Alterations in autonomic nervous control of heart rate among tourists at 2700 and 3700 m above sea level

Effect of Acute Altitude Exposure on Anaerobic Threshold Assessed by a Novel Electrocardiogram-Based Method

Background: Acute altitude has a relevant impact on exercise physiology and performance. Therefore, the positive impact on the performance level is utilized as a training strategy in professional as well as recreational athletes. However, ventilatory thresholds (VTs) and lactate thresholds (LTs), as established performance measures, cannot be easily assessed at high altitudes. Therefore, a noninvasive, reliable, and cost-effective method is needed to facilitate and monitor training management at high altitudes. High Alt Med Biol. 25:94-99, 2024. Methods: In a cross-sectional setting, a total of 14 healthy recreational athletes performed a graded cycling exercise test at sea level (Munich, Germany: 512 m/949 mbar) and high altitude (Zugspitze: 2,650 m/715 mbar). Anaerobic thresholds (ATs) were assessed using a novel method based on beat-to-beat repolarization instability (dT) detected by Frank-lead electrocardiogram (ECG) monitoring. The ECG-based ATs (ATdT°) were compared to routine LTs assessed according to Dickhuth and Mader. Results: After acute altitude exposure, a decrease in AT was detected using a novel ECG-based method (ATdT°: 159.80 ± 52.21 W vs. 134.66 ± 34.91 W). AtdT° levels correlated significantly with LTDickhuth and LTMader, at baseline (rDickhuth/AtdT° = 0.979; p < 0.001) (rMader/AtdT° = 0.943; p < 0.001), and at high altitude (rDickhuth/AtdT° = 0.969; p < 0.001) (rMader/AtdT° = 0.942; p < 0.001). Conclusion: Assessment of ATdT is a reliable method to detect performance alterations at altitude. This novel method may facilitate the training management of athletes at high altitudes.

Cardiovascular physiology and pathophysiology at high altitude

Oxygen is vital for cellular metabolism; therefore, the hypoxic conditions encountered at high altitude affect all physiological functions. Acute hypoxia activates the adrenergic system and induces tachycardia, whereas hypoxic pulmonary vasoconstriction increases pulmonary artery pressure. After a few days of exposure to low oxygen concentrations, the autonomic nervous system adapts and tachycardia decreases, thereby protecting the myocardium against high energy consumption. Permanent exposure to high altitude induces erythropoiesis, which if excessive can be deleterious and lead to chronic mountain sickness, often associated with pulmonary hypertension and heart failure. Genetic factors might account for the variable prevalence of chronic mountain sickness, depending on the population and geographical region. Cardiovascular adaptations to hypoxia provide a remarkable model of the regulation of oxygen availability at the cellular and systemic levels. Rapid exposure to high altitude can have adverse effects in patients with cardiovascular diseases. However, intermittent, moderate hypoxia might be useful in the management of some cardiovascular disorders, such as coronary heart disease and heart failure. The aim of this Review is to help physicians to understand the cardiovascular responses to hypoxia and to outline some recommendations that they can give to patients with cardiovascular disease who wish to travel to high-altitude destinations.

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.

Adrenergic control of the cardiovascular system in deer mice native to high altitude

Studies of animals native to high altitude can provide valuable insight into physiological mechanisms and evolution of performance in challenging environments. We investigated how mechanisms controlling cardiovascular function may have evolved in deer mice (Peromyscus maniculatus) native to high altitude. High-altitude deer mice and low-altitude white-footed mice (P. leucopus) were bred in captivity at sea level, and first-generation lab progeny were raised to adulthood and acclimated to normoxia or hypoxia. We then used pharmacological agents to examine the capacity for adrenergic receptor stimulation to modulate heart rate (f_H) and mean arterial pressure (P_mean) in anaesthetized mice, and used cardiac pressure-volume catheters to evaluate the contractility of the left ventricle. We found that highlanders had a consistently greater capacity to increase f_H via pharmacological stimulation of β₁-adrenergic receptors than lowlanders. Also, whereas hypoxia acclimation reduced the capacity for increasing P_mean in response to α-adrenergic stimulation in lowlanders, highlanders exhibited no plasticity in this capacity. These differences in highlanders may help augment cardiac output during locomotion or cold stress, and may preserve their capacity for α-mediated vasoconstriction to more effectively redistribute blood flow to active tissues. Highlanders did not exhibit any differences in some measures of cardiac contractility (maximum pressure derivative, dP/dt_max, or end-systolic elastance, E_es), but ejection fraction was highest in highlanders after hypoxia acclimation. Overall, our results suggest that evolved changes in sensitivity to adrenergic stimulation of cardiovascular function may help deer mice cope with the cold and hypoxic conditions at high altitude.

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High-altitude deer mice have evolved increased aerobic capacity in hypoxia.

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Cardiovascular regulation was examined in normoxia and chronic hypoxia.

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Highland mice had increased capacity for β₁-adrenergic stimulation of heart rate.

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Hypoxia reduced vascular α-adrenergic sensitivity in lowland but not highland mice.

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Cardiac ejection fraction was elevated in highland mice in chronic hypoxia.

Effects of acute hypobaric hypoxia on thermoregulatory and circulatory responses during cold air exposure

Background

The thermoregulatory responses during simultaneous exposure to hypoxia and cold are not well understood owing to the opposite reactions of vasomotor tone in these two environments. Therefore, the purpose of this study was to investigate the influences of hypobaric hypoxia on various thermoregulatory responses, including skin blood flow (SkBF) during cold exposure.

Methods

Ten subjects participated in two experimental conditions: normobaric normoxia with cold (NC, barometric pressure (P_B) = 760 mmHg) and hypobaric hypoxia with cold (HC, P_B = 493 mmHg). The air temperature was maintained at 28 °C for 65 min and gradually decreased to 19 °C for both conditions. The total duration of the experiment was 135 min.

Results

The saturation of percutaneous oxygen (SpO₂) was maintained at 98–99% in NC condition, but decreased to around 84% in HC condition. The rectal and mean skin temperatures showed no significant differences between the conditions; however, the forehead temperature was higher in HC condition than in NC condition. The pulse rate increased in HC condition, and there was a strong negative relationship between SpO₂ and pulse rate (r = − 0.860, p = 0.013). SkBF and blood pressure showed no significant differences between the two conditions.

Conclusion

These results suggest that hypobaric hypoxia during cold exposure did not alter the overall thermoregulatory responses. However, hypobaric hypoxia did affect pulse rate regardless of cold exposure.

Systemic Blood Pressure Variation During a 12-Hour Exposure to Normobaric Hypoxia (4500 m)

This study was aimed at evaluating a potential association between blood pressure variation and acute mountain sickness (AMS) during acute exposure to normobaric hypoxia. A total of 77 healthy subjects (43 males, 34 females) were exposed to a simulated altitude of 4500 m for 12 hours. Peripheral oxygen saturation, heart rate, systemic blood pressure, and Lake Louise AMS scores were recorded before and during (30 minutes, 3, 6, 9, and 12 hours) hypoxic exposure. Blood pressure dips were observed at 3-hour mark. However, systolic blood pressure fell more pronounced from baseline during the initial 30 minutes in normobaric hypoxia (-17.5 vs. -11.0 mmHg, p = 0.01) in subjects suffering from AMS (AMS⁺; n = 56) than in those remaining unaffected from AMS (AMS^-; n = 21); values did not differ between groups over the subsequent time course. Our data may suggest a transient autonomic dysfunction resulting in a more pronounced blood pressure drop during initial hypoxic exposure in AMS⁺ compared with AMS^- subjects.

Urinary physiology and hypoxia: a pilot study of moderate-altitude trekking effects on urodynamic indexes

Exposure to high altitude is one of the most widely used models to study the adaptive response to hypoxia in humans. However, little is known about the related effects on micturition. The present study addresses the adaptive urinary responses in four healthy adult lowlanders, comparing urodynamic indexes at Kathmandu [1,450 m above sea level (a.s.l.); K1450] and during a sojourn in Namche Bazar (3,500 m a.s.l.; NB3500). The urodynamic testing consisted of cistomanometry and bladder pressure/flow measurements. Anthropometrics, electrocardiographic, and peripheral capillary oxygen saturation data were also collected. The main findings consisted of significant reductions in bladder power at maximum urine flow by ~30%, bladder contractility index by 13%, and infused volume both at first (by 57%) and urgency sensation (by 14%) to urinate, indicating a reduced cystometric capacity, at NB3500. In addition to the urinary changes, we found that oxygen saturation, body mass index, body surface area, and median RR time were all significantly reduced at altitude. We submit that the hypoxia-related parasympathetic inhibition could be the underlying mechanism of both urodynamic and heart rate adaptive responses to high-altitude exposure. Moreover, increased diuresis and faster bladder filling at altitude may trigger the anticipation of being able to void, a common cause of urgency. We believe that the present pilot study represents an original approach to the study of urinary physiology at altitude.

Influence of Training Models at 3,900-m Altitude on the Physiological Response and Performance of a Professional Wheelchair Athlete: A Case Study

Sanz-Quinto, S, López-Grueso, R, Brizuela, G, Flatt, AA, and Moya-Ramón, M. Influence of training models at 3,900-m altitude on the physiological response and performance of a professional wheelchair athlete: A case study. J Strength Cond Res 33(6): 1715-1723, 2019-This case study compared the effects of two training camps using flexible planning (FP) vs. inflexible planning (IP) at 3,860-m altitude on physiological and performance responses of an elite marathon wheelchair athlete with Charcot-Marie-Tooth disease (CMT). During IP, the athlete completed preplanned training sessions. During FP, training was adjusted based on vagally mediated heart rate variability (HRV) with specific sessions being performed when a reference HRV value was attained. The camp phases were baseline in normoxia (BN), baseline in hypoxia (BH), specific training weeks 1-4 (W1, W2, W3, W4), and Post-camp (Post). Outcome measures included the root mean square of successive R-R interval differences (rMSSD), resting heart rate (HRrest), oxygen saturation (SO2), diastolic blood pressure and systolic blood pressure, power output and a 3,000-m test. A greater impairment of normalized rMSSD (BN) was shown in IP during BH (57.30 ± 2.38% vs. 72.94 ± 11.59%, p = 0.004), W2 (63.99 ± 10.32% vs. 81.65 ± 8.87%, p = 0.005), and W4 (46.11 ± 8.61% vs. 59.35 ± 6.81%, p = 0.008). At Post, only in FP was rMSSD restored (104.47 ± 35.80%). Relative changes were shown in power output (+3 W in IP vs. +6 W in FP) and 3,000-m test (-7s in IP vs. -16s in FP). This case study demonstrated that FP resulted in less suppression and faster restoration of rMSSD and more positive changes in performance than IP in an elite wheelchair marathoner with CMT.

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 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.

Cardiovascular System Response to Carbon Dioxide and Exercise in Oxygen-Enriched Environment at 3800 m

Background: This study explores the responses of the cardiovascular system as humans exercise in an oxygen-enriched room at high altitude under various concentrations of CO₂. Methods: The study utilized a hypobaric chamber set to the following specifications: 3800 m altitude with 25% O₂ and different CO₂ concentrations of 0.5% (C1), 3.0% (C2) and 5.0% (C3). Subjects exercised for 3 min three times, separated by 30 min resting periods in the above-mentioned conditions, at sea level (SL) and at 3800 m altitude (HA). The changes of heart rate variability, heart rate and blood pressure were analyzed. Results: Total power (TP) and high frequency power (HF) decreased notably during post-exercise at HA. HF increased prominently earlier the post-exercise period at 3800 m altitude with 25% O₂ and 5.0% CO₂ (C3), while low frequency power (LF) changed barely in all tests. The ratios of LF/HF were significantly higher during post-exercise in HA, and lower after high intensity exercise in C3. Heart rate and systolic blood pressure increased significantly in HA and C3. Conclusions: Parasympathetic activity dominated in cardiac autonomic modulation, and heart rate and blood pressure increased significantly after high intensity exercise in C3.

Comparison of heart rate variability between resting state and external-cuff-inflation-and-deflation state: a pilot study

Heart rate variability (HRV) has been widely used in clinical research to provide an insight into the autonomic control of the cardiovascular system. Measurement of HRV is generally performed under a relaxed resting state. The effects of other conditions on HRV measurement, such as running, mountaineering, head-up tilt, etc, have also been investigated. This study aimed to explore whether an inflation-and-deflation process applied to a unilateral upper arm cuff would influence the HRV measurement. Fifty healthy young volunteers aged between 21 and 30 were enrolled in this study. Electrocardiogram (ECG) signals were recorded for each subject over a five minute resting state followed by a five minute external-cuff-inflation-and-deflation state (ECID state). A one minute gap was scheduled between the two measurements. Consecutive RR intervals in the ECG were extracted automatically to form the HRV data for each of the two states. Time domain (SDNN, RMSSD and PNN50), frequency domain (LFn, HFn and LF/HF) and nonlinear (VLI, VAI and SampEn) HRV indices were analyzed and compared between the two states. In addition, the effects of mean artery pressure (MAP) and heart rate (HR) on the aforementioned HRV indices were assessed for the two states, respectively, by Pearson correlation analysis. The results showed no significant difference in all aforementioned HRV indices between the resting and the ECID states (all p  >  0.05). The corresponding HRV indices had significant positive correlation (all p  <  0.01) between the two states. None of the indices showed MAP-related change (all p  >  0.05) for either state. Besides, none of the indices showed HR-related change (all p  >  0.05) for either state except the index of VLI in the resting state. To conclude, this pilot study suggested that the applied ECID process hardly influenced those commonly used HRV indices. It would thus be applicable to simultaneously measure both blood pressure and HRV indices in clinical practice.

Heart rate variability and critical flicker fusion frequency changes during and after parachute jumping in experienced skydivers

PURPOSE

The purpose of this study was (1) to further explore the heart rate dynamics and assess a potential cardiovascular risk in response to 4000 m jumps in experienced skydivers; (2) to assess whether there is an impact of such jumps on skydivers' cortical arousal or not, which may impact their decision making processes.

METHOD

18 experienced skydivers performed successive jumps from a plane at 4000 m of height. Heart rate dynamics and cortical arousal were assessed by the use of heart rate variability and Critical Flicker Fusion Frequency (CFFF), respectively.

RESULTS

CFFF did not differ between the three measurement time points (p > 0.05). Mean heart rate increased during the jump (p < 0.001) and came back to pre-jump values after the jump (p < 0.001). Percentage of the differences of successive NN intervals greater than 50 ms (pNN50) decreased during the jump (p < 0.001) and kept lower values after the jump compared to pre-jump (p < 0.05). High-frequency power (HF) did not differ during the jump (p > 0.05) but decreased after the jump compared to both pre-jump (p < 0.01) and jump (p < 0.05). Sample entropy decreased during the jump (p < 0.001) and came back to pre-jump values after the jump (p > 0.05).

CONCLUSION

These results confirm a vagal input reduction associated with a rise of the sympathetic tone during the jump and suggests that the experienced skydiver is not exposed to a high cardiovascular risk. This study also shows that environmental stresses induced by free fall could not hamper the perceptual vigilance of experienced skydivers.

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.

Correlation between blood pressure changes and AMS, sleeping quality and exercise upon high-altitude exposure in young Chinese men

BACKGROUND

Excessive elevation of arterial blood pressure (BP) at high altitude can be detrimental to our health due to acute mountain sickness (AMS) or some AMS symptoms. This prospective and observational study aimed to elucidate blood pressure changes induced by exposure to high-altitude hypoxia and the relationships of these changes with AMS prevalence, AMS severity, sleep quality and exercise condition in healthy young men.

METHODS

A prospective observational study was performed in 931 male young adults exposed to high altitude at 3,700 m (Lhasa) from low altitude (LA, 500 m). Blood pressure measurement and AMS symptom questionnaires were performed at LA and on day 1, 3, 5, and 7 of exposure to high altitude. Lake Louise criteria were used to diagnose AMS. Likewise, the Athens Insomnia Scale (AIS) and the Epworth Sleepiness Scale (ESS) were filled out at LA and on day 1, 3, and 7 of exposure to high altitude.

RESULTS

After acute exposure to 3,700 m, diastolic blood pressure (DBP) and mean arterial blood pressure (MABP) rose gradually and continually (P < 0.05). Analysis showed a relationship with AMS for only MABP (P < 0.05) but not for SBP and DBP (P > 0.05). Poor sleeping quality was generally associated with higher SBP or DBP at high altitude, although inconsistent results were obtained at different time (P < 0.05). SBP and Pulse BP increased noticeably after high-altitude exercise (P < 0.05).

CONCLUSIONS

Our data demonstrate notable blood pressure changes under exposure to different high-altitude conditions: 1) BP increased over time. 2) Higher BP generally accompanied poor sleeping quality and higher incidence of AMS. 3) SBP and Pulse BP were higher after high-altitude exercise. Therefore, we should put more effort into monitoring BP after exposure to high altitude in order to guard against excessive increases in BP.

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.

Heart rate variability during two sequential mountaineering expeditions

This study was undertaken to assess the duration of altitude acclimatization retention in individuals after initial exposure to a maximum altitude of 5360 m during a mountaineering expedition. Spectral heart rate variability analysis accompanied by an assessment of acute mountain sickness using the Lake Louise Scoring System was performed during two sequential mountaineering expeditions to altitudes of 5360 m and 5642 m, with a period of 30 days between each expedition. Subjects displayed varying degrees of alterations in heart rate variability during the initial expedition, which indicated differing degrees of dysadaptation and stress development. Their Lake Louise Scores accounted for the presence of acute mountain sickness throughout the trip. During the subsequent expedition, the subjects' heart rate variability measures were within the normal range, and there were no signs of acute mountain sickness. All three subjects who underwent step-by-step exposure to altitudes of 5360 m displayed differing degrees of alterations in heart rate variability in conjunction with differing degrees of acute mountain sickness. All subjects also developed acclimatization to hypoxic conditions at this altitude, which was preserved for 30 days, and was sufficient to prevent them from showing any significant alterations in heart rate variability when re-exposed to the same altitude.

Effects of altitude in high-rise building on the autonomic nervous modulation in healthy subjects

This study intended to study the effects of altitude in the high-rise building on the automatic nervous modulation in healthy subjects. Heart rate variability (HRV) analysis was performed to assess the automatic nervous modulation of the subjects at three different altitudes in the air-conditioned high-rise building, i.e., the first basement (4 m beneath sea level), the 31st floor (133 m above sea level), and the 46 th floor (200 m above sea level). We found that the heart rate was significantly decreased, whereas the standard deviation of RR intervals (SD(RR)), total power and high frequency power were significantly increased when the subject was elevated to a higher altitude. The normalized low frequency power and low-/high-frequency power ratio on the 31st and 46 th floors were significantly different between genders; however, no such difference was found on the first basement. The age correlated significantly and positively with the percentage change in the SD(RR) and coefficient of variation of RR intervals when the subjects were elevated from the first basement to the 46 th floor. In conclusion, higher altitude in an air-conditioned high-rise building can lead to an increase in HRV/vagal modulation. The stay at a higher altitude in a high-rise building may lead to increased overall HRV and vagal modulation of a subject, especially for the elder people and the people who had a small HRV at ground level.

'Cross-adaptation': habituation to short repeated cold-water immersions affects the response to acute hypoxia in humans

Adaptation to an environmental stressor is usually studied in isolation, yet these stressors are often encountered in combination in the field, an example being cold and hypoxia at altitude. There has been a paucity of research in this area, although work with rodents indicates that habituation to repeated short cold exposures has a cross-adaptive effect during hypoxia. The present study tested the hypothesis that cross-adaptation is also possible with humans. Thirty-two male volunteers were exposed to 10 min bouts of normoxic and hypoxic (FIO2 0.12) rest and exercise (100 W on a recumbent cycle ergometer). These were repeated after a 96 h interval, during which participants completed six, 5 min immersions in either cold (12°C, CW) or thermoneutral water (35°C, TW). Venous blood samples were taken immediately after each bout, for determination of catecholamine concentrations. A three-lead ECG was recorded throughout and the final 5 min of each bout was analysed for heart rate variability using fast fourier transformations (and displayed as log transformed data (ln)). In comparison with the first hypoxic exercise exposure, the second exposure of the CW group resulted in an increased ln high frequency (ln HF) power (P < 0.001) and reduced adrenaline (P < 0.001) and noradrenaline concentrations (P < 0.001). Adrenaline and noradrenaline concentrations were lower in the CW group during the second hypoxic exercise compared to the TW group (P = 0.042 and P = 0.003), but ln HF was not. When separated into hypoxic sensitive and hypoxic insensitive subgroups, ln HF was higher in the hypoxic sensitive CW group during the second hypoxic exercise than in any of the other subgroups. Cold habituation reduced the sympathetic response (indicated by the reduced catecholamine concentrations) and elevated the parasympathetic activity (increased ln HF power) to hypoxic exercise. These data suggest a generic autonomic cross-adaptive effect between cold habituation and exposure to acute hypoxia in humans.

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.

Systemic hypoxia affects cardiac autonomic activity and vascular hemodynamic control modulated by physical stimulation

This study investigates how various hypoxic interventions affect cardiac autonomic activity and hemodynamic control during posture change and the Valsalva maneuver. Ten healthy sedentary men exposed to 12, 15 and 21% O(2) for 1 h in a normobaric hypoxia chamber in a random order. Before and after various O(2) concentrations were administered, subjects performed the sit-up test and Valsalva maneuver, respectively. An impedance plethysmography was utilized to measure blood pressure (BP) and vascular hemodynamics, whereas spectral analysis of heart rate variability (HRV) was performed to determine cardiac autonomic activity. Analytical results can be summarized as follows: while the patient rests in a supine position, exposure to 12% O(2) reduces the ratio of lower to upper extremity systolic BP, which is accompanied by (1) suppressed arterial reactive hyperemia and increased venous flow resistance, as well as (2) decreased total power and high frequency (HF) and increased low frequency (LF) and the ratio of LF to HF. Moreover, the hypoxia-induced changes of time and frequency domains in HRV at resting supine disappear following the sit-up test, whereas this hypoxic exposure attenuates the BP and heart rate responses to the Valsalva maneuver. Conversely, resting and physical stimuli-mediated HRV and vascular hemodynamic values are unaltered by both 15 and 21% O(2) exposures. We conclude that acute hypoxic exposure affects cardiovascular autonomic functions, with reactions determined by the intervening O(2) concentrations. Moreover, the BP and cardiac autonomic responses to 12% O(2), but not 15% O(2), exposure are depressed while performing posture change and the Valsalva maneuver.

Effect of rapid ascent to high altitude on autonomic cardiovascular modulation

BACKGROUND

Effect of acute hypobaric hypoxia on autonomic nervous activities remains unclear. We evaluated the effect of rapid ascent to high altitude on autonomic cardiovascular modulation and compared the differences between the subjects with and without acute mountain sickness (AMS).

METHOD

Twenty-seven unacclimatized healthy subjects were included for this study. The sleep and study altitude (3180 m) was reached by car from low level (555 m) within 3 hours. The stationary spectral heart rate variability was measured 3 days before ascent (T0), 2 nights at high altitude (T1 and T2), and 2 days after descent (T3). AMS occurrence was evaluated by the Lake Louise score system.

RESULTS

At high altitude, RR intervals (RRI), standard deviation of RRI (SDRR), total power (TP), low-frequency power (LF), high-frequency power (HF), and normalized HF decreased significantly but normalized LF and LF/HF ratio increased significantly in subjects irrespective of AMS. AMS developed in 13 of 27 (48.1%) subjects. Compared with the data at T1, SDRR, TP, LF, and HF increased at T2 in AMS group but decreased in non-AMS group, and the differences in these variables (data at T2 minus data at T1) between the 2 groups showed statistical significance.

CONCLUSIONS

After rapid ascent to high altitude, autonomic nervous activities were suppressed and sympathetic activity was relatively predominant. At high altitude, the discordant changes in SDRR, TP, LF, and HF may reflect varying capacity of acute hypobaric hypoxic adaptation between the subjects with and without AMS.

The effect of sucrose ingestion on autonomic nervous system function in young subjects during acute moderate hypoxia

Cardiac arrhythmias are associated with an increase in sympathetic activity (reflected in increased heart rate) and a simultaneous decrease in rhythmical fluctuations of sympathetic activity [reflected in decreased heart rate variability (HRV)]. As hypoxia is a well known trigger for cardiac arrhythmias, and carbohydrate loading a known sympatho-excitatory stimulus, the present study investigated if carbohydrate loading affects the cardiac response to acute hypoxic challenge. Fourteen subjects ingested a sucrose solution or an equal volume of water and spectral analysis of HRV was used to determine HRV components in normoxia and acute, normobaric hypoxia. Compared to the control condition, ingestion of carbohydrates increased heart rate, spectral power of nLF (P < 0.02) and LF/HF ratio (P < 0.003), and decreased spectral power of nHF (P < 0.03) during hypoxia. Carbohydrate ingestion thus intensified cardiac autonomic modulation during acute hypoxia and may therefore act as a beneficial protective mechanism against the disturbances of cardiac rhythm in hypoxic conditions.

EEG, ECG and oxygen concentration changes from sea level to a simulated altitude of 4000m and back to sea level

In order to describe how high altitude affects the body during a one night stay at 4000m experiments were performed in a hypobaric chamber and compared to a study on Dachstein (mountain in Austria, 2700m). Ten subjects had to perform a reaction time task at different altitudes. The EEG and ECG were recorded simultaneously. Additionally, the oxygen saturation of the blood was measured at different altitudes and the subjects filled out a Lake Louise questionnaire that describes the degree of altitude mountain sickness (AMS). After elevation from 134m to 4000m in the hypobaric chamber heart-rate increased from 68.9bpm to 81.6bpm, RMSSD (root mean square of squared differences of adjacent heart beat intervals) decreased from 54.3ms to 33.3ms, the LF/HF ratio increased from 2.5 to 3.9 and oxygen saturation decreased to 82.7% after 11h at 4000m altitude. The Lake Louise Score (LSS) reached 3.4 after one night at 4000m. EEG beta activity between 14Hz and 18Hz was attenuated at 4000m and also after return to 134m. The results indicate that the subjects were not able to adapt to 4000m within 12h in the hypobaric chamber. Even after 1h after the return to 134m all parameters are still affected from the night at 4000m altitude. ECG and EEG changes are in line with results obtained at 2700m height at Dachstein.

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.

Peripheral circulation monitored by surface temperature and autonomic nervous function in hypobaric hypoxic environment: effects of submaximal exercise

Hypothermia and frostbite are frequently seen in accidents in remote wilderness environment, especially in hypobaric hypoxic conditions. The aim of this study was to clarify how hypobaric hypoxic conditions affects peripheral circulation. Peripheral skin temperature and autonomic nervous functions were assessed in two 1000-m ascent exercises. Subjects (n = 15) ascended from 1000 m above sea level in Study 1, and ascended from 2400 m in Study 2. Conditions other than environmental oxygen pressure were mostly identical in both studies. The autonomic nervous activities were decreased solely in Study 2. The relative sympathetic activity was significantly increased in the lower barometric pressure in Study 2 (p < 0.01). Peripheral skin temperature was significantly decreased after the exercise in Study 2 (p < 0.01). In conclusion, hypobaric hypoxia itself induced peripheral low temperature during exercise at high altitudes. Relative sympathetic hyperactivity may be responsible for the compromised peripheral circulation.

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.

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.

Autonomic cardiovascular regulation in subjects with acute mountain sickness

The aims of this study were 1) to evaluate whether subjects suffering from acute mountain sickness (AMS) during exposure to high altitude have signs of autonomic dysfunction and 2) to verify whether autonomic variables at low altitude may identify subjects who are prone to develop AMS. Forty-one mountaineers were studied at 4,559-m altitude. AMS was diagnosed using the Lake Louise score, and autonomic cardiovascular function was explored using spectral analysis of R-R interval and blood pressure (BP) variability on 10-min resting recordings. Seventeen subjects (41%) had AMS. Subjects with AMS were older than those without AMS ( P < 0.01). At high altitude, the low-frequency (LF) component of systolic BP variability (LFSBP) was higher ( P = 0.02) and the LF component of R-R variability in normalized units (LFRRNU) was lower ( P = 0.001) in subjects with AMS. After 3 mo, 21 subjects (43% with AMS) repeated the evaluation at low altitude at rest and in response to a hypoxic gas mixture. LFRRNU was similar in the two groups at baseline and during hypoxia at low altitude but increased only in subjects without AMS at high altitude ( P < 0.001) and did not change between low and high altitude in subjects with AMS. Conversely, LFSBP increased significantly during short-term hypoxia only in subjects with AMS, who also had higher resting BP ( P < 0.05) than those without AMS. Autonomic cardiovascular dysfunction accompanies AMS. Marked LFSBP response to short-term hypoxia identifies AMS-prone subjects, supporting the potential role of an exaggerated individual chemoreflex vasoconstrictive response to hypoxia in the genesis of AMS.

Effects of a fast cable car ascent to an altitude of 2700 meters on EEG and ECG

In the Eastern Alps, the Dachstein massif with a height of almost 3000 m is an ideal location for investigating the effects of changes in altitude on the human body. Within a few minutes, a cable car facilitates an ascent from 1702 to 2700 m above sea level, where the partial pressure of oxygen is about 550 mmHg (as compared to 760 mmHg at sea level). In this study, 10 healthy subjects performed a reaction time task at 990 m and 2700 m in altitude. The subjects were instructed to perform a right hand index finger movement as fast as possible after a green light flashed (repeated 50 times). The corresponding electrocardiogram (ECG) and the electroencephalogram (EEG) were recorded. From the ECG heart rate and heart rate variability measures in the time and frequency domain were calculated. An event-related desynchronization/synchronization (ERD/ERS) analysis was performed with the EEG data. Finally, the EEG activity and the ECG parameters were correlated. The study showed that with the fast ascent to 2700 m the heart rate increased and the heart rate variability measures decreased. The correlation analysis indicated a close relationship between the EEG activity and the heart rate and heart rate variability. Furthermore it was shown for the first time that the beta ERS in the 14-18 Hz frequency range (post-movement beta ERS) was significantly reduced at high altitude. Very interesting also is the loss of correlation between EEG activity and cardiovascular measures during finger movement at high altitude. The suppressed post-movement beta ERS at the altitude of 2700 m may be interpreted as results of an increased cortical excitability level when compared with the reference altitude at 990 m above sea level.

Relationship between arterial oxygen saturation and heart rate variability at high altitudes

Autonomic nervous systems have important roles for survival of victims under hypobaric hypoxic condition. In the present study, we assessed the correlation between arterial oxygen saturation (Sp O 2 ) and heart rate variability (HRV) to identify the autonomic nervous responsiveness among trekkers at high altitude (n = 21). HRV was analyzed by the maximum entropy method. Sp O 2 among subjects at 3456 m (495 mm Hg) was 80% +/- 5% (mean +/- SD; range, 69%-93%). Sp O 2 and percentile entropy, and Sp O 2 and low-frequency variability, had positive correlation ( r = 0.455 and 0.518, respectively). Sp O 2 value among subjects with mountain sickness symptoms was not different from that among subjects without the symptoms. In conclusion, autonomic responses among high-altitude trekkers may be blunted under hypobaric hypoxic conditions. Deterioration of autonomic function measured by HRV might be more sensitive to hypoxia than clinical symptoms at high altitudes.

Autonomic control of the cardiovascular system during acclimatization to high altitude: effects of sildenafil

Both acute hypoxia and sildenafil may influence autonomic control through transient cardiovascular effects. In a double-blind study, we investigated whether sildenalfil (Sil) could interfere with cardiovascular effects of hypoxia. Twelve healthy men [placebo (Pla) n = 6; Sil, n = 6] were exposed to an altitude of 4,350 m during 6 days. Treatment was continuously administered from 6 to 8 h after arrival at altitude (3 × 40 mg/day). The autonomic control on the heart was assessed by heart rate variability (HRV) during sleep at sea level (SL) and between day 1–2 and day 5–6 in hypoxia. Arterial pressure (AP) and total peripheral resistances (TPR) were obtained during daytime. There was no statistical difference between groups in HRV, AP, and TPR throughout the study. Hypoxia induced a decrease in R-R interval and an increase in AP in both groups. Low frequency-to-high frequency ratio increased at day 1–2 (Pla, P = 0.04; Sil, P = 0.02) and day 5–6 (Pla and Sil, P = 0.04) vs. SL, whereas normalized high-frequency power decreased only in Pla ( P = 0.04, day 1–2 vs. SL). Normalized low-frequency power increased at high altitude (Pla and Sil, P = 0.04, day 5–6 vs. SL). TPR decreased at day 2 in Pla ( P = 0.02) and tended to normalize at day 6 ( P = 0.07, day 6 vs. day 2). Acute hypoxia induced a decrease in parasympathetic and increase in sympathetic tone, which tended to be reversed with acclimatization. Sil had no deleterious effects on the cardiovascular response to high-altitude exposure and its control by the autonomic nervous system.

Heart rate variability in rats acclimatized to high altitude

The aims of this study were to relate heart morphology and functions changes to heart rate variability (HRV) components after acclimatization to high altitude and to define whether preadaptation to hypoxia could modulate HRV responses to acute hypoxic stress. Doppler-echocardiographic studies of the left ventricle were performed in female Wistar rats before, during, and after a 10-week exposure to moderate hypobaric hypoxia (CH rats, approximately 4000 m simulated) or normoxia (N rats, approximately 55 m). Right ventricular morphology and function and pulmonary artery pressure were evaluated using heart catheterization. Spectral analysis of HRV was studied after exposure in conscious unrestrained rats in normoxia and during acute hypoxic stress. Necropsy right ventricular hypertrophy and intraventricular and pulmonary artery hypertension were found in CH rats compared with N rats. Echocardiographic left ventricular morphology and functions were similar between the groups after exposures. Compared to the control group, CH rats had similar heart rates and HRV components when measured in normoxia. During acute hypoxic stress, HRV decreased in all rats, but less in CH rats. These results support the hypothesis that long-term mild hypoxia may moderate sympathetic activation induced by acute hypoxia and that right ventricular hypertrophy cannot be the direct cause of such a shift in sympathovagal nerve interaction during acute hypoxic stress.