses during hypoxemia were enhanced after 14-day IH exposures. Normobaric-poikilocapnic hypoxia was induced with inhalation of 10% O2 for 5-6 min interspersed with 4 min recovery on eight nonsmokers. Heart rate (HR), arterial O2 saturation (SaO 2), and end-tidal O2 (PetO 2) were continuously monitored during cyclic normoxia and hypoxia. These variables were compared during the first and fifth hypoxic bouts between day 1 and day 14. There was a rightward shift in the oxyhemoglobin equilibrium response following 14-day IH exposures, as indicated by the greater PetO 2 (an index of arterial Po2) at 50% of SaO 2 on day 14 compared with day 1 [33.9 ± 1.5 vs. 28.2 ± 1.3 mmHg (P = 0.005) during the first hypoxic bout and 39.4 ± 2.4 vs. 31.4 ± 1.5 mmHg (P = 0.006) during the fifth hypoxic bout] and by the augmented gains of ΔSaO 2/ΔPetO 2 (i.e., deoxygenation) during PetO 2 from 65 to 40 mmHg in the first (1.12 ± 0.08 vs. 0.80 ± 0.02%/mmHg, P = 0.001) and the fifth (1.76 ± 0.31 vs. 1.05 ± 0.06%/mmHg, P = 0.024) hypoxic bouts. Repetitive IH exposures attenuated (P = 0.049) the tachycardiac response to hypoxia while significantly enhancing normoxic R-R interval variability in low-frequency and high-frequency spectra without changes in arterial blood pressure at rest or during hypoxia. We conclude that 14-day IH exposures enhance arterial O2 delivery and improve vagal control of HR during hypoxic hypoxemia.

Baroreflex and chemoreflex controls of sympathetic activity following intermittent hypoxia

There is a large amount of evidence linking obstructive sleep apnea (OSA), and the associated intermittent hypoxia that accompanies it, with the development of hypertension. For example, cross-sectional studies demonstrate that the prevalence of hypertension increases with the severity of OSA (Bixler et al., 2000; Grote et al., 2001) and an initial determination of OSA is associated with a three-fold increase for future hypertension (Peppard et al., 2000). Interestingly, bouts of intermittent hypoxia have also been shown to affect sympathetic output associated with the baroreflex and chemoreflex, important mechanisms in the regulation of arterial blood pressure. As such, the possibility exists that changes in the baroreflex and chemoreflex may contribute to the development of chronic hypertension observed in OSA patients. The aim of the current article is to briefly review the response of the baroreflex and chemoreflex to intermittent hypoxic exposure and to evaluate evidence for the hypothesis that modification of these autonomic reflexes may, at least in part, support the comorbidity observed between chronic hypertension and OSA.

Intermittent hypobaric hypoxia applicability in myocardial infarction prevention and recovery

Intermittent hypobaric hypoxia (IHH) has been the focus of important research in cardioprotection, and it has been associated with several mechanisms. Intermittent hypobaric hypoxia inhibits prolyl hydroxylases (PHD) activity, increasing the stabilization of hypoxia-inducible factor-1 (HIF-1) and activating crucial adaptative genes. It has been hence suggested that IHH might be a simple intervention, which may offer a thoughtful benefits to patients with acute myocardial infarction and no complications. Nevertheless, several doubts exist as to whether IHH is a really safe technique, with little to no complications in post-myocardial infarction patients. Intermittent hypobaric hypoxia might produce instead unfavourable changes such as impairment of vascular hemodynamics and hypertensive response, increased risk of hemoconcentration and thrombosis, cardiac rhythm perturbations, coronary artery disease and heart failure, insulin resistance, steatohepatitis and even high-altitude pulmonary oedema in susceptible or nonacclimatized patients. Although intermittent and chronic exposures seem effective in cardioprotection, IHH safety issues have been mostly overlooked, so that assorted concerns should be raised about the opportunity to use IHH in the post-myocardial infarction period. Several IHH protocols used in some studies were also aggressive, which would hamper their widespread introduction within the clinical practice. As such, further research is needed before IHH can be widely advocated in myocardial infarction prevention and recovery.

Hypobaric intermittent hypoxia attenuates hypoxia-induced depressor response

Background

Hypobaric intermittent hypoxia (HIH) produces many favorable effects in the cardiovascular system such as anti-hypertensive effect. In this study, we showed that HIH significantly attenuated a depressor response induced by acute hypoxia.

Methodology/Principal Findings

Sprague-Dawley rats received HIH in a hypobaric chamber simulating an altitude of 5000 m. The artery blood pressure (ABP), heart rate (HR) and renal sympathetic nerve activity (RSNA) were recorded in anesthetized control rats and rats received HIH. The baseline ABP, HR and RSNA were not different between HIH and control rats. Acute hypoxia-induced decrease in ABP was significantly attenuated in HIH rat compared with control rats. However, acute hypoxia-induced increases in HR and RSNA were greater in HIH rat than in control rats. After removal of bilateral ascending depressor nerves, acute hypoxia-induced depressor and sympathoexcitatory responses were comparable in control and HIH rats. Furthermore, acute hypoxia-induced depressor and sympathoexcitatory responses did not differ between control and HIH groups after blocking ATP-dependent K⁺ channels by glibenclamide. The baroreflex function evaluated by intravenous injection of phenylephrine and sodium nitroprusside was markedly augmented in HIH rats compared with control rats. The pressor and sympathoexcitatory responses evoked by intravenous injection of cyanide potassium were also significantly greater in HIH rats than in control rats.

Conclusions/Significance

Our findings suggest that HIH suppresses acute hypoxia-induced depressor response through enhancement of baroreflex and chemoreflex function, which involves activation of ATP-dependent K⁺ channels. This study provides new information and underlying mechanism on the beneficiary effect of HIH on maintaining cardiovascular homeostasis.

Role of intermittent hypoxia in the treatment of bronchial asthma and chronic obstructive pulmonary disease

PURPOSE OF REVIEW

The purpose of this review is to describe the impact that exposure to intermittent hypoxic training (IHT) could have on bronchial asthma and chronic obstructive pulmonary disease (COPD). This is of particular interest, as an increasing number of patients suffer from severe symptoms of bronchial asthma and COPD and desire more effective and efficient treatment options with fewer side effects.

RECENT FINDINGS

Exposure to IHT has been shown to raise baroreflex sensitivity to normal levels and to selectively increase hypercapnic ventilatory response, total exercise time, total haemoglobin mass, and lung diffusion capacity for carbon monoxide in COPD patients. However, evidence proving that IHT leads to health benefit effects in bronchial asthma patients has not been produced by recent literature.

SUMMARY

Recent research outlines the value of IHT as a therapeutic strategy for the treatment of COPD patients, leading to more efficient ventilation. Additionally, IHT might represent an attractive method to complement the known beneficial effects of exercise training and to rebalance early autonomic dysfunction in COPD patients. Future research examining the potential risks and benefits of IHT could pave the way for the development of new therapeutic approaches for patients suffering from bronchial asthma and COPD.

Baroreflex sensitivity of an arterial wall during rotary blood pump assistance

It is well known that the baroreflex system is one of the most important indicators of the pathophysiology in hypertensive patients. We can check the sensitivity of the baroreflex by observing heart rate (HR) responses; however, there is no simple diagnostic method to measure the arterial behavior in the baroreflex system. Presently, we report the development of a method and associated hardware that enables the diagnosis of baroreflex sensitivity by measuring the responses of both the heart and the artery. In this system, the measurements are obtained by monitoring an electrocardiogram and a pulse wave recorded from the radial artery or fingertip. The arterial responses were measured in terms of the pulse wave velocity (PWV) calculated from the pulse wave transmission time (PTT) from the heart to the artery. In this system, the HR change corresponding to the blood pressure change in time series sequence was observed. Slope of the changes in blood pressure and HR indicated the sensitivity of the baroreflex system of the heart. This system could also measure the sensitivity of the baroreflex system of an artery. Changes in the PWV in response to the blood pressure changes were observed. Significant correlation was observed in the time sequence between blood pressure change and PWV change after calculating the delay time by cross-correlation. The slope of these parameter changes was easily obtained and it demonstrated the sensitivity of the baroreflex system of an artery. We evaluated this method in animal experiments using rotary blood pump (RBP) with undulation pump ventricular assist device, and PTT elongation was observed in response to increased blood pressure with RBP assistance. Furthermore, when tested clinically, decreased sensitivity of the baroreflex system in hypertensive patients was observed. This system may be useful when we consider the ideal treatment and follow-up of patients with hypertension.

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.

Cerebral hypoperfusion during hypoxic exercise following two different hypoxic exposures: independence from changes in dynamic autoregulation and reactivity

We examined the effects of exposure to 10–12 days intermittent hypercapnia [IHC: 5:5-min hypercapnia (inspired fraction of CO2 0.05)-to-normoxia for 90 min ( n = 10)], intermittent hypoxia [IH: 5:5-min hypoxia-to-normoxia for 90 min ( n = 11)] or 12 days of continuous hypoxia [CH: 1,560 m ( n = 7)], or both IH followed by CH on cardiorespiratory and cerebrovascular function during steady-state cycling exercise with and without hypoxia (inspired fraction of oxygen, 0.14). Cerebrovascular reactivity to CO2 was also monitored. During all procedures, ventilation, end-tidal gases, blood pressure, muscle and cerebral oxygenation (near-infrared spectroscopy), and middle cerebral artery blood flow velocity (MCAv) were measured continuously. Dynamic cerebral autoregulation (CA) was assessed using transfer-function analysis. Hypoxic exercise resulted in increases in ventilation, hypocapnia, heart rate, and cardiac output when compared with normoxic exercise ( P < 0.05); these responses were unchanged following IHC but were elevated following the IH and CH exposure ( P < 0.05) with no between-intervention differences. Following IH and/or CH exposure, the greater hypocapnia during hypoxic exercise provoked a decrease in MCAv ( P < 0.05 vs. preexposure) that was related to lowered cerebral oxygenation ( r = 0.54; P < 0.05). Following any intervention, during hypoxic exercise, the apparent impairment in CA, reflected in lowered low-frequency phase between MCAv and BP, and MCAv-CO2 reactivity, were unaltered. Conversely, during hypoxic exercise following both IH and/or CH, there was less of a decrease in muscle oxygenation ( P < 0.05 vs. preexposure). Thus IH or CH induces some adaptation at the muscle level and lowers MCAv and cerebral oxygenation during hypoxic exercise, potentially mediated by the greater hypocapnia, rather than a compromise in CA or MCAv reactivity.

Performance of runners and swimmers after four weeks of intermittent hypobaric hypoxic exposure plus sea level training

This double-blind, randomized, placebo-controlled trial examined the effects of 4 wk of resting exposure to intermittent hypobaric hypoxia (IHE, 3 h/day, 5 days/wk at 4,000-5,500 m) or normoxia combined with training at sea level on performance and maximal oxygen transport in athletes. Twenty-three trained swimmers and runners completed duplicate baseline time trials (100/400-m swims, or 3-km run) and measures for maximal oxygen uptake (VO(2max)), ventilation (VE(max)), and heart rate (HR(max)) and the oxygen uptake at the ventilatory threshold (VO(2) at VT) during incremental treadmill or swimming flume tests. Subjects were matched for sex, sport, performance, and training status and divided randomly between hypobaric hypoxia (Hypo, n = 11) and normobaric normoxia (Norm, n = 12) groups. All tests were repeated within the first (Post1) and third weeks (Post2) after the intervention. Time-trial performance did not improve in either group. We could not detect a significant difference between groups for a change in VO(2max), VE(max), HR(max), or VO(2) at VT after the intervention (group x test interaction P = 0.31, 0.24, 0.26, and 0.12, respectively). When runners and swimmers were considered separately, Hypo swimmers appeared to increase VO(2max) (+6.2%, interaction P = 0.07) at Post2 following a precompetition taper and increased VO(2) at VT (+8.9 and +12.1%, interaction P = 0.007 and 0.006, at Post1 and Post2). We conclude that this "dose" of IHE was not sufficient to improve performance or oxygen transport in this heterogeneous group of athletes. Whether there are potential benefits of this regimen for specific sports or training/tapering strategies may require further study.