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PubMed1984

Simultaneous modeling of pharmacokinetics and pharmacodynamics with a nonparametric pharmacodynamic model.

We describe a variation on an approach to simultaneous modeling of pharmacokinetics (PK) and pharmacodynamics (PD). Both approaches model the often-observed time lag between plasma drug concentration (Cp) and drug effect (E) in non-steady-state experiments by postulating an E site whose concentration (Ce) is kinetically linked to Cp by a first-order process. With the linking model, the time lag can be removed from the data and the underlying concentration-response (Ce-E) relationship can be estimated. The original method requires the analyst to postulate a particular parametric form for the Ce-E model, whereas ours does not. It estimates the rate constant of the linking model as the value that causes the hysteresis curve (Ce vs E points connected in time order) to collapse to a single curve that represents the (empirical) Ce-E relationship. The method is presented as an algorithm and is tested by means of simulation and a real-world example. The results suggest that the method can faithfully estimate the Ce-E curve for a variety of PD models and degrees of experimental error when its basic assumption of time-invariant PD holds.

Dose-Response Relationship, Drug↗

PubMed2002

Compartment modeling of MTBE in the generic environment and estimations of the aquatic MTBE input in Germany using the EQC model.

The use of the gasoline additive methyl tert-butyl ether (MTBE) has caused serious concern about groundwater and surface water contamination. The behavior of MTBE in the two most relevant compartments, surface water and air in a generic environment and in a simulated German environment is investigated using the equilibrium criterion (EQC) model. Due to lack of literature data, the half-life time of MTBE in river water is estimated to about 80-120 d (105 d) at 18 degrees C and roughly 1.5 a (year)(533 d) at 4 degrees C from a batch experiment. The EQC model considers four compartments, air, surface water, soil and sediment in an environment of typically 100,000 km2 with about 10% of the area covered with water. The user can progress through the tiered sequence of Level I to III with increasing complexity which reveals more information about the the fate of the considered chemical. The equilibrium mass distribution of MTBE calculated with the Level I model shows that 87% partitions into air and 13% into surface water at 10 degrees C. The results of the Level II calculations indicate that 50% of MTBE in the air is transported from the system and 38% in the air is degraded at 10 degrees C. The resulting total persistence time of 3 d for MTBE in the generic environment of the Level II model can be compared to the calculated value for chlorobenzene. The MTBE input into water is significantly more sensitive to the 'mode of entry' than input into air. The MTBE concentration in surface water is almost exclusively the result of direct emission into water, whereas the atmosphere can additionally be loaded by volatilization from water. The total aquatic MTBE emission in Germany and the average MTBE concentration in German surface waters were roughly estimated to 20-80 t a(-1) (tons per year)(50 t a(-1)) and 50 ng L(-1), respectively. Surface water concentrations calculated with the underlying assumptions of the model can neither be explained by exposure through waste water and industrial effluents nor with an estimated loss of industrially used MTBE in Germany. For the year-round scenario at 10 degrees C, MTBE concentrations of 19 ng L(-1) (surface water) and 167 ng m(-3) (air) result. However, it remains unclear whether the assumptions of the model, the lack of analyses from industrial effluents or both are responsible for the difference. Additional aquatic emission sources could result from gasoline transport on and storage near rivers. The comparison of winter and summer scenarios shows that in summer, atmospheric (25%) and aqueous (50%) concentrations are lower than in winter due to higher degradation rates.

Air Pollutants↗

PubMedDate not supplied

Development of a chronic skin defect model and a study of cytokine secretion using the model.

In this study, we established a delayed healing chronic type wound model in order to investigate the etiology of chronic wound healing, including wound contraction. Establishment of the model was important for clarification of the mechanism(s) of chronic wound healing and wound contraction and for use in evaluating therapeutic efficacy. A pedicled skin flap was raised beneath the panniculus carnosus membrane on the backs of mice, and after the loose connective tissue at the base of the flap was completely removed surgically, the flap was replaced and sutured. Seven days after surgery, a full-thickness defect measuring 1.5 x 1.5 cm was made in the center of the skin flap. At that time, a defect of the same size, including the panniculus carnosus membrane, was made in another group of mice as controls, and changes with time in wound area were compared between the two groups. The exudate retained on the wound surface was collected, and various cytokines contained in the exudate were measured. In the control group, the wound rapidly contracted and almost completely epithelialized and closed 21 days after surgery. On the other hand, the wound area was significantly larger in the delayed model than in the control animals during the observation period, revealing a delay in wound contraction. Transforming growth factor-beta, interleukin-1beta, and tumor necrosis factor-alpha in the exudates from the wound of the model were significantly higher than in those of the control group, whereas interleukin-6 was low in the model. From these results, it was concluded that this model could be a useful experimental system for studies on wound contraction as well as clarifying the mechanism of so called chronic type wounds.

Animals↗

PubMed1999

Treating depression in staff-model versus network-model managed care organizations.

OBJECTIVE: To compare primary care providers' depression-related knowledge, attitudes, and practices and to understand how these reports vary for providers in staff or group-model managed care organizations (MCOs) compared with network-model MCOs including independent practice associations and preferred provider organizations. DESIGN: Survey of primary care providers' depression-related practices in 1996. SETTING AND PARTICIPANTS: We surveyed 410 providers, from 80 outpatient clinics, in 11 MCOs participating in four studies designed to improve the quality of depression care in primary care. MEASUREMENTS AND MAIN RESULTS: We measured knowledge based on depression guidelines, attitudes (beliefs about burden, skill, and barriers) related to depression, and reported behavior. Providers in both types of MCO are equally knowledgeable about treating depression (better knowledge of pharmacologic than psychotherapeutic treatments) and perceive equivalent skills in treating depression. However, compared with network-model providers, staff/group-model providers have stronger beliefs that treating depression is burdensome to their practice. While more staff/group-model providers reported time limitations as a barrier to optimal depression treatment, more network-model providers reported limited access to mental health specialty referral as a barrier. Accordingly, these staff/group-model providers are more likely to treat patients with major depression through referral (51% vs 38%) or to assess but not treat (17% vs 7%), and network-model providers are more likely to prescribe antidepressants (57% vs 6%) as first-line treatment. CONCLUSIONS: Whereas the providers from staff/group-model MCOs had greater access to and relied more on referral, the providers from network-model organizations were more likely to treat depression themselves. Given varying attitudes and behaviors, improving primary care for the treatment of depression will require unique strategies beyond enhancing technical knowledge for the two types of MCOs.

Chi-Square Distribution↗

PubMed2003

Studies of turbulence models in a computational fluid dynamics model of a blood pump.

Computational fluid dynamics (CFD) is used widely in design of rotary blood pumps. The choice of turbulence model is not obvious and plays an important role on the accuracy of CFD predictions. TASCflow (ANSYS Inc., Canonsburg, PA, U.S.A.) has been used to perform CFD simulations of blood flow in a centrifugal left ventricular assist device; a k-epsilon model with near-wall functions was used in the initial numerical calculation. To improve the simulation, local grids with special distribution to ensure the k-omega model were used. Iterations have been performed to optimize the grid distribution and turbulence modeling and to predict flow performance more accurately comparing to experimental data. A comparison of k-omega model and experimental measurements of the flow field obtained by particle image velocimetry shows better agreement than k-epsilon model does, especially in the near-wall regions.

Equipment Design↗

PubMedDate not supplied

Modeling incomplete longitudinal and cross-sectional data using latent growth structural models.

In this paper we describe some mathematical and statistical models for identifying and dealing with changes over age. We concentrate specifically on the use of a latent growth structural equation model approach to deal with issues of: (1) latent growth models of change, (2) differences in longitudinal and cross-sectional results, and (3) differences due to longitudinal attrition. This is a methodological paper using simulated data, but we base our models on practical and conceptual principles of modeling change in developmental psychology. Our results illustrate both benefits and limitations using structural models to analyze incomplete longitudinal data.

Aging↗

PubMed2002

Testing the grief to personal growth model using structural equation modeling.

The belief that loss can result in growth has been hypothesized for centuries.Yet, traditional grief theories have viewed grief work as a process of resolving grief and returning to normal. Formal conceptualizations of grief to growth models have been recently delineated by several grief theorists. The Grief to Personal Growth model represents one emergent perspective of the qualitative changes resulting from the loss of a loved one. The model delineates a pathway through grief that indicates the bereft experience despair and detachment followed by intrusive thoughts and later avoidance of intense preoccupation with grief. Social support is shown to facilitate the bereft as they reconstruct their lives and find new meaning in life. A second path indicates that some bereaved individuals become mired in grief and need help to proceed toward personal growth. The model was tested in a sample of bereaved parents using structural equation modeling as a method of theory testing. The results of testing this model are presented within a framework of theory testing as a mechanism to bridge the gaps between theory, practice, and research. Implications for practice are considered.

Adult↗

PubMed2005

[Aquatic ecosystem modelling approach: temperature and water quality models applied to Oualidia and Nador lagoons].

The objective of this work is to develop an aquatic ecosystem and apply it on Moroccan lagoon systems. This model will keep us abreast of the yearly development of the main parameters that characterize these ecosystems while integrating all the data that have so far been acquired. Within this framework, a simulation model of the thermal system and a model of the water quality have been elaborated. These models, which have been simulated on the lagoon of Oualidia (North of Morocco) and validated on the lagoon of Nador (North West Mediterranean), permit to foresee the cycles of temperature of the surface and the parameters of the water quality (dissolved oxygen and biomass phytoplankton) by using meteorological information, specific features and in situ measurements in the studied sites. The elaborated model, called Zero-Dimensional, simulates the average conduct of the site during the time of variable states that are representatives of the studied ecosystem. This model will provide answers for the studied phenomena and is a work tool adequate for numerical simplicity.

Ecosystem↗

PubMed2003

Modeling sorbent injection for mercury control in baghouse filters: II--pilot-scale studies and model evaluation.

Activated carbon injection for Hg control in a 500-lb/hr pilot-scale coal-fired furnace equipped with a fabric filter for particulate control was evaluated at different operating conditions. The pilot-scale tests showed that Hg removal was improved at lower temperatures and higher C/Hg ratios. The two-stage mathematical model developed to describe Hg removal using powdered activated carbon injection upstream of a baghouse filter was used to obtain Langmuir isotherm parameters as a function of temperature by fitting the model to a subset of experimental data. The predictive capability of the model was then tested by comparing model calculations with additional experimental data from this system obtained using different operating temperatures and sorbent to Hg ratios. Model predictions were in good agreement with experimentally measured Hg removal efficiency. Based on the model predictions, Hg removal in the duct appears to be limited and higher C/Hg ratio, lower operating temperature, and longer cleaning cycle of the baghouse filter should be utilized to achieve higher Hg removal in this system.

Adsorption↗

PubMed2006

Development of an empirical model to estimate real-world fine particulate matter emission factors: the traffic air quality model.

The purpose of the study was to quantify the impact of traffic conditions, such as free flow and congestion, on local air quality. The Borman Expressway (I-80/94) in Northwest Indiana is considered a test bed for this research because of the high volume of class 9 truck traffic traveling on it, as well as the existing and continuing installation of the Intelligent Transportation System (ITS) to improve traffic management along the highway stretch. An empirical traffic air quality (TAQ) model was developed to estimate the fine particulate matter (PM2.5) emission factors (grams per kilometer) based solely on the measured traffic parameters, namely, average speed, average acceleration, and class 9 truck density. The TAQ model has shown better predictions that matched the measured emission factor values more than the U.S. Environmental Protection Agency (EPA)-PART5 model. During congestion (defined as flow-speeds < 50 km/hr [30 mi/hr]), the TAQ model, on average, overpredicted the measured values only by a factor of 1.2, in comparison to a fourfold underprediction using the EPA-PART5 model. On the other hand, during free flow (defined as flow-speeds > 80 km/hr [50 mi/hr]), the TAQ model was conservative in that it overpredicted the measured values by 1.5-fold.

Air Pollutants↗

PubMed2006

Phyloclimatic modeling: combining phylogenetics and bioclimatic modeling.

We investigate the impact of past climates on plant diversification by tracking the "footprint" of climate change on a phylogenetic tree. Diversity within the cosmopolitan carnivorous plant genus Drosera (Droseraceae) is focused within Mediterranean climate regions. We explore whether this diversity is temporally linked to Mediterranean-type climatic shifts of the mid-Miocene and whether climate preferences are conservative over phylogenetic timescales. Phyloclimatic modeling combines environmental niche (bioclimatic) modeling with phylogenetics in order to study evolutionary patterns in relation to climate change. We present the largest and most complete such example to date using Drosera. The bioclimatic models of extant species demonstrate clear phylogenetic patterns; this is particularly evident for the tuberous sundews from southwestern Australia (subgenus Ergaleium). We employ a method for establishing confidence intervals of node ages on a phylogeny using replicates from a Bayesian phylogenetic analysis. This chronogram shows that many clades, including subgenus Ergaleium and section Bryastrum, diversified during the establishment of the Mediterranean-type climate. Ancestral reconstructions of bioclimatic models demonstrate a pattern of preference for this climate type within these groups. Ancestral bioclimatic models are projected into palaeo-climate reconstructions for the time periods indicated by the chronogram. We present two such examples that each generate plausible estimates of ancestral lineage distribution, which are similar to their current distributions. This is the first study to attempt bioclimatic projections on evolutionary time scales. The sundews appear to have diversified in response to local climate development. Some groups are specialized for Mediterranean climates, others show wide-ranging generalism. This demonstrates that Phyloclimatic modeling could be repeated for other plant groups and is fundamental to the understanding of evolutionary responses to climate change.

Acclimatization↗

PubMed2003

A mathematical model of phytoremediation for petroleum-contaminated soil: model development.

We present a simple model for root length density that combines the generally accepted spatial (exponential decrease with depth) and temporal (sinusoidal) variability of root length. Parameters in this model for root length density can be determined from assumed or measured information regarding the annual biomass turnover, maximum standing biomass, and maximum depth of root penetration. The root length density model, coupled with information regarding the average root lifespan, gives specific root growth and senescence functions that are the forcing functions for the phytoremediation model. We present a screening level mathematical model for phytoremediation that accounts for the growth and senescence of roots in the system. This is an important factor for recalcitrant, immobile compounds found in weathered crude oil contaminated soils. The phytoremediation model is based on variable volume compartments that have individual first-order degradation rate constants; as the roots move through the soil, the soil cycles through the rhizosphere zone, decaying root zone and bulk soil zone. Thus, although the oil is immobile, as the roots penetrate through the soil the oil is brought into contact with the rhizosphere.

Algorithms↗

PubMed2000

Long-term memory for spatial and temporal mental models includes construction processes and model structure.

There is a strong case that people construct and manipulate mental models in working memory but relatively little evidence that mental models are preserved in long-term memory. Instead people may remember an episodic construction trace: a record of the operations used to construct a mental model (Payne, 1993). Experiment 1 investigated memory for determinate spatial descriptions (which describe a single configuration of objects) and indeterminate spatial descriptions (which describe two equally plausible configurations). Recognition performance was impaired when the overlap between the episodic construction trace of a description at learning and at test was disrupted by reordering the sentences within a description. Participants were better at remembering the gist of determinate descriptions than that of indeterminate descriptions. For indeterminate descriptions, provided differences in gist recognition were controlled, participants showed better memory for the original description. Experiment 2 showed a similar pattern of results with temporal descriptions. A third experiment manipulated the similarity between foils in the recognition test and the original descriptions to provide further evidence for both episodic construction trace and remembered mental models. In combination, these results favour a hybrid account of memory for mental models, which includes information about both construction processes and model structure.

Adult↗

PubMed1999

Feasibility of an in vitro microbiological model as an alternative to the rabbit eye model.

Ocular inserts of gentamicin sulfate with polyvinyl alcohol (PVA) 1.5%, 2.0%, and 2.5% and a combination of methyl cellulose 2% and Eudragit NE 30D 30%, 35%, and 40% w/w of methyl cellulose were fabricated by a casting technique. The inserts were sterilized by gamma radiation at 25 kGy and tested for sterility. The microbiological efficacy of the ocular inserts against Staphylococcus aureus ATCC 6538P and Pseudomonas aeruginosa NCIM 2200 was evaluated by developing an in vitro microbiological model and an in vivo noninvasive rabbit eye model. Parameters of the in vitro microbiological model were varied, and the results correlated with a noninvasive rabbit eye model. The in vitro model proved to be a viable alternative to the rabbit eye model in evaluating the microbiological efficacy of gentamicin sulfate ocular inserts.

Administration, Topical↗

PubMed2003

Dissolution and absorption modeling: model expansion to simulate the effects of precipitation, water absorption, longitudinally changing intestinal permeability, and controlled release on drug absorption.

A previously described model for simulating drug dissolution, absorption, and pharmacokinetics has been expanded beyond the original application of simulating immediate-release dosage forms to include simulation of drug precipitation, water absorption from the gastrointestinal tract, changing gastrointestinal permeability, disintegration, and controlled-release and dissolution from a GITS-type dosage form. A mathematical description of the model is presented as well as a retrospective analysis of nifedipine to demonstrate the utility of the model. The fourth-order Runge-Kutta numerical method was used to solve the series of coupled differential equations used to simulate the process of dissolution, absorption, and drug disposition. The model was able to simulate the clinically demonstrated effect for drug particle size on nifedipine plasma concentrations for an immediate-release dosage form. Further simulations indicated that drug particle size was less important for a GITS-type dosage form at a release rate of 1.7 mg/hr compared to rate of 17 mg/hr. Hypothetical calculations simulated the potential effect of drug precipitation, water absorption, and changing permeability on drug plasma concentrations. The expanded model increases the utility of a previously described model in providing guidance in drug development and selection.

Biological Availability↗

PubMed1990

Flow cytometric analysis and modeling of cell-cell adhesive interactions: the neutrophil as a model.

The immune function of granulocytes, monocytes, lymphocytes, and other specialized cells depends upon intercellular adhesion. In many cases the molecules mediating leukocyte cell adhesion belong to the Leu-CAM superfamily of adhesive molecules. To elucidate the events of homotypic aggregation in a quantitative fashion, we have examined the aggregation of neutrophils stimulated with formyl peptides, where aggregate formation is a transient reversible cell function. We have mathematically modeled the kinetics of aggregation using a linear model based on particle geometry and rates of aggregate formation and breakup. The time course was modeled as a three-phase process, each phase with distinct rate constants. Aggregate formation was measured on the flow cytometer; singlets and larger particles were distinguished using the intravital stain LDS-751. Aggregation proceeded rapidly after stimulation with formyl peptide (CHO-nle-leu-phe-nle-tyr-lys). The first phase lasted 30-60 s; this was modeled with the largest aggregation rate and smallest rate of disaggregation. Aggregate formation plateaued during the second phase which lasted up to 2.5 min. This phase was modeled with an aggregation rate nearly an order of magnitude less than that of the initial fast phase, whereas the disaggregation rate for this phase did not change significantly. A third phase where disaggregation predominated, lasted the remaining 2-3 min and was modeled with a four to fivefold increase of the disaggregation rate. The mechanism of cell-cell adhesion in the plateau phase was probed with the monoclonal antibody IB4 to the CD18 subunit of the adhesive receptor CR3. Based on these studies it appears that new aggregates do not form to a large degree after the first phase of aggregate formation is complete. However, new adhesive contact sites may form within the contact region of these adherent cells to keep the aggregates together.

Cell Adhesion↗

PubMed2002

Modelling fields induced in humans by 50/60 Hz magnetic fields: reliability of the results and effects of model variations.

This paper presents a comparison of anatomically realistic human models and numerical codes in the dosimetry of power frequency magnetic fields. The groups at the University of Victoria and the National Radiological Protection Board have calculated the induced electric fields in both their 'UVic and 'NORMAN' models using independently developed codes. A detailed evaluation has been performed for a uniform magnetic field at 60 Hz. Comparisons of all dosimetric metrics computed in each particular model agree within 2% or less. Since in situ measurements cannot be performed in humans, and achievable accuracy of measurements in models and animals is not likely to be better than 10-15%, the comparisons presented should provide confidence limits on computational dosimetry. An evaluation of the effect of model size, shape and resolution has also been performed and further illuminated the reasons for differences in induced electric fields for various human body models.

Algorithms↗

PubMed2005

Body composition modeling in the calf using an equivalent circuit model of multi-frequency bioimpedance analysis.

An equivalent electrical circuit model is used to describe the response of different tissue components in the calf to multi-frequency current. This model includes seven electrical components: skin resistance, contact capacitance, fat resistance, fat capacitance, extracellular resistance, intracellular resistance and cell membrane capacitance. Calf bioimpedance was measured on 30 pts using a multi-frequency bioimpedance device (Xitron 4200) with a range of frequency from 5 kHz to 1000 kHz. MRI was performed on each measured calf to provide body composition components: fat, muscle mass and bone. An equivalent circuit containing seven parameters (P1, P2, P3, P4, Q1, Q2, Q3) was constructed to represent the model. To identify the effect of different body compositions on their parameters, subjects were subgrouped according to (1) their range of fat mass: F1>0.4 kg, F2>0.4 & F2<0.25 kg and F3<0.25 kg; (2) their range of muscle mass: M1>1.2 kg, M2<1.2 & M2>1.0 kg and M3<0.25 kg. Curve fitting and simulation programs (Matlab Toolbox) were used to obtain the solution of the electrical equations. The results show a decrease in impedance with an increase in excitation frequency that differed among subjects with different fat contents. Simulation results show a high correlation (R2>0.98) between the bioimpedance measurements and the value calculated from the model. There are significant differences in parameters P1 (32.5+/-5.9 versus 26+/-4.4, p<0.05), P3 (-15,330+/-3352 versus -10,973+/-3448, p<0.05) and P4 (42,640 versus 24,191, p<0.05) between groups F1 and F3. P2 is significantly different (1045+/-442 versus 1407+/-349, p<0.05) between groups M1 and M2. The parameters that characterize the bioimpedance data depend upon many more tissue characteristics of electrical properties than those incorporated in current models and they are affected by aspects of body composition that are not considered in the fitting of bioimpedance data. This study shows a new model and methodology to analyze bioimpedance data and further work is likely to lead to much better understanding of electrical properties of body tissue.

Algorithms↗