The Effect of Sex on Heart Rate Variability at High Altitude

Combining hypoxia with thermal stimuli in humans: physiological responses and potential sex differences

Increasing prevalence of native lowlanders sojourning to high altitudes (>2,500 m) for recreational, occupational, military, and competitive reasons has generated increased interest in physiological responses to multistressor environments. Exposure to hypoxia poses recognized physiological challenges that are amplified during exercise and further complicated by environments that might include combinations of heat, cold, and high altitude. There is a sparsity of data examining integrated responses in varied combinations of environmental conditions, with even less known about potential sex differences. How this translates into performance, occupational, and health outcomes requires further investigation. Acute hypoxic exposure decreases arterial oxygen saturation, resulting in a reflex hypoxic ventilatory response and sympathoexcitation causing an increase in heart rate, myocardial contractility, and arterial blood pressure, to compensate for the decreased arterial oxygen saturation. Acute altitude exposure impairs exercise performance, for example, reduced time to exhaustion and slower time trials, largely owing to impairments in pulmonary gas exchange and peripheral delivery resulting in reduced V̇o2max. This exacerbates with increasing altitude, as does the risk of developing acute mountain sickness and more serious altitude-related illnesses, but modulation of those risks with additional stressors is unclear. This review aims to summarize and evaluate current literature regarding cardiovascular, autonomic, and thermoregulatory responses to acute hypoxia, and how these may be affected by simultaneous thermal environmental challenges. There is minimal available information regarding sex as a biological variable in integrative responses to hypoxia or multistressor environments; we highlight these areas as current knowledge gaps and the need for future research.

The relationship between liver function and neurophysiological factors in depressed individuals: a cross-sectional study using an integrated "East meets West" medicine approach

Introduction

Depression is a common mental disorder worldwide. The pathology of depression may involve the dysregulation of neurotransmitters and immunity and produce genetic and environmental effects. Traditional Chinese Medicine (TCM) has been practiced for several thousand years and has a different understanding of depression compared to Western medicine. However, this approach has not been widely accepted by scientific communities as TCM mainly focuses on clinical practice.

Methods

In this study, we conducted a cross-sectional study among 100 participants in a rehabilitation hospital to analyze the plausible pathways linking TCM-based liver function and depression, which we hypothesized in a prior theoretical review.

Results

A significant relationship between adrenocorticotropic hormone and TCM-based liver function was found (r = 0.211, p = 0.041). Cortisol was significantly associated with norepinephrine (r = 0.243, p = 0.015) and adrenocorticotropic hormone (r = 0.302, p < 0.001). A positive significant relationship was also found between norepinephrine and adrenocorticotropic hormone (r = 0.272, p < 0.001). There was no significant relationship between the ratio from low frequency to high frequency and TCM-based liver function (p = 0.690).

Discussion

These results suggest that TCM-based liver function can be interpreted using the hypothalamic-pituitary-adrenal axis. This is a pioneering study to examine the mechanisms of depression in relation to liver function by integrating Eastern and Western medical approaches. The findings of this study are valuable for a deeper understanding of depression and public education.

Using Heart Rate Variability Methods for Health-Related Outcomes in Outdoor Contexts: A Scoping Review of Empirical Studies

Heart rate variability (HRV) is a psychophysiological variable that is often used in applied analysis techniques to indicate health status because it provides a window into the intrinsic regulation of the autonomic nervous system. However, HRV data analysis methods are varied and complex, which has led to different approaches to data collection, analysis, and interpretation of results. Our scoping review aimed to explore the diverse use of HRV methods in studies designed to assess health outcomes in outdoor free-living contexts. Four database indexes were searched, which resulted in the identification of 17,505 candidate studies. There were 34 studies and eight systematic reviews that met the inclusion criteria. Just over half of the papers referenced the 1996 task force paper that outlined the standards of measurement and physiological interpretation of HRV data, with even fewer adhering to recommended HRV recording and analysis procedures. Most authors reported an increase in parasympathetic (n = 23) and a decrease in systematic nervous system activity (n = 20). Few studies mentioned methods-related limitations and challenges, despite a wide diversity of recording devices and analysis software used. We conclude our review with five recommendations for future research using HRV methods in outdoor and health-related contexts.

Detecting driver fatigue using heart rate variability: A systematic review

Driver fatigue detection systems have potential to improve road safety by preventing crashes and saving lives. Conventional driver monitoring systems based on driving performance and facial features may be challenged by the application of automated driving systems. This limitation could potentially be overcome by monitoring systems based on physiological measurements. Heart rate variability (HRV) is a physiological marker of interest for detecting driver fatigue that can be measured during real life driving. This systematic review investigates the relationship between HRV measures and driver fatigue, as well as the performance of HRV based fatigue detection systems. With the applied eligibility criteria, 18 articles were identified in this review. Inconsistent results can be found within the studies that investigated differences of HRV measures between alert and fatigued drivers. For studies that developed HRV based fatigue detection systems, the detection performance showed a large variation, where the detection accuracy ranged from 44% to 100%. The inconsistency and variation of the results can be caused by differences in several key aspects in the study designs. Progress in this field is needed to determine the relationship between HRV and different fatigue causal factors and its connection to driver performance. To be deployed, HRV-based fatigue detection systems need to be thoroughly tested in real life conditions with good coverage of relevant driving scenarios and a sufficient number of participants.

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.

Neurophysiological Markers for Monitoring Exercise and Recovery Cycles in Endurance Sports

The current study analyzes the suitability and reliability of selected neurophysiological and vegetative nervous system markers as biomarkers for exercise and recovery in endurance sport. Sixty-two healthy men and women, endurance trained and moderately trained, performed two identical acute endurance tests (running trial 1 and running trial 2) followed by a washout period of four weeks. Exercise protocol consisted of an acute running trial lasting 60 minutes. An intensity corresponding to 95% of the heart rate at individual anaerobic threshold for 40 minutes was followed by 20 minutes at 110%. At pre-exercise, post-exercise, three hours post-exercise and 24 hours post-exercise, experimental diagnostics on Brain-derived neurotrophic factor (BDNF), heart rate variability (HRV), Stroop Color and Word Test (SCWT), and Short-Form McGill Pain Questionnaire (SF-MPQ) were performed. Significant changes over time were found for all parameters (p < .05). Furthermore, there was an approached statistical significance in the interaction between gender and training status in BDNF regulation (F₍₃₎ = 2.43; p = 0.06), while gender differences were found only for LF/HF-ratio (3hPoEx, F₍₃₎ = 3.40; p = 0.002). Regarding the reliability, poor ICC-values (< 0.5) were found for BDNF, Stroop sensitivity and pNN50, while all other parameters showed moderate ICC-values (0.5-0.75). Plasma-BDNF, SCWT performance, pain perception and all HRV parameters are suitable exercise-sensitive markers after an acute endurance exercise. Moreover, pain perception, SCWT reaction time and all HRV parameters show a moderate reliability, others rather poor. In summary, a selected neurophysiological and vegetative marker panel can be used to determine exercise load and recovery in endurance sports, but its repeatability is limited due to its vaguely reliability.

A change of heart: mechanisms of cardiac adaptation to acute and chronic hypoxia

Over the last 100 years, high‐altitude researchers have amassed a comprehensive understanding of the global cardiac responses to acute, prolonged and lifelong hypoxia. When lowlanders are exposed to hypoxia, the drop in arterial oxygen content demands an increase in cardiac output, which is facilitated by an elevated heart rate at the same time as ventricular volumes are maintained. As exposure is prolonged, haemoconcentration restores arterial oxygen content, whereas left ventricular filling and stroke volume are lowered as a result of a combination of reduced blood volume and hypoxic pulmonary vasoconstriction. Populations native to high‐altitude, such as the Sherpa in Asia, exhibit unique lifelong or generational adaptations to hypoxia. For example, they have smaller left ventricular volumes compared to lowlanders despite having larger total blood volume. More recent investigations have begun to explore the mechanisms underlying such adaptive responses by combining novel imaging techniques with interventions that manipulate cardiac preload, afterload, and/or contractility. This work has revealed the contributions and interactions of (i) plasma volume constriction; (ii) sympathoexcitation; and (iii) hypoxic pulmonary vasoconstriction with respect to altering cardiac loading, or otherwise preserving or enhancing biventricular systolic and diastolic function even amongst high altitude natives with excessive erythrocytosis. Despite these advances, various areas of investigation remain understudied, including potential sex‐related differences in response to high altitude. Collectively, the available evidence supports the conclusion that the human heart successfully adapts to hypoxia over the short‐ and long‐term, without signs of myocardial dysfunction in healthy humans, except in very rare cases of maladaptation.

Using machine learning to determine the correlation between physiological and environmental parameters and the induction of acute mountain sickness

Background

Recent studies on acute mountain sickness (AMS) have used fixed-location and fixed-time measurements of environmental and physiological variable to determine the influence of AMS-associated factors in the human body. This study aims to measure, in real time, environmental conditions and physiological variables of participants in high-altitude regions to develop an AMS risk evaluation model to forecast prospective development of AMS so its onset can be prevented.

Results

Thirty-two participants were recruited, namely 25 men and 7 women, and they hiked from Cuifeng Mountain Forest Park parking lot (altitude: 2300 m) to Wuling (altitude: 3275 m). Regression and classification machine learning analyses were performed on physiological and environmental data, and Lake Louise Acute Mountain Sickness Scores (LLS) to establish an algorithm for AMS risk analysis. The individual R² coefficients of determination between the LLS and the measured altitude, ambient temperature, atmospheric pressure, relative humidity, climbing speed, heart rate, blood oxygen saturation (SpO₂), heart rate variability (HRV), were 0.1, 0.23, 0, 0.24, 0, 0.24, 0.27, and 0.35 respectively; incorporating all aforementioned variables, the R² coefficient is 0.62. The bagged trees classifier achieved favorable classification results, yielding a model sensitivity, specificity, accuracy, and area under receiver operating characteristic curve of 0.999, 0.994, 0.998, and 1, respectively.

Conclusion

The experiment results indicate the use of machine learning multivariate analysis have higher AMS prediction accuracies than analyses utilizing single varieties. The developed AMS evaluation model can serve as a reference for the future development of wearable devices capable of providing timely warnings of AMS risks to hikers.

Hypoxia Differentially Affects Healthy Men and Women During a Daytime Nap With a Dose-Response Relationship: a Randomized, Cross-Over Pilot Study

Context: The use of daytime napping as a countermeasure in sleep disturbances has been recommended but its physiological evaluation at high altitude is limited. Objective: To evaluate the neuroendocrine response to hypoxic stress during a daytime nap and its cognitive impact. Design, Subject, and Setting: Randomized, single-blind, three period cross-over pilot study conducted with 15 healthy lowlander subjects (8 women) with a mean (SD) age of 29(6) years (Clinicaltrials identifier: NCT04146857, https://clinicaltrials.gov/ct2/show/NCT04146857?cond=napping&draw=3&rank=12). Interventions: Volunteers underwent a polysomnography, hematological and cognitive evaluation around a 90 min midday nap, being allocated to a randomized sequence of three conditions: normobaric normoxia (NN), normobaric hypoxia at FiO2 14.7% (NH15) and 12.5% (NH13), with a washout period of 1 week between conditions. Results: Primary outcome was the interbeat period measured by the RR interval with electrocardiogram. Compared to normobaric normoxia, RR during napping was shortened by 57 and 206 ms under NH15 and NH13 conditions, respectively (p < 0.001). Sympathetic predominance was evident by heart rate variability analysis and increased epinephrine levels. Concomitantly, there were significant changes in endocrine parameters such as erythropoietin (∼6 UI/L) and cortisol (∼100 nmol/L) (NH13 vs. NN, p < 0.001). Cognitive evaluation revealed changes in the color-word Stroop test. Additionally, although sleep efficiency was preserved, polysomnography showed lesser deep sleep and REM sleep, and periodic breathing, predominantly in men. Conclusion: Although napping in simulated altitude does not appear to significantly affect cognitive performance, sex-dependent changes in cardiac autonomic modulation and respiratory pattern should be considered before napping is prescribed as a countermeasure.

Sexual Dimorphism of Dexamethasone as a Prophylactic Treatment in Pathologies Associated With Acute Hypobaric Hypoxia Exposure

Dexamethasone can be taken prophylactically to prevent hypobaric hypoxia-associated disorders of high-altitude. While dexamethasone-mediated protection against high-altitude disorders has been clinically evaluated, detailed sex-based mechanistic insights have not been explored. As part of our India-Leh-Dexamethasone-expedition-2020 (INDEX 2020) programme, we examined the phenotype of control (n = 14) and dexamethasone (n = 13) groups, which were airlifted from Delhi (∼225 m elevation) to Leh, Ladakh (∼3,500 m), India, for 3 days. Dexamethasone 4 mg twice daily significantly attenuated the rise in blood pressure, heart rate, pulmonary pressure, and drop in SaO₂ resulting from high-altitude exposure compared to control-treated subjects. Of note, the effect of dexamethasone was substantially greater in women than in men, in whom the drug had relatively little effect. Thus, for the first time, this study shows a sex-biased regulation by dexamethasone of physiologic parameters resulting from the hypoxic environment of high-altitude, which impacts the development of high-altitude pulmonary hypertension and acute mountain sickness. Future studies of cellular contributions toward sex-specific regulation may provide further insights and preventive measures in managing sex-specific, high-altitude–related disorders.

The Effect of High-Altitude Acclimatisation on Ultra-Short Heart Rate Variability

Introduction

High-altitude (HA) exposure affects heart rate variability (HRV) and has been inconsistently linked to acute mountain sickness (AMS). The influence of increasing HA exposure on ultra-short HRV and its relationship to gold standard HRV measures at HA has not been examined.

Methods

This was a prospective observational study of adults aged ≥ 18 years undertaking a HA trek in the Dhaulagiri region of the Himalayas. Cardiac inter-beat-intervals were obtained from a 10-s recording of supra-systolic blood pressure (Uscom BP⁺ device) immediately followed by 300 s single lead ECG recording (CheckMyHeart device). HRV was measured using the RMSSD (root mean square of successive differences of NN intervals) at sea level (SL) in the United Kingdom and at 3,619, 4,600, and 5,140 m at HA. Oxygen saturations (SpO₂) were measured using finger-based pulse oximetry. The level of agreement between the 10 and 300 s RMSSD values were examined using a modified Bland–Altman relative-difference analysis.

Results

Overall, 89 participants aged 32.2 ± 8.8 years (range 18–56) were included of which 70.8% were men. HA exposure (SL vs. 3,619 m) was associated with an initial increase in both 10 s (45.0 [31.0–82.0]) vs. 58.0 [33.0–119.0] ms) and 300 s (45.67 [33.24–70.32] vs. 56.48 [36.98–102.0] ms) in RMSSD. Thereafter at 4,600 and 5,140 m both 10 and 300 s RMSSD values were significantly lower than SL. From a total of 317 paired HRV measures the 10 and 300 s RMSSD measures were moderately correlated (Spearman r = 0.66; 95% CI: 0.59–0.72; p < 0.0001). The median difference (bias) in RMSSD values (300 s − 10 s) was −2.3 ms with a lower and upper limit of agreement of −107.5 and 88.61 ms, respectively with no differences with altitude. Overall, 293/317 (92.4%) of all paired HRV values fell within the 95% CI limits of agreement. Neither HRV method was predictive of AMS.

Conclusion

Increasing HA affects ultra-short HRV in a similar manner to gold-standard 300 s. Ultra-short HRV has a moderate agreement with 300 s measurements. HRV did not predict AMS.

Effect of resisted exercise on autonomic cardiac modulation in elderly women: STROBE-compliant study

Heart rate variability (HRV) is an important cardiac health marker, with lower values indicating a reduction in vagal control of cardiac rhythm and decreasing significantly with advancing age. In this study, we evaluated the effects of strength exercises for the upper and lower limbs on cardiac autonomic modulation in elderly women.We registered 29 participants using a portable RS800CX heart rate monitor to record the RR intervals. For the collection of HRV data, the volunteers remained seated at rest for 10 minutes. After the rest period, the participants performed the exercises. Immediately after the exercise protocol, the subjects remained seated at rest for 30 minutes. HRV indices were analyzed in the following periods: rest, 0 to 10 minutes, 5 to 10 minutes, 10 to 20 minutes, and 20 to 30 minutes. Systolic arterial pressure and diastolic arterial pressure were measured in the following periods: rest, immediately after exercise, and 30 minutes after exercise.Regarding the mean of the RR intervals, heart rate, and indexes of the time and frequency domains surveyed, there were no statistically significant differences between the 4 moments in the protocols for upper and lower limbs. No significant differences were found in systolic and diastolic pressures between the 3 time points surveyed in the protocols for the upper and lower limbs.Resistance exercises performed with low-intensity loads and a greater number of repetitions did not promote significant variations in cardiac autonomic modulation and blood pressure levels, showing good safety in elderly women.

Duration at high altitude influences the onset of arrhythmogenesis during apnea

PURPOSE

Autonomic control of the heart is balanced by sympathetic and parasympathetic inputs. Excitation of both sympathetic and parasympathetic systems occurs concurrently during certain perturbations such as hypoxia, which stimulate carotid chemoreflex to drive ventilation. It is well established that the chemoreflex becomes sensitized throughout hypoxic exposure; however, whether progressive sensitization alters cardiac autonomic activity remains unknown. We sought to determine the duration of hypoxic exposure at high altitude necessary to unmask cardiac arrhythmias during instances of voluntary apnea.

METHODS

Measurements of steady-state chemoreflex drive (SS-CD), continuous electrocardiogram (ECG) and SpO₂ (pulse oximetry) were collected in 22 participants on 1 day at low altitude (1045 m) and over eight consecutive days at high-altitude (3800 m). SS-CD was quantified as ventilation (L/min) over stimulus index (P_ETCO₂/SpO₂).

RESULTS

Bradycardia during apnea was greater at high altitude compared to low altitude for all days (p < 0.001). Cardiac arrhythmias occurred during apnea each day but became most prevalent (> 50%) following Day 5 at high altitude. Changes in saturation during apnea and apnea duration did not affect the magnitude of bradycardia during apnea (ANCOVA; saturation, p = 0.15 and apnea duration, p = 0.988). Interestingly, the magnitude of bradycardia was correlated with the incidence of arrhythmia per day (r = 0.8; p = 0.004).

CONCLUSION

Our findings suggest that persistent hypoxia gradually increases vagal tone with time, indicated by augmented bradycardia during apnea and progressively increased the incidence of arrhythmia at high altitude.

Alterations of Cardiovascular Complexity during Acute Exposure to High Altitude: A Multiscale Entropy Approach

Stays at high altitude induce alterations in cardiovascular control and are a model of specific pathological cardiovascular derangements at sea level. However, high-altitude alterations of the complex cardiovascular dynamics remain an almost unexplored issue. Therefore, our aim is to describe the altered cardiovascular complexity at high altitude with a multiscale entropy (MSE) approach. We recorded the beat-by-beat series of systolic and diastolic blood pressure and heart rate in 20 participants for 15 min twice, at sea level and after arrival at 4554 m a.s.l. We estimated Sample Entropy and MSE at scales of up to 64 beats, deriving average MSE values over the scales corresponding to the high-frequency (MSE_HF) and low-frequency (MSE_LF) bands of heart-rate variability. We found a significant loss of complexity at heart-rate and blood-pressure scales complementary to each other, with the decrease with high altitude being concentrated at Sample Entropy and at MSE_HF for heart rate and at MSE_LF for blood pressure. These changes can be ascribed to the acutely increased chemoreflex sensitivity in hypoxia that causes sympathetic activation and hyperventilation. Considering high altitude as a model of pathological states like heart failure, our results suggest new ways for monitoring treatments and rehabilitation protocols.

Normalization of heart rate variability with taurine and meldonium complex in post-infarction patients with type 2 diabetes mellitus

The purpose of this study is to scrutiny the Dynamics of heart rate variability (HRV) in patients with PICS with 2nd type DM against the background of Taurine (TN) and meldonium (ME). The results of the investigations prove the decrease of the oxidative stress, which is basis of DACN, under the influence of sulfur-containing amino acid taurine (TN), and meldonium (ME) — a competitive inhibitor of gamma-butyrobetaine hydroxylase. Biochemical mechanisms of synergistic action of ME and TN are also described. The results of the studies of 98 patients with PICS and concomitant 2nd type diabetes mellitus were analyzed. They were distributed by simple randomization method into two groups, comparable according to age and sex: the main group (MG) (n = 68): and group of comparison (GoC) (n = 30). HRV was evaluated twice daily at the Cardiosense HMEGG system: at baseline and after 12 weeks of treatment. For the assessment of HRV the frequency and spectral parameters were used. While evaluating the different methods of treatment, their influence on the range of spectral and time indices of HRV was determined (p = 0.001 by the criterion of Kruskall-Wallis). It was learned that the combined application of ME and TN gives a statistically significant (p <0.01) increase of SDNN, HF at night, pNN — on 50% by day (p <0.01, p <0.001 and p <0.01 respectively), and statistically significant decrease in LF at night, compared to GHG.

Recovery time and heart rate variability following extreme endurance exercise in healthy women

The relationship between autonomic function and recovery following prolonged arduous exercise in women has not been examined. We undertook an exploratory study that aimed to examine the temporal change in linear and nonlinear measures of heart rate variability (HRV) following prolonged arduous exercise in the form of first all‐female (mean age 32.7 ± 3.1 years) team to attempt an unassisted Antarctic traverse. HRV analysis was performed before and 1, 4, and 15 days postexpedition. The traverse was completed in 61 days. There was a significant paired reduction in heart rate, LnLF, LF:HF, DFAα1 between baseline and 15 days postexercise in the same environment. Conversely, RMSSD, LnHF and HFnu, SD1:SD2, and SampEn significantly increased. DFAα2 levels significantly fell from baseline to Day 1 postexercise. In conclusion, we observed a significant latent increase in relative parasympathetic dominance and RR interval irregularity at 15 days post prolonged arduous exercise, versus pre‐exercise baseline, in a group of very fit and healthy adult women.

The Magnitude of Diving Bradycardia During Apnea at Low-Altitude Reveals Tolerance to High Altitude Hypoxia

Acute mountain sickness (AMS) is a potentially life-threatening illness that may develop during exposure to hypoxia at high altitude (HA). Susceptibility to AMS is highly individual, and the ability to predict it is limited. Apneic diving also induces hypoxia, and we aimed to investigate whether protective physiological responses, i.e., the cardiovascular diving response and spleen contraction, induced during apnea at low-altitude could predict individual susceptibility to AMS. Eighteen participants (eight females) performed three static apneas in air, the first at a fixed limit of 60 s (A1) and two of maximal duration (A2-A3), spaced by 2 min, while SaO₂, heart rate (HR) and spleen volume were measured continuously. Tests were conducted in Kathmandu (1470 m) before a 14 day trek to mount Everest Base Camp (5360 m). During the trek, participants reported AMS symptoms daily using the Lake Louise Questionnaire (LLQ). The apnea-induced HR-reduction (diving bradycardia) was negatively correlated with the accumulated LLQ score in A1 (r _s = -0.628, p = 0.005) and A3 (r _s = -0.488, p = 0.040) and positively correlated with SaO₂ at 4410 m (A1: r = 0.655, p = 0.003; A2: r = 0.471, p = 0.049; A3: r = 0.635, p = 0.005). Baseline spleen volume correlated negatively with LLQ score (r _s = -0.479, p = 0.044), but no correlation was found between apnea-induced spleen volume reduction with LLQ score (r _s = 0.350, p = 0.155). The association between the diving bradycardia and spleen size with AMS symptoms suggests links between physiological responses to HA and apnea. Measuring individual responses to apnea at sea-level could provide means to predict AMS susceptibility prior to ascent.

Physiological Responses in Humans Acutely Exposed to High Altitude (3480 m): Minute Ventilation and Oxygenation Are Predictive for the Development of Acute Mountain Sickness

The importance of arterial oxygen saturation for the prediction of acute mountain sickness (AMS) is still a matter of debate. Reasons for discrepancies may result from varying laboratory or field conditions and their interactions. Thus, we analyzed data from our prior high-altitude studies, including participants of a broad range of age of both sexes (20 males and 20 females, aged between 20 and 67 years) under strictly standardized conditions of pre-exposure and acute exposure to real high altitude (3480 m). A set of resting cardiovascular, respiratory, hematological, and metabolic variables were recorded at high altitude (Testa Grigia, Plateau Rosa, 3480 m; Swiss-Italian boarder) after performing pretests at low altitude (Innsbruck, 600 m, Austria). Our analyses indicate that (1) smaller changes in resting minute ventilation (VE) and a larger decrease of peripheral oxygen saturation (SpO₂) during the first 3 hours of acute exposure to high altitude were independent predictors for subsequent development of AMS (90% correct prediction), (2) there are no differences of responses between sexes, and (3) there is no association of responses with age. Considering the independent effects of both responses (VE and SpO₂) may be of clinical/practical relevance. Moreover, the presented data derived from a broad age range of both sexes might be of interest for comparative purposes.

Blood Pressure, Heart Rate Variability, and Adiposity in Caribbean Pre-pubertal Children

Childhood obesity prevalence has increased over the last 30 years. The Heart Rate Variability (HRV) studies performed in adults suggest a possible relation between abnormal autonomic regulation and hypertension in the situation of overweight or obesity. Objective: The aims of this study were to explore the early relationships between adiposity and blood pressure and HRV in pre-pubertal children. Methods: Data were collected during the medical examination of the follow-up at 7 years of the TIMOUN mother-child cohort in Guadeloupe. Body Mass Index z-score (zBMI), sum of tricipital and subscapular skinfold thickness, percentage of fat mass, and Waist-to-Height Ratio were measured. A global corpulence score was computed using a Principal Component Analysis (PCA). Systolic Blood Pressure (SBP) and HRV parameters (cardiac holter monitoring) were collected under 2 conditions (calm and tachycardial period). Relations between HRV, SBP, each adiposity indicator and the corpulence score were studied with restricted cubic splines models, and linear regression models. The age at adiposity rebound (AR) was estimated from the individual growth curves. Results: 575 children were included in the SBP study (mean age: 7.7 years, from 85 to 99 months). SBP was linearly correlated with the corpulence score and the zBMI. An increase of 1 in the zBMI was associated with an increase of 2.3 (±0.28) mmHg in SBP. The effect-size of zBMI on SBP was higher in children with early age at AR. Compared to children with normal BMI, children with a zBMI <™2SD had their RMSSD, SDNN, LF and HF indicators in tachycardial conditions significantly reduced by -30, -21, -37, and -48%, respectively. In boys with a zBMI >2SD, we observed a global increase in all HRV parameters (under tachycardial conditions), particularly the LF [β = 0.43 (±0.18)]. Conclusion: In pre-pubertal period a positive correlation between adiposity excess and SBP was observed with significant changes of HRV in boys, arguing for an early abnormal autonomic regulation and for early preventive intervention in the infancy period, particularly in case of overweight or obesity. Thinness was associated with a reduction in almost all the HRV parameters studied, when compared to normal corpulence, suggesting a decrease in autonomic influence.

Hypoxia-induced vagal withdrawal is independent of the hypoxic ventilatory response in men

Hypoxia increases heart rate (HR) in humans by sympathetic activation and vagal withdrawal. However, in anaesthetized dogs hypoxia increases vagal activity and reduces HR if pulmonary ventilation does not increase and we evaluated whether that observation applies to awake humans. Ten healthy males were exposed to 15 min of normoxia and hypoxia (10.5% O2), while respiratory rate and tidal volume were volitionally controlled at values identified during spontaneous breathing in hypoxia. End-tidal CO2 tension was clamped at 40 mmHg by CO2 supplementation. β-Adrenergic blockade by intravenous propranolol isolated vagal regulation of HR. During spontaneous breathing, hypoxia increased ventilation by 3.2 ± 2.1 l/min ( P = 0.0033) and HR by 8.9 ± 5.5 beats/min ( P < 0.001). During controlled breathing, respiratory rate (16.3 ± 3.2 vs. 16.4 ± 3.3 breaths/min) and tidal volume (1.05 ± 0.27 vs. 1.06 ± 0.24 l) were similar for normoxia and hypoxia, whereas the HR increase in hypoxia persisted without (8.6 ± 10.2 beats/min) and with (6.6 ± 5.6 beats/min) propranolol. Neither controlled breathing ( P = 0.80), propranolol ( P = 0.64), nor their combination ( P = 0.89) affected the HR increase in hypoxia. Arterial pressure was unaffected ( P = 0.48) by hypoxia across conditions. The hypoxia-induced increase in HR during controlled breathing and β-adrenergic blockade indicates that hypoxia reduces vagal activity in humans even when ventilation does not increase. Vagal withdrawal in hypoxia seems to be governed by the arterial chemoreflex rather than a pulmonary inflation reflex in humans.

NEW & NOTEWORTHY Hypoxia accelerates the heart rate of humans by increasing sympathetic activity and reducing vagal activity. Animal studies have indicated that hypoxia-induced vagal withdrawal is governed by a pulmonary inflation reflex that is activated by the increased pulmonary ventilation in hypoxia. The present findings, however, indicate that humans experience vagal withdrawal in hypoxia even if ventilation does not increase, indicating that vagal withdrawal is governed by the arterial chemoreflex rather than a pulmonary inflation reflex.

VARIABILITY OF THE HEART RHYTHM AS AN ADDITIONAL MARKER FOR DETERMINING VEGETATIVE FUNCTIONS IN PATIENTS WITH CHRONIC CEREBRAL ISCHEMIA

Aim. Practical cardiology is in constant search for non-invasive vascular risk markers. Heart rhythm reflects the body's response to various stimuli of the external and internal environment. Heart rate variability (HRV) has a prognostic and diagnostic value and allows timely identification of conditions that threaten life. The results of an instrumental examination of heart rhythm fluctuations in patients suffering from chronic cerebral ischemia against the background of angina pectoris of different functional classes allows to evaluate the prognosis of the disease and select the appropriate treatment. Materials and methods. An assessment of the state of the mechanisms of regulation of physiological functions in patients suffering from chronic cerebral ischemia against the background of angina pectoris of different functional classes was obtained according to spectral and temporal analysis of heart rate variability using electrocardiographic monitoring. The spectral characteristics of the heart rate variability were studied: HF (high frequency), LF (low frequency), VLF (very low frequency). Results. Heart rhythm regulation in patients with chronic cerebral ischemia occurred under the influence of neurohumoral mechanisms. The imbalance of functional systems was caused by changes in the autonomic nervous system, which disrupted the normal functioning of the sympathetic and parasympathetic parts. We marked decrease in the activity of the parasympathetic autonomic nervous system, which changed the indices of spectral analysis, while the high-frequency component of the spectrum was characterized by a decrease, while the low-frequency component was characterized by an increase. The progression of stable angina of tension (SAT) in patients with chronic cerebral ischemia (CCI) occurred with disruption of the autonomic nervous system (ANS) and was associated with a shift in the physiological response towards sympathetic activity. This was particularly pronounced in patients in group 2 with CCI on the background of SAT III FC, as the regulatory mechanisms were in a critical state of tension against the background of long-term chronic ischemia, they showed a high level of humoral modulation of regulatory mechanisms, which was manifested by excessive VLF and high-frequency oscillations. Conclusions. A connection was established between the autonomic nervous system and chronic cerebral ischemia, which was expressed in the imbalance of the ANS, associated with reliable signs of the dominant sympathetic system, which was associated with the progression of stable angina of tension.

High Altitude Affects Nocturnal Non-linear Heart Rate Variability: PATCH-HA Study

Background: High altitude (HA) exposure can lead to changes in resting heart rate variability (HRV), which may be linked to acute mountain sickness (AMS) development. Compared with traditional HRV measures, non-linear HRV appears to offer incremental and prognostic data, yet its utility and relationship to AMS have been barely examined at HA. This study sought to examine this relationship at terrestrial HA. Methods: Sixteen healthy British military servicemen were studied at baseline (800 m, first night) and over eight consecutive nights, at a sleeping altitude of up to 3600 m. A disposable cardiac patch monitor was used, to record the nocturnal cardiac inter-beat interval data, over 1 h (0200-0300 h), for offline HRV assessment. Non-linear HRV measures included Sample entropy (SampEn), the short (α1, 4-12 beats) and long-term (α2, 13-64 beats) detrend fluctuation analysis slope and the correlation dimension (D2). The maximal rating of perceived exertion (RPE), during daily exercise, was assessed using the Borg 6-20 RPE scale. Results: All subjects completed the HA exposure. The average age of included subjects was 31.4 ± 8.1 years. HA led to a significant fall in SpO₂ and increase in heart rate, LLS and RPE. There were no significant changes in the ECG-derived respiratory rate or in any of the time domain measures of HRV during sleep. The only notable changes in frequency domain measures of HRV were an increase in LF and fall in HFnu power at the highest altitude. Conversely, SampEn, SD1/SD2 and D2 all fell, whereas α1 and α2 increased (p < 0.05). RPE inversely correlated with SD1/SD2 (r = -0.31; p = 0.002), SampEn (r = -0.22; p = 0.03), HFnu (r = -0.27; p = 0.007) and positively correlated with LF (r = 0.24; p = 0.02), LF/HF (r = 0.24; p = 0.02), α1 (r = 0.32; p = 0.002) and α2 (r = 0.21; p = 0.04). AMS occurred in 7/16 subjects (43.8%) and was very mild in 85.7% of cases. HRV failed to predict AMS. Conclusion: Non-linear HRV is more sensitive to the effects of HA than time and frequency domain indices. HA leads to a compensatory decrease in nocturnal HRV and complexity, which is influenced by the RPE measured at the end of the previous day. HRV failed to predict AMS development.

Heart rate variability as predictive factor for sudden cardiac death

Sudden cardiac death (SCD) represents about 25% of deaths in clinical cardiology. The identification of risk factors for SCD is the philosopher's stone of cardiology and the identification of non-invasive markers of risk of SCD remains one of the most important goals for the scientific community.The aim of this review is to analyze the state of the art around the heart rate variability (HRV) as a predictor factor for SCD.HRV is probably the most analyzed index in cardiovascular risk stratification technical literature, therefore an important number of models and methods have been developed.Nowadays, low HRV has been shown to be independently predictive of increased mortality in post- myocardial infarction patients, heart failure patients, in contrast with the data of the general population.Contrariwise, the relationship between HRV and SCD has received scarce attention in low-risk cohorts. Furthermore, in general population the attributable risk is modest and the cost/benefit ratio is not always convenient.The HRV evaluation could become an important tool for health status in risks population, even though the use of HRV alone for risk stratification of SCD is limited and further studies are needed.

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

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