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PubMed1997

Vibration, acceleration, gravitation, and movement: activity controlled rate adaptive pacing during treadmill exercise testing and daily life activities.

Activity-based sensors for rate adaptive pacing have been available for several years and now include several different types: vibration; acceleration; gravitation; and movement. However, a systematic comparison evaluating the relative advantages and disadvantages of these various sensors has received little study. The purpose of the present study was to compare these sensor subtypes using treadmill testing and an outdoor test circuit, which simulated daily life activities and included both uphill and downhill walking. Pacemakers were strapped on the chest of healthy volunteers and connected to one channel of an ambulatory recording device, which also recorded the subject's intrinsic heart rate. The pacemakers were programmed using an initial treadmill test to standardize the rate responsive parameters for each device. Nine different pacemaker models were studied including 3 vibration-based (Elite, Synchrony, Metros), 4 acceleration-based (Relay, Excel, Ergos, Trilogy), 1 gravitational-based (Swing), and 1 movement-based (Sensorithm) device. All devices demonstrated a prompt rate response with casual walking on flat ground. The vibration-, gravitational-, and movement-based pacemakers showed a pronounced rate decline during more strenuous work, e.g., walking uphill. This phenomenon was absent in the accelerometer-based units. In particular, the vibration- and movement-based units showed a higher rate with walking downhill compared to uphill. An optimally tuned rate behavior on the treadmill usually did not provide an optimal rate behavior during daily activities and there was a tendency to overstimulation during low workload. The development of the two newest sensors (gravitational and movement) did not result in an improved performance of rate response behavior. Overall, the accelerometer-based pacemakers simulated or paralleled sinus rate behavior the most closely.

Acceleration↗

PubMed1985

Increased gravitational stress does not alter maximum expiratory flow.

We measured maximum expiratory flow-volume (MEFV) curves in six seated subjects during normal (+1 Gz) and increased (+2 and +3 Gz) gravitational stress. Full MEFV curves, initiated at total lung capacity, were recorded, as were partial MEFV curves, initiated at approximately 60% of the vital capacity. Data were acquired in all subjects breathing air at +1 and +2 Gz; results were available for three subjects breathing 80% He-20% O2 at +1 and +2 Gz, and in two subjects, results were obtained at +3 Gz. Changes in gravitational stress were not associated with changes of either full or partial MEFV curves. The known increase in differences of regional lung volume and recoil caused by increased gravitational stress did not influence maximum expiratory flow. Though increased gravitational stress probably changed regional emptying sequences little during full MEFV maneuvers, substantial changes of emptying sequence were expected during partial maneuvers. It is possible that such changes in emptying sequence occurred but were not associated with changes in maximum flow because the latter was determined by choking in central airways common to all regions.

Adult↗

PubMed2004

Daily short-period gravitation can prevent functional and structural changes in arteries of simulated microgravity rats.

This study was designed to clarify whether simulated microgravity-induced differential adaptational changes in cerebral and hindlimb arteries could be prevented by daily short-period restoration of the normal distribution of transmural pressure across arterial vasculature by either dorsoventral or footward gravitational loading. Tail suspension (Sus) for 28 days was used to simulate cardiovascular deconditioning due to microgravity. Daily standing (STD) for 1, 2, or 4 h, or +45 degrees head-up tilt (HUT) for 2 or 4 h was used to provide short-period dorsoventral or footward gravitational loading as countermeasure. Functional studies showed that Sus alone induced an enhancement and depression in vasoconstrictor responsiveness of basilar and femoral arterial rings, respectively, as previously reported. These differential functional alterations can be prevented by either of the two kinds of daily gravitational loading treatments. Surprisingly, daily STD for as short as 1 h was sufficient to prevent the differential functional changes that might occur due to Sus alone. In morphological studies, the effectiveness of daily 4-h HUT or 1-h STD in preventing the differential remodeling changes in the structure of basilar and anterior tibial arteries induced by Sus alone was examined by histomorphometry. The results showed that both the hypertrophic and atrophic changes that might occur, respectively, in cerebral and hindlimb arteries due to Sus alone were prevented not only by daily HUT for 4 h but also by daily STD even for 1 h. These data indicate that daily gravitational loading by STD for as short as 1 h is sufficient to prevent differential adaptational changes in function and structure of vessels in different anatomic regions induced by a medium-term simulated microgravity.

Adaptation, Physiological↗

PubMed1982

Gravitational effects on the distribution of pulmonary blood flow: hemodynamic misconceptions.

In the upright individual the apex of the lung receives relatively little blood. This has often been explained by the low pulmonary arterial pressure which is said to be just sufficient to raise the blood to the apex. It is believed that pulmonary arterial pressure must overcome the pressure due to gravity. This misconception overlooks the fact that the siphon principle applies to the vascular system in which the gravitational pressure of venous blood counterbalances the gravitational pressure of blood in the arteries and vice versa. Accordingly, the perfusion or driving pressure (P1-P2) between arteries and veins at any horizontal level of the lung remains unchanged, irrespective of body position. Intravascular pressure at any point is the algebraic sum of dynamic pressure causing flow (cardiogenic) and the pressure of blood due to gravity which does not cause flow. In the upright position, since the dynamic pressure in the pulmonary circuit is low, the drop in gravitational pressure at the apex of the lung reduces significantly the intravascular and, consequently, the transmural pressure in these vessels. The pulmonary microvessels being highly compliant undergo collapse and increase their resistance to flow. The reduction in apical flow is, therefore, a consequence of increased vascular resistance and not a matter of raising the blood against gravity. Gravitational pressure of blood per se neither hinders upward flow nor favors downward flow.

Biomechanical Phenomena↗

PubMed2005

Gravitational shunts in longstanding overt ventriculomegaly in adults.

OBJECTIVE: A new entity of chronic hydrocephalus was introduced recently in the international literature: longstanding overt ventriculomegaly in adults. Previous experience with this disorder has demonstrated that shunt therapy for such patients involves a considerable risk of overdrainage. In the present study, we aimed to clarify whether this risk could be avoided by use of gravitational shunts. METHODS: A total of 26 adults (age range, 17-75 yr) with macrocephaly and progressive hydrocephalus symptoms underwent implantation of either an adjustable Codman Hakim gravity-assisted shunt (Codman Medos, LeLocle, Switzerland) plus an Aesculap-Miethke ShuntAssistant (Miethke KG, Kleinmachnow, Germany) or a nonadjustable gravitational shunt (Aesculap-Miethke gravity-assisted valve). The follow-up period averaged 29 +/- 13 months (range, 6-48 mo). RESULTS: Significant sustained clinical improvement was achieved in 87% of patients. In more than 90% of patients, Evans index decreased postoperatively by less than 10%. No correlation was documented between the degree of ventricle width reduction and clinical improvement. Only two patients developed subdural hematoma, which was caused by insufficient hydrostatic pressure compensation owing to errors in estimation of intraperitoneal pressure. CONCLUSION: Unlike conventional differential pressure shunts, gravitational shunts can be used in the treatment of high-risk patients with longstanding overt ventriculomegaly in adults. Significant risk of overdrainage can be avoided. Gravitational shunts offer a viable alternative to endoscopic third ventriculostomy, provided the choice and adjustment of the shunt opening pressure is based on a correct assessment of the hydrostatic pressure to be compensated for.

Adolescent↗

PubMed1993

Regional cutaneous microvascular flow responses during gravitational and LBNP stresses.

The most significant cardiovascular event during the transition to microgravity is the redistribution of vascular transmural pressures that results from the loss of hydrostatic gradients along the length of the body. The well-documented effects of this redistribution include facial venous engorgement, headache, and a significant decrease in leg volume. These effects predominantly represent bulk fluid volume shifts, especially in the venous macro- and microcirculation, where volume is a direct function of pressure, related by the mechanical compliance of the vascular compartment. When considering the effect of gravitational pressure alterations on microcirculatory blood flow and volume, however, this direct monotonic relationship no longer applies. Regional microvascular perfusion is largely a function of local arteriolar tone, which is subject to a variety of central and local controls. Lower body venous pooling during application of footward gravitational stress unloads arterial and cardiopulmonary baroreceptors, increasing sympathetic arteriolar tone to elicit vasoconstriction and a general decrease in microvascular perfusion. The same stimulus also triggers an increase in the levels of circulating vasoactive hormones, such as norepinephrine and angiotensin II, further augmenting arteriolar tone. Vasomotor tone is also mediated by local mechanisms such as myogenic autoregulation and veno-arteriolar reflexes, which enhance microvascular tone in response to elevated local arteriolar and venular pressure, respectively. Due to the regional variability of local hydrostatic pressures, microvascular flow responses to gravitational stress probably vary along the length of the body. Although these differences in local autoregulation have been observed previously during whole-body tilting, they have not been investigated during application of artificial gravitational stresses, such as lower body negative pressure (LBNP) or +Gz centrifugation. Although these stresses can create equivalent G-levels at the feet, they result in distinct distributions of vascular transmural pressure along the length of the body, and should consequently elicit different magnitudes and distributions of microvascular response. In the present study, the effects of whole-body tilting and LBNP on the level and distribution of microvascular flows within skin along the length of the body were compared.

Adult↗

PubMed2004

Gravitation in pathogeny of essential hypertension.

The purpose of this research is the study of changes of a systemic hemodynamics under passive orthostatic test for a healthy persons and an ill with Essential Hypertension (EH) and analysis of a possible role of the gravitational factor in a Pathogeny of this disease. For an ill with EH reduction of Stroke Volume and Cardiac Output were reliably lower in an orthostatic position. Increasing of a Total peripheral vascular resistance was twice less for ill. Considerable differences in reaction of cardiovascular system to gravitational influence for an ill with Essential Hypertension are stipulated by changes in central regulation of circulation and in the structure of a vascular wall. It allows to assume influence of gravitation at early stages of a Pathogeny of the given disease. The detection of hyper reactivity of a cardiovascular system to influence of gravitation can indicate the first stage of the disease.

Adaptation, Physiological↗

PubMed2001

Dust-acoustic modes in self-gravitating plasmas with dust size distributions.

Using a kinetic description, dust-acoustic waves are considered for dusty plasmas containing, besides the electrons and ions, dust particles with continuous mass (size) distributions. For broad size spectra, self-gravitational effects cannot be neglected anymore because in the competition between electromagnetic and gravitational forces, the scale tips over towards gravitation for the heavier dust grains. Self-gravitational effects are clearly interwoven with the grain size distribution and here the effects of different power-law size distributions on the propagation, damping, and instability of low-frequency waves are discussed.

Journal Article↗

PubMed2003

Incomplete relaxation in a two-mass one-dimensional self-gravitating system.

Due to the apparent ease with which they can be numerically simulated, one-dimensional gravitational systems were first introduced by astronomers to explore different modes of gravitational evolution. These include violent relaxation and the approach to thermal equilibrium. Careful work by dynamicists and statistical physicists has shown that several claims made by astronomers regarding these models were incorrect. Unusual features of the evolution include the development of long lasting structures on large scales, which can be thought of as one-dimensional analogs of Jupiter's red spot or a galactic spiral density wave or bar. The existence of these structures demonstrates that in gravitational systems evolution is not entirely dominated by the second law of thermodynamics and also appears to contradict the Arnold diffusion ansatz. Thus it is correct to assert that the one-dimensional planar sheet gravitational system is the nonextensive analog of the Fermi-Pasta-Ulam model of dynamical systems. This paper is an extension of a preliminary study where we conclusively showed mass segregation and equipartition of kinetic energy in a two-mass planar sheet system for the first time. Here we employ both mean-field theory and dynamical simulation to more thoroughly probe the statistical and ergodic properties of these systems. Valuable information is obtained from local and global time averaging, and temporal and spatial correlation functions. Using these tools we show that the system appears to approach the equilibrium distribution on very long time scales, but the relaxation is incomplete.

Journal Article↗

PubMed2000

First search for gravitational wave bursts with a network of detectors

We report the initial results from a search for bursts of gravitational radiation by a network of five cryogenic resonant detectors during 1997 and 1998. This is the first significant search with more than two detectors observing simultaneously. No gravitational wave burst was detected. The false alarm rate was lower than 1 per 10(4) yr when three or more detectors were operating simultaneously. The typical threshold was H approximately 4x10(-21) Hz-1 on the Fourier component at approximately 10(3) Hz of the gravitational wave strain amplitude. New upper limits for amplitude and rate of gravitational wave bursts have been set.

Journal Article↗

PubMed2001

Managing gravitational eczema and allergic contact dermatitis.

Venous leg ulceration is a common chronic problem in the community, and gravitational eczema is a common complication of it. The presence of gravitational eczema can lead to further ulceration and may also impair wound healing. Treatment often requires more than one topical preparation, to gain control and prevent relapse. Knowledge of the benefits and risks of these therapeutic modalities is essential, in this article we aim to provide a practical approach to the treatment of gravitational eczema. We also discuss allergic contact dermatitis, a common complication of gravitational eczema.

Administration, Topical↗

PubMed1994

Simulation of gravitational field variation on fluid-filled biological membranes.

The knowledge of the behavior of biological organs in a gravitational field is important to understand the functioning of the human body in the aerospace environment. The disturbances in biological transport processes in microgravity have indicated adverse effects on humans engaged in space operations. The relationship between the deformations in the biological organs and the transport phenomena that take place in them has been long established and widely reported in biological sciences and engineering literature. A number of soft tissue organs such as brain, lungs, heart, kidney, bladder, stomach, and the circulatory system can be modeled as fluid-filled membranes. In this investigation, a mathematical model of a fluid-filled biological membrane is developed, and its deformation and spatial configuration in a variable gravitational field are calculated. The variation in the gravitational field in the range 1g to zero-g is simulated by partial submergence of the fluid-filled membrane which, by virtue of buoyancy, gains an effective density as if it is in a different gravitational field. The equations of motion are derived using the theory of large elastic deformations and numerically solved in conjunction with a constitutive equation suitably selected for the biological membrane.

Biological Transport↗

PubMed1993

Subcutaneous compliance and gravitational adaptation in snakes.

Previous studies have implicated morphological adaptations as important counter-measures to gravitational stresses on the circulatory systems of arboreal or climbing snakes. Such features include tight skin and relatively non-compliant tissue compartments that oppose edema formation, but these traits have not been previously studied quantitatively. To provide information on this subject, interstitial fluid pressures were measured in eleven species of snakes using slit-end catheters positioned in subcutaneous tissue located at the base of the tail. Interstitial pressures in all species were typically 0 to +2 mm Hg in normally hydrated tissue, but varied widely when snakes were active or when the tail was curved at the site of measurement. Local compliance of the free fluid space was determined from measurements of pressure while saline was infused via the catheter tip. Such measurements varied from 0.18 microliters/mm Hg in Philodryas baroni, an arboreal species, to 2.3 microliters/mm Hg in Crotalus adamanteus, a ground-dwelling, terrestrial species. In general, compliance of the subcutaneous tissue space was greatest (P < 0.05) in non-climbing and aquatic species of snakes that do not face problems of gravitational edema in dependent tissues. Presumably, the compliance measurements reflect adaptive structural differences related to requirements for counteracting gravitational stresses in the various species.

Adaptation, Biological↗

PubMed1994

Effects of gravitational forces on single joint arm movements in humans.

We have examined the kinematics and muscle activation patterns of single joint elbow movements made in the vertical plane. Movements of different amplitudes were performed during a visual, step-tracking task. By adjusting shoulder position, both elbow flexion and extension movements were made under three conditions: (a) in the horizontal plane, (b) in the vertical plane against gravity, and (c) in the vertical plane with gravity. Regardless of the gravitational load, all movements were characterized by time symmetric velocity profiles. In addition, no differences were found in the relationships between movement duration, peak velocity, and movement amplitude in movements with or against gravity. The pattern of muscle activation was influenced however, by the gravitational load. Both flexion and extension movements made with gravity were characterized by a reciprocally organized pattern of muscle activity in which phasic agonist activity was followed by phasic antagonist activity. Flexion and extension movements made against gravity were characterized by early phasic antagonist activity occurring at about the same time as the initial agonist burst. These findings suggest that EMG patterns are modified in order to preserve a common temporal structure in the face of different gravitational loads.

Adult↗

PubMed1998

Hand trajectories of vertical arm movements in one-G and zero-G environments. Evidence for a central representation of gravitational force.

The purpose of the present experiment was to study the way in which the central nervous system (CNS), represents gravitational force during vertical arm pointing movements. Movements in upward (against gravity) and downward (with gravity) directions, with two different mass loads (hand empty and with a hand-held 0.5-kg weight) were executed by eight subjects in a normal gravitational environment. Movements by two cosmonauts, in the two directions, were also tested in a state of weightlessness. Analyses focused upon finger trajectories in the sagittal plane. Subjects in a normal gravitational environment showed curved paths for both directions and weight conditions. In addition, downward movements showed significantly smaller curvatures than upward movements. Movement times were approximately the same for all the experimental conditions. Curvature differences between upward and downward movements persisted during space flight and immediately postflight. Movement times from both cosmonauts increased slightly during flight, but returned to normal immediately on reentry in a one-G environment. Results from the present study provide evidence that gravity is centrally represented in an anticipatory fashion as a driving force during vertical arm movement planning.

Adult↗

PubMedDate not supplied

Gravitational effects of the period of high tides and the new moon on lunacy.

In this report, the established timing of terrestrial tidal gravity fluxes is examined to assess the role of the full moon per se in modern gravitational lunacy theory. The results show that the principal tidal gravity fluxes are semidiurnal, with lesser diurnal and even smaller fortnightly components. There are no uniquely monthly components that would correspond to the period of the full moon. This means that the gravitational effects of the new moon are equivalent to those of the full moon. Furthermore, the gravitational effects associated with the times of high tide are even greater than those associated with the moon phases. Using the technique of reductio ad absurdum, I suggest that lunacy effects, if indeed there are any, should occur twice each day (high tides) but should be more pronounced during the new moon and full moon (spring tides). On the basis of this analysis, I would recommend that all studies that have compared hospital records with the full moon be redone to coincide with the proper timing as found in this report.

Atmospheric Pressure↗

PubMed1979

A model for vestibular function in altered gravitational states.

During evolution, the vestibular organ was made to serve mainly two purposes: 1) to guide eye movements during sharp turns, so that the point of fixation in the visual field can be kept steady, a function accomplished by the semicircular canal system and 2) to indicate the terrestrial vertical, so that upright posture and gait can be maintained even in the dark. The otolith system serves the latter purpose. Since the function of the semicircular canal system does not depend on gravity, it is not grossly disturbed by gravitational levels different from 1 g. The proper function of the otolith system depends entirely on the presence of a gravitational force vector of 9.8/m/sec2 directed towards the center of the earth. This system therefore malfunctions when the amplitude of the combined gravito-inertial load is different from 1 g and also when the direction of the sensed gravitational pull is "contaminated" by additional inertial reactive forces as during horizontal acceleration. The effect of such inertial stimulations is probably even stronger in a weightless environment, in which case the background stimulation of terrestrial gravity is absent. Moreover, minor mass differences between the otolithic membranes of the left and right inner ear, even if well compensated on the ground, might lead to malcompensation in weightlessness as well as in hypergravity. A hypothetical model is developed to describe in the central nervous system compensating mechanisms in hypo- and hypergravitational states. The "space-sled" is introduced as a new research tool and recommendations are made for a prophylactic training regimen to reduce or prevent space sickness.

Adaptation, Physiological↗

PubMed2001

Fos-related antigens are involved in the transcriptional responses of locus coeruleus neurons to altered gravitational fields in rats.

Locus coeruleus (LC) neurons, which have widespread projections to the whole brain, respond to natural stimulation of macular receptors. Using immunocytochemistry we investigated whether rats exposed to altered gravitational fields showed changes in Fos and Fos-related antigen (FRA) protein levels in the LC. Fos protein is induced very rapidly and returns to basal levels within hours after stimulation, while FRAs persist for days or weeks after induction. Adult male albino rats (Fisher 344) were sacrificed at different time points during a space flight (NASA Neurolab Mission, STS-90) and the numbers of Fos- and FRA-positive cells in the LC were counted and compared to those in ground-based control rats. No significant changes in Fos protein expression were detected in the LC under different space flight conditions. In contrast, the number of FRA-positive cells increased on average to 167% of that of the controls at FD2, i.e. when gravity increased from 1 to 3 g during the launch before reaching about 0 g. FRA-labeled neurons then decreased to 46% of control values at FD14, i.e. after adaptation to 0 g, but increased again to 317% of control values at R + 1, when the animals were exposed to an increase in gravitational force from 0 to 1.5-1.8 g before reaching 1 g during landing. The number of labeled cells was 193% of the control values at R + 13, i.e. after readaptation to 1 g. Thus gravitational force appears to be very effective in inducing a long-term increase in FRA protein expression in the LC. Because activity in the noradrenergic LC neurons may increase Fos expression in several target structures, we postulate that the long-lasting induction of FRAs in the LC at FD2, and more prominently at R + 1, may contribute to the long-term molecular changes which probably occur in the brain during adaptation to 0 g and readaptation to 1 g.

Adaptation, Physiological↗