Orthostatic intolerance and motion sickness after parabolic flight

Because it is not clear that the induction of orthostaticintolerance in returning astronauts always requires prolonged exposure to microgravity, we investigated orthostatic tolerance and autonomic cardiovascular function in 16 healthy subjects before and after thebrief micro- and hypergravity of parabolic flight. Concomitantly, weinvestigated the effect of parabolic flight-induced vomiting onorthostatic tolerance, R-wave-R-wave interval and arterial pressurepower spectra, and carotid-cardiac baroreflex and Valsalva responses.After parabolic flight 1) 8 of 16 subjects could not tolerate 30 min of upright tilt (compared to 2 of 16 before flight); 2) 6 of 16 subjects vomited; 3) new intoleranceto upright tilt was associated with exaggerated falls in totalperipheral resistance, whereas vomiting was associated with increasedR-wave-R-wave interval variability and carotid-cardiac baroreflexresponsiveness; and 4) the proximate mode of new orthostaticfailure differed in subjects who did and did not vomit, with vomitersexperiencing comparatively isolated upright hypocapnia and cerebralvasoconstriction and nonvomiters experiencing signs and symptomsreminiscent of the clinical postural tachycardia syndrome. Resultssuggest, first, that syndromes of orthostatic intolerance resemblingthose developing after space flight can develop after a brief (i.e.,2-h) parabolic flight and, second, that recent vomiting can influencethe results of tests of autonomic cardiovascular function commonlyutilized in returning astronauts.

     

Gravity effects on cellulose assembly

The effect of microgravity on cellulose synthesis using the model system of Acetobacter xylinum was the subject of recent investigations using The National Aeronautics and Space Administration's Reduced Gravity Laboratory, a modified KC-135 aircraft designed to produce 20 sec of microgravity during the top of a parabolic dive. Approximately 40 parabolas were executed per mission, and a period of 2 x g was integral to the pullout phase of each parabola. Cellulose biosynthesis was initiated on agar surfaces, liquid growth medium, and buffered glucose during parabolic flight and terminated with 2.0% sodium azide or 50.0% ethanol. While careful ground and in-flight controls indicated normal, compact ribbons of microbial cellulose, data from five different flights consistently showed that during progression into the parabola regime, the cellulose ribbons became splayed. This observation suggests that some element of the parabola (the 20 sec microgravity phase, the 20 sec 2 x g phase, or a combination of both) was responsible for this effect. Presumably the cellulose I alpha crystalline polymorph normally is produced under strain, and the microgravity/hypergravity combination may relieve this stress to produce splayed ribbons. An in-flight video microscopy analysis of bacterial motions during a parabolic series demonstrated that the bacteria continue to synthesize cellulose during all phases of the parabolic series. Thus, the splaying may be a reflection of a more subtle alteration such as reduction of intermicrofibrillar hydrogen bonding. Long-term microgravity exposures during spaceflight will be necessary to fully understand the cellulose alterations from the short-term microgravity experiments.     

Shift in arm-pointing movements during gravity changes produced by aircraft parabolic flight.

It has been shown that target-pointing arm movements without visual feedback shift downward in space microgravity and upward in centrifuge hypergravity. Under gravity changes in aircraft parabolic flight, however, arm movements have been reported shifting upward in hypergravity as well, but a downward shift under microgravity is contradicted. In order to explain this discrepancy, we reexamined the pointing movements using an experimental design which was different from prior ones. Arm-pointing movements were measured by goniometry around the shoulder joint of subjects with and without eyes closed or with a weight in the hand, during hyper- and microgravity in parabolic flight. Subjects were fastened securely to the seat with the neck fixed and the elbow maintained in an extended position, and the eyes were kept closed for a period of time before each episode of parabolic flight. Under these new conditions, the arm consistently shifted downward during microgravity and mostly upward during hypergravity, as expected. We concluded that arm-pointing deviation induced by parabolic flight could be also be valid for studying the mechanism underlying disorientation under varying gravity conditions.     

G-suit inflation to 50 mmHg alters the cardiovascular transients when entering micro-G in parabolic flight

In the present experiments it was decided to have each test-subject serve as his own control by fitting the test-subjects with a G-suit and comparing the condition of inflated G-suit to the normal situation. G-suit inflation was intended to only displace blood on the venous side of the circulation, not to increase total peripheral resistance. Therefore, a very modest inflation of 50 mmHg was applied. This was considered sufficient to expel most of the blood from the venous pool in abdomen and legs, even under the condition of increased G-loading in the pull-up phase. The parabolas were to be undergone in three body positions: standing upright, sitting and supine. The prediction of the experimental outcome was that we would find no difference between transients with and without G-suit inflation in the supine position, that an initial overshoot in pressure and stroke volume in the upright position would be very much damped by the G-suit, even more in the standing than in the sitting position. Studies were performed in 5 flights of NASA's KC-135, in January 1993. Per flight 40 parabolas were flown in an adapted 'roller coaster profile', i.e. 0-G phases were followed by a 2-G pull-out phase, after a very brief 1-G phase again followed by the next 2-G pull-up phase. This sequence was flown for 10 parabolas, then a 1-G horizontal flight period was inserted. The first 3 parabolas of each set of 10 the subjects were sitting upright, seat belt fastened. The next three they were standing, feet stuck under a load strap on the floor, stabilizing themselves by a grip on the ceiling. Then three parabolas were flown with the test-subject supine, loosely attached to the floor by a load strap and further aided by a grip to another strap on the floor. The last parabola of a set was used as 'spare' to repeat any failed maneuver.     

Vestibular function and space motion sickness

The vestibular system plays an important role in intersensory interactions and gravitation is a natural stimulus for its receptors. Weightlessness alters the input signals of the otoliths and their effect on the pattern and dynamics of changes in the vestibular function (VF), which is accompanied by development of space adaptation syndrome (SAS) and space motion sickness (SMS). These changes occur both during the spaceflight (SF) and after returning to Earth, but the mechanisms of their development are still poorly understood and require special studies. In total, 47 Russian cosmonauts (crewmembers of long-term International Space Station (ISS) missions) have participated in the studies into VF before and after SF and nine of them, in onboard studies during SF (129–215 days) as a part of the Virtual space experiment (stage 1). Electro- and video-oculography are used to record spontaneous eye movements (SpEM), static vestibular–ocular responses during head tilts to the right or left shoulder (static otolith–cervical–ocular reflex, OCOR), and dynamic vestibular-ocular response during the head rotation around the longitudinal axis of the body. The examination is accompanied by personal and questionnaire survey on subjective responses and complaints of cosmonauts about SAS and SMS. Significant changes in SpEM (drifts of eyes, spontaneous and gaze-evoked nystagmus, and arbitrary saccades) and a decrease in OCOR (statistically significant decrease in the amplitude of ocular counter-rolling in response to head tilts up to its absence or inversion, an atypical OCOR) are observed during SF. An atypical OCOR is observed at the beginning of adaptation to weightlessness in seven of the nine cosmonauts (the first one to two weeks of SF) and repeatedly throughout the flight in all cosmonauts regardless of whether it is their first flight or not. Atypical vestibular responses after SF, similar to the responses during SF, are observed in several cosmonauts by day 9 after flight. It has been shown that atypical OCOR variants are more frequently observed in the subjects lacking any previous space experience, as well as a more pronounced decrease in this response with a concurrent increase in the response of the semicircular canals. It is also demonstrated that repeated SFs lead to a considerable shortening in the after-flight readaptation to terrestrial conditions and a considerable decrease in the degree of vestibular disorders. In the initial period of SF, the changes in VF are correlated with the complaints and manifestations of SAS and SMS; however, the complaints and the corresponding symptoms are unobservable during the further flight despite significant changes in the VF state. The patterns of the VF disorders associated with the impact of weightlessness and observed during and after SF are very similar, allowing these disorders to be regarded as SAS and SMS of different severities (intensities).     

Relationship between stroke volume and sympathetic nerve activity: new insights about autonomic mechanisms of syncope

Head-up tilt table experiments conducted in astronauts prior to and immediately after the NASA Neurolab Space Mission (STS-90) revealed that a reduction in stroke volume induced by moving from the supine to upright posture was associated with increased muscle sympathetic nerve activity (MSNA). Although this finding was not unexpected, lower average stroke volume and greater average MSNA measured after space flight in both supine and upright postures were positioned in a linear fashion on the same stroke volume-MSNA stimulus-response relationship as the average pre-flight stroke volume and MSNA responses. Since all astronauts who participated in the Neurolab orthostatic experiments completed the 10-min tilt table tests, these observations supported the notion that sympathetic reflex responses were not altered but functioned adequately after space flight in non-presyncopal subjects. In contrast to the Neurolab results, development of orthostatic hypotension and presyncopal events reported in astronauts during standing after space flight have been accompanied by attenuated peripheral vasoconstriction and less elevation in plasma concentrations of norepinephrine. The association between circulating norepinephrine (NE) and peripheral vascular resistance in presyncopal astronauts after space flight led to the conclusion that postflight presyncope can be attributed to a combination of inherently low-resistance responses, a strong dependence on volume status, and relative hypoadrenergic function. In the present investigation, we used graded levels of lower body negative pressure (LBNP) to produce linear reductions in stroke volume and performed direct measurements of MSNA to test the hypotheses that (1) elevations in MSNA during central hypovolemia are proportional (i.e., linear) with reductions in stroke volume and; (2) that the slope of the stroke volume-MSNA relationship will be reduced in presyncopal subjects.     

On the analysis of hand synergies during grasping in weightlessness

The aim of this paper was to analyse how the strategies implemented by the Central Nervous System to control the hand during grasping are modified under microgravity conditions. Two right-handed subjects carried out simple grasping tasks during parabolic flights. The trajectories of the fingers of the hand were recorded using a sensorised glove and processed in order to extract a variable (here indicated as K) which can indicated the degree of synergies existing among the fingers. The results showed that K was quite small during the trial at 1g while becoming significantly greater than 1 during the first parabolas. Then, the value k decreased to the values at 1 g after some parabolas. These results suggested a possible adaptation process of the manipulation abilities during the permanence at 0g conditions. Future extensive trials will be performed in order to confirm these preliminary results.     

Perception of spatial orientation in different g-levels

Spatial orientation perception was examined in experiments on the MIR94 mission, during parabolic flight, and in ground control experiments. Space flight subjects were asked to remember their initial orientation, then turned and, with eyes closed, signal the ceiling direction. Ground-based control subjects were asked to indicate their original starting position while rotating on their backs. Results indicate a sensory conflict related to the vestibular system can lead to spatial disorientation.     

A sustained hyper-g load as a tool to simulate space sickness

In April 1989 the three European scientist astronauts of the D1 Spacelab Mission were exposed to a 1.5 hours +3G centrifuge run in supine position, resulting in a linear acceleration along the subjects' x-axis. Afterwards, severe motion sickness symptoms were provoked by head movements (Sickness Induced by Centrifugation: SIC). The astronauts mentioned close similarities with what they experienced in space during the D1-Spacelab Mission in 1985, where head movements also provoked motion sickness symptoms (Space Adaptation Syndrome: SAS). Moreover, the astronauts agreed that the rank order of their susceptibility to SAS was the same as for SIC. It was therefore postulated that with this method SAS could be simulated on earth. Additionally, in otolith function tests following the centrifuge run, changes in visual-vestibular interaction were observed, which replicated objective findings obtained with the same astronauts immediately after the D1 Spacelab Mission. During the last couple of years a series of experiments has been carried out to determine the nature of the stimulus causing SIC, the incidence of SIC, and the underlying cardio-vascular and/or vestibular mechanisms. These experiments were carried out on several astronauts and some 50 'normal' healthy subjects. In the next sections the main findings of all these experiments and the implications are summarized.     

Feasibility of real-time 3D echocardiography in weightlessness during parabolic flight

Aim of the study was to test the feasibility of transthoracic real-time 3D (Philips) echocardiography (RT3D) during parabolic flight, to allow direct measurement of heart chambers volumes modifications during the parabola. One RT3D dataset corresponding to one cardiac cycle was acquired at each gravity phase (1 Gz, 1.8 Gz, 0 Gz, 1.8 Gz) during breath-hold in 8 unmedicated normal subjects (41 +/- 8 years old) in standing upright position. Preliminary results, obtained by semi-automatically tracing left ventricular (LV) and left atrial (LA) endocardial contours in multiple views (Tomtec), showed a significant (p<0.05) reduction, compared to 1 Gz, of LV and LA volumes with 1.8 Gz, and a significant increase with 0 Gz. Further analysis will focus on the right heart.     

The effects of a change in gravity on the dynamics of prehension

The objective of this study was to measure the forces applied on an object manipulated in different gravitational fields attained during parabolic flights. Eight subjects participated flights (ES) and four were inexperienced (NES). They had to move continuously an instrumented object up and down in three different gravitational conditions (1 g, 1.8 g, 0 g). In 1 g, the grip force precisely anticipated the fluctuations of load force which was maximum and minimum at the bottom and at the top of the arm trajectory respectively. When the gravity changed (0 g and 1.8 g), the grip-load force coupling persisted for all the subjects from the first parabola. While the ES immediately exerted a grip force appropriate to the gravity, the NES dramatically increased their grip when faced with hyper and microgravity for the first time. Then, they progressively released their grip until a continuous grip-load force relationship with regard to 1 g was established after the fifth parabola. We suggest that each new gravitational field is rapidly incorporated into an internal model within the CNS which can then be reused as required by the occasion.     

Behavioral reactions of the bat Carollia perspicillata to abrupt changes in gravity.

As part of an ongoing survey of the behavioral responses of vertebrates to abrupt changes in gravity, we report here on the reactions of bats (Carollia perspicillata) exposed to altered gravity during parabolic aircraft flight. In microgravity, mammals typically behave as if they were upside-down and exhibit repetitive righting reflexes, which often lead to long axis rolling. Since bats, however, normally rest upside-down, we hypothesized that they would not roll in microgravity. Only one of three specimens attempted to fly during microgravity. None rolled or performed any righting maneuvers. During periods of microgravity the bats partially extended their forearms but kept their wings folded and parallel to the body. Between parabolas and occasionally during microgravity the bats groomed themselves. Both the extended limbs and autogrooming may be stress responses to the novel stimulus of altered gravity. This is the first behavioral record of Chiroptera in microgravity.     

Gaze Behavior in One-Handed Catching and Its Relation with Interceptive Performance: What the Eyes Can't Tell

In ball sports, it is usually acknowledged that expert athletes track the ball more accurately than novices. However, there is also evidence that keeping the eyes on the ball is not always necessary for interception. Here we aimed at gaining new insights on the extent to which ocular pursuit performance is related to catching performance. To this end, we analyzed eye and head movements of nine subjects catching a ball projected by an actuated launching apparatus. Four different ball flight durations and two different ball arrival heights were tested and the quality of ocular pursuit was characterized by means of several timing and accuracy parameters. Catching performance differed across subjects and depended on ball flight characteristics. All subjects showed a similar sequence of eye movement events and a similar modulation of the timing of these events in relation to the characteristics of the ball trajectory. On a trial-by-trial basis there was a significant relationship only between pursuit duration and catching performance, confirming that keeping the eyes on the ball longer increases catching success probability. Ocular pursuit parameters values and their dependence on flight conditions as well as the eye and head contributions to gaze shift differed across subjects. However, the observed average individual ocular behavior and the eye-head coordination patterns were not directly related to the individual catching performance. These results suggest that several oculomotor strategies may be used to gather information on ball motion, and that factors unrelated to eye movements may underlie the observed differences in interceptive performance.     

Cardiovascular and Valsalva responses during parabolic flight

We investigated the integrated cardiovascularresponses of 15 human subjects to the acute gravitational changes(micro- and hypergravity portions) of parabolic flight. Measurementswere made with subjects quietly seated and while subjects performed controlled Valsalva maneuvers. During quiet, seated, parabolic flight,mean arterial pressure increased during the transition into microgravity but decreased as microgravity was sustained. Thedecrease in mean arterial pressure was accompanied by immediate reflexive increases in heart rate but by absent (orlater-than-expected) reflexive increases in total vascular resistance.Mean arterial pressure responses in Valsalva phasesII_l, III, and IV wereaccentuated in hypergravity relative to microgravity(P < 0.01, P < 0.01, andP < 0.05, respectively), butaccentuations differed qualitatively and quantitatively from thoseinduced by a supine-to-seated postural change in 1 G. This study is thefirst systematic evaluation of temporal and Valsalva-related changes incardiovascular parameters during parabolic flight. Results suggest thatarterial baroreflex control of vascular resistance may be modified byalterations of cardiopulmonary, vestibular, and/or otherreceptor activity.

     

Study protocol to examine the effects of spaceflight and a spaceflight analog on neurocognitive performance: extent,longevity, and neural bases

Background

Long duration spaceflight (i.e., 22 days or longer) has been associated with changes in sensorimotor systems, resulting in difficulties that astronauts experience with posture control, locomotion, and manual control. The microgravity environment is an important causal factor for spaceflight induced sensorimotor changes. Whether spaceflight also affects other central nervous system functions such as cognition is yet largely unknown, but of importance in consideration of the health and performance of crewmembers both in- and post-flight. We are therefore conducting a controlled prospective longitudinal study to investigate the effects of spaceflight on the extent, longevity and neural bases of sensorimotor and cognitive performance changes. Here we present the protocol of our study.

Methods/design

This study includes three groups (astronauts, bed rest subjects, ground-based control subjects) for which each the design is single group with repeated measures. The effects of spaceflight on the brain will be investigated in astronauts who will be assessed at two time points pre-, at three time points during-, and at four time points following a spaceflight mission of six months. To parse out the effect of microgravity from the overall effects of spaceflight, we investigate the effects of seventy days head-down tilted bed rest. Bed rest subjects will be assessed at two time points before-, two time points during-, and three time points post-bed rest. A third group of ground based controls will be measured at four time points to assess reliability of our measures over time. For all participants and at all time points, except in flight, measures of neurocognitive performance, fine motor control, gait, balance, structural MRI (T1, DTI), task fMRI, and functional connectivity MRI will be obtained. In flight, astronauts will complete some of the tasks that they complete pre- and post flight, including tasks measuring spatial working memory, sensorimotor adaptation, and fine motor performance. Potential changes over time and associations between cognition, motor-behavior, and brain structure and function will be analyzed.

Discussion

This study explores how spaceflight induced brain changes impact functional performance. This understanding could aid in the design of targeted countermeasures to mitigate the negative effects of long-duration spaceflight.

     

Recording of blood pressure,heart rate and aortic nerve activity during parabolic flight in the rat via radio-telemetry

Exposure to microgravity induces cardiovascular deconditioning characterized by orthostatic hypotension when astronauts return to the earth. In order to understand the mechanism of cardiovascular deconditioning, it is necessary to clarify the changes in hemodynamics and the cardiovascular regulation system over the period of space flight. The telemetry system applied to freely moving animals will be a useful and appropriate technique for this kind of long term study of the cardiovascular system in the conscious animal during space flight. The purpose of the present study is twofold: firstly, to observe the detailed changes of arterial pressure and heart rate (HR) during microgravity elicited by the parabolic flight in order to study the acute effect of microgravity exposure on the cardiovascular system; and secondly, to test the feasibility of the telemetry system for recording blood pressure, HR and autonomic nervous activities continuously during space flight.     

Using Low Levels of Stochastic Vestibular Stimulation to Improve Balance Function

Low-level stochastic vestibular stimulation (SVS) has been associated with improved postural responses in the medio-lateral (ML) direction, but its effect in improving balance function in both the ML and anterior-posterior (AP) directions has not been studied. In this series of studies, the efficacy of applying low amplitude SVS in 0–30 Hz range between the mastoids in the ML direction on improving cross-planar balance function was investigated. Forty-five (45) subjects stood on a compliant surface with their eyes closed and were instructed to maintain a stable upright stance. Measures of stability of the head, trunk, and whole body were quantified in ML, AP and combined APML directions. Results show that binaural bipolar SVS given in the ML direction significantly improved balance performance with the peak of optimal stimulus amplitude predominantly in the range of 100–500 μA for all the three directions, exhibiting stochastic resonance (SR) phenomenon. Objective perceptual and body motion thresholds as estimates of internal noise while subjects sat on a chair with their eyes closed and were given 1 Hz bipolar binaural sinusoidal electrical stimuli were also measured. In general, there was no significant difference between estimates of perceptual and body motion thresholds. The average optimal SVS amplitude that improved balance performance (peak SVS amplitude normalized to perceptual threshold) was estimated to be 46% in ML, 53% in AP, and 50% in APML directions. A miniature patch-type SVS device may be useful to improve balance function in people with disabilities due to aging, Parkinson’s disease or in astronauts returning from long-duration space flight.     

Head out of water immersion as simulation study: an heart rate variability study

During head out of water immersion (HOI), the hydrostatic pressure on the tissues of the lower limbs causes an increase in thoracic blood volume and a high vascular perfusion. This blood shift results in changes in autonomic balance. The aim of this study was to evaluate the activity of the autonomic nervous system, as obtained from the analysis of heart rate variability (HRV), during HOI at 2 different temperatures and compare these results with data obtained during parabolic flight. In HOI, two different positions (sitting and standing) were compared. Results showed a shift to vagal activity as well during HOI as in microgravity during parabolic flight.     

Bone markers during a 6-month space flight: effects of vitamin K supplementation

Rapid bone loss is a serious health problem for astronauts during long lasting missions in space. We have recorded the changes of biochemical markers for bone metabolism in one of the astronauts during the 6-month space flight of the EUROMIR-95 mission. Immediately after launch both bone resorption markers and urinary calcium excretion increased about two fold, whereas bone formation markers remained unchanged. After 12 1/2 weeks the astronaut received vitamin K1 (10 mg/day for 6 weeks). Vitamin K is known to be involved in the formation of gamma-carboxyglutamate (Gla) in proteins, such as the calcium-binding bone Gla-proteins osteocalcin and matrix Gla-protein. Concomitant with the start of vitamin K treatment, the calcium-binding capacity of osteocalcin increased, and so did the urinary excretion of free Gla. This is suggestive for a subclinical vitamin K-deficiency in the astronaut before vitamin K-supplementation. During periods of high vitamin K status markers for bone formation (osteocalcin and bone alkaline phosphatase) had increased as compared to the first part of the flight. The mean increases were 14 and 23%, respectively. Our data suggest that increased intake of vitamin K may contribute to counteracting microgravity-induced loss of bone mass during long lasting space missions, but need confirmation in more astronauts.     

Static and dynamic postural control in long-term microgravity: evidence of a dual adaptation

The adaptation of dynamicmovement-posture coordination during forward trunk bending wasinvestigated in long-term weightlessness. Three-dimensionalmovement analysis was carried out in two astronauts during a 4-momicrogravity exposure. The principal component analysis was applied tojoint-angle kinematics for the assessment of angular synergies. Theanteroposterior center of mass (CM) displacement accompanying trunkflexion was also quantified. The results reveal that subjects kepttypically terrestrial strategies of movement-posture coordination. Thetemporary disruption of joint-angular synergies observed at subjects'first in-flight session was promptly recovered when repetitive sessionsin flight were analyzed. The CM anteroposterior shift was consistently<3-4 cm, suggesting that subjects could dynamically control theCM position throughout the whole flight. This is in contrast to theobserved profound microgravity-induced disruption of the quasi-staticbody orientation and initial CM positioning. Although this study wasbased on only two subjects, evidence is provided that static anddynamic postural control might be under two separate mechanisms,adapting with their specific time course to the constraints of microgravity.