Comparison of a 4-day confinement and head-down tilt on endocrine response and cardiovascular variability in humans

Human Adaptations to Multiday Saturation on NASA NEEMO

Human adaptation to extreme environments has been explored for over a century to understand human psychology, integrated physiology, comparative pathologies, and exploratory potential. It has been demonstrated that these environments can provide multiple external stimuli and stressors, which are sufficient to disrupt internal homeostasis and induce adaptation processes. Multiday hyperbaric and/or saturated (HBS) environments represent the most understudied of environmental extremes due to inherent experimental, analytical, technical, temporal, and safety limitations. National Aeronautic Space Agency (NASA) Extreme Environment Mission Operation (NEEMO) is a space-flight analog mission conducted within Florida International University’s Aquarius Undersea Research Laboratory (AURL), the only existing operational and habitable undersea saturated environment. To investigate human objective and subjective adaptations to multiday HBS, we evaluated aquanauts living at saturation for 9–10 days via NASA NEEMO 22 and 23, across psychologic, cardiac, respiratory, autonomic, thermic, hemodynamic, sleep, and body composition parameters. We found that aquanauts exposed to saturation over 9–10 days experienced intrapersonal physical and mental burden, sustained good mood and work satisfaction, decreased heart and respiratory rates, increased parasympathetic and reduced sympathetic modulation, lower cerebral blood flow velocity, intact cerebral autoregulation and maintenance of baroreflex functionality, as well as losses in systemic bodyweight and adipose tissue. Together, these findings illustrate novel insights into human adaptation across multiple body systems in response to multiday hyperbaric saturation.

Cardiovascular System Under Simulated Weightlessness: Head-Down Bed Rest vs. Dry Immersion

Background

The most applicable human models of weightlessness are −6° head-down bed rest (HDBR) and head-out dry immersion (DI). A detailed experimental comparison of cardiovascular responses in both models has not yet been carried out, in spite of numerous studies having been performed in each of the models separately.

Objectives

We compared changes in central hemodynamics, autonomic regulation, plasma volume, and water balance induced by −6° HDBR and DI.

Methods

Eleven subjects participated in a 21-day HDBR and 12 subjects in a 3-day DI. During exposure, measurements of the water balance, blood pressure, and heart rate were performed daily. Plasma volume evolution was assessed by the Dill–Costill method. In order to assess orthostatic tolerance time (OTT), central hemodynamic responses to orthostatic stimuli, and autonomous regulation, the 80° lower body negative pressure–tilt test was conducted before and right after both exposures.

Results

For most of the studied parameters, the changes were co-directional, although they differed in their extent. The changes in systolic blood pressure and total peripheral resistance after HDBR were more pronounced than those after DI. The OTT was decreased in both groups: to 14.2 ± 3.1 min (vs. 27.9 ± 2.5 min before exposure) in the group of 21-day HDBR and to 8.7 ± 2.1 min (vs. 27.7 ± 1.2 min before exposure) in the group of 3-day DI.

Conclusions

In general, cardiovascular changes during the 21-day HDBR and 3-day DI were co-directional. In some cases, changes in the parameters after 3-day DI exceeded changes after the 21-day HDBR, while in other cases the opposite was true. Significantly stronger effects of DI on cardiovascular function may be due to hypovolemia and support unloading (supportlessness).

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.

Five days of head-down-tilt bed rest induces noninflammatory shedding of L-selectin

Head-down-tilt bed rest (HDTBR) is a popular model, simulating alterations of gravitation during space missions. The aim of this study was to obtain a better insight into the complexly orchestrated regulations of HDTBR-induced immunological responses, hypothesizing that artificial gravity can mitigate these HDTBR-related physiological effects. This crossover-designed 5 days of HDTBR study included three protocols with no, or daily 30 min of centrifugation or 6 × 5 min of centrifugation. Twelve healthy, male participants donated blood pre-HDTBR, post-HDTBR, and twice during HDTBR. Cellular immune changes were assessed either by enumerative and immune cell phenotyping assays or by functional testing of responses to either recall antigens or receptor-dependent activation by chemotactic agents N-formyl-methionyl-leucyl-phenylalanine (fMLP) and with TNF-α. The expression of the adhesion molecule L-selectin (CD62L) on the surface of granulocytes and its shedding into plasma samples were measured. In parallel, other humoral factor, such as interleukin-6 and interleukin-8, parameters of endothelial damage (glycocalyx) were determined. Hematocrit and hemoglobin were significantly increased during HDTBR. Although immune functional tests did not indicate a change in the immune performance, the expression of CD62L on resting granulocytes was significantly shed by 50% during HDTBR. Although the latter is normally associated to an activation of inflammatory innate immune responses and during interaction of granulocytes with the endothelium, CD62L shedding was, however, not related either to a systemic inflammatory alteration or to shedding of the endothelial glycocalyx during bed rest. This suggests a noninflammatory or “mechanical” shedding related to fluid shifts during head-down intervention and not to an acute inflammatory process.

Cardiovascular responses to lower body negative pressure before and after 4 h of head-down bed rest and seated control in men and women

Cardiovascular deconditioning after a 4-h head-down bed rest (HDBR) might be a consequence of the time of day relative to pre-HDBR testing, or simply 4 h of confinement and inactivity rather than the posture change. Ten men and 11 women were studied during lower body negative pressure (LBNP) before and after 4-h HDBR and 4-h seated posture (SEAT) as a control for time of day and physical inactivity effects to test the hypotheses that cardiovascular deconditioning was a consequence of the HDBR posture, and that women would have a greater deconditioning response. Following HDBR, men and women had lower blood volume, higher heart rate with a greater increase during LBNP, a greater decrease of stroke volume during LBNP, lower central venous pressure, smaller inferior vena cava diameter, higher portal vein resistance index with a greater increase during LBNP, but lower forearm vascular resistance, lower norepinephrine, and lower renin. Women had lower vasopressin and men had higher vasopressin after HDBR, and women had lower pelvic impedance and men higher pelvic impedance. Following SEAT, brachial vascular resistance was reduced, thoracic impedance was elevated, the reduction of central venous pressure during LBNP was changed, women had higher angiotensin II whereas men had lower levels, and pelvic impedance increased in women and decreased in men. Cardiovascular deconditioning was greater after 4-h HDBR than after SEAT. Women and men had similar responses for most cardiovascular variables in the present study that tested the responses to LBNP after short-duration HDBR compared with a control condition.

From space to Earth: advances in human physiology from 20 years of bed rest studies (1986-2006)

Bed rest studies of the past 20 years are reviewed. Head-down bed rest (HDBR) has proved its usefulness as a reliable simulation model for the most physiological effects of spaceflight. As well as continuing to search for better understanding of the physiological changes induced, these studies focused mostly on identifying effective countermeasures with encouraging but limited success. HDBR is characterised by immobilization, inactivity, confinement and elimination of Gz gravitational stimuli, such as posture change and direction, which affect body sensors and responses. These induce upward fluid shift, unloading the body's upright weight, absence of work against gravity, reduced energy requirements and reduction in overall sensory stimulation. The upward fluid shift by acting on central volume receptors induces a 10-15% reduction in plasma volume which leads to a now well-documented set of cardiovascular changes including changes in cardiac performance and baroreflex sensitivity that are identical to those in space. Calcium excretion is increased from the beginning of bed rest leading to a sustained negative calcium balance. Calcium absorption is reduced. Body weight, muscle mass, muscle strength is reduced, as is the resistance of muscle to insulin. Bone density, stiffness of bones of the lower limbs and spinal cord and bone architecture are altered. Circadian rhythms may shift and are dampened. Ways to improve the process of evaluating countermeasures--exercise (aerobic, resistive, vibration), nutritional and pharmacological--are proposed. Artificial gravity requires systematic evaluation. This review points to clinical applications of BR research revealing the crucial role of gravity to health.

Role of individual predisposition in orthostatic intolerance before and after simulated microgravity

Orthostatic intolerance (OI) is a major problem after spaceflight. Its etiology remains uncertain, but reports have pointed toward an individual susceptibility to OI. We hypothesized that individual predisposition plays an important role in post-bed rest OI. Twenty-four healthy male subjects were equilibrated on a constant diet, after which they underwent tilt-stand test (pre-TST). They then completed 14-16 days of head-down-tilt bed rest, and 14 of the subjects underwent repeat tilt-stand test (post-TST). During various phases, the following were performed: 24-h urine collections and hormonal measurements, plethysmography, and cardiovascular system identification (a noninvasive method to assess autonomic function and separately quantify parasympathetic and sympathetic responsiveness). Development of presyncope or syncope defined OI. During pre-TST, 11 subjects were intolerant and 13 were tolerant. At baseline, intolerant subjects had lower serum aldosterone (P < 0.01), higher excretion of potassium (P = 0.01), lower leg venous compliance (P = 0.03), higher supine parasympathetic responsiveness (P = 0.02), and lower standing sympathetic responsiveness (P = 0.048). Of the 14 subjects who completed post-TST, 9 were intolerant and 5 were tolerant. Intolerant subjects had lower baseline serum cortisol (P = 0.03) and a higher sodium level (P = 0.02) compared with tolerant subjects. Thus several physiological characteristics were associated with increased susceptibility to OI. We propose a new model for OI, whereby individuals with greater leg venous compliance recruit compensatory mechanisms (activation of the renin-angiotensin-aldosterone system and sympathetic nervous system, and withdrawal of the parasympathetic nervous system) in the face of daily postural challenges, which places them at an advantage to face orthostatic stress. With head-down-tilt bed rest, the stimulus to recruit compensatory mechanisms disappears, and differences between the two subgroups attenuate.

Effects of Space Flight and −6° Bed Rest on the Neuroendocrine Response to Metabolic Stress in Physically Fit Subjects

The aim of this study was to evaluate the association of plasma epinephrine (EPI) and norepinephrine (NE) responses to insulin-induced hypoglycemia (ITT) 3 weeks before the space flight (SF), on the fifth day of SF, on days 2 and 16 after landing in the first Slovak astronaut, and before and on the fifth day of prolonged bed rest (BR) in 15 military aircraft pilots, aged 33.5 +/- 1.4 years, body mass index (BMI) 26.5 +/- 0.7 kg/m(2), maximal oxygen uptake (VO(2max)) 55.2 +/- 2.4 mL/kg/min, who volunteered for the study. ITT was induced by i.v. administrations of 0.1 IU/kg body weight insulin (Actrapid HM) in a bolus. Insulin administration led to a comparable hypoglycemia in preflight, actual flight conditions, and before and after bed rest. ITT led to a pronounced increase in EPI levels and moderate increase in NE in preflight studies. However, an evidently reduced plasma elevation of EPI was found after insulin administration during SF and during BR. Thus, during the real microgravity in SF and simulated microgravity in BR, ITT activates the adrenomedullary system to less extent that at conditions of the Earth's gravitation. Post-flight changes in EPI and NE did not differ from those of preflight values, since SF was relatively short (8 days) and the readaptation to Earth's gravitation was fast. It seems that an increased blood flow in brain might be responsible for the reduced EPI response to insulin. Responses to ITT in physically fit subjects indicate the stimulus specificity of the deconditioning effect of 5 days of bed rest on the stress response.

Heart rate variability and short duration spaceflight: relationship to post-flight orthostatic intolerance

Background

Upon return from space many astronauts experience symptoms of orthostatic intolerance. Research has implicated altered autonomic cardiovascular regulation due to spaceflight with further evidence to suggest that there might be pre-flight autonomic indicators of post-flight orthostatic intolerance. We used heart rate variability (HRV) to determine whether autonomic regulation of the heart in astronauts who did or did not experience post-flight orthostatic intolerance was different pre-flight and/or was differentially affected by short duration (8 – 16 days) spaceflight. HRV data from ten-minute stand tests collected from the 29 astronauts 10 days pre-flight, on landing day and three days post-flight were analysed using coarse graining spectral analysis. From the total power (P_TOT), the harmonic component was extracted and divided into high (P_HI: >0.15 Hz) and low (P_LO: = 0.15 Hz) frequency power regions. Given the distribution of autonomic nervous system activity with frequency at the sinus node, P_HI/P_TOT was used as an indicator of parasympathetic activity; P_LO/P_TOT as an indicator of sympathetic activity; and, P_LO/P_HI as an estimate of sympathovagal balance.

Results

Twenty-one astronauts were classified as finishers, and eight as non-finishers, based on their ability to remain standing for 10 minutes on landing day. Pre-flight, non-finishers had a higher supine P_HI/P_TOT than finishers. Supine P_HI/P_TOT was the same pre-flight and on landing day in the finishers; whereas, in the non-finishers it was reduced. The ratio P_LO/P_HI was lower in non-finishers compared to finishers and was unaffected by spaceflight. Pre-flight, both finishers and non-finishers had similar supine values of P_LO/P_TOT, which increased from supine to stand. Following spaceflight, only the finishers had an increase in P_LO/P_TOT from supine to stand.

Conclusions

Both finishers and non-finishers had an increase in sympathetic activity with stand on pre-flight, yet only finishers retained this response on landing day. Non-finishers also had lower sympathovagal balance and higher pre-flight supine parasympathetic activity than finishers. These results suggest pre-flight autonomic status and post-flight impairment in autonomic control of the heart may contribute to orthostatic intolerance. The mechanism by which higher pre-flight parasympathetic activity might contribute to post-flight orthostatic intolerance is not understood and requires further investigation.

Renal, Endocrine, and Cardiovascular Responses to Bed Rest in Male Subjects on a Constant Diet

Background

Exposure to actual and simulated microgravity induces cardiovascular deconditioning through a variety of factors. Although the mechanisms involved remain uncertain, one involves alterations in volume-regulating systems—the hypothesis being tested in this study. To maximize our ability to detect subtle changes in the volume-regulating systems, subjects were studied on a high-average salt intake to maximally suppress these systems basally.

Methods

Fourteen healthy male subjects underwent 14-day head-down tilt bed rest (HDTB) during which a constant 200 mEq sodium, 100 mEq potassium diet was maintained. Daily 24-hour urine collection was performed; plasma renin activity, serum aldosterone, plethysmography, and cardiovascular system identification were performed during a control period (pre-HDTB) and at the end of HDTB (end HDTB).

Results

Sodium excretion increased initially (pre-HDTB = 182.8 ± 10.4 mEq/total volume; early HDTB = 236.4 ± 13.0; p = .002) and then returned to baseline values. Potassium excretion increased 4 days after the initiation of HDTB and remained elevated thereafter (pre-HDTB = 82.2 ± 2.4/total volume; mid- to late HDTB = 89.4 ± 2.1; p = .02). Plasma renin activity increased significantly with HDTB (pre-HDTB = 1.28 ± 0.21 ng/mL/h; end HDTB = 1.69 ± 0.18; p = .01), but serum aldosterone did not change. A significant decrease in autonomic responsiveness and an increase in leg compliance were observed.

Conclusions

We conclude that even in the presence of a high-average salt intake diet, simulated microgravity leads to renal, cardioendocrine, and cardiovascular system alterations that likely contribute to cardiovascular deconditioning.

Bursting into space: alterations of sympathetic control by space travel

AIM

Astronauts return to Earth with reduced red cell masses and hypovolaemia. Not surprisingly, when they stand, their heart rates may speed inordinately, their blood pressures may fall, and some may experience frank syncope. We studied autonomic function in six male astronauts (average +/- SEM age: 40 +/- 2 years) before, during, and after the 16-day Neurolab space shuttle mission.

METHOD

We recorded electrocardiograms, finger photoplethysmographic arterial pressures, respiration, peroneal nerve muscle sympathetic activity, plasma noradrenaline and noradrenaline kinetics, and cardiac output, and we calculated stroke volume and total peripheral resistance. We perturbed autonomic function before and during spaceflight with graded Valsalva manoeuvres and lower body suction, and before and after the mission with passive upright tilt.

RESULTS

In-flight baseline sympathetic nerve activity was increased above pre-flight levels (by 10-33%) in three subjects, in whom noradrenaline spillover and clearance also were increased. Valsalva straining provoked greater reductions of arterial pressure, and proportionally greater sympathetic responses in space than on Earth. Lower body suction elicited greater increases of sympathetic nerve activity, plasma noradrenaline, and noradrenaline spillover in space than on Earth. After the Neurolab mission, left ventricular stroke volume was lower and heart rate was higher during tilt, than before spaceflight. No astronaut experienced orthostatic hypotension or pre-syncope during 10 min of post-flight tilting.

CONCLUSION

We conclude that baseline sympathetic outflow, however measured, is higher in space than on earth, and that augmented sympathetic nerve responses to Valsalva straining, lower body suction, and post-flight upright tilt represent normal adjustments to greater haemodynamic stresses associated with hypovolaemia.

Effects of rapid saline infusion on orthostatic intolerance and autonomic tone after 20 days bed rest

To test whether acute volume expansion can normalize orthostatic intolerance and autonomic tone after prolonged bed rest (BR), 23 men were subjected to 20 days BR. Left ventricular (LV) echocardiography was performed during the lower body negative pressure (LBNP) test before and after BR with and without preceding rapid infusion of saline (1,500 ml/30 min). Saline infusion restored heart rate, LV dimension, and stroke volume during LBNP, increased cardiac output (from 4.1 +/- 1 to 5.3 +/- 1 L/min), and normalized LBNP tolerance time (from 11 +/- 4 to 23 +/- 6 minutes). In 9 men, a Holter electrocardiogram was recorded on the day before BR, the fourth and twentieth days of BR, and the day after BR. The high-frequency component of heart rate variability during sleep gradually decreased and reached the lowest level on the day after BR (100%, 66 +/- 16%, 39 +/- 18%, 10 +/- 8%). Thus, restoring decreased blood volume is an effective countermeasure for orthostatic intolerance after BR. However, decreased vagal tone persisted, suggesting reset autonomic tone.

Influence of microgravity on astronauts' sympathetic and vagal responses to Valsalva's manoeuvre

When astronauts return to Earth and stand, their heart rates may speed inordinately, their blood pressures may fall, and some may experience frank syncope. We studied brief autonomic and haemodynamic transients provoked by graded Valsalva manoeuvres in astronauts on Earth and in space, and tested the hypothesis that exposure to microgravity impairs sympathetic as well as vagal baroreflex responses. We recorded the electrocardiogram, finger photoplethysmographic arterial pressure, respiration and peroneal nerve muscle sympathetic activity in four healthy male astronauts (aged 38-44 years) before, during and after the 16 day Neurolab space shuttle mission. Astronauts performed two 15 s Valsalva manoeuvres at each pressure, 15 and 30 mmHg, in random order. Although no astronaut experienced presyncope after the mission, microgravity provoked major changes. For example, the average systolic pressure reduction during 30 mmHg straining was 27 mmHg pre-flight and 49 mmHg in flight. Increases in muscle sympathetic nerve activity during straining were also much greater in space than on Earth. For example, mean normalized sympathetic activity increased 445% during 30 mmHg straining on earth and 792% in space. However, sympathetic baroreflex gain, taken as the integrated sympathetic response divided by the maximum diastolic pressure reduction during straining, was the same in space and on Earth. In contrast, vagal baroreflex gain, particularly during arterial pressure reductions, was diminished in space. This and earlier research suggest that exposure of healthy humans to microgravity augments arterial pressure and sympathetic responses to Valsalva straining and differentially reduces vagal, but not sympathetic baroreflex gain.

Adaptations to a 7-day head-down bed rest with thigh cuffs

PURPOSE

Thigh cuffs were two elastic strips fixed at the upper part of each thigh, which limits the shift of fluid from the legs into the cardio-thoracic region. The purpose of this study was to examine the effects of thigh cuffs on hormonal and plasma volume responses and orthostatic tolerance during a 7-day head-down bed rest (HDBR).

METHODS

Orthostatic tolerance, plasma volume, total body water, blood volume-regulating hormones, and hydro-electrolyte responses were measured in eight healthy men (age range, 25-40 yr), using thigh cuffs 10 h daily during 7 d of -6 degrees HDBR.

RESULTS

Thigh cuffs worn during HDBR attenuated the decrease in plasma volume observed after HDBR (thigh cuffs: -5.85 +/- 0.95% vs control: -9.09 +/- 0.82%, P < or = 0.05). During this experiment, there was no significant change in total body water. Thus, the hypovolemia did not result from a loss of water but from a fluid shift from the blood compartment into the interstitial and/or intracellular compartment. Hormonal responses during HDBR and stand test were not modified by the thigh cuffs. Thigh cuffs had no significant effect on the clinical symptoms of orthostatic intolerance after HDBR.

CONCLUSIONS

Thigh cuffs worn during HDBR blunted the decrease in plasma volume but did not reduce orthostatic intolerance; thus, they are not a completely effective countermeasure. Furthermore, hypovolemia seems to be necessary but not sufficient to induce orthostatic intolerance after HDBR.

Does edema formation occur in the rabbit brain exposed to head-down tilt?

Earlier studies showed that exposure to microgravity caused cephalad fluid shift, increased capillary pressure in the head, and produced facial edema and nasal congestion. In the present study, edema formation in the brain was investigated in rabbits exposed to simulated microgravity, head-down tilt (HDT), by measuring water content and histological examinations. Water content in the brain tissues of rabbits exposed to 2 and 8 days of HDT did not increase significantly compared with that of control animals. Neither vital staining using Evans blue nor immunohistochemical examination demonstrated extravasation of plasma constituents in the brain tissues of the HDT rabbits. Although marked congestion was noted in the brain, hematoxylin and eosin staining did not show edematous changes, such as distension of the perivascular and pericellular spaces and vacuolar appearance, in the tissues obtained from HDT rabbits. Transmission electron microscopy revealed that tight junctions of the capillary endothelium were intact in the HDT rabbits. These results suggest that either HDT up to 8 days does not cause brain edema in rabbits or it induces only a slight brain edema which is hard to be demonstrated by measurement of water content or histological examinations.

Influences of thigh cuffs on the cardiovascular system during 7-day head-down bed rest

Thigh cuffs, presently named “bracelets,” consist of two straps fixed to the upper part of each thigh, applying a pressure of 30 mmHg. The objective was to evaluate the cardiac, arterial, and venous changes in a group of subjects in head-down tilt (HDT) for 7 days by using thigh cuffs during the daytime, and in a control group not using cuffs. The cardiovascular parameters were measured by echography and Doppler. Seven days in HDT reduced stroke volume in both groups (−10%; P < 0.05). Lower limb vascular resistance decreased more in the cuff group than in the control group (−29 vs. −4%; P < 0.05). Cerebral resistance increased in the control group only (+6%; P < 0.05). The jugular vein increased (+45%; P < 0.05) and femoral and popliteal veins decreased in cross-sectional area in both groups (−45 and −8%, respectively; P < 0.05). Carotid diameter tended to decrease (−5%; not significant) in both groups. Heart rate, blood pressure, cardiac output, and total resistance did not change significantly. After 8 h with thigh cuffs, the cardiac and arterial parameters had recovered their pre-HDT level except for blood pressure (+6%; P < 0.05). Jugular vein size decreased from the pre-HDT level (−21%; P < 0.05), and femoral and popliteal vein size increased (+110 and +136%, respectively; P < 0.05). The thigh cuffs had no effect on the development of orthostatic intolerance during the 7 days in HDT.

Influences of thigh cuffs on the cardiovascular system during 7-day head-down bed rest

Thigh cuffs, presently named "bracelets," consist of two straps fixed to the upper part of each thigh, applying a pressure of 30 mmHg. The objective was to evaluate the cardiac, arterial, and venous changes in a group of subjects in head-down tilt (HDT) for 7 days by using thigh cuffs during the daytime, and in a control group not using cuffs. The cardiovascular parameters were measured by echography and Doppler. Seven days in HDT reduced stroke volume in both groups (-10%; P < 0.05). Lower limb vascular resistance decreased more in the cuff group than in the control group (-29 vs. -4%; P < 0.05). Cerebral resistance increased in the control group only (+6%; P < 0.05). The jugular vein increased (+45%; P < 0.05) and femoral and popliteal veins decreased in cross-sectional area in both groups (-45 and -8%, respectively; P < 0.05). Carotid diameter tended to decrease (-5%; not significant) in both groups. Heart rate, blood pressure, cardiac output, and total resistance did not change significantly. After 8 h with thigh cuffs, the cardiac and arterial parameters had recovered their pre-HDT level except for blood pressure (+6%; P < 0.05). Jugular vein size decreased from the pre-HDT level (-21%; P < 0.05), and femoral and popliteal vein size increased (+110 and +136%, respectively; P < 0.05). The thigh cuffs had no effect on the development of orthostatic intolerance during the 7 days in HDT.

Contributory factors to orthostatic intolerance after simulated weightlessness

Various factors may contribute to orthostatic intolerance (OI) observed after space flights or simulated weightlessness such as bed rest experiments: individual physical and physiological factors (arterial blood pressure (BP), height), physiological changes induced by real or simulated weightlessness (hypovolaemia, increase in venous distensibility), and space flight or simulation conditions (duration and counter-measure application). Our purpose was to test which of these factors were dominant in contributing to the OI. This was assessed in 47 healthy men participating in bed rest experiments of 4, 14, 28, 30 and 42 days, with or without counter-measures (medical stockings, lower-body negative pressure (LBNP), LBNP + muscular exercise). Nineteen subjects did not finish the orthostatic test (60 degrees head-up tilt or stand test) after bed rest. The occurrence of OI was associated with greater height, low resting BP, greater changes in resting lower-limb venous distensibility throughout the bed rest, and absence of counter-measures.

Autonomic control of cardiovascular dynamics during weightlessness

Measuring cardiovascular dynamics is a new method of assessing the autonomic regulation of the cardiovascular system, it provides an easily-implemented non-invasive way of monitoring the effects of weightlessness on this regulatory function. The major findings of studies on cardiovascular dynamics during actual or simulated weightlessness are presented, taking into account the recent consensus on this approach. Future improvements of these studies are discussed.

Changes in the sympathetic nervous system induced by 42 days of head-down bed rest

Changes in autonomic nervous system activity could be linked to the orthostatic intolerance (OI) that individuals suffer after a spaceflight or head-down bed rest (HDBR). We examined this possibility by assessing the sympathetic nervous system activity during 42 days of HDBR in seven healthy men. Heart rate variability was studied with the use of power spectral analysis, which provided indicators of the sympathetic (SNSi) and parasympathetic (PNSi) nervous system influences on the heart. Urinary catecholamines and the spontaneous baroreflex sensitivity were measured. Urinary catecholamines decreased by 21.3%, showing a decrease in SNSi. Heart rate variability was greatly reduced during 42 days of HDBR with a drop in PNSi but with no significant changes in SNSi. The baroreflex sensitivity was greatly reduced (30.7%) on day 42 of HDBR. These results suggest a dissociation between the catecholamine response and the SNSi of the heart rate. This dissociation could be the consequence of an increase in beta-adrenergic receptor density and/or activity induced by a decrease in catecholamines during HDBR. The subjects who suffered from OI also had a greater sympathetic response and much lower baroreflex sensitivity when supine than those who finished the stand test. However, the mean response of all subjects indicated that the sympathetic activity (catecholamine excretion) was probably slightly inhibited during HDBR and could contribute to OI.