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Results for “Models, Theoretical”

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At least 307 records · Page 17Linked to original sources

PubMed1998

Cost-effectiveness of the polymerase chain reaction versus smear examination for the diagnosis of tuberculosis in Kenya: a theoretical model.

SETTING: A major out-patient tuberculosis clinic in Nairobi, Kenya. OBJECTIVE: To ascertain the cost-effectiveness of the polymerase chain reaction (PCR) for the diagnosis of tuberculosis in an urban setting in a developing country. DESIGN: A cost-effectiveness analysis of PCR and direct smear microscopy examination based on theoretical modelling. The cost-effectiveness was expressed in costs per correctly diagnosed tuberculosis patient for each of the two diagnostic techniques. Data were obtained from the literature, from the staff and the register at the health facility and from structured interviews with patients. Assumptions were made when no data were available. RESULTS: The PCR is expected to be more specific and sensitive than the routine procedure for diagnosis, but it is also more costly. The routine procedure based on direct smear microscopy turned out to be 1.8 times as cost-effective as PCR. CONCLUSION: It is concluded that the PCR method can potentially be a cost-effective screening procedure for tuberculosis, provided that the largest contributing cost component, the costs of the PCR-kit, can be reduced substantially.

Bacteriological Techniques↗

PubMed2004

A theoretical model of type I collagen proteolysis by matrix metalloproteinase (MMP) 2 and membrane type 1 MMP in the presence of tissue inhibitor of metalloproteinase 2.

One well documented family of enzymes responsible for the proteolytic processes that occur in the extracellular matrix is the soluble and membrane-associated matrix metalloproteinases. Here we present the first theoretical model of the biochemical network describing the proteolysis of collagen I by matrix metalloproteinases 2 (MMP2) and membrane type 1 matrix metalloproteinases (MT1-MMP) in the presence of the tissue inhibitor of metalloproteinases 2 (TIMP2) in a bulk, cell-free, well stirred environment. The model can serve as a tool for describing quantitatively the activation of the MMP2 proenzyme (pro-MMP2), the ectodomain shedding of MT1-MMP, and the collagenolysis arising from both of the enzymes. We show that pro-MMP2 activation, a process that involves a trimer formation of the proenzyme with TIMP2 and MT1-MMP, is suppressed at high inhibitor levels and paradoxically attains maximum only at intermediate TIMP2 concentrations. We also calculate the conditions for which pro-MMP2 activation is maximal. Furthermore we demonstrate that the ectodomain shedding of MT1-MMP can serve as a mechanism controlling the MT1-MMP availability and therefore the pro-MMP2 activation. Finally the proteolytic synergism of MMP2 and MT1-MMP is introduced and described quantitatively. The model provides us a tool to determine the conditions under which the synergism is optimized. Our approach is the first step toward a more complete description of the proteolytic processes that occur in the extracellular matrix and include a wider spectrum of enzymes and substrates as well as naturally occurring or artificial inhibitors.

Animals↗

PubMed1995

Coeur en santé St-Henri--a heart health promotion programme in a low income, low education neighbourhood in Montreal, Canada: theoretical model and early field experience.

STUDY OBJECTIVE: Coeur en santé St-Henri is a five year, community based, multifactorial, heart health promotion programme in a low income, low education neighbourhood in Montreal, Canada. The objectives of this programme are to improve heart-healthy behaviours among adults of St-Henri. This paper describes the theoretical model underlying programme development as well as our early field experience implementing interventions. DESIGN: The design of the intervention programme is based on a behaviour change model adapted from social learning theory, the reasoned action model, and the precede-proceed model. The Ottawa charter for health promotion provided the framework for the development of specific interventions. Each intervention is submitted to formative, implementation, and impact evaluations using simple and inexpensive methods. PARTICIPANTS: The target population consists of adults living in St-Henri, a neighbourhood of 23,360 residents. Because of costs constraints, the intervention strategy targets women more specifically. The community is one of the poorest in Canada with 46% of the population living below the poverty line and 20% being very poor. The age-sex adjusted ischaemic heart disease mortality in 1985-87 was 317 per 100,000 compared with 126 per 100,000 in an affluent adjacent neighbourhood. RESULTS: Thirty nine distinct interventions have been developed and tested in the community, eight related to tobacco, 10 to diet, seven to physical activity, and 14 which are multifactorial. The interventions include smoking cessation and healthy recipes contests, a menu labelling and healthy food discount programme in restaurants, a point of choice nutrition education campaign, healthy eating and smoking cessation workshops, a walking club, educational material, print and electronic media campaigns, heart health fairs, and community events. CONCLUSION: An integrated heart health promotion programme is feasible in low income urban neighbourhoods but not all interventions are successful. Such a programme requires substantial energy and resources as well as long term commitment from public health departments.

Cardiovascular Diseases↗

PubMed2003

Theoretical models of cognitive aging and implications for translational research in medicine.

PURPOSE: We provide an overview of theoretical models of cognitive aging and present empirical research that uses these models to explain older patients' medical behaviors and to develop interventions for improving the delivery of health information and services to older adults. DESIGN AND METHODS: Theoretical accounts of age and cognition are summarized and are related to key research findings, including age differences in comprehension of medical information, adherence, and use of medical technologies. The implications of cognitive aging theories for designing optimal medical environments and enhancing adherence are discussed. RESULTS: Age declines in basic cognitive abilities such as working memory capacity limit older adults' ability to comprehend and recall several types of novel medical information. In contrast, automatic processes and environmental cues can benefit older adults, as shown by age increases in compliance for practiced or mentally imaged health behaviors, but can also be dangerous, as shown by older adults' greater belief in false but familiar health statements. Last, cognitive aging is shown to be a useful model for investigating cognitive disorders. IMPLICATIONS: Cognitive aging theories are important for understanding older patient behaviors and suggest age-related declines as well as gains in performance in medical domains. In addition, models of cognitive aging are useful in guiding the development of interventions that improve health care in older patients and in furthering our understanding of cognitive dysfunction in populations other than older adults.

Aged↗

PubMed1999

Nursing home care quality: a multidimensional theoretical model integrating the views of consumers and providers.

This exploratory study was undertaken to discover the defining dimensions of nursing home care quality from the viewpoint of consumers of nursing home care. Eleven focus groups were conducted in five Missouri communities. The seven dimensions of the consumer multidimensional model of nursing home care quality are: staff, care, family involvement, communication, environment, home, and cost. The views of consumers and families are compared with the results of a previous study of providers of nursing home services. An integrated, multidimensional theoretical model is presented for testing and evaluation. An instrument based on the model is being tested to observe and score the dimensions of nursing home care quality.

Attitude of Health Personnel↗

PubMed2006

Theoretical modeling of the oxidation of hydrazine by iron(II) phthalocyanine in the gas phase. Influence of the metal character.

The hydrazine oxidation by iron(II) phthalocyanine (Fe(II)Pc) has been studied using an energy profile framework through quantum chemistry theoretical models calculated in the gas phase at the density functional theory B3LYP/LACVP(d) level. We applied two models of charge-transfer mechanisms previously reported (J. Phys. Chem. A 2005, 109, 1196) for the hydrazine oxidation mediated by Co(II)Pc. Model 1 consists of an alternated loss of one electron and one proton, involving anionic and neutral species. Model 2 considers an alternated loss of two electrons and two protons and includes anionic, neutral, and cationic species. Both applied models describe how the charge-transfer process occurs. In contrast with the obtained results for Co(II)Pc, we found that the hydrazine oxidation mediated by Fe(II)Pc is a fully through-bond charge-transfer mechanism. On the other hand, the use of different charge-transfer descriptors (spin density, electronic population, condensed Fukui function) showed a major contribution of the iron atom in comparison with the cobalt atom in the above-mentioned process. These results could explain the higher catalytic activity observed experimentally for Fe(II)Pc in comparison with Co(II)Pc. The applied theoretical models are a good starting point to rationalize the charge-transfer process of hydrazine oxidation mediated by Fe(II)Pc.

Ferrous Compounds↗

PubMed2001

Capillary electrophoretic separation of uncharged polymers using polyelectrolyte engines. Theoretical model.

We recently demonstrated that the molecular mass distribution of an uncharged polymer sample can be analyzed using free-solution capillary electrophoresis of DNA-polymer conjugates. In these conjugates, the DNA is providing the electromotive force while the uncharged polydisperse polymer chains of the sample retard the DNA engine with different amounts of hydrodynamic drag. Here we present a theoretical model of this new analytical method. We show that for the most favourable, diffusion-limited electrophoresis conditions, there is actually an optimal DNA size to achieve the separation of a given polymer sample. Moreover, we demonstrate that the effective friction coefficient of the polymer chains is related to the stiffness of the two polymers of the conjugate, thus offering a method to estimate the persistence length of the uncharged polymer through mobility measurements. Finally, we compare some of our predictions with available experimental results.

Biopolymers↗

PubMed2006

Theoretical model for a tetrad of hydrogen bonds and its application to interpretation of infrared spectra of salicylic acid.

Theoretical model of vibrational interactions in hydrogen-bonded salicylic acid dimer is presented which takes into account the adiabatic couplings between high- and low-frequency O-H and O...O stretching vibrations, resonance interactions between both intermolecular hydrogen bonds and between inter- and intramolecular hydrogen bonds, and Fermi resonance between the O-H stretching fundamental and the first overtone of the O-H in-plane bending vibrations. The model is used for theoretical simulation of the nu(s) stretching bands of salicylic acid and its OD derivative at 300 K. The effect of deuteration is successfully reproduced by our model. Infrared, far infrared, Raman, and low-frequency Raman spectra of the polycrystalline salicylic acid and its deuterated derivative have been measured. The geometry and experimental frequencies are compared with the results of density-functional theory calculations performed at the B3LYP6-31 ++ G**, B3LYP/cc-pVTZ, B3PW916-31 ++ G**, and B3PW91/cc-pVTZ levels. O-H, O-D, and O...O stretching frequencies are used in theoretical simulation of the nu(s) stretching bands.

Journal Article↗

PubMed2004

Theoretical modeling study of the necrotic field during high-intensity focused ultrasound surgery.

BACKGROUND: High-intensity focused ultrasound (HIFU) is a type of micro-invasive treatment of tumor. It is important to create a guideline for the volume and position of the necrotic field by means of a theoretical model, this being one of the key problems in the clinic application of HIFU. MATERIAL/METHODS: Based on cellular thermo-necrotic theory, a computational model of temperature distribution and, therefore, the necrotic field was developed. A multi-transducer system with non-interferential and self-focused property was devised to provide experimental verification of the theoretical model. RESULTS: The necrotic field resulting from the modeling simulation did not show any evident difference from that of the experiment. The necrotic field of the same heating duration (4 seconds) was found with both modeling and experiment to be spheroid, with its center at the system's geometric focus and a volume of 1x1x2 cm3. CONCLUSIONS: The model of this study may predict the practical volume of the necrotic field as well as the time needed for it to form.

Computer Simulation↗

PubMed2003

Prediction of the amount of urban waste solids by applying a gray theoretical model.

Urban waste solids are now becoming one of the most crucial environmental problems. There are several different kinds of technologies normally used for waste solids disposal, among which landfill is more favorable in China than others, especially for urban waste solids. Most of the design works up to now are based on a roughly estimation of the amount of urban waste solids without any theoretical support, which lead to a series problems. To meet the basic information requirements for the design work, the amount of the urban waste solids was predicted in this research by applying the gray theoretical model GM (1,1) through non-linear differential equation simulation. The model parameters were estimated with the least square method (LSM) by running a certain MATALAB program, and the hypothesis test results show that the residual between the prediction value and the actual value approximately comply with the normal distribution N (0, 0.21(2)), and the probability of the residual within the range ( -0.17, 0.19) is more than 95%, which indicate obviously that the model can be well used for the prediction of the amount of waste solids and those had been already testified by the latest two years data about the urban waste solids from Loudi City of China. With this model, the predicted amount of the waste solids produced in Loudi City in the next 30 years is 8049000 ton in total.

Cities↗

PubMed2004

Theoretical model for the formation of caveolae and similar membrane invaginations.

We study a physical model for the formation of bud-like invaginations on fluid lipid membranes under tension, and apply this model to caveolae formation. We demonstrate that budding can be driven by membrane-bound proteins, provided that they exert asymmetric forces on the membrane that give rise to bending moments. In particular, caveolae formation does not necessarily require forces to be applied by the cytoskeleton. Our theoretical model is able to explain several features observed experimentally in caveolae, where proteins in the caveolin family are known to play a crucial role in the formation of caveolae buds. These include 1), the formation of caveolae buds with sizes in the 100-nm range and 2), that certain N- and C-termini deletion mutants result in vesicles that are an order-of-magnitude larger. Finally, we discuss the possible origin of the morphological striations that are observed on the surfaces of the caveolae.

Animals↗

PubMed1986

An experimental test of new theoretical models for the electrokinetic properties of biological membranes. The effect of UO2++ and tetracaine on the electrophoretic mobility of bilayer membranes and human erythrocytes.

For a large smooth particle with charges at the surface, the electrophoretic mobility is proportional to the zeta potential, which is related to the charge density by the Gouy-Chapman theory of the diffuse double layer. This classical model adequately describes the dependence of the electrophoretic mobility of phospholipid vesicles on charge density and salt concentration, but it is not applicable to most biological cells, for which new theoretical models have been developed. We tested these new models experimentally by measuring the effect of UO2++ on the electrophoretic mobility of model membranes and human erythrocytes in 0.15 M NaCl at pH 5. We used UO2++ for these studies because it should adsorb specifically to the bilayer surface of the erythrocyte and should not change the density of fixed charges in the glycocalyx. Our experiments demonstrate that it forms high-affinity complexes with the phosphate groups of several phospholipids in a bilayer but does not bind significantly to sialic acid residues. As observed previously, UO2++ adsorbs strongly to egg phosphatidylcholine (PC) vesicles: 0.1 mM UO2++ changes the zeta potential of PC vesicles from 0 to +40 mV. It also has a large effect on the electrophoretic mobility of vesicles formed from mixtures of PC and the negative phospholipid phosphatidylserine (PS): 0.1 mM UO2++ changes the zeta potential of PC/PS vesicles (10 mol % PS) from -13 to +37 mV. In contrast, UO2++ has only a small effect on the electrophoretic mobility of either vesicles formed from mixtures of PC and the negative ganglioside GM1 or erythrocytes: 0.1 mM UO2++ changes the apparent zeta potential of PC/GM1 vesicles (17 mol % GM1) from -11 to +5 mV and the apparent zeta potential of erythrocytes from -12 to -4 mV. The new theoretical models suggest why UO2++ has a small effect on PC/GM1 vesicles and erythrocytes. First, large groups (e.g., sugar moieties) protruding from the surface of the PC/GM1 vesicles and erythrocytes exert hydrodynamic drag. Second, charges at the surface of a particle (e.g., adsorbed UO2++) exert a smaller effect on the mobility than charges located some distance from the surface (e.g., sialic acid residues).

Electrophoresis↗

PubMed2002

[Borderline personality disorder and attentional biases. Theoretical models and empirical findings].

The purpose of this review is to analyse nature, degree and consequences of attentional biases in patients with borderline personality disorder based on existing literature. The clinical importance of these phenomena in patients with borderline personality disorder is strengthened and the link between theoretical models, empirical findings and therapeutic interventions is elaborated. This link between selective attention and borderline personality disorder is demonstrated on the basis of studies, which indicate a context between borderline pathology, affective disorders, anxiety disorders, and attention-deficit/hyperactivity disorders and also give evidence to neuropsychological deficits in patients with borderline personality disorder. The present article comparatively describes three theoretical models of borderline personality disorder and their possible links to attentional biases. The dialectical behavior approach postulates a pronounced attentional narrowing to emotional stimuli in connection with deficits regarding affect regulation. The cognitive approach assumes that rigid dysfunctional schemata initiate a cognitive-emotional circuit, which leads to a reinforcement of the symptomatology as a result of a biased perception and memory. Additionally the present article specifies the trauma approach which emphasizes the relation between borderline pathology and childhood trauma with recourse to theories of attentional biases in patients with traumatic experiences. Preliminary empiricial findings suggest that patients with borderline personality disorder demonstrate a general attentional bias for any emotional negative stimuli and not a selective attentional bias to borderline-specific stimuli. Further studies should clarify, whether therapeutical interventions with the aim to influence attentional processes represent a useful complement to established therapies in patients with borderline personality disorder.

Attention↗

PubMed2005

Theoretical model of prion propagation: a misfolded protein induces misfolding.

There is a hypothesis that dangerous diseases such as bovine spongiform encephalopathy, Creutzfeldt-Jakob, Alzheimer's, fatal familial insomnia, and several others are induced by propagation of wrong or misfolded conformations of some vital proteins. If for some reason the misfolded conformations were acquired by many such protein molecules it might lead to a "conformational" disease of the organism. Here, a theoretical model of the molecular mechanism of such a conformational disease is proposed, in which a metastable (or misfolded) form of a protein induces a similar misfolding of another protein molecule (conformational autocatalysis). First, a number of amino acid sequences composed of 32 aa have been designed that fold rapidly into a well defined native-like alpha-helical conformation. From a large number of such sequences a subset of 14 had a specific feature of their energy landscape, a well defined local energy minimum (higher than the global minimum for the alpha-helical fold) corresponding to beta-type structure. Only one of these 14 sequences exhibited a strong autocatalytic tendency to form a beta-sheet dimer capable of further propagation of protofibril-like structure. Simulations were done by using a reduced, although of high resolution, protein model and the replica exchange Monte Carlo sampling procedure.

Amino Acid Sequence↗

PubMed2006

A theoretical model evaluating the angular distribution of luminescence emission in X-ray scintillating screens.

The aim of this study was to examine the angular distribution of the light emitted from radiation-excited scintillators in medical imaging detectors. This distribution diverges from Lambert's cosine law and affects the light emission efficiency of scintillators, hence it also affects the dose burden to the patient. In the present study, the angular distribution was theoretically modeled and was used to fit experimental data on various scintillator materials. Results of calculations revealed that the angular distribution is more directional than that predicted by Lambert's law. Divergence from this law is more pronounced for high values of light attenuation coefficient and thick scintillator layers (screens). This type of divergence reduces light emission efficiency and hence it increases the incident X-ray flux required for a given level of image brightness.

Diagnostic Imaging↗

PubMed1991

A theoretical model for the histamine H2-receptor.

We describe an electronic and conformational study of histamine H2-receptor ligands of the imidazole series, in which the possibilities of configurational isomerism (the thiourea group) and N3H and N1H tautomerism (imidazole ring) were considered. The results suggest that the conformational flexibility of the molecules and the properties of the imidazole ring are of special importance in the display of H2-receptor activity. A theoretical model of histamine H2-receptor interactions is proposed on the basis of these and other results. A very important characteristic of our model is its ability to explain H2-receptor activation by compounds which differ structurally, and to explain antagonism at the same receptor. The stereospecificity of rigid analogues of cimetidine and tiotidine, and the importance of chain length in flexible histamine H2-antagonists are also accounted for.

Burimamide↗

PubMed2003

A theoretical model for the regulation of Sex-lethal, a gene that controls sex determination and dosage compensation in Drosophila melanogaster.

Cell fate commitment relies upon making a choice between different developmental pathways and subsequently remembering that choice. Experimental studies have thoroughly investigated this central theme in biology for sex determination. In the somatic cells of Drosophila melanogaster, Sex-lethal (Sxl) is the master regulatory gene that specifies sexual identity. We have developed a theoretical model for the initial sex-specific regulation of Sxl expression. The model is based on the well-documented molecular details of the system and uses a stochastic formulation of transcription. Numerical simulations allow quantitative assessment of the role of different regulatory mechanisms in achieving a robust switch. We establish on a formal basis that the autoregulatory loop involved in the alternative splicing of Sxl primary transcripts generates an all-or-none bistable behavior and constitutes an efficient stabilization and memorization device. The model indicates that production of a small amount of early Sxl proteins leaves the autoregulatory loop in its off state. Numerical simulations of mutant genotypes enable us to reproduce and explain the phenotypic effects of perturbations induced in the dosage of genes whose products participate in the early Sxl promoter activation.

Animals↗

PubMed1999

Articular cartilage biomechanics: theoretical models, material properties, and biosynthetic response.

Articular cartilage has unique material properties that enable the cartilage to perform its physiological functions over a lifetime and under a wide range of loading conditions. Numerous studies have investigated the relationship between cartilage properties and composition/structure. For cartilage transplantation and regeneration, it is necessary to know how cartilage maintains its functionality and how cartilage responds to the ever-changing mechanical environment. In this review, we discuss theoretical and experimental studies on the behavior of articular cartilage to load. In the first part, the composition and structure of articular cartilage is presented. In the second part, theoretical models of the mechanical behavior of cartilage, experimental methods for the determination of cartilage properties, and material properties for normal, pathologic, and repair cartilage are summarized. In the third part, the relationship between mechanical loading of the cells and their corresponding biological responses are discussed. The goal for treating joint degeneration in the future lies in cartilage regeneration rather than prosthetic replacement. In order to achieve this goal, it has to be understood how structure and function, metabolic and biochemical properties, and biomechanical performance of articular cartilage can be restored.

Animals↗