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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Venous compliance and fluid shift measurements on Spacelab IML-1

During the first few hours of a human spaceflight mission, a headward fluid shift out of the abdomen, pelvis, and legs initiates a number of adaptive cardiovascular responses, including a loss of intravascular and extravascular fluid volume. On return to earth, these cardiovascular changes may lead to debilitating symptoms of orthostatic intolerance in an unprotected astronaut. To test the hypothesis that an inflight increase in compliance of the leg veins may contribute to this condition, measurements of lower leg fluid shift and bulk venous compliance were collected from crew members during the eight-day First International Microgravity Laboratory shuttle mission. An ultrasonic limb plethysmograph, in conjunction with two compression cuffs encircling the calf and thigh, was used to determine bulk compliance of the underlying veins over a range of negative and positive transmural pressures. The data from inflight experiment sessions were compared to preflight and postflight sessions. The preliminary results indicate that the volume of the lower leg decreased by over 10 percent by the sixth day of flight, but there was no apparent change in venous compliance.

Leiski, D.↗
NASA NTRS1992

Spectral components of human cardiovascular responses to step changes in Lower Body Negative Pressure (LBNP) before and after 22 hour of 6 deg head down bed rest

Changes in autonomic outflow to peripheral organs during the development of bedrest induced orthostatic intolerance have not been determined. Recent studies have indicated that spectral analysis provides an indirect assessment of these changes. Eight male subjects were studied before and after 22 hours of 6 degree head down bedrest plus Lasix (40 mg. P.P.). Cardiovascular spectra (using an autoregressive technique) were determined for heart rate (HR, ECG), arterial pressure (AP, Finapres), radial artery flow (RF, Hokansen) and respiration rate (RR, BoMed). Spectra were obtained from 2.5 minute segments during control, lower body negative pressure (minus 10, 20, 30, 40, 50 mmHg) and recovery. Bedrest increased HR spectra power in the low frequency (.001 to .041 Hz) range, increased RF power in the low and mid (.04 to .18 Hz) range and increased AP power in the high (.18 to .50 Hz) frequency range. Increasing levels of lower body negative pressure decreased HR power and increased RF power in the high frequency range and decreased AP power in the low frequency range. Since spectral power of HR in the high frequency range has been shown to indicate parasympathetically mediated regulation and power in the low and mid frequency ranges indicates a sympathetic / parasympathetic mixture, then both bedrest and lower body negative pressure appeared to shift sympathetic / parasympathetic balance toward sympathetic regulation of HR. The interpretation of the spectral content of AP and RF with respect to their autonomic origins remains unclear.

Knapp, C. F.↗
NASA NTRS1992

Effects of weightlessness on human baroreflex function

Impaired cardiovascular function, characterized by orthostatic intolerance and reduced exercise capacity, is a result of space travel. We hypothesized that postflight baroreflex dysfunction may contribute. We studied the vagally mediated carotid baroreceptor-cardiac reflex response of 6 astronauts before, during, and after the ten day SLS-l mission. A series of R-waves triggered pressure and suction steps (from 40 to minus 65 mmHg) were delivered to a neck chamber during held expirtation. Resulting R-R interval changes were plotted against carotid distending pressure (systolic - neck pressure), and curve parameters calculated. After an initial rise, the operational point declined consistently during the flight and reached a nadir on landing day, but had recovered to preflight levels by L + 4. Slope and range of the response declined throughout the flight, were slightly recovered by the time measurements were made on landing day, but still were reduced on L + 4. These data indicate that space flight results in a significant impairment of the carotid baroreceptor cardiac reflex response.

Fritsch, Janice M.↗
NASA NTRS1992

Artificial gravity: How much, how often, how long?

The argument is not overwhelming for the need to provide a continuous 1G environment using tethers or other means of spinning a spacecraft in order to maintain crew health in planetary exploration. Even on Earth, we spend a maximum of 16 hours in 1G (upright). Sporadic evidence over the years has suggested that somewhere between 30-minutes and 4-hours of 1G may suffice to prevent the deconditioning effects of bedrest (orthostatic intolerance and the rise in calcium excretion). However, it is not known what the minimum requirements are, whether they vary for different physiological systems and whether passive zero gravity or the enhancement of the effects of activity conducted in an increased G field are more effective. It is similarly not known what the optimal duration and frequency of the G stimulus is, and how time of day might alter its effectiveness. Since acceleration level and duration appear to be physiologically interactive, it seems feasible to hypothesize that periodic acceleration exposures to greater than 1G levels provided by some on-board centrifuge, would suffice and should be explored.

Burton, R.↗
NASA NTRS1995

Calf venous compliance measured with head-up tilt equals supine calf compliance

Elevated calf compliance may contribute to orthostatic intolerance following space flight and bed rest. Calf venous compliance is measured conventionally with venous occlusion plethysmography in supine subjects. With this well-established technique, subjects undergo inflation of a pressure cuff around the thigh just above the knee, which increases calf venous pressure. A plethysmograph simultaneously measures calf volume elevation. Compliance equals calf volume elevation per mm Hg thigh occlusion (calf venous) pressure in relaxed legs of the supine subjects. Compliance may also be measured during stepwise head-up tilt (HUT) as calf volume elevation per mm Hg gravitational venous pressure elevation produced by HUT. However, during HUT on a tilt table with a footplate, calf muscles activate to counteract gravity: this is an obvious and natural response to gravitational force. Such muscle activation conceivably could reduce calf compliance, yet relatively little calf muscle activation occurs during HUT and orthostasis (less than 10% of maximal voluntary levels; 4, 8, 10). Also, this activation produces minimal calf volume change (less than 0.3%; 3). Therefore, we hypothesized that calf compliance measured with HUT equals that measured with supine venous occlusion.

Watenpaugh, D. E.↗
NASA NTRS1995

Effects of 30 day simulated microgravity and recovery on fluid homeostasis and renal function in the rat

Transition from a normal gravitational environment to that of microgravity eventually results in decreased plasma and blood volumes, increasing with duration of exposure to microgravity. This loss of vascular fluid is presumably due to negative fluid and electrolyte balance and most likely contributes to the orthostatic intolerance associated with the return to gravity. The decrease in plasma volume is presumed to be a reflection of a concurrent decrease in extracellular fluid volume with maintenance of normal plasma-interstitial fluid balance. In addition, the specific alterations in renal function contributing to these changes in fluid and electrolyte homeostasis are potentially responding to neuro-humoral signals that are not consistent with systemic fluid volume status. We have previously demonstrated an early increase in both glomerular filtration rate and extracellular fluid volume and that this decreases towards control values by 7 days of simulated microgravity. However, longer duration studies relating these changes to plasma volume alterations and the response to return to orthostasis have not been fully addressed. Male Wistar rats were chronically cannulated, submitted to 30 days heat-down tilt (HDT) and followed for 7 days after return to orthostasis from HDT. Measurements of renal function and extracellular and blood volumes were performed in the awake rat.

Tucker, Bryan J.↗
NASA NTRS1996

Dextroamphetamine: a pharmacologic countermeasure for space motion sickness and orthostatic dysfunction

Dextroamphetamine has potential as a pharmacologic agent for the alleviation of two common health effects associated with microgravity. As an adjuvant to Space Motion Sickness (SMS) medication, dextroamphetamine can enhance treatment efficacy by reducing undesirable Central Nervous System (CNS) side effects of SMS medications. Secondly, dextroamphetamine may be useful for the prevention of symptoms of post-mission orthostatic intolerance caused by cardiovascular deconditioning during spaceflight. There is interest in developing an intranasal delivery form of dextroamphetanmine for use as a countermeasure in microgravity conditions. Development of this dosage form will require an analytical detection method with sensitivity in the low ng range (1 to 100 ng/mL). During the 1995 Summer Faculty Fellowship Program, two analytical methods were developed and evaluated for their suitability as quantitative procedures for dextroamphetamine in studies of product stability, bioavailability assessment, and pharmacokinetic evaluation. In developing some of the analytical methods, beta-phenylethylamine, a primary amine structurally similar to dextroamphetamine, was used. The first analytical procedure to be evaluated involved hexane extraction and subsequent fluorescamine labeling of beta-phenylethylamine. The second analytical procedure to be evaluated involved quantitation of dextroamphetamine by an Enzyme-Linked ImmunoSorbent Assay (ELISA).

Snow, L. Dale↗
NASA NTRS1993

[Volume Homeostasis and Renal Function in Rats Exposed to Simulated and Actual Microgravity]

This project has investigated mechanisms that influence alterations in compartmental fluid and electrolyte balance in microgravity and evaluates countermeasures to control renal fluid and electrolyte losses. Determining the alterations due to space flight in fluid compartments and renal function is an important component in understanding long term adaptation to spaceflight and the contribution to post-flight orthostatic intolerance. Four definition phase studies and two studies examining neuro-humoral and vascular mechanisms have been completed.

Tucker, Bryan J.↗
NASA NTRS1996

Effect of +Gz Acceleration on the Oxygen Uptake-Excercise Load Relationship during Lower Extremity Ergometer Excercise

Long term spaceflight and habitation of a space station and/or the moon require that astronauts be provided with sufficient environmental and physiological support so that they can not only function in microgravity but be returned to earth safely. As the duration of habitation in microgravity increase the effects of the concomitant deconditioning of body systems becomes a concern for added exercise in space and for reentry to Earth gravity. Many countermeasures have been proposed to maintain proper functioning of the body, but none have proved sufficient, especially when the cost of crew time spent in these activities is considered. The issue of appropriate countermeasures remains unresolved. Spaceflight deconditioning decreases tolerance to +Gz acceleration, head to foot, the direction which is experienced during reentry; the result is that the crew member is more prone to becoming pre-syncopal or syncopal, thus exacerbating the orthostatic intolerance. All ground-based research using microgravity analogues has produced this same lowered G tolerance. When intermittent exposure to +1 to +4 Gz acceleration training was used, some alleviation of orthosatic intolerance and negative physiological effects of deconditioning occurred. Exercise alone was not as effective; but the added G force was. The physiological responses to acceleration added to exercise training have not been clearly shown. We will test the hypothesis that there will be no difference in the exercise oxygen uptake-exercise load relationship with added +Gz acceleration. We wi also compare oxygen uptake during graded exercise-acceleration loads in the human-powered short arm centrifuge with those from normal supine exercise loads. The human-powered short arm centrifuge was built by NASA engineers at Ames Research Center.

Jackson, Catherine G. R.↗
NASA NTRS1995

Clinical Aspects of the Control of Plasma Volume at Microgravity and During Return to One Gravity

Plasma volume is reduced by 10%-20% within 24 to 48 h of exposure to simulated or actual microgravity. The clinical importance of microgravity-induced hypovolemia is manifested by its relationship with orthostatic intolerance and reduced VO2max after return to one gravity (1G). Since there is no evidence to suggest plasma volume reduction during microgravity is associated with thirst or renal dysfunctions, a diuresis induced by an immediate blood volume shift to the central circulation appears responsible for microgravity-induced hypovolemia. Since most astronauts choose to restrict their fluid intake before a space mission, absence of increased urine output during actual spaceflight may be explained by low central venous pressure (CVP) which accompanies dehydration. Compelling evidence suggests that prolonged reduction in CVP during exposure to microgravity reflects a 'resetting' to a lower operating point which acts to limit plasma volume expansion during attempts to increase fluid intake. In groudbase and spaceflight experiments, successful restoration and maintenance of plasma volume prior to returning to an upright posture may depend upon development of treatments that can return CVP to its baseline 10 operating point. Fluid-loading and LBNP have not proved completely effective in restoring plasma volume, suggesting that they may not provide the stimulus to elevate the CVP operating point. On the other, exercise, which can chronically increase CVP, has been effective in expanding plasma volume when combined with adequate dietary intake of fluid and electrolytes. The success of designing experiments to understand the physiological mechanisms of and development of effective countermeasures for the control of plasma volume in microgravity and during return to one gravity will depend upon testing that can be conducted under standardized controlled baseline condi

Convertino, Victor A.↗
NASA NTRS1998

Human Physiological Responses to Cycle Ergometer Leg Exercise During +Gz Acceleration

Spaceflight and bed-rest deconditioning decrease maximal oxygen uptake (aerobic power), strength, endurance capacity, and orthostatic tolerance. In addition to extensive use of muscular exercise conditioning as a countermeasure for the reduction in aerobic power (VO(sub 2max)), stimuli from some form of +Gz acceleration conditioning may be necessary to attenuate the orthostatic intolerance component of this deconditioning. Hypothesis: There will be no significant difference in the physiological responses (oxygen uptake, heart rate, ventilation, or respiratory exchange ratio) during supine exercise with moderate +Gz acceleration.

Chou, J. L.↗
NASA NTRS1999

Cardiovascular Session Summary

It was apparent that the bed-rest and spaceflight data indicated that decreases in plasma volume and cardiac atrophy along with cardiac remodeling were fundamental changes which predisposed many astronauts to post flight orthostatic intolerance. Despite the recently acquired in-flight and post-flight muscle sympathetic nerve activity findings suggesting that the sympathetic nerve responses were appropriate there remains significant contrary data from bed-rest studies, post- flight stand tests and hind-limb unweighted rat studies that suggest that the vasoconstrictive responses were compromised at least insufficient in susceptible individuals. The key issues raised is whether a diminished increase in sympathetic activity from baseline without changes in 254 First Biennial Space Biomedical Investigators'Workshop Cardiovascular peak response or receptor adaptations is an abnormal response or is an individual variance of response to the accentuated decrease in stroke volume. Data relating autonomic neural control of heart rate were presented to suggest that the vagal and sympathetic control of heart rate was attenuated. Also, bed-rest and space flight induced attenuated baroreflex control of heart rate was shown to be restored to pre-bedrest function by one bout of maximal dynamic exercise. However, these data were confounded by relying on the use of R-R interval as a measure of efferent responses of the baroreflex during a condition in which the baseline heart rate was changed. Clearly the idea that the autonomic control of heart rate may be changed by microgravity needs further investigation. This direction is suggested despite the fact that in the triple product (HR x SV x TPR = MAP) assessment of the regulation of arterial blood pressure during orthostasis the role of the HR reflex may be less influential than that associated. with cardiac atrophy (SV changes) and aberrant sympathetic vasoconstriction (resistance) changes. Although sympathetic nerve activity responses in-flight and post-flight on neurolab appeared appropriate, enough bed-rest and post-flight stand test data, along with animal model data suggest that vasoconstriction was compromised. The mechanism of this compromised vasoconstriction needs to be delineated. Other major findings concerning microgravity and physiological regulatory systems are that: I . Thermoregulatory adaptation appear to suggest some decrements in the control of cutaneous vasodilation and sweating; 2. Calcium resorption and dietary calcium need to be defined for differing durations of spaceflight, especially as the effects of excess calcium on vasomotor function appears to be detrimental; 3. Neurohumoral mechanisms of microgravity induced changes in neural function and the regulation of plasma volume and total body water, bone resorption and autonomic neural control of the circulation need further delineation; 4. As performance of work tasks become prolonged, the mechanisms of blood pressure regulation in microgravity needs to be used in the recovery period from prolonged work tasks.

Raven, Peter↗
NASA NTRS2000

Development of Countermeasures and Exercise Protocols to Reduce the Effects of Microgravity

I have helped scientists at NASA-JSC in analyzing data from many projects. Some of the major ones are: (1) cardiovascular responses to lower body negative pressure (LBNP) following bed rest, (2) the effects of dietary sodium, (3) in-flight cycle exercise mitigates reduced oxygen consumption at submaximal heart rates following space flight, (4) exercise thermoregulation after 13 days of head down bed rest, and (5) bed rest induced orthostatic intolerance. Many of the projects have now been completed and some of them are in the process of being published and others have been presented at national meetings. These projects have helped me be a true statistician and given me a real-life perspective of how interesting and complicated data can be. As a by-product of of these involvements I have been able to write and publish some methodological research that have applications in NASA and elsewhere. For instance, while I was at JSC, I happened to meet Dr. Al Feiveson and got into a discussion of the Space Shuttle Reliability. This led us to rethink about the way the data on the accelerated life testing of space shuttle pressure vessels had been analyzed. This has resulted in a major statistical paper and the paper has appeared in one of the top journals in the field of Statistics. A review of the paper by the editor of the journal was published in AmStatNews, a copy is attached with this report. I have presented these findings at the national/international statistics conference and at other places. I have also written another paper on reliability and a paper on calibration techniques that have applications in the engineering and the biomedical branches of NASA. Further, I am currently in the process of writing at least two more papers that have direct applications in NASA related studies.

Kulkarni, Pandurang M.↗
NASA NTRS2000

Blood Volume: Importance and Adaptations to Exercise Training, Environmental Stresses and Trauma/Sickness

This paper reviews the influence of several perturbations (physical exercise, heat stress, terrestrial altitude, microgravity, and trauma/sickness) on adaptations of blood volume (BV), erythrocyte volume (EV), and plasma volume (PV). Exercise training can induced BV expansion; PV expansion usually occurs immediately, but EV expansion takes weeks. EV and PV expansion contribute to aerobic power improvements associated with exercise training. Repeated heat exposure induces PV expansion but does not alter EV. PV expansion does not improve thermoregulation, but EV expansion improves thermoregulation during exercise in the heat. Dehydration decreases PV (and increases plasma tonicity) which elevates heat strain and reduces exercise performance. High altitude exposure causes rapid (hours) plasma loss. During initial weeks at altitude, EV is unaffected, but a gradual expansion occurs with extended acclimatization. BV adjustments contribute, but are not key, to altitude acclimatization. Microgravity decreases PV and EV which contribute to orthostatic intolerance and decreased exercise capacity in astronauts. PV decreases may result from lower set points for total body water and central venous pressure, which EV decrease bay result form increased erythrocyte destruction. Trauma, renal disease, and chronic diseases cause anemia from hemorrhage and immune activation, which suppressions erythropoiesis. The re-establishment of EV is associated with healing, improved life quality, and exercise capabilities for these injured/sick persons.

Sawka, Michael N.↗
NASA NTRS1994

Effect of intermittent standing and walking on physiological changes induced by head-down bed rest

Continuous exposure to gravity may not be necessary to prevent compromised physiological function resulting from exposure to microgravity. However, minimum gravity (G) exposure requirements, effectiveness of passive Gz versus activity in a G field, and optimal G stimulus amplitude, duration, and frequency are unknown. To partially address these questions, a 4-day, 6 degree head-down bed rest (HDBR) study (one ambulatory control day, 4 full HDBR days, one recovery day) was conducted. Nine males, 30-50 yr, were subjected to four different +1 Gz (head-foot) exposure protocols (periodic standing or controlled walking for 2 or 4 h/day in 15 min doses), plus a continuous HDBR (0 Gz) control. Standing 4 h completely prevented and standing 2 h partially prevented post-HDBR orthostatic intolerance. Both walking conditions (2 h and 4 h) attenuated the decrease in peak VO2 and prevented the increased urinary Ca2+ excretion associated with HDBR. Both 4 h conditions (standing and walking) attenuated plasma volume loss during HDBR. It was concluded that various physiological systems benefit differentially from passive +1 Gz or activity in +1 Gz and the duration (2 h vs. 4 h) of the stimulus may be an important moderating factor.

Vernikos, J.↗
NASA NTRS1994

Autogenic Feedback Training Applications for Man in Space

Finding an effective treatment for the motion sickness-like symptoms that occur in space has become a high priority for NASA. This paper reviews the back-round research and procedures of an experiment designed to prevent space motion sickness in shuttle crewmembers. The preventive method used, Autogenic - Feedback Training (AFT) involves training subjects to control voluntarily several of their own physiological responses to environmental stressors. AFT has been used reliably to increase tolerance to motion sickness during around based tests in over 300 men and women under a variety of conditions that induce motion sickness, and preliminary evidence from space suggests that AFT may be an effective treatment for space motion sickness as well. Other applications of AFT described include; (1) a potential treatment for post flight orthostatic intolerance, a serious biomedical problem resulting from long duration exposure to micro-g and (2) improving pilot performance during emergency flying conditions.

Cowings, Patricia S.↗
NASA NTRS1994

Sixteen-Day Bedrest Significantly Increases Plasma Colloid Osmotic Pressure

Upon exposure to microgravity, astronauts lose up to 10% of their total plasma volume, which may contribute to orthostatic intolerance after space flight. Because plasma colloid osmotic pressure (COP) is a primary factor maintaining plasma volume, our objective was to measure time course changes in COP during microgravity simulated by 6 deg. head-down tilt (HDT). Seven healthy male subjects (30-55 years of age) were placed in HDT for 16 days. For the purpose of another study, three of the seven subjects were chosen to exercise on a cycle ergometer on day 16. Blood samples were drawn immediately before bedrest on day 14 of bedrest, 18-24 hours following exercise while all subjects were still in HDT and 1 hour following bedrest termination. Plasma COP was measured in all 20 microliter EDTA-treated samples using an osmometer fitted with a PM 30 membrane. Data were analyzed with paired and unpaired t-tests. Plasma COP on day 14 of bedrest (29.9 +/- 0.69 mmHg) was significantly higher (p less than 0.005) than the control, pre-bedrest value (23.1 +/- 0.76 mmHg). At one hour of upright recovery after HDT, plasma COP remained significantly elevated (exercise: 26.9 +/- 0.87 mmHg; no exercise: 26.3 +/- 0.85 mmHg). Additionally, exercise had no significant effect on plasma COP 18-24 hours following exercise (exercise: 27.8 +/- 1.09 mmHg; no exercise: 27.1 +/- 0.78 mmHg). Our results demonstrate that plasma COP increases significantly with microgravity simulated by HDT. However, preliminary results indicate exercise during HDT does not significantly affect plasma COP.

Hargens, Alan R.↗
NASA NTRS1995

Adaptation to Space: An Introduction

The cardiovascular and musculoskeletal systems are normally exposed to gradients of blood pressure and weight on Earth. These gradients increase blood pressure and tissue weight in dependent tissues of the body. Exposure to actual and simulated microgravity causes blood and tissue fluid to shift from the legs to the head. Studies of humans in space have documented facial edema, space motion sickness, decreased plasma volume, muscle atrophy, and loss of bone strength. Return of astronauts to Earth is accompanied by orthostatic intolerance, decreased neuromuscular coordination, and reduced exercise capacity. These factors decrease performance during descent from orbit and increase risk during emergency egress from the spacecraft. Models of simulated microgravity include 6 deg head-down tilt, immersion, and prolonged horizontal bedrest. Head-down tilt is the most accepted model and studies using this model of up to one year have been performed in Russia. Animal models which offer clear insights into the role of gravity on vertebrates include the developing giraffe and snakes from various habitats. Finally, possible countermeasures to speed readaptation of astronauts to gravity after prolonged space flight will be discussed.

Hargens, Alan R.↗