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PubMed1995

Dynamic simulation of the laboratory-scale controlled ecological life support system.

There are a number of design and control issues that need to be resolved in order to make a crop growth chamber an integral part of a controlled ecological life support system (CELSS) capable of supporting life on extended space missions. A modeling and simulation effort, along with the construction of an experimental testbed, are underway at NASA Ames Research Center to explore the long-term dynamic behavior of closed-loop life support systems. One problem that has been isolated for investigation is the stability and robustness of closed-loop systems over extended periods of time. Currently a crop growth chamber is being integrated with a solid waste processor to study closure of the carbon loop. A dynamic simulation model of the system was developed to evaluate system design options and operational alternatives. The model was also used to simulate the impact of system buffer size on the dynamic behavior of conditions inside the crop growth chamber.

Biomass↗
PubMed2005

[A probabilistic model of cardiac electrical activity based on a cellular automata system].

INTRODUCTION AND OBJECTIVES: Mathematical models of cardiac electrical activity may help to elucidate the electrophysiological mechanisms involved in the genesis of arrhythmias. The most realistic simulations are based on reaction-diffusion models and involve a considerable computational burden. The aim of this study was to develop a computer model of cardiac electrical activity able to simulate complex electrophysiological phenomena but free of the large computational demands required by other commonly used models. MATERIAL AND METHOD: A cellular automata system was used to model the cardiac tissue. Each individual unit had several discrete states that changed according to simple rules as a function of the previous state and the state of the neighboring cells. Activation was considered as a probabilistic process and was adjusted using restitution curves. In contrast, repolarization was modeled as a deterministic phenomenon. Cell currents in the model were calculated with a prototypical action potential that allowed virtual monopolar and bipolar electrograms to be simulated at any point in space. RESULTS: Reproducible flat activation fronts, propagation from a focal stimulus, and reentry processes that were stable and unstable in two dimensions (with their corresponding electrograms) were obtained. The model was particularly suitable for the simulation of the effects observed in curvilinear activation fronts. Fibrillatory conduction and stable rotors in two- and three-dimensional substrates were also obtained. CONCLUSIONS: The probabilistic cellular automata model was simple to implement and was not associated with a high computational burden. It provided a realistic simulation of complex phenomena of interest in electrophysiology.

Computer Simulation↗
PubMed2004

Effects of real and simulated microgravity on response of sympathoadrenal system to various stress stimuli.

Changes in plasma levels of epinephrine (EPI) and norepinephrine (NE) were investigated in humans exposed to physical exercise (WL), to psychic stressor (mental arithmetic test, MAT), and to oral glucose administration (oGTT) before and during a stay in microgravity (real space flight, SF) or in simulated microgravity (head-down bed rest, HDBR). A permanent cannula inserted into the cubital vein and a special appliance, Plasma-03, were used for blood collection, plasma separation, and freezing of samples during SF. Plasma EPI, NE, dihydroxyphenylglycol (DHPG), and dihydroxyphenylalanine (DOPA) levels were measured by the high-pressure liquid chromatography (HPLC) method. Basal plasma EPI, NE, DHPG, and DOPA levels were found within the range of control values during SF. Preflight WL produced high increase in plasma NE and moderate elevation of plasma EPI, DHPG, and DOPA levels. Exaggerated exercise induced increases in plasma NE, DHPG, EPI, and DOPA levels were demonstrated in real microgravity. A return to preflight responses of sympathoadrenal system was seen after the landing. Plasma EPI, NE, and DHPG responses to MAT were relatively small, but increased during SF. During the oGTT the plasma EPI levels were slightly reduced in microgravity. Similarly as in SF, WL in HDBR was followed by significantly exaggerated responses of plasma catecholamines. These results show that both somatic and psychological stressors are able to induce an increased activation of sympathoadrenal system during SF or simulated microgravity in HDBR.

Adrenal Glands↗
PubMed1984

Echocardiographic simulation of pericardial fluid accumulation by right pleural effusion.

On two-dimensional echocardiography, in the subcostal four-chamber view, right pleural effusion can present as a large echo-free space adjacent to the echo-free space representing the right atrium, thus simulating a loculated pericardial effusion or a pericardial cyst. On occasion, the echo-free space representing the right pleural effusion can bulge into the right atrium, distorting the right atrial shape, and thereby produce an echocardiographic appearance that may be confused with an indentation of the right atrial wall by pericardial effusion. The latter has recently been described as a sign of pericardial tamponade.

Diagnosis, Differential↗
PubMed2005

Simulation of water flow and nitrogen transport for a Bulgarian experimental plot using SWAP and ANIMO models.

Unsaturated zone models are useful tools in predicting effects of measures and can be used to optimise agricultural practice aiming to minimise the impact on the environment. However, current soil models have a varying degree of abstraction level referring to simulated processes in time and space. In the framework of an EU funded project the SWAP (Soil-Water-Atmosphere-Plant) and ANIMO (Agricultural-Nutrient-Model) models were tested for an experimental arable plot in Bulgaria. SWAP was used to simulate water flow in the soil while ANIMO describes nitrogen movement and transformations. The objectives of this study are: (i) to show results of the combined application of water and nitrogen dynamics of originally Dutch models SWAP and ANIMO for specific Bulgarian soil and hydrological conditions; (ii) to calibrate and evaluate SWAP and ANIMO models by comparing numerical results with field measurements collected for an arable field in western Bulgaria and (iii) to analyse possible contamination of groundwater due to agricultural practice in the considered region. Further a short description of the experimental plot, as well as information about parameters of the investigated soil profiles, is provided. The obtained SWAP results evidenced that the model gives sufficient adaptation for soil water dynamics. The simulations of ANIMO for nitrogen cycle show greater divergence with observations but are satisfactory precise for the purposes of assessing land use impact on groundwater quality. In general, differences between model results and field measurements do not exceed 10-15%. For the experimental plot predictions indicate nitrate-N concentrations less then 5 mg/l in deeper soil compartments and low downward annual flux containing 0.133 kg N/ha. These results indicate that there is no serious pollution of the shallow groundwater table by nitrogen resulting from land use and agricultural activities.

Agriculture↗
PubMed1986

Alveolar flooding: a computer simulation.

A validated computer simulation of pulmonary microvascular exchange (J.L. Bert and K.L. Pinder, Microvasc. Res., 27 (1984) 51-70) has been extended to include exchange with the air space (alveoli). Equations which hypothetically describe characteristics associated with this additional compartment and the exchange of both fluid and plasma proteins between the lung tissue and the alveolar space are presented. These are incorporated into the simulation which has been used to predict the behavior of the pulmonary microvascular exchange system including alveolar flooding. The predicted trends associated with alveolar flooding are reasonable. However, due to the lack of specific experimental or clinical findings, the simulation remains essentially unvalidated. The effect on alveolar flooding and interstitial edema of the parameters associated with the additional relationships is presented and discussed. Primarily, results for perturbations in circulatory pressure are presented. Additionally, changes in permeability characteristics are shown and discussed.

Blood Proteins↗
PubMed1979

Exercise response to simulated weightlessness.

Two bed rest analog studies of space flight were performed; one 14 d and the other 28 d in duration. Exercise response was studied in detail during the 28 d study and following both the 14 d and 28 d studies. This paper relates the results of these studies to physiologic changes noted during and following space flight. The most consistent change noted after both bed rest and space flight is an elevated heart rate during exercise. A second consistent finding is a postflight or postbed rest reduction in cardiac stroke volume. Cardiac output changes were variable. The inability to simulate inflight activity levels and personal exercise makes a direct comparison between bed rest and the results from specific space flights difficult.

Adult↗
PubMed2006

A single ventilator for multiple simulated patients to meet disaster surge.

OBJECTIVES: To determine if a ventilator available in an emergency department could quickly be modified to provide ventilation for four adults simultaneously. METHODS: Using lung simulators, readily available plastic tubing, and ventilators (840 Series Ventilator; Puritan-Bennett), human lung simulators were added in parallel until the ventilator was ventilating the equivalent of four adults. Data collected included peak pressure, positive end-expiratory pressure, total tidal volume, and total minute ventilation. Any obvious asymmetry in the delivery of gas to the lung simulators was also documented. The ventilator was run for almost 12 consecutive hours (5.5 hours of pressure control and more than six hours of volume control). RESULTS: Using readily available plastic tubing set up to minimize dead space volume, the four lung simulators were easily ventilated for 12 hours using one ventilator. In pressure control (set at 25 mm H2O), the mean tidal volume was 1,884 mL (approximately 471 mL/lung simulator) with an average minute ventilation of 30.2 L/min (or 7.5 L/min/lung simulator). In volume control (set at 2 L), the mean peak pressure was 28 cm H2O and the minute ventilation was 32.5 L/min total (8.1 L/min/lung simulator). CONCLUSIONS: A single ventilator may be quickly modified to ventilate four simulated adults for a limited time. The volumes delivered in this simulation should be able to sustain four 70-kg individuals. While further study is necessary, this pilot study suggests significant potential for the expanded use of a single ventilator during cases of disaster surge involving multiple casualties with respiratory failure.

Adult↗
PubMed2005

Development of a force field for Li(2)SiF(6).

A force field has been developed for Li(2)SiF(6) for subsequent use in Molecular Dynamics (MD) simulations involving Li(+) and SiF(2-) (6) ions in a polymer electrolyte host. Both ab initio calculations and available empirical data have been used. The force field has been verified in simulations of the crystal structure of Li(2)SiF(6) in two different space groups: P321 and P3(-)m1. The use of MD simulation to assess the correct space group for Li(2)SiF(6) shows that it is probably P321.

Journal Article↗
PubMed2003

Efficient simulation of three-dimensional anisotropic cardiac tissue using an adaptive mesh refinement method.

A recently developed space-time adaptive mesh refinement algorithm (AMRA) for simulating isotropic one- and two-dimensional excitable media is generalized to simulate three-dimensional anisotropic media. The accuracy and efficiency of the algorithm is investigated for anisotropic and inhomogeneous 2D and 3D domains using the Luo-Rudy 1 (LR1) and FitzHugh-Nagumo models. For a propagating wave in a 3D slab of tissue with LR1 membrane kinetics and rotational anisotropy comparable to that found in the human heart, factors of 50 and 30 are found, respectively, for the speedup and for the savings in memory compared to an algorithm using a uniform space-time mesh at the finest resolution of the AMRA method. For anisotropic 2D and 3D media, we find no reduction in accuracy compared to a uniform space-time mesh. These results suggest that the AMRA will be able to simulate the 3D electrical dynamics of canine ventricles quantitatively for 1 s using 32 1-GHz Alpha processors in approximately 9 h.

Algorithms↗
PubMed1979

Continuous metabolic and cardiovascular measurements on a monkey subject during a simulated 6-day Spacelab mission.

In May 1977 the NASA Ames Research Center and NASA Johnson Space Center jointly conducted a test of the feasibility of performing various types of biological and medical experiments during future Spacelab flight missions. One of the experiments was designed to examine the effects of weightlessness on several basic parameters of metabolic and cardiovascular function in an adult nonhuman primate. The subject animal, a 10 kg male pig-tailed monkey (Macaca nemestrina), was prepared 3 months before the test with a surgically implanted NASA/Ames biotelemetry unit to provide continuous measurements of body temperature and selected cardiovascular parameters. Three days before the simulated "launch" the animal was inserted into a fiberglass pod system to provide continuous measurement of respiratory gas exchange. The pod system was installed in a Spacelab mock-up and all physiological data channels were recorded continuously for 2.6 days prior to "launch", for 6.3 days during "flight", and for 1.8 days after "landing". The experiment was tended entirely by the Spacelab flight crew during the "flight" period. Statistically significant diurnal variation was noted in oxygen consumption and carbon dioxide production rates, body temperature and heart rate, but not in respiratory quotient or blood pressure. A transient increase of the parametric values was noted during the latter portion of the "flight" period, possibly occasioned by increased flight crew activity. The high quality of the continuous data obtained during the 10.7-day total experiment period demonstrates the feasibility of performing sound physiological experimentation on nonhuman primates in the Spacelab environment.

Adaptation, Physiological↗
PubMed1980

Firing behaviour in stochastic nerve membrane models with different pore densities.

A stochastic nerve membrane model with a two-state pore system was investigated by computer simulation in the uniform (space-clamped) case. The model was based upon the Hodgkin-Huxley equations for the giant axon in squid, but where both the maximal membrane conductances and the rate constants were changed systematically. This was done in order to simulate nerve membranes of small axons, where both of these parameters are smaller than in squid. It was found that the effects upon the firing behaviour due to a finite number of pores were not greatly affected by changes in these parameters. When the specific injected current was calculated relative to the maximal membrane conductances, the threshold for firing was increased somewhat, and the frequency-current relationship became slightly more linear when the maximal conductances (or pore density) were decreased, or the rate constants increased. In the discussion it is shown how the results obtained could be applied qualitatively to the firing behaviour of nerve cells, and that firing in small nerve cells should be significantly influenced by the stochastic effects of a finite number of pores. Gating currents were also discussed, and their effects were found to be insignificant in small nerve cells.

Action Potentials↗
PubMed2004

Factors governing diffusing molecular signals in brain extracellular space.

Volume transmission involving the migration of chemical signals through brain extracellular space is primarily mediated by diffusion. This review summarizes the biophysical basis of diffusion in the brain and describes how tortuosity and volume fraction of the extracellular space modify the process. Recent work using both Monte Carlo simulation and experimental measurements suggests that the extracellular space may consist of both well-connected regions and dead-space microdomains. The review concludes with a brief overview of previous work on diffusion in the aging rat brain.

Aging↗
PubMed2002

Direct estimation of the partition function from computer simulation.

We propose an approximate method for directly estimating the partition function of classical, many-body model systems. The accessible part of the phase space is determined from a single simulation. We introduce the method for the hard-sphere fluid and solid. The best performance is found in the dense fluid regime, close to freezing density and in the solid crystal. Defining hard-core effective diameters, the method can be applied to systems with soft-core interactions. We present results of exploratory calculations for the Lennard-Jones liquid.

Journal Article↗
PubMed1990

Space Station Freedom crew training.

The nature of the Space Station Freedom Program presents an array of new and enhanced challenges which need to be addressed en route to developing an effective and affordable infrastructure for crew training. Such an infrastructure is essential for the safety and success of the program. The three major challenges that affect crew training are the long lifetime of the program (thirty years), the interdependence of successive increments, and the participation of the three International Partners (Canada, European Space Agency, and Japan) and a myriad of experimenters. This paper addresses these major challenges as they drive the development of a crew training capability and the actual conduct of crew training.

Astronauts↗
PubMed2006

Fluorescence and REMPI spectroscopy of jet-cooled isolated 2-phenylindene in the S1 state.

We investigated the spectroscopy of the first excited singlet electronic state S1 of 2-phenylindene using both fluorescence excitation spectroscopy and resonantly enhanced multiphoton ionization spectroscopy. Moreover, we investigated the dynamics of the S1 state by determining state-selective fluorescence lifetimes up to an excess energy of approximately 3400 cm(-1). Ab initio calculations were performed on the torsional potential energy curve and the equilibrium and transition state geometries and normal-mode frequencies of the first excited singlet state S1 on the CIS level of theory. Numerous vibronic transitions were assigned, especially those involving the torsional normal mode. The torsional potentials of the ground and first excited electronic states were simulated by matching the observed and calculated torsional frequency spacings in a least-squares fitting procedure. The simulated S1 potential showed very good agreement with the ab initio potential calculated on the CIS/6-31G(d,p) level of theory. TDDFT energy corrections improved the match with the simulated S(1) torsional potential. The latter calculation yielded a torsional barrier of V2 = 6708 cm(-1), and the simulation a barrier of V2 = 6245 cm(-1). Ground-state normal-mode frequencies were calculated on the B3LYP/6-31G(d,p) level of theory, which were used to interpret the infrared spectrum, the FDS spectrum of the transition and hot bands of the FES spectrum. The fluorescence intensities of the nu49 overtone progression could reasonably be reproduced by considering the geometry changes upon electronic excitation predicted by the ab initio calculations. On the basis of the torsional potential calculations, it could be ruled out that the uniform excess energy dependence of the fluorescence lifetimes is linked to the torsional barrier in the excited state. The rotational band contour simulation of the transition yielded rotational constants in close agreement to the ab initio values for both electronic states. Rotational coherence signals were obtained by polarization-analyzed, time-resolved measurements of the fluorescence decay of the transition. The simulation of these signals yielded corroborating evidence as to the quality of the ab initio calculated rotational constants of both states. The origin of the anomalous intensity discrepancy between the fluorescence excitation spectrum and the REMPI spectrum is discussed.

Bridged Bicyclo Compounds↗