Reduced autonomic activity during stepwise exposure to high altitude

Baroreflex and chemoreflex interaction in high-altitude exposure: possible role on exercise performance

The hypoxic chemoreflex and the arterial baroreflex are implicated in the ventilatory response to exercise. It is well known that long-term exercise training increases parasympathetic and decreases sympathetic tone, both processes influenced by the arterial baroreflex and hypoxic chemoreflex function. Hypobaric hypoxia (i.e., high altitude [HA]) markedly reduces exercise capacity associated with autonomic reflexes. Indeed, a reduced exercise capacity has been found, paralleled by a baroreflex-related parasympathetic withdrawal and a pronounced chemoreflex potentiation. Additionally, it is well known that the baroreflex and chemoreflex interact, and during activation by hypoxia, the chemoreflex is predominant over the baroreflex. Thus, the baroreflex function impairment may likely facilitate the exercise deterioration through the reduction of parasympathetic tone following acute HA exposure, secondary to the chemoreflex activation. Therefore, the main goal of this review is to describe the main physiological mechanisms controlling baro- and chemoreflex function and their role in exercise capacity during HA exposure.

Effects of Acute Hypoxia on Heart Rate Variability in Patients with Pulmonary Vascular Disease

Pulmonary vascular diseases (PVDs), defined as arterial or chronic thromboembolic pulmonary hypertension, are associated with autonomic cardiovascular dysregulation. Resting heart rate variability (HRV) is commonly used to assess autonomic function. Hypoxia is associated with sympathetic overactivation and patients with PVD might be particularly vulnerable to hypoxia-induced autonomic dysregulation. In a randomised crossover trial, 17 stable patients with PVD (resting PaO₂ ≥ 7.3 kPa) were exposed to ambient air (FiO₂ = 21%) and normobaric hypoxia (FiO₂ = 15%) in random order. Indices of resting HRV were derived from two nonoverlapping 5-10-min three-lead electrocardiography segments. We found a significant increase in all time- and frequency-domain HRV measures in response to normobaric hypoxia. There was a significant increase in root mean squared sum difference of RR intervals (RMSSD; 33.49 (27.14) vs. 20.76 (25.19) ms; p < 0.01) and RR50 count divided by the total number of all RR intervals (pRR50; 2.75 (7.81) vs. 2.24 (3.39) ms; p = 0.03) values in normobaric hypoxia compared to ambient air. Both high-frequency (HF; 431.40 (661.56) vs. 183.70 (251.25) ms²; p < 0.01) and low-frequency (LF; 558.60 (746.10) vs. 203.90 (425.63) ms²; p = 0.02) values were significantly higher in normobaric hypoxia compared to normoxia. These results suggest a parasympathetic dominance during acute exposure to normobaric hypoxia in PVD.

Hypobaric hypoxia and cardiac baroreflex sensitivity in young women

We sought to determine the effects of prolonged moderate hypobaric hypoxia (HH) on cardiac baroreflex sensitivity (cBRS) in young women and whether these effects are a consequence of the reduced arterial oxygen (O₂) tension and/or increased pulmonary ventilation in HH. We hypothesized that HH would reduce cBRS and that this effect would be counteracted by acute restoration of the inspiratory partial pressure of O₂ ([Formula: see text]) and/or voluntary attenuation of pulmonary ventilation. Twelve healthy women (24.0 ± 4.2 yr) were studied before (day 0) and twice during a sojourn in a hypobaric chamber (∼8 h, day 1; 4 days, day 4) where barometric pressure corresponded to ∼3,500-m altitude. Minute ventilation (V̇e; pneumotachometer), heart rate (electrocardiogram), and arterial pressure (finger volume clamp method) were recorded. cBRS was calculated using transfer function analysis between systolic pressure and RR interval. Assessments were made during 1) spontaneous breathing and (in HH only), 2) controlled breathing (reducing V̇e by ∼1 to 2 L/min), and 3) breathing a hyperoxic gas mixture that normalized [Formula: see text]. During spontaneous breathing, HH decreased cBRS (12.5 ± 7.1, 8.9 ± 4.4, and 7.4 ± 3.0 ms/mmHg on days 0, 1, and 4, respectively; P = 0.018). The normalization of [Formula: see text] increased cBRS (10.6 ± 3.3 and 10.7 ± 6.1 ms/mmHg on days 1 and 4) in HH compared with values observed during spontaneous breathing (P < 0.001), whereas controlled breathing had no effect on cBRS (P = 0.708). These findings indicate that ongoing arterial chemoreflex activation by the reduced arterial O₂ tension, independently of the hypoxic ventilatory response, reduces cBRS in young women exposed to extended HH.NEW & NOTEWORTHY We examined the effects of prolonged hypobaric hypoxia (corresponding to ∼3,500-m altitude) on cardiac baroreflex sensitivity (cBRS) in young women and investigated underlying mechanisms. We found that cBRS was reduced in hypoxia and that this reduction was attenuated by acute restoration of inspiratory oxygen partial pressure but not by volitional restraint of pulmonary ventilation. These findings help to elucidate the role of arterial chemoreflex mechanisms in the control of cBRS during hypobaric hypoxia in young women.

Left Ventricular, Left Atrial and Right Ventricular Strain Modifications after Maximal Exercise in Elite Ski-Mountaineering Athletes: A Feasibility Speckle Tracking Study

Eleven world elite ski-mountaineering (Ski-Mo) athletes were evaluated for pronounced echocardiographic physiological remodeling as the primary aim of our feasibility speckle tracking study. In this context, sports-related cardiac remodeling was analyzed by performing two-dimensional echocardiography, including speckle tracking analysis of the left atrium (LA), right ventricle (RV) and left ventricular (LV) global longitudinal strain (LV-GLS) at rest and post-peak performance. The feasibility echocardiographic speckle tracking analysis was performed on eleven elite Ski-Mo athletes, which were obtained in 2022 during the annual medical examination. The obtained data of the professional Ski-Mo athletes (11 athletes, age: 18-26 years) were compared for different echocardiographic parameters at rest and post-exercise. Significant differences were found for LV-GLS mean (p = 0.0036) and phasic LA conduit strain pattern at rest and post-exercise (p = 0.0033). Furthermore, negative correlation between LV mass and LV-GLS (p = 0.0195, r = -0.69) and LV mass Index and LV-GLS (p = 0.0253, r = -0.66) at rest were elucidated. This descriptive reporting provided, for the first time, a sport-specific dynamic remodeling of an entire elite national team of the Ski-Mo athlete's left heart and elucidated differences in the dynamic deformation pattern of the left heart.

Remote ischemic preconditioning enhances aerobic performance by accelerating regional oxygenation and improving cardiac function during acute hypobaric hypoxia exposure

Remote ischemic preconditioning (RIPC) may improve exercise performance. However, the influence of RIPC on aerobic performance and underlying physiological mechanisms during hypobaric hypoxia (HH) exposure remains relatively uncertain. Here, we systematically evaluated the potential performance benefits and underlying mechanisms of RIPC during HH exposure. Seventy-nine healthy participants were randomly assigned to receive sham intervention or RIPC (4 × 5 min occlusion 180 mm Hg/reperfusion 0 mm Hg, bilaterally on the upper arms) for 8 consecutive days in phases 1 (24 participants) and phase 2 (55 participants). In the phases 1, we measured the change in maximal oxygen uptake capacity (VO₂max) and muscle oxygenation (SmO₂) on the leg during a graded exercise test. We also measured regional cerebral oxygenation (rSO₂) on the forehead. These measures and physiological variables, such as cardiovascular hemodynamic parameters and heart rate variability index, were used to evaluate the intervention effect of RIPC on the changes in bodily functions caused by HH exposure. In the phase 2, plasma protein mass spectrometry was then performed after RIPC intervention, and the results were further evaluated using ELISA tests to assess possible mechanisms. The results suggested that RIPC intervention improved VO₂max (11.29%) and accelerated both the maximum (18.13%) and minimum (53%) values of SmO₂ and rSO₂ (6.88%) compared to sham intervention in hypobaric hypoxia exposure. Cardiovascular hemodynamic parameters (SV, SVRI, PPV% and SpMet%) and the heart rate variability index (Mean RR, Mean HR, RMSSD, pNN50, Lfnu, Hfnu, SD1, SD2/SD1, ApEn, SampEn, DFA1and DFA2) were evaluated. Protein sequence analysis showed 42 unregulated and six downregulated proteins in the plasma of the RIPC group compared to the sham group after HH exposure. Three proteins, thymosin β4 (Tβ4), heat shock protein-70 (HSP70), and heat shock protein-90 (HSP90), were significantly altered in the plasma of the RIPC group before and after HH exposure. Our data demonstrated that in acute HH exposure, RIPC mitigates the decline in VO₂max and regional oxygenation, as well as physiological variables, such as cardiovascular hemodynamic parameters and the heart rate variability index, by influencing plasma Tβ4, HSP70, and HSP90. These data suggest that RIPC may be beneficial for acute HH exposure.

Daily Exposure to Mild Intermittent Hypoxia Reduces Blood Pressure in Male Patients with Obstructive Sleep Apnea and Hypertension

Rationale: Daily exposure to mild intermittent hypoxia (MIH) may elicit beneficial cardiovascular outcomes. Objectives: To determine the effect of 15 days of MIH and in-home continuous positive airway pressure treatment on blood pressure in participants with obstructive sleep apnea and hypertension. Methods: We administered MIH during wakefulness 5 days/week for 3 weeks. The protocol consisted of twelve 2-minute bouts of hypoxia interspersed with 2 minutes of normoxia. End-tidal carbon dioxide was maintained 2 mm Hg above baseline values throughout the protocol. Control participants were exposed to a sham protocol (i.e., compressed air). All participants were treated with continuous positive airway pressure over the 3-week period. Results are mean ± SD. Measurements and Main Results: Sixteen male participants completed the study (experimental n = 10; control n = 6). Systolic blood pressure at rest during wakefulness over 24 hours was reduced after 15 days of MIH (142.9 ± 8.6 vs. 132.0 ± 10.7 mm Hg; P < 0.001), but not following the sham protocol (149.9 ± 8.6 vs. 149.7 ± 10.8 mm Hg; P = 0.915). Thus, the reduction in blood pressure from baseline was greater in the experimental group compared with control (-10.91 ± 4.1 vs. -0.17 ± 3.6 mm Hg; P = 0.003). Modifications in blood pressure were accompanied by increased parasympathetic and reduced sympathetic activity in the experimental group, as estimated by blood pressure and heart rate variability analysis. No detrimental neurocognitive and metabolic outcomes were evident following MIH. Conclusions: MIH elicits beneficial cardiovascular and autonomic outcomes in males with OSA and concurrent hypertension. Clinical trial registered with www.clinicaltrials.gov (NCT03736382).

Platelets and High-Altitude Exposure: A Meta-Analysis

Wang, Yuliang, Xuewen Huang, Weibo Yang, and Qingxian Zeng. Platelets and high-altitude exposure: a meta-analysis. High Alt Med Biol. 23:43-56, 2022. Background: How high-altitude hypoxia influences platelets is controversial. We attempted to quantify the impact of high-altitude exposure on platelets through meta-analysis. Methods: We systematically searched electronic databases (PubMed, Embase, Web of Science, VIP, Wanfang, and CNKI) and identified articles reporting an association between platelet count (PC) or platelet indices (platelet distribution width, mean platelet volume [MPV], and plateletcrit) and high-altitude exposure. The mean and standard deviation were extracted, and the standard mean difference (SMD) was estimated using random-effects models. Stata 15.3 was used to analyze statistical data. Results: Thirty-two studies were ultimately included. For acute high-altitude hypoxia (1-14 days), no significant difference was detected, even in patients with acute mountain disease. For the chronic high-altitude hypoxia (≥1 month) group, a significant decrease in PC (SMD [95% confidence interval, CI] = -0.34 [-0.63 to -0.04]) and increase in MPV (SMD [95% CI] = 1.55 [0.60 to 2.49]) were detected compared with those in the control group. Subgroup analysis showed that the tendency was more obvious in the group with longer exposure (≥1 year). Moreover, the PC of the chronic mountain sickness group was less compared with the healthy altitude control group (SMD [95% CI] = -1.82 [-2.74 to -0.91]). Conclusion: A reduced PC and an increased MPV are associated with chronic exposure to high-altitude hypoxia. Moreover, acute high-altitude exposure has no significant influence on platelets.

Longitudinal Changes and Recovery in Heart Rate Variability of Young Healthy Subjects When Exposure to a Hypobaric Hypoxic Environment

Background: The autonomic nervous system (ANS) is crucial for acclimatization. Investigating the responses of acute exposure to a hypoxic environment may provide some knowledge of the cardiopulmonary system’s adjustment mechanism.

Objective: The present study investigates the longitudinal changes and recovery in heart rate variability (HRV) in a young healthy population when exposed to a simulated plateau environment.

Methods: The study followed a strict experimental paradigm in which physiological signals were collected from 33 healthy college students (26 ± 2 years, 171 cm ± 7 cm, 64 ± 11 kg) using a medical-grade wearable device. The subjects were asked to sit in normoxic (approximately 101 kPa) and hypoxic (4,000 m above sea level, about 62 kPa) environments. The whole experimental process was divided into four stable resting measurement segments in chronological order to analyze the longitudinal changes of physical stress and recovery phases. Seventy-six time-domain, frequency-domain, and non-linear indicators characterizing rhythm variability were analyzed in the four groups.

Results: Compared to normobaric normoxia, participants in hypobaric hypoxia had significantly lower HRV time-domain metrics, such as RMSSD, MeanNN, and MedianNN (p &lt; 0.01), substantially higher frequency domain metrics such as LF/HF ratio (p &lt; 0.05), significantly lower Poincaré plot parameters such as SD1/SD2 ratio and other Poincaré plot parameters are reduced considerably (p &lt; 0.01), and Refined Composite Multi-Scale Entropy (RCMSE) curves are reduced significantly (p &lt; 0.01).

Conclusion: The present study shows that elevated heart rates, sympathetic activation, and reduced overall complexity were observed in healthy subjects exposed to a hypobaric and hypoxic environment. Moreover, the results indicated that Multiscale Entropy (MSE) analysis of RR interval series could characterize the degree of minor physiological changes. This novel index of HRV can better explain changes in the human ANS.

Similar Supine Heart Rate Variability Changes During 24-h Exposure to Normobaric vs. Hypobaric Hypoxia

Purpose: This study aimed to investigate the differences between normobaric (NH) and hypobaric hypoxia (HH) on supine heart rate variability (HRV) during a 24-h exposure. We hypothesized a greater decrease in parasympathetic-related parameters in HH than in NH.

Methods: A pooling of original data from forty-one healthy lowland trained men was analyzed. They were exposed to altitude either in NH (F_IO₂ = 15.7 ± 2.0%; PB = 698 ± 25 mmHg) or HH (F_IO₂ = 20.9%; PB = 534 ± 42 mmHg) in a randomized order. Pulse oximeter oxygen saturation (S_pO₂), heart rate (HR), and supine HRV were measured during a 7-min rest period three times: before (in normobaric normoxia, NN), after 12 (H12), and 24 h (H24) of either NH or HH exposure. HRV parameters were analyzed for time- and frequency-domains.

Results: S_pO₂ was lower in both hypoxic conditions than in NN and was higher in NH than HH at H24. Subjects showed similarly higher HR during both hypoxic conditions than in NN. No difference in HRV parameters was found between NH and HH at any time. The natural logarithm of root mean square of the successive differences (LnRMSSD) and the high frequency spectral power (HF), which reflect parasympathetic activity, decreased similarly in NH and HH when compared to NN.

Conclusion: Despite S_pO₂ differences, changes in supine HRV parameters during 24-h exposure were similar between NH and HH conditions indicating a similar decrease in parasympathetic activity. Therefore, HRV can be analyzed similarly in NH and HH conditions.

Monitoring stress and allostatic load in first responders and tactical operators using heart rate variability: a systematic review

BACKGROUND

Awareness of the cumulative stress placed on first responders and tactical operators is required to manage acute fatigue, which can impair occupational performance, and may precipitate negative chronic health outcomes. The aim of this review was to investigate the utility of heart rate variability (HRV) to monitor stress and allostatic load among these populations.

METHODS

A systematic search of Academic Search Complete, MEDLINE complete, PsycINFO, SPORTDiscus and Scopus databases was conducted. Eligibility criteria: original peer reviewed research articles, written in English, published between 1985 and 2020, using human participants employed as a first responder or tactical operator, free from any psychological disorder.

RESULTS

Of the 360 articles screened, 60 met the inclusion criteria and were included for full text assessment. Articles were classified based on single or repeated stressor exposure and the time of HRV assessment (baseline, during stressor, post stressor). Singular stressful events elicited a reduction in HRV from baseline to during the event. Stressors of greater magnitude reduced HRV for extended durations post stressor. Lower resting HRV was associated with lower situational awareness and impaired decision-making performance in marksmanship and navigation tasks. There were insufficient studies to evaluate the utility of HRV to assess allostatic load in repeated stressor contexts.

CONCLUSION

A reduction in HRV occurred in response to acute physical and cognitive occupational stressors. A slower rate of recovery of HRV after the completion of acute occupational stressors appears to occur in response to stressors of greater magnitude. The association between lower HRV and lower decision-making performance poses as a useful tool but further investigations on within subject changes between these factors and their relationship is required. More research is required to investigate the suitability of HRV as a measure of allostatic load in repeated stress exposures for fatigue management in first responder and tactical operators.

Global REACH 2018: The carotid artery diameter response to the cold pressor test is governed by arterial blood pressure during normoxic but not hypoxic conditions in healthy lowlanders and Andean highlanders

NEW FINDINGS

What is the central question of this study? What is the impact of oxygen on the circulatory responses to an isocapnic cold pressor test (CPT) in lowlanders and Andean highlanders? What is the main finding and its importance? Overall, the circulatory responses to an isocapnic CPT were largely unaltered with acute normobaric hypoxia and chronic hypobaric hypoxia exposure in lowlanders. However, the relationship between mean arterial pressure and common carotid artery diameter was dampened in hypoxic conditions. Furthermore, there were no differences in the circulatory responses to the CPT between lowlanders and Andean highlanders with lifelong exposure to high altitude.

ABSTRACT

The impact of oxygen on the circulatory responses to a cold pressor test (CPT) in lowlanders and Andean highlanders remains unknown. Our hypotheses were as follows: (i) in lowlanders, acute normobaric and hypobaric hypoxia would attenuate the common carotid artery (CCA) diameter response to the CPT compared with normobaric normoxia; (ii) Andean highlanders would exhibit a greater CCA diameter response compared with lowlanders; and (iii) a positive relationship between CCA diameter and blood pressure in response to the CPT would be present in both lowlanders and highlanders. Healthy lowlanders (n = 13) and Andean highlanders (n = 8) were recruited and conducted an isocapnic CPT, which consisted of a 3 min foot immersion into water at 0-1°C. Blood pressure (finger photoplethysmography) and CCA diameter and blood flow (Duplex ultrasound) were recorded continuously. The CPT was conducted in lowlanders at sea level in isocapnic normoxic and hypoxic conditions and after 10 days of acclimatization to 4300 m (Cerro de Pasco, Peru) in hypoxic and hyperoxic conditions. Andean highlanders were tested at rest at high altitude. The main findings were as follows: (i) in lowlanders, normobaric but not hypobaric hypoxia elevated CCA reactivity to the CPT; (ii) no differences in response to the CPT were observed between lowlanders and highlanders; and (iii) although hypobaric hypoxaemia reduced the relationship between CCA diameter and blood pressure compared with normobaric normoxia (P = 0.132), hypobaric hyperoxia improved this relationship (P = 0.012), and no relationship was observed in Andean highlanders (P = 0.261). These data demonstrate that the circulatory responses to a CPT were modified by oxygen in lowlanders, but were unaltered with lifelong hypoxic exposure.

Minimal Influence of Hypobaria on Heart Rate Variability in Hypoxia and Normoxia

Introduction

The present study evaluated the putative effect of hypobaria on resting HRV in normoxia and hypoxia.

Methods

Fifteen young pilot trainees were exposed to five different conditions in a randomized order: normobaric normoxia (NN, P_B = 726 ± 5 mmHg, F_IO₂ = 20.9%), hypobaric normoxia (HN, P_B = 380 ± 6 mmHg, F_IO₂≅40%), normobaric hypoxia (NH, P_B = 725 ± 4 mmHg, F_IO₂≅11%); and hypobaric hypoxia (HH at 3000 and 5500 m, HH3000 and HH5500, P_B = 525 ± 6 and 380 ± 8 mmHg, respectively, F_IO₂ = 20.9%). HRV and pulse arterial oxygen saturation (SpO₂) were measured at rest seated during a 6 min period in each condition. HRV parameters were analyzed (Kubios HVR Standard, V 3.0) for time (RMSSD) and frequency (LF, HF, LF/HF ratio, and total power). Gas exchanges were collected at rest for 10 min following HRV recording.

Results

SpO₂ decreased in HH3000 (95 ± 3) and HH5500 (81 ± 5), when compared to NN (99 ± 0). SpO₂ was higher in NH (86 ± 4) than HH5500 but similar between HN (98 ± 2) and NN. Participants showed lower RMSSD and total power values in NH and HH5500 when compared to NN. In hypoxia, LF/HF ratio was greater in HH5500 than NH, whereas in normoxia, LF/HF ratio was lower in HN than NN. Minute ventilation was higher in HH5500 than in all other conditions.

Discussion

The present study reports a slight hypobaric effect either in normoxia or in hypoxia on some HRV parameters. In hypoxia, with a more prominent sympathetic activation, the hypobaric effect is likely due to the greater ventilation stimulus and larger desaturation. In normoxia, the HRV differences may come from the hyperoxic breathing and slight breathing pattern change due to hypobaria in HN.

Effects of a Single Power Strength Training Session on Heart Rate Variability When Performed at Different Simulated Altitudes

Álvarez-Herms, Jesús, Sonia Julià-Sánchez, Hannes Gatterer, Francisco Corbi, Gines Viscor, and Martin Burtscher. Effects of a single power strength training session on heart rate variability when performed at different simulated altitudes. High Alt Med Biol. 21:292-296, 2020. Background: This study assessed heart rate variability (HRV) after a single power strength training session performed at different hypoxic levels. Materials and Methods: Eight physically active subjects (31.1 ± 4.3 years; 177.6 ± 3.0 cm; 70.1 ± 5.2 kg) performed 6 bouts of 15-second continuous maximal jump exercises interspersed by 3 minutes of rest at different altitude levels (total volume of each session: 20 minutes). The normoxic hypoxia levels were FiO₂ low altitude: 20.9%; moderate altitude: 16.5%; and high altitude: 13.5%. Results: Average power output during the jumps was similar for all conditions (≅3150 W). Twenty-four hours before (PRE) and 24 hours after (POST) each training session, HRV parameters (R-R, square root of the mean of the sum of differences between intervals [RMSSD], pNN50, and very low frequency, low frequency, and high frequency) were determined without resulting in significant statistical differences, neither from PRE to POST nor between conditions (p > 0.05). Conclusions: This study showed a negligible perturbation of HRV parameters 24 hours after a single power strength session up to a hypoxic level equivalent to 4000 m. Further studies are needed to determine the hypoxia-dependent threshold and intensities of training loads affecting HRV.

Acute mountain sickness: Do different time courses point to different pathophysiological mechanisms?

Acute mountain sickness (AMS) is a syndrome of nonspecific symptoms (i.e., headache, anorexia, nausea, vomiting, dizziness, and fatigue) that may develop in nonacclimatized individuals after rapid exposure to altitudes ≥2,500 m. In field studies, mean AMS scores usually peak after the first night at a new altitude. Analyses of the individual time courses of AMS in four studies performed at 3,450 m and 4,559 m revealed that three different patterns are hidden in the above-described overall picture. In 41% of those who developed AMS (i.e., AMS-C score >0.70), symptoms peaked on day 1, in 39%, symptoms were most prominent on day 2, and in 20%, symptoms were most prominent on day 3. We suggest to name the different time courses of AMS type I, type II, and type III, respectively. Here, we hypothesize that the variation of time courses of AMS are caused by different pathophysiological mechanisms. This assumption could explain why no consistent correlations between an overall assessment of AMS and single pathophysiological factors have been found in a large number of studies over the past 50 yr. In this paper, we will briefly review the fundamental mechanisms implicated in the pathophysiology of AMS and discuss how they might contribute to the three different AMS time courses.

Blood pressure response to exposure to moderate altitude in patients with COPD

Purpose

Patients with COPD might be particularly susceptible to hypoxia-induced autonomic dysregulation. Decreased baroreflex sensitivity (BRS) and increased blood pressure (BP) variability (BPV) are markers of impaired cardiovascular autonomic regulation and there is evidence for an association between decreased BRS/increased BPV and high cardiovascular risk. The aim of this study was to evaluate the effect of short-term exposure to moderate altitude on BP and measures of cardiovascular autonomic regulation in COPD patients.

Materials and methods

Continuous morning beat-to-beat BP was noninvasively measured with a Finometer^® device for 10 minutes at low altitude (490 m, Zurich, Switzerland) and for 2 days at moderate altitude (2,590 m, Davos Jakobshorn, Switzerland) – the order of altitude exposure was randomized. Outcomes of interest were mean SBP and DBP, BPV expressed as the coefficient of variation (CV), and spontaneous BRS. Changes between low altitude and day 1 and day 2 at moderate altitude were assessed by ANOVA for repeated measurements with Fisher’s exact test analysis.

Results

Thirty-seven patients with moderate to severe COPD (mean±SD age 64±6 years, FEV₁ 60%±17%) were included. Morning SBP increased by +10.8 mmHg (95% CI: 4.7–17.0, P=0.001) and morning DBP by +5.0 mmHg (95% CI: 0.8–9.3, P=0.02) in response to altitude exposure. BRS significantly decreased (P=0.03), whereas BPV significantly and progressively increased (P<0.001) upon exposure to altitude.

Conclusion

Exposure of COPD patients to moderate altitude is associated with a clinically relevant increase in BP, which seems to be related to autonomic dysregulation.

Clinical trial registration

ClinicalTrials.gov (NCT01875133).

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.

Effects of Different Training Intensity Distributions Between Elite Cross-Country Skiers and Nordic-Combined Athletes During Live High-Train Low

Purpose: To analyze the effects of different training strategies (i.e., mainly intensity distribution) during living high – training low (LHTL) between elite cross-country skiers and Nordic-combined athletes.

Methods: 12 cross-country skiers (XC) (7 men, 5 women), and 8 male Nordic combined (NC) of the French national teams were monitored during 15 days of LHTL. The distribution of training at low-intensity (LIT), below the first ventilatory threshold (VT1), was 80% and 55% in XC and NC respectively. Daily, they filled a questionnaire of fatigue, and performed a heart rate variability (HRV) test. Prior (Pre) and immediately after (Post), athletes performed a treadmill incremental running test for determination of V˙O_2max and V˙O₂ at the second ventilatory threshold (V˙O_{2V T2}), a field roller-skiing test with blood lactate ([La-]) assessment.

Results: The training volume was in XC and NC, respectively: at LIT: 45.9 ± 6.4 vs. 23.9 ± 2.8 h (p < 0.001), at moderate intensity: 1.9 ± 0.5 vs. 3.0 ± 0.4 h, (p < 0.001), at high intensity: 1.2 ± 0.9 vs. 1.4 ± 02 h (p = 0.05), in strength (and jump in NC): 7.1 ± 1.5 vs. 18.4 ± 2.7 h, (p < 0.001). Field roller-skiing performance was improved (-2.9 ± 1.6%, p < 0.001) in XC but decreased (4.1 ± 2.6%, p < 0.01) in NC. [La-] was unchanged (-4.1 ± 14.2%, p = 0.3) in XC but decreased (-27.0 ± 11.1%, p < 0.001) in NC. Changes in field roller-skiing performance and in [La-] were correlated (r = -0.77, p < 0.001). V˙O_2max increased in both XC and NC (3.7 ± 4.2%, p = 0.01 vs. 3.7 ± 2.2%, p = 0.002) but V˙O_{2V T2} increased only in XC (7.3 ± 5.8%, p = 0.002). HRV analysis showed differences between XC and NC mainly in high spectral frequency in the supine position (HF_SU). All NC skiers showed some signs of overreaching at Post.

Conclusion: During LHTL, despite a higher training volume, XC improved specific performance and aerobic capacities, while NC did not. All NC skiers showed fatigue states. These findings suggest that a large amount of LIT with a moderate volume of strength and speed training is required during LHTL in endurance athletes.

Smartphone-Enabled Heart Rate Variability and Acute Mountain Sickness

INTRODUCTION

The autonomic system and sympathetic activation appears integral in the pathogenesis of acute mountain sickness (AMS) at high altitude (HA), yet a link between heart rate variability (HRV) and AMS has not been convincingly shown. In this study we investigated the utility of the smartphone-derived HRV score to predict and diagnose AMS at HA.

METHODS

Twenty-one healthy adults were investigated at baseline at 1400 m and over 10 days during a trek to 5140 m. HRV was recorded using the ithlete HRV device.

RESULTS

Acute mountain sickness occurred in 11 subjects (52.4%) at >2650 m. HRV inversely correlated with AMS Scores (r = -0.26; 95% CI, -0.38 to -0.13: P < 0.001). HRV significantly fell at 3700, 4100, and 5140 m versus low altitude. HRV scores were lower in those with both mild (69.7 ± 14.0) and severe AMS (67.1 ± 13.1) versus those without AMS (77.5 ± 13.1; effect size n = 0.043: P = 0.007). The HRV score was weakly predictive of severe AMS (AUC 0.74; 95% CI, 0.58-0.89: P = 0.006). The change (delta) in the HRV Score (compared with baseline at 1400 m) was a moderate diagnostic marker of severe AMS (AUC 0.80; 95% CI, 0.70-0.90; P = 0.0004). A fall in the HRV score of >5 had a sensitivity of 83% and specificity of 60% to identify severe AMS (likelihood ratio 1.9). Baseline HRV at 1400 m was not predictive of either AMS at higher altitudes.

CONCLUSIONS

The ithlete HRV score can be used to help in the identification of severe AMS; however, a baseline score is not predictive of future AMS development at HA.

Effects of rapid ascent on the heart rate variability of individuals with and without acute mountain sickness

PURPOSE

Through time- and frequency-domain analysis, we compared the effects of acute hypobaric hypoxia on the changes in heart rate variability (HRV) following night sleeping and morning awakening in individuals with and without acute mountain sickness (AMS).

METHOD

Thirty-nine nonacclimatised healthy individuals were transported by bus from sea level to 3150 m within 3 h. Short-term HRV was measured two times a day-before sleeping (BS) and after awakening (AA)- at 3 days before ascent (T0), two consecutive nights at 3150 m (T1 and T2), and 2 days after descent (T3). AMS was diagnosed using the self-reported Lake Louise score questionnaire.

RESULT

AMS developed in 19 of 39 participants (48.7%). At sea level, individuals had higher HRV at AA than at BS, and the trend of increased HRV at AA remained unchanged at high altitude, irrespective of AMS. At T1 BS, low-frequency power in normalised unit was significantly lower in participants with AMS than in those without AMS. Compared with those at T1 BS, the square root of the mean squared differences of successive normal-normal (NN) intervals, the number of interval differences of successive NN intervals more than 50 ms (NN50), and the proportion derived by dividing NN50 by the total number of NN intervals at T1 AA significantly increased in participants without AMS but nonsignificantly decreased in those with AMS.

CONCLUSION

After rapid ascent, individuals with AMS did not demonstrate sympathetic hyperactivity but did exhibit withdrawal of cardiac vagal modulation in the morning following the first night's sleep.

A comparison of two methods of heart rate variability assessment at high altitude

Heart rate variability (HRV) is a useful index of autonomic function and has been linked to the development of high altitude (HA) related illness. However, its assessment at HA has been undermined by the relative expense and limited portability of traditional HRV devices which have mandated at least a minute heart rate recording. In this study, the portable ithlete^™ HRV system, which uses a 55 s recording, was compared with a reference method of HRV which utilizes a 5 min electrocardiograph recording (CheckMyHeart^™ ). The root mean squares of successive R-R intervals (RMSSD) for each device was converted to a validated HRV score (lnRMSSD × 20) for comparison. Twelve healthy volunteers were assessed for HRV using the two devices across seven time points at HA over 10 days. There was no significant change in the HRV values with either the ithlete (P = 0·3) or the CheckMyHeart^™ (P = 0·19) device over the seven altitudes. There was also a strong overall correlation between the ithlete^™ and CheckMyHeart^™ device (r = 0·86; 95% confidence interval: 0·79-0·91). The HRV was consistently, though non-significantly higher with ithlete^™ than with the CheckMyHeart^™ device [mean difference (bias) 1·8 l; 95% CI -12·3 to 8·5]. In summary, the ithlete^™ and CheckMyHeart^™ system provide relatively similar results with good overall agreement at HA.

Heart rate variability during sleep at high altitude: effect of periodic breathing

BACKGROUND

Heart rate variability (HRV) during sleep in normal subjects at high altitude shows a decrease in parasympathetic tone associated with an increase in the sympathetic one, which tends to be reversed with acclimatization. However, periodic breathing (PB) during sleep may influence this effect detected by HRV spectral analysis.

PURPOSE

The aim of our study was to investigate HRV during sleep periodic breathing (PB) at high altitude in normal subjects at two different times of acclimatization, i.e., two different levels of hypoxemia.

METHODS

Recordings of six healthy climbers (aged between 33 and 40 years), at sea level (SL) and at Everest North Base Camp (5180 m), during the first (BC1) and the tenth (BC2) overnight unattended polygraphy, were analyzed. PB was commonplace in all subjects at high altitude to a variable extent. At SL and at BC1 and BC2, HRV was evaluated overnight and separately during clear regular breathing (RB) and PB.

RESULTS

A mean overnight beat-by-beat series interval (RR) reduction at acute environmental hypoxic exposure that resumed to SL values after 10-day sojourn was observed. This reduction was mostly due to RR during RB, while during PB, RR values were not different from SL. Higher peaks of tidal volume were associated with higher HRV.

CONCLUSIONS

The present study shows that in healthy subjects, PB with central apneas increases the amplitude of RR oscillations, and these oscillations are tightly related to respiratory amplitude. Oxygenation does not influence this phenomenon. Therefore, oscillations in ventilation itself should be taken into account when investigating HRV.

Cold pressor response in high landers versus low landers

BACKGROUND

Native high landers face two main environmental challenges i.e. hypobaric hypoxia and low ambient temperatures. Both factors contribute to increased sympathetic stimulation and increased blood pressure. Despite these challenges, subjects living at high altitude have lower systolic and diastolic pressures as compared to subjects living in plains. Present study investigated cold pressor test (CPT) which is a potential predictor of future hypertension in high landers and low landers Materials and Methods: Vascular reactivity in terms of changes in systolic and diastolic blood pressure and heart rate in response to cold pressor test has been compared in high lander (n=45) and low lander (n=46) population.

RESULTS

Systolic and diastolic blood pressure changes and heart rate changes with cold pressor test are lower in high landers as compared to low landers. Females in both the groups in general exhibited greater cold pressor response than males.

CONCLUSION

Hypo-reactive cold pressor test is due to higher parasympathetic tone and lower sympathetic tone. Decreased cold pressor response in high landers reflects another adaptive modulation of sympatho-vagal activity that enables them to stay in hypobaric atmosphere and lower temperatures without undue autonomic stress.

Effects of acute hypoxia on heart rate variability, sample entropy and cardiorespiratory phase synchronization

BACKGROUND

Investigating the responses of autonomic nervous system (ANS) in hypoxia may provide some knowledge about the mechanism of neural control and rhythmic adjustment. The integrated cardiac and respiratory system display complicated dynamics that are affected by intrinsic feedback mechanisms controlling their interaction. To probe how the cardiac and respiratory system adjust their rhythms in different simulated altitudes, we studied heart rate variability (HRV) in frequency domain, the complexity of heartbeat series and cardiorespiratory phase synchronization (CRPS) between heartbeat intervals and respiratory cycles.

METHODS

In this study, twelve male subjects were exposed to simulated altitude of sea level, 3000 m and 4000 m in a hypobaric chamber. HRV was assessed by power spectral analysis. The complexity of heartbeat series was quantified by sample entropy (SampEn). CRPS was determined by cardiorespiratory synchrogram.

RESULTS

The power spectral HRV indices at all frequency bands depressed according to the increase of altitude. The SampEn of heartbeat series increased significantly with the altitude (P < 0.01). The duration of CRPS epochs at 3000 m was not significantly different from that at sea level. However, it was significantly longer at 4000 m (P < 0.01).

CONCLUSIONS

Our results suggest the phenomenon of CRPS exists in normal subjects when they expose to acute hypoxia. Further, the autonomic regulation has a significantly stronger influence on CRPS in acute hypoxia. The changes of CRPS and HRV parameters revealed the different regulatory mechanisms of the cardiac and respiratory system at high altitude.

Autonomic cardiovascular responses in acclimatized lowlanders on prolonged stay at high altitude: a longitudinal follow up study

Acute exposure to hypobaric hypoxia at high altitude is reported to cause sympathetic dominance that may contribute to the pathophysiology of high altitude illnesses. The effect of prolonged stay at high altitude on autonomic functions, however, remains to be explored. Thus, the present study aimed at investigating the effect of high altitude on autonomic neural control of cardiovascular responses by monitoring heart rate variability (HRV) during chronic hypobaric hypoxia. Baseline electrocardiography (ECG) data was acquired from the volunteers at mean sea level (MSL) (<250 m) in Rajasthan. Following induction of the study population to high altitude (4500–4800 m) in Ladakh region, ECG data was acquired from the volunteers after 6 months (ALL 6) and 18 months of induction (ALL 18). Out of 159 volunteers who underwent complete investigation during acquisition of baseline data, we have only included the data of 104 volunteers who constantly stayed at high altitude for 18 months to complete the final follow up after 18 months. HRV parameters, physiological indices and biochemical changes in serum were investigated. Our results show sympathetic hyperactivation along with compromise in parasympathetic activity in ALL 6 and ALL 18 when compared to baseline data. Reduction of sympathetic activity and increased parasympathetic response was however observed in ALL 18 when compared to ALL 6. Our findings suggest that autonomic response is regulated by two distinct mechanisms in the ALL 6 and ALL 18. While the autonomic alterations in the ALL 6 group could be attributed to increased sympathetic activity resulting from increased plasma catecholamine concentration, the sympathetic activity in ALL 18 group is associated with increased concentration of serum coronary risk factors and elevated homocysteine. These findings have important clinical implications in assessment of susceptibility to cardio-vascular risks in acclimatized lowlanders staying for prolonged duration at high altitude.

Effects of high altitude on sleep and respiratory system and theirs adaptations

High-altitude (HA) environments have adverse effects on the normal functioning body of people accustomed to living at low altitudes because of the change in barometric pressure which causes decrease in the amount of oxygen leading to hypobaric hypoxia. Sustained exposure to hypoxia has adverse effects on body weight, muscle structure and exercise capacity, mental functioning, and sleep quality. The most important step of acclimatization is the hyperventilation which is achieved by hypoxic ventilatory response of the peripheral chemoreceptors. Hyperventilation results in increase in arterial carbon-dioxide concentration. Altitude also affects sleep and cardiac output, which is the other determinant of oxygen delivery. Upon initial exposure to HA, the resting pulse rate increases rapidly, but with acclimatization, heart rate and cardiac output tend to fall. Another important component that leads to decrease in cardiac output is the reduction in the stroke volume with acclimatization. During sleep at HA, the levels of CO2 in the blood can drop very low and this can switch off the drive to breathe. Only after the body senses a further drop in O2 levels breathing is started again. Periodic breathing is thought to result from instability in the control system through the hypoxic drive or the response to CO2.

Heart rate variability changes at 2400 m altitude predicts acute mountain sickness on further ascent at 3000-4300 m altitudes

OBJECTIVE

If the body fails to acclimatize at high altitude, acute mountain sickness (AMS) may result. For the early detection of AMS, changes in cardiac autonomic function measured by heart rate variability (HRV) may be more sensitive than clinical symptoms alone. The purpose of this study was to ascertain if the changes in HRV during ascent are related to AMS.

METHODS

We followed Lake Louise Score (LLS), arterial oxygen saturation at rest (R-SpO(2)) and exercise (Ex-SpO(2)) and HRV parameters daily in 36 different healthy climbers ascending from 2400 m to 6300 m altitudes during five different expeditions.

RESULTS

After an ascent to 2400 m, root mean square successive differences, high-frequency power (HF(2 min)) of HRV were 17-51% and Ex-SpO(2) was 3% lower in those climbers who suffered from AMS at 3000 to 4300 m than in those only developing AMS later (≥5000 m) or not at all (all p < 0.01). At the altitude of 2400 m RMSSD(2 min) ≤ 30 ms and Ex-SpO(2) ≤ 91% both had 92% sensitivity for AMS if ascent continued without extra acclimatization days.

CONCLUSIONS

Changes in supine HRV parameters at 2400 m were related to AMS at 3000-4300 m Thus, analyses of HRV could offer potential markers for identifying the climbers at risk for AMS.

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.

Sentra PM (a Medical Food) and Trazodone in the Management of Sleep Disorders

Sleep disorders are a common and poorly treated disease state. This double blind, four arm placebo-controlled, randomized trial compared (1) low dose trazodone, (2) Sentra PM, a neurotransmitter based medical food, (3) the joint administration of trazodone and the medical food Sentra PM and (4) placebo. There were 111 subjects studied in 12 independent sites. Subjects underwent baseline screening, informed consent and an initial sleep questionnaire. After 14 days subjects underwent a second evaluation by questionnaire. At baseline and Day 14 the subjects underwent 24 hour ECG recordings that were analyzed in the frequency domain of heart rate variability. The specific high frequency parasympathetic autonomic nervous system activity was analyzed. The primary endpoints were sleep latency and parasympathetic autonomic nervous system improvement in sleeping hours. The results showed improvement in sleep latency for the Sentra PM and combination of Sentra PM and trazodone (−41 and −56 minutes P < 0.001). There was an improvement in quality of sleep for the amino acid formulation Sentra PM and the combination (3.86 and 6.48 Likert units on a 10 point scale P < 0.001). There was an activation of circadian activity percent at night in the medical food and combination groups while there was no change in parasympathetic activity in either the placebo or trazodone group. These data indicate that Sentra PM can improve the quality of sleep, the response to trazodone as a sleep medication and parasympathetic autonomic nervous system activity.

Reduced baroreflex sensitivity and pulmonary dysfunction in alcoholic cirrhosis: effect of hyperoxia

Patients with cirrhosis exhibit impaired regulation of the arterial blood pressure, reduced baroreflex sensitivity (BRS), and prolonged QT interval. In addition, a considerable number of patients have a pulmonary dysfunction with hypoxemia, impaired lung diffusing capacity (Dl(CO)), and presence of hepatopulmonary syndrome (HPS). BRS is reduced at exposure to chronic hypoxia such as during sojourn in high altitudes. In this study, we assessed the relation of BRS to pulmonary dysfunction and cardiovascular characteristics and the effects of hyperoxia. Forty-three patients with cirrhosis and 12 healthy matched controls underwent hemodynamic and pulmonary investigations. BRS was assessed by cross-spectral analysis of variabilities between blood pressure and heart rate time series. A 100% oxygen test was performed with the assessment of arterial oxygen tensions (Pa(O(2))) and alveolar-arterial oxygen gradient. Baseline BRS was significantly reduced in the cirrhotic patients compared with the controls (4.7 +/- 0.8 vs. 10.3 +/- 2.0 ms/mmHg; P < 0.001). The frequency-corrected QT interval was significantly prolonged in the cirrhotic patients (P < 0.05). There was no significant difference in BRS according to presence of HPS, Pa(O(2)), Dl(CO), or Child-Turcotte score, but BRS correlated with metabolic and hemodynamic characteristics. After 100% oxygen inhalation, BRS and the QT interval remained unchanged in the cirrhotic patients. In conclusion, BRS is significantly reduced in patients with cirrhosis compared with controls, but it is unrelated to the degree of pulmonary dysfunction and portal hypertension. Acute hyperoxia does not significantly revert the low BRS or the prolonged QT interval in cirrhosis.

Combining hypoxic methods for peak performance

New methods and devices for pursuing performance enhancement through altitude training were developed in Scandinavia and the USA in the early 1990s. At present, several forms of hypoxic training and/or altitude exposure exist: traditional 'live high-train high' (LHTH), contemporary 'live high-train low' (LHTL), intermittent hypoxic exposure during rest (IHE) and intermittent hypoxic exposure during continuous session (IHT). Although substantial differences exist between these methods of hypoxic training and/or exposure, all have the same goal: to induce an improvement in athletic performance at sea level. They are also used for preparation for competition at altitude and/or for the acclimatization of mountaineers. The underlying mechanisms behind the effects of hypoxic training are widely debated. Although the popular view is that altitude training may lead to an increase in haematological capacity, this may not be the main, or the only, factor involved in the improvement of performance. Other central (such as ventilatory, haemodynamic or neural adaptation) or peripheral (such as muscle buffering capacity or economy) factors play an important role. LHTL was shown to be an efficient method. The optimal altitude for living high has been defined as being 2200-2500 m to provide an optimal erythropoietic effect and up to 3100 m for non-haematological parameters. The optimal duration at altitude appears to be 4 weeks for inducing accelerated erythropoiesis whereas <3 weeks (i.e. 18 days) are long enough for beneficial changes in economy, muscle buffering capacity, the hypoxic ventilatory response or Na(+)/K(+)-ATPase activity. One critical point is the daily dose of altitude. A natural altitude of 2500 m for 20-22 h/day (in fact, travelling down to the valley only for training) appears sufficient to increase erythropoiesis and improve sea-level performance. 'Longer is better' as regards haematological changes since additional benefits have been shown as hypoxic exposure increases beyond 16 h/day. The minimum daily dose for stimulating erythropoiesis seems to be 12 h/day. For non-haematological changes, the implementation of a much shorter duration of exposure seems possible. Athletes could take advantage of IHT, which seems more beneficial than IHE in performance enhancement. The intensity of hypoxic exercise might play a role on adaptations at the molecular level in skeletal muscle tissue. There is clear evidence that intense exercise at high altitude stimulates to a greater extent muscle adaptations for both aerobic and anaerobic exercises and limits the decrease in power. So although IHT induces no increase in VO(2max) due to the low 'altitude dose', improvement in athletic performance is likely to happen with high-intensity exercise (i.e. above the ventilatory threshold) due to an increase in mitochondrial efficiency and pH/lactate regulation. We propose a new combination of hypoxic method (which we suggest naming Living High-Training Low and High, interspersed; LHTLHi) combining LHTL (five nights at 3000 m and two nights at sea level) with training at sea level except for a few (2.3 per week) IHT sessions of supra-threshold training. This review also provides a rationale on how to combine the different hypoxic methods and suggests advances in both their implementation and their periodization during the yearly training programme of athletes competing in endurance, glycolytic or intermittent sports.

Alternations of heart rate variability at lower altitude in the predication of trekkers with acute mountain sickness at high altitude

OBJECTIVE

To determine the change and relationship of spectral components of heart rate variability (HRV) measurements in subjects with or without acute mountain sickness (AMS) at both low and high altitude.

DESIGN

A prospective study.

SETTING

A 12-day itinerary by trekking to the Namche Bazaar, 3440 m in Nepal.

PARTICIPANTS

A total of 32 subjects were recruited.

INTERVENTIONS

The alternations were measured by heart rate (HR), arterial oxygen saturation (SpO(2)), and spectral analysis of HRV at sea level, 1317 m, 3440 m, 1317 m, and sea level, respectively.

MAIN OUTCOME MEASURES

Spectral analysis of HRV.

RESULTS

There were statistically significant increases in HR and decreases in SpO(2) in all subjects at high altitude. In HRV, the values of R-R interval, total variance, high frequency (HF), low frequency (LF), and HF% were significantly lower at 3440 m than at sea level, respectively (P < 0.05). The subjects with AMS had significantly lower total variance, HF, and HF%, respectively, but higher LF:HF ratio (P < 0.05) at 3440 m. Subjects with both HF% < 20% (nu) and LF:HF ratio > 1.3 measured at 1317 m had odds ratios of 7.00 (95% confidence interval, 1.11 to 44.06; P = 0.047) to get AMS at 3440 m.

CONCLUSIONS

The HRV measurements in total variances, HF, and HF% in trekkers with AMS were statistically significantly lower at high altitude. HF% < 20% (nu) or LF:HF ratio > 1.3 at lower altitudes could be an important predication parameter of trekkers with AMS at higher altitudes.

Effect of hypobaric hypoxia on heart rate variability during exercise: a pilot field study

The influence of hypoxia on heart rate variability (HRV) has been studied under resting conditions with mixed results. Differences have been found in physiological responses to normobaric versus hypobaric hypoxia. Our aim was to study the influence of hypobaric hypoxia on HRV during physical exercise to determine whether HRV changes due to the exercise-induced heart rate (HR) increase or whether hypoxia itself exerts an influence. We tested nine healthy non-acclimatised white males (age = 43 +/- 7 years) at 400 and 4,200 m during exercises. At 400 m HRV was measured at 50% and 75% maximal oxygen uptake (VO(2) max). At 4,200 m HR was kept equal as during exercise at 400 m by adjusting the intensity of step testing. The Poincaré plot as a non-linear method of HRV analysis was used, where the shape of the ellipse depending on HRV is expressed by two parameters, SD1 and SD2 (correlating to parasympathetic activity and both sympathetic and parasympathetic activity, respectively). We established a decrease in SD2 and an insignificant decrease in SD1 at medium HR at 4,200 m compared to 400 m. Both parameters showed similar tendencies during high-intensity exercise. Our results indicate that hypobaric hypoxia itself exerts an influence on HRV at a moderate HR.

Impact of intermittent hypoxia on long-term facilitation of minute ventilation and heart rate variability in men and women: do sex differences exist?

Following exposure to intermittent hypoxia, respiratory motor activity and sympathetic nervous system activity may persist above baseline levels for over an hour. The present investigation was designed to determine whether sustained increases in minute ventilation and sympathovagal (S/V) balance, in addition to sustained depression of parasympathetic nervous system activity (PNSA), were greater in men compared with women following exposure to intermittent hypoxia. Fifteen healthy men and women matched for age, race, and body mass index were exposed to eight 4-min episodes of hypoxia during sustained hypercapnia followed by a 15-min end-recovery period. The magnitude of the increase in minute ventilation during the end-recovery period, compared with baseline, was similar in men and women (men, 1.52 +/- 0.03; women, 1.57 +/- 0.02 fraction of baseline; P < 0.0001). In contrast, depression of PNSA and increases in S/V balance were evident during the end-recovery period, compared with baseline, in men (PNSA, 0.66 +/- 0.06 fraction of baseline, P < 0.0001; S/V balance, 2.8 +/- 0.7 fraction of baseline, P < 0.03) but not in women (PNSA, 1.27 +/- 0.19 fraction of baseline, P = 0.3; S/V balance, 1.8 +/- 0.6 fraction of baseline, P = 0.2). We conclude that a sustained increase in minute ventilation, which is indicative of long-term facilitation, is evident in both men and women following exposure to intermittent hypoxia and that this response is independent of sex. In contrast, sustained alterations in autonomic nervous system activity were evident in men but not in women.

During hypoxic exercise some vasoconstriction is needed to match O2 delivery with O2 demand at the microcirculatory level

To test the hypothesis that the increased sympathetic tonus elicited by chronic hypoxia is needed to match O(2) delivery with O(2) demand at the microvascular level eight male subjects were investigated at 4559 m altitude during maximal exercise with and without infusion of ATP (80 mug (kg body mass)(-1) min(-1)) into the right femoral artery. Compared to sea level peak leg vascular conductance was reduced by 39% at altitude. However, the infusion of ATP at altitude did not alter femoral vein blood flow (7.6 +/- 1.0 versus 7.9 +/- 1.0 l min(-1)) and femoral arterial oxygen delivery (1.2 +/- 0.2 versus 1.3 +/- 0.2 l min(-1); control and ATP, respectively). Despite the fact that with ATP mean arterial blood pressure decreased (106.9 +/- 14.2 versus 83.3 +/- 16.0 mmHg, P < 0.05), peak cardiac output remained unchanged. Arterial oxygen extraction fraction was reduced from 85.9 +/- 5.3 to 72.0 +/- 10.2% (P < 0.05), and the corresponding venous O(2) content was increased from 25.5 +/- 10.0 to 46.3 +/- 18.5 ml l(-1) (control and ATP, respectively, P < 0.05). With ATP, leg arterial-venous O(2) difference was decreased (P < 0.05) from 139.3 +/- 9.0 to 116.9 +/- 8.4(-1) and leg .VO(2max) was 20% lower compared to the control trial (1.1 +/- 0.2 versus 0.9 +/- 0.1 l min(-1)) (P = 0.069). In summary, at altitude, some degree of vasoconstriction is needed to match O(2) delivery with O(2) demand. Peak cardiac output at altitude is not limited by excessive mean arterial pressure. Exercising leg .VO(2peak) is not limited by restricted vasodilatation in the altitude-acclimatized human.

Oxygen dependence of tyrosine hydroxylase

The effects of dioxygen on tyrosine hydroxylase (TH) activity was studied, measuring the formation of DOPA from tyrosine, (3)H(2)O from 3,5-(3)H-tyrosine, or by direct oxygraphic determination of oxygen consumption. A high enzyme activity was observed during the initial 1-2 min of the reactions, followed by a decline in activity, possibly related to a turnover dependent substoichiometrical oxidation of enzyme bound Fe(II) to the inactive Fe(III) state. During the initial reaction phase, apparent K (m)-values of 29-45 microM for dioxygen were determined for all human TH isoforms, i.e. 2-40 times higher than previously reported for TH isolated from animal tissues. After 8 min incubation, the K (m) (O(2))-values had declined to an average of 20 +/- 4 microM. Thus, TH activity may be severely limited by oxygen availability even at moderate hypoxic conditions, and the enzyme is rapidly and turnover dependent inactivated at the experimental conditions commonly employed to measure in vitro activities.

Cardiovascular adjustments for life at high altitude

The effects of hypobaric hypoxia in visitors depend not only on the actual elevation but also on the rate of ascent. There are increases in sympathetic activity resulting in increases in systemic vascular resistance, blood pressure and heart rate. Pulmonary vasoconstriction leads to pulmonary hypertension, particularly during exercise. The sympathetic excitation results from hypoxia, partly through chemoreceptor reflexes and partly through altered baroreceptor function. Systemic vasoconstriction may also occur as a reflex response to the high pulmonary arterial pressures. Many communities live permanently at high altitude and most dwellers show excellent adaptation although there are differences between populations in the extent of the ventilatory drive and the erythropoiesis. Despite living all their lives at altitude, some dwellers, particularly Andeans, may develop a maladaptation syndrome known as chronic mountain sickness. The most prominent characteristic of this is excessive polycythaemia, the cause of which has been attributed to peripheral chemoreceptor dysfunction. The hyperviscous blood leads to pulmonary hypertension, symptoms of cerebral hypoperfusion, and eventually right heart failure and death.

The autonomic nervous system at high altitude

The effects of hypobaric hypoxia in visitors depend not only on the actual elevation but also on the rate of ascent. Sympathetic activity increases and there are increases in blood pressure and heart rate. Pulmonary vasoconstriction leads to pulmonary hypertension, particularly during exercise. The sympathetic excitation results from hypoxia, partly through chemoreceptor reflexes and partly through altered baroreceptor function. High pulmonary arterial pressures may also cause reflex systemic vasoconstriction. Most permanent high altitude dwellers show excellent adaptation although there are differences between populations in the extent of the ventilatory drive and the erythropoiesis. Some altitude dwellers, particularly Andeans, may develop chronic mountain sickness, the most prominent characteristic of which being excessive polycythaemia. Excessive hypoxia due to peripheral chemoreceptor dysfunction has been suggested as a cause. The hyperviscous blood leads to pulmonary hypertension, symptoms of cerebral hypoperfusion, and eventually right heart failure and death.

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

Intermittent hypoxia (IH), which refers to the discontinuous use of hypoxia to reproduce some key features of altitude acclimatization, is commonly used in athletes to improve their performance. However, variations of IH are also used as a model for sleep apnea, causing sustained sympathoexcitation and hypertension in animals and, thus, raising concerns over the safety of this model. We tested the hypothesis that chronic IH at rest alters autonomic control of arterial pressure in healthy trained individuals. Twenty-two young athletes (11 men and 11 women) were randomly assigned to hypobaric hypoxia (simulated altitude of 4,000-5,500 m) or normoxia (500 m) in a double-blind and placebo-controlled design. Both groups rested in a hypobaric chamber for 3 h/day, 5 days/wk for 4 wk. In the sitting position, resting hemodynamics, including heart rate (HR), blood pressure (BP), cardiac output (Q(c), C(2)H(2) rebreathing), stroke volume (SV = Q(c)/HR), and total peripheral resistance (TPR = mean BP/Q(c)), were measured, dynamic cardiovascular regulation was assessed by spectral and transfer function analysis of cardiovascular variability, and cardiac-vagal baroreflex function was evaluated by a Valsalva maneuver, twice before and 3 days after the last chamber exposure. We found no significant differences in HR, BP, Q(c), SV, TPR, cardiovascular variability, or cardiac-vagal baroreflex function between the groups at any time. These results suggest that exposure to intermittent hypobaric hypoxia for 4 wk does not cause sustained alterations in autonomic control of BP in young athletes. In contrast to animal studies, we found no secondary evidence for sustained physiologically significant sympathoexcitation in this model.

Effects of intermittent hypoxia on heart rate variability during rest and exercise

Changes in heart rate variability induced by an intermittent exposure to hypoxia were evaluated in athletes unacclimatized to altitude. Twenty national elite athletes trained for 13 days at 1200 m and either lived and slept at 1200 m (live low, train low, LLTL) or between 2500 and 3000 m (live high, train low, LHTL). Subjects were investigated at 1200 m prior to and at the end of the 13-day training camp. Exposure to acute hypoxia (11.5% O(2)) during exercise resulted in a significant decrease in spectral components of heart rate variability in comparison with exercise in normoxia: total power (p < 0.001), low-frequency component. LF (p < 0.001), high-frequency component, HF (p < 0.05). Following acclimatization, the LHTL group increased its LF component (p < 0.01) and LF/HF ratio during exercise in hypoxia after the training period. In parallel, exposure to intermittent hypoxia caused an increased ventilatory response to hypoxia. Acclimatization modified the correlation between the ventilatory response to hypoxia at rest and the difference in total power between normoxia and hypoxia (r (2) = 0.65, p < 0.001). The increase in total power, LF component, and LF/HF ratio suggests that intermittent hypoxic training increased the response of the autonomic nervous system mainly through increased sympathetic activity.