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.     

Lower body negative pressure (LBNP) as a countermeasure for long term spaceflight

The article presents the current status of lower body negative pressure (LBNP) as a countermeasure for preventing orthostatic intolerance after space flight or bed rest. Devices discussed include the Chibis vacuum suit, the Anthrorack device, a collapsible device, and an inflatable device. Two bed rest studies examined the effect of LBNP and exercise on orthostatic tolerance; plasma volume; vasopressin, plasma renin activity, and catecholamines; and side effects.     

A new application of LBNP to measure orthostatic tolerance before and after 0-g simulation (water immersion)

Lower body negative pressure (LBNP) remains an important device for the generation of orthostatic stress in the space flight environment as well as a tool to measure inflight and postflight changes in orthostatic response.

These applied levels of LBNP have typically not exceeded 50–60 mm Hg negative pressure. Information is incomplete as to the levels of absolute LBNP orthostatic tolerance, and the factors responsible for their variance. A better definition of the tolerance limits for males and females could be expected to aid the evaluation of lower levels of LBNP.

An LBNP device was built to study absolute orthostatic tolerance; additionally, another LBNP device was constructed to permit orthostatic tolerance testing directly after a controlled water immersion period.

Absolute LBNP orthostatic tolerance patterns are analyzed for a group of males and females (series I). A preliminary statement on the variations of LBNP orthostatic tolerance after limited periods of water immersion and bed rest is also provided (series II).     

Skylab experiment M-092: results of the first manned mission

Blood pressure at 30-sec intervals, heart rate, and percentage increase in leg volume continuously were recorded during a 25-min protocol in the M092 Inflight Lower Body Negative Pressure (LBNP) experiment carried out in the first manned Skylab mission. These data were collected during six tests on each crewman over a 5-month preflight period. The protocol consisted of a 5-min resting control period, 1 min at -8, 1 min at -16, 3 min at -30, 5 min at -40, and 5 min at -50 mm Hg LBNP. A 5-min recovery period followed. Inflight tests were performed at approximately 3-day intervals through the 28-day mission. Individual variations in cardiovascular responses to LBNP during the preflight period continued to be demonstrated in the inflight tests. Measurements of the calf indicated that a large volume of fluid was shifted out of the legs early in the flight and that a slower decrease in leg volume, presumably due to loss of muscle tissue, continued throughout the flight. Resting heart rates tended to be low early in the flight and to increase slightly as the flight progressed. Resting blood pressure varied but usually was characterized by slightly elevated systolic blood pressure, lower diastolic pressure, and higher pulse pressures than during preflight examinations. During LBNP inflight a much greater increase in leg volume occurred than in preflight tests. Large increases occurred even at the smallest levels of negative pressure, suggesting that the veins of the legs were relatively empty at the beginning of the LBNP. The greater volume of blood pooled in the legs was associated with greater increases of heart rate and diastolic pressure and larger falls of systolic and pulse pressure than seen in preflight tests. The LBNP protocol represented a greater stress inflight, and on three occasions it was necessary to stop the test early because of impending syncopal reactions. LBNP responses inflight appeared to predict the degree of postflight orthostatic intolerance. Postflight responses to LBNP during the first 48 hours were characterized by marked elevations of heart rate and instability of blood pressure. In addition, systolic and diastolic pressures were typically elevated considerably both at rest and also during stress. The time required for cardiovascular responses to return to preflight levels was much slower than in the case of Apollo crewmen.     

Sympathetic nervous system and spaceflight

Purpose: Orthostatic stability on Earth is maintained through sympathetic nerve activation sufficient to increase peripheral vascular resistance and defend against reductions of blood pressure. Orthostatic instability in astronauts upon return from space missions has been linked to blunted vascular resistance responses to standing, introducing the possibility that spaceflight alters normal function between sympathetic efferent traffic and vascular reactivity.Methods: We evaluated published results of spaceflight and relevant ground-based microgravity simulations in an effort to determine responses of the sympathetic nervous system and consequences for orthostatic stability.Results: Direct microneurographic recordings from humans in space revealed that sympathetic nerve activity is increased and preserved in the upright posture after return to Earth (STS-90). However, none of the astronauts studied during STS-90 presented with presyncope postflight, leaving unanswered the question of whether postflight orthostatic intolerance is associated with blunted sympathetic nerve responses or inadequate translation into vascular resistance.Conclusions: There is little evidence to support the concept that spaceflight induces fundamental sympathetic neuroplasticity. The available data seem to support the hypothesis that regardless of whether or not sympathetic traffic is altered during flight, astronauts return with reduced blood volumes and consequent heightened baseline sympathetic activity. Because of this, the ability to withstand an orthostatic challenge postflight is directly proportional to an astronaut's maximal sympathetic activation capacity and remaining sympathetic reserve.     

System identification of dynamic closed-loop control of total peripheral resistance by arterial and cardiopulmonary baroreceptors.

Prolonged exposure to microgravity in space flight missions (days) impairs the mechanisms responsible for defense of arterial blood pressure (ABP) and cardiac output (CO) against orthostatic stress in the post-flight period. The mechanisms responsible for the observed orthostatic intolerance are not yet completely understood. Additionally, effective counter measures to attenuate this pathophysiological response are not available. The aim of this study was to investigate the ability of our proposed system identification method to predict closed-loop dynamic changes in TPR induced by changes in mean arterial pressure (MAP) and right atrial pressure (RAP). For this purpose we designed and employed a novel experimental animal model for the examination of arterial and cardiopulmonary baroreceptors in the dynamic closed-loop control of total peripheral resistance (TPR), and applied system identification to the analysis of beat-to-beat fluctuations in the measured signals. Grant numbers: NAG5-4989.     

Evaluation of spontaneous baroreflex response after 28 days head down tilt bedrest

The spontaneous baroreflex response was evaluated during supine rest and head up tilt (60 degrees) before and immediately after a 28 day 6 degrees HDT bedrest in 6 healthy adult men (age 30-42 years). Sequences of 3 or more beats where RR-interval and systolic blood pressure changed in the same direction were used to evaluate baroreflex response slope (BRS). Prior to bedrest, the mean BRS and RR-interval were 18.0 +/- 3.9 ms/mm Hg and 926 +/- 61 ms at rest and 10.5 +/- 2.5 ms/mm Hg and 772 +/- 63 ms during the first 10 min of 60 degrees tilt. Following bedrest, these values changed to 15.6 +/- 2.7 ms/mm Hg and 780 +/- 53 ms at rest, and to 6.5 +/- 1.2 ms/mm Hg and 636 +/- 44 ms during tilt. Thus, (1) the spontaneous baroreflex can be evaluated in human subjects during experiments of orthostatic stress; (2) the baroreflex slope was reduced on going from supine to the head up tilt position; and (3) 28 days of bedrest reduced the spontaneous baroreflex slope.     

Spacelab Life Sciences flight experiments: an integrated approach to the study of cardiovascular deconditioning and orthostatic hypotension

The microgravity environment of spaceflight produces rapid cardiovascular changes which are adaptive and appropriate in that setting, but are associated with significant deconditioning and orthostatic hypotension on return to Earth's gravity. The rapidity with which these space flight induced changes appear and disappear provides an ideal model for studying the underlying pathophysiological mechanisms of deconditioning and orthostatic hypotension, regardless of etiology. Since significant deconditioning is seen after flights of very short duration, muscle atrophy due to inactivity plays, at most, a small role. These changes in circulatory control associated with cephalad fluid shifts, rather than inactivity per se, are probably more important factors. In order to test this hypothesis in a systematic way, a multidisciplinary approach which defines and integrates inputs and responses from a wide variety of circulatory sub-systems is required. The cardiovascular experiments selected for Spacelab Life Sciences flights 1 and 2 provide such an approach. Both human and animal models will be utilized. Pre- and post-flight characterization of the payload crew includes determination of maximal exercise capacity (bicycle ergometry), orthostatic tolerance (lower body negative pressure), alpha and beta adrenergic sensitivity (isoproterenol and phenylephrine infusions), baroreflex sensitivity (ECG-gated, stepwise changes in carotid artery transmural pressure with a pneumatic neck collar), and responses to a 24 h period of 5 deg head-down tilt. Measurements of cardiac output (CO2 and C2H2 rebreathing), cardiac chamber dimensions (phased-array 2-dimensional echocardiography), direct central venous pressure, leg volume (Thornton sock), limb blood flow and venous compliance (occlusion plethysmography), blood and plasma volumes, renal plasma flow and glomerular filtration rates, and various hormonal levels including catecholamines and atrial natriuretic factor will also be obtained. The central venous catheter will be inserted immediately pre-launch and monitored with heart rate and blood pressure in-flight until cardiac output, respiratory gas exchange and quantitative 2D echocardiography measurements can be performed. In-flight hemodynamic measurements will be repeated at rest and during submaximal exercise daily and also during maximal exercise midway through the flight to document the timecourse and extent of cardiovascular changes in the payload crew. Parallel studies are planned for the animals. In addition to measurements of right atrial and aortic pressures and cardiac output, a dorsal micro-circulatory chamber will allow determinations of changes in capillary and venular architecture and function in six of the rats. The techniques and findings from many of the SLS-1 and 2 supporting studies have already yielded significant information about circulatory regulation in patients with both hypo- and hypertension. The flight experiments themselves will provide new data to test the validity of both animal and human models currently used for simulating the fluid shifts of a micro-gravity environment. The development of effective countermeasures, not only for short and long duration space travellers, but also for Earth-bound medical patients can then be physiologically based on experimental data rather than anecdote.     

Effect of physical fitness and training on physiological responses to hypogravity.

The studies on the orthostatic tolerance during the hypodynamics exposure seem to be significant in connection with the selection, training and health maintenance of astronauts. Using male human subjects of various physical fitness levels, fluctuations of their physical fitness through 2 weeks of vigorous athletic training were measured in many parameters. For some of the subjects, the effects of 6 hr thermal neutral water immersion exposure in head out supine position on the physical fitness parameters and orthostatic tolerability were compared before training with after training. The results obtained were as follows: (1) Before training, orthostatic tolerability before hypodynamics exposure increased, following the physical fitness levels; the value after the hypodynamics exposure decreased in all the cases, but no differences were observed between the physical fitness levels. (2) As a result of training an increase of the physical fitness capacity was observed. The increase of orthostatic tolerability before hypodynamics exposure was noticed except for athletes. (3) Before hypodynamics exposure the urinary excretion of noradrenaline on non-athlete subjects increased as the physicsl fitness level increased. The values were decreased by physical training, the more so the better the physical fitness. After hypodynamics exposure the same relation was observed. But for athletes the values remain more stable and the decrease by hypodynamics exposure was not so distinctive. Such decreased reaction to hypodynamic conditions seems to reveal the neuro hormonal mechanism for the detrimental adaptation of athletes to hypodynamics. These results suggest that stable athletes do not always have low orthostatic tolerability, but do not respond well to hypodynamic conditions, at least from the orthostatic point of view. The mechanism seems related to sympathetic nerve activity.     

Effects of 12 days exposure to simulated microgravity on central circulatory hemodynamics in the rhesus monkey

Central circulatory hemodynamic responses were measured before and during the initial 9 days of a 12-day 10 degrees head-down tilt (HDT) in 4 flight-sized juvenile rhesus monkeys who were surgically instrumented with a variety of intrathoracic catheters and blood flow sensors to assess the effects of simulated microgravity on central circulatory hemodynamics. Each subject underwent measurements of aortic and left ventricular pressures, and aortic flow before and during HDT as well as during a passive head-up postural test before and after HDT. Heart rate, stroke volume, cardiac output, and left ventricular end-diastolic pressure were measured, and dP/dt and left ventricular elastance was calculated from hemodynamic measurements. The postural test consisted of 5 min of supine baseline control followed by 5 minutes of 90 degrees upright tilt (HUT). Heart rate, stroke volume, cardiac output, and left ventricular end-diastolic pressure showed no consistent alterations during HDT. Left ventricular elastance was reduced in all animals throughout HDT, indicating that cardiac compliance was increased. HDT did not consistently alter left ventricular +dP/dt, indicating no change in cardiac contractility. Heart rate during the post-HDT HUT postural test was elevated compared to pre-HDT while post-HDT cardiac output was decreased by 52% as a result of a 54% reduction in stroke volume throughout HUT. Results from this study using an instrumented rhesus monkey suggest that exposure to microgravity may increase ventricular compliance without alternating cardiac contractility. Our project supported the notion that an invasively-instrumented animal model should be viable for use in spaceflight cardiovascular experiments to assess potential changes in myocardial function and cardiac compliance.     

Interaction of mental and orthostatic stressors

We assessed hemodynamic responses induced by orthostatic and mental stressors, using passive head up tilt (HUT) and mental arithmetic (MA), respectively. The 15 healthy males underwent three protocols: (1) HUT alone, (2) MA in supine position and (3) MA+HUT, with sessions randomized and ≥2 weeks apart. In relation to baseline, HUT increased heart rate (HR) (+20.4±7.1 bpm; p<0.001), mean blood pressure (MBP) (+4.7±11.3 mmHg; p<0.05), diastolic blood pressure (DBP) (+6.1±11.6 mmHg; p<0.05) and total peripheral resistance (TPR) (+155±232 dyne*s/cm⁵; p<0.001) but decreased stroke volume (SV) (?33.1±13.4 ml; p<0.001) and cardiac output (CO) (?0.6±1.0 l/min; p<0.01). MA increased HR (+8.0±6.0 bpm; p<0.001), systolic blood pressure (SBP) (+9.0±7.7 mmHg; p<0.001), MBP (+10.0±6.5 mmHg; p<0.001), DBP (+9.5±7.2 mmHg; p<0.001) and CO (+0.6±0.8 l/min; p<0.01). MA+HUT increased HR (+28.8±8.4 bpm; p<0.001), SBP (+4.6±14.3 mmHg; p<0.05), MBP (+11.2±11.6 mmHg; p<0.001), DBP (+13.5±10.1 mmHg; p<0.001) and TPR (+160±199 dyne*s/cm⁵; p<0.001) but SV (?34.5±14.6 ml; p<0.001) decreased. Mental challenge during orthostatic challenge elicited greater increases in heart rate, despite similar reductions in stroke volume such as those during orthostatic stress alone. Overall, cardiac output decreases were less with combinations of mental and orthostatic challenges in comparison to orthostasis alone. This would suggest that carefully chosen mental stressors might affect orthostatic responses of people on standing up. Therefore, additional mental loading could be a useful countermeasure to alleviate the orthostatic responses of persons, particularly in those with histories of dizziness on standing up or on return to earth from the spaceflight environment of microgravity.     

Does mental arithmetic before head up tilt have an effect on the orthostatic cardiovascular and hormonal responses?

Passive head up tilt (HUT) and mental arithmetic (MA) are commonly used for providing mental and orthostatic challenges, respectively. In animal experiments, even a single exposure to a stressor has been shown to modify the response to subsequent stress stimulus. We investigated whether MA applied before HUT elicits synergistic responses in orthostatic heart rate (HR), cardiac output (CO), heart rate variability and arterial blood pressure. The 15 healthy young males were subjected to two randomized protocols: (a) HUT and (b) HUT preceded by MA, with sessions randomized and ≥2 weeks apart. Beat to beat continuous hemodynamic variables were measured and saliva samples taken for hormonal assay. HUT alone increased HR from 59±7 (baseline) to 80±10 bpm (mean±SD) and mean blood pressure (MBP) from 88±10 to 91±14 mmHg. HUT results after MA were not different from those with HUT alone. The activity of alpha amylase showed differences during the experiments irrespective of the protocols. We conclude that mental challenge does not affect orthostatic cardiovascular responses when applied before; the timing of mental loading seems to be critical if it is intended to alter cardiovascular responses to upright standing.     

Fluid volumes changes induced by spaceflight

The blood volume (BV), plasma volume (PV), and extracellular fluid volume changes produced in crewmembers during spaceflights of 11-84 days were compared to changes after 14 or 28 days of bedrest. Spaceflight and bedrest produce approximately equal BV changes but the recorded PV change after spaceflight was less. However, the diurnal change in PV may explain the smaller decreases recorded after spaceflight. The cardiovascular deconditioning caused by spaceflight and bedrest was compared using the mean heart rate response to lower body negative pressure (LBNP) testing at -50 mmHg pressure. These tests show approximately equal LBNP produced heart rate changes after bedrest and spaceflight. A countermeasure which includes 4 hr of LBNP treatment at -30 mmHg and the ingestion of one l. of saline was studied and found capable of returning the heart rate response and the PV of bedrested subjects to control (prebedrest) levels suggesting that it would be useful to the crewmembers after a spaceflight.     

Cardiovascular effects of simulated zero-gravity in humans

Head-down and head-up [correction of heat-up] tilted bedrest (5 degrees) and head out water immersion (HOWI) for 6 hr were compared. Parameters: Cardiac output (rebreathing method), blood pressure (arm cuff), forearm blood flow (venous occlusion plethysmography), total peripheral (TPR), and forearm vascular (FVR) resistances, Hct, Hb, relative plasma volume (PV) changes, and plasma catecholamines (single-isotope assay). During HOWI there was as expected a decrement in TPR, FVR, Mean arterial pressure (MAP, from 100 to 80 mmHg), Hct, and PV, and--as a new finding--catecholamines, which were 30-50% lower compared with both +5 and -5 degrees bedrest. During head down tilt, MAP was elevated (to 100-110 mmHg) and catecholamines did not fall, while TPR and EVR slowly decreased over 6 hr. HOWI is a stronger stimulus than -5 degrees bedrest, probably because HOWI elevates central venous pressure more markedly emptying the peripheral veins, while bedrest permits a distension of veins, which induces an increase in sympathetic nervous activity.     

Usefulness of daily +2Gz load as a countermeasure against physiological problems during weightlessness.

Adaptation to head-down-tilt bed rest as a simulated microgravity leads to an abnormality of reflex control of circulation, hypovolemia and reduction of exercise capacity. We hypothesized that this cardiovascular deconditioning and reduction of exercise capacity could be prevented by a daily 1 hr centrifugation at +2Gz. To test this hypothesis, twenty healthy male subjects underwent 4 day of 6 degrees head-down-tilt bed rest. Ten of them were exposed to a +2Gz load for up to 30 min twice per day (the Gz group). The remaining 10 were not exposed to a Gz load (the no-Gz group). We estimated autonomic cardiovascular control by power spectral analysis of blood pressure and R-R interval variability, and baroreflex regulation by the transfer function analysis and the sequence method, before and after bed rest. Further, we measured hematocrit as an index of changes in plasma volume and maximal oxygen consumption as an index of exercise capacity, before and after bed rest. Result: In the no-Gz group, heart rate increased after bed rest. The high frequency power of R-R interval variability as an index of cardiac parasympathetic nervous activity, baroreflex gains estimated by transfer function analysis and the sequence method as index of the integrated arterial-cardiac baroreflex function decreased significantly. Associated with these changes, the ratio of low to high frequency power of R-R as an indicator of cardiac sympathovagal balance tended to increase after bed rest in the no-Gz group. However, those showed no significant changes after bed rest in the Gz group. Hematocrit increased after bed rest in the no-Gz group. It also tended to increase in the Gz group, however it did not achieve statistical significance. Maximal oxygen consumption decreased significantly to similar extent in both the groups. Conclusion: This result suggested that 1) a daily 1hr +2Gz load produced by a centrifuge might eliminate the changes in autonomic cardiovascular control during simulated weightlessness; 2) furthermore, it might partly reverse hypovolemia induced by bed rest; 3) however, it could not prevent the decreases in exercise capacity.     

Skylab experiment results: hematology studies

These studies were designed and coordinated to evaluate specific aspects of man's immunologic and hematologic systems which might be altered by or respond to the space flight environment. The biochemical functions investigated included cytogenetic damage to blood cells, immune resistance to disease, regulation of plasma and red cell volumes, metabolic processes of the red blood cell, and physical chemical aspects of red blood cell functions. Only minor changes were observed in the functional capacity of erythrocytes as determined by measuring the concentrations of selected intracellular enzymes and metabolites. Tests of red cell osmotic regulation indicated some elevation in the activity of the metabolic dependent Na-K pump, with no significant alterations in the cellular Na and K concentrations or osmotic fragility. A transient shift in red cell specific-gravity profile was observed on recovery, possibly related to changes in cellular water content. Measurements of hemoconcentration (hematocrit, hemoglobin concentration, red cell count) indicated significant fluctuations postflight, reflecting observed changes in red cell mass and plasma volume. There was no apparent reticulocytosis during the 18 days following the first manned Skylab mission in spite of a significant loss in red cell mass. However, the reticulocyte count and index did increase significantly 5 to 7 days after completion of the second, longer duration, flight. There were no significant changes in either the while blood cell count or differential. However, the capacity of lymphocytes to respond to an in vitro mitogenic challenge was repressed postflight, and appeared to be related to mission duration. The cause of this repression is unknown at this time. Only minor differences were observed in plasma protein patterns. In the second mission there were changes in the proteins involved in the coagulation process which suggested a hypercoagulative condition.     

Respiratory mechanics after 180 days space mission (EUROMIR'95)

The present study reports data on respiratory function of lung and chest wall following the 180 days long European - Russian EuroMir '95 space mission. Data reported refer to two subjects studied before the mission, on day 9 and 175 in flight and on days 1, 10, 12, 27 and 120 after return. In-flight vital capacity (VC) and expiratory reserve volume (ERV) were similar to those in supine posture, namely approximately 5% and approximately 30% less than in sitting posture. On day 1 after return, VC was reduced by approximately 30% in both postures. This reflected a decrease in ERV (approximately 0.5 L) and in IC (inspiratory capacity, approximately 1.7 L) that could be attributed to a marked weakening of the respiratory muscles. Regain of normal preflight values barely occurred 120 days after return. Post-flight pressure-volume curves of the lung, chest wall and total respiratory system are equal to preflight ones. The pressure-volume curve of the lung in supine posture is displaced to the right relative to sitting posture and shows a lower compliance. As far as the lung in-flight condition resembles that occurring in supine posture, this implies a lower compliance, a greater amount of blood in the pulmonary microvascular bed, a more homogeneous lung perfusion and therefore a greater microvascular filtration rate towards lung interstitium.     

Blood volume regulating hormones response during two space related simulation protocols: four-week confinement and head-down bed-rest

The volume of regulating hormones (renin, aldosterone, arginine vasopressin and atrial natriuretic factor), electrolytes and creatinine concentrations, and blood pressure were measured in two different four-week experimental protocols: respectively -6 degrees head-down bed-rest (5 subjects) and confinement (6 subjects). We observed a significant increase (P < 0.01 at D2 vs D-5) of systolic blood pressure during confinement and a different level of response for some hormones, especially for arginine vasopressin (300% increase during confinement instead of 50% during bed-rest). The renin-angiotensin-aldosterone system was enhanced during confinement and head-down bed-rest. In both conditions, we obtained a similar pattern of response for blood volume regulating hormones. During confinement, two main factors were inactivity and stress activation of the sympathetic nervous system. In the bed-rest study the response is principally due to the fluid shift and blood volume adaptation but it is not possible to exclude the role of inactivity and stress.     

The human cardiovascular system during space flight

Purpose of the work is to analyze and to summarize the data of investigations into human hemodynamics performed over 20 years aboard orbital stations Salyut-7 and Mir with participation of 26 cosmonauts on space flights (SF) from 8 to 438 days in duration. The ultrasonic techniques and occlusive plethysmography demonstrated dynamics of changes in the cardiovascular system during SF of various durations. The parameters of general hemodynamics, the pumping function of the heart and arterial circulation in the brain remained stable in all the space flights; however, there were alterations in peripheral circulation associated with blood redistribution and hypovolemie in microgravity. The anti-gravity distribution of the vascular tone decayed gradually as unneeded. The most considerable changes were observed in leg vessels, equally in arteries (decrease in resistance) and veins (increase in maximum capacity). The lower body negative pressure test (LBNP) revealed deterioration of the gravity-dependent reactions that changed for the worse as SF duration extended. The cardiovascular deconditioning showed itself as loss of descent acceleration tolerance and orthostatic instability in the postflight period.     

A mathematical and experimental simulation of the hematological response to weightlessness

Two ground-based methods of weightlessness simulation--a computer model of erythropoiesis feedback regulation and bedrest--were used to investigate the mechanisms which lead to loss of red cell mass during spaceflight. Both methods were used to simulate the first Skylab mission of 28 days. Human bedrest subjects lose red cell mass linearly with time and in this study the loss was 6.7% at the end of four weeks (compared to 14% in Skylab). Postbedrest recovery of red cell mass was delayed for two weeks during which time a further decline in this quantity was noted. This is consistent with the first Skylab mission but not with the two longer flights of two and three months. Hemoconcentration, observed early in the study, was essentially maintained despite red cell loss because of continued loss of plasma volume. The computer model, using the time-varying hematocrit data to estimate red cell production rates, predicted dynamic behavior of plasma volume and red cell mass that was in close agreement with the measured values. The results support the hypothesis that red cell loss during supine bedrest is a normal physiological feedback process in response to hemoconcentration enhanced tissue oxygenation and suppression of red cell production. In contrast, the delayed postbedrest recovery of red cell mass was more difficult to explain, especially in the light of enhanced reticulocyte indices observed at the onset on ambulation. Model simulation suggested the possibilities, still to be experimentally demonstrated, that this period was marked by some combination of increased oxygen-hemoglobin affinity, small reductions in mean red cell life span, ineffective erythropoiesis, or abnormal reticulocytosis. The question of whether hemoconcentration is the sole contributor to spaceflight red cell losses also remains to be resolved.