International Workshop on Cardiovascular Rearch in Space: Body Fluid Metabolism and Control of Intravascular Volume
It is generally admitted that initial changes induced by weightlessness or simulated weightlessness are characterized by a cephalad fluid shift that induces an increase in central venous pressure(1). This central hypervolemia leads to adaptation including sodium loss hypovolemia, altered baroreceptor reflexes, and endocrine modifications. When the hydrostatic gradient reappears (return to gravity after a spaceflight) or after simulation studies by head-down bed rest, a state of cardiovascular deconditioning occurs with orthostatic hypotension. The potential mechanisms underlying the lower orthostatic tolerance are depicted in Figure 1 and are well documented, but few data relate these changes to confinement and isolation, environmental factors very often associated with spaceflights or with simulations including bed rest.
The role of environmental factors and psychosocial interactions is well described in several reviews(3,4,9,10,13). One of the first descriptions of the importance of environmental factors in bed-rest experiments was reported by Sandler and Winter (11) in a study on physiological responses of women to bed rest. To their surprise, they found that the ambulatory control subjects also exhibited a degree of deconditioning, which was thought to result from the stress of confinement. Even if one thinks that a control ambulatory period could modify the sympathetic nervous system and the cardiovascular adaptations to orthostatism, the interpretation of fluid and diuresis modifications seems less obvious.
Nevertheless, such alterations in body fluids are described by Radziszewski(8) in a 46-d study of life in a confined space (diving chamber with a pressure very close to exterior). He observed a significant decrease in hematocrit throughout the experiment and a progressive and important increase in diuresis and potassium, calcium, and creatinine excretion the first 3-4 d of confinement.
We made similar observations in a 4-d confinement study (seeFig. 2 (12)) that was in fact the control experiment for a 4-d head-down bed-rest study. The comparison of confinement and bed rest in the same subjects was very interesting. Daily water balance followed the same patterns in both situations and was negative on the first day. Plasma-active renin was also increased in both situations on day 4. Finally, five of eight subjects experienced presyncope after bed rest, and two of eight had signs of presyncope after confinement. Lin et al.(5) also show the same cardiovascular deconditioning after a 7-d saturation dive at 31 atmosphere absolute.
These observations were confirmed by three studies organized by the European Space Agency (ESA). In 1990, ESA undertook an experimental program on the psychological problems that could be encountered by space crews to investigate the role of confinement and isolation on water metabolism and volume regulating hormones.
In the first confinement experiment (Isolation Study for the European Manned Space Infrastructure: ISEMSI), six men of different nationalities were isolated for 28 d (6). The second experiment(Experimental Campaign for European Manned Space Infrastructure: EXEMSI) was performed with three men and one woman for 60 d. In the third investigation(Human Behavior Study: HUBES), three men were isolated for 135 d in a ground simulation of the manned space mission EUROMIR 95.
Sandal et al. (9) showed that during EXEMSI crews developed interpersonal tensions which resulted in crises. The one woman in EXEMSI was characterized by males as a peacemaker. Psychological issues have a great impact on homeostasis and on hormones(4,9,13).
In the three situations (ISEMSI, EXEMSI, and HUBES), an increase in urine output was observed at the beginning of the confinement. The time course of hormone concentrations was different in ISEMSI and EXEMSI(7). In ISEMSI, the basal renin concentrations were very high compared with EXEMSI (4-fold augmentation), and a significant increase in renin and arterial pressure was observed. Unfortunately, cardiovascular deconditioning was not determined in these experiments.
Plasma volume increase by training or decrease by inactivity is well documented (14) and has nothing to do with weightlessness. Recently a decrease in plasma erythropoietin has been described in situations of head-down bed rest and isolation-confinement(2). Probably an important part of plasma volume decrease in bed rest is related to inactivity.
The aim of this overview is to emphasize the role of environmental and psychological factors in weightlessness simulation, even in animal studies. our hypothesis is that the endocrine and cardiovascular modifications described during space flight or simulation studies are actually linked in part to environmental conditions (specially confinement, isolation, and inactivity) as described in Figure 3. This means also that the results of simulation studies could be misinterpreted without a control confinement study present in the protocol.
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2. Gunga, H. C., K. Kirsch, F. Baartz, et al. Erythropoietin under real and simulated microgravity conditions in humans. J. Appl. Physiol.
3. Harrison, A. A., Y. A. Clearwater, and C. P. McKay.From Antarctica to Outer Space: Life in Isolation and Confinement
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6. Maillet, A., H. C. Gunga, G. Gauquelin, et al. Effects of 28-day isolation (ESA-ISEMSI'90) on blood pressure and blood volume regulating hormones. Aviat. Space Environ. Med.
7. Maillet, A., S. Normand, H. C. Gunga, et al. Hormonal, water balance and electrolyte changes during sixty-day confinement.Adv. Space Biol. Med.
8. Radziszewski, E. Effets physiologiques chez l'homme du confinement de longue durée en atmosphère enrichie en dioxyde de carbone: application à la détermination des limites admissibles de CO2
dans la vie en espace clos (Ph.D. Thesis). University of Lyon, Lyon, France, 1987, pp. 329.
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10. Sandler, H. and J. Vernikos. Inactivity: Physiological Effects
. Orlando, FL: Academic Press, 1986, pp.205.
11. Sandler, H. and D. L. Winter. Physiological Responses of Women to Simulated Weightlessness
. Washington, DC: National Aeronautics and Space Administration (NASA), 1978, SP-430.
12. Sigaudo, D., J. O. Fortrat, A. Maillet, et al. Comparison of a 4-day confinement and head-down tilt on endocrine response and cardiovascular variability. Eur. J. Appl. Physiol.
13. Ursin, H., R. Vaernes, I. Endresen, and M. Warncke. Physiological and psychological aspects of living in confined space under stress. SPRI Polar Symposia
14. Xiangrong, S., W. G. Squires, J. W. Williamson, et al. Aerobic fitness: I. response of volume regulating hormones to head-down tilt.Med. Sci. Sports Exerc.
ENDOCRINE AND CARDIOVASCULAR SYSTEM; ISOLATION; CONFINEMENT