Venous distensibility and venous emptying in the legs during a 42-day -6 degrees head-down bedrest.
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Abstract:
Exposure to actual or simulated microgravity is known to result in changes in lower limb venous compliance or distensibility which may play a role in post-bedrest or postflight orthostatic intolerance. Venous deconditioning has only been described in terms of changes in vascular compliance or distensibility. But a complete understanding of changes in venous hemodynamics and cardiovascular regulation occurring under these conditions has to take into account changes in emptying capacities of the veins which influence venous return, cardiac filling, and cardiac output regulation. Moreover, few data are available about the course of changes in venous hemodynamics for periods of simulated microgravity longer than 4 weeks. The purpose of this investigation was to measure parameters of venous compliance and venous emptying before, during, and after a 42-day period of bedrest at -6 degrees head-down tilt for a better understanding of long term venous physiological adaptation to microgravity.Keywords:
Orthostatic intolerance
Venous return curve
Head-Down Tilt
Deconditioning
Venous pressure
Lower body
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Maladaptation
Human physiology
Weightlessness
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Session MP1 includes short reports on: (1) Orthostatic Tests after 42 Days of Simulated Weightlessness; (2) Effects of 12 Days Exposure to Simulated Microgravity on Central Circulatory Hemodynamics in the Rhesus Monkey; (3) Increased Sensitivity and Resetting of Baroflex Control of Exercise Heart Rate After Prolonged Bed-Rest; (4) Complex Cardiovascular Dynamics and Deconditioning During Head-down Bed Rest; (5) The Cardiovascular Effects of 6 Hours of Head-down Tilt Upon Athletes and Non-athletes; (6) Individual Susceptibility to Post-spaceflight Orthostatic Intolerance: Contributions of Gender-related and Microgravity-related Factors; (7) Cassiopee Mission 1996: Comparison of Cardiovascular Alteration after Short and Long-term Spaceflights; (8) Cerebral and Femoral Flow Response to LBNP during 6 Month MIR Spaceflights (93-95); and (9) Cerebrovascular Changes due to Spaceflight and Postflight Presyncope.
Orthostatic intolerance
Spaceflight
Presyncope
Deconditioning
Weightlessness
Head-Down Tilt
Lower body
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Exposure to actual or simulated microgravity is known to result in changes in lower limb venous compliance or distensibility which may play a role in post-bedrest or postflight orthostatic intolerance. Venous deconditioning has only been described in terms of changes in vascular compliance or distensibility. But a complete understanding of changes in venous hemodynamics and cardiovascular regulation occurring under these conditions has to take into account changes in emptying capacities of the veins which influence venous return, cardiac filling, and cardiac output regulation. Moreover, few data are available about the course of changes in venous hemodynamics for periods of simulated microgravity longer than 4 weeks. The purpose of this investigation was to measure parameters of venous compliance and venous emptying before, during, and after a 42-day period of bedrest at -6 degrees head-down tilt for a better understanding of long term venous physiological adaptation to microgravity.
Orthostatic intolerance
Venous return curve
Head-Down Tilt
Deconditioning
Venous pressure
Lower body
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Citations (0)
To affirm the protective effect of inflated cuffs.The protective effect against the cardiovascular deconditioning of inflated cuffs on the upper parts of thighs and arms during the 1st through 10th days of 21 d bedrest has been reported previously. The effect of the cuffs during the rest of the 21 d bedrest were studied in this paper. Five subjects in cuffs group whose orthostatic tolerance were well maintained continued bed rest for 7 d without inflated cuffs. For the last 4 days of this period, inflated cuffs were again applied. Orthostatic tolerance of subjects in cuffs group with and without cuffs were measured.Two of the five subjects suffered orthostatic intolerance on the 17th day. No sign of orthostatic intolerance were observed by the end of bedrest.(1) the cardiovascular deconditioning developed when inflated cuffs were not used, and (2) the use of inflated cuffs for 4 d improves the orthostatic tolerance.
Orthostatic intolerance
Deconditioning
Head-Down Tilt
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Introduction: Limb venous compliance is a major factor in determining the extent of blood pooling during orthostatic stress. We measured the filling of the calf and of its major veins at different postures and quantified the venous contribution to the total volume shift of the calf. The results were also used to determine the venous elastance and to gain some information on the filling of the small veins. Methods: Twelve healthy volunteers participated in the study. The calf volume was measured with strain gauge plethysmography and the veins were imaged with ultrasound in the following positions: supine, sitting with both legs horizontal, sitting with one leg suspended, and supine with one leg raised. Cross-sectional areas of the imaged veins were calculated from the measured diameters. Hydrostatic change in the venous pressure was assumed when calculating the venous elastance. Results: The maximal increase in the calf area was up to 1 cm 2 . The large veins accounted for approximately 30% of the total volume shift of the calf except in cases where the subjects had one leg suspended. Then, the contribution of the large veins was less than 20%. The estimated elastance of the large veins was 80 ± 14 mmHg, which may be regarded as substantial. Conclusions: The study suggests that there is a distinct difference in the filling of the small and the large leg veins. The small veins may be the principal contributors to the total compliance of the calf when the venous pressure is above 30 mmHg.
Supine position
Plethysmograph
Venous return curve
Sitting
Orthostatic intolerance
Venous pressure
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-6 degrees head down bedrest (HDBR) 10d was used to simulate weightlessness. 12 volunteers were randomed into control group and cuffs group, with 6 in each group. Inflated cuffs on the upper part of thighs and arms were performed 12 h/d in cuffs group during bedrest. No countermeasures were applied in the control group. During head up tilt (HUT) +75 degrees 20 min orthostatic tolerance test after HDBR, 5 in control group became presyncopal, while only one in cuffs group. The study indicates that the use of inflated cuffs provides a protective effect against the cardiovascular deconditioning after bedrest.
Head-Down Tilt
Deconditioning
Weightlessness
Orthostatic intolerance
Spaceflight
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Study of time-course variations of venous pressure in the jugular veins can be used to identify individual adaptation to prolonged head-down tilt. The method of measuring blood return to the right heart presented here can be employed to evaluate the efficacy of countermeasures during head-down tilt and to predict orthostatic tolerance after it.
Venous return curve
Hypokinesia
Head-Down Tilt
Jugular vein
Venous pressure
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Supine position
Head-Down Tilt
Deconditioning
Venous return curve
Orthostatic intolerance
Stroke
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Susceptibility of healthy astronauts to orthostatic hypotension and presyncope is exacerbated upon return from spaceflight. Hypo-volemia is suspected to play an important role in cardiovascular deconditioning following exposure to spaceflight, which may lead to increased peripheral resistance, attenuated arterial baroreflex, and changes in cardiac function. The effect of altered gravity during space flight and planetary transition on human cardiovascular function is of critical importance to maintenance of astronaut health and safety. A promising countermeasure for post-flight orthostatic intolerance is fluid loading used to restore loss fluid volume by giving crew salt tablets and water prior to re-entry. Eight men and eight women will be tested during two, 6-hour exposures to 6o HDT: 1) fluid loading, 2) no fluid loading. Before and immediately after each HDT, subjects will perform a stand test to assess their orthostatic tolerance. Physiological measures (e.g., ECG, blood pressure, peripheral blood volume) will be continuously monitored while echocardiography measures are recorded at 30-minute intervals during HDT and stand tests. Preliminary results (N=4) clearly show individual differences in responses to this countermeasure and the time course of physiological changes induced by HDT.
Spaceflight
Presyncope
Orthostatic intolerance
Head-Down Tilt
Weightlessness
Deconditioning
Intravascular volume status
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Besides microgravity, inactivity is likely to play a role in the cardiovascular deconditioning after space flights and weightlessness simulations. The aim of the study was to compare the effects of a 4‐day head‐down bed rest (HDBR) (−68) and a 4‐day confinement (C) on cardiovascular responses to orthostatic stress. Eight male subjects underwent head‐up tilt (HUT) (+60°) and lower‐body negative pressure (LBNP) (−20, −30, −40 and −50 mmHg) before (D−1) and at the end (R1) of each situation. Blood pressure, heart rate variability (HRV) and spontaneous baroreflex slope (SBS) were determined. The HDBR reduced orthostatic tolerance, as five subjects presented orthostatic hypotension during the HUT at R1, compared with two subjects at D−1. These same two subjects presented orthostatic hypotension after confinement. The main findings, after HDBR, included reductions in RR interval and total spectral power and a decrease in the parasympathetic indicator (PNS) in favour of a decrease in vagal tone; the increase in the sympathetic indicator (SNS) was not significant. After confinement, the RR interval was also significantly reduced and PNS decreased, but not significantly. RR interval and PNS were further reduced during HUT and LBNP, reflecting a withdrawal of parasympathetic activity. SBS was reduced after HDBR ( P <0·05) and confinement ( P =0·05), with a further reduction during HUT and LBNP without difference between D−1 and R1. This experiment showed that a 4‐day HDBR leads to impaired baroreflex function and changes in autonomic balance, which may contribute to orthostatic intolerance. Although less significant, similar patterns of changes in the autonomic nervous system were observed after confinement, suggesting an influence of the inactivity in cardiovascular deconditioning.
Orthostatic intolerance
Lower body
Weightlessness
Deconditioning
Spaceflight
Presyncope
Tilt table test
Head-Down Tilt
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Citations (29)