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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Effects of Local Compression on Peroneal Nerve Function in Humans

A new apparatus was developed to compress the anterior compartment selectively and reproducibly in humans. Thirty-five normal volunteers were studied to determine short-term thresholds of local tissue pressure that produce significant neuromuscular dysfunction. Local tissue fluid pressure adjacent to the deep peroneal nerve was elevated by the compression apparatus and continuously monitored for 2-3 h by the slit catheter technique. Elevation of tissue fluid pressure to within 35-40 mm Hg of diastolic blood pressure (approx. 40 mm Hg of in situ pressure in our subjects) elicited a consistent progression of neuromuscular deterioration including, in order, (a) gradual loss of sensation, as assessed by Semmes-Weinstein monofilaments, (b) subjective complaints, (c) reduced nerve conduction velocity, (d) decreased action potential amplitude of the extensor digitorum brevis muscle, and (e) motor weakness of muscles within the anterior compartment. Generally, higher intracompartment at pressures caused more rapid deterioration of neuromuscular function. In two subjects, when in situ compression levels were 0 and 30 mm Hg, normal neuromuscular function was maintained for 3 h. Threshold pressures for significant dysfunction were not always the same for each functional parameter studied, and the magnitudes of each functional deficit did not always correlate with compression level. This variable tolerance to elevated pressure emphasizes the need to monitor clinical signs and symptoms carefully in the diagnosis of compartment syndromes. The nature of the present studies was short term; longer term compression of myoneural tissues may result in dysfunction at lower pressure thresholds.

Hargens, Alan R.↗
NASA NTRS1995

Atomic Clocks and Variations of the FIne Structure Constant

We describe a new test for possible variations of the fine structure constant alpha by comparisons of rates between clocks based on hyperfine transitions in alkali atoms with different atomic number Z. H-maser, Cs, and Hg(+) clocks have a different dependence on alpha via relativistic contributions of order (Z-alpha)(sup 2). Recent H-maser vs Hg(+) clock comparison data improve laboratory limits on a time variation by 100-fold to give dot-alpha less than or equal to 3.7 x 10(exp -14)/yr. Future laser cooled clocks (Be(+), Rb, Cs, Hg(+), etc.), when compared, will yield the most sensitive of all tests for dot-alpha/alpha.

Prestage, John D.↗
NASA NTRS1999

Crystal Growth of II-VI Semiconducting Alloys by Directional Solidification

This research study is investigating the effects of a microgravity environment during the crystal growth of selected II-VI semiconducting alloys on their compositional, metallurgical, electrical and optical properties. The on-going work includes both Bridgman-Stockbarger and solvent growth methods, as well as growth in a magnetic field. The materials investigated are II-VI, Hg(1-x)Zn(x)Te, and Hg(1-x)Zn(x)Se, where x is between 0 and 1 inclusive, with particular emphasis on x-values appropriate for infrared detection and imaging in the 5 to 30 micron wavelength region. Wide separation between the liquidus and solidus of the phase diagrams with consequent segregation during solidification and problems associated with the high volatility of one of the components (Hg), make the preparation of homogeneous, high-quality, bulk crystals of the alloys an extremely difficult nearly an impossible task in a gravitational environment. The three-fold objectives of the on-going investigation are as follows: (1) To determine the relative contributions of gravitationally-driven fluid flows to the compositional redistribution observed during the unidirectional crystal growth of selected semiconducting solid solution alloys having large separation between the liquidus and solidus of the constitutional phase diagram; (2) To ascertain the potential role of irregular fluid flows and hydrostatic pressure effects in generation of extended crystal defects and second-phase inclusions in the crystals; and, (3) To obtain a limited amount of "high quality" materials needed for bulk crystal property characterizations and for the fabrication of various device structures needed to establish ultimate material performance limits. The flight portion of the study was to be accomplished by performing growth experiments using the Crystal Growth Furnace (CGF) manifested to fly on various Spacelab missions.

Lehoczky, Sandor L.↗
NASA NTRS1994

Comparison of Gait During Treadmill Exercise While Supine in Lower Body Negative Pressure (LBNP), Supine with Bungee Resistance and Upright in Normal Gravity

The purpose of this study is to compare footward forces, gait kinematics, and muscle activation patterns (EMG) generated during supine treadmill exercise against LBNP with the same parameters during supine bungee resistance exercise and upright treadmill exercise. We hypothesize that the three conditions will be similar. These results will help validate treadmill exercise during LBNP as a viable technique to simulate gravity during space flight. We are evaluating LBNP as a means to load the musculoskeletal and cardiovascular systems without gravity. Such loading should help prevent physiologic deconditioning during space flight. The best ground-based simulation of LBNP treadmill exercise in microgravity is supine LBNP treadmill exercise on Earth because the supine footward force vector is neither directed nor supplemented by Earth's gravity. Previous results from HR-95 ("Dynamics of footward force and leg intramuscular pressure during exercise against supine LBNP and upright standing in normal gravity") indicate that supine plantar-/dorsiflexion exercise in LBNP at 100 mm Hg produces similar ground reaction forces, musculoskeletal stress, and VO2 to those during upright exercise against Earth's gravity. However, elevations of leg volume and heart rate indicate that cardiovascular stress during 100 mm Hg LBNP exercise exceeds that during 1 g exercise. Therefore, the need arose to reduce the cardiovascular stress of LBNP, while maintaining LBNP-induced reaction forces. To this end, we determined that mild plantar-/dorsiflexion exercise during LBNP significantly improves tolerance to LBNP via musculovenous pumping and sympathoexcitation; more intense exercise such as walking and running may further improve LBNP tolerance. In addition, two methodological advances have permited us to simulate upright 1 g exercise better with supine LBNP exercise. First, a newly-designed waist seal allows decreased levels of LBNP (50-60 mm Hg) to produce a footward force equaling one body weight

Boda, Wanda↗
NASA NTRS2001

Effects of Hypergravity Exposure on Prolactin Levels in Pre-parturient , Parturient and Lactating Rat Dams

We analyzed the effects of 2.0-g, 1.75-g and 1.5-g hypergravity exposure on plasma concentrations of the lactotrophic hormone, prolactin (PRL), in female rats on pre-parturient (Gestation Day 20), parturient (Post-natal day 0) and lactating (P10) days. PRL levels have been found to be reduced in rat dams around the time of birth following exposure to gravitational loads varying from 2.16 to 3.14-g (Megory et. al., Aviation, Space and Environs 1129-1135, 1984). It has also been reported that at these high gravitational loads, neonatal mortality has been extremely high, suggesting a possible interaction between dam PRL concentration and neonatal outcome. We have previously reported no significant differences in PRL levels of parturient (PO) and lactating (P6 & P 15) dams when exposed to 1.5-g hypergravity, but did observe a slight elevation of PRL on PO and P 15, with a decrease on P6. In the present study, time-bred pregnant dams were exposed to either continuous 2.0-g, 1.75-g or 1.5-g centrifugation, beginning on Gestational day (G) 11 of the rats' 22-day pregnancy. We observed no significant differences in PRL concentrations between SC and any of the HG conditions. On G20 and PO, PRL concentrations of the 2.0-g and 1.5-g groups were slightly elevated as compared to SC. Similar to what we previously reported. PRL secretion was elevated in both HG and SC conditions on the day of birth relative to later during lactation, but on P10 it appeared to be reduced in HG relative to SC dams. These findings suggests that hypergravity slightly elevates plasma concentration of PRL in pre-parturient and lactating rat dams, with effects most pronounced during the periparturitional period and in a direction opposite to that observed following microgravity exposure.

Baer. Lisa A.↗
NASA NTRS1994

Fluid Redistribution and Heart Rate in Humans During Whole-Body Tilting, G(z) Centrifugation, and Lower Body Negative Pressure

Gravity creates blood pressure gradients which redistribute body fluids towards the feet. Positive G(z) centrifugation and lower body negative pressure (LBNP) have been proposed to simulate these and other effects of gravity during long-term existence in microgravity. We hypothesized that the magnitude of upper-to-lower body fluid redistribution would increase according to the following order: short-arm centrifugation (SAC), long-arm centrifugation (LAC), head-up tilt (HUT), and LBNP. To test this hypothesis, we employed strain gauge plethysmography of the neck, thigh and calf during HUT and supine SAC and LAC up to lG(z) at the feet, and during supine LBNP to 100 mm Hg. Supine 100 mm Hg LBNP generates footward force and produces transmural blood pressures in the foot approximately equal to 1 G(z) (90 deg) HUT. Heart rate was measured via cardiotachometry. Control measurements were made while supine. SAC and LAC elicited similar increases in thigh volume at 1 G(z) (2.3 +/- 0.4 and 2.1 +/- 0.1%, respectively; mean +/- se, n greater than or equal to 7). At 100 mm Hg LBNP, thigh volume increased (3.4 +/- 0.3%) significantly more than during l G(z) centrifugation (p less than 0.05). Surprisingly, due to a paradoxical 0.6% reduction of thigh volume between 0.8 and 1.0 G(z) HUT, thigh volume was increased only 0.6 +/- 0.3% at 1 G(z) HUT. The calf demonstrated similar, although less definitive, responses to the various gravitational stimuli. Neck volume tended to decrease less during HUT than during the other stimuli. Heart rate increased similarly during HUT (18 +/- 2 beats/min) and LAC (12 +/- 2 beats/min), and exhibited still greater elevation during LBNP (29 +/- 4 beats/min), yet did not increase during SAC. These results suggest upright posture activates mechanisms that counteract footward fluid redistribution which are not activated during supine applications of simulated gravity. LAC more closely approximated effects of normal gravity (HUT) than LBNP. Therefore, when considering LBNP to simulate gravity, these findings support efforts to reduce the cardiovascular stress imposed by LBNP, while preserving the gravity-like force generated by LBNP.

Watenpaugh, D. E.↗
NASA NTRS2003

Effects of repeated Valsalva maneuver straining on cardiac and vasoconstrictive baroreflex responses

INTRODUCTION: We hypothesized that repeated respiratory straining maneuvers (repeated SM) designed to elevate arterial BPs (arterial baroreceptor loading) would acutely increase baroreflex responses. METHODS: We tested this hypothesis by measuring cardiac baroreflex responses to carotid baroreceptor stimulation (neck pressures), and changes in heart rate and diastolic BP after reductions in BP induced by a 15-s Valsalva maneuver in 10 female and 10 male subjects at 1, 3, 6, and 24 h after performing repeated SM. Baroreflex responses were also measured in each subject at 1, 3, 6, and 24 h at the same time on a separate day without repeated SM (control) in a randomized, counter-balanced cross-over experimental design. RESULTS: There was no statistical difference in carotid-cardiac and peripheral vascular baroreflex responses measured across time following repeated SM compared with the control condition. Integrated cardiac baroreflex response (deltaHR/ deltaSBP) measured during performance of a Valsalva maneuver was increased by approximately 50% to 1.1 +/- 0.2 bpm x mm Hg(-1) at 1 h and 1.0 +/- 0.1 bpm x mm Hg(-1) at 3 h following repeated SM compared with the control condition (0.7 +/- 0.1 bpm x mm Hg(-1) at both 1 and 3 h, respectively). However, integrated cardiac baroreflex response after repeated SM returned to control levels at 6 and 24 h after training. These responses did not differ between men and women. CONCLUSIONS: Our results are consistent with the notion that arterial baroreceptor loading induced by repeated SM increased aortic, but not carotid, cardiac baroreflex responses for as long as 3 h after repeated SM. We conclude that repeated SM increases cardiac baroreflex responsiveness which may provide patients, astronauts, and high-performance aircraft pilots with protection from development of orthostatic hypotension.

NASA Discipline Cardiopulmonary↗
NASA NTRS2002

Lower-body negative-pressure exercise and bed-rest-mediated orthostatic intolerance

PURPOSE: Supine, moderate exercise is ineffective in maintaining orthostatic tolerance after bed rest (BR). Our purpose was to test the hypothesis that adding an orthostatic stress during exercise would maintain orthostatic function after BR. METHODS: Seven healthy men completed duplicate 15-d 6 degrees head-down tilt BR using a crossover design. During one BR, subjects did not exercise (CON). During another BR, subjects exercised for 40 min.d(-1) on a supine treadmill against 50-60 mm Hg LBNP (EX). Exercise training consisted of an interval exercise protocol of 2- to 3-min intervals alternating between 41 and 65% (.)VO(2max). Before and after BR, an LBNP tolerance test was performed in which the LBNP chamber was decompressed in 10-mm Hg stages every 3 min until presyncope. RESULTS: LBNP tolerance, as assessed by the cumulative stress index (CSI) decreased after BR in both the CON (830 +/- 144, pre-BR vs 524 +/- 56 mm Hg.min, post-BR) and the EX (949 +/- 118 pre-BR vs 560 +/- 44 mm Hg.min, post-BR) conditions. However, subtolerance (0 to -50 mm Hg LBNP) heart rates were lower and systolic blood pressures were better maintained after BR in the EX condition compared with CON. CONCLUSION: Moderate exercise performed against LBNP simulating an upright 1-g environment failed to protect orthostatic tolerance after 15 d of BR.

NASA Center JSC↗
NASA NTRS2002

Selective norepinephrine reuptake inhibition as a human model of orthostatic intolerance

BACKGROUND: Observations in patients with functional mutations of the norepinephrine transporter (NET) gene suggest that impaired norepinephrine uptake may contribute to idiopathic orthostatic intolerance. METHODS AND RESULTS: We studied the effect of the selective NET blocker reboxetine and placebo in a randomized, double-blind, crossover fashion on cardiovascular responses to cold pressor testing, handgrip testing, and a graded head-up tilt test (HUT) in 18 healthy subjects. In a subset, we determined isoproterenol and phenylephrine sensitivities. Subjects ingested 8 mg reboxetine or placebo 12 hours and 1 hour before testing. In the supine position, heart rate was 65+/-2 bpm with placebo and 71+/-3 bpm with reboxetine. At 75 degrees HUT, heart rate was 84+/-3 and 119+/-4 bpm with placebo and with reboxetine (P<0.0001). Mean arterial pressure was 85+/-2 with placebo and 91+/-2 mm Hg with reboxetine while supine (P<0.01) and 88+/-2 mm Hg and 90+/-3 mm Hg at 75 degrees HUT. Blood pressure responses to cold pressor and handgrip testing were attenuated with reboxetine. Reboxetine increased the sensitivity to the chronotropic effect of isoproterenol and the pressor effect of phenylephrine. Vasovagal reactions occurred in 9 subjects on placebo and in 1 subject on reboxetine. CONCLUSIONS: Selective NET blockade creates a phenotype that resembles idiopathic orthostatic intolerance. This observation supports the hypothesis that disordered norepinephrine uptake mechanisms can contribute to human cardiovascular disease. Our study also suggests that NET inhibition might be useful in preventing vasovagal reactions.

Non-NASA Center↗
NASA NTRS2000

Sympathetically mediated hypertension in autonomic failure

BACKGROUND: Approximately 50% of patients with primary autonomic failure have supine hypertension. We investigated whether this supine hypertension could be driven by residual sympathetic activity. METHODS AND RESULTS: In patients with multiple system atrophy (MSA) or pure autonomic failure (PAF), we studied the effect of oral yohimbine on seated systolic blood pressure (SBP), the effect of ganglionic blockade (with trimethaphan) on supine SBP and plasma catecholamine levels, and the effect of alpha(1)-adrenoreceptor blockade (phentolamine) on supine SBP. The SBP response to yohimbine was greater in patients with MSA than in those with PAF (area under the curve, 2248+/-543 versus 467+/-209 mm Hg. min; P=0.022). MSA patients with a higher supine SBP had a greater response than those with a lower supine SBP (3874+/-809 versus 785+/-189 mm Hg. min; P=0. 0017); this relationship was not seen in PAF patients. MSA patients had a marked depressor response to low infusion rates of trimethaphan; the response in PAF patients was more variable. Plasma norepinephrine decreased in both groups, but heart rate did not change in either group. At 1 mg/min, trimethaphan decreased supine SBP by 67+/-8 and 12+/-6 mm Hg in MSA and PAF patients, respectively (P<0.0001). Cardiac index and total peripheral resistance decreased in MSA patients by 33.4+/-5.8% and 40.7+/-9.5%, respectively (P=0. 0015). Patients having a depressor response to trimethaphan also had a depressor response to phentolamine. In MSA patients, the pressor response to yohimbine and the decrease in SBP with 1 mg/min trimethaphan were correlated (r=0.98; P=0.001). CONCLUSIONS: Residual sympathetic activity drives supine hypertension in MSA. It contributes to, but does not completely explain, supine hypertension in PAF.

Non-NASA Center↗
NASA NTRS2000

The pressor response to water drinking in humans : a sympathetic reflex?

BACKGROUND: Water drinking increases blood pressure profoundly in patients with autonomic failure and substantially in older control subjects. The mechanism that mediates this response is not known. METHODS AND RESULTS: We studied the effect of drinking tap water on seated blood pressure in 47 patients with severe autonomic failure (28 multiple system atrophy [MSA], 19 pure autonomic failure patients [PAF]). Eleven older controls and 8 young controls served as control group. We also studied the mechanisms that could increase blood pressure with water drinking. Systolic blood pressure increased profoundly with water drinking, reaching a maximum of 33+/-5 mm Hg in MSA and 37+/-7 in PAF mm Hg after 30 to 35 minutes. The pressor response was greater in patients with more retained sympathetic function and was almost completely abolished by trimethaphan infusion. Systolic blood pressure increased by 11+/-2.4 mm Hg in elderly but not in young controls. Plasma norepinephrine increased in both groups. Plasma renin activity, vasopressin, and blood volume did not change in any group. CONCLUSIONS: Water drinking significantly and rapidly raises sympathetic activity. Indeed, it raises plasma norepinephrine as much as such classic sympathetic stimuli as caffeine and nicotine. This effect profoundly increases blood pressure in autonomic failure patients, and this effect can be exploited to improve symptoms due to orthostatic hypotension. Water drinking also acutely raises blood pressure in older normal subjects. The pressor effect of oral water is an important yet unrecognized confounding factor in clinical studies of pressor agents and antihypertensive medications.

Non-NASA Center↗
NASA NTRS1998

[Evaluation of super dwarf wheat growth and development in greenhouse "Svet" during cultivation in inhabited pressurized chamber]

Goals of the 3-month experiment GREENHOUSE using the equipment of greenhouse SVET (ECO-PSY-95) were to feature growth and development of wheat through the entire cycle of ontogeny under the maximally mimicked MIR environment, and to try out the procedures and timeline of space experiment GREENHOUSE-2 as a part of the fundamental biology investigations within the MIR/NASA space science program. Irradiation intensity (PAR) was 65 W/m2 and 38 W/m2 in the experiment and laboratory control, respectively. Values of other environmental parameters were MIR average (18-25 degrees C, relative air humidity in the interval between 40% and 75%, total gas pressure of about 660 to 860 mm Hg, partial oxygen pressure within the range from 140 to 200 mm Hg, partial carbon dioxide pressure up to 7 mm Hg). Experimental results showed that wheat cultivation in inhabited chamber under a modified lighting unit providing greater irradiation of the crop area produced more plant mass although seed production dropped. Low grain content in ears could be the aftermath of the gaseous trace contaminants in the chamber atmosphere.

NASA Discipline Life Support Systems↗
NASA NTRS1997

The hypertension of autonomic failure and its treatment

We studied the incidence and severity of supine hypertension in 117 patients with severe primary autonomic failure presenting to a referral center over a 9-year period. Patients were uniformly characterized by disabling orthostatic hypotension, lack of compensatory heart rate increase, abnormal autonomic function tests, and unresponsive plasma norepinephrine. Fifty-four patients had isolated autonomic impairment (pure autonomic failure). Sixty-three patients had central nervous system involvement in addition to autonomic impairment (multiple-system atrophy). Patients were studied off medications, in a metabolic ward, and on a controlled diet containing 150 mEq of sodium. Fifty-six percent of patients had supine diastolic blood pressure > or =90 mm Hg. The prevalence of hypertension was slightly greater in females (63%) than in males (52%). Potential mechanisms responsible for this hypertension were investigated. No correlation was found between blood volume and blood pressure. Similarly, plasma norepinephrine (92+/-15 pg/mL) and plasma renin activity (0.3+/-0.05 ng/mL per hour) were very low in the subset of patients with pure autonomic failure and supine hypertension (mean systolic/diastolic pressure, 177 +/- 6/108 +/- 2 mm Hg, range 167/97 to 219/121). Supine hypertension represents a challenge in the treatment of orthostatic hypotension. We found these patients to be particularly responsive to the hypotensive effects of transdermal nitroglycerin. Doses ranging from 0.025 to 0.1 mg/h decreased systolic blood pressure by 36+/-7 mm Hg and may effectively treat supine hypertension overnight, but the dose should be individualized and used with caution.

NASA Discipline Regulatory Physiology↗
NASA NTRS1992

Blood pressure responses to LBNP in nontrained and trained hypertensive rats

To study the influences of 16 wk of endurance training on the reflex regulation of resting blood pressure, nontrained (NT) and trained (T) female hypertensive rats (SHR) were subjected to conditions of lower body negative pressure (LBNP). Measurements of muscle cytochrome oxidase activity and run time to exhaustion indicated that the animals were endurance trained. The rats (NT = 6, T = 7) were tranquilized with 300-600 micrograms.kg-1 diazepam (IV) before heart rates and blood pressures were measured over a range of 2.5-10.0 mm Hg of negative pressure. When subjected to conditions of LBNP, the reflex tachycardia of the T group was greater than the NT at the lower (-2.5 and -5.0 mm Hg) negative pressures. Although arterial pressure declines were similar in both groups, the T group experienced significantly less of a decline in central venous pressure than the NT animals. When chlorisondamine was used as a ganglionic blocker (2.5 mg.kg-1, IV), the fall in CVP at 10 mm Hg negative pressure was greater for the NT group while the fall in the initial systemic arterial pressure was more for the T group. From these results we concluded that training had altered the interaction between cardiopulmonary and arterial baroreflexes in these hypertensive rats and a nonneural component had been altered such as cardiac function.

Non-NASA Center↗
NASA NTRS1992

Mechanisms of alpha 1-adrenergic vascular desensitization in conscious dogs

To investigate the mechanisms of alpha 1-adrenergic vascular desensitization, osmotic minipumps containing either saline (n = 9) or amidephrine mesylate (AMD) (n = 9), a selective alpha 1-adrenergic receptor agonist, were implanted subcutaneously in dogs with chronically implanted arterial and right atrial pressure catheters and aortic flow probes. After chronic alpha 1-adrenergic receptor stimulation, significant physiological desensitization to acute AMD challenges was observed, i.e., pressor and vasoconstrictor responses to the alpha 1-adrenergic agonist were significantly depressed (p < 0.01) compared with responses in the same dogs studied in the conscious state before pump implantation. However, physiological desensitization to acute challenges of the neurotransmitter norepinephrine (NE) (0.1 micrograms/kg per minute) in the presence of beta-adrenergic receptor blockade was not observed for either mean arterial pressure (MAP) (30 +/- 7 versus 28 +/- 5 mm Hg) or total peripheral resistance (TPR) (29.8 +/- 4.9 versus 28.9 +/- 7.3 mm Hg/l per minute). In the presence of beta-adrenergic receptor plus ganglionic blockade after AMD pump implantation, physiological desensitization to NE was unmasked since the control responses to NE (0.1 micrograms/kg per minute) before the AMD pumps were now greater (p < 0.01) than after chronic AMD administration for both MAP (66 +/- 5 versus 32 +/- 2 mm Hg) and TPR (42.6 +/- 10.3 versus 23.9 +/- 4.4 mm Hg/l per minute). In the presence of beta-adrenergic receptor, ganglionic, plus NE-uptake blockade after AMD pump implantation, desensitization was even more apparent, since NE (0.1 micrograms/kg per minute) induced even greater differences in MAP (33 +/- 5 versus 109 +/- 6 mm Hg) and TPR (28.1 +/- 1.8 versus 111.8 +/- 14.7 mm Hg/l per minute). The maximal force of contraction induced by NE in the presence or absence of endothelium was significantly decreased (p < 0.05) in vitro in mesenteric artery rings from AMD pump dogs compared with saline control dogs. Furthermore, alpha 1-adrenergic receptor density, as determined by [3H]prazosin binding in membrane preparations from vessels in the mesentery, was decreased (8.2 +/- 1.0 versus 18.4 +/- 1.4 fmol/mg protein, p < 0.001) without any change in Kd in the AMD pump dogs compared with the saline pump dogs.(ABSTRACT TRUNCATED AT 400 WORDS).

Non-NASA Center↗
NASA NTRS1995

New Tests for Variations of the Fine Structure Constant

We describe a new test for possible variations of the fine structure constant, by comparisons of rates between clocks based on hyperfine transitions in alkali atomos with different atomic number Z. H- maser, Cs and Hg+ clocks have a different dependence on ia relativistic contributions of order (Z. Recent H-maser vs Hg+ clock comparison data improves laboratory limits on a time variation by 100-fold to giveFuture laser cooled clocks (Be+, Rb, Cs, Hg+, etc.), when compared, will yield the most senstive of all tests for.

Fine Structure↗
NASA NTRS2006

Miniaturized Mercury Ion Clock for Ultrastable Deep Space Applications

We have recently completed a prototype ion-clock physics package based on Hg ions shuttled between a quadrupole and a 16-pole rf trap. With this architecture we have demonstrated short-term stability ~2-3x10-13 at 1 second, averaging to 10-15 at 1 day. This development shows that H-maser quality stabilities can be produced in a small clock package, comparable in size to an ultra-stable quartz oscillator required for holding 1-2x10-13 at 1 second. This performance was obtained in a sealed vacuum configuration where only a getter pump was used to maintain vacuum. The vacuum tube containing the traps has now been under sealed vacuum conditions for nearly 1.5 years with no measurable degradation of ion trapping lifetimes or clock short-term performance. Because the tube is sealed, the Hg source and Neon buffer gas are held indefinitely, for the life of the tube. There is no consumption of Hg in this system unlike in a Cs beam tube where lifetime is often limited by Cs depletion.

mercury ion clocks↗
NASA NTRS2009

In-flight Assessment of Lower Body Negative Pressure as a Countermeasure for Post-flight Orthostatic Intolerance

Introduction. We investigated the efficacy of combining fluid loading with sustained lower body negative pressure (LBNP) to reverse orthostatic intolerance associated with weightlessness during and immediately after Space Shuttle missions. Methods. Shuttle astronauts (n=13) underwent 4 hours of LBNP at -30 mm(Hg) and ingested water and salt ( soak treatment) during flight in two complementary studies. In the first study (n=8), pre-flight heart rate (HR) and blood pressure (BP) responses to an LBNP ramp (5-min stages of -10 mm(Hg) steps to -50 mm(Hg) were compared to responses in-flight one and two days after LBNP soak treatment. In the second study (n=5), the soak was performed 24 hr before landing, and post-flight stand test results of soak subjects were compared with those of an untreated cohort (n=7). In both studies, the soak was scheduled late in the mission and was preceded by LBNP ramp tests at approximately 3-day intervals to document the in-flight loss of orthostatic tolerance. Results. Increased HR and decreased BP responses to LBNP were evident early in-flight. In-flight, one day after LBNP soak, HR and BP responses to LBNP were not different from pre-flight, but the effect was absent the second day after treatment. Post-flight there were no between-group differences in HR and BP responses to standing, but all 5 treatment subjects completed the 5-minute stand test whereas 2 of 7 untreated cohort subjects did not. Discussion. Exaggerated HR and BP responses to LBNP were evident within the first few days of space flight, extending results from Skylab. The combined LBNP and fluid ingestion countermeasure restored in-flight LBNP HR and BP responses to pre-flight levels and provided protection of post-landing orthostatic function. Unfortunately, any benefits of the combined countermeasure were offset by the complexity of its implementation, making it inappropriate for routine application during Shuttle flights.

Charles, J. B.↗