Sleep, napping and alertness during an overwintering mission at Belgrano II Argentine Antarctic station.

During Antarctic isolation personnel are exposed to extreme photoperiods. A frequent observation is a sleep onset phase delay during winter. It is not known if, as a result, daytime sleeping in the form of naps increases. We sought to assess sleep patterns - with focus on daytime sleeping - and alertness in a Latin American crew overwintering in Argentine Antarctic station Belgrano II. Measurements were collected in 13 males during March, May, July, September and November, and included actigraphy and psychomotor vigilance tasks. Sleep duration significantly decreased during winter. A total of eight participants took at least one weekly nap across all measurement points. During winter, the nap onset was delayed, its duration increased and its efficiency improved. We observed a significant effect of seasonality in the association of evening alertness with sleep onset. Our results replicate previous findings regarding sleep during overwintering in Antarctica, adding the description of the role of napping and the report of a possible modulatory effect of seasonality in the relation between sleep and alertness. Napping should be considered as an important factor in the scheduling of activities of multicultural crews that participate in Antarctica.

Orthostatic Intolerance Is Independent of the Degree of Autonomic Cardiovascular Adaptation after 60 Days of Head-Down Bed Rest.

Spaceflight and head-down bed rest (HDBR) can induce the orthostatic intolerance (OI); the mechanisms remain to be clarified. The aim of this study was to determine whether or not OI after HDBR relates to the degree of autonomic cardiovascular adaptation. Fourteen volunteers were enrolled for 60 days of HDBR. A head-up tilt test (HUTT) was performed before and after HDBR. Our data revealed that, in all nonfainters, there was a progressive increase in heart rate over the course of HDBR, which remained higher until 12 days of recovery. The mean arterial pressure gradually increased until day 56 of HDBR and returned to baseline after 12 days of recovery. Respiratory sinus arrhythmia and baroreflex sensitivity decreased during HDBR and remained suppressed until 12 days of recovery. Low-frequency power of systolic arterial pressure increased during HDBR and remained elevated during recovery. Three subjects fainted during the HUTT after HDBR, in which systemic vascular resistance did not increase and remained lower until syncope. None of the circulatory patterns significantly differed between the fainters and the nonfainters at any time point. In conclusion, our data indicate that the impaired orthostatic tolerance after HDBR could not be distinguished by estimation of normal hemodynamic and/or neurocardiac data.

Cardiovascular autonomic adaptation in lunar and martian gravity during parabolic flight.

PURPOSE: Weightlessness has a well-known effect on the autonomic control of the cardiovascular system. With future missions to Mars in mind, it is important to know what the effect of partial gravity is on the human body. We aim to study the autonomic response of the cardiovascular system to partial gravity levels, as present on the Moon and on Mars, during parabolic flight. METHODS: ECG and blood pressure were continuously recorded during parabolic flight. A temporal analysis of blood pressure and heart rate to changing gravity was conducted to study the dynamic response. In addition, cardiovascular autonomic control was quantified by means of heart rate (HR) and blood pressure (BP) variability measures. RESULTS: Zero and lunar gravity presented a biphasic cardiovascular response, while a triphasic response was noted during martian gravity. Heart rate and blood pressure are positively correlated with gravity, while the general variability of HR and BP, as well as vagal indices showed negative correlations with increasing gravity. However, the increase in vagal modulation during weightlessness is not in proportion when compared to the increase during partial gravity. CONCLUSIONS: Correlations were found between the gravity level and modulations in the autonomic nervous system during parabolic flight. Nevertheless, with future Mars missions in mind, more studies are needed to use these findings to develop appropriate countermeasures.

Is autonomic modulation different between European and Chinese astronauts?

PURPOSE: The objective was to investigate autonomic control in groups of European and Chinese astronauts and to identify similarities and differences. METHODS: Beat-to-beat heart rate and finger blood pressure, brachial blood pressure, and respiratory frequency were measured from 10 astronauts (five European taking part in three different space missions and five Chinese astronauts taking part in two different space missions). Data recording was performed in the supine and standing positions at least 10 days before launch, and 1, 3, and 10 days after return. Cross-correlation analysis of heart rate and systolic pressure was used to assess cardiac baroreflex modulation. A fixed breathing protocol was performed to measure respiratory sinus arrhythmia and low-frequency power of systolic blood pressure variability. RESULTS: Although baseline cardiovascular parameters before spaceflight were similar in all astronauts in the supine position, a significant increase in sympathetic activity and a decrease in vagal modulation occurred in the European astronauts when standing; spaceflight resulted in a remarkable vagal decrease in European astronauts only. Similar baseline supine and standing values for heart rate, mean arterial pressure, and respiratory frequency were shown in both groups. Standing autonomic control was based on a balance of higher vagal and sympathetic modulation in European astronauts. CONCLUSION: Post-spaceflight orthostatic tachycardia was observed in all European astronauts, whereas post-spaceflight orthostatic tachycardia was significantly reduced in Chinese astronauts. The basis for orthostatic intolerance is not apparent; however, many possibilities can be considered and need to be further investigated, such as genetic diversities between races, astronaut selection, training, and nutrition, etc.

Inhalation/Exhalation ratio modulates the effect of slow breathing on heart rate variability and relaxation.

Slow breathing is widely applied to improve symptoms of hyperarousal, but it is unknown whether its beneficial effects relate to the reduction in respiration rate per se, or, to a lower inhalation/exhalation (i/e) ratio. The present study examined the effects of four ventilatory patterns on heart rate variability and self-reported dimensions of relaxation. Thirty participants were instructed to breathe at 6 or 12 breaths/min, and with an i/e ratio of 0.42 or 2.33. Participants reported increased relaxation, stress reduction, mindfulness and positive energy when breathing with the low compared to the high i/e ratio. A lower compared to a higher respiration rate was associated only with an increased score on positive energy. A low i/e ratio was also associated with more power in the high frequency component of heart rate variability, but only for the slow breathing pattern. Our results show that i/e ratio is an important modulator for the autonomic and subjective effects of instructed ventilatory patterns.

Circadian rhythm of autonomic cardiovascular control during Mars500 simulated mission to Mars.

INTRODUCTION: The Mars500 project was conceived to gather knowledge about the psychological and physiological effects of living in an enclosed environment during 520 d as would be required for a real mission to Mars. Our objective was to investigate the circadian profile of heart rate variability (HRV) in the context of the Mars500 study. METHODS: Before, during, and after confinement, 24-h EKG records were obtained from the six crewmembers who participated in the mission. Autonomic activity was evaluated through time and frequency domain indexes of HRV analysis. Circadian rhythmicity was assessed both by averaging hourly HRV along wake and sleep scheduled periods and by fitting a 24-h harmonic to the hourly means. RESULTS: During confinement, wake HRV showed (mean +/- SE) a progressive increase in mean RR interval (from 778 +/- 24 ms to 916 +/- 42 ms), and in the amplitude (values are wavelet power coefficients) of very low (from 13.3 +/- 0.3 to 14.1 +/- 0.2) and high (from 7.8 +/- 0.4 to 8.3 +/- 0.3) frequency components. During sleep, the relative amplitude of the high frequency component of HRV decreased (from 11.8 +/- 1.6 to 9.4 +/- 1.8 normalized units). Overall, sleep-wake differences of HRV showed a progressive decrease of the relative amplitude of the high frequency component. Also, circadian HRV rhythms were dampened during confinement. DISCUSSION: Data revealed diminished amplitude of the rest-activity pattern of the autonomic nervous system parasympathetic function. Reduced daylight exposure and mood changes could account for this observation.

Adaptation of autonomic heart rate regulation in astronauts after spaceflight.

BACKGROUND: Spaceflight causes changes in the cardiovascular control system. The aim of this study was to evaluate postflight recovery of linear and nonlinear neural markers of heart rate modulation, with a special focus on day-night variations. MATERIAL/METHODS: Twenty-four-hour Holter ECG recordings were obtained in 8 astronauts participating in space missions aboard the International Space Station (ISS). Data recording was performed 1 month before launch, and 5 and 30 days after return to Earth from short- and long-term flights. Cardiovascular control was inferred from linear and nonlinear heart rate variability (HRV) parameters, separately during 2-hour day and 2-hour night recordings. RESULTS: No remarkable differences were found in the postflight recovery between astronauts from short- and long-duration spaceflights. Five days after return to Earth, vagal modulation was significantly decreased compared to the preflight condition (day: p=0.001; night: p=0.019), while the sympathovagal balance was strongly increased, but only at night (p=0.017). A few nonlinear parameters were reduced early postflight compared to preflight values, but these were not always statistically significant. No significant differences remained after 30 days of postflight recovery. CONCLUSIONS: Our results show that 5 days after return from both short- and long-duration space missions, neural mechanisms of heart rate regulation are still disturbed. After 1 month, autonomic control of heart rate recovered almost completely.

Sleep-wake differences in heart rate variability during a 105-day simulated mission to Mars.

INTRODUCTION: In prolonged spaceflights the effect of long-term confinement on the autonomic regulation of the heart is difficult to separate from the effect of prolonged exposure to microgravity or other space-related stressors. Our objective was to investigate whether the sleep-wake variations in the autonomic control of the heart are specifically altered by long-term confinement during the 105-d pilot study of the Earth-based Mars500 project. METHODS: Before (pre), during (T1: 30, T2: 70, andT3: 100 d), and after (post) confinement, 24-h EKG records were obtained from the six crewmembers that participated in the mission. Sleep and wake periods were determined by fitting a square wave to the data. Autonomic activity was evaluated through time and frequency domain indexes of heart rate variability (HRV) analysis during wake and sleep periods. RESULTS: During confinement, wake HRV showed decreased mean heart rate and increased amplitude at all frequency levels, particularly in the very low (pre: 13.3 +/- 0.2; T1: 13.9 +/- 0.3; T2: 13.9 +/- 0.2; T3: 13.9 +/- 0.2; post: 13.2 +/- 0.2) and high (pre: 7.6 +/- 0.4; T1: 8.3 +/- 0.5; T2: 8.2 +/- 0.4; T3: 8.1 +/- 0.4; post: 7.6 +/- 0.3) frequency components (values expressed as mean +/- SE of wavelet power coefficients). Sleep HRV remained constant, while sleep-wake high frequency HRV differences diminished. DISCUSSION: The observed autonomic changes during confinement reflect an increase in parasympathetic activity during wake periods. Several factors could account for this observation, including reduced daylight exposure related to the confinement situation.

Heart rate variability after heart transplantation: a 10-year longitudinal follow-up study.

BACKGROUND: Cross-sectional studies suggest, by use of heart rate variability (HRV), that partial re-innervation of the sinus node may occur after heart transplantation (HTx). Our aim was to test this hypothesis by examining HRV in long-term longitudinal follow-up study of HTx recipients. METHODS AND RESULTS: 14 HTx recipients (11 men) were studied 1-48 (median 13) months (baseline) and 119-172 (median 141) months after HTx (follow-up). At baseline and follow-up, electrocardiographic RR interval was continuously recorded in the supine position for 20 min. The signals were analyzed in the time domain and in the frequency domain by means of power spectral analysis. RR-interval decreased significantly from baseline to follow-up (p<0.05). This was associated with an increase of total power (p<0.001), absolute low frequency (p<0.001), and high frequency power (p<0.001), but unchanged relative low frequency and high frequency power. CONCLUSIONS: The observed changes in HRV during long-term follow-up after HTx are compatible with partial re-innervation of the cardiac sinus node, as has been suggested by cross-sectional studies.

Haemodynamic adaptation during sudden gravity transitions.

Haemodynamic responses during parabolic flight were studied. The hypothesis that haemodynamic changes may be counteracted by a transient vagal reflex during acute gravity transitions was tested. ECG, arterial pressure and respiration were recorded continuously in seven male subjects during parabolic flight. Beat-to-beat haemodynamic parameters were estimated. In the supine position no significant differences were shown among the different gravity phases. In the upright position, significant within-group differences were observed across gravity phases for all parameters. Postural differences in haemodynamic data disappeared during the microgravity phase and were enlarged during hypergravity phases. Detailed temporal analysis of cardiac time series in standing subjects confirmed the hypothesized biphasic response of initial parasympathetic modulation: a sharp increase of RRI within 3-5 s followed by a 10% decrease in the remaining period of microgravity (p < 0.001); a sharp increase in SAP within 2-4 s followed by a slow decrease of 25%. Significant within-group differences were observed in the standing position for mean RRI (836 ± 170 ms, p = 0.003), DAP (66 ± 8 mmHg, p < 0.001), MAP (139 ± 12 mmHg, p = 0.001), RRI HF amplitude (17.6 ± 7.5 ms, p < 0.001), SV (146 ± 5%, p < 0.001) and SVR (73 ± 10%, p = 0.020). In standing subjects, the initial baroreflex-mediated vagal heart rate response is limited to a transition period at early microgravity lasting about 3-5 s, followed by a gradual heart rate recovery during the remaining 15-17 s due to a parasympathetic withdrawal. The resultant increase in cardiac output induces a baroreflex-mediated systemic vasodilatation, which may be the driving force for a decreased arterial pressure in weightlessness.

Cardiovascular autonomic adaptation to long-term confinement during a 105-day simulated Mars mission.

INTRODUCTION: Long-term confinement and microgravity may entail alteration in the regulation of the cardiovascular system. A 105-d pilot study of a Mars mission simulation was conducted to test the cardiovascular response to slow-paced breathing and mental stress. METHODS: Finger blood pressure and beat-to-beat heart rate were monitored in six male volunteers taking part in a 105-d Mars mission simulation. Data were collected before, during (Days 35-38, 70-72, and 100), and after confinement. Recordings were performed in the sitting position during 5-min spontaneous breathing, 3-min 12 cycle/min breathing, 3-min 6 cycle/min breathing, and 5-min mental task performance. RESULTS: We found significant U-shaped changes across the confinement period in systolic arterial pressure (SAP), diastolic arterial pressure (DAP), and mean arterial pressure (MAP). In the first month of confinement, mental task performance significantly lowered SAP by 34.23 mmHg and MAP by 19.89 mmHg compared to spontaneous breathing, whereas these changes were reversed during other periods. Furthermore, no differences in arterial pressure and heart rate were found between spontaneous, 12 cycle/min, and 6 cycle/min breathing. CONCLUSIONS: Our findings are in line with and extend previous findings on the alteration of blood pressure regulation due to long-term confinement.

Effects of mental stress on autonomic cardiac modulation during weightlessness.

Sustained weightlessness affects all body functions, among these also cardiac autonomic control mechanisms. How this may influence neural response to central stimulation by a mental arithmetic task remains an open question. The hypothesis was tested that microgravity alters cardiovascular neural response to standardized cognitive load stimuli. Beat-to-beat heart rate, brachial blood pressure, and respiratory frequency were collected in five astronauts, taking part in three different short-duration (10 to 11 days) space missions to the International Space Station. Data recording was performed in supine position 1 mo before launch; at days 5 or 8 in space; and on days 1, 4, and 25 after landing. Heart rate variability (HRV) parameters were obtained in the frequency domain. Measurements were performed in the control condition for 10 min and during a 5-min mental arithmetic stress task, consisting of deducting 17 from a four-digit number, read by a colleague, and orally announcing the result. Our results show that over all sessions (pre-, in-, and postflight), mental stress induced an average increase in mean heart rate (Delta7 +/- 1 beats/min; P = 0.03) and mean arterial pressure (Delta7 +/- 1 mmHg; P = 0.006). A sympathetic excitation during mental stress was shown from HRV parameters: increase of low frequency expressed in normalized units (Delta8.3 +/- 1.4; P = 0.004) and low frequency/high frequency (Delta1.6 +/- 0.3; P = 0.001) and decrease of high frequency expressed in normalized units (Delta8.9 +/- 1.4; P = 0.004). The total power was not influenced by mental stress. No effect of spaceflight was found on baseline heart rate, mean arterial pressure, and HRV parameters. No differences in response to mental stress were found between pre-, in-, and postflight. Our findings confirm that a mental arithmetic task in astronauts elicits sympathovagal shifts toward enhanced sympathetic modulation and reduced vagal modulation. However, these responses are not changed in space during microgravity or after spaceflight.

Respiratory variability preceding and following sighs: a resetter hypothesis.

Respiratory behavior is characterized by complex variability with structured and random components. Assuming that both a lack of variability and too much randomness represent suboptimal breathing regulation, we hypothesized that sighing acts as a resetter inducing structured variability. Spontaneous breathing was measured in healthy persons (N=42) during a 20min period of quiet sitting using the LifeShirt(®) System. Four blocks of 10 breaths with a 50% window overlap were determined before and after spontaneous sighs. Total respiratory variability of minute ventilation was measured using the coefficient of variation and structured (correlated) variability was quantified using autocorrelation. Towards a sigh, total variability gradually increased without concomittant changes in correlated variability, suggesting that randomness increased. After a sigh, correlated variability increased. No changes in variability were found in comparable epochs without intermediate sighs. We conclude that a sigh resets structured respiratory variability, enhancing information processing in the respiratory system.

Adaptation of heart rate and blood pressure to short and long duration space missions.

To what extent does going to space affect cardiovascular function? Although many studies have addressed this question, the answer remains controversial. Even for such primary parameters as heart rate (HR) and blood pressure (BP) contradictory results have been presented. The purpose of this investigation was to evaluate HR and arterial BP in 11 male astronauts who each took part in nine different space missions aboard the International Space Station (ISS), for up to 6 months. Pre-flight HR and BP readings were obtained in both the standing and supine positions on Earth and were taken as reference values. Our results show that HR and arterial BP in space equal pre-flight supine values. In all subjects, HR and mean arterial BP (MAP) were lower in space compared with pre-flight standing (both p<0.05). HR in space was well maintained at pre-flight supine level for up to 6 months in all astronauts while MAP tended to adapt to a level in between the ground-based standing and supine positions. Also pulse pressure (PP) decreased over the course of long duration spaceflight. In conclusion, our data indicate that weightlessness relaxes the circulation in humans for an extended duration of up to 6 months in space.

Cardiac autonomic regulation under hypnosis assessed by heart rate variability: spectral analysis and fractal complexity.

OBJECTIVE: This study examined the effects of hypnosis on autonomic cardiac control. We hypothesized a modification of autonomic modulation of the heart rate with an enhanced vagal tone during hypnosis compared to baseline. METHODS: In 12 healthy subjects (6 men and 6 women, 22.2 +/- 1.0 years of age) ECG was recorded at baseline and during hypnosis. Heart rate variability parameters were obtained in the frequency domain (LFnu: low frequency normalized units, and HFnu: high frequency normalized units) and from nonlinear analysis methods (detrended fluctuation analysis, DFA). RESULTS: Compared to the control condition, hypnosis showed a significantly decreased LFnu, a significantly increased HFnu, and a significantly decreased LF/HF. DFA showed a significantly decreased short-range similarity. Heart rate remained unchanged. CONCLUSION: Autonomic cardiac tone is significantly modified during hypnosis by shifting the balance of the sympathovagal interaction toward an enhanced parasympathetic modulation, accompanied by a reduction of the sympathetic tone and a decreased short-range similarity but without a concomitant change in heart rate. Central and secondary autonomous nervous system changes induced by hypnosis are a possible explanation for our results. Another highly probable explanation is given by a variation in the depth of respiration. Hypnosis appears to prevent the autonomic responses expected during neutral stimulation.

Complexity of cardiovascular regulation in small animals.

Oscillations of heart rate and blood pressure are related to the activity of the underlying control mechanism. They have been investigated mostly with linear methods in the time and frequency domains. Also, in recent years, many different nonlinear analysis methods have been applied for the evaluation of cardiovascular variability. This review presents the most commonly used nonlinear methods. Physiological understanding is obtained from various results from small animals.

Steep fall in cardiac output is main determinant of hypotension during drug-free and nitroglycerine-induced orthostatic vasovagal syncope.

BACKGROUND: How much of the hypotension occurring during postural syncope is cardiac output-mediated and how much can be ascribed to a fall in systemic vascular resistance are unknown. The contribution of both determinants may be influenced by the use of vasoactive drugs. OBJECTIVE: The purpose of this study was to assess the determinants of hypotension during drug-free and nitroglycerine (NTG)-induced vasovagal presyncope in routine tilt table testing. METHODS: In this retrospective study, a total of 56 patients (37 female; age 36 +/- 19 years) with suspected vasovagal syncope and a positive tilt test at two clinical centers were selected. In 29 patients, presyncope was provoked by 0.4 mg sublingual NTG, administered in the 60 degrees head-up tilt position. In the other 27 patients, presyncope was provoked by passive tilt alone. Finger arterial pressure was monitored continuously, and left ventricular stroke volume was computed from pressure pulsations. RESULTS: After NTG administration, heart rate rose, and peak heart rate was similar in all patients. Use of NTG did not affect circulatory patterns precipitating a vasovagal response. On average in all patients, marked hypotension was mediated by an approximately 50% fall in cardiac output, whereas systemic vascular resistance was well maintained until presyncope. CONCLUSION: Hypotension during routine tilt testing is cardiac output-mediated, and the mechanism appears independent of the use of 0.4 mg sublingual NTG. The study data challenge the conventional idea of systemic vasodilation as the overriding cause of hypotension during postural syncope.

Tilt training increases the vasoconstrictor reserve in patients with neurally mediated syncope evoked by head-up tilt testing.

AIMS: Tilt training is a useful therapeutic option in neurally mediated syncope (NMS). We tested the hypothesis that tilt training will restore orthostatic tolerance by increasing the degree of vasomotor reserve during sustained orthostatic stress. METHODS AND RESULTS In this follow-up study we enrolled 17 patients (age 31 +/- 22 years, 11 females) with a clinical diagnosis of NMS and two consecutive positive tilt tests. The head-up tilt test was repeated day after day: one session per day. All patients were instructed to continue a programme of daily standing training at home. Follow-up tilt testing was performed after a period of 6 weeks in 14 patients. ECG and finger arterial blood pressure (Portapres) were recorded during subsequent tilt testing. Left ventricular stroke volume (SV), cardiac output, and systemic vascular resistance were computed from the pressure pulsations (Modelflow). Spontaneous cardiac baroreflex sensitivity was estimated by cross-spectral analysis of heart rate (HR) and systolic blood pressure. In all patients, orthostatic tolerance was restored after 4.1 +/- 0.9 tilt sessions, median 4. The follow-up tilt test was also negative in all patients. This was accompanied by a significant rise in systemic vascular resistance to compensate for a postural reduction in SV in the sustained head-up tilt position. No evidence could be provided of altered baroreflex control of HR after tilt training. CONCLUSION: Tilt training restores orthostatic tolerance at least in part by increasing the amount of vasoconstriction that can ultimately be made available during sustained orthostatic stress. The increased vasoconstrictor reserve is preserved after 6 weeks of continued standing training at home.

Cardiovascular function and gravity transitions during parabolic flight.

Cardiovascular function and gravity transitions during parabolic flight.