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PubMed1997

Computer simulation to support policy making in the control of pseudorabies.

A further integration of international markets makes a coordinated policy against contagious animal infections increasingly important. In the future, stricter demands are to be expected concerning the control and eradication of such infections. To anticipate these demands, a computer simulation model is created in which scenarios can be evaluated with respect to epidemiological and economic effects of the infections and control strategies. In this paper, the simulation model is described for Pseudorabies in swine. In the model, the population of herds is subdivided into two main herd types: breeding and finishing. Each herd is in one of 24 states per herd type. The states are based on (1) the reproduction ratio R which is the number of secondary cases caused by one infectious herd, (2) the prevalence for each value of R and (3) the expected number of infectious animals in an infectious herd within each prevalence range and for each R. The different values of R are based on experiments and field data in which different vaccination strategies were used. The transition matrix with the probabilities of every transition from one state to another is calculated on a weekly base. With this matrix the distribution of herds over states from week to week is derived. To include a dynamic element in the transition probabilities, the number of newly infectious herds per week is a function of animal and other contacts, including aerial, material and personal contacts. Calculations show that the infection in the Dutch swine population will not disappear without vaccination. With a vaccination scheme in which sows are vaccinated 3 times per year and fattening pigs 1 time per cycle the infection will ultimately be eradicated, but 2 vaccinations per cycle for fattening pigs are needed to eradicate the infection within an acceptable timespan (i.e. 2 to 3 years). The latter strategy will become compulsory in the Netherlands from October 1st 1995.

Animals↗
PubMed1994

Importance of collateral vessels in aortic coarctation: computer simulation at rest and exercise using transmission line elements.

Coarctation of the aorta causes arterial hypertension in the upper body and a low blood pressure downstream. Collateral blood vessels compensate by reducing the downstream pressure drop. To study the effect of various coarctation and collateral properties, we designed a computer model of the arterial circulation. The model contains a flow source and a library of subroutines for the lines and connectors. Distributed friction and wall viscoelasticity effects are included. Computer simulation was performed, using published values for vessel dimensions, in an arterial model with a coarctation and one lumped collateral. Rest and two levels of exercise (by increased heart rate) were studied. Without a collateral, we found the downstream pressure of the model was extremely dependent on the size of the coarctation. A collateral vessel reduced the pressure difference between the up- and downstream circulations. For a severe coarctation, the length and the diameter of the collateral were the main factors determining the downstream pressure and flow, whereas wall stiffness of the collateral had little influence. The relationship between mean pressure drop and cardiac output in coarctation was also dependent on the peripheral resistance in different flow beds, especially during exercise.

Aortic Coarctation↗
PubMed1994

Computer simulations of the effects of different synaptic input systems on the steady-state input-output structure of the motoneuron pool.

1. The effects of different types of synaptic input on the steady-state input-output relations of the mammalian motoneuron pool were investigated by the use of computer simulations. The properties of the simulated motor units and their synaptic inputs were based as closely as possible on the experimental data from studies in the cat hindlimb. 2. Three basic types of synaptic input systems were simulated: postsynaptic, presynaptic, and neuromodulatory. The effects of these inputs on three aspects of the system input-output structure were studied: gain, precision, and motor-unit type utilization. 3. The gain analyses were based on a simulation of the steady-state homonymous Ia input. The gain of this steady-state Ia "reflex" was found to be determined largely by the slope of the pool input-output function. Precision was evaluated in two ways, from the amplitudes of the quantal steps due to motor-unit recruitment and from the sensitivity of the input-output function to noise. The pattern of motor-unit type utilization allowed indirect assessment of fatigue resistance: the larger the percentage of force generated by FF units, the lower the fatigue resistance. 4. A uniformly distributed input (i.e., one that generates equal input in all motoneurons) generates outputs that are solely determined by the intrinsic properties of the motor units. Thus the gain, precision, and motor-unit type patterns generated by a uniform input were used as the basis with which the effects of all other input systems were compared. 5. Postsynaptic excitatory inputs with nonuniform distributions within the pool did influence gain. The greatest effect was the increase mediated by the rubrospinal excitatory input (27% increase at 30% of maximal force). However, this input also greatly decreased both fatigue resistance and precision, due to increased activation of FF units at low force levels. In contrast, the Ia input slightly decreased gain (12% decrease at 30% of maximum force) while slightly increasing fatigue resistance and precision. 6. The simulated neuromodulatory input was based on the monoaminergic reticulospinal effect on motoneurons. Gain was generally increased by the monoaminergic input. However, the magnitude of the increase strongly depended on whether the monoaminergic effects were largest on S units (giving a 20% increase at 30% of maximum force), equal on all types (52%), or largest on FF units (102%). Presynaptic inhibition reduced gain with no effect whatsoever on fatigue resistance or precision. 7. Therefore Ia reflex gain was modifiable by all three types of input: postsynaptic, presynaptic, and neuromodulatory.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1995

Using the standard staircase to measure the point of subjective equality: a guide based on computer simulations.

In experiments that measure a point of subjective equality, it is necessary to employ a psychophysical technique that measures the 50% point (P50) on the psychometric function. The standard staircase is an attractive candidate, because its simplicity makes it easy to understand and implement. However, Pentland (1980, Perception & Psychophysics, 28, 377-379) found his own maximum likelihood method (Best PEST) to be considerably less variable than the standard staircase. Here, computer simulations were used to study the effects of manipulating several parameters of the standard staircase in order to try to find a variant that would be competitive with the performance of Best PEST. A graphical summary of some of the standard staircase implementations that were found to be most stable is presented in order that investigators may choose easily the parameters best suited to their requirements. A comparison with an implementation of Pentland's Best PEST failed to replicate Pentland's finding, suggesting that investigators should have no misgivings about the standard staircase in experiments that measure a point of subjective equality.

Algorithms↗
PubMed1997

Formation of the branching pattern of blood vessels in the wall of the avian yolk sac studied by a computer simulation.

The present study was performed to provide data to support the notion previously believed but not proved experimentally or theoretically, that blood vessels are formed by the selection of capillaries in the network. In an attempt to understand the mechanism of formation of blood vessel branching structures, the transformation of a capillary network to a branching system in the wall of quail yolk sac was successively recorded by a series of photographs, and a computer simulation was carried out for the process of in vivo vascularization based on the photographs. The simulation demonstrated that a positive feedback system participated in the formation of a branching structure. That is, vessels which had been much used were enlarged, whereas less used vessels were reduced in their size and finally extinguished. The enlarged vessels became major components of the branching system. As the body of an embryo grew, it was observed that polygonal capillary networks enlarged, which led each polygon of the network to divide into a few finer polygons. Then, some of the capillary vessels were again selected and formed a branching system. This process repeated during the body growth, indicating that the vascular system developed adaptively to the body growth. A region where the growth was fast, received much blood flow and produced finer networks of capillaries. Thus, it was experimentally demonstrated for the first time that capillaries in the network are successively selected by a positive feedback mechanism and form blood vessels.

Animals↗
PubMed1995

Application of R-charts to detection of random errors: a computer simulation study.

The application of R-charts to quality control in clinical chemistry was investigated. These charts are widely used in statistical process control and plot the range (ie the difference between the highest and the lowest values) of a set of controls. A set of five control serums was considered. A computer simulation program was developed for this investigation. It showed a power function that is remarkably insensitive to systematic errors. Consequently these charts may be a useful tool in discriminating between systematic and random errors, helping the trouble-shooting significantly, if used concurrently with Levey-Jennings' charts. Nevertheless, as several control samples are necessary, this technique is not cost-effective for short analytical runs.

Bias↗
PubMedDate not supplied

Use of computer simulations for quantitation of 31P ISIS MRS results.

The difficulties in quantitation of in vivo 31P spectra are exacerbated by the fact that, in general, coils with inhomogeneous B1 fields are used with in vivo samples. A general method for quantitation of in vivo 31P MRS results obtained with the ISIS localization method was developed using computer simulations. The simulation calculates the preparation of the sample magnetization throughout the sample by the ISIS pulse sequence, as well as the sensitivity of signal reception. The calculation accounts for both the B1 field and the B0 gradients applied to the sample. The sensitivity of the experiment is expressed by integration of the simulated signal over the sample, assuming a homogeneous sample. The primary advantage of this approach is that a separate localization experiment on a phantom of known concentration is not required each time parameters of the localization experiment, such as dimensions or location of the localized volume, are altered. In addition, the simulations indicate the degree of contamination (signal from outside of the localized volume) that occurs, and provide a means of comparing different executions of the ISIS experiment. Experiments were performed on phantoms to verify the simulations, and experimental results on human brain and liver are reproduced to show that this approach provides reasonable estimates of metabolite levels in terms of molar concentrations.

Computer Simulation↗
PubMed1990

[Basic study on criteria for the reactivity of non-stress tests by means of computer simulation].

The non-stress test has become one of the most popular tests for antenatal surveillance in modern obstetrics. However, the fact that there are so many criteria for 'reactivity' in this test has made it rather difficult to interpret the results of these tests and compare them with each other. Using computer simulation of the occurrence of acceleration in fetal heart beats, the sensitivity and specificity of eight criteria which are in general use were studied. Diagnosis by Brown's criteria produced the best results for both sensitivity and specificity, but the long time necessary for this test is a drawback. However, the predictive value positive (probability of poor fetal outcome with nonreactivity) is only 62% even according to Brown's criteria when the test is done for antenatal care screening of an unrestricted population. The results of the test are far more convincing when it is reactive because the predictive value negative is more than 96%, even when strictly interpreted.

Computer Simulation↗
PubMedDate not supplied

[Teaching medical diagnosis by computer simulation].

A program of computer-assisted teaching, called AEDM, has been developed to simulate, with a conversational type terminal, the natural process of medical decision making. This system rests on two softwares, 'teaching' and 'inquiring', with 10 000 PASCAL instructions and the database of a computer-assisted diagnostic system (about 100 MO). The study describes the objectives and the characteristics of a system based on a simulation program; using natural procedures and non-specialized software is useful to develop computer-assisted teaching concerning different fields of pathology.

Computer-Assisted Instruction↗
PubMed1995

The computer simulation of RNA folding pathways using a genetic algorithm.

A procedure for simulating the RNA folding process using the principles of genetic algorithm is proposed. The method allows one to simulate a folding pathway of RNA, including such processes as disruption of temporarily formed structures, the folding of a molecule during its synthesis and pseudoknot formation. The simulations are able to predict functional metastable foldings and kinetically driven transitions to more stable structures. The analysis of free energies for intermediate foldings allows estimation of the ranges of kinetic refolding barriers and suggests that in some RNAs the selective evolutionary pressure suppresses the possibilities for alternative structures that could form in the course of transcription. It is shown that the folding pathway simulation can result in structure predictions that are more consistent with phylogenetically proven structures than minimum energy solutions. This suggest that RNA folding kinetics is very important for the formation of functional RNA structures. Therefore, apart form its value for predictions of RNA structures, the proposed computer simulations can be a powerful tool in the studies of RNA folding features.

Algorithms↗
PubMed1995

Hemodynamic assessment of the development and rupture of intracranial aneurysms using computational simulations.

Intracranial aneurysms manifest themselves as sacculations within a weakened region of the vessel wall and pose substantial neurological risks upon rupture. A primary factor in the development and rupture stages of an aneurysm is hemodynamics and its degrading effects on the aneurysm wall. Wall dynamics and hemodynamics within a fully developed aneurysm were investigated using computational simulation techniques. To study wall dynamics, the aneurysm was modeled as a thing spherical shell with linearly elastic and plastic (viscoelastic) wall behavior. The sensitivity of this model to the biophysical parameters which describe it will assist in the quantitative assessment of factors predisposing to aneurysm rupture and subarachnoid hemorrhage. Flow dynamics simulations were performed for spherical aneurysms with rigid walls. We observed the development and motion of an annular vortex within the lateral sacculation. We also simulated a ruptured aneurysm by placing a tear near the neck of the aneurysm. Flow patterns showed blood flowing out during the initial stages of the flow, but displayed an inflow of blood soon thereafter, as the internal pressure dropped. These results are substantiated by the clinical observations that turbulent flow is observed within the aneurysm as evidenced by reduced bruits.

Aneurysm, Ruptured↗
PubMedDate not supplied

The shape of the proximal isovelocity surface area varies with regurgitant orifice size and distance from orifice: computer simulation and model experiments with color M-mode technique.

The hemispheric proximal isovelocity surface area method for quantification of mitral regurgitant flow (i.e., Qc = 2 pi r2v), where 2 pi r2 is the surface area and v is the velocity at radius r, was investigated as distance from the orifice was increased. Computer simulations and steady flow model experiments were performed for orifices of 4, 6, and 8 mm. Flow rates derived from the centerline velocity and hemispheric assumption were compared with true flow rates. Proximal isovelocity surface area shape varied as distance from each orifice was increased and could only be approximated from the hemispheric equation when a certain distance was exceeded: > 7, > 10, and > 12 mm for the 4, 6, and 8 mm orifices, respectively. Prediction of relative error showed that the best radial zone at which to make measurements was 5 to 9, 6 to 14 and 7 to 17 mm for the 4, 6, and 8 mm orifices, respectively. Although effects of a nonhemispheric shape could be compensated for by use of a correction factor, a radius of 8 to 9 mm can be recommended without the use of a correction factor over all orifices studied if a deviation in calculated as compared with true flow of 15% is considered acceptable. These measurements therefore have implications for the technique in clinical practice.

Blood Flow Velocity↗
PubMed1995

Computer simulation of occlusal discrepancies resulting from different mounting techniques.

The effect of arbitrary mounting of maxillary casts on occlusal relationships was investigated in this study. Maxillary casts of 31 volunteers were mounted on an articulator by use of two split cast bases. This mounting was done first with the arbitrary face bow and second with a hinge bow. Three reference points were defined and measured on each maxillary cast with a three-dimensional digitizer. The measurements were taken from the arbitrarily mounted cast and from the cast mounted according to the hinge axis. Opening and closing movements that were transferred according to the hinge axis. Opening and closing movements that were transferred from the articulator to the mouth of the patient were simulated by a computer based on measurements of the reference points. The results revealed that the use of an arbitrary face bow causes a deviation of the hinge-axis points from the precise axis of more than 5 mm in 77% of the cases. Resulting occlusal errors depended on the angles between the arbitrary and precise axes and the direction of the axis shifts. The occlusal error is roughly proportional to the shift or tilting of the hinge axis in millimeters or degrees. For a given deviation of the arbitrary and precise axes, the occlusal error is proportional to the record height. For a record height of 2 mm or more, an occlusal error of more than 0.1 mm will occur. An average occlusal error of more than 0.1 mm would most likely lead to the necessity of extensive selective grinding of occlusal discrepancies in the patient's mouth.

Adult↗
PubMed1989

Computer simulation and experimental validation of the electrophoretic behavior of proteins.

A mathematical model of the electrophoretic behavior of proteins is presented. The Debye-Hückel-Henry theory is used for the description of protein mobility, which has the important result of making net mobility a function of ionic strength. A net charge vs pH relationship and a diffusion coefficient are required to describe a specific protein. The model is employed for the computer simulation of three distinct electrophoretic modes: isoelectric focusing, isotachophoresis, and zone electrophoresis. The validity of the model is tested by comparing simulation with experimental data. Excellent qualitative agreement was found.

Computer Simulation↗
PubMed1977

Computer simulation of rat heart metabolism after adding glucose to the perfusate.

An experiment where perfused rat hearts receiving no substrate are suddenly given glucose with insulin in the perfusate is simulated with a computer model of cardiac energy metabolism. Mitochondrial metabolism is quantitatively reorganized under cytoplasmic control, with fatty acid oxidation undergoing a two-step decrease. There is an unspanning of the Krebs cycle (different reactions going at different rates) due primarily to slowing of alpha-ketoglutarate dehydrogenase; this ends when cytoplasmic glucose reaches a new steady state. Mitochondria in vitro are known to have higher pH than their surroundings; it is found here that this also holds in situ. Under these conditions, glycolysis is coherently substrate controlled, as is phosphofructokinase, usually considered the typical example of an allosteric enzyme. Limitations on simple methods of analyzing metabolic data of this type, e.g., use of lactate/pyruvate ratios to calculate NADH/NAD ratios, are discussed. Here a large volume of enzyme and other biochemical information has been integrated into a physiologically meaningful system.

Amino Acids↗
PubMed1984

Cardiopulmonary resuscitation by intrathoracic pressure variations--in vivo studies and computer simulation.

The effect of intrathoracic pressure variations on the hemodynamics of dogs with cardiac arrest were studied experimentally and simulated on a computer. High intrathoracic pressure (up to 90 mm Hg) was generated by lung inflation with passive and active modes of external fixation. Abdominal binding was found to be essential for the generation of high intrathoracic pressure. Remarkable Doppler flow signals were detected over the femoral artery with each lung inflation. Blood gases measured after 30 minutes of cardiac fibrillation in dogs together with intrathoracic pressure variations showed well oxygenated arterial blood with metabolic acidosis. A computer model was used to explore the effects of intrathoracic pressure variations over a large range of parameters. For intrathoracic pressure of 50/0 mm Hg. the mathematical model predicted maximal flow of 663 ml/min, occurring at a rate of 115 cpm, with a duty cycle of 58%. The heart showed only minor volume changes during the cycle, indicating its main function as a passive conduit during cardiopulmonary resuscitation. The data show that intrathoracic pressure variation with no direct heart compression can cause systemic blood flow of the magnitude occurring in most cardiopulmonary resuscitation techniques.

Animals↗
PubMed1994

Estimation of metabolic flux rates in liver purine catabolism of tumour-bearing mice by computer simulation of radioactive tracer experiments.

Mouse hepatocytes from healthy control mice and from Ehrlich ascites tumour-bearing mice were used for tracer-kinetic studies of purine catabolism of liver cells during different periods of tumour growth. The dynamics of the radioactive tracers were modelled mathematically by a system of differential equations. Computer simulations, i.e. direct fitting of numerical solutions of these equations to the observed time-courses of metabolites and specific radioactivities, enables one to estimate unknown kinetic parameters of a simplified model of pathways of hepatic purine catabolism in tumour-bearing mice. There occurred great differences of metabolic flux rates between control hepatocytes, hepatocytes of mice during the proliferating period of tumour growth (6th day after inoculation of the tumour) and hepatocytes of mice during the resting period of tumour growth (12th day after inoculation of the tumour). The final purine degradation of hepatocytes prepared during the proliferating period was lower in comparison with that of control hepatocytes, but it was markedly higher in hepatocytes prepared during the resting period of tumour growth. The changes in hepatocyte purine catabolism during the proliferating period of tumour growth argue for transitions which aim at the maintenance of high purine nucleotide levels in the liver itself rather than for an increased nucleoside and nucleobase supply for the tumour. This suggestion is in accordance with the increased ATP level of the liver during the proliferating phase of tumour growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗