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PubMed1968

Isolation, confinement, and sensory deprivation.

Long-duration space flight involves sensory monotony, isolation, and confinement. Obviously, data from other such environments are relevant to our concerns; and the application of the concept of arousal both to interpersonal relations and to task performance in space can point to appropriate selection, training, and spacecraft design features. In the context of space flight, all of these leads remain conjectural--simulation as well as laboratory research and initiation of long-duration flights will be the crucial test of the hypotheses.

Adaptation, Psychological↗
PubMed1990

A mathematical model for vasogenic brain edema.

A two-dimensional model of vasogenic brain edema was constructed using the FEM. The model incorporates the following physical parameters: cerebrovascular hydraulic conductivity, metabolic water production, tissue hydraulic conductivity and compliance, tissue and plasma osmotic and hydrostatic pressures, CSF hydrostatic pressure and absorption, intracerebral stress, and shift of brain tissue. These parameters, related by partial differential equations, were approximated numerically in time and space by the FEM. The results of computer simulation of brain edema by the FEM were compared with experimental data of cold injury brain edema in cats. Computer simulation predicted the observed changes in interstitial pressure, extension of edema, intracerebral stress distribution, and displacement.

Animals↗
PubMed2000

Evaluation of psychological effects due to bed rest.

The psychological condition of astronauts is an important factor for ensuring mission success in limited space. Head-down tilting bed rest is a well-accepted method by which to simulate an acute stage of human adaptation to the weightless state in space flight. In our previous studies, the enhancement of depressive and neurotic levels occurred during a 20-day horizontal bed rest. In this study, we attempted to examine the depressive and neurotic levels, the mood status, and behavioral tendency of the subjects and to analyze the changes of 24-hour urinary excretion of 17-hydroxycorticosteroid-glucronides (17-OHCS) for an indicator of changes in the endocrine system due to physical and psychological stress during a 20-day 6-degrees head-down tilting bed rest (BR).

17-Hydroxycorticosteroids↗
PubMed2003

Structure of lithium benzilate hemihydrate solved by simulated annealing and difference Fourier synthesis from powder data.

The crystal structure of lithium benzilate hemihydrate (C(14)H(11)O(3)(-)Li(+).0.5H(2)O) was solved from synchrotron powder diffraction data. This compound crystallizes in the monoclinic space group P2(1)/a. The structure was solved via the direct space search for two benzilate fragments using the simulated-annealing program DASH, localization of the lithium ions and water molecule from a difference Fourier map, and a restrained Rietveld refinement (R(wp) = 0.0687). The structure is a coordination polymer of [Li(2)(C(14)H(11)O(3))(2).H(2)O](2) tetramers building helical fourfold one-dimensional channels parallel to [010]. Inside the channels the tetrahedral coordination spheres of the lithium ions contain hydroxyl and carbonyl groups, and water molecules. The water molecule functions as the cohesive entity forming extended hydrogen-bonded chains running along [010], and bifurcated donor hydrogen bonds with the two nearest carboxylates. At the outer edge of the channels, weaker intermolecular C-H.Ph hydrogen bonds along [100] and [001] contribute to the supramolecular aggregation of the structure.

Benzilates↗
PubMed2003

Photon beam relative dose validation of the DPM Monte Carlo code in lung-equivalent media.

Validation experiments have been conducted using 6 and 15 MV photons in inhomogeneous (water/lung/water) media to benchmark the accuracy of the DPM Monte Carlo code for photon beam dose calculations. Small field sizes (down to 2 x 2 cm2) and low-density media were chosen for this investigation because the intent was to test the DPM code under conditions where lateral electronic disequilibrium effects are emphasized. The treatment head components of a Varian 21EX linear accelerator, including the jaws (defining field sizes of 2 x 2, 3 x 3 and 10 x 10 cm2), were simulated using the BEAMnrc code. The phase space files were integrated within the DPM code system, and central axis depth dose and profile calculations were compared against diode measurements in a homogeneous water phantom in order to validate the phase space. Results of the homogeneous phantom study indicated that the relative differences between DPM calculations and measurements were within +/- 1% (based on the rms deviation) for the depth dose curves; relative profile dose differences were on average within +/- 1%/1 mm. Depth dose and profile measurements were carried out using an ion-chamber and film, within an inhomogeneous phantom consisting of a 6 cm slab of lung-equivalent material embedded within solid water. For the inhomogeneous phantom experiment, DPM depth dose calculations were within +/- 1% (based on the rms deviation) of measurements; relative profile differences at depths within and beyond the lung were, on average, within +/- 2% in the inner and outer beam regions, and within 1-2 mm distance-to-agreement within the penumbral region. Relative point differences on the order of 2-3% were within the estimated experimental uncertainties. This work demonstrates that the DPM Monte Carlo code is capable of accurate photon beam dose calculations in situations where lateral electron disequilibrium effects are pronounced.

Algorithms↗
PubMedDate not supplied

[Change in entropy of the free polypeptide chain during formation of hydrogen bonds].

Formation probabilities of different hydrogen bonds between carbonyl oxygen and amide hydrogen were determined by Monte Carlo simulations using a computer model in the space of sterically allowable conformations of alanine and glycine oligopeptides, and the corresponding entropy losses for the peptide backbone, T delta S, were calculated. The model was studied at different criteria of steric interactions. Comparison with the data of other authors showed the values of T delta S to be mainly determined by overall extent and type of the state space and to be only slightly dependent on its energy profile. Both short-range and long-range steric interactions were shown to prevent hydrogen bonding, especially in alanine peptides. In the model studied, the initiation of alpha(R)-helices is associated with T delta S = 8-10 kT, and prior formation of a 3/10-turn or one three-center H-bond does not appreciably decrease this entropy barrier. Elongation of the alpha(R)-helix by one residue leads to T delta S = 3.0-3.7 kT, the helices begin to stabilize after at least three sequential H-bonds are formed. The difference in the probability of insertion of Ala and Gly into the helix is lower than it follows from comparison of their mobility. The results could be explained assuming that factors different from helical H-bonds take part in the stabilization of the helices. One may suppose upon modeling of folding that even three sequential H-bonds are unable to fix the structure of a flexible peptide loop, while the elongation of alpha(R)-helices in the supersecondary helix-loop-helix structure is favorable as long as the loop conformation remains nearly optimal.

Computer Simulation↗
PubMedDate not supplied

[Simulation of liquid transcapillary exchanges].

The exchange between capillaries and the interstitial spaces are analysed using Wiederhielm's technique, and programmed in Basic language. The physiological variants represented are the hydrostatic pressures: arteriolar, venular and interstitial, and the oncotic, plasmatic and interstitial pressures. This technique enables us to analyses the relations between filtration, reabsorption. Plasmatic protein leaks, and lymphatic flux. It also enables us to simulate oedema of venous obstruction or lymphatic oedema, or protein deficiency oedema.

Arterioles↗
PubMed2004

Vibrational coupling, isotopic editing, and beta-sheet structure in a membrane-bound polypeptide.

The N-acetylated hexapeptide WLLLLL (AcWL5) partitions into lipid membranes and is believed to assemble into an antiparallel beta-sheet. As a test of this structural assignment, the peptide bonds of residues 2-6 were labeled with (13)C and allowed to adsorb onto a supported lipid membrane. Peptides bound to the membrane were examined for evidence of coupling between the labeled vibrational modes in adjacent beta-strands with internal reflection infrared spectroscopy. Experimental results indicate that the amide I absorption band in D(2)O (i.e., amide I') attributable to (13)C is selectively enhanced when the label is at any one of several positions along the peptide backbone. Simulations employing an excitonic model with through-bond and through-space interactions were performed on AcWL5 models in parallel and antiparallel beta-sheet configurations. The simulations yield spectra in good agreement with the experimental results, accounting for the enhancement of both (13)C band intensities and band frequencies. They also yield insight into the physical origin and structure selectivity of the distinctive amide I' band shapes that arise in isotopically edited spectra. It is concluded that the beta-sheet formed by membrane-bound AcWL5 is indeed antiparallel, and the enhancement of (13)C bands in the infrared spectra of these peptides is caused by both interstrand and intrastrand coupling to (12)C modes.

Amides↗
PubMed2004

Molecular dynamics simulations of a guaiacyl beta-O-4 lignin model compound: examination of intramolecular hydrogen bonding and conformational flexibility.

The dynamical conformational behavior of a guaiacyl beta-O-4 lignin model compound has been investigated by molecular simulations. The potential energy surface of the molecule in vacuum has been examined by means of an adiabatic map, showing a large accessible conformational space with multiple energy minima separated by low barriers. Molecular dynamics simulations have been performed in vacuum and with explicit solvent molecules for 10 and 2.1 ns, respectively. Molecular dynamics trajectories recorded in vacuum have shown the molecule to be flexible and to visit a large number of conformations. Many intramolecular H-bonds have been observed, existing for more than 90% of the total simulation time. The presence of explicit solvent molecules induces a significant broadening of some regions of the accessible conformational space and also largely reduces the statistical significance of intramolecular H-bonding. Intramolecular H-bonds observed in vacuum do not persist significantly and are preferentially exchanged with intermolecular H-bonds to the surrounding solvent molecules. The theoretical results are in good agreement with experimental NMR data that do not support the existence of strong and persistent intramolecular H-bonds in solution but instead indicate that H-bonds to solvent predominate. Finally, both molecular modeling and NMR approaches predict the guaiacyl beta-O-4 structure to be flexible and indicate that intramolecular H-bonds are not strong and persistent enough to confer rigidity to the molecule in solution.

Guaiacol↗
PubMed1995

Structure refinement with molecular dynamics and a Boltzmann-weighted ensemble.

Time-averaging restraints in molecular dynamics simulations were introduced to account for the averaging implicit in spectroscopic data. Space- or molecule-averaging restraints have been used to overcome the fact that not all molecular conformations can be visited during the finite time of a simulation of a single molecule. In this work we address the issue of using the correct Boltzmann weighting for each member of an ensemble, both in time and in space. It is shown that the molecular- or space-averaging method is simple in theory, but requires a priori knowledge of the behaviour of a system. This is illustrated using a five-atom model system and the small cycle peptide analogue somatostatin. When different molecular conformers that are separated by energy barriers insurmountable on the time scale of a simulation contribute significantly to a measured NOE intensity, the use of space- or molecule-averaged distance restraints yields a more appropriate description of the measured data than conventional single-molecule refinement with or without application of time averaging.

Algorithms↗
PubMed1993

Body position and volume status as determinants of cardiovascular responses to transition into microgravity in parabolic flight.

The condition of microgravity during spaceflight imposes a new challenge to the cardiovascular system and to its homeostatic mechanisms. Initial fluids shifts from the dependent parts to the upper parts of the body are supposed to induce a plethora of effects which eventually lead to the well-known puffy faces and chicken legs' of astronauts. At the same time some 2-3 kgs. in fluid is lost in urine and by diminished uptake. For research into these longer-term effects of spaceflight extensive physiologic experiments are required in space. In view of the high cost and the logistic problems related to space-research much work is done in simulation experiments like bedrest or head down tilt studies. For the very initial effects of micro-G parabolic flight can be used. In parabolic flights we have addressed the question of immediate cardiovascular effects of the transition into microgravity. Since a parabola will last for not more than some 25 seconds, one may expect to observe mainly changes in the outflow of the autonomic nervous system, reflecting in blood pressure and heart rate as easily measurable parameters. Such changes can be expected to be caused by the sudden disappearance of hydrostatic effects and the shifts of fluid from pools where it is kept under the influence of gravity. Hydrostatic effects will play a role in the position of the baroreceptors with respect to the heart: in the upright position the carotid sinuses are some 25 cm above heart level, consequently they observe a lower pressure than that at the heart. When this effect disappears in micro-G a suddenly increased pressure will be observed and the baroreflex is called into action. On the venous side blood will rush to the right atrium when it is no longer pulled down in the compliant vessels of the abdomen and legs. This may be expected to lead to increased pressures on the low-pressure side of the heart. Apart from changes in filling of the left heart this may lead to autonomic nervous effects on systemic blood pressure and heart rate as well.

Adult↗
PubMed2006

Determination of the initial beam parameters in Monte Carlo linac simulation.

For Monte Carlo linac simulations and patient dose calculations, it is important to accurately determine the phase space parameters of the initial electron beam incident on the target. These parameters, such as mean energy and radial intensity distribution, have traditionally been determined by matching the calculated dose distributions with the measured dose distributions through a trial and error process. This process is very time consuming and requires a lot of Monte Carlo simulation experience and computational resources. In this paper, we propose an easy, efficient, and accurate method for the determination of the initial beam parameters. We hypothesize that (1) for one type of linacs, the geometry and material of major components of the treatment head are the same; the only difference is the phase space parameters of the initial electron beam incident on the target, and (2) most linacs belong to a limited number of linac types. For each type of linacs, Monte Carlo treatment planning system (MC-TPS) vendors simulate the treatment head and calculate the three-dimensional (3D) dose distribution in water phantom for a grid of initial beam energies and radii. The simulation results (phase space files and dose distribution files) are then stored in a data library. When a MC-TPS user tries to model their linac which belongs to the same type, a standard set of measured dose data is submitted and compared with the calculated dose distributions to determine the optimal combination of initial beam energy and radius. We have applied this method to the 6 MV beam of a Varian 21EX linac. The linac was simulated using EGSNRC/BEAM code and the dose in water phantom was calculated using EGSNRC/DOSXYZ. We have also studied issues related to the proposed method. Several common cost functions were tested for comparing measured and calculated dose distributions, including chi2, mean absolute error, dose difference at the penumbra edge point, slope of the dose difference of the lateral profile, and the newly proposed Kappaalpha factor (defined as the fraction of the voxels with absolute dose difference less than alpha%). It was found that the use of the slope of the lateral profile difference or the difference of the penumbra edge points may lead to inaccurate determination of the initial beam parameters. We also found that in general the cost function value is very sensitive to the simulation statistical uncertainty, and there is a tradeoff between uncertainty and specificity. Due to the existence of statistical uncertainty in simulated dose distributions, it is practically impossible to determine the best energy/radius combination; we have to accept a group of energy/radius combinations. We have also investigated the minimum required data set for accurate determination of the initial beam parameters. We found that the percent depth dose curves along or only a lateral profile at certain depth for a large field size is not sufficient and the minimum data set should include several lateral profiles at various depths as well as the central axis percent depth dose curve for a large field size.

Body Burden↗
PubMed1985

[Fundamental and clinical studies on aztreonam in the field of obstetrics and gynecology].

Aztreonam (AZT) was studied for the transfer into the pelvic dead space exudate in 5 patients who received radical hysterectomy due to uterocervical cancer, and for the clinical efficacy in the treatment of 5 cases of obstetrical and gynecological infections. Transfer into the pelvic dead space exudate Data analysis was performed by the simulation curves prepared from pharmacokinetic parameters using the two-compartment model. On examination, the serum concentration of cubital venous serum was found to be 53.20 micrograms/ml at 1 hour after the start of 1-hour intravenous drip infusion of 1 g of AZT. Also, the shifting concentration of the pelvic dead space exudate was found to have reached its peak value of 12.79 micrograms/ml at 3.22 hours after the start of the infusion, and still showed the value of more than 7 micrograms/ml even at 8 hours after the start of the infusion. The AUC was 112.81 micrograms X hr/ml. Clinical evaluation AZT was administered at a daily dose of 2-4 g in 2 divided doses by intravenous injection and intravenous drip infusion for 60 minutes. Clinical efficacy of the 5 patients with acute and moderate degree obstetrical and gynecological infections was; excellent in 1 case, good in 3 cases and poor in 1 case, with the overall efficacy rate of 80.0%.

Adult↗
PubMedDate not supplied

Recovery of bacteria from reprocessed high flux dialyzers after bacterial contamination of the header spaces and O-rings.

The Centers for Disease Control (CDC) have received reports of bacteremia in patients on high flux dialysis attributed to contamination of dialyzer header spaces or o-rings. A study was performed in which header spaces and o-rings of Hemoflow F-80 dialyzers (Fresenius AG, Bad Homburg, FRG) were exposed to an aqueous suspension of Xanthomonas maltophilia and Mycobacterium chelonae for 1 hour. After exposure, the dialyzers were reprocessed manually with 4% formaldehyde, 4% Renalin, 2.5% Renalin, or sterile water (SW) as a control, or with an automated reprocessing machine using 3.25% Renalin. After 48 hours the blood compartment (BC) was drained and rinsed twice with 500 ml of SW. Each BC sample was cultured. To simulate dialysis, separate circulates of SW were pumped through the DC and the BC. After 15 minutes, the BC circulate was cultured, headers were unscrewed, and o-rings, header caps, and fiber bundle ends were cultured. For each germicide, bacteria were recovered in low numbers, primarily from the o-rings and the o-ring groove in the header caps. In 38 tests, a total of 60 of 342 assays (17.5%) were positive. In only one of these tests one bacterial colony forming unit (cfu) was recovered from the BC circulate during simulated dialysis. It was concluded that if header spaces and o-rings are contaminated, bacteria could be sealed protectively from the germicide. However, concentrations of surviving bacteria were low, probably outside the BC, and did not effectively contaminate the BC circulate during simulated dialysis.

Bacterial Adhesion↗
PubMed1991

Heterogeneity of monoclonal antibody distribution and radiation dose in tumors: a modeling analysis.

For successful use of monoclonal antibodies and their conjugates for diagnosis and therapy, it is helpful to understand both macroscopic and microscopic aspects of antibody distribution. Antibody distribution in a tumor is simulated by splicing together information on global pharmacokinetics, transport across the capillary wall, diffusive penetration through the tumor interstitial space, and antigen-antibody interaction. One interesting implication of this simulation is a microscopic dosimetry for radioimmunotherapy. The information of microscopic radioconjugate distribution will enable us to calculate absorbed dose in a tumor at the microscopic scale. The first step is to simulate the spatial antibody concentration profile in a tumor as a function of time after intravenous (bolus) injection, using reasonable values for the parameters involved. The second step is to calculate, also as a function of time, the absorbed radiation dose distribution resulting from each concentration profile. Parameter values for IgG pharmacology and a radiation point source function for I-131 are used to explore the effect of affinity on the antibody distribution and consequent absorbed dose in the tumor. The geometry simulated corresponds to a spherical nodule of densely packed tumor cells. Absorbed doses are calculated for radiation from a single nodule and for a cubic lattice of such nodules. This modeling analysis demonstrates that 1) antigen-antibody binding in tumors can retard antibody percolation; 2) high antibody affinity at a given dose tends to decrease antibody percolation and result in a heterogeneous distribution; 3) heterogeneous antibody distribution results in heterogeneous absorbed dose. This is more apparent in the case of radiation from a single nodule or small tumors. PERC and PERC-RAD, the computer program packages developed for these analyses, provide a convenient and flexible way to assess the impact of macroscopic and microscopic parameters on the distribution of immunoconjugates (PERC) and the consequent absorbed radiation dose in tumors (PERC-RAD). This mathematical model and the general principles developed here can be applied as well as to other biological ligands and beta-emitters.

Antibodies, Monoclonal↗
PubMed1998

Geometric effects on resistivity measurements with four-electrode probes in isotropic and anisotropic tissues.

We studied via computer simulation the effects of electrode diameter, electrode length, interelectrode spacing, and tissue size on the accuracy of measured tissue resistivities and anisotropy ratios obtained with the widely used four-electrode technique. Such measurements commonly assume an ideal situation in which the four electrodes are infinitesimally small and the tissue is semi-infinite. Our study shows that these geometric factors can significantly affect measured resistivities, particularly for anisotropic tissues. The measured anisotropy ratio is decreased by either 1) increasing the electrode diameter or length relative to the interelectrode spacing of the probe or 2) decreasing tissue size. We have provided an equation for estimating errors in the measured anisotropy ratio from the parameters of electrode and tissue geometries. The simulation findings are supported by our in vitro experimental results.

Animals↗
PubMed1985

The use of suspension models and comparison with true weightlessness: "a resumé".

The topics for this workshop were selected to illustrate the manner in which particular physiological and biochemical phenomena can be assessed using specific animal model systems. Our speakers presented data which offer comparisons of experiments in earthside laboratories with experiments using subjects which have experienced weightlessness in response to orbital flight. In addition, there have been speculative assertions that the earthside experiments mimic or simulate responses which one may expect to see with exposure to micro or zero gravity conditions. The topics focused on at least three areas that appear to be amenable to earthside investigations; in addition, they serve as primary areas for flight experimentation. As the result of animal and human exposures to microgravity experienced during orbital space flight, there is ample evidence of pathophysiological functional and structural alterations in muscle, bone, cardiovascular and related body fluid shift responses. This Animal Model Workshop represents the first of a series of sessions planned for further meetings of the IUPS Commission on Gravitational Physiology.

Animals↗