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Results for “Models, Theoretical”

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PubMed1981

Speculations on the mechanism of action of hallucinogenic indolealkylamines.

In this review we attempt to develop a fluid theoretical model which is being used as a strategy-base for future experimentation. The first two sections (A and B) describe how we have conducted our research, and present the perspective value of each. This is important because the research strategies developed in these laboratories over the last 5 years combine in vitro and in vivo pharmacological techniques as a means of understanding mechanisms of drug action. Sections C and D attempt to describe how we interpret our data and how we have utilized these data to formulate hypotheses concerning drug mechanisms. The last section of this review sets forth our own ideas on how we believe hallucinogenic agents produce their effects and presents some original data, which we feel, allows us to develop the overall hypotheses presented.

Animals↗

PubMed1985

The stochastic determinants of hospital-bed supply.

This paper uses a general queuing-theoretic model to derive more appropriate measures of hospital quality, such as turn-away probabilities and patient's expected waiting time, than have been used in recent empirical work [e.g., Joskow (1980)].

Health Facility Size↗

PubMed1987

Working conditions and the relationship between schooling and health.

This paper's main objective is to investigate an occupational linkage in the empirically observed positive relationship between schooling and health. Combining results from the health economics literature and labor economic studies of occupational choice, a theoretical model is developed which allows schooling to affect health directly and indirectly, through choice of work environment and other market inputs in health production. The model is estimated using data from the 1980 Health Interview Survey, combined with occupational information from the Dictionary of Occupational Titles. The empirical results indicate the existence of a small occupation effect in the schooling-health relationship, but this effect is conservatively estimated and biased downward given the presence of unobserved differences in individuals' initial health status. The general findings highlight the importance of investigating the health effects of occupational hazards using longitudinal data and comprehensive measures of hazards which include stress at work.

Educational Status↗

PubMed1990

The role of clinical supervision in the education of student psychiatric nurses: a theoretical approach.

A system of clinical supervision for student psychiatric nurses is suggested as an educational response to the rapidly changing role of the nurse. In addition to the small but expanding body of literature from the field of psychiatric nursing, research from social work, counselling, psychotherapy and teaching is also considered. A theoretical model is outlined in which supervision of students is by qualified and skilled practitioners. This is neither therapeutic nor didactic, but rather a 'parallel process' in which the supervisor-student relationship is employed as a tool to explore the student-client relationship and the therapeutic process.

Humans↗

PubMed1996

LET spectra of trapped anomalous cosmic rays in low-Earth orbit.

Observations aboard Cosmos satellites discovered trapped anomalous cosmic rays (ACRs), tracked the variation in their intensity in 1986-1988, and measured their fluence, spectrum, and composition at solar minimum in the previous solar cycle. The MAST instrument aboard the SAMPEX satellite has observed trapped anomalous cosmic rays in the present solar cycle, confirmed the general features of the Cosmos data, and provided the first detailed observations of trapped ACRs. In this paper we apply theoretical modeling of trapped ACRs, which is shown to provide a reasonably good description of both the Cosmos and SAMPEX data, to calculate the integral linear-energy-transfer (LET) spectra due to trapped ACRs in typical low-Earth orbits. We compare these calculations with the LET spectra produced by galactic cosmic rays (GCRs) and non-trapped ACRs in order to assess the relative radiation hazard posed by trapped ACRs.

Cosmic Radiation↗

PubMed1986

Physician-induced demand by Irish GPs.

Approximately one-third of the Irish population receive all medical care services free. GPs (general practitioners) treat both public and private patients, and are remunerated on a fee-for-service basis by the state for public patients, and by the patient, at a higher rate, for private patients. In 1981, the first author conducted a national survey of Irish medical care utilization, asking whether patients' most recent GP visits resulted in a return visit being arranged. This measure of self-referral by GPs is significantly and strongly associated with the ratio of GPs to population, and negatively with the ratio to population of persons eligible for free services, and with area per capita income. All three results are as hypothesized from a theoretical model, and point to significant self-interested physician-induced demand by Irish GPs.

Adolescent↗

PubMed1990

The relative influence of the community and the health system on work performance: a case study of community health workers in Colombia.

A central component of the primary health care approach in developing countries has been the development and utilization of community-based health workers (CHWs) within the national health system. While the use of these front line workers has the potential to positively influence health behavior and health status in rural communities, there continues to be challenges to effective implementation of CHW programs. Reports of high turnover rates, absenteeism, poor quality of work, and low morale among CHWs have often been associated with weak organizational and managerial capacity of government health systems. However, no systematic research has examined the contribution of work-related factors to CHW job performance. The research reported in this paper examines the relative influence of reward and feedback factors associated with the community compared to those associated with the health system on the performance of CHWs. The data are drawn from a broader study of health promoters (CHWs) conducted in two departments (provinces) in Colombia in 1986. The research was based on a theoretical model of worker performance that focuses on job related sources of rewards and feedback. A survey research design was employed to obtain information from a random sample of rural health promoters (N = 179) and their auxiliary nurse supervisors about CHW performance and contributing factors. The findings indicate that feedback and rewards from the community have a greater influence on work performance (defined as degree of perceived goal attainment on job tasks) than do those stemming from the health system.(ABSTRACT TRUNCATED AT 250 WORDS)

Colombia↗

PubMed1994

Domestic violence--an emerging health issue.

The aim of this paper is to explain some of the aspects included in the concept of family violence, to summarize the principal theoretical models proposed to explain it, as well as to describe the principal syndromes included within the scope of the concept: battered woman, courtship violence, child-abuse, child sexual-abuse, child witnessing violence, sibling violence and elderly violence. Particular emphasis is put on the description of the effects that different types of family violence cause on the individuals' health.

Battered Women↗

PubMed1995

Differential binding of RNA polymerase to the wild type Mu mom promoter and its C independent mutant: a theoretical analysis.

Using the theoretical model for DNA bending we have analyzed the Mu mom promoter wild type and its mutant tin7 which showed differential binding to the RNA polymerase. We have demonstrated here the structural change as a result of the point mutation which may be responsible for the altered binding of RNA polymerase. Analysis using both sets of parameters essentially gives the same result.

Bacteriophage mu↗

PubMedDate not supplied

Finite amplitude distortion of the pulsed fields used in diagnostic ultrasound.

A simple theoretical model of the non-linear propagation of pulsed focused acoustic beams is described. It enables the distortion of the peak cycle of the pulse to be calculated from a few experimentally measured parameters. The model is discussed, and justified for application to the fields from medical ultrasonic diagnostic equipment. It is particularly relevant for specifying the degree of distortion present, as might be required by future written standards for diagnostic equipment performance. Preliminary experimental verification of the model is reported.

Acoustics↗

PubMed1996

Membrane potential changes in activated cells: connection with cytosolic calcium oscillator.

The activation of many cells by hormones, growth factors or neurotransmitters can lead to a periodic variation of the cell membrane potential. A theoretical model that describes this behaviour on the basis of the cytosolic calcium oscillator is developed. The main assumption of this model stressed that the plasma membrane potential is mainly a K+ diffusion potential. It is shown that the oscillations of the intracellular calcium concentration can induce solely the membrane potential oscillations, without any further interaction mechanisms between fluxes through the ionic channels from the plasma membrane. A one-pool model was used to account for Ca2+ release from the intracellular stores. The model and the numerical calculations are based on the theory of nonlinear dynamical systems.

Animals↗

PubMed1993

Rural-urban migration, informal sector and development policies: a theoretical analysis.

"A theoretical model of rural-urban migration has been developed with special reference to the informal sector. The wage rate and employment in the informal sector are determined endogenously. The paper shows the simultaneous existence of open unemployment and informal sector in the urban area in migration equilibrium. The effects of alternative subsidy policies on unemployment and welfare of the workers are studied." The model is intended primarily for use in analyzing trends and policies in developing countries.

Asia↗

PubMed1994

Restoration of high-tilt electron micrographs using a focus series.

Electron microscopy tomography requires high-tilt projections. However, the variation in defocus over a highly tilted specimen results in a blurred micrograph, where the blur is spatially varying blur can cause the micrograph to deviate from an acceptable approximation of the projection of the specimen's density function. In practice, this has been one factor limiting reconstructions of large cellular areas. We derive an algorithm to restore, or deblur, high-tilt electron micrographs using multiple defocused versions of the micrograph. This algorithm consists of digitization and registration of the micrographs, identification of the defocus blur using either a theoretical model or a zero-tilt focus series, and spatially varying restoration using the multiple high-tilt defocused micrographs. Numerical results using simulated and real data are presented, demonstrating the effectiveness of this algorithm.

Algorithms↗

PubMed1995

Effect of increasing vascular hydraulic conductivity on delivery of macromolecular drugs to tumor cells.

PURPOSE: To examine the assumption that increasing the hydraulic conductivity Lp of microvessels (by such strategies as radiation, hyperthermia, or inducing inflammation) improves the transport of macromolecular drugs to tumors. METHODS AND MATERIALS: A theoretical model is used to investigate the effect of varying Lp on macromolecular transport in spherical tumor nodules. The model is generalized to nonspherical tumors. The equations governing fluid flow in the tumors are solved numerically to obtain the pressure in the interior, and macromolecule fluxes are deduced using Starling's equation. RESULTS: Because of the interaction of two opposing effects, the filtration rate at the tumor center is maximized at an "optimum" value of Lp, which depends strongly on tumor size. Only for tumor nodules smaller than about 0.2 cm across is this value lower than the actual estimated value. By considering spheroidal tumor nodules, it is found that dependence on shape is much weaker than dependence on size, except for extremely elongated or flattened shapes. CONCLUSIONS: While total fluid filtration for the entire tumor region always increases with increasing Lp, the local filtration rate at points inside the tumor decreases if the tumor is larger than about 0.2 cm, leading to a less uniform distribution of the drug, and therapeutic disadvantage. Increased vessel leakiness is an unlikely explanation for reported experimental findings that hyperthermia and inflammation result in more uniform distributions of monoclonal antibodies throughout tumors much larger than 0.2 cm.

Antibodies, Monoclonal↗

PubMed1996

On the relation between size of necrosis and diameter of tumor spheroids.

PURPOSE: In many previous experimental studies on multicellular tumor spheroids, the spheroid diameter at which central necrosis develops has been determined to be twice the thickness of the viable cell rim measured at a later stage of spheroid growth. This procedure tacitly assumes that there is a linear relation between the diameter of necrosis and that of the whole spheroid over the entire range of emergence and growth of necrosis. However, some experimental investigations have demonstrated that necroses do not grow gradually with spheroid diameter, but show a rapid initial increase, once a few cells have died. The present article offers an explanation for this phenomenon, which is derived from basic diffusion theory. METHODS AND MATERIALS: A theoretical relation between sizes of spheroids and of their central necroses is developed, which is based on the assumption that formation of necrosis is caused by depletion of substrates or accumulation of metabolic waste products. In a second part, the theoretical model is fitted to experimental data from the literature, and oxygen consumption rate as a function of spheroid size is determined. RESULTS: It turns out that the model closely mimics the experimentally observed behavior described above. These experimental results, therefore, do not furnish any evidence for assuming other hypotheses of necrosis formation. Resulting O2 consumption rates are well in the range of previously published data. In all cases, approximations to the measured data are better than the corresponding linear squares fits. CONCLUSION: At least in some tumor cell lines, depletion of substrates or accumulation of waste products can explain formation of necrosis without the assumption of any additional mechanisms. Moreover, the model presented in this article offers an alternative way of determining the turnover rate of a substrate or metabolic waste product provided that depletion/accumulation of this substance represents the cause for necrosis development.

Least-Squares Analysis↗

PubMed1990

An observation of increased contrast due to both T1 and T2 weighting in a synthetic image.

An observation was made that subtracted images generated by an SR/IR interleaved pulse sequence on a low field strength magnetic resonance imager (M&D Technology Ltd, Aberdeen, Scotland) contained contrast between pathological and normal tissues that was not explained by either T1 or T2 weighting alone. A theoretical model for this image has been derived and its validity confirmed by phantom experiments. It is shown that the subtracted image is dependent on the product of T1 and T2 prolongation for contrast. This type of image might be of use in screening for unknown pathologies and such techniques may shorten routine patient imaging times.

Gadolinium↗

PubMed1990

Photochemistry of uridine on high intensity laser UV irradiation.

The method of high performance liquid chromatography was applied to investigate the products of single-quantum and two-quantum photo-reactions which occur when an aqueous solution of uridine is irradiated with high intensity nanosecond and picosecond laser UV pulses. A comparison of the experimental results obtained with theoretical models enables a number of photophysical parameters of the highly excited electronic states of the uridine molecule to be determined.

Chromatography, High Pressure Liquid↗

PubMed2005

Toward a theoretical approach to medical error reporting system research and design.

The release of the Institute of Medicine (Kohn et al., 2000) report "To Err is Human", brought attention to the problem of medical errors, which led to a concerted effort to study and design medical error reporting systems for the purpose of capturing and analyzing error data so that safety interventions could be designed. However, to make real gains in the efficacy of medical error or event reporting systems, it is necessary to begin developing a theory of reporting systems adoption and use and to understand how existing theories may play a role in explaining adoption and use. This paper presents the results of a 9-month study exploring the barriers and facilitators for the design of a statewide medical error reporting system and discusses how several existing theories of technology acceptance, adoption and implementation fit with many of the results. In addition we present an integrated theoretical model of medical error reporting system design and implementation.

Equipment Design↗