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At least 19 recordsLinked to original sources
PubMed1991

Studies of controlled reperfusion after ischemia. XXIII. Deleterious effects of simulated thrombolysis preceding simulated coronary artery bypass grafting with controlled blood cardioplegic reperfusion.

This study tests whether simulated thrombolysis before controlled reperfusion (i.e., simulated coronary artery bypass) causes reperfusion injury that obviates the benefits of subsequent controlled reperfusion and results in unnecessary ventricular arrhythmias. Fifteen dogs underwent acute occlusion of the left anterior descending coronary artery. In 10 dogs we simulated thrombolysis after 1 hour of ischemia (delivering 10% to 15% of control flow at 5 ml/min), followed 1 hour later by either normal blood reperfusion at systemic pressure (to simulate percutaneous transluminal coronary angioplasty) in five dogs or regionally controlled blood cardioplegic reperfusion on bypass in five others to simulate coronary bypass. In five dogs ischemia was prolonged to 2 hours, and the initial reperfusate was blood cardioplegic solution on total vented bypass (to simulate primary coronary bypass). All hearts receiving simulated thrombolysis (100%) after 1 hour of ischemia had reperfusion-induced ventricular fibrillation. All hearts treated by simulated angioplasty recovered regional contractility (56% of control systolic shortening), whereas there was no (0%) recovery of spontaneous contractility after subsequent blood cardioplegic reperfusion, and only two (40%) dogs had contractile reserve capacity (6% +/- 49%). Conversely, surgically controlled blood cardioplegic reperfusion without preceding low-flow normal blood reperfusion after 2 hours of ischemia resulted in no ventricular arrhythmias (0%; p less than 0.05 versus simulated coronary artery bypass after simulated thrombolysis), 72% +/- 7% (p less than 0.05 versus simulated coronary artery bypass after simulated thrombolysis) recovery of regional contractility (ultrasonic crystals), and 114% +/- 11% (p less than 0.05 versus simulated coronary artery bypass after simulated thrombolysis) recovery of contractile reserve with calcium chloride stimulation. We conclude that controlled reperfusion (simulating coronary artery bypass) with blood cardioplegic solution produces immediate functional recovery and avoids the ventricular fibrillation that follows simulated thrombolysis despite the need for prolonged ischemic time. Preceding controlled reperfusion by normal blood reperfusion (simulated thrombolysis) shortens the ischemic time but nullifies immediate functional recovery possible by simulated coronary bypass and produces unnecessary arrhythmias.

Angioplasty, Balloon, Coronary↗
PubMed1989

Simulator sickness in U.S. Navy flight simulators.

Flight simulators have become a major factor in pilot training. A general finding from Navy research on simulator design is that equipment features that offer faithful representation improve pilot performance and promote pilot acceptance. To the extent that an aircraft produces motion sickness, its simulator should induce the same result. However, reports of simulator sickness appear to be increasing and a shortcoming in simulation is implied when these effects occur in simulators during maneuvers that do not occasion them in the aircraft. This article presents incidence data from surveys of the 10 simulators at 6 different Naval/Marine Corps Air Stations. Approximately 1,200 simulator flights were recorded. Some severe motion sickness symptoms were recorded and some simulators induced unsteadiness afterwards. Individuals experiencing effects may be at risk if they drive themselves home or return to demanding activities at work. The simulators which exhibited the highest incidences of sickness were helicopter simulators with cathode ray tube (CRT) infinity optics and six-degrees-of-freedom moving base systems. Of those studied, fixed-wing, fixed-base, dome displays had relatively low incidence of simulator sickness.

Aircraft↗
PubMed1990

Simulator sickness in an army simulator.

Simulator sickness describes a symptom reported by aircrew during or after flight simulator training. Some features are common to motion sickness but others, which are unusual during real flight, are believed to result specifically from the simulator environment. This paper describes the results of a questionnaire study examining the incidence and factors influencing simulator sickness in any army training system. Case histories are described and conclusions drawn with respect to health and safety, training and the effect on flight operations. One hundred and fifteen aircrew were registered in the questionnaire study. Data were collected from a history questionnaire, a post-sortie report and a delayed report form. Sixty-nine per cent of aircrew gave a history of symptoms in the simulator and 59.9 per cent experienced at least one symptom during the study period although few symptoms were rated as being other than slight. Only 3.6 per cent of subjects reported symptoms of disequilibrium. Comparative analysis of the results was performed after scoring symptoms to produce a sickness rating. This showed: association between simulator-induced sickness and greater flying experience; adaptation to the simulator environment; a history of sea sickness may predict susceptibility to simulator sickness; and no association of crew role and simulator sickness. Although some authorities believe simulator sickness to be a potential flight safety hazard there was little evidence from this study. Guidelines for the prevention of the problem are presented now that many factors have been identified. A general policy to 'ground' aircrew for a period following simulator training is not necessary, but severe cases should be assessed individually.

Adult↗
PubMed1990

Using molecular dynamics simulations on crambin to evaluate the suitability of different continuum dielectric and hydrogen atom models for protein simulations.

Molecular dynamics simulations of enzymes with enough explicit waters of solvation to realistically account for solute-solvent interactions can burden the computational resources required to perform the simulation by more than two orders of magnitude. Since enzyme simulations even with an implicit solvation model can be imposing for a supercomputer, it is important to assess the suitability of different continuum dielectric models for protein simulations. A series of 100-picosecond molecular dynamics simulations were performed on the X-ray crystal structure of the protein crambin to examine how well computed structures, obtained using seven continuum dielectric and two hydrogen atom models, agreed with the X-ray structure. The best level of agreement between computed and experimental structures was obtained using a constant dielectric of 2 and the all-hydrogen model. Continuum dielectric models of 1, 1r, and 2r also led to computed structures in reasonably good agreement with the X-ray structure. In all cases, the all-hydrogen model gave better agreement than the united atom model, although, in one case, the difference was not significant. Dielectric models of 4, 80, and 4r with either hydrogen model yielded significantly poorer fits. It is especially noteworthy that the observed trends did not semiquantitatively converge until about 50 picoseconds into the simulations, suggesting that validation studies for protein calculations based on energy minimizations or short simulations should be viewed with caution.

Computer Simulation↗
PubMed1976

Computer simulation of the precordial QRS complex: effects of simulated changes in ventricular wall thickness and volume.

The cardiac electric field generated by depolarization of the human ventricle is simulated with a computer model which utilizes 1,500 dipoles. The configuration of the ventricles utilized in the model assumed that the cross-sectional shape of the left ventricle was circular and the right ventricular free wall was a portion of an ellipse. The torso was assumed to be homogeneous and infinite. The activation sequence was based on the measurements of Durrer. The depolarizational wave was simulated by dipole layers. The output of the model is presented as a standard multilead precordial ECG. The ECG complexes generated by the model closely resemble the precordial QRS complexes of normal man. Simulated increases in wall thickness (1 to 2.2 X control) were associated with changes in the calculated precordial QRS complexes which were characteristic of left ventricular hypertrophy. Voltage (R in V5 or V6 and S in V1) and QRS duration increased linearly as a function of calculated left ventricular mass. Increases in ventricular activation time were related nonlinearly to changes in left ventricular mass and did not occur in the absence of a simulated increase in wall thickness. The effects of simulated changes in left ventricular volume (0.6 to 3.0 X control) on the QRS complex were mainly dependent on the resultant increase in left ventricular mass. This model may be useful in simulating the precordial QRS complexes that result from isolated or combined changes in ventricular volume or wall thickness or other disorders of the heart. Furthermore, it may be useful whenever a simulation of a QRS generator is needed.

Cardiac Volume↗
PubMed1991

A simulation model of AIDS in San Francisco: II. Simulations, therapy, and sensitivity analysis.

The HIV and AIDS incidences each year for homosexual men in San Francisco are estimated from data. A computer simulation model for HIV transmission dynamics and progression to AIDS is used to reconstruct the HIV epidemic. Using some a priori parameter estimates, simulations are found that give good fits to the incidence data. In the stimulations the populations is divided into risk groups whose sexual activities are found to be strongly connected. There is saturation in the high-risk group, but changes in sexual behavior are more important in obtaining adequate fits. The simulation modeling yields useful parameter estimates, but the remaining uncertainty in parameter values implies that the simulation forecasts are also uncertain. Changes in HIV incidence lead to changes in AIDS incidence about 6-10 years later. Simulation models with and without zidovudine treatment both fit the incidence data; thus the effects of therapy on AIDS incidence are unclear. The fits of the simulation model are most sensitive to the yearly migration rate, the number of stages in the progression to AIDS, and the average number of new sexual partners per month; thus better estimates of these parameters would be desirable.

Acquired Immunodeficiency Syndrome↗
PubMed1987

Simulation of genetic control of reproduction in beef cows. I. Simulation model.

A stochastic computer simulation model was constructed to predict the reproductive performance of beef cows as a function of the postpartum interval from calving to first estrus and the single-service conception rate. Phenotypic values for single-service conception rate were obtained by truncation of an assumed underlying normal phenotypic distribution at a point corresponding to a cumulative frequency of conception of .70. The underlying phenotypic distribution was derived as the sum of independent, normally-distributed additive genetic, permanent environmental and temporary environmental effects. The phenotypic frequency distribution for postpartum interval has been shown to be skewed toward large intervals. This distribution was simulated by combining normal distributions of additive genetic and permanent environmental effects with a strongly skewed Pearson III gamma distribution of temporary environmental effects. The simulated management system involved a 63-d breeding season, thereby allowing females up to three opportunities to conceive. The reproductive outputs that were simulated from postpartum interval and single-service conception rate included date of first service, first-service conception rate, number of services received, annual conception rate, calving date and postpartum interval. A normally-distributed 205-d weaning weight was also simulated and converted to an actual weaning weight by adjustment for calving date. General simulation results in terms of frequencies, means and standard deviations were compared with values found in the literature and found to provide a reasonable model of bovine reproductive performance.

Animals↗
PubMed1990

The simulation of aerial movement--IV. A computer simulation model.

A computer simulation model of human airborne movement is described. The body is modelled as 11 rigid linked segments with 17 degrees of freedom which are chosen with a view to modelling twisting somersaults. The accuracy of the model is evaluated by comparing the simulation values of the angles describing somersault, tilt and twist with the corresponding values obtained from film data of nine twisting somersaults. The maximum deviations between simulation and film are found to be 0.04 revolutions for somersault, seven degrees for tilt and 0.12 revolutions for twist. It is shown that anthropometric measurement errors, from which segmental inertia parameters are calculated, have a small effect on a simulation, whereas film digitization errors can account for a substantial part of the deviation between simulation and film values.

Anthropometry↗
PubMed1992

Simulator sickness in the U.S. Army UH-60A Blackhawk flight simulator.

Realistic training in aviation now includes the use of sophisticated flight simulators which can provide near total perception of aircraft flight. Reports of simulator sickness seem to be increasing in simulators with multi-directional motion, computer-generated imagery, and infinity optics. A voluntary, subjective, symptom-oriented questionnaire was completed by 141 pilots after training in the UH-60A 2B38 Blackhawk flight simulator. Thirty-six percent reported positive symptoms of motion sickness. Pilots may be at risk if they fly too soon after simulator training and a mandatory grounding time is advised.

Computer Simulation↗
PubMed1991

Computer simulation model of swine production systems: III. A dynamic herd simulation model including reproduction.

A dynamic herd simulation model for a swine production unit has been developed. The model is discrete and stochastic and the appropriate random distributions have been defined from the literature or empirically estimated. Theoretical concepts and relationships presented previously are used to simulate growth and the overall life cycle of individual animals. Simulated animals are young pigs (from birth to slaughter age), gilts, and sows. Animal events are farrowing, weaning, mating, end-nursery (transfer from nursery to the growing-finishing facility), and slaughter and are processed individually using a next-event scheduling orientation package. Animals within the herd are culled for different health and reproductive reasons. Genetic parameters, diet composition, and herd management are input variables chosen by the user. The effects of nutrition, genetics, season, and other factors on several reproductive parameters are considered. Model evaluation is performed by studying the output of the model under specified herd situations. The herd model adequately simulates animal flow and dynamics of the herd.

Animal Nutritional Physiological Phenomena↗
PubMed1990

Genetic implications of a simulation model of litter size in swine based on ovulation rate, potential embryonic viability and uterine capacity: II. Simulated selection.

Direct selection for ovulation rate, uterine capacity, litter size and embryo survival and selection for indexes of ovulation rate with each of the remaining traits were simulated for a swine population. The relationships among these traits were determined from a simulation model that assumed that litter size was always less than or equal to both ovulation rate and uterine capacity. Heritabilities of ovulation rate and uterine capacity were assumed to be .25 and .20, respectively, and uncorrelated genetically and phenotypically. No additional genetic variation was assumed. Responses to weak selection pressure were simulated by recurrent updating of phenotypic variances and covariances combined with the heritabilities of ovulation rate and uterine capacity. Two indexes of ovulation rate and uterine capacity each resulted in 37% greater increase in litter size than direct selection for litter size. Indexes of ovulation rate and either litter size or embryo survival increased litter size by 21% more than direct selection for litter size. Selection for ovulation rate, uterine capacity or embryo survival was 6, 35 and 79%, respectively, less effective than direct selection for litter size. Responses to intense selection pressure were determined by direct simulation of genotypes and phenotypes of individuals. The two indexes of ovulation rate and uterine capacity exceeded direct selection for litter size by 39 and 27%. The indexes of ovulation rate and either litter size or embryo survival exceeded direct selection for litter size by 19 and 13%, respectively. Intense selection for ovulation rate or uterine capacity decreased selection response by 26 and 67%, respectively, relative to direct selection for litter size. Intense selection for embryo survival decreased litter size slightly.

Animals↗
PubMed1992

Cardiovascular simulation using a multiple modeling method on a digital computer--simulation of interaction between the cardiovascular system and angiotensin II.

A cardiovascular system model that simulates interactive responses to drugs has been developed on a small digital computer. The overall model basically consists of three models. The first is a momentum transport model that represents relations between blood pressure and flow in the cardiovascular system. In this model, the cardiovascular system is divided into 14 components and modeled by using equivalent electrical circuits. The second is a mass transport model comprising 14 compartments corresponding to the respective components of the cardiovascular system. This model represents the distribution of the administered drug in the various cardiovascular components. The third is an interaction model that represents the relationships between the momentum and mass transport models. This model causes variations in the resistance and capacitance parameters of the momentum transport model as a function of the current drug concentrations in the appropriate compartments of the mass transport model. The capacitances representing the ventricles are varied in a time-dependent fashion to simulate the beat of the heart. Simulation is performed by using the Euler method to solve a system of 28 ordinary differential equations governing the momentum and mass transport models on a 32-bit microcomputer, a Macintosh II. The model was assessed by performing two demonstrations of the cardiovascular response to the vasopressor angiotensin II (AT II). They first examined the interaction between the cardiovascular system and AT II. The effect of AT II on the cardiovascular system was incorporated into the interaction model. Administration of AT II as a constant infusion (200 micrograms/hr) resulted in an elevation of mean arterial pressure from approximately 100 to 150 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II↗
PubMed1991

Computer simulations of cell-target encounter including biased cell motion toward targets: single and multiple cell-target simulations in two dimensions.

In order for immune cells to carry out many of their functions, including clearance of infectious agents from tissue, they must first encounter their targets in the tissue. This encounter process is often the rate-limiting step in the overall function. Most immune cells exhibit chemotactic ability, and previous continuum models for encounter rates and dynamics have shown that chemotaxis can be a great advantage to cells by greatly increasing encounter rates relative to those for randomly moving cells. This paper describes computer simulations of discrete cell-target encounter events in two dimensions, for the two cases considered by the continuum models: where only a single cell and a single target are present, and where many cells and targets are present. The results of these simulations verify our previous model predictions that a small amount of chemotactic bias dramatically decreases the encounter time, while further increases in the amount of bias have a much smaller effect. Chemotactic ability is shown to be an important determinant of the kinetics of target clearance, and its effects depend on the initial cell-target ratio and the initial distributions of cells and targets. To the best of our knowledge, this work provides the first computer simulations of particle-target encounter in which there is biased motion of particles toward their targets, and is therefore of general interest beyond specific application to immune cell function.

Animals↗
PubMed1992

A method for computer simulation of ultrasound Doppler color flow images--II. Simulation results.

A computational method of simulating Doppler color flow images has been developed. It is based on a point-scattering model of moving blood and surrounding tissue and is capable of treating the entire flow image generation process. Simulated images of parabolic flow dynamics in a cylindrical vessel are presented to show the statistical nature of the map of velocity estimates and to demonstrate the effects of wall filters and different display schemes. Quantitative results of extracted velocity profiles are included and indicate the usefulness of the simulation method for studying the quantitative capabilities of flow imaging.

Blood Flow Velocity↗
PubMed1992

Application of restrained minimization, simulated annealing and molecular dynamics simulations for the conformational analysis of oligosaccharides.

The purpose of the present study was to determine the confidence with which the small number of 1H NMR nuclear Overhauser effect (NOE) distance constraints measurable across glycosidic linkages in oligosaccharides could be used for solution conformational analysis. This was assessed by use of these constraints in restrained molecular mechanical minimization of the tetrasaccharide Gal beta 1----4(Fuc alpha 1----3)Glc-NAc beta 1----3Gal, a model compound of the Lewis-X antigenic determinant. This presents a particularly severe test case in view of extreme resonance overlap and a dearth of inter-residue distance constraints. It is concluded that these constraints, when used in conventional restrained minimization, result in the generation of 'virtual conformations' and local minima about glycosidic linkages. However, these restraints are nevertheless found to be useful in the initial stages of a conformational analysis strategy involving restrained minimization combined with dynamical simulated annealing to define more accurately the global minimum energy configuration, together with molecular dynamics simulation to explore conformational mobility about this minimum. Theoretical ROE values calculated over the time course of the MD simulation, using a formalism appropriate for the time scale of the internal motion, are compared with those obtained experimentally in the oligosaccharide.

Carbohydrate Conformation↗
PubMed1986

Growth of gram-positive mastogenic bacteria in normal, simulated bulk tank, and mastitic milk held at simulated fluctuating temperatures of farm bulk tank.

Growth of Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus agalactiae, Streptococcus dysgalactiae, Streptococcus bovis, and Streptococcus uberis was studied in normal milk, simulated bulk tank milk, and aseptic mastitic milk held at simulated fluctuating temperatures of farm bulk tank for 48 h. With the exception of S. bovis, growth rates of the other five bacteria were similar in both normal and simulated bulk tank milk. Mastitic milk inhibited growth of all bacteria studied. A 24-h adjustment period occurred before most of the bacteria started growing. The mastitis level in a dairy herd may be monitored by cultures of bulk tank milk samples and by calculations as discussed in this study.

Animals↗
PubMed1989

Mucus transport in a miniaturized simulated cough machine: effect of constriction and serous layer simulant.

The transport of mucus gel simulant (MGS) in a constricted simulated cough machine, using blood plasma as a serous layer simulant (SLS), was investigated. MGS was prepared from locust bean gum solutions crosslinked with varying amounts of added borate to produce gels of varying spinnability (filance). The model trachea was a plexiglass channel of rectangular cross-section with the plane bottom surface. The upper surface included a sinusoidal protrusion which provided a flow convergence with minimum gaps of 6, 4 and 2 mm. Experiments for mucus transport were conducted for these minimum gaps, as well as for the non-convergent case (12 mm gap). Miniaturization of sample quantity was achieved by keeping the MGS layer depth constant (0.5 or 1 mm) but reducing the zone of loading from 13.4 cm to 1 cm, thus reducing the sample requirement to as little as 0.2 ml. MGS transport was determined as the minimum displacement of a line of marker dye placed in the MGS layer at the point of minimum constriction gap. It was shown that in all cases (dry as well as with SLS), MGS transport increased as the minimum constriction gap between the plane and the convergent top surface decreased. This increase was further enhanced if an SLS of lesser viscosity was used. It was also found that the transport of MGS increased as the depth of MGS layer increased or as the filance decreased in both non-constricted and constricted cases. The relationship between MGS transport and filance was maintained even in the presence of an SLS layer.

Airway Obstruction↗
PubMed1990

CT simulator: a new 3-D planning and simulating system for radiotherapy: Part 1. Description of system.

A real time CT-linked 3-D treatment planning system, called a CT simulator, has been developed. The basic system consists of a CT scanner, a multi-image display component, a treatment planning device with real time visual optimization, and a laser beam projecting component. All the components are connected on line. The system can be conveniently used for 3-D planning and simulation for radiation therapy within a reasonably short period of time.

Computer Simulation↗