Clinical effects of thigh cuffs during a 7-day 6 degrees head-down bed rest

Thigh cuffs are used by Russian cosmonauts to limit the fluid shift induced by space flight. A ground simulation using the head-down bed rest (HDBR) model was performed to assess the effects of thigh cuffs on clinical tolerance and orthostatic adaptation. 8 male healthy volunteers (32.4 +/- 1.9 years) participated twice in a 7-day HDBR--one time with thigh cuffs (worn daily from 9 am to 7 pm) (TC) and one time without (WTC). Orthostatic tolerance was assessed by a 10 minute stand test and by a LBNP test (5 min at -15, -30, -45 mmHg) before (BDC-1) and at the end of the HDBR period (R+1). Plasma volume was measured before and at the end of HDBR by the Evans blue dye dilution technique. Thigh cuffs limits headache due to fluid shift, as well as the loss in plasma volume (TC: -5.85 +/- 0.95%; WTC: -9.09 +/- 0.82%, p<0.05). The mean duration of the stand test (R+1) did not differ in the two group (TC 7.1 +/- 1.3 min; WTC 7.0 +/- 1.0 min). The increase in HR and decrease in diastolic blood pressure were slightly but significantly larger without thigh cuffs. Duration of the LBNP tests did not differ with thigh cuffs. Thigh cuffs limit the symptoms due to fluid shift and the loss in plasma volume. They partly reduced the increase in HR during orthostatic stress but had no effect on duration of orthostatic stress tests.

Use of thigh pressure cuffs to modulate simulated microgravity-induced changes in the skin measured with high-resolution B-scan ultrasound

OBJECTIVE

The aim of the present study was to evaluate the fluid shift in a simulated microgravity experiment and to test the use of thigh cuffs to help alleviate the problem.

METHODS

The change in skin thickness was assessed by a 20 MHz B-scan ultrasound device. This was performed on eight volunteers who underwent two successive 7-day periods of -6 degrees anti-orthostatic bed-rest, with or without the daytime use of thigh cuffs. The thigh cuffs were used to counteract the development of facial oedema.

RESULTS

In the control group (without thigh cuffs), the results showed a steady increase in skin thickness of the combined dermis and hypodermis of the forehead and a reduction of the thickness of this tissue on the tibia. For the countermeasure group, although thigh cuffs were only employed during the daytime - being removed at night - their use reduced the amplitude and kinetics of the fluid shift, resulting in greater beneficial effects at the end of the day than early in the morning.

CONCLUSION

These results of objective measurements of skin made using a non-invasive high frequency ultrasonography method confirm reports by cosmonauts of a reduction in facial oedema and a more 'comfortable' adaptation to microgravity by the use of thigh cuffs during space flight. This system is potentially promising for investigating fluid shifts in the skin and may prove useful in the evaluation of some oedematous skin diseases, as well as their therapy.

Weightlessness as an accelerated model of nutritional disturbances

Food intake and eating patterns, body functions and composition are significantly altered by short-duration space flight. Prolonged missions lasting weeks or months further aggravate these changes, and are responsible for acute or chronic physical impairments at return to ground conditions. Current projects of missions to Mars, resulting in 2 years of microgravity conditions, stress the critical need for the development of optimal nutritional programs and physical countermeasures to prevent body mass and function alterations. This review outlines ground models of microgravity simulation, summarizes the major effects of weightlessness on body composition, protein metabolism, hormonal pattern, and muscle function, and addresses contradictory findings related to the oxidative stress secondary to space flight. Potential countermeasures, such as nutrient intake and physical conditioning, as well as areas of interest for future research both in ground and space medicine, are discussed.

Endocrine responses to 7 days of head-down bed rest and orthostatic tests in men and women

The objective of this study was to investigate plasma volume (PV), total body water, hormones and hydroelectrolyte responses in eight males (25-40 years) and eight females (25-31 years) during 7 days of exposure to simulated microgravity (-6 degrees head-down bed rest, HDBR). Bed rest is a model that has commonly been used to simulate spaceflight. Heart rate (HR), blood pressure (BP) and vasoactive hormone responses were studied before and after HDBR during a 10-min stand test. No change in total body water and body mass was noted in either sex. The decrease in PV was similar in both men (9.1 +/- 1.4%) and women (9.4 +/- 0.8%). Urinary normetanephrine (NMN) was decreased during HDBR in both sexes. Urinary metanephrine (MN) and plasma catecholamines were unchanged. Daily urinary excretion of urea, an indirect index of protein breakdown, was increased only in the female subjects during HDBR. Plasma active renin (AR) and aldosterone were increased in both sexes, but urinary atrial natriuretic peptide (ANP) and arginine vasopressin (AVP) were unchanged throughout the study. Also, the hormonal responses to 7 days of HDBR were comparable between men and women. Moreover, the results show similar cardiovascular and endocrine responses to standing after HDBR. However, the orthostatic intolerance following HDBR was associated with a blunted increase in noradrenaline (NA) only in the women during the stand test. It is concluded that: (i) 7 days of physical inactivity achieved during HDBR resulted in a reduced sympathetic activity in both sexes and alterations in protein metabolism in women and (ii) standing after HDBR resulted in an attenuated release of noradrenaline in women.

Effect of prolonged head-down bed rest on complex cardiovascular dynamics

We postulated that a change in complex dynamics of the cardiovascular system could be involved in the orthostatic intolerance observed after simulated weightlessness. Supine recordings of 1024 consecutive pulse intervals and systolic blood pressures were obtained on 7 subjects adapted to a 42 day head-down bed rest (day 22 and 42) but also before and 6 days after head-down bed rest (-6 degrees). Coarse graining spectral analysis was used to extract the non-harmonic (fractal) component from each time series. The power spectral densities of this fractal component are inversely proportional to their frequency (1/f beta). We fitted an inverse power law estimate to the fractal component to determine the spectral exponent beta. The complex dynamics of blood pressure and heart rate variability were also analyzed by correlation dimension and non-linear prediction. Bed rest induced orthostatic intolerance in 4 subjects. There was a significant increase in the spectral exponent beta of RR-interval variability during and after head-down bed rest (before: 1.039 +/- 0.090; during: 1.552 +/- 0.080 and 1.547 +/- 0.100; after: 1.428 +/- 0.040). Analysis of the blood pressure dynamics indicated lower correlation dimensions during head-down bed rest and higher coefficients of predictability after head-down bed rest. Complexity alterations of RR-interval and blood pressure variability were not linked with one another during head-down bed rest. These alterations seemed to be correlated with the orthostatic intolerance observed after bed rest. These results suggest a change of the integration level of cardiovascular autonomic regulation.

No effect of venoconstrictive thigh cuffs on orthostatic hypotension induced by head-down bed rest

Orthostatic intolerance (OI) is the most serious symptom of cardiovascular deconditioning induced by head-down bed rest or weightlessness. Wearing venoconstrictive thigh cuffs is an empirical countermeasure used by Russian cosmonauts to limit the shift of fluid from the lower part of the body to the cardio-cephalic region. Our aim was to determine whether or not thigh cuffs help to prevent orthostatic hypotension induced by head-down bed rest. We studied the effect of thigh cuffs on eight healthy men. The cuffs were worn during the day for 7 days of head-down bed rest. We measured: orthostatic tolerance (stand tests and lower body negative pressure tests), plasma volume (Evans blue dilution), autonomic influences (plasma noradrenaline) and baroreflex sensitivity (spontaneous baroreflex slope). Thigh cuffs limited the loss of plasma volume (thigh cuffs: -201 +/- 37 mL vs. control: -345 +/- 42 mL, P < 0.05), the degree of tachycardia and reduction in the spontaneous baroreflex sensitivity induced by head-down bed rest. However, the impact of thigh cuffs was not sufficient to prevent OI (thigh cuffs: 7.0 min of standing time vs. control: 7.1 min). Decrease in absolute plasma volume and in baroreflex sensitivity are known to be important factors in the aetiology of OI induced by head-down bed rest. However, dealing with these factors, using thigh cuffs for example, is not sufficient to prevent OI. Other factors such as venous compliance, microcirculatory changes, peripheral arterial vasoconstriction and vestibular afferents must also be considered.

Characterisation of gravity-induced facial skin oedema using biophysical measurement techniques

BACKGROUND/AIMS

In humans, the microgravity environment can be expected to induce swelling of facial tissues and shrinking of the tissues in the lower limbs, together with a loss in body weight. To evaluate fluid shifts in skin, the head-down bed-rest model was used. The aim of the present study was to evaluate the appearance of facial oedema in subjects undergoing anti-orthostatic bed-rest at an angle of -10 degrees.

METHODS

The forehead of each of four subjects was measured before and after 1, 10 and 24 h in this head-down tilt position. At these time points, interstitial fluid migration and facial oedema were assessed using a high resolution B-scan ultrasound and a device for measuring the skin's mechanical properties.

RESULTS

The results obtained showed a progressive increase in dermal thickness and initial stress, and a reduction in stiffness and elasticity of the skin during the study period.

CONCLUSIONS

This preliminary study has demonstrated the feasibility of the method in measuring fluid displacement and retention in the skin. Furthermore, it highlights the influence of fluids on the mechanical behaviour of the skin. These techniques could be used for studying the redistribution of liquid masses during periods spent in space.

Orthostatic tolerance and hormonal changes in women during 120 days of head-down bed rest

BACKGROUND

Women will be included as mission specialists in the upcoming International Space Station program. This paper describes the changes in volume-regulating hormones and determines the degree of degradation in orthostatic tolerance in a group of women after 120 d of bed rest. The aim of this study was to test a countermeasure program to be used by women during long-duration spaceflights.

METHODS

For 120 d of -6 degrees head-down bed rest (HDBR), eight healthy women were assigned either to a no-countermeasure (No-CM, n = 4), or to a countermeasure (CM, n = 4) group. In the countermeasure group, exercise began after 2 wk, pharmacological agents were given during the 1st and 3rd mo, and the "Centaur" suit was worn on the last day of bed rest and during the day time for several days after bed rest. Diet supplements were taken during the 1st and 4th mo of HDBR. Tilt tests were run before and after HDBR.

RESULTS

After the HDBR, none of the CM subjects, had pre-syncopal or syncopal symptoms during tilt tests: BP was well maintained in the CM group, while heart rate and BP changed in the No-CM group. In plasma, atrial natriuretic peptide (ANP) increased in both groups and remained high throughout HDBR, while aldosterone increased and remained elevated in the No-CM group. Natriuresis was decreased during HDBR.

CONCLUSION

The CM protocols used during this study were efficient and prevented orthostatic intolerance for the four CM subjects. It would be necessary to obtain more data regarding this set of CM protocols on female subjects to lead to statistical and formal conclusions.

Effects of a three-day head-down tilt on renal and hormonal responses to acute volume expansion

To clarify whether exposure to 6 degrees head-down tilt (HDT) leads to alterations in body fluid volumes and responses to a saline load similar to those observed during space flight we investigated eight healthy subjects during a 4-day, 6 degrees HDT and during a time-control ambulatory period with cross-over. Compared with the ambulatory period, HDT was associated with greater urinary excretion of water and sodium (UV, U(Na)V) from 0 to 12 h (cumulated UV 1,781 +/- 154 vs. 1,383 +/- 170 ml, P < 0.05; cumulated U(Na)V 156 +/- 14 vs. 117 +/- 9 mmol, P < 0.05), and with higher plasma atrial natriuretic factor (ANF) at 4 h. Hemoglobin and hematocrit increased over the first 24 h, and blood and plasma volumes were decreased after 48 h of HDT (P < 0.05). Plasma renin activity (PRA) and aldosterone did not differ between the two groups. With prolongation of HDT, UV and U(Na)V returned close to baseline values. On the fourth HDT day, a 30-min infusion of 20 ml/kg isotonic saline was performed, while a large oral water load maintained a high urine output. The ambulatory period experiment was done with the subjects in the acute supine posture. Sodium excreted within 4 h of loading was 123 +/- 8 mmol during HDT vs. 168 +/- 16 mmol during the ambulatory period (P < 0.05). The increase in plasma ANF and decrease in PRA were greater during HDT than during the ambulatory period (ANF 30 +/- 5 vs. 13 +/- 4 pg/ml, P < 0.05; PRA -1.4 +/- 0.4 vs. -0.5 +/- 0.2 ng. ml(-1). h(-1), P < 0.05). Our data suggest that after a 3-day HDT period, thoracic volume receptor loading returns to the level seen in the upright position, leading to blunted responses to volume expansion, compared with acute supine control.

Observations in energy and macronutrient intake during prolonged bed-rest in a head-down tilt position

OBJECTIVE

to report observations in energy and macronutrient intakes, and body weight during prolonged bed-rest in a head down tilt (HDT) position.

DESIGN

open study, each subject was his own control, and was studied during 14 days of baseline, 42 days of -6 degrees HDT bed-rest, and 12 days of recovery.

SUBJECTS

eight healthy young man were recruited, one dropped out.

METHODS

energy and macronutrient content of the diet were calculated from weighed amounts of food consumed and French food composition tables.

RESULTS

body weight declined during HDT (74.0+/-3.2 to 71.8+/-3.2 kg, P<< 0.001) and increased during recovery (72.7+/-3.2 kg, P<< 0. 001). Energy intake decreased during HDT (by 17% after 4-5 weeks) and increased during recovery but remained lower than during baseline (P<< 0.001). During HDT fat intake, expressed by a percentage of energy, decreased (P<< 0.01) while carbohydrate increased (P= 0.04); protein intake did not change (P= 0.08). The reverse trends were observed during recovery.

CONCLUSIONS

the present study reports a spontaneous reduction in energy and relative fat intake during prolonged HDT bed rest. We believe that these findings have implications for the clinical setting.

Hormonal changes during a 20-week confinement

BACKGROUND

When the European Space Agency planned the EUROMIR'95 long-duration flight with a European astronaut on board the Russian orbital MIR station, it organized simultaneously a ground simulation, called the Human Behaviour Study, of this manned space mission. The ground simulation was a confinement experiment, and this paper describes the changes in volume-regulating hormones that occurred during and after 20 weeks of confinement.

METHODS

In a normobaric diving chamber, 3 subjects were confined for 135 d. Arterial pressure, plasma concentrations of blood volume-regulating hormones (active renin and arginine-vasopressin), and urinary variables (aldosterone, arginine-vasopressin, and metabolites of catecholamines) were measured before, during, and after confinement.

RESULTS

Arterial pressure was increased from week 1 until week 15 of confinement, while heart rate was elevated from week 6 until the end of the simulation. Plasma active renin was elevated throughout the confinement (after week 6). Urine volume increased transitively on the first 2 d of confinement.

CONCLUSIONS

The results obtained during this long-term confinement experiment have major importance regarding concerns about spaceflight and bed rest data, because we observed hormonal changes during the experiment that normally are assigned to the fluid shift that occurs in weightlessness or in the head-down tilt position (i.e., an increase of renin, an increase of urinary volume during the first two days, and a decreased urinary cyclic guanosine monophosphate.

Effects of a 60-day confinement on the blood pressure, hormonal responses and body fluids of a mixed crew

During the EXEMSI experiment, an international crew of 4 subjects (1 woman and 3 men) was confined for 60 days in a normobaric diving chamber (with 1060 mbar atmospheric pressure) to simulate life in a space station and to assess the effects of confinement on psychological and physiological factors. Blood pressure and blood volume regulating hormones (atrial natriuretic peptide, renin, aldosterone) and urine data (24-h urine outputs, ionogram) were measured before (BDC: baseline data collection), during (D: day) and after (R: recovery) confinement. We also measured energy expenditure and total body water, 14 days before, and after 27 days of confinement, by the double-labeled water method. We found a marked increase in 24-h urine output during most of the confinement in the men and the woman. Body weight (-1.8 +/- 0.9 kg) and energy expenditure (-1064 +/- 143 kcal/d, p<0.01) decreased in the 3 men. The total body water (TBW) decreased by 1.5 +/- 1.2 l in the men. Stress was not indicated by plasma and urine catecholamines but plasma growth hormone was elevated on D2 (p<0.01 vs. BDC) in the men. This study shows that confinement conditions can modify body fluid (increases in 24-h urine outputs and TBW changes) and energetic metabolisms.

MEDES clinical research facility as a tool to prepare ISSA space flights

This new multi-disciplinary medical experimentation center provides the ideal scientific, medical and technical environment required for research programs and to prepare international space station Alpha (ISSA) missions, where space and healthcare industries can share their expertise. Different models are available to simulate space flight effects (bed-rest, confinement,...). This is of particular interest for research in Human psychology, physiology, physiopathology and ergonomics, validation of biomedical materials and procedures, testing of drugs, and other healthcare related products. This clinical research facility (CRF) provides valuable services in various fields of Human research requiring healthy volunteers. CRF is widely accessible to national and international, scientific, medical and industrial organisations. Furthermore, users have at their disposal the multi-disciplinary skills of MEDES staff and all MEDES partners on a single site.

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.

Cardiovascular responses to orthostatic tests after a 42-day head-down bed-rest

Cardiovascular responses to orthostatic tests were studied before and after a prolonged 42 day-head-down bed-rest (HDBR;-6 degrees) experiment simulating a long duration space flight. Seven men participating in the experiment underwent stand tests (10 min) and lower body negative pressure (LBNP) tests (5 min at -25, -35, -45 mmHg). Heart rate variability and spontaneous baroreflex response slope (SBS) were analysed to assess autonomic nervous system responses. Changes in plasma volume (PV) were assessed at the end of HDBR. At the end of HDBR, four subjects could not complete the stand tests and one could not complete the LBNP test. A higher stressed heart rate with standing (+ 44% before and + 57% after HDBR) and LBNP exposure (+ 19% before and + 34% after HDBR) were observed. A decrease in blood pressure (BP) reflecting a reduced vasomotor response was only observed with standing (mean BP + 21% before and -8% after HDBR); LBNP was less sensitive probably because it was performed 6 h after the stand test. The PV decreased by 10.6%. A decline in spectrum total power reflecting a reduced variance of RR-interval, a decrease in parasympathetic activity and an increase in sympathetic one were observed at the end of HDBR. The reduced parasympathetic indicator and SBS would suggest that the vagal nerve component of the cardiovascular control had been diminished. Except for a lower BP when standing after HDBR, no significant difference was observed between finishers and non-finishers. Autonomic nervous system changes including reduced vasomotor responses constituted important contributors to the orthostatic intolerance observed here and after space flights. Some autonomic and PV changes seemed to be opposite to those observed with training and would suggest a role of reduced physical activity in cardiovascular changes induced by HDBR.

Cardiovascular and hormonal changes induced by isolation and confinement

The cardiovascular changes induced by microgravity are in general described as the result of the loss of hydrostatic pressure. Other factors are also important: restricted environment with 1) elimination of mobility, action and 2) isolation always found in space environment or during simulation studies. Several studies indicate an analogy between microgravity and confinement. The results of simulation studies could be misinterpreted without a control confinement study present in the protocol.

Erythropoietin under real and simulated microgravity conditions in humans

The aim of this study was to analyze the time course of erythropoietin (EPO) during Earth-bound microgravity simulations such as bed rest, isolation and confinement (IC), head-down tilt (HDT; -6 degrees), and immersion to evaluate which factors could contribute to alterations in EPO under real microgravity conditions during and after short- (< 10 days) and long-term (> 6 mo) spaceflights. During bed rest (24h), no significant changes in EPO could be observed. Subjects confined in a diving chamber facility for 60 days showed a decrease in EPO. In the recovery period a slight increase was observed, but EPO concentrations did not reach the pre-IC control level. In the control period before HDT, subjects showed normal resting values for EPO, but on day 2 of HDT the EPO concentrations were decreased (P < 0.01). Later the EPO levels remained below the control value and were increased after HDT (P < 0.05). After immersion (24 h) increased EPO concentrations could be determined (P < 0.05). During a short-term spaceflight the astronauts showed in-flight (day 4) decreased and unchanged EPO concentrations. During a long-term spaceflight, 24 h after recovery, the cosmonaut showed slightly elevated EPO concentration, which increased markedly during the following days. It is concluded that 1) HDT (-6 degrees) causes a rapid decrease in EPO in humans, 2) IC per se leads to diminished EPO concentrations, 3) EPO regulation in humans during short- and long-term spaceflights might be different, 4) changes in central blood volume, i.e., central venous pressure, seem to be involved in the modulation of EPO production and release under simulated and real microgravity conditions, and 5) the HDT (-6 degrees) Earth-bound simulation reflects mostly the changes in EPO production and release observed under real microgravity conditions in humans.

Diurnal rhythms of plasma renin activity, atrial natriuretic peptide and arterial pressure during head-down bed rest in humans

The effects of prolonged head-down bed rest on the rhythms of several parameters (blood pressure, heart rate, haematocrit, plasma renin activity (PRA), atrial natriuretic peptide (ANP) were assessed in six healthy men, aged 33 (SEM 2) years, who were submitted to bed rest for 28 days (D1-28). Systolic and diastolic blood pressure (BPs and BPd) and heart rate were measured at 0700 and 1900 hours; circulating PRA and ANP were determined from blood samples drawn at 0800, 1000, 1200, 1500, 1800 and 2200 hours before bed rest (D - 5), D1, 2, 7, 20, 27 during bed rest and post bed rest (D + 2). The BPs was the lowest at 0700 hours and increased at 1900 hours. There was a significant difference between values during all the measurements. The BPd and heart rate were lower at 0700 hours before and after bed rest and no significant difference appeared between these two values during the bed rest. The PRA and ANP concentrations were more stable during bed rest, and had not returned to original rhythmicity 2 days after bed rest. The mean daily concentration of ANP decreased during bed rest. It would seem from this study that changes occur in those rhythms during bed rest.

Cardiovascular and hormonal response during a 4-week head-down tilt with and without exercise and LBNP countermeasures

To determine whether exercise and Lower Body Negative Pressure (LBNP) during 28 days of -6 degrees head-down tilt (HDT) would modify orthostatic tolerance and blood volume regulating hormones, twelve healthy men were assigned to either a no- countermeasure (No-CM, n=6), or a countermeasure (CM, n=6) group. LBNP sessions consisted of 15 minutes exposure to -30 mm Hg, on days 16, 18, 20 and 22-28 of HDT. Muscular exercise began on day 8 and consisted of combined graded dynamic and isometric resistance bilateral leg exercise on a specially designed supine ergometer, in two sessions of 15-20 min. each, every day, 6 days per week. A tilt test was performed before and at the end of HDT. Changes in resting plasma volume from control day (D-5) to HDT day 24 were -11.2% for No-CM and -2.2% for CM. After HDT three among the 6 subjects of the No-CM group presented presyncopal or syncopal symptoms, no tilt test was interrupted in CM group. Atrial Natriuretic Peptide (ANP) decreased at day 7 for the two groups and remained low during all the HDT period for No-CM group only. Plasma Renin Activity and Aldosterone increased at day 7 and remained elevated for the two groups. Norepinephrine and epinephrine were unchanged. Elevated diuresis and natriuresis were evident during the first day of HDT. However, renal excretory patterns were different between the two groups: indeed, a decrease of Na+, ANP and cGMP was observed only in No-CM at Day 13 during HDT. Our data showed that the subjects of the No-CM group experienced a greater increase in heart rate and a decrease in systolic blood pressure during tilt tests after HDT; nevertheless, after HDT, blood pressure was better maintained in CM group during the tilt test. The plasma volume decrease measured at the end of HDT was significantly lower in CM group, in contrast, these countermeasures were ineffective in preventing at least certain changes in blood volume regulating hormones.

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

The aim of this study was to determine the effects of a 4-day head-down tilt (HDT; -6 degrees) and 4-day confinement on several indicators that might reflect a state of cardiovascular deconditioning on eight male subjects. Measurements were made of endocrine responses, heart rate variability and spontaneous baroreflex response (SBR) slope before, during and after each intervention. Plasma volume decreased by 10 percent after the 4-day HDT. The concentration of active renin was increased and that of urinary atrial natriuretic peptide decreased during the 4-day experiment in both groups. Plasma arginine vasopressin concentration decreased significantly only after 4-day confinement. After the 4-day HDT, one of the spectrum analysis parameters was statistically changed: the parasympathetic indicator decreased significantly (P <0.05) whereas the sympathetic indicator and the total power spectrum were unaltered. After 4-day confinement spectrum analysis parameters were not statistically altered. A significant decrease of SBR (P <0.05) was noticed only after the 4-day HDT. These data would suggest that exposure to a 4-day HDT was sufficient to induce a cardiovascular deconditioning which may have been induced by confinement and inactivity.

Hormonal, water balance, and electrolyte changes during sixty-day confinement

The EXEMSI experiment has made it clear that it is difficult to perform psychological and physiological protocols satisfactorily in the same study. It is, therefore, essential that the objectives of study be defined clearly before the start. While behavioral and psychological studies may be possible and provide valid results for a small group of mixed gender, it is more difficult to conduct valid physiological studies due to large differences between individuals and even in the same individual over time. As stated before, it is unusual in space research on humans and even during space simulation studies to have large and homogeneous groups of subjects. The consequence is that the results remain tentative. For a better understanding of the physiological data collected during the ISEMSI ad EXEMSI experiments, they should be correlated with the results of the psychological studies. One of the conclusions drawn from the ISEMSI experiment was that confinement provides a valuable parallel to other simulations of weightlessness, such as bedrest. The same pattern of changes in parameters like the blood volume regulating hormones renin and aldosterone was observed as in bedrest. After the EXEMSI study we can say that the conditions imposed by confinement, high work load, and stress, potentiate these effects. This implies that in using head-down bedrest as a weightlessness simulation the confinement effects must be identified by setting adequate control conditions for the head-down position, for short-term as well as for long-term simulations. Indeed, we have seen in the two isolation studies that confinement may have its effects at the beginning of the isolation period (EXEMSI) as well as during the entire isolation period (ISEMSI). In planning for EXEMSI we wanted to obtain more insight in some of the phenomena observed during ISEMSI by the introduction of new techniques such as the doubly labeled water method for determination of total body water. However, in some cases the opposite effects of those encountered in ISEMSI were found. This was probably due to the many changes in the experimental scenario, like number of subjects, mixed gender, living space per subject, and workload. Thus, for future isolation studies the operational scenario should be better examined and preferably standardized. Nevertheless, in such studies as well as in long-term sojourns in a space station, the crew size will not be larger than that of the EXEMSI crew. Physiologists will, therefore, have to become familiar with the study of small groups of subjects and to try to overcome the problems of large individual differences and statistical analysis of data from small groups.

Body weight and body composition during sixty days of isolation

The aim of this study was to find the mechanisms leading to the weight changes that have frequently been observed during isolation and in spaceflight. Isolation studies with small groups impose limitations on the measurements that can be performed to simple, noninvasive methods. In this study the simple parameters of body weight and body composition, along with sodium and potassium excretion, were determined in three males and one female subject before, during and after 60 days of isolation. Our assumption was that application of these simple methods might provide valuable information, when measurements are done on a daily basis and when the pre- and post-isolation periods are taken into account. Three subjects gained weight before isolation, while one lost weight. All four subjects gradually lost weight during isolation, 1-4% of their weight on the first day of isolation. During the first post-isolation week weight remained stable. During isolation one subject lost body fat, whereas another lost body water and lean body mass, but gained body fat. The urinary electrolyte excretion pattern reflected the changes in body composition: sodium loss coincided with a decrease of total body water, and potassium loss with a decrease of lean body mass. The Bioelectrical Impedance Analysis method, used in defining changes in body composition, provided data in good agreement with those obtained with the double-labeled water method. The results reported here are in agreement with observations reported by other investigators with respect to the body weight changes and the body composition. However, it is still not understood why some subjects lose fat and others gain fat under identical conditions. Psychological factors may be involved in these individual differences. Two further points have become clear from these studies: (1) the pre- and post-isolation periods should be taken into account, (2) urinary electrolyte excretion must be seen in the context of changes in body composition, not only in the context of kidney function.

Blood volume regulating hormones, fluid and electrolyte modifications during 21 and 198-day space flights (Altair-MIR 1993)

During the Altair MIR '93 mission we studied several parameters involved in blood volume regulation. The experiment was done on two cosmonauts before (B-60, B-30), during (D6, D12, D18 for French and D7, D12, D17 for Russian) and after the flight (R+1, R+3 and R+7). Space flight durations were different for two cosmonauts: for the Russian the flight duration was 198 days and for the French 21 days. On board the MIR station only urinary (volume and electrolytes, atrial natriuretic peptide (ANP), cyclic guanosine monophosphate (cGMP) and catecholamines) and salivary (cGMP and cortisol) samples were collected, centrifuged and stored in freezer. Lithium was used as a tracer to know exactly the 24 h urine output (CNES urine collection Kit). Before and after flight, blood was drawn with an epicite needle and vacutainer system for hormonal assays (renin, antidiuretic hormone, cGMP, ANP and aldosterone) in two positions: after 30 min rest in upright seated position and after 90 min of supine position. Salivary samples were collected simultaneously. During flight a decrease of diuresis and ANP and an increase of osmolality were found. No modifications of hematocrit, but an increase of salivary cGMP and cortisol were also observed. The decrease of urinary ANP is in favor of hypovolemia as described in previous flights. The postflight examinations revealed changes in fluid-electrolyte metabolism which indicate a hypohydration status and a stimulation of hormonal system responsible for water and electrolyte retention in order to readapt to the normal gravity.

Spectral analysis of blood pressure variability in heart transplant patients

The cardiac transplant patient provides a unique model for the study of blood pressure variability in the absence of heart rate variability. We examined the harmonic and fractal components of blood pressure variability in 14 heart transplant patients (12 men, 2 women; 21 to 62 years of age) and in age-and sex-matched control subjects during seated rest, supine rest, and supine rest with fixed-pace breathing (12 respirations per minute). Heart rate was faster in transplant patients than in control subjects, with much less heart rate variability (P < .0001). Spectral analysis of blood pressure variability revealed no difference in total power for either systolic or diastolic pressure, but transplant patients had less low-frequency (0 to 0.15 Hz) harmonic spectral power in both systolic (P < .01) and diastolic (P < .03) pressure and more high-frequency power (0.15 to 0.5 Hz) in diastolic pressure than control subjects. The ratio of high-frequency power in diastolic relative to systolic pressure was consistently higher (P < .0001) in the transplant patients (0.29 to 0.51) than in control subjects (0.11 to 0.13). The slope of the fractal component of systolic pressure was approximately 1.8 in both transplant patients and control subjects. This was greater than the slope for heart rate variability (approximately 1.1 in control subjects). These data provide clear evidence of independence of the fractal component of heart rate and blood pressure variabilities in both transplant patients and control subjects. The heart rate component of the arterial baroreflex minimized high-frequency diastolic pressure changes while contributing to low-frequency variations in both systolic and diastolic pressures.