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At least 487 records · Page 27Linked to original sources
PubMed2003

A utility-theoretic model for QALYs and willingness to pay.

Despite the widespread use of quality-adjusted life years (QALY) in economic evaluation studies, their utility-theoretic foundation remains unclear. A model for preferences over health, money, and time is presented in this paper. Under the usual assumptions of the original QALY-model, an additive separable presentation of the utilities in different periods exists. In contrast to the usual assumption that QALY-weights do solely depend on aspects of health-related quality of life, wealth-standardized QALY-weights might vary with the wealth level in the presented extension of the original QALY-model resulting in an inconsistent measurement of QALYs. Further assumptions are presented to make the measurement of QALYs consistent with lifetime preferences over health and money. Even under these strict assumptions, QALYs and WTP (which also can be defined in this utility-theoretic model) are not equivalent preference-based measures of the effects of health technologies on an individual level. The results suggest that the individual WTP per QALY can depend on the magnitude of the QALY-gain as well as on the disease burden, when health influences the marginal utility of wealth. Further research seems to be indicated on this structural aspect of preferences over health and wealth and to quantify its impact.

Attitude to Health↗
PubMedDate not supplied

[Dynamic interaction of membrane transport of cations and biosynthesis in the regulation of animal cell growth. A theoretical model].

Possibility of direct participation of the cell volume stabilization system in biosynthesis and growth control in animal cells is postulated and theoretically treated in terms of general membraneous cell model. The states of proliferative quiescence or proliferative activity were attributed, respectively, to the states of stability or non-stability of dynamical interactions between ion-dependent volume regulating system and biosynthetic apparatus which controls metabolic renewal and current number of ion transporters. The results showed that in the case of fixed Na+/K+ pumping ratio the cellular steady-state is quite stable. Necessary conditions are described when labialization of Na+/K+ exchange stoichiometry must initiate metabolic instability and cellular growth. The loss of the cell membrane ability to link together different fluxes may thus be accounted for the uncontrolled cellular and tissue growth. Quantitative criteria for the cell growth capacity are obtained depending on the high enough requiring level of transmembrane ion asymmetry and low enough value of intracellular electric potential.

Animals↗
PubMed2006

Prevalence of sexually transmitted infections among adolescents in Kampala, Uganda, and theoretical models for improving syndromic management.

PURPOSE: To estimate the prevalence of treatable sexually transmitted infections (STI) in adolescents visiting a youth health clinic. To evaluate the algorithm for management of the abnormal vaginal discharge (AVD) syndrome recommended in Uganda's national guidelines and compare it with other theoretical flowchart models. METHODS: Sexually experienced adolescents who were visiting an urban youth health clinic in Kampala, Uganda were examined and interviewed (with their consent) about their socio-demographic background, sexual risk factors, and genital symptoms. Samples taken for Chlamydia trachomatis (CT), Neisseria gonorrhoea (NG), and Trichomonas vaginalis (TV) were analyzed by polymerase chain reaction (PCR). Rapid plasma reagin (RPR) was used with confirming treponema pallidum hem agglutination (TPHA) for syphilis diagnosis. One hundred ninety-nine females and 107 males were examined. Performance of the national algorithm was compared with different theoretical algorithms. RESULTS: Prevalence of CT, NG, TV and syphilis was 4.5%, 9.0%, 8.0%, and 4.0%, respectively, for girls and 4.7%, 5.7%, 0%, and 2.8%, respectively, for boys. We found that 20.6% of the females and 13.2% of the males had at least one STI. The national AVD flow chart had a sensitivity of 61%, a specificity of 38.5% and a positive predictive value (PPV) of 11.6%. All the models had PPV of less than 20% and sensitivity less than 85%. The best performing algorithm using risk and protective factors, rather than symptoms, implicated a sensitivity/specificity and PPV of 82.6%/47% and 17.3%, respectively (p = .012). CONCLUSIONS: An algorithm for management of STI using behavioral and demographic factors in this population demonstrated enhanced sensitivity, specificity, and PPV.

Adolescent↗
PubMed1980

A theoretical model for predicting parameter changes in soft contact lenses due to bending.

Equations are presented for the theoretical determination of back vertex poser changes in soft contact lenses due to bending. The main assumption is that the volume of the lens remains unchanged. A description of changes in other lens parameters, namely, central thickness, overall diameter, and anterior radii, is presented for both spherical and toric lenses. In general, power changes predicted by the proposed model are less than those suggested by previous workers in this field. Changes however occur in other parameters which have previously been regarded as remaining constant such as the central thickness of the lens. These changes are of small degree but have significant bearing on back vertex power.

Contact Lenses, Hydrophilic↗
PubMed2001

A theoretical model of a plant antifreeze protein from Lolium perenne.

Antifreeze proteins (AFPs), found in certain organisms enduring freezing environments, have the ability to inhibit damaging ice crystal growth. Recently, the repetitive primary sequence of the AFP of perennial ryegrass, Lolium perenne, was reported. This macromolecular antifreeze has high ice recrystallization inhibition activity but relatively low thermal hysteresis activity. We present here a theoretical three-dimensional model of this 118-residue plant protein based on a beta-roll domain with eight loops of 14-15 amino acids. The fold is supported by a conserved valine hydrophobic core and internal asparagine ladders at either end of the roll. Our model, which is the first proposed for a plant AFP, displays two putative, opposite-facing, ice-binding sites with surface complementarity to the prism face of ice. The juxtaposition of the two imperfect ice-binding surfaces suggests an explanation for the protein's inferior thermal hysteresis but superior ice recrystallization inhibition activity and activity when compared with fish and insect AFPs.

Amino Acid Motifs↗
PubMedDate not supplied

Do black-box models of thermoregulation still have any research value? Contribution of system-theoretical models to the analysis of thermoregulation.

The aims and the usefulness of modelling the thermoregulatory system are outlined by demonstrating applications and results of simulation on different levels of complexity. It is shown that both very simple one-loop models and complex models based on spatially distributed parameters have contributed to a better understanding of the system, but that current issues primarily require the latter type. However, mathematical modelling must be performed in conjunction with experimental studies and must be adapted to the amount of basic physiological data. Future fields of modelling are the adaptive mechanisms and the interactions of systems.

Animals↗
PubMed2005

[The meaning of hospitalization for the pre-school child: a theoretical model].

This study was aimed at comprehending the meaning of being hospitalized for pre-school children. The theoretical references were the Symbolic Interactionism and Vygotsky's theory on the child's symbolic play. The methodological referential was the Theory Based on Data. Eleven children between the ages of 3 and 6 took part in the survey. The strategies for data collection were: participative observation, interviews with the children mediated by the Therapeutic Toy, and interviews with the mothers. The analysis of the data made possible the construction of the technical model Growing up with the mother's protective presence, which puts in evidence the vulnerability, the child's force and the protection given by the mother to face the mystery and the fear of hospitalization.

Child↗
PubMed2004

Theoretical model of interfacial polymerization.

We present a theoretical description for the creation of a thin polymeric layer through the interfacial polymerization of two immiscible, low molecular weight liquids. The theory specifically takes into account the effects of polydispersity on the formation of the polymer film at the liquid-liquid interface. Consequently, we can describe the structure of the growing film and the molecular weight distribution of the resultant polymer chains. We focus on a model system where alternating AB copolymers are formed at the interface between phase-separated, low molecular weight species A and B. It is assumed that any A(B) unit can reversibly attach to any available B(A) unit or B(A)-ended chain. The formation of the copolymer layer is described by a system of reaction-diffusion equations, which detail the chemical evolution and diffusive dynamics of the polydisperse mixture of AB copolymers around the interface, and the evolution of the interface itself. Using this model, we determine the effects of the chemical reaction rates and the initial conditions on the kinetics of forming the AB copolymer layer and the structure of this film.

Journal Article↗
PubMedDate not supplied

[A theoretical model of the mechanism of DNA compactization by polycations].

DNA compactization in the presence of polycationic ligands was analysed theoretically. The concept is substantiated which states that the formation of polycationic bridges between the DNA regions adjoining in the chain is the principal mechanism of compactization. A phase diagram is plotted for DNA transition into the compact state in the coordinates of solution ionic strength and ligand concentration. The ligand binding with DNA under compactization conditions is shown to be characterized by positive cooperativity. DNA compactization by protamines and histones HI is discussed in terms of the results obtained.

DNA↗
PubMed1998

Theoretical modelling of arteriovenous malformation rupture risk: a feasibility and validation study.

PURPOSE: To explore the feasibility of using a theoretical computational model to simulate the risk of spontaneous arteriovenous malformation (AVM) haemorrhage. METHODS: Data from 12 patients were collected from a prospective databank which documented the angioarchitecture and morphological characteristics of the AVM and the feeding mean arterial pressure (FMAP) measured during initial superselective angiography prior to any treatment. Using the data, a computational model of the cerebral circulation and the AVM was constructed for each patient (patient-specific model). Two model risk (Risk(model)) calculations (haemodynamic- and structural-weighted estimates) were performed by using the patient-specific models. In our previously developed method of haemodynamic-weighted estimate, Risk(model) was calculated with the simulated intranidal pressures related to its maximal and minimal values. In the method of structural-weighted estimate developed and described in this paper, the vessel mechanical properties and probability calculation were considered in more detail than in the haemodynamic-weighted estimate. Risk(model) was then compared to experimentally determined risk which was calculated using a statistical method for determining the relative risk of having initially presented with AVM haemorrhage, termed Risk(exp). RESULTS: The Risk(model) calculated by both haemodynamic- and structural-weighted estimates correlated with experimental risks with chi2 = 6.0 and 0.64, respectively. The risks of the structural-weighted estimate were more correlated to experimental risks. CONCLUSIONS: Using two different approaches to the calculation of AVM haemorrhage risk, we found a general agreement with independent statistical estimates of haemorrhagic risk based on patient data. Computational approaches are feasible; future work can focus on specific pathomechanistic questions. Detailed patient-specific computational models can also be developed as an adjunct to individual patient risk assessment for risk-stratification purposes.

Cerebral Hemorrhage↗
PubMed1977

A theoretical model of regionally ischemic myocardium.

The isometric tension development of a one-dimensional regionally ischemic muscle was analyzed theoretically. The model consist of a one-dimensional normal segment in series with a one-dimensional ischemic segment. Each segment is modeled as a three-element muscle. The inputs to the various elements, except the contractile element in ischemic segment, were obtained from published data for cat papillary muscles. To be consistent with segment length measurements on ischemic canine hearts, it was assumed that the ischemic contractile element contracted normally at the beginning of contraction and then at some tension, TM, fell behind in its rate of tension development compared to the contractile element in the normal segment. Rate of tension development of various lengths of the ischemic segment and strengths of the ischemic contractile element. At the tension, TM, the ischemic segment begins undergoing paradoxical expansion and, simultaneously, as a result of the expansion. the time derivative of the tension produced by the regionally ischemic muscle exhibits a sudden decrease.

Animals↗
PubMed2000

Forgiveness: toward an integration of theoretical models.

Studies on forgiveness have only recently emerged in the psychological literature. Despite evidence that forgiveness is associated with positive therapeutic outcomes, the concept has received little theoretical consideration in mainstream psychology. Existing definitions and models of forgiveness differ widely, and little attempt has been made to integrate these diverse approaches. Based on a review of the Medline and Psychlit databases, the present article evaluates current conceptualizations of forgiveness in the psychological literature. The values and limitations of each approach are discussed with reference to two case studies: a close interpersonal relationship and a human rights violation presented to the Truth and Reconciliation Commission in South Africa. A new theoretical model, integrating the most valuable aspects of existing approaches, is proposed.

Attitude↗
PubMed2004

[Simulation of skeletal muscles dielectric behaviour with theoretical model].

The dielectric behavior of the frog skeletal muscle cells in the 100 Hz-100 MHz range was simulated with dielectric ellipsoidal-shell model, and the model's parameters were put forward for the first time. It could serve as a theoretical foundation on which to make analysis of muscle fatigue, muscle malnutrition and muscle atrophies in the future.

Animals↗
PubMed2006

Resource holding potential, subjective resource value, and game theoretical models of aggressiveness signalling.

Empirical evidence suggests that aggressiveness (willingness to enter into, or escalate an aggressive interaction) may be more important than the ability to win fights in some species. Both empirical and theoretical traditions treat aggressiveness as a distinct property from the ability (RHP) or motivation (subjective resource value) to win a fight. I examine how these three traits are clearly distinct when modelled using a simple strategic model of escalation. I then examine game theoretical models of agonistic communication and demonstrate that models in which aggressiveness is signalled require: (1) a trait, aggressiveness, which is neither a correlate, nor consequence of RHP or motivation, (2) a handicap which negates any benefit to be gained through the use of a particular signal, and (3) the absence of any other asymmetry which could be used to assign roles to players. I conclude that it is unlikely that these assumptions are ever met, and that empirical examples of "aggressiveness" are far more likely to represent long-term differences in subjective resource value.

Aggression↗
PubMed2002

The impact of factitious disorder on the physician-patient relationship. An epistemological model.

Theoretical models for physician-patient communication in clinical practice are described in literature, but none of them seems adequate for solving the communication problem in clinical practice that emerges in case of factitious disorder. Theoretical models generally imply open communication and respect for the autonomy of the patient. In factitious disorder, the physician is confronted by lies and (self)destructive behaviour of the patient, who in one way or another tries to involve the physician in this behaviour. It is no longer controversial that the physician should communicate his consideration of a factitious disorder without insistence that the patient accepts this diagnosis. However, the balance between patient autonomy and open communication on the one hand, and the preservation of the patient's health, physician integrity and of a constructive physician-patient relationship on the other is easily disrupted. In this article, an epistemological model is described to facilitate a positive outcome of confrontation in treatment of factitious disorder. Analysing the problem in terms of systems theory will help the physician to assess what information is appropriate to use in which phase of the patient's treatment, while preserving the physician-patient relationship.

Communication↗
PubMed1998

Theozymes and compuzymes: theoretical models for biological catalysis.

A theozyme is a theoretical enzyme constructed by computing the optimal geometry for transition-state stabilization by functional groups. It is created in order to permit quantitative assessment of catalytic function. Theozymes have been used to elucidate the role of transition-state stabilization in the mechanisms underlying enzyme- and antibody-catalyzed hydroxyepoxide cyclizations, eliminations and decarboxylations, peptide and ester hydrolyses, and pericyclic and radical reactions. The enzymes studied include orotodine monophosphate decarboxylase, HIV protease and ribonucleotide reductase.

Catalysis↗
PubMed2002

Clinical observations supporting a theoretical model of choriocapillaris blood flow in treatment of choroidal neovascularization associated with age-related macular degeneration.

PURPOSE: To report clinical observations consistent with conclusions from a previous theoretical investigation indicating that photocoagulation of choroidal neovascularization (CNV) efferent vessels can be, in some instances, an effective treatment. DESIGN: Interventional case series. METHODS: In five eyes with age-related macular degeneration (five patients with mean age +/- SD of 65 +/- 11 years, range 55-79 years) requiring treatment of CNV. In each case, the appropriate treatment was location and photocoagulation of the CNV efferent vessels, since the afferent vessels were not identifiable or were located beneath the fovea. Targeted vessels were determined to be draining vessels by analysis of pretreatment high-speed indocyanine green (ICG) angiograms, and successful vessel closure by photocoagulation was demonstrated by posttreatment ICG angiograms. The eyes subsequently were followed from 2 to 12 months. RESULTS: After photocoagulation of efferent vessels, CNV-related retinal edema was significantly reduced or resolved within 1 to 4 days. Visual acuity became stabilized in three eyes and improved in two eyes. In a few days, metamorphopsia disappeared in four of the eyes and was stable for a period longer than the duration of the associated efferent vessel closure. Initial efferent vessel closure by photocoagulation persisted on average for 7 to 15 days, after which additional treatment was required. It is significant that in no case did hemorrhage result from the photocoagulation treatment. CONCLUSIONS: These observations are consistent with the earlier theoretical study prediction that photocoagulation of efferent CNV vessels can be effective in reducing CNV-associated edema. That no hemorrhage was induced by photocoagulation is consistent with the theoretical concept that there appears to be no direct hydrostatic connection between the CNV and its afferent vessels. Rather, that connection appears to be a functional one made through the choriocapillaris, which may dissipate excess CNV hydrostatic pressure produced by occlusion of a draining vessel. However, this finding is not intended to be a recommendation to attempt CNV efferent vessel photocoagulation.

Aged↗