Heart Rate Variability , Standard of Measurement , Physiological Interpretation and Clinical Use in Mountain Marathon Runners during Sleep and after Acclimatization at 3480 m

Fluctuations in autonomic cardiovascular regulation during exposure to high altitude may increase the risk of heart attack during waking and sleep. This study compared heart rate variability (HVR) and its components during sleep at low altitude and after 30 41 hours of acclimatization at high altitude (3480 m) in five mountain marathon runners controlled for diet, drugs, light-dark cycle and jet lag. In comparison to sea level, RR-intervals during sleep at high altitude decreased significantly (P < 0.001). The significant increase in sympathetic autonomic cardiovascular modulation at high altitude protects against excessive oxygen deprivation during sleep. Increases in R-R intervals can require longer periods of acclimatization at 3480 m to mitigate the effects of altitude/hypoxia on sympathetic tone, thus reducing cardiovascular distress at rest during waking and sleep and probably before during and after athletic performance at altitude.


Introduction 1.Heart Rate Variability
Intrinsic to pacemaker tissues, cardiac automaticity is regulated by the central nervous systems (CNS).Control of the cardiac cycle is also mediated by local and autonomic nervous system components: the parasympathetic influence on heart rate is modulated by acetylcholine released by the vagus nerve on the sinoatrial node and the sympathetic influence by the release of epinephrine and norepinephrine.Under resting conditions, vagal modulation and tone predominate at the level of the sinoatrial node.Vagal and sympathetic activities interact constantly [1 hereinafter Task Force 1996].
In the sympathovagal and thoracic systems, CNS control and influences on the autonomic mechanism can be physiologically and voluntarily cut off to different degrees during relaxed attentive waking and involuntarily during the progressive deepening of slow-wave sleep [1].
Alterations in the autonomic nervous system may give rise to cardiovascular and/or cerebrovascular diseases and have been frequently associated with death in humans.Research into predisposition to arrhythmias and increased sympathetic activity or reduced vagal activity has led to the development of quantitative markers of autonomic activity [1].
The Task Force [1] has suggested that cardiovascular changes can be investigated non-invasively by electrocardiography (ECG) and by common parameters derived from ECG such as heart rate variability (HRV), i.e., the variation in the duration of two consecutive R-R intervals.R-R interval variations during resting condition are precisely tuned by reflexes directed to the sinus node and modulated by central (vasomotor and respiratory centre) and peripheral (arterial pressure and respiratory movements) oscillators, particularly during high altitude exposure.Analysis of R-R intervals provides information about the state and function of central oscillators, sympathetic and vagal efferent activities, humoral cardiac factors, and sinus node characteristics [1].
It has also been reported by the Task Force [1] and by Lanfranchi et al. [2] that the risk of cardiac disease can be evaluated by means of spectral analysis of the variability in the R-R interval in order to determine highfrequency ([HF] 0.15 -0.4 Hz) rhythm, which primarily reflects respiratory-driven vagal modulation of sinus rhythm, and low-frequency ([LF] 0.04 -0.15 Hz) rhythm, which appears to have a widespread neuronal genesis.LF is also considered as a marker of sympathetic modulation (expressed in normalized units) and/or as a parameter that includes both sympathetic and vagal modulation.Thermoregulation-related HRV, so-called very low frequency fluctuation ([VLF] <0.04 Hz) rhythm is also used to analyze HRV.However, explanation of the VLF component of HRV is less defined than the LF or the HF component.VLF, LF and HF power are usually measured in absolute values of power (milliseconds squared [ms 2 ]).LF and HF can be also measured in normalized units (NU) to emphasize the controlled and balanced behavior of the two branches of the autonomic nervous system, as well as baroreflex responsiveness to beat-to-beat variations in arterial blood pressure [1].Normalization of LF and HF power tends to minimize the effect of the changes in the total power on the values of these two components.Normalized units and absolute values of LF and HF power should both be calculated to provide a better measurement of the degree of autonomic modulation rather than just the level of autonomic tone [1].
To date, little has been reported about the effect of different environments (type of nature, physical activity, emotional circumstances, environment of the group) on HRV analysis [1].

Hypobaric-Hypoxic Conditions
Hypoxia affects ventilator control circuits and autonomic cardiovascular regulatory mechanisms in normal subjects and in those with cardiac and/or respiratory failure.In hypobaric-hypoxic conditions, HRV analysis can be considered as an expression of the changes in respiratory frequency oscillation and of respiratory sinus arrhythmia not mediated by the beta adrenergic block, yet modulated by the vagus nerve; furthermore, changes in the cardiac vagal nervous system result in proportional changes in R-R intervals.During exposure to hypobaric-hypoxic and during waking conditions, HRV is reduced, with a relative increase in the LF component.In mountaineers, the relative increase in the LF component is thought to be due to increased sympathetic modulation of the sinus node in response to high altitude.Acute exposure to hypobaric-hypoxic conditions at high altitude increases the risk of cardiovascular stroke, heart attack and death [2].

Sleep-Wake Cycle
At low altitudes, HR is normally higher during daytime hours and lower at night.During wakefulness, HRV oscillates in relation to physical activity; during the sleep cycle it changes with the passage from non-rapid eye movement (NREM) to rapid eye movement (REM) sleep to awakening periods during sleep (W) episodes [3].Changes in HR may precede changes on the electroencephalogram (EEG).Shorter R-R intervals are believed to reflect sympathetic dominance and are associated with waking and REM sleep, while longer R-R intervals reflect vagal dominance probably coincident with sleep dampening.This allows for HRV analysis in the LF, HF, and LH:HF frequency domains as a tool for exploring sympathovagal balance continuously during sleep at altitude.To our knowledge, HRV during the nocturnal sleepwake cycle at high altitudes in humans has been less investigated than at low altitude in humans and animals.

Environment and Physical Activity of Mountain Marathon Athletes
Mountain marathoners, also called sky runners, are athletes who perform marathons and races at high altitude.
Their anthropometric characteristics are similar to those of marathoners competing at sea level.Various physiological, biochemical, hematological and psychological parameters studied in these athletes during waking, before, during, and after races have shown that changes in these parameters are transient, promptly return to normal, and produce no evident clinical symptoms or diseases.
Unlike the situation described in Lanfranchi and coworkers [2], the medical staff involved in this study, during the long-term follow-up of a group of mountain marathon athletes during training and athletic competition from sea level to 5500 m, has never recorded signs of acute mountain sickness (AMS) [4][5][6][7][8][9][10][11][12][13].Regular exercise is thought to modify autonomic balance and accelerate the safe recovery of physiological sympathovagal interaction [1].Exercise and training of the mountain marathon runners might have decreased their risk of cardiovascular mortality and sudden cardiac death at low and high altitudes, as well as prevented syncope episodes, which can occur after the end of races at high altitude [12].

Mountain Environments
When undertaken in mountain environments, studies on humans typically lack the controlled conditions of the laboratory.High-altitude research, as in the present study, is limited to a small number of anthropometrically controlled subjects.Even so, for future high and very high altitude expeditions, studies on anthropometrically controlled subjects transiently and naturally exposed to the mountain environment can give, albeit under less rigorously controlled conditions, important insights into HRV that may not completely be gleaned from sea-level laboratory studies.The data presented here were collected during sleep, in sea-level native mountain marathon runners, at 122 m and at an altitude of 3480 m, in clinostatic position so as to avoid, or at least reduce, autonomic mechanisms correlated with the central and peripheral autonomic nervous systems and effects related to the time of eating, jet lag, light-dark cycle and motor activities [14].

Aim of the Study
Given the direct and indirect physiological effect of hypobaric-hypoxia on the cardiovascular system [15], fluctuations in autonomic cardiovascular regulation during exposure to high altitude [2], the increased risk of heart attack, and the instability of the cardiovascular system during sleep, this study analyzed HRV by calculating the average of the spectral component of stacked series of sequential power spectra from short ECG segments lasting 0.5 minutes [16] during sleep: at sea level and between 30 and 41 h of acclimatization at 3480 m altitude in a small sample of anthropometrically well characterized mountain marathon runners [4][5][6][7][8][9][10][11][12][13].The analysis of HRV during sleep at 3480 m can provide firmer ground for studying and diagnosing overtraining at high altitude of mountain marathon runners exposed to hypobaric-hypoxic conditions.HRV measurements at high altitude may offer useful data for standard physiological evaluation and for formulating recommendations on increasing or reducing acclimatization time to defined hypobarichypoxic conditions and reduce cardiovascular distress at low altitude and during performance at high altitude in particular.
The average body weight for the five subjects was 65.8 ± 4 kg; the average height was 176 ± 3.7 cm and the average aerobic power was 61.4 ± 2.7 ml·kg −1 ·min −1 .The use of any drugs, dietary and neuroactive supplements was suspended for one week before the start of the study.Effects of light-dark and jet-lag interference on acclimatization were excluded by the location of the study.

Polysomnographic Recording
Polysomnographic recording procedures were carried out in accordance with Directive 86/609/EEC for experimental human care.Informed consent prior to each experimental session was given by all five subjects and by the international medical staff of the Federation of Sport at Altitude.The study was conducted during normal sleep time (between 10 p.m. and 9 a.m.).Workouts were suspended on the days the measurements were taken.
Polysomnographic recordings were taken in dedicated dark, isolated, silent rooms at 122 m (Milan, Italy) at a barometric pressure (P B ) of 742 ± 7.7 mm Hg and after an acclimatization period of 30 -32 or 38 -41 h after reaching 3480 m at a P B of 495.4 ± 3.19 mm Hg.The recordings were performed to study the electroencephalogram (EEG), submental electromyogram (EMG), electrooculogram (EOG), electrocardiogram (ECG) and the percent of peripheral arterial oxygen saturation (%SpaO 2 ) signals.All signals were amplified and registered at a sampling rate of 250 Hz, then analyzed off-line according to standard criteria (Somnological 3, EmblaMedcare, Flaga ® , Monza, Italy).Electrocardiographic recordings were taken with a bipolar derivation from two cardiac electrodes placed in V 2 in the fourth left intercostal region along the sternum, and in V 4 in the fifth left intercostal region on the hemiclavear line.
The polysomnographic tracings containing the ECG signals were scored as follows: awakening during sleep (W), S1 + S2 and S3 + S4 of slow-wave sleep, also called NREM sleep, and REM sleep, according to standard criteria developed by Rechtschaffen and Kales [17] in 30-second artifact-free epochs [16].
Automatic analysis of HRV values was performed using Somnological 3 software (Embla  ), autoregressive model, order 12, following the rules of the Task Force [1] in a total of stacked series of sequential 30-second artifact-free epochs of awakening during sleep (W), S1 + S2 and S3 + S4 of NREM sleep, and of REM sleep.This analysis focused on the average of the R-R intervals, the power of the very low frequency (VLF), low frequency (LF) and high frequency (HF), the normalized unit of the low Frequency and high frequency (LF RR NU and HF RR NU), which were obtained by dividing the power of each component by the total variance, from which the VLF was subtracted, and by multiplying them by 100, and on the total power (TP) [Task Force 1].

Statistical Analysis
The results are expressed as the mean ± standard deviation (SD) or standard error mean (SEM).Probabilities lower than P < 0.05 were accepted as significant.ANOVA and post hoc tests were also performed.Simple linear regression analysis between the (%SpaO 2 , PCO 2 ) (personal observations) and the ECG parameters was also performed using the Stat-View program.

Results
None of the five subjects ever experienced AMS during the present study.There were no differences in the time of evaluation of the subjects.During sleep the %SpaO 2 and PCO 2 at high altitude was significantly lower than that recorded at sea level (average ±SD: %SpaO 2 80 ± 3.64 at 3480 m vs 95.6 ± 0.85 at 122 m; P < 0.05; PCO 2 : 28.25 ± 2.33 at 3480 m vs 41.21 ± 3.38 at 122 m).

Sea Level (122 m)
At 122 m, the averages of the recorded R-R intervals were similar between all sleep stages and ranged from 1223 ms during the awakening period during sleep (W), 1300 ms during S1 + S2 and 1262 ms during S3 + S4 of NREM sleep, to 1272 ms during REM sleep (Table 1).The averages of the R-R intervals recorded at 122 m during the nocturnal sleep-wake cycle suggested a prevalence of vagal tone during all four sleep stages and in all five subjects.

High Altitude (3480 m)
At 3480 m, the averages of the R-R intervals recorded during sleep ranged from 935 ms during W, 1054 ms during S1 + S2 and 993 ms during S3 + S4 of NREM sleep, to 990 ms during REM sleep.
The averages of the R-R intervals recorded at altitude during the awakening period during sleep (W: 935 ± 88) were significantly shorter than those recorded during S1 + S2 (1054 ± 73; P < 0.05) (Table 1).The averages of the R-R intervals recorded during sleep suggested an increase in sympathetic tone during W, S3 + S4 NREM sleep and REM sleep, and a persistent significant increase in vagal tone during the light phases of NREM sleep.

Sea Level (122 m) and High Altitude (3480 m)
Changes in ECG, with a reduction in the R-R intervals (ms) recorded during sleep, became evident between 30 and 41 h of acclimatization at 3480 m compared to measurements taken at 122 m during sleep: signs of sinus arrhythmia during periodic breathing; during S1 + S2 of NREM sleep, and during REM sleep were evident (Table 1).In all five mountain marathon runners, the averages of the R-R intervals during W, S1 + S2 and S3 + S4 of NREM sleep, and REM sleep were significantly shorter at altitude than those recorded at sea level (P < 0.01 -P < 0.001).
Simple regression analysis between the average %SpaO 2 during sleep at low and at high altitudes was significantly correlated (DF 1,9 R-squared 0.477, coefficient 623.58,F-test 7,31 , P = 0.0269, t = 2.704) with the average changes in R-R intervals.Footnotes to Table 2: (1) One-way ANOVA showed that in the five subjects the averages of the total power of very low frequency (VLF, ms 2 ) (<0.04 Hz) (thermoregulation-related HRV) differed significantly between stages (DF = 7, 32,39 ; F = 5.15; P < 0.0002).Post-hoc comparison with Fisher analysis of the mean revealed a significant difference between the VLF values recorded during W at 122 m and those during stages S1 + S2 at 122 m (P < 0.05), and a significant difference between the VLF values recorded during stages S1 + S2 at 3480 m and those during stages S3 + S4 at 122 m and S3 + S4 at 3480 m.There was also a significant difference between the mean of the VLF recorded during stages S3 + S4 at 122 m and those during REM sleep at 122 m and at 3480 m (P < 0.05).

Total Power of Very
There was a significant difference between the average of the VLF recorded during stages S3 + S4 at 3480 m and those during REM sleep at 122 m and at 3480 m (P < 0.05).* Post-hoc comparison with Student'S t-test showed that the power in the VLF range of the R-R intervals measured in 190 30-second epochs during the waking period during sleep (W) at 122 m was significantly higher (P < 0.0046) than that measured in 226 30-second signal epochs during stages S3 + S4 of NREM sleep at 122 m in all five subjects.** Post-hoc comparison with Student's t-test of the mean of the average of the power in the VLF range measured in 271 30-second epochs during the awakening period during sleep (W) at 3480 m was significantly higher (P < 0.0007) than that measured in 106 30-second signal epochs during stages S3+S4 of NREM sleep at 3480 m in all five subjects.Overall, the averaged data recorded during sleep at 3480 m demonstrate that during the deepening of synchronized sleep the thermoregulation-related component (VLF) of HRV decreased, with a physiological increase in vagal tone.During REM sleep the thermoregulationrelated component of HRV approached that recorded during the awakening period during sleep (W) indicating an increase in the sympathetic tone .

Sea Level (122 m) and High Altitude (3480 m)
The average of the total power of very low frequency (VLF, ms 2 ) recorded during S1 + S2 of NREM sleep (5979 ± 6471) was significantly shorter at high altitude than that recorded sea level (10,545 ± 9710; P < 0.005) in only 1/5 (GM) mountain marathon runners (Table 2).3).These data suggest that the average values of LF, a marker of sympathetic modulation, decreased during the deepening phases of NREM sleep.During REM sleep the average values of LF significantly increased, approaching a value similar to that observed during W.

High Altitude (3480 m)
At 3480 m, the averages of LF were: 6272 ms 2 during W; 7516 ms 2 during S1 + S2; 3805 ms 2 during S3 + S4; and 5715 ms 2 during REM sleep (Table 3).The results demonstrate that the average values of LF during sleep at altitude did not change significantly between the different sleep stages (Table 3).

Sea Level (122 m) and High Altitude (3480 m)
The averages of total power of low frequency (LF ms   Footnotes to Table 4: (1) One-way ANOVA revealed significant differences in the averages of the total power in low frequency (0.04 -0. .These data show a great variability between mountain marathon runners in LF RR NU when the data set recorded at 122 m is compared with that recorded at 3480 m.Analysis of the data recorded in individual mountain marathon runners showed an increase in the average LF RR NU at 3480 m, demonstrating an increase in the marker for sympathetic modulation.The average HF at 3480 m recorded in individual mountain marathon runners was generally lower than that recorded at 122 m, suggesting a decrease in the marker of vagal tone at altitude.Footnotes to Table 5: (1) One-way ANOVA revealed a significant difference between stages in the averages of the total power in the high frequency (HF, ms 2 ) (0.15-0.4 Hz) range (DF = 7 32 , 39 ; F = 2,576, P = 0,0317).Post-hoc Fisher analysis showed significant differences between the averages of HF during W at 122 m and during stages S3 + S4 at 122 m (P < 0.05).There was a significant difference between the values measured during W at 3480 m and those during stages S3 + S4 at 122 m (P < 0.05

Sea Level (122 m) and High Altitude (3480 m)
The average of the LF:HF ratio (  Footnotes to Table 6: (1) One-way ANOVA of the averages of the total power in the high frequency (0.04 -0.15 Hz) range in normalized units (HF-RR -NU) showed significant differences between conditions (DF7 ,32,39 ; F 4071; P < 0.0027).Post-hoc analysis demonstrated a significant difference between the average HF-RR -NU during W at 122 m and that during stages S3 + S4 at 122 m (P < 0.05).The averages of HF-RR -NU during W at 3480 m differed significantly from those during stages S3 + S4 at 122 (P < 0.05).The averages of HF-RR-NU during W at 122 m differed significantly from those during stages S3 + S4 at 122 m (P < 0.05).The average of HF-RR -NU during stages S1 + S2 at 122 m differed significantly from that during stages S3 + S4 at 3480 m (P < 0.05).The average of HF-RR-NU during stages S1 + S2 at 3480 m differed significantly from that during stages S3 + S4 at 122 m (P < 0.05).The average of HF-RR-NU during stages S3 + S4 at 122 m differed significantly from that during stages S3 + S4 at 3480 m (P < 0.05).Footnotes to Table 7: (1) One-way ANOVA showed no significant differences in the LF/HF ratio in the different conditions.* Student's t-test showed that the average of the total power of the LF/HF ratio during the waking state during sleep (W) (190 epochs) at 122 m was generally higher than the average LF/HF ratio during stages S3 + S4 (226 epochs) at the same altitude in the five subjects (P < 0.0110).** Student's t-test showed that the average of the total power of the LF/HF ratio during stages S3 + S4 in a total of 226 epochs of 30 seconds at low altitude was significantly lower than that in 175 epochs of REM sleep at 122 m in the five subjects (P < 0.011335).

Discussion
Fluctuations in autonomic cardiovascular regulation during exposure to high-altitude environment may increase the risk of heart attack.This study compared heart rate variability (HVR) and its components during sleep: at low altitude and after 30 -41 hours of acclimatization at high altitude (3480 m) in mountain marathon runners controlled for diet, drugs, light-dark cycle and jet lag differences.At altitude, RR-intervals became significantly shorter (P < 0.001).The significant changes in sympathetic/parasympathetic autonomic cardiovascular modulation at high altitude can protect against excessive oxygen deprivation, particularly during sleep, and thus lower the risk of heart attack.Increase in R-R intervals during wake and sleep may require longer periods of acclimatization at 3480 m to mitigate the effects of altitude/hypoxia on the sympathetic tone of the mountain marathon runners, thus reducing cardiovascular distress.

Background
Molecular oxygen is essential for all higher forms of life and brain cell function.During evolution, humans and other animal species developed molecular, biochemical and physiological mechanisms to optimize oxygen utilization efficacy.Humans respond to acute or chronic exposure to hypobaric-hypoxia by resetting the pO 2 balance to ensure brain and heart cell function.Understanding how the body adjusts its biochemical and physiological cellular needs may help to better define the health risks associated with activities in hypobaric-hypoxia conditions and can aid in identifying appropriate therapeutic non-pharmacological and/or pharmacological treatments.Our ability to breathe and to modify breathing according to the amount of available ambient oxygen and to our body's demands (particularly those of the brain, heart and lungs) is essential for survival.Failure to breathe or an inadequate oxygen supply, especially to the brain, contributes to cardiorespiratory distress.In hypoxic conditions, distribution of cardiac output to the cardiovascular beds is subordinate to a preprogrammed priority program for preservation of the organism.In hypoxic conditions, the majority of neuronal systems of the brain increase and/or change their firing rate in order to modulate in a cyclic manner the discharge of the neurons involved in regulating the oxygen supply in mammals [15,16,18,19].
Acute and chronic exposure to hypoxemia leads to all sorts of disturbances that suggest impaired excitatory neuronal functions involved in the behavioral and metabolic integration of autonomic control and arousal [15].Footnotes to Table 8: (1) One-way ANOVA showed a significant difference in the average of the total power (TP) between conditions (DF 7, 32,39 ; F = 5433; P = 0.0004).Post-hoc Fisher analysis revealed a significant difference between the average of the TP recorded at 122 m during waking (W) and that recorded at 122 m during stages S1 + S2 (P < 0.05)and during stages S3 + S4 (P < 0.05).Post-hoc Fisher analysis revealed a significant difference between the average of the TP recorded at 122 m during waking (W) and that at 3480 m during stages S1 + S2 (P < 0.05) and stages S3 + S4 (P < 0.05).Post-hoc Fisher analysis revealed a significant difference between the average of the TP recorded at 122 m during waking (W) and that at 3480 m during REM sleep (P < 0.05).Post-hoc Fisher analysis revealed a significant difference between the average of the TP recorded at 3480 m during stages S3 + S4 and that at 122 m during stages S3 + S4 (P < 0.05).Post-hoc Fisher analysis revealed a significant difference between the average of the TP recorded at 122 m during stages S1 + S2 and that at 3480 m during REM sleep (P < 0.05).Post-hoc Fisher analysis revealed a significant difference between the average of the TP recorded at 3480 m during stages S1 + S2 and that at 122 m during REM sleep (P < 0.05).Post-hoc Fisher analysis revealed a significant difference between the average of the TP recorded at 122 m during stages S3 + S4 and that at 122 m during REM sleep (P < 0.05).Post-hoc Fisher revealed a significant difference between the average of the TP recorded at 3480 m during stages S3 + S4 and that at 122 m during REM sleep (P < 0.05).Post-hoc Fisher analysis revealed a significant difference between the average of the TP recorded at 3480 m during stages S3 + S4 and that at 3480 m during REM sleep (P < 0.05) in the five subjects.* Post-hoc analysis with Student's t-test showed a significant difference (P < 0.05) between the average of the total power (TP, ms 2 ), in a physiologically defined number of 30-second signal epochs, according to standard criteria developed by Rechtschaffen and Kales (1968), recorded during the waking state during sleep (W) (190 epochs) and that during stages S3 + S4 (226 epochs) at 122 m (P < 0.0068) in the five subjects.** Post-hoc analysis with Student's t-test showed a significant difference (P < 0.028) between the average of the total power (TP, ms 2 ), in a physiologically defined number of 30-second signal epochs, according to standard criteria developed by Rechtschaffen and Kales (1968), recorded during the waking state during sleep (W) (271 epochs) and that during stages S3 + S4 (106 epochs) at 3480 m in the five subjects.*** Post-hoc analysis with Student's t-test showed a significant difference (P < 0.003341) between the average of the total power (TP, ms 2 ), in a physiologically defined number of 30-second signal epochs, according to standard criteria developed by Rechtschaffen and Kales (1968), recorded during stages S3 + S4 (226 epochs) and that during REM sleep (175 epochs) at 122 m in the five subjects.**** Post-hoc analysis with Student's t-test showed a significant difference (P < 0.002613) between the average of the total power (TP, ms 2 ), in a physiologically defined number of 30-second signal epochs, according to standard criteria developed by Rechtschaffen and Kales (1968), recorded during stages S3 + S4 (106 epochs) and that during REM sleep (113 epochs) at 3480 m in the five subjects.
Research into the causes of cardiovascular mortality, experimental evidence for a predisposition to fatal arrhythmias, signs of tonic or phasic increased sympathetic activity, and reduced parasympathetic-cholinergic-vagal activity has advanced efforts for the development of quantitative markers of heart rate variability (HRV) (Task Force, [1]).

Heart Rate Variability during Wake and Sleep
HRV has been established as a non-invasive tool to study cardiac autonomic activity and proposed as a predictor for evaluating the increased risk of cardiac death.Interactions of changes in cardiac autonomic nervous modulation in various types of subjects are under study by the Task Force [1].An overnight declining trend of HRV has been found to increase during sleep [14].Otzenberger and co-workers [14] demonstrated that overnight profiles of the R-R intervals are related to changes in the sleep EEG mean frequency sign, which reflects the depth of sleep.
During attentive-to-quiet waking and from the lighter to the deeper phases of sleep, HRV has been shown to be affected by vagal/sympathetic modulation and control.

Heart Rate Variability and Mountain Marathon Runners
Past studies showed that the high-altitude endurance performance of mountain marathoners is appreciably reduced at 5200 m.Based on their clinical characteristics, mountain marathoners can be considered as a physiological model for studying cardiovascular alterations, of long-lasting stay and exercise at high altitude.Over the last 12 years, the time course of cardiovascular changes in the mountain marathon runners has been assessed before, during, and after the end of races at altitude.In sealevel native mountain marathoners, long-lasting training -from sea level to altitude-can lower the HR, during rest and exercise, associated with a decrease in sympathetic modulation and a rise in parasympathetic activity [20].As in normal subjects, so too in well-trained mountain marathoners cardiovascular modifications can change continuously over a 24-h period during acclimatization and the nocturnal sleep-wake-cycle at low and high altitudes.In mountain marathoners, just as in normal subjects, HRV alterations at altitude may include altered resting HR due to changes in the vagal and sympathetic components, besides the appearance of sinus arrhythmia during waking and periodically during nocturnal S1 + S2 NREM sleep and REM sleep breathing.As suggested by the Task Force [1], the present study data were obtained during sleep and as such may add valuable insights for research into HRV.With this study we also wanted to determine whether 20 years of training between 122 m and 5200 m altitude could have exerted, in addition to the changes reported elsewhere [9,12], a high impact on the autonomic/cardiovascular systems of the mountain marathon runners during the nocturnal sleepwake cycle at low and at high altitudes.
Our study documents that, in mountain marathoners, the nocturnal sleep-wake cycle at 122 m is highly influence by autonomic parasympathetic activity and by increased autonomic parasympathetic activity during the deepening of NREM sleep.The results also show a physiological decrease in parasympathetic activity versus an increase in sympathetic activity during all phases of the nocturnal sleep-wake cycle at 3480 m. Surprisingly, our study also documents that the nocturnal sleep-wake cycle at 3480 m is still characterized by an increase in autonomic parasympathetic activity during light S1 + S2 NREM sleep.At both altitudes, the trend of the balance of parasympathetic/sympathetic activity recorded during REM sleep is similar to that recorded during the awakening state (W).

Sea Level (122 m)
The averages of the R-R intervals recorded at 122 m did not change significantly between nocturnal awakenings during sleep and sleep stages (R-R interval range, 1200 -1300 ms).The average R-R interval at 122 m, during the awaking state of sleep, was similar to that recorded during the wake state in 12 trained endurance athletes who performed a minimum of 3 h of aerobic activity per week [21].

High Altitude (3480 m)
The majority of the ECG alterations in the five mountain runners, during sleep at an altitude of 3480 m and at a barometric pressure (P B ) of 495 mm Hg, and between 30 and 41 h of acclimatization were: signs of sinus arrhythmia during the early stages (S1 + S2) of NREM sleep and during REM sleep.At 3480 m, the average of the R-R intervals we recorded during S1 + S2 of NREM sleep were significantly longer than those observed during the awaking period (W) during sleep.The longest average R-R intervals were recorded during S1 + S2 at 3480 m.These results suggest that, at altitude, there was still a significant elevation of parasympathetic tone during S1 + S2 of slow-wave sleep versus the waking state during sleep at 3480 m.

Sea Level (122 m) and High Altitude (3480 m)
There was a nonpathological, significant reduction in the R-R intervals during the awakening period during sleep (W), S1 + S2, S3 + S4 of NREM and REM sleep stages, and after 30 -41 h of acclimatization to an altitude of 3480 m, versus the averages of the R-R intervals observed during the same stages at 122 m.
The longest average R-R intervals were recorded during the light stage of slow-wave-sleep (S1 + S2) at 122 m (suggesting high vagal activation), and the lowest average R-R intervals were recorded during the awakening period during sleep (W) at 3480 m (indicating an increase in noradrenergic activation).Overall, the average R-R intervals at high altitude were significantly lower than those recorded at 122 m, with the average R-R intervals we recorded at 3480 m, even during the awakening period (W) during sleep (935 ms).The absolute value of the R-R intervals Bernardi and co-workers [22] found in 10 sea-level native subjects at low altitude was 1002 ± 45 ms and 809 ± 116 ms in 3 high-altitude native subjects living at low altitude.In that study, the average R-R intervals in the 10 sea-level native subjects and in 3 highaltitude native subjects (living at low altitude) exposed to 4970 m for 3 days were 775 ± 57 and 749 ± 47, respectively.The delta of the averages of the R-R intervals that we calculated in our subjects after exposure to 3480 m was similar to the delta of the R-R intervals Bernardi and co-workers [22] reported.In our study, the average R-R intervals recorded at 122 m and 3480 m, during all sleep stages, were also longer than those observed by Lanfranchi and co-workers [2] in 41 mountaineers, during waking, with and without AMS, and after acute exposure to approximately 4500 m.Overall, the average R-R intervals we recorded at 122 m and 3480 m demonstrate that frequent exposure to altitudes between 122 m and 5500 m, for more than 20 years, may have improved the efficacy of vagal modulation at low and high altitude during nocturnal awakening and sleep.

Simple Linear Regression Analysis between the R-R Intervals and the %SpaO 2
Linear regression analysis showed a significant negative correlation between the changes in the average %SpaO 2 and the changes in the average R-R intervals.The significant negative correlation between %SpaO 2 and R-R intervals may indicate that the average quantity of oxygen supplied during nocturnal sleep stages may be directly responsible for the R-R interval changes observed during nocturnal sleep, particularly at high altitude.

Total Power of Very Low Frequency [(VLF, ms 2 ) (<0.04 Hz)] Such as the Thermoregulation-Related HRV
The thermoregulatory-related very low frequency (VLF) rhythm may be related to thermoregulatory changes or chemical and acid-base equilibrium changes, or both.Interestingly, the changes in the VLF are significantly corelated with the PCO 2 changes (personal observations, DF

High Altitude (3480 m)
In altitude there was a significant increase in sympathetic tone throughout the awakening period (W) during sleep, the S1 + S2, and the REM sleep.The sympathetic tone decreased during the deep phase of slow wave sleep (S3 + S4).

Sea Level (122 m) and High Altitude (3480 m)
No significant changes were found between the average LF recorded at 122 m and that recorded at 3480 m.However, in some subjects, a significant decrease emerged between the average of LF recorded during the awakening period (W) during sleep, S1 + S2, S3 + S4, and REM sleep at 122 m versus the average of LF recorded at 3480 m.
The average LF recorded in all 5 mountain marathoners and in all 4 stages of desynchronization and synchronization at 122 m and at 3480 m (Table 3) were longer than the average LF Ako and co-workers [3] observed in 7 subjects during sleep at sea level.
We also found a decrease in the average LF during S3 + S4 at 122 m and 3480 m compared with the average LF recorded during S1 + S2.The average LF recorded in our study during the desynchronized REM sleep state, at 122 m and 3480 m, returned to similar values we recorded during the desynchronized awakening period (W) during sleep.A comparison of LF indices in the different sleep stages recorded at 122 m and at 3480 m revealed a decrease in the LF from the waking to the deep stages of sleep.The average LF during REM sleep was similar to the average LF recorded during waking.Like those of Ako and co-workers [3], our data suggest that sympathetic nervous activity decreases as sleep deepens and concomitantly increases during desynchronization of the awakening period (W) during sleep and even more so during desynchronization during REM sleep.In our study, the average LF, a marker of sympathetic modulation, during the awakening period (W) during sleep, S1 + S2, S3 + S4, and REM sleep at 122 m and at 3480 m was lower.

LF RR NU
The aim of this study was also to verify whether autonomic variables at low altitude could predict prodromic signs of AMS when subjects were exposed to high altitude or whether subjects experienced AMS at high altitude.Autonomic cardiovascular function was also explored by measuring widespread neuronal genesis rhythm low frequency in normalized unit (LF RR NU) to study sympathetic modulation.
Lanfranchi and co-workers [2] reported a substantial increase in the LF RR NU component at altitude, suggesting an increase in sympathetic modulation in response to hypoxia at 4500 m.During exposure to high altitude, besides shorter R-R intervals, we also found a relative, though not significant, increase in the LF RR NU component.
We agree with Lanfranchi and co-workers [2] that this increase in the LF RR NU component may reflect an increase in the sympathetic modulation of HR.During exposure to 3480 m, R-R interval variability decreased as the LF component increased, suggesting an increase in sympathetic modulation in response to hypobaric-hypoxia.The average LF RR NU Lanfranchi and co-workers [2] recorded in 24 subjects without AMS after exposure to approximately 4500 m during waking was similar to the average we recorded in our subjects during the awakening period (W) during sleep at 3480 m (73 vs 75, respectively).
LF RR NU has been said to be a quantitative marker of the cardiac vigil (Vanderwalle et al. [23]) and sympathetic outflow of the autonomic nervous system.By means of spectral analysis of subjects in supine position, the Task Force [1] calculated an average LF RR NU of 54, which is fairly similar to the average values in 4/5 of our subjects: 47 in CS, 45 in DC, and 39 in SS in supine position during S3 + S4 at 122 m.Overall, the average LF RR NU reported by the Task Force [1] is somewhat lower than our data (54 vs 39 -89, respectively).
From a steady-state HRV analysis, Murrel and coworkers [24] found an average LF RR NU of 77 in the standing position and premarathon conditions, an average between 72 and 92 immediately after a marathon, and an average of 73 in the standing position at 48 h postmarathon.These data appear similar to the data we recorded at 122 m and 3480 m during the awakening, S1 + S2, S3 + S4, and REM stages of sleep.
Specifically, at 122 m there was a decrease in LF RR NU as sleep depth increased; during the desynchronized phase of sleep (the awakening state), the LF RR NU reached the values observed during REM sleep.At 3480 m there was a decrease in LF RR NU as sleep deepened; during the desynchronized phase of sleep (the awakening state) the LF RR NU reached the values recorded during the REM phases at 3480 m.

High Frequency
Efferent vagal activity is a major contributor to the high frequency (HF) component (Task Force, [1]) which primarily reflects respiration-driven vigil modulation of the sinus rhythm.Other factors such as voluntary controlled respiration, cold stimulation of the face or rotational motor stimuli (see above) can occur physiologically during sleeping and might confound results.Spectral analysis of stationary supine subjects revealed an average normal HF close to 975 ms 2 (Task Force, [1]).Saito and co-workers [25] reported an average HF of 780 ± 211 ms 2 in 21 subjects during waking at low altitude and an average HF between 61 ± 44 and 112 ± 128 ms 2 after exposure for 4 -6 h at P B 495 mm Hg.Lanfranchi and co-workers [2] reported an average HF between 102 ± 37 and 157 ± 43 ms 2 in subjects exposed to approximately 4500 m, or between 135 ± 30 and 64 ± 4 ms 2 in those with no signs of AMS.Briefly, in the subjects without AMS there was a non significant increase in the HF component.At low altitude, the average HF in our five mountain marathon runners increased as sleep deepened and decreased during REM sleep to levels similar to those observed in the waking state.The average HF at altitude was 1572 ms 2 during the awakening state, 1610 ms 2 during S1 + S2, 1480 ms 2 during S3 + S4, and 1159 ms 2 during REM sleep.The average HF decreased during the sleep stages.Surprisingly, the average HF of 1159 ms 2 during REM sleep at high altitude was lower than the average observed during the desynchronized state during waking.
As reported by the Task Force [1], we also found the HF vigil component of the power spectrum to be augmented during NREM sleep at low altitude.

HF RR NU
The average of total power expressed in high frequency RR normalized units (HF RR NU) is considered a marker of vagal modulation of the autonomic system.The values that we found at low altitude during the awakening period (W) during sleep, during S2 + S3 and S3 + S4, and during REM sleep were similar to those Murrell and coworkers [24] recorded at premarathon, postmarathon, and 48 h postmarathon assessment under controlled normal breathing and standing conditions.Our HF RR NU data recorded during the awakening period (W) during sleep at 122 and were similar to those they reported during REM sleep (23.47 versus 22.64, respectively).In their evaluation of LF RR NU, the average HF (controlled breathing) was 58.45 during the premarathon stage, 45 during, and 53 at 48 h postmarathon The average HF RR NU decreased immediately after the race and at 48 h postmarathon.All these values are very close to the average LF RR NU we recorded in the subjects during S3 + S4 sleep at low altitude.
Of note is that HF RR NU at low and at high altitudes increases at sleep onset and peaks during the deep stage of sleep (S3 + S4) at altitude (56.91).After exposure to 3480 m at a P B of 495 mm Hg for 30 -41 h there was a non significant reduction in HF RR NU across all sleep stages.The average HF RR NU during the awakening period (W) during sleep at 3480 m was lower but still simi-lar to that at 122 m.These values were similar, though lower, than those Murrell and co-workers [24] recorded immediately and at 48 h post-marathon at altitude.
Our HF RR NU data recorded during sleep at 122 m and 3480 m were also similar to those Lanfranchi and coworkers [2] reported in subjects without AMS and those with AMS (16 and 31, respectively) during the supposed diurnal state and after exposure to 4500 m.
Compared with the HF RR NU reported by the Task Force [1], our data recorded during the awakening state, S1 + S2 and S3 + S4 and REM sleep at sea level and at 3480 m were 21.31 and 31.11respectively.

LF:HF Ratio
Exercise and training at low and high altitudes may have modified autonomic nervous system balance and thus the LF:HF ratio.The LF:HF ratio is considered by some investigators as the mirror of sympathovagal balance or a reflection of sympathetic modulation (Ako et al. [3]).In agreement with Ako and co-workers [3], we observed, significantly at low altitude, but indeed also at high altitude, a significant decrease in the LF:HF component as sleep deepened.In agreement with Ako and co-workers [3], we also observed that the sympathetic nervous system is activated during the two desynchronisation phases of REM sleep and deactivated during NREM sleep at low and high altitude.
In detail, we analyzed the dynamics of nocturnal fluctuations of autonomic nerve activities in sleep stages classified by Rechtschaffen and Kales's criteria.These criteria have not yet been investigated in association with HRV indices, particularly during hypoxic conditions.Ako and co-workers [3] found an increase in the LF:HF ratio (2.51 ± 0.17) during REM sleep and a significant linear decrease during NREM sleep (S 1 2.30 ± 0.29; S 2 1.85 ± 0.09; S 3 0.78 ± 0.06; S 4 2.51 ± 0.17).Like Ako and co-workers [3], we noted a reduction in the LF:HF ratio at 122 m and at 3480 m during NREM sleep (S1 + S2 122 m 3.1187; S1 + S2 3480 m 5.1242; S3 + S4 122 m 1.6733; S3 + S4 3480 m 3.2109) and an increase during REM sleep (REM 122 m 4.4739; REM 3480 m 6.9132) to the level found during the awaking period (W) during sleep (W 122m 4.6446 and W 3480 m 6.0036).We also found that the average LF:HF ratio during S3 + S4 of NREM sleep at 122 m (LF:HF = 1.6733) was similar to that reported by the Task Force [1] (LF:HF 1.5 -2.0) in the spectral analysis of stationary supine 5-minute recordings.In our study, the average LF:HF ratio at 122 m and 3480 m during the awaking period (W) during sleep (W 122 m 4.6446 and W 3480 m 6.0036, respectively) was lower yet similar to the average LF:HF ratio Lanfranchi and co-workers [2] found during the waking state at 4500 m in mountaineers with and without AMS (3.4 ± 1.3 and 8.3 ± 14, respectively).
Saito and co-workers [25] found an average LF:HF ratio of 2.6 ± 0.8 at P B 760 mm Hg at sea level during the relaxed waking state.This was similar to the value we found (1.6733) during S3 + S4 at 122 m.They reported an average LF:HF ratio of 3.5 ± 2.3 at 3456 m and at a P B of 495 mm Hg which was similar to the average of 3.2109 that we recorded during S3 + S4 at 3480 m.The average LF:HF ratio that we found in all five mountain runners during S3 + S4 (1.6733 ± 1.0847) at 122 m was similar to that reported by the Task Force [1].In some of the mountain marathon runners, at low altitude, at 122 m, and after 30 -41 h of acclimatization at 3480 m during the different stages of desynchronized and synchronized sleep, the average LF:HF ratio reached the normal values reported by the Task Force [1] (between 1.5 and 2.0).The LF:HF ratio value of 2, which is considered normal by the Task Force [1], was recorded at 122 m in: 1/5 subjects (SS) during the awakening period (W) of sleep; 3/5 subjects (CS, DC, SS) during S1 + S2; and 5/5 subjects during S3 + S4; 2/5 subjects (CS, SS) during the REM sleep stage.A LF:HF ratio value of 2 was reached after 30 -41 h of acclimatization at 3480 m by: 1/5 subjects (GM) during the awakening period (W) of sleep; 1/5 subjects (CS) during S1 + S2; and 2/5 subjects (CS, GM) during S3 + S4.No changes were noted between the average LF:HF ratio recorded in all five mountain runners at 122 m and after 30 -41 h of acclimatization at 3480 m and during all four stages of sleep: awakening period (W) during sleep; S1 + S2; S3 + S4; and REM sleep.These results suggest highly controlled regulation of sympathovagal balance on exposure to moderate hypoxia conditions.As reported in Table 7, several mountain marathon runners showed very high control of sympathovagal balance: their LF:HF ratio was similar to that reported by the Task Force [1] or did not differ between 122 m and 3480 m, after 30 -41 h of acclimatization, during the awakening period (W) during sleep, during S1 + S2; during S3 + S4, or during the REM sleep.Cornolo and co-workers [20] reported an average LF:HF ratio of ~6.0 in trained high-altitude native subjects 6 -8 h after the end of a marathon performed between 4100 and 4400 m.This was similar to the average LF:HF ratio that we recorded at 3480 m during the desynchronized state of sleep, specifically, during the awakening period (W) during sleep (6.0036 ± 5.0276) and during the REM sleep at the same altitude (6.9132 ± 3.6806).

Total Power
We observed significant parallel changes in TP recorded at low (P B about 740 mm Hg) and high (P B 495 mm Hg) altitude between the awakening period (W) during sleep and S3 + S4 of sleep.We also found significant parallel changes in the TP recorded during REM sleep and the average of TP recorded during S3 + S4 at 122 and 3480 m.These changes may reflect changes in cardiac, respiratory and sympathovagal balance.In general, in individual mountain marathon runners we found a decrease in the total power at altitude.The components of HRV are thought to be influenced by both neuronal and humoral factors (Task Force, [1]).Saito and co-workers [25] reported that humoral control of the autonomic nervous system plays an important role in the survival of victims under hypoxic conditions at high altitude.

Functional Significance
Physical training at low and high altitudes may have induced marked autonomic adaptations in the mountain marathon runners over the course of their years of training.The resting heart rate was lower at sea level because parasympathetic modulation predominates over higher sympathetic control at high altitude.By constantly maintaining their training levels at low and high altitudes for more than 20 years, the mountain runners may have shifted their cardiovascular autonomic control toward a greater parasympathetic modulation, particularly at sea level.Rest and training under hypoxia conditions may have balanced the prevalence of parasympathetic modulation, as well as improved control of phasic sympathetic responses.
We agree with Lanfranchi et al. [2], that HRV analysis has proved to be essential for investigating the mountain sickness, the balance of the sympathetic and parasympathetic function of the autonomic nervous system at low and high altitude in mountain marathon runners.HRV analysis could also be very useful for evaluating decreases in parasympathetic and increases in sympathetic tone of the autonomic nervous system during acclimatization at altitude of mountain marathon runners who generally have a tonic prevalence of parasympathetic tone at low altitude.R-R interval variability was observed in the mountain marathon runners during sleep at 122 m (i.e., a controlled increase in parasympathetic tone during NREM sleep and an increased sympathetic tone during the awakening period (W) during sleep and during REM sleep).At high altitude, the RR-intervals we observed were similar to those Sacknoff and co-workers [21] reported in athletes and controls.Moreover, the major ECG alterations evident during sleep, at an altitude of 3480 m, at a P B of 495 mm Hg, and after 30 -41 hours of acclimatization, were an increase in HR, together with signs of sinus arrhyth-mia during periodic breathing in S1 + S2 of NREM sleep and in REM sleep.
This study did not find abnormal reductions in the R-R interval during the awakening period (W) during sleep, S1 + S2, S3 + S4 and REM sleep, between 30 and 41 h of acclimatization at 3480 m in these well-trained mountain marathon runners.
The HRV data observed at altitude suggest that the runners possess an intrinsic high parasympathetic tone that at high altitude promptly compensates the physiological increase in sympathetic tone due to a periodic, significant decrease in peripheral oxygen saturation during sleep.
Not in group average but in several individual runners we found a significant reduction in the LF component at high altitude which may be due to the rhythmic increase of sympathetic discharge of the central brain stem center.
In comparison to the average calculated at low altitude, we also found a significant decrease in the average of the power of HF during S3 + S4 of NREM sleep at 3480 m.This significant reduction probably indicates a decrease in the vagal parasympathetic components of the brainstem generator.
The study demonstrates that cardiovascular modifications during sleep and after 30 -41 h of acclimatization at high altitude can occur even in mountain marathoners who have benefited from 20 years of training at low and high altitude.Physicians should therefore take the results of HRV analysis into account to improve the training and performance, particularly at altitude.

6 . 1 .
Sea Level (122 m)In all five subjects, the average of the HF RR NU analyzed

The LF Appears to Have a Widespread Neuronal Genesis and Is Considered as a Marker of Sympathetic Modulation or a Marker of Both Sympathetic and Vagal Modulation 3
2 ) (0.04 -0.15 Hz)] Range..3.1.Sea Level (122 m) At 122 m, the averages of LF were: 7997 ms 2 during W; 7400 ms 2 during S1 + S2; 4881 ms 2 during S3 + S4; and 8579 ms 2 during REM sleep.The changes in the average total power in LF were significantly shorter during S3 + S4 than those observed during REM sleep (4881 ± 2041 vs 8579 ± 3473; P < 0.02) (Table

LF RR NU Is Considered as a Marker of Sympathetic Modulation 3.4.1. Sea Level (122 m) In
These data indicate that the marker of sympathetic modulation (LF RR NU) decreased during deepening of sleep.During REM sleep, the average value of LF RR NU was similar to that recorded during the awakening period during sleep (W), suggesting that both desynchronized states were supported by an increase in sympathetic tone.

Table 3 . Averages of the total power in the low frequency (LF, ms 2 ) (0.04 -0.15 Hz) range appear to have a widespread neu- ronal genesis and are considered as a marker of sympathetic modulation or a marker of both sympathetic and vagal modula- tion. The LF average was calculated in a physiologically defined number of 30-second signal epochs, defined according to the standard criteria developed by Rechtschaffen and Kales (1968), recorded during the waking period during sleep (W), stages S1 + S2 and S3 + S4 of NREM and REM sleep at 122 m and after 30 -41 h of acclimatisation at 3480 m in the five subjects. (1)
No. of subjects Mean S.E.M.No. of subjects Mean S.E.M.

No. of epochs 226 106 No. of subjects Mean S.E.M. No. of subjects Mean S.E.M. Delta
sFootnotes to Table3:(1)One-way ANOVA revealed no significant changes in the averages of the total power in the low frequency (LF, ms 2 ) (0.04 -0.15 Hz) range (DF 7, 32 , 39 , F test 1,3, P = 0,2819).* Post-hoc comparison with Student's t-test of the average LF value recorded in 175 30-second epochs during REM sleep at sea level was significantly higher (P < 0.02) than that recorded in 226 30-second epochs during stages S3 + S4 of NREM sleep at 122 m in all five subjects.

of epochs 226 106 No. of subjects Mean S.E.M. No. of subjects Mean S.E.M. Delta subjects subjects Average 5 64.76 * 11.45 5 66.62 13.36 1.86 n.s.
12.71 at 3480 m vs 57.67 ± 17.92 at 122 m; P < 0.0001)], while in 1/5 the average LF RR NU was significantly shorter (GM: 59.53 ± 11.40 at 3480 m vs 70.19 ± 10.08 at 122 m; P < 0.0168) in S3 + S4.The average LF RR NU recorded during S3 + S4 at high altitude was significantly longer than that recorded at 122 m in 3/5 subjects [(MR: 73.90 ± 7.93 at 3480 m vs 63.73 ± 12.98 at 122 m; P < 0.0150); (DC: 74.40 ± 11.30 at 3480 m vs 45.84 ± 16.91 at 122 m; P < 0.0001); (SS: 75.93 ± 8.95 at 3480 m vs 39.52 ± 16.07 at 122 m; P < 0.0001)].The average LF RR NU recorded during REM sleep at high altitude was significantly longer than that recorded at 122 m in 4/5 subjects [(CS: 79.53 ± 5.87 at 3480 m vs 67.97 RR-NU average during stages S3 + S4 at 122 m differed significantly from that during REM sleep at 122 m (P < 0.05).The LF-RR -NU component of the R-R intervals during stages S3 + S4 at 122 m differed significantly from that during REM sleep at 3480 m (P < 0.05).The LF-RR -NU average during stages S3 + S4 at 3480 m differed from that during REM sleep at 3480 m (P < 0.05) in all five subjects.* Comparison with Student's t-test showed a significant difference between the average of the LF-RR-NU recorded in 190 30-second epochs during the waking period during sleep (W) at 122 m (P < 0.03) and that recorded in 226 30-second epochs during stages S3 + S4 of NREM sleep at 122 m in all five subjects.** Comparison with Student's t-test showed a significant difference (P < 0.0192) between the average of the LF-RR -NU in stages S3 + S4 of NREM sleep and that during REM sleep at 122 m in all five subjects.

Table 5 )
. The averages of the HF recorded during the nocturnal sleep-wake cycle at 3480 m suggested a reduction in vagal tone throughout all sleep stages and during awakening during sleep.Compared with the values calculated at sea level, there was a significant decrease in the average HF recorded at altitude during S1 + S2 of NREM sleep in 4/5 subjects [(CS: 2485 ± 1178 at 122 m vs 1513 ± 443 at 3480 m; P < 0.0001); (MR: 2892 ± 906 at 122 m vs 1382 ± 546 at

Table 5 . Averages of the total power in the high frequency (HF, ms 2 ) (0.15 -0.4 Hz) range, which primarily reflects respira- tory-driven vagal modulation of sinus rhythm, in a natural, physiologically defined number of 30-second signal epochs, ac- cording to the standard criteria developed by Rechtschaffen and Kales (1968), during the waking period during sleep (W), stages S1 + S2 and S3 + S4 of NREM and REM sleep measured at 122 m and after 30 -41 h of acclimatisation at 3480 m in the five subjects. (1)
No. of subjects Mean S.E.M.No. of subjects Mean S.E.M.

Table 7
) during W was significantly shorter at 3480 m than at 122 m in 1/5 subjects (GM: 6.1615 ± 2.4214 at 122 m vs 2.3017 ± 1.2563 ± 3.7020 at 122 m); (SS: 4.1387 ± 3.9490 at 3480 m vs 2.7194 ± 2.0375 at 122 m)].At 122 m and at 3480 m there was a decrease in the LF:HF ratio during deepening of NREM sleep, indicating

Table 8 )
decline in sympathovagal balance.At 122 m and at 3480 m the average values of LF:HF during REM sleep were similar to those observed during the awakening period during sleep (W).During W, the average of the total power (TP, ms 2 ) re- *** Student's t-test showed that the average of the total power of the LF/HF ratio during stages S3 + S4 (106 epochs) at 3480 m was significantly lower than that during REM sleep (113 epochs) at high altitude in the five subjects (P < 0.036835).a 3.8.3.Sea Level (122 m) and High Altitude (3480 m) (