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Results for “Models, Theoretical”

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At least 289 records · Page 16Linked to original sources

PubMed1984

A theoretical model for myocardial tissue deformability.

Early detection of cardiac disease is based on the quantitative interpretation of left ventricular wall motion throughout the cardiac pumping cycle. Wall deformations result from complex fluid-wall interactions wherein muscle fibre orientation, intraventricular pressure and regional variations of myocardial wall rheology play a crucial role. A reliable theoretical model would be of intrinsic value in aiding the cardiologist in his interpretation of clinical diagnostic results, particularly through the incorporation of microprocessor-based algorithms permitting automatic processing of clinical data within the framework of such a model. As a step in this direction, a theoretical analysis is formulated for a relatively simple characterization of the left ventricle in terms of a truncated ellipsoidal shell. The myocardial wall contains contractile muscle fibres of known orientation. The stress tensor is derived on the basis of an inviscid fluid-fibre continuum. Principal stresses are calculated in terms of regional wall deformations and intraventricular pressure. These are determined from an inviscid fluid dynamic model for left ventricular contraction, subject to an appropriate Neumann condition on wall velocity as obtained from cineangiography. Local "defects" in wall velocity simulate the inhibition of wall contractility associated with the development of myocardial infarct. The theoretical model makes it possible to evaluate local variations in wall stress at those sites and to calculate both regional and overall changes in heart work as a noninvasive indicator of cardiomyopathies or valvular defects. Graphic results are presented depicting the role of myocardial tissue rheology on the dynamics of cardiac performance during the ejection phase, on the basis of the present theoretical model.

Blood Pressure↗

PubMed2003

[Theoretical model of a cost-effectiveness analysis of combined enalapril-nitrendipine therapy for treating hypertension].

OBJECTIVE: Cost-effectiveness analysis of combined enalapril-nitrendipine therapy (E/N), as second-line therapy for light or moderate hypertension. DESIGN: Theoretical model of cost-effectiveness, based on the norms of hypertension treatment in primary care, the considered view of a panel of experts and the direct costs of health resources and purchase of medication. SETTING: Spanish National Health system. PARTICIPANTS: Simulation of 1000 patients with hypertension, with a time horizon of one year. INTERVENTIONS: After a prior failure of the first-line treatment with either enalapril or nitrendipine, an evaluation was made of the possibilities of increasing dosage of the first-line treatment, changing the drug or administering the E/N combination. MAIN MEASUREMENT: The likelihoods, in the primary care context, of controlling diastolic pressure, of abandonment and of using the two strategies or not were measured, as were the use of health resources in each situation, and costs of resource use and of medication. RESULTS: The cost-effectiveness quotient of the combined E/N treatment was consistently more efficient than the increase in dose or change to another drug. This was so, whether the treatment was started with enalapril (301.06 euros vs 337.97 euros and 588.42 euros) or with nitrendipine (331.5 euros vs 469.88 euros and 579.76 euros). CONCLUSIONS: Combined therapy (E/N) is, on the basis of the assumptions made in the model, an efficient therapy option. Therefore, it can be recommended for prescription.

Cost-Benefit Analysis↗

PubMed1996

Analysis of a theoretical model based on similarity for studying RNA base pairings.

A theoretical model for studying RNA base pairing is presented, based on similarity measures of the bases and corresponding indexes. Similarity calculations are made by evaluating atomic importance in molecules, a method that uses an original method for the calculation of electronic energy. The application of the model to both Watson-Crick and non-Watson-Crick pairings is commented on. Some theoretical considerations concerning the capability of the genetic code to repair dangerous mutations contribute to the ongoing debate.

Animals↗

PubMed1983

B-scan texture classification: a study using physical and theoretical models.

Recent clinical trials, in our own centre and elsewhere, have demonstrated the ability of A- and B-scan texture analysis techniques to provide useful in vivo separation of certain disease conditions. The present paper reports studies, using physical and theoretical models of tissues, aimed at identifying the particular sets of textural features which are most effective in achieving such separations. Using a 3.5 MHz, single element manual scanner, a total of 300 B-scans have been collected from four physical models consisting of gel-embedded 'point' scatterers, with differing mean separations. Additionally, computer modelling has been used to obtain a further 500 B-scans from 14 statistically different scattering situations. The total set of B-scans thus obtained has been subjected to a variety of texture classification procedures which have previously been developed for application in other imaging fields, such as cytology and aerial photography. The results are discussed in terms of: i) the features which are best suited to the classification of B-scans, and ii) the manner in which the resulting classification schemes are sensitive to variables of the ultrasonic scanning instrumentation.

Biometry↗

PubMed1988

A theoretical model for the hydraulic conductance of human dentine.

A theoretical model was devised that describes the dependence of the hydraulic conductance of dentine disks on the decrease in dentine tubule radius and dentine tubule density (number per area of cross-section) in the pulp-enamel direction. A parameter that describes the decrease in tubule radius was calculated from experimental results from the literature.

Chemical Phenomena↗

PubMed1975

A theoretical model for lipid monolayer phase transitions.

We present a theoretical model for the liquid-expanded to liquid-condensed phase transition observed in many phospholipid monolayer films. The total two-dimensional pressure in the model is the sum of the hydrocarbon chain pressure and the surface pressure. The hydrocarbon chain pressure is calculated in an exteded version of a model published earlier. The surface pressure results from a lowering of the surface tension in the monolayer over that of pure water, thus producing a force on a Langmuir float. When these two contributions are added, pi/A isotherms are obtained which have slope discontinuities very similar to those observed experimentally. The results indicate that a successful model for lipid phase behavior must consider the interactions between head groups and water as well as cooperative hydrocarbon chain melting.

Binding Sites↗

PubMed1983

A theoretical model for surface polarizability in phospholipid membranes.

A simple theoretical model based on statistical thermodynamics is proposed in order to correlate surface polarizability in membranes with frequency shifts in the electronic transitions of fluorescence probes. This model predicts a different behaviour for the absorption and emission processes of probes in lipid environments and suggests that as a first approximation these spectroscopic variations can be linearly related to the deformability (polarizability) of the ionic heads of the phospholipids. Some preliminary spectroscopic measurements suggest that this hypothesis might be a correct interpretation of the electronic shift observed in the spectral analysis of phospholipid membranes.

Anilino Naphthalenesulfonates↗

PubMed1995

Theoretical model of iontophoresis utilized in transdermal drug delivery.

A theoretical model of in vitro iontophoresis that utilizes radial mass transport is presented. The applicability of radial transport to the skin morphology of different species is determined by the density of low-resistance pores, which represents the density of skin appendages such as hair follicles or sweat glands. Radial transport is expected to dominate in the case where appendage density is low, in contrast to the linear transport mechanism that would dominate in the case of skin with high appendage density. Parameters such as the effective surface area of highly conductive pores and boundary-layer thickness are defined for the radial and linear transport limits and evaluated for the iontophoretic flux of pindolol hydrochloride through porcine skin. Based upon a decrease in the calculated boundary-layer thickness and an increase in drug flux with current density to a limiting value, it is concluded that radial mass transport is the dominant mechanism of iontophoretic transport under the experimental conditions described.

Administration, Cutaneous↗

PubMed2001

A control theoretic model of the forearm.

In this paper, a control theoretic model of the forearm is developed and analyzed, and a computational method for predicting muscle activations necessary to generate specified motions is described. A detailed geometric model of the forearm kinematics, including the carrying angle and models of how the biceps and the supinator tendons wrap around the bones, is used. Also, including a dynamics model, the final model is a system of differential equations where the muscle activations play the role of control signals. Due to the large number of muscles, the problem of finding muscle activations is redundant, and this problem is solved by an optimization procedure. The computed muscle activations for ballistic movements clearly recaptures the triphasic ABC (Activation-Braking-Clamping) pattern. It is also transparent, from the muscle activation patterns, how the muscles cooperate and counteract in order to accomplish desired motions. A comparison with previously reported experimental data is included and the model predictions can be seen to be partially in agreement with the experimental data.

Biomechanical Phenomena↗

PubMed1985

A theoretical model for calcium absorption from the intestinal lumen.

A theoretical model is developed for the absorption of calcium in the human gut and a mathematical description of the model is written. For simplicity, only three compartments are considered to derive a kinetic equation which can be fit to plasma activity measurements following oral ingestion of radiotracer calcium. Initial tests show good success in correlating total fractional absorption of calcium.

Calcium↗

PubMed2004

A theoretical model of the photoabsorption spectra of carbon buckyonions.

A theoretical model has been developed to predict the photoabsorption spectra of spherical carbon buckyonions in the region dominated by the pi-plasmon feature. This model makes use of the microscopic electronic structure of the system, which is provided by an effective Huckel one-electron model. The important screening effects are treated within the random phase approximation, whose form is an extension to the dynamic case of the one derived in a previous work for the static polarizabilities of these species. A systematic analysis as a function of the buckyonion size is performed. We compare the spectra obtained in this way with those derived from a different representation of the electron motion, namely a two-dimensional spherical electron gas, and from a classical dielectric model.

Journal Article↗

PubMed1971

Kinetics of heterogeneous chemical reactions: a theoretical model for the accumulation of pesticides in soil.

A theoretical model for the accumulation of pesticides in soil has been proposed and discussed from the viewpoint of heterogeneous reaction kinetics with a basic aim to understand the complex nature of soil processes relating to the environmental pollution. In the bulk of soil, the pesticide disappears by diffusion and a chemical reaction; the rate processes considered on the surface of soil are diffusion, chemical reaction, vaporization, and regular pesticide application. The differential equations involved have been solved analytically by the Laplace-transform method.

Chemical Phenomena↗

PubMed2005

How much do doctors use quality of life information in primary care? Testing the trans-theoretical model of behaviour change.

This study aimed to find out whether General Practitioners (GPs) use quality of life (QOL) information in primary care, to explore their reasoning and to assess any barriers to use. A second purpose was to see whether the Trans-Theoretical Model (TTM) of behaviour change could be applied to doctor's use of QOL information in primary care. A representative, cross-sectional sample of 800 GPs was approached in a national postal survey; 280 (38%) provided qualitative and quantitative information. Most GPs said that QOL was interesting and important. Users had seen more information and scales, and were more aware of its uses; only 8% had ever used formal standardised questionnaires. The main barriers to implementation were a shortage of time and information, and experience with using QOL assessment. A sizable minority wanted to know more. Seventy-one percent would use QOL to monitor treatment effectiveness. The five stages of the TTM were used to identify whether GP's were predisposed to use QOL information from their knowledge levels and values. While there was some support for the model, the TTM did not sensitively and reliably discriminate between users and non-users on all variables, and so has limited value here.

Attitude of Health Personnel↗

PubMed2005

Factors affecting resident performance: development of a theoretical model and a focused literature review.

PURPOSE: The clinical performances of physicians have come under scrutiny as greater public attention is paid to the quality of health care. However, determinants of physician performance have not been well elucidated. The authors sought to develop a theoretical model of physician performance, and explored the literature about factors affecting resident performance. METHOD: Using expert consensus panel, in 2002-03 the authors developed a hypothesis-generating model of resident performance. The developed model had three input factors (individual resident factors, health care infrastructure, and medical education infrastructure), intermediate process measures (knowledge, skills, attitudes, habits), and final health outcomes (affecting patient, community and population). The authors used factors from the model to focus a PubMed search (1967-2002) for all original articles related to the factors of individual resident performance. RESULTS: The authors found 52 original studies that examined factors of an individual resident's performance. They describe each study's measurement instrument, study design, major findings, and limitations. Studies were categorized into five domains: learning styles/personality, social/financial factors, practice preferences, personal health, and response to job environment. Few studies examined intermediate or final performance outcomes. Most were single-institution, cross-sectional, and survey-based studies. CONCLUSIONS: Attempting to understand resident performance without understanding factors that influence performance is analogous to examining patient adherence to medication regimens without understanding the individual patient and his or her environment. Based on a systematic review of the literature, the authors found few discrete associations between the factors of individual resident and the resident's actual job performance. Additionally, they identify and discuss major gaps in the educational literature.

Adult↗

PubMed2000

Scattering of ultrasound in cancellous bone: predictions from a theoretical model.

An understanding of the interaction between acoustic waves and cancellous bone is needed in order to realize the full clinical potential of ultrasonic bone measurements. Scattering is likely to be of central importance but has received little attention to date. In this study, we adopted a theoretical model from the literature in which scattering was assumed to be proportional to the mean fluctuation in sound speed, and bone was considered to be a random continuum containing identical scatterers. The model required knowledge only of sound speeds in bone and marrow, porosity, and scatter size. Predicted attenuation, broadband ultrasonic attenuation (BUA) and backscatter coefficient were obtained for a range of porosities and scatterer sizes, and were found to be comparable to published values for cancellous bone. Trends in predicted BUA with porosity agreed with previous experimental observations. All three predicted acoustic parameters showed a non-linear dependence on scatterer size which was independent of porosity. These data confirm the value of the scattering approach and provide the first quantitative predictions of the independent influence of structure and porosity on bone acoustic properties.

Bone and Bones↗

PubMed1989

Calculation of intermolecular interaction strengths in the P beta' phase in lipid bilayers. Implications for theoretical models.

The existence of the P beta' phase in certain lipid bilayers is evidence that molecular interactions between lipids are capable of producing unusual large-scale structures at or near biological conditions. The problem of identifying the specific intermolecular interactions responsible for the structures requires construction of theoretical models capable of clear predictions of the observable consequences of postulated intermolecular interactions. To this end we have carried out a twofold modeling effort aimed at understanding the ripple phase. First, we have performed detailed numerical calculations of potential energies of interaction between pairs and triplets of lipid molecules having different chain tilt angles and relative vertical alignments. The calculations support the notion that chain tilting in the gel phase is a result of successive 3-5-A displacements of neighboring molecules perpendicular to the bilayer plane rather than long-range cooperative chain tilting. Secondly, we have used these results as a guide to formulate a new lattice model for lipid bilayer condensed phases. The new model is less complex than our earlier model and it includes interactions which are, based on the energy calculations, more likely to be responsible for the ripple phase. In a certain limit the model maps onto the chiral clock model, a model of much interest in condensed matter theory. In this limit the model exhibits a chain-tilted ordered phase followed by (as temperature increases) a modulated phase followed by a disordered phase. Within this limit we discuss the properties of the model and compare structures of the modulated phase exhibited by the model with experimental data for the P beta' phase in lipid bilayers.

Lipid Bilayers↗

PubMed2002

A qualitative study of the theoretical models used by UK mental health nurses to guide their assessments with family caregivers of people with dementia.

Some researchers suggest that nurse training does not provide adequate theoretical knowledge to guide mental health nurses' work with carers of people with dementia. In recent years theoretical guidance for working with carers has emerged in the nursing literature. However, little attention has been given to theory practice links. This study used interpretative phenomenological analysis to investigate the theoretical model used by nurses to guide carer assessments. During the analysis the data evolved into a description of the model underlying the nurses' work. In the discussion this model was compared with the stress process model (SPM). This comparison highlighted striking similarities between the themes in the nurses' model of carer stress and the theoretical constructs of the SPM. However, the SPM describes influential links between different constructs which were absent in the nurses' model. The research illustrates developmental training needs for mental health nursing to improve the efficacy of assessments and therefore, interventions.

Adult↗

PubMed2003

A theoretical model of oxygen delivery and metabolism for physiologic interpretation of quantitative cerebral blood flow and metabolic rate of oxygen.

The coupling of cerebral blood flow (CBF) and metabolic rate of oxygen (CMRO2) during physiologic and pathophysiologic conditions remains the subject of debate. In the present study, we have developed a theoretical model for oxygen delivery and metabolism, which describes the diffusion of oxygen at the capillary-tissue interface and the nonlinear nature of hemoglobin (Hb) affinity to oxygen, allowing a variation in simple-capillary oxygen diffusibility, termed "effective oxygen diffusibility (EOD)." The model was used to simulate the relationship between CBF and CMRO2, as well as oxygen extraction fraction (OEF), when various pathophysiologic conditions were assumed involving functional activation, ischemia, hypoxia, anemia, or hypo- and hyper-capnic CBF variations. The simulations revealed that, to maintain CMRO2 constant, a variation in CBF and Hb required active change in EOD. In contrast, unless the EOD change took place, the brain allowed small but significant nonlinear change in CMRO2 directly dependent upon oxygen delivery. Application of the present model to quantitative neuroimaging of CBF and CMRO2 enables us to evaluate the biologic response at capillary level other than Hb- and flow-dependent properties of oxygen transport and may give us another insight regarding the physiologic control of oxygen delivery in the human brain.

Biological Transport↗