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Results for “Models, Theoretical”

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At least 379 records · Page 21Linked to original sources

PubMed2002

A theoretical model for the effects of reduced hemoglobin-oxygen affinity on tumor oxygenation.

PURPOSE: To develop a theoretical model for oxygen delivery to tumors, and to use the model to simulate the effects of changing the affinity of hemoglobin for oxygen on tumor oxygenation. METHODS AND MATERIALS: Hemoglobin affinity is expressed in terms of P(50), the partial pressure of oxygen (Po(2)) at half saturation. Effects of changing P(50) on arterial Po(2) are predicted using an effective vessel approach to describe diffusive oxygen transport in the lungs, assuming fixed systemic oxygen demand and fixed blood flow rate. The decline in oxygen content of blood as it flows through normal tissue before entering the tumor region is assumed fixed. The hypoxic fraction of the tumor region is predicted using a three-dimensional simulation of diffusion from a network of vessels whose geometry is derived from observations of tumor microvasculature in the rat. RESULTS: In air-breathing rats, predicted hypoxic fraction decreases with moderate increases in P(50), but increases with further increases of P(50), in agreement with previous experimental results. In rats breathing hyperoxic gases, and in humans breathing either normoxic or hyperoxic gases, increased P(50) is predicted to improve tumor oxygenation. CONCLUSIONS: The results support the administration of synthetic agents to increase P(50) during radiation treatment of tumors.

Animals↗

PubMed2001

Religion and medicine III: developing a theoretical model.

In this third of a four-article series on religion and medicine, I describe a theoretical model to illustrate the complex pathways by which religion may influence physical health. Genetic factors, childhood training, psychological and social influences, health behaviors, and healthcare practices are discussed as part of this model. Considerable space is given to recent advances in psychoneuroimmunology and to stress-induced cardiovascular changes that demonstrate physiological pathways by which cognitive, emotional, and behavioral processes may influence susceptibility to disease and disease course. I also discuss research illustrating the important role that social support plays in moderating the physiological effects of stress and improving health outcomes. If religious beliefs and practices improve coping, reduce stress, prevent or facilitate the resolution of depression, improve social support, promote healthy behaviors, and prevent alcohol and drug abuse, then a plausible mechanism exists by which physical health may be affected.

Down-Regulation↗

PubMed1983

The problem of cell debris in pulse-cytophotometry: a probability theoretic model.

The problem of cell debris in pulse-cytophotometry is considered on the basis of a probability-theoretic model and the results are compared with the model of a pure exponential function for the portions of the DNA fragments in the channels of the flow cyclometer, which is generally used in practice. The model is based on the assumption that the possible decomposition of the DNA content of a single cell into some parts as a consequence of the necessary preparation of the cell material for measurement can be interpreted as a Poisson point process. Therefore the number of divisions of the DNA content of a cell is assumed as a Poisson random variable. It follows a well-defined distribution function of the DNA content in the common population of intact cells and fragments depending on the apriori distribution before pretreatment. This distribution determines a theoretical histogram which can be compared with the measurements. The results corroborate the assumption of a pure exponential course for the portions of DNA fragments only in the area of very small DNA contents. For greater DNA contents there are differences between the pure exponential course and the model described here depending on the a priori distribution and on the intensity of the cell disintegration process. As a byproduct, an estimation of the fraction of disintegrated cells is obtained with respect to the fraction of fragments. A corresponding computer program was applied to a number of experimental histograms. The results have shown that the theory describes the measured histograms satisfactory in most cases.

Cell Cycle↗

PubMed1998

Unifying genetic and game theoretic models of kin selection for continuous traits.

A framework is presented for unifying single locus genetic and game theoretic models of continuous traits under frequency-dependent selection when there are interactions among relatives. This framework serves two purposes. First, it is used to determine how "games between relatives" must be modeled to be genetically valid. There are two commonly employed phenotypic approaches used in this setting, and we demonstrate that, although some of their predictions are always genetically valid, others are invalid in general, and this is true for both haploid asexual and diploid sexual organisms. In particular, we show that both approaches obtain the correct equilibrium and convergence stability conditions, but neither obtains the correct condition for evolutionary stability. Unlike earlier results for discrete trait matrix games (Hines & Maynard Smith, 1979), there is no simple correspondence between phenotypic and genetic predictions, and we provide two examples to illustrate this point. It is possible however, to obtain these earlier results within the present setting by restricting attention to a particular class of fitness functions. These results demonstrate that, even when selection is weak, phenotypic models can fail if fitness is frequency-dependent. The second purpose is to determine when population mean inclusive fitness effect provides an adaptive topography in games between relative. Our results show that the fitness function must have a special form for this to be true, and this form differs between haploid and diploid organisms.

Animals↗

PubMed2004

Comparison of theoretical models of crossover behavior near the 3He liquid-vapor critical point.

Parametric expressions, based on three different theoretical models, are used to calculate the isothermal susceptibility, specific heat, and order parameter along the critical isochore and coexistence curve from the asymptotic region to the crossover region. These models are (i) the minimal-subtraction renormalization scheme, (ii) the massive renormalization scheme within the phi(4) model, and (iii) the crossover parametric model based on the crossover Landau model. We fit these theories globally to experimental measurements of the isothermal susceptibility and specific heat along the critical isochore and coexistence curve, and to the coexistence curve of 3He near its liquid-vapor critical point. All of the theories provide good agreement with the experimental measurements within the reduced temperature range |t| < or =2 x 10(-2) . The differences in the fits between the theories and the correlations between the adjustable model parameters are discussed.

Journal Article↗

PubMedDate not supplied

[Nursing and the environmental question: proposal of a theoretical model for the professional practice].

Considering the side effects of environmental changes over the population's health, a theoretical model is proposed in this study in order to incorporate ecologic matters into the nursing practices. The reference for this work is the eco-socialist-marxist theory. The model is based on the analysis of the capitalist economic process, its production technologies and consumption. It is known that this economic model generates ecoinequalities and anthropogenic impacts that rebound on the health-disease profile of the population. The nursing action, permeated by ecological awareness, can prevent and also combat ecoinequalities and destructive human actions on the environment.

Environment↗

PubMed1993

Learned helplessness as an interacting variable with self-care agency: testing a theoretical model.

This article describes the theoretical development and initial testing of a model outlining the interaction of the concepts of self-care agency and learned helplessness in healthy working adults. Orem's theory of self-care and the reformulated learned helplessness theory are discussed as the theoretical basis for the study. The self-care agency conditioning factors, age and gender, were examined for relationships to the main variables. In a descriptive, correlational design, the hypothesis, that learned helplessness was inversely related to self-care agency, was supported (r = -.57). Neither age nor gender was related to main variables in the population. Implications for nursing research, self-care theory clarification, and nursing practice are discussed.

Adult↗

PubMed1994

Penicillin G extraction from model media using an emulsion liquid membrane: a theoretical model of product decomposition.

To confirm the applicability for the extraction of penicillin G by an emulsion liquid membrane (ELM), the degree of decomposition of penicillin G during extraction was theoretically calculated. Decomposition was less than 1% provided that the initial sodium carbonate concentration in the internal phase was correctly determined, which proved the applicability of the ELM process. The procedure to determine the initial carbonate concentration in the internal phase was also described in order that the pH in the internal phase should be within the relatively stable range for penicillin G at the end of the extraction.

Biotechnology↗

PubMed2006

A study on the dispersions of piezoelectric plate with laser ultrasound measurement and theoretical modeling.

This research is focused in the measurement and modeling for the dispersion relations of Lamb waves propagating in LiNbO(3) and LiTaO(3) plates. A theoretical model based on a stiffness matrix method with recursion computation algorism is used to provide numerical calculations for the dispersion relations. Also, a dry, noncontact laser ultrasound technique is used to measured dispersion multi-mode dispersion relations. For all the experiments, the measured dispersion curves show good agreement with the theoretical calculation, indicating the reliabilities in the measurement and modeling.

Journal Article↗

PubMed1975

A theoretical model for the covalent assembly of immunoglobulins. Application to the assembly of human immunoglobulin G in vitro.

A simple theoretical model for the formation of interchain disulfide bonds (covalent assembly) in immunoglobulins has been developed. This model successfully simulates the experimentally determined sequence of disulfide bond formation in vitro for human immunoglobulin G1 and G4 (Petersen, J. G. L., and Dorrington, K. J. (1974) J. Biol. Chem. 249, 5633-5641). The model appears to be generally applicable suggesting that the various pathways of covalent assembly observed in vivo and in vitro reflect structural differences between the immunoglobulin classes and subclasses. The rate of assembly, however, is dependent upon environmental factors.

Disulfides↗

PubMed2004

Forward model theoretical basis for a superconducting imaging surface magnetoencephalography system.

A novel magnetoencephalography (MEG) system was designed at Los Alamos National Laboratory (LANL) that incorporates a helmet-shaped superconductor in order to increase the signal to noise ratio. The magnetic field perturbations caused by the superconducting surface must be included in the forward physics for accurate source localization. In this paper, the theoretical basis for the forward model that calculates the field of any magnetic source in the presence of an arbitrarily shaped superconducting surface is presented. Appropriate magnetic field integral equations are derived that provide a description of the physics of the forward model. These equations are derived starting from Maxwell's equations in the presence of inhomogeneous media, with the appropriate boundary conditions for a superconductor. A discretized version of this equation is then compared with known analytic solutions for simple superconducting surface geometries.

Algorithms↗

PubMed1989

[Theoretical models of causative factors in depression: a review of the literature for nursing].

This literature review was undertaken to explore theoretical models of depression for their potential usefulness in nursing research and practice. Depression has been accounted for by numerous theories or models of causation: 11 theories selected from psychology, medicine and psychoanalysis and supported by empirical or experimental research were reviewed. These theories identify a variety of precipitating and predisposing factors that may affect the individual's depression. Aggression-turned-inward theory, object loss theory, ego functioning theory, personality organization theory, behavioral theory, learned helplessness theory, cognitive theory, genetic factors, and biological theories conceptualize predisposing factors. Only life stressors theory identifies precipitating factors. Each of these theories contributes to an understanding of depression, but many of them use overlapping and interrelated factors. It is also evident from recent research that there are multiple causes for depression involving an interactive effect among predisposing and precipitating factors that are both biological and psychological in origin. That is, a single theory is not useful, but perhaps a unified theory could be developed that would be helpful to nursing. This review points to the need for continuing development and testing of theories that would integrate the multiple conceptualizations of depression.

Depression↗

PubMed1990

Mechanical pathophysiology of some heart diseases: a theoretical model study.

Sarcomere dynamics are related to the global left ventricular (LV) function in some representative pathological states, by using a theoretical model which combines sarcomere function, LV fibrous structure and geometry with the haemodynamic loading conditions. The analysis shows that pressure (concentric) hypertrophy due to hypertension or aortic stenosis is associated with an increase of the normal endocardial-to-epicardial gradient(s) of oxygen demand, which may be one of the causes for the development of endocardial fibrosis. The analysis also indicates that sarcomere shortening is relatively normal in compensated volume (eccentric) hypertrophy. Mitral stenosis demonstrates a case of decreased LV function, secondary to a chronic decrease in LV end diastolic volume, with sarcomeres that operate at their lowest length range. Conversely, the sarcomere function is depressed in cardiomyopathy; the heart's pumping function is maintained by appropriate adjustment mechanisms. However, the sarcomeres show minimal shortening and function at their highest length range with low (or zero) functional reserve. The study thus provides a quantitative tool that relates global LV function to local sarcomere dynamics in various pathological states.

Heart↗

PubMed2004

A theoretical model for simulating the outcome of mechanism based in vitro toxicity testing strategies.

In order to investigate the fundamental principles that influence the optimal selection of toxicity test methods for the evaluation of chemical hazards, it is useful to have a design model to explore possible alternative testing strategies. In general, our lack of detailed knowledge of the mechanisms of toxicity (including not only the early events in the interaction of chemicals with biological systems, but the sequence of events that lead to experimentally measurable toxicity) limits the development of realistic dynamic models of the toxicological process for individual chemicals. We report here the development of a theoretical model that includes two independent hypothetical mechanisms of toxicity. The mechanisms are designed to be qualitatively similar to known mechanisms of action. The model is exercised to simulate the experimental data that would be obtained for a collection of "test" and "validation" chemicals using a single or a combination of two toxicity tests. The data generated are used to evaluate the "relevance" of the testing strategy based on the two proposed in vitro toxicity tests.

Aging↗

PubMed1983

A game-theoretical model of parasite virulence.

The evolution of parasitic reproductive rates, relative infectiousness and severity of disease are considered using a game-theoretical model in which parasites compete within hosts. Each parasite's fitness is assumed to be directly proportional to the product of its reproductive rate (lambda) and the length of time (T) over which it reproduces. An increase in a parasite's reproductive rate is assumed to increase its host's disease-induced mortality rate (alpha) and consequently, through host death, to decrease T. By maximizing the total number of propagules that individuals produce with respect to their individual reproductive rates, we show that competitors within a host may be favored by natural selection to reproduce at rates below their maximum potential rates. Whether competitors behaving with such restraint can coexist at a Nash equilibrium is shown to depend on the functional form of alpha (lambda) and on the number of competitors within a host. While an individual's restraint benefits its within-host competitors through increased host longevity, the model does not invoke group selection. In the model, selection favors an individual's restraint when such behavior increases the individual's total number of propagules. Concurrent increases in the absolute and relative fitness of an individual's within-host competitors can be consequences of such individual selection.

Animals↗

PubMed1986

The effect of perimetric stimulation on evoked potential distribution--a theoretical model.

A schematic cortical projection of the human visual field is combined with the concept of dipole planar generators to provide a theoretical model of the scalp distribution of evoked potentials from visual field stimuli. The computations illustrate the importance of gyral surfaces as the principal generators of recorded evoked potentials. Taking several zones of the visual field projection on the striate area as an example, the model is used to produce horizontal profiles of evoked potential amplitude. This indicates the limitation and possibilities of mapping the cortical projections in man and relating these to perimetric findings.

Animals↗

PubMedDate not supplied

[Oscillations in population densities of the bacterial prey-predator couple Escherichia coli-Bdellovibrio bacteriovorus: experimental study and theoretical model].

Oscillations in population densities in the bacterial predator-prey Bdellovibrio bacteriovorus-Escherichia coli system were investigated both experimentally and theoretically. Experimental conditions for observing (damped) oscillations were first determined in a closed system and then used in an open system, i.e. in a chemostat, when an adequate flux of nutritive medium was added. The experimentally observed oscillations were always unstable with poorly reproducible amplitude and period. A theoretical model was used in order to explain this behaviour. It was first presented and satisfactorily tested for the same bacterial couple operating in a closed system, and allowed an experimental determination of its kinetic parameters. When adapted to open system conditions, it yielded computer-simulations which showed oscillations of the population densities in good agreement with those experimentally observed. It also showed that stable oscillations were not possible, the only "focus" in the predator-prey-density plane being an unstable one with no surrounding limit cycle.

Bdellovibrio↗

PubMed1992

Axial length and scleral thickness effect on susceptibility to glaucomatous damage: a theoretical model implementing Laplace's law.

Laplace's law relates the pressure inside a hollow sphere with its radius and the tension in its walls. A theoretical model implementing Laplace's law in the eye globe is presented. The physical model may help to explain certain aspects in glaucomatous disk damage such as higher susceptibility of myopic eyes to glaucomatous damage and a possible explanation for glaucoma nerve head damage in low tension glaucoma.

Disease Susceptibility↗