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PubMed1992

Computer simulation of erythrocyte transit in the cerebrocortical capillary network.

The relationship between erythrocyte flow path length and transit time was studied using a probabilistic computer simulation of erythrocyte transit in morphometrically reconstructed cerebrocortical microvascular network of the rat. The results suggest (1) a near-cubic relationship between erythrocyte flow and vessel diameter, (2) preferential pathways of erythrocyte flow, (3) an exponential relationship between erythrocyte transit time and flow path length.

Animals↗

PubMed1989

Computer-simulation methods in human linkage analysis.

In human linkage analysis, many statistical problems without analytical solution could be solved by ad hoc Monte Carlo procedures were efficient computer-simulation methods available for members of family pedigrees. In this paper, a general method is described for randomly generating genotypes at one or more marker loci, given observed phenotypes at loci linked among themselves and with the markers. The method is based on a well-known expansion of the multivariate probability of genotypes, given phenotypes, into a product of conditional univariate probabilities that may be viewed as corresponding to conditionally independent univariate random variables. This representation allows a recursive evaluation of the univariate probabilities that can be implemented in a surprisingly simple manner by carrying out successive "risk calculations" with respect to marker genotypes, given observed phenotypes and marker genotypes already generated. Potential applications to various unresolved problems are discussed. The method is applied to 28 published families analyzed for genetic linkage between hereditary motor and sensory neuropathy I and the Duffy (FY) blood group locus and confirms heterogeneity of hereditary motor and sensory neuropathy I. An implementation of the simulation methods developed in the LINKAGE program package will be available later in 1989.

Chromosomes, Human↗

PubMed1987

Hemodynamic evaluation of arterial stenoses by computer simulation.

A new method to assess the hemodynamic severity of arterial stenoses was proposed and evaluated. It is based on a previously developed finite element computer simulation model for laminar-separated flow in arteries of axially varying cross-section; the present modification allows use of angiographic stenosis shapes acquired by automatic edge-detection algorithms. The method was validated by comparing its results with published experimental and theoretic results for ideal stenosis shapes. At moderate flowrates (Reynolds number = 500), poststenosis flow separation was predicted for moderately severe (75% area reduction) but not for mild (25%) stenoses. For high flowrates (Reynolds number = 900) in a severe stenosis (89%), stagnation and reversed flow were predicted and the experimental nondimensional pressure drop of 48.5 was correctly determined. Bernoulli's Equation, which neglects viscosity, predicted a drop of only 40. For a severe stenosis (89%), even at low Reynolds numbers (50), reversed flow agreeing with other theoretic solutions was predicted. Predictions are especially useful at low flow rates, where experiments are difficult to conduct. The height of the curve on the graph of nondimensional pressure gradient vs. Reynolds number reflects the hemodynamic severity of a particular stenosis; these curves were predicted for moderate and severe ideal stenoses and agree with experiments. A similar analysis is applied to an actual human coronary artery stenosis, and the results are demonstrated to have use in assessing interventions during angiography.

Arterial Occlusive Diseases↗

PubMed1989

Programmed electrical stimulation at potential ventricular reentry circuit sites. Comparison of observations in humans with predictions from computer simulations.

The purpose of this study was to define specific types of resetting responses to programmed electrical stimulation during human ventricular tachycardia and to use computer simulations of reentry circuits to assess the possible mechanisms and pacing site location relative to the reentry circuit for each type of response. The effects of scanning single stimuli at 35 left ventricular endocardial sites during sustained monomorphic ventricular tachycardia in 12 patients were studied. In considering alterations in QRS configuration and the delay between the stimulus and the advanced QRS, we identified three types of resetting responses to scanning stimuli consistent with stimulation at sites in or near the reentry circuit at 12 abnormal endocardial sites in eight patients. Type 1: all capturing stimuli were followed after a delay by early QRS complexes that had the same configuration as the tachycardia complexes. Type 2: late stimuli reset tachycardia as in type 1 but early stimuli reset the tachycardia after altering the QRS configuration. Type 3: late stimuli reset tachycardia as in type 1, but early stimuli advanced tachycardia with a short stimulus to QRS delay without altering the QRS configuration. In the simulations, premature depolarization of sites in the circuit produced orthodromic and antidromic wavefronts. The orthodromic wavefront propagated through the circuit and exited the circuit at the same site as did the previous tachycardia wavefronts and advanced the tachycardia without altering the configuration of the advanced QRS. The antidromic wavefront of relatively late stimuli was confined within or near the circuit by collision with the orthodromic wavefront of the preceding tachycardia beat and failed to alter ventricular activation distant from the circuit. Therefore, the QRS configuration after the stimulus was unchanged. A type 1 response occurred when all capturing stimuli produced this effect. However, with increasing stimulus prematurity, the antidromic wavefront propagated farther before colliding with an orthodromic wavefront, and under some conditions, it exited the circuit from a site other than the original circuit "exit," and altered the ventricular activation sequence distant from the circuit and, therefore, the QRS configuration, producing a type 2 pattern. The type 3 pattern occurred when the antidromic wavefront of early premature beats captured the original circuit exit. The effect of a stimulus was dependent on the stimulus prematurity, the relative conduction times from the stimulation site to the potential sites of "exit" from the circuit, and the timing of the excitable gap at the stimulation site.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiac Pacing, Artificial↗

PubMed1992

Computer simulation of the conformational behaviour of angiotensinogen (6-13) renin substrate.

The conformational behaviour of the biologically active angiotensinogen (6-13) fragment has been investigated by computer simulations. A large sample of conformers has been generated using the Monte-Carlo procedure, then analysed using classification and partition methods. Seven families can describe the conformational distribution. About 40% of conformers are fully extended, 28% are folded at the C-terminal His6-Pro7-Phe8-His9 level and the others are folded at different levels. The study highlights the extreme flexibility of the angiotensinogen fragment.

Amino Acid Sequence↗

PubMed1988

Acoustic effects in in-the-ear hearing aid response: results from a computer simulation.

The response of a hearing aid depends on the design of the instrument and on the characteristics of the individual ear. In this paper a computer simulation of an in the ear (ITE) hearing aid is used to determine the effects on the hearing aid response caused by variations in the size of the ear canal, the magnitude of the eardrum impedance, and the vent size and damping. The simulation results indicate that, for an unvented hearing aid, changes in the size of the ear canal or the eardrum impedance shift the average sound pressure level at the eardrum but have relatively small effects on the overall shape of the frequency response. A vented instrument presents a more complicated situation since the vent modifies the low-frequency response in a predictable manner but can have unexpectedly pronounced effects at high frequencies due to the acoustic feedback.

Acoustic Impedance Tests↗

PubMedDate not supplied

Qualitative probability versus quantitative probability in clinical diagnosis: a study using a computer simulation.

The use of Bayes' theorem as a diagnostic tool in clinical medicine normally requires an input of exact probability estimates. However, humans tend to think in categories ("likely," "unlikely," etc.) rather than in terms of exact probability. A computer simulation of the presenting features of a case of pelvic infection has been used to compare the effects of quantitative and qualitative probability estimates on the diagnostic accuracy of Bayes' theorem. For the commoner conditions (prior probability greater than or equal to 0.2) the use of a two- or three-category system is virtually equivalent to the use of exact probability. However, uncommon conditions (prior probability less than or equal to 0.03) are completely ignored by the qualitative system. It is concluded that the use of simple categories of probability is acceptable for a Bayesian diagnostic system provided that the target conditions have a relatively high prior probability.

Artificial Intelligence↗

PubMed1992

[Computer simulation of the 10-year outcome of a cohort of heroin addicts in Ile-de-France].

The aim of this study is to develop a computer simulation model to describe imaginary cohorts of heroin users in the Ile de France (greater Paris) region. Because of the lack of well-designed cohort studies in France, this model is based on epidemiological data found in various different studies. The results of the simulation reveal horrifying outcomes in ten years' time: in the more optimistic scenario (an annual non-Aids mortality rate of 1%, an annual rate of permanent addiction curtailment of 10% and a syringe exchange rate of 10%), only 50% of heroin users curtail their consumption permanently without being HIV carriers after 10 years, 16% are HIV carriers and 10% have died. In conclusion, medical care should play an important part in the management of drug users, and secondary prevention aimed at reducing the health problems involved in heroin use should be intensified.

Acquired Immunodeficiency Syndrome↗

PubMed1987

Computer simulation of the Wolff-Parkinson-White syndrome utilizing a human heart model.

The relationship between the location of an accessory pathway and body surface ECG waveforms was investigated by computer simulation of the WPW syndrome using a human heart model. The 3-dimensional heart model is composed of 50,000 units so as to represent the details of ventricular excitation. Ten typical accessory pathways near the A-V groove were examined, which correspond to Gallagher's classification of the WPW syndrome based on their surgical experience. Twelve-lead ECG waveforms were calculated for these 10 cases and were compared with the clinical observations of Gallagher et al. The polarities of the delta waves at 40 msec after the onset of the QRS were listed. The agreement of our model with the clinical data obtained by Gallagher et al is 87%. Details of changes in the ECG waveforms were also examined by changing ther terminal location of the accessory pathway along the radial and longitudinal directions in the ventricles, and it was found that the ECG waveforms of the limb leads are more sensitive to this difference in the preexcited location than those of the chest leads. The simulated epicardial isochronic patterns also agree with the measurements.

Computer Simulation↗

PubMedDate not supplied

Computer simulation of arcuate keratotomy for astigmatism.

BACKGROUND: The development of refractive corneal surgery involves numerous attempts to isolate the effect of individual factors on surgical outcome. Computer simulation of refractive keratotomy allows the surgeon to alter variables of the technique and to isolate the effect of specific factors independent of other factors, something that cannot easily be done in any of the currently available experimental models. METHODS: We used the finite element numerical method to construct a mathematical model of the eye. The model analyzed stress-strain relationships in the normal corneoscleral shell and after astigmatic surgery. The model made the following assumptions: an axisymmetric eye, an idealized aspheric anterior corneal surface, transversal isotropy of the cornea, nonlinear strain tensor for large displacements, and near incompressibility of the corneoscleral shell. The eye was assumed to be fixed at the level of the optic nerve. The model described the acute elastic response of the eye to corneal surgery. RESULTS: We analyzed the effect of paired transverse arcuate corneal incisions for the correction of astigmatism. We evaluated the following incision variables and their effect on change in curvature of the incised and unincised meridians: length (longer, more steepening of unincised meridian), distance from the center of the cornea (farther, less flattening of incised meridian), depth (deeper, more effect), and the initial amount of astigmatism (small effect). CONCLUSIONS: Our finite element computer model gives reasonably accurate information about the relative effects of different surgical variables, and demonstrates the feasibility of using nonlinear, anisotropic assumptions in the construction of such a computer model. Comparison of these computer-generated results to clinically achieved results may help refine the computer model.

Animals↗

PubMed1992

Computer simulations of enzymatic reactions: examination of linear free-energy relationships and quantum-mechanical corrections in the initial proton-transfer step of carbonic anhydrase.

Computer simulation approaches can provide a powerful tool for correlating the structure of enzymes with their catalytic activity. One of the most effective ways of simulating enzymatic reactions is provided by the empirical valence bond method. The general applicability of this method has been demonstrated in several enzymatic reactions and it is reexamined here in a study of the initial proton-transfer step in the catalytic reaction of carbonic anhydrase. The simulations produce a rate constant which is in agreement with the observed kinetic data and emphasizes the importance of the electrostatic effect associated with the catalytic zinc ion. The calculations are also used to examine the validity of linear free-energy relationships (LFERs) in enzyme catalysis and to evaluate quantum-mechanical corrections of the calculated rate constant. It is found that LFERs are valid in the present case and it is argued that this reflects the fact that the protein responds linearly to the development of electrostatic forces during the reaction. It is concluded that the present approach can be used to augment experimental studies in establishing the general validity of LFERs. It is noted, however, that such relationships are much more valid for transitions between different resonance structures than for transitions between reactants and product states.

Carbonic Anhydrases↗

PubMed1989

Computer simulations of figure-ground discrimination in the visual system of the fly.

Figure-ground discrimination and pattern discrimination represent one of the most important problems in computer vision. Based on computational neuronal network model proposed by Reichardt et al. for figure-ground discrimination in the visual system of the fly, entire systematic computer simulations were carried out under nonstationary conditions. Results show that the model network and simulations have predictive power for behavioural experimental results. This paper proposes that motion perception and elementary pattern discrimination in human visual system may be mediated by some kind of figure-ground system with movement detectors as input layer.

Animals↗

PubMed1990

The behaviour of common measures of association used to assess a vaccination programme under complex disease transmission patterns--a computer simulation study of malaria vaccines.

Case-control studies have been evoked as important alternatives to randomized clinical trials in the evaluation of infectious disease interventions. Using computer simulations, we compare the behaviour of common measures of association derived from case-control studies in the context of a malaria vaccine programme administered under complex transmission conditions. Several simplifying assumptions of previous workers have been relaxed and the simulated conditions are endemic rather than epidemic. The common estimators of association used in case-control studies remain unbiased only in limited circumstances. The term dependent happenings, first defined by Ross in 1916, is resurrected. Since the number of people becoming infected is dependent on the number of people already infected, control programmes in infectious diseases produce direct as well as indirect effects. Three different study designs with different pairs of comparison populations are defined. The choice of comparison population can be used to differentiate direct from indirect effects. In order to clarify the direct effects of a vaccination programme the comparison groups must be subjected to identical transmission intensities. In contrast, the referent group must remain unaffected by consequences of the intervention to determine indirect effects.

Brazil↗

PubMed1992

Computer simulation for the prediction of separation as a function of pH for reversed-phase high-performance liquid chromatography. I. Accuracy of a theory-based model.

Computer simulation software (DryLab I/mp) is described for predicting high-performance liquid chromatographic separation as a function of changes in mobile phase pH. Three experimental runs with pH (only) varied are used to derive values of pKa plus capacity factors (k') for the ionized and non-ionized form of each ionizable solute. Various tests of the experimental data then allow classification of each solute as acidic, basic, neutral (including strong or weak acids or bases) and amphoteric. Experimental data are reported for the separation of several substituted anilines as a function of pH and solvent composition (%B). Experimental requirements for the accurate prediction of separation (ca. +/- 2-4% in alpha) as a function of pH are discussed. The reliability of the software is demonstrated for three different samples: mixtures of (a) substituted benzoic acids, (b) substituted anilines and (c) catecholamine-related compounds.

Aniline Compounds↗

PubMedDate not supplied

Computer simulation of the circulatory system during support with a rotary blood pump.

In order to investigate the performance characteristics of nonpulsatile centrifugal pumps and their interaction with the pulsating heart, a computer simulation model was developed. A model of the pump, including the cannulae and the interference of the cannula and the left atrial wall, was added to a simulator of the circulation. It was implemented on a PC-AT using the analog simulating software AGO1000. An interval of 1 msec real-time per calculation step was evaluated to provide stable results. The calculation speed was approximately 100 msec per step. First simulation studies showed that both the positioning and resistance of the cannulae exert a major influence on system performance and equalized the different pressure-flow characteristics of vaneless and impeller pumps (difference of dp/dQ decreases from more than 6 times without cannulae to 130% with cannulae). In addition, the effects of suction in the atrial cannula, and changes of ventricular performance during changes in pumping power, could be properly simulated. The comparison with animal tests proved that the model is a suitable tool for testing control algorithms for centrifugal pumps.

Algorithms↗

PubMed1988

Theoretical considerations on the "spine of hydration" in the minor groove of d(CGCGAATTCGCG).d(GCGCTTAAGCGC): Monte Carlo computer simulation.

A theoretical description of aqueous hydration in the minor groove of a B-form DNA is presented on the basis of a liquid-state Monte Carlo computer simulation on a system consisting of the oligonucleotide duplex d(CGCGAATTCGCG).d(GCGCTTAAGCGC) in a canonical B-form together with 1777 water molecules contained in a hexagonal prism cell and treated under periodic boundary conditions. The results are analyzed in terms of solvent density distributions. The calculated minor-groove solvent density shows considerable localization, indicative of discrete solvation sites and providing theoretical evidence for a well-defined ordered water structure. In the AATT sequence, this corresponds to the "spine of hydration" described by H. R. Drew and R. E. Dickerson [(1981) J. Mol. Biol. 151, 535-556] based on the x-ray crystal structure of the dodecamer hydrate. We find, however, that the calculated ordered water structure also extends into the CGCG flanking sequences, supported by the N2 hydrogen bond donors of the guanine residues and indicating that the spine of hydration could thus extend throughout the minor groove of a B-form DNA. This provides a possible explanation of the positive binding entropies observed by L. A. Marky and K. J. Breslauer [(1984) Proc. Natl. Acad. Sci. USA 84, 4359-4363] for both A.T and C.G sequences on the complexation of netropsin to the minor groove of DNAs. Implications of these results with regard to the thermodynamic stability of DNA in water and the sequence specificity of the minor groove hydration are discussed.

Computer Simulation↗

PubMed1991

Computer simulations of chondrocytic clone behaviour in rabbit growth plates.

The growth behaviour of chondrocytic clones in the cell columns of the proximal tibial growth plates of young rabbits was modelled in computer simulations. Simulations were performed, modelling either clones in large groups of columns or clones in one single column. The former were based on morphological data and measurements of cell columns from an earlier study while the latter utilised previous findings of cellular kinetics in rabbit growth plates. Simulation results that resembled most closely the actual observations on rabbit growth plates were those in which a distribution of values was assumed both for clone length (ranging from 1000 to 2000 microns) and for the lengths of the discontinuities between clones. When the assumption was made in the models that the disappearing (metaphyseal) end of an 'old' clone moved more rapidly than the developing (epiphyseal) end of a 'new' clone, replacing the former, the length of the discontinuity between these two clones increased with time. This assumption, which could be modelled in the simulations of clones in a single column based on cell growth behaviour, was found to provide an explanation for an earlier finding that there are more short columns at the epiphyseal side than at the metaphyseal side of a growth plate.

Animals↗

PubMed1991

Computer simulation model of swine production systems: II. Modeling body composition and weight of female pigs, fetal development, milk production, and growth of suckling pigs.

Theoretical concepts and relationships used to develop a deterministic computer simulation model of female pigs during their reproductive life are described. The model predicts, in a continuous form, body composition and weight of female pigs, fetal development, sow milk yield, and growth of suckling pigs according to genotype, diet, and management conditions. The model simulates growth of adult female pigs. Dietary nutrients are used first for maintenance and second for fetal growth or milk production. Any surplus is retained in the body. Energy and protein body reserves are mobilized when a nutrient deficit occurs during gestation or lactation. However, the rates of body protein and fat accretion as well as the fat/protein ratio are limited by boundaries, the values of which depend on the nutritional and physiological status of the pig. The model's ability to simulate sow body weight changes and composition, fetal growth, milk production, and suckling pig's growth is illustrated.

Animals↗