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PubMed1992

A theoretical model for respirator cartridge service life for binary systems: application to acetone/styrene mixtures.

A theoretical model, developed previously to assess respirator cartridge service life, was applied to various acetone/styrene binary assault systems. Experimental data, collected for several binary mixtures differing only with respect to the concentration of each of the two compounds, were interpreted in terms of the model. Styrene concentrations varied from 228 to 1578 ppm; the range of acetone concentrations was 92-985 ppm. The specific influence of the compound assault concentrations on respirator cartridge service life was carefully characterized, as break-through curves were generated for both acetone and styrene for each of several different binary systems. Specifically, experimental data for each system were used to determine values of the following theoretical parameters: k'1, tau 1, k'2, tau 2, and Am. These parameters were employed with the theory to generate complete theoretical breakthrough curves and to determine the time-dependence of the weight of each compound adsorbed by the respirator cartridge carbon bed. An interesting phenomenon observed for the acetone/styrene systems was the displacement (from the carbon) of previously adsorbed acetone molecules by styrene molecules. Acetone breakthrough was observed first in each of the systems studied. Following the onset of this breakthrough, the acetone breakthrough concentration was enhanced by the displacement of acetone from the carbon bed by the adsorption of styrene. The theoretical model accurately predicts both this enhancement and the associated breakthrough characteristics of styrene. In addition, the theory is capable of predicting the ratio of the number of displaced acetone molecules to the corresponding number of displacing styrene molecules. For these studies, this ratio ranged from 0.3 to 0.7. The service life of respirator cartridges exposed to acetone/styrene mixtures depends on the assault concentration of each compound and is significantly influenced (shortened) by the displacement phenomenon.

Acetone↗
PubMed2005

Structure of plant photosystem I revealed by theoretical modeling.

Photosystem (PS) I is a large membrane protein complex vital for oxygenic photosynthesis, one of the most important biological processes on the planet. We present an "atomic" model of higher plant PSI, based on theoretical modeling using the recent 4.4 angstroms x-ray crystal structure of PSI from pea. Because of the lack of information on the amino acid side chains in the x-ray structural model and the high cofactor content in this system, novel modeling techniques were developed. Our model reveals some important structural features of plant PSI that were not visible in the crystal structure, and our model sheds light on the evolutionary relationship between plant and cyanobacterial PSI.

Amino Acid Sequence↗
PubMed2004

Evaluation of a theoretical model to quantify the sources of metabolic cost in walking.

OBJECTIVE: To evaluate the validity of a theoretical model of walking in which the oxygen uptake (V(O2)) is described as a function of speed by an equation in the form y = ax + b, with constant a representing the metabolic cost for performing the walking movement, and constant b representing the sum of the metabolic costs for basal metabolism and maintaining balance and posture. DESIGN: Repeated-measures analysis of variance was used to analyze our theoretical model. In a human exercise research laboratory, 12 healthy male subjects walked on a level treadmill at speeds of 0.39-1.83 m/sec under a control condition, while wearing "instability" shoes with peripheral vision obstructed, and with 2-kg weights around each wrist. Total transported mass was the same under each condition through the carriage of 4 kg in a back pack during the control and instability conditions. Outcome was determined by equations describing (V(O2)) as a function of speed and by kinematics of the center of mass. RESULTS: The constant b was higher (P < 0.01) for the instability condition than the other conditions, but constant a did not differ among the conditions. However, external work was greatest (P < 0.05) for the instability condition. CONCLUSIONS: Because the kinematic data demonstrate that the instability condition increased the metabolic cost for performing the walking movement compared with the control condition, and there was no difference among conditions in constant a, the theoretical model seems invalid.

Adult↗
PubMed2000

A novel method for analysis of the periodicity of chondrogenic patterns in limb bud cell culture: correlation of in vitro pattern formation with theoretical models.

To experimentally examine whether the pattern changes predicted by theoretical models of pattern formation actually occur in a limb bud cell culture system, we developed a practical method to automatically measure the periodicity of chondrogenic patterns in vitro. The method utilizes binary image processing to quantify the total number of peak and valley pixels in a pattern to obtain the average interval between stripes in the chondrogenic pattern, and we named it the peak length method. The reliability of the peak length method was examined by using computer simulation results. The peak length method enabled us quantitatively obtain the average interval between chondrogenic islands, and the values obtained by this method were closely correlated with the average intervals obtained by manual measurement and two-dimensional Fourier transformation. The average intervals obtained by the peak length method were shown to be stable over a wide range of pattern variations that are frequently observed in actual experiments. By applying the peak length method to actual experimental data, we compared the validity of two theoretical models of pattern formation (cell sorting model and reaction-diffusion model) and it was concluded that the peak-length method is a useful tool to quantitatively analyze chondrogenic patterns in limb micromass culture and to relate theoretical predictions and experimental results of pattern formation.

Animals↗
PubMed2004

A theoretical model of job retention for home health care nurses.

BACKGROUND: Predicted severe nursing shortages and an increasing demand for home health care services have made the retention of experienced, qualified nursing staff a priority for health care organizations. AIMS: The purpose of this paper is to describe a theoretical model of job retention for home health care nurses. METHODS: The theoretical model is an integration of the findings of empirical research related to intent to stay and retention, components of Neal's theory of home health care nursing practice and findings from earlier work to develop an instrument to measure home health care nurses' job satisfaction. DISCUSSION: The theoretical model identifies antecedents to job satisfaction of home health care nurses. The antecedents are intrinsic and extrinsic job characteristics. The model also proposes that job satisfaction is directly related to retention and indirectly related to retention though intent to stay. Individual nurse characteristics are indirectly related to retention through intent to stay. The individual characteristic of tenure is indirectly related to retention through autonomy, as an intrinsic characteristic of job satisfaction, and intent to stay. CONCLUSIONS: The proposed model can be used to guide research that explores gaps in knowledge about intent to stay and retention among home health care nurses.

Community Health Nursing↗
PubMed2001

State legislation concerning individuals with dementia: an evaluation of three theoretical models of policy formation.

PURPOSE: This study examined the formation of state government policies concerning persons with dementia. In particular, we identified variables associated with the passage of pertinent state laws by testing three theoretical models of policy formation: the iron triangle, the policy system, and an integrated model. METHODS: We sampled 44 states and counted the number of laws concerning individuals with dementia that passed during the 1998 legislative sessions. We constructed nine independent variables to represent the three theoretical models of state policy formation. Then the number of legislative actions were regressed onto each model. RESULTS: The integrated model that consisted of political actors, legislative features, and environmental inputs met goodness-of-fit criteria for a maximum likelihood regression analysis (chi2/df = 1.55). The advocacy effort of the Alzheimer's Association, number of legislative champions, supply of special care beds for persons with dementia, and recent policy activity were related significantly with increased legislative activity concerning individuals with dementia. IMPLICATIONS: Policy responses concerning persons with dementia increasingly have been created within state governments, and we identified variables associated with the passage of pertinent state laws. This research also contributed to the advancement of comparative state policy research by contrasting the three theoretical models of policy formation.

Aged↗
PubMed1997

A theoretical model of erosion and macromolecular drug release from biodegrading microspheres.

A theoretical model is outlined for predicting the time evolution of total mass, mean molecular weight, and drug release for the case of a spherical bulk-eroding microsphere, prepared by a double emulsification procedure and containing a hydrophilic drug, such as a protein or peptide. Explicit analytical formulae are derived for calculating the time evolution of measurable macroscopic characteristics, such as drug release or mean molecular weight. Microsphere hydration, polymer erosion, and drug release phases are each described. Results indicate that polymer degradation by only random-chain scission or only end scission (or unzipping) cannot explain experimentally observed kinetics of particle mass loss and molecular weight change; thus, a combined model (incorporating both random and end scission) is proposed. A general methodology for determining the microscopic transport coefficients (such as polymer degradation rate constant or drug diffusion coefficient) from erosion and release data is outlined. This paradigm is applied to the specific case of 50:50 poly(D,L-lactic-co-glycolic acid (PLGA) microspheres encapsulating glycoprotein 120 (gp 120), a candidate AIDS vaccine. Predictions permit comparisons with experimental data for mean weight- and number-averaged molecular weights, as well as for mass loss and protein release. Other comparisons are made with data appearing in the literature for release of tetanus toxoid from PLA and PLGA microspheres of variable molecular weight. Agreement between theory and experiment is observed.

AIDS Vaccines↗
PubMed1990

A theoretical model for the practice of residential treatment.

This paper presents a theoretical model describing the practice of psychiatric residential treatment for children and adolescents. The emphasis is on forty practice principles, guiding concepts which dictate the specific treatment techniques and administrative procedures for the Southern Oregon Adolescent Study and Treatment Center. These principles are grouped into six clusters, each a critical area of concern for residential treatment: program organization, physical environment, program personnel, clinical practices, therapeutic milieu, and interpersonal relationships.

Adolescent↗
PubMed2000

Occupational disability related to back pain: application of a theoretical model of work disability using prospective cohorts of manual workers.

BACKGROUND: A new model of work disability was developed based on the assumption that four different groups of workers are present at the beginning of a prospective epidemiologic study: one group of workers without back pain, and three groups of workers with back pain and a gradient of work disability. The goal of this research was to verify if these groups comprise workers at different levels of risk of occurrence of complete work disability related to back injury. METHODS: Prospective cohorts of manual workers (n=578) were followed for 1 year to document the risk of occurrence of complete disability related to back injury. RESULTS: The results showed that the workers who presented with back pain without work disability at the beginning of the study were at less risk compared to all the other workers in the cohort. Moreover, an effect modification was found between the workers who initially presented with back pain without work disability and a past history of compensation for back injury, adding credence to the non-similarity of these workers to the others. CONCLUSIONS: Based on these results, further studies should focus on improving the knowledge of the characteristics of these workers leading to a better understanding of how to prevent occupational low-back pain.

Adult↗
PubMed2004

Analysis of SNPs in pooled DNA: a decision theoretic model.

Systematic analysis of the genetic background of complex diseases using single nucleotide polymorphisms (SNPs) affords a tremendous amount of genotypings. To reduce the amount of genotypings necessary and hence the overall cost of a case-control study with SNPs, the genotyping is often performed in two stages. In the first, the DNA of all cases and all controls are mixed into two pools and genotyped for each SNP. The frequency of both alleles is determined in both pooled DNA samples. If different frequencies are observed in the pools of cases and controls, genotyping is performed individually in the second stage and analyzed conventionally. However, so far no well-founded algorithm is available to guide the decision on whether to genotype a SNP individually. In this report, an approach is introduced for the decision on individual genotyping based on the results from pooled DNA. The analysis is modeled as a decision process with the specific goal to decide on whether to genotype a specific SNP individually. For a given situation, the resulting decision criteria are aimed to be optimal for those conducting the study. Different loss functions and decision rules are presented. Using Monte-Carlo simulations, we show that for a given situation, the genotyping rates and hence the costs can be reduced remarkably while maintaining acceptable overall error rates.

Case-Control Studies↗
PubMed2006

Nonlinear local electrovascular coupling. I: A theoretical model.

Here we present a detailed biophysical model of how brain electrical and vascular dynamics are generated within a basic cortical unit. The model was obtained from coupling a canonical neuronal mass and an expandable vasculature. In this proposal, we address several aspects related to electroencephalographic and functional magnetic resonance imaging data fusion: (1) the impact of the cerebral architecture (at different physical levels) on the observations; (2) the physiology involved in electrovascular coupling; and (3) energetic considerations to gain a better understanding of how the glucose budget is used during neuronal activity. The model has three components. The first is the canonical neural mass model of three subpopulations of neurons that respond to incoming excitatory synaptic inputs. The generation of the membrane potentials in the somas of these neurons and the electric currents flowing in the neuropil are modeled by this component. The second and third components model the electrovascular coupling and the dynamics of vascular states in an extended balloon approach, respectively. In the first part we describe, in some detail, the biophysical model and establish its face validity using simulations of visually evoked responses under different flickering frequencies and luminous contrasts. In a second part, a recursive optimization algorithm is developed and used to make statistical inferences about this forward/generative model from actual data.

Brain↗
PubMedDate not supplied

A theoretical model for preventing osteoarthritis-related disability.

Osteoarthritis (OA) affects many aspects of life for affected individuals. Effective interventions to prevent and restore function must be based upon an understanding of what contributes to OA and its associated disabilities. A hypothetical OA disability model built upon the previous work of Nagi (1991), Verbrugge and Jette (1994), the International Classification of Functioning of World Health Organization (World Health Organization, 2001), and other scientific findings is proposed. The model includes a main disease pathway, which describes the sequence of events from OA-associated impairments to disabilities. Contextual factors influencing the process include individual characteristics, psychological state, coping style, comorbidities, social support, and physical environment. The model provides a useful conceptual framework for understanding the OA disability process from a biopsychosocial perspective and for guiding rehabilitation nursing interventions in OA care.

Activities of Daily Living↗
PubMed2003

Inter- and intramolecular potential for the N-formylglycinamide-water system. A comparison between theoretical modeling and empirical force fields.

An intramolecular NEMO potential is presented for the N-formylglycinamide molecule together with an intermolecular potential for the N-formylglycinamide-water system. The intramolecular N-formylglycinamide potential can be used as a building block for the backbone of polypeptides and proteins. Two intramolecular minima have been obtained. One, denoted as C5, is stabilized by a hydrogen bonded five member ring, and the other, denoted as C7, corresponds to a seven membered ring. The interaction between one water molecule and the N-formylglycinamide system is also studied and compared with Hartree-Fock SCF calculations and with the results obtained for some of the more commonly used force fields. The agreement between the NEMO and SCF energies for the complexes is in general superior to that of the other force fields. In the C7 region the surfaces obtained from the intramolecular part of the commonly used force fields are too flat compared to the NEMO potential and the ab initio calculations. We further analyze the possibility of using a charge distribution obtained from one conformation to describe the charge distribution of other conformations. We have found that the use of polarizabilities and generic dipoles can model most of the changes in charge density due to the different geometry of the new conformations, but that one can expect additional errors in the interaction energies that are of the order of 1 kcal/mol.

Algorithms↗
PubMed2005

Delay in seeking care for symptoms of acute myocardial infarction: applying a theoretical model.

Thirty percent of people who experience symptoms of acute myocardial infarction (AMI) do not seek care until more than 2-6 hours after onset of symptoms, increasing their risk for morbidity and mortality. Using a model based on two frameworks, the common sense model of illness representation (CSM) and goal expectancy, variables associated with delay were examined to identify the most salient predictors of delay in seeking care for AMI. Hierarchical regression analysis revealed that the set of illness representation components from the CSM was a significant predictor of time to seek care, but individually, only recognition of symptoms as being caused by the heart was significant. Providing accurate information on symptoms of AMI may lead to early recognition, reduced delay, and reduced morbidity and mortality.

Adult↗
PubMed1994

Formation and resolution of brain edema associated with brain tumors. A comprehensive theoretical model and clinical analysis.

The purpose of this study is to quantify the relative contribution of the mechanisms in the absorption of edema fluid. The convection/diffusion and the comprehensive bulk flow model were applied for the finite element analysis of peritumoral brain edema. For clinical analysis, 90 meningiomas studied by MRI were selected. Serial CT scan and MRI were performed at 0, 2, 4, 6 hours after injection of Iopamidol or Gadpenteic acid respectively. Then the tracer distributions in the edematous brain was analyzed. The tracer movement in the brain is well represented by the convection/diffusion equation. The absence of the preferential fluid flow directing toward the ventricle indicates that a limited role of CSF sink action into the ventricle. From capillary surface area (240 cm2/g brain), capillary hydraulic conductivity (1.8 x 10(-8) ml/cmH2O/cm2/min) and the simulated average tissue pressure of 9.8 mmHg, maximum absorption rate into capillaries was estimated to be 0.003 ml/h/cm3 brain tissue. Considering the limited role of edema fluid clearance into the ventricle, the results indicate a possible role of subarachnoid CSF space for the clearance of edema fluid. The clearance of edema fluid into subarachnoid CSF space should be studied quantitatively. Finally, unification of the convection/diffusion and the comprehensive bulk flow model will provide a more quantitative analysis of edema formation and resolution by using MRI and tracer studies.

Blood-Brain Barrier↗
PubMed1990

A theoretical model of endochondral ossification and bone architectural construction in long bone ontogeny.

The role of mechanical stresses in the formation of endochondral ossification patterns and the construction of basic bone architecture in human long bones is investigated using a three-dimensional generalized model of long bone development. The distribution of mechanical stress which is created in developing bones as a result of intermittent mechanical loading is calculated using a computer model that mathematically represents the bone's geometry, material properties and loading conditions. The process of endochondral ossification is simulated by iteratively converting cartilaginous regions of the computer model to bone, based on the calculated intermittent hydrostatic and shear stress distributions. Once local regions of mineralized bone have formed, these regions are remodeled according to an algorithm which relates bone density to a mechanical stress stimulus. The results simulated the correct sequence of the appearance of morphological structures which are common to long bones in the human appendicular skeleton. These developmental structures include the site of the first endochondral bone and the secondary ossification center and the tubular nature of long bones. Our results suggest that mechanical loading histories may influence bone morphogenesis beginning from the early stages of endochondral ossification and continuing throughout life. The stress-based algorithms may be part of the 'rules of construction' or 'developmental constraints' which guide limb ontogeny.

Algorithms↗