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PubMed1993

The predictive performance of a system model for enflurane closed-circuit inhalational anesthesia.

BACKGROUND: Previously, the authors described a system model for closed-circuit inhalational anesthesia, and demonstrated close agreement between end-tidal isoflurane concentrations measured in their clinical study and those predicted by the model. The predictive performance of their model has not, however, been tested for anesthetics featuring nonpulmonary elimination (NPE). METHODS: The authors quantified the predictive performance of two versions (A and C) of the model in 50 patients by comparing the predicted and the measured alveolar concentration-time profiles after bolus injections of liquid enflurane into the expiratory limb of the closed system. Version A did not incorporate NPE, but version C emulated NPE by adopting the irreversible loss of a fraction of the enflurane present in the arterial hepatic blood flow (0.131, derived from a mass balance study performed by others). For each concentration measured by mass spectrometry, the authors used computer simulations of version A and C to calculate a predicted concentration for both versions. For each patient, the authors calculated the bias (indicating systematic over- or underprediction) and the scatter of the prediction errors (indicating typical error size). RESULTS: The authors administered a total of 379 ml of liquid enflurane via 466 injections. A total of 18,432 alveolar concentrations (one per 10-s period; average concentration = 0.96 vol%) were measured. The bias and the scatter, both given as mean (and SD), were 10.0 (13.1)% and 11.8 (3.9)% for version A and -0.8 (11.4)% and 11.4 (2.8)% for C. The bias for version C was closer to zero; the scatters were similar. CONCLUSIONS: Version C incorporating NPE performs better than version A. The accuracy that was obtained should encourage the use of version C for clinical, teaching, research, economic, and ecologic purposes.

Adult↗

PubMed1997

Neural networks as predictors of outcomes in alcoholic patients with severe liver disease.

We developed and evaluated neural networks as predictors of outcomes in alcoholic patients with severe liver disease using commonly available clinical and laboratory values. Hospital charts of 144 patients were reviewed. Nine variables (five laboratory, four clinical) were recorded along with in-hospital death or survival. Data were organized into separate development and validation sets. Neural network predictions of survival were compared with those of the Maddrey discriminant function and logistic regression models developed on the same data. Model performance was evaluated by comparing areas under receiver-operating characteristic (ROC) curves and the distributions of model scores. Survivors had significantly different laboratory and clinical characteristics, the most important being a higher prothrombin time, lower bilirubin, and lower incidence of encephalopathy. Neural network performance was significantly better than that of the Maddrey score (ROC areas, 81.5% vs. 73.8%; P = .04). The ROC area for neural networks was similar to that of logistic regression (ROC area 78.2%; P = .3), but the neural networks were more successful in classifying patients into low- and high-risk groups (P < .001). A neural network score with laboratory data from hospital-day 7 improved prognostic accuracy further to 84.3%. After adjusting for baseline risk, the neural network change in illness severity was still a significant predictor of mortality (P = .001). Neural networks using clinical and laboratory data showed a high prognostic accuracy for predicting mortality in alcoholic patients with severe liver disease.

Adult↗

PubMed1985

Studies of parallel barrier performance by acoustical modeling.

An investigation is presented into the performance of parallel barrier configurations, using acoustical scale modeling. A realistic geometry is investigated, with the source being positioned over a paved roadway and the receiver over grass-covered ground. The grass-covered ground surface was properly modeled in terms of its impedance. Results were obtained for a range of barrier types, and demonstrate that frequency dependent effects are evident in barrier insertion loss data. In most cases, the barrier on the far side of the source did not significantly affect sound levels at the receiver. The most effective barrier design was found to be that of a gradual grass-covered slope up to an upright, thin barrier.

Acoustics↗

PubMed1990

Occupational performance measures: a review based on the guidelines for the client-centred practice of occupational therapy.

In 1987, Health and Welfare Canada and the Canadian Association of Occupational Therapists Task Force recommended that work go forward to develop an outcome measure for occupational therapy which reflects the Occupational Performance Model. The first step in this process was to review critically those outcome measures which assess occupational performance and that are currently available in the literature. This paper will present the review process, describe in more detail eight assessments that fulfilled many of the review criteria, discuss the limitations of these measures using the "Guidelines for the Client-centred Practice of Occupational Therapy as the framework, and make recommendations for the development of a new outcome measure for use in occupational therapy.

Activities of Daily Living↗

PubMedDate not supplied

Performance optimization of left ventricular assistance. A computer model study.

Performance of temporary parallel left ventricular assistance was investigated and the theoretic conditions leading to optimal behavior of the mechanical system were explored. Computer models of nonpulsatile and pulsatile left ventricular assist devices (LVADs) were incorporated into a previously reported closed-loop simulation of the canine cardiovascular system. Assuming the assisted heart was capable of recovery, LVAD performance was assessed based on both myocardial oxygen balance and cardiac output. With a synchronous LVAD, and operating in a counterpulsation mode, these variables were sensitive to the phasing of pump ejection. Maximum reduction in cardiac oxygen consumption, maximum increase in oxygen availability, and maximum increase in cardiac output with the atrio-aortic device were obtained when pump ejection immediately followed aortic valve closure. These variables were directly proportional to the magnitude of bypass volume. The pulsatile asynchronous and nonpulsatile LVAD models affected oxygen balance in a similar manner, but neither performed so well as the synchronous model when equal bypass volumes were used. Ventricular uptake of blood provided a further 27% decrease in oxygen consumption and further 78% increase in oxygen availability than atrial uptake. In summary, the model predicted that the pulsatile synchronous LVAD, filling from the ventricle during heart systole and ejecting into either the ascending or descending aorta just after ventricular systole, would be most beneficial to both myocardial oxygen balance and cardiac output.

Animals↗

PubMed1999

Prospective evaluation of a logistic model based on sonographic morphologic and color Doppler findings developed to predict adnexal malignancy.

To assess prospectively a logistic model based on sonographic morphologic and color Doppler findings, which had been developed to predict adnexal malignancy, 167 consecutive and unselected patients (mean age, 45.7 yr; range, 17 to 81 yr; 113 [67.7%] premenopausal and 54 [32.3%] postmenopausal) diagnosed as having an adnexal mass and scheduled for surgery were prospectively included in this study. All patients were evaluated by transvaginal color Doppler ultrasonography. The probability of adnexal malignancy was estimated prior to surgery, applying a logistic model developed previously. A probability of malignancy greater than 75% was considered to assess model performance. Sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were calculated for the model. In all cases definitive histopathologic diagnosis was obtained. One hundred and twenty-five (74.9%) benign and 42 (25.1%) malignant tumors were found. The sensitivity, specificity, positive predictive value, and negative predictive value of the model were 85.7% (95% confidence intervals, 71.4% to 94.6%), 100% (95% confidence intervals, 97.1% to 100%), 100% (95% confidence intervals, 90.3% to 100%), and 95.4% (95% confidence intervals, 90.3% to 98.3%), respectively. Overall accuracy was 96.4% (95% confidence intervals, 91.3% to 98.7%). Our results confirm the validity of the proposed logistic model in predicting adnexal malignancy.

Adnexal Diseases↗

PubMed1997

Probabilistic gas and bubble dynamics models of decompression sickness occurrence in air and nitrogen-oxygen diving.

Probabilistic models of the occurrence of decompression sickness (DCS) with instantaneous risk defined as the weighted sum of bubble volumes in each of three parallel-perfused gas exchange compartments were fit using likelihood maximization to the subset of the USN Primary Air and N2-O2 database [n = 2,383, mean P(DCS) = 5.8%] used in development of the USN LE1 probabilistic models. Bubble dynamics with one diffusible gas in each compartment were modeled using the Van Liew equations with the nucleonic bubble radius, compartmental volume, compartmental bulk N2 diffusivity, compartmental N2 solubility, and the N2 solubility in blood x compartmental blood flow as adjustable parameters. Models were also tested that included the effects of linear elastic resistance to bubble growth in one, two, or all three of the modeled compartments. Model performance about the training data and separate validation data was compared to results obtained about the same data using the LE1 probabilistic model, which was independently implemented from published descriptions. In the most successful bubble volume model, BVM(3), diffusion significantly slows bubble growth in one of the modeled compartments, whereas mechanical resistance to bubble growth substantially accelerates bubble resolution in all compartments. BVM(3) performed generally on a par with LE1, despite inclusion of 12 more adjustable parameters, and tended to provide more accurate incidence-only estimates of DCS probability than LE1, particularly for profiles in which high fractional O2 gas mixes are breathed. Values of many estimated BVM(3) parameters were outside of the physiologic range, indicating that the model emerged from optimization as a mathematical descriptor of processes beyond bubble formation and growth that also contribute to DCS outcomes. Although incomplete as a mechanistic description of DCS etiology, BVM(3) remains applicable to a wider variety of decompressions than LE1 and affords a conceptual framework for further refinements motivated by mechanistic principles.

Decompression Sickness↗

PubMed2002

Diagnostic cost groups (DCGs) and concurrent utilization among patients with substance abuse disorders.

OBJECTIVE: To assess the performance of Diagnostic Cost Groups (DCGs) in explaining variation in concurrent utilization for a defined subgroup, patients with substance abuse (SA) disorders, within the Department of Veterans Affairs (VA). DATA SOURCES: A 60 percent random sample of veterans who used health care services during Fiscal Year (FY) 1997 was obtained from VA administrative databases. Patients with SA disorders (13.3 percent) were identified from primary and secondary ICD-9-CM diagnosis codes. STUDY DESIGN: Concurrent risk adjustment models were fitted and tested using the DCG/HCC model. Three outcome measures were defined: (1) "service days" (the sum of a patient's inpatient and outpatient visit days), (2) mental health/substance abuse (MH/SA) service days, and (3) ambulatory provider encounters. To improve model performance, we ran three DCG/HCC models with additional indicators for patients with SA disorders. DATA COLLECTION: To create a single file of veterans who used health care services in FY 1997, we merged records from all VA inpatient and outpatient files. PRINCIPAL FINDINGS: Adding indicators for patients with mild/moderate SA disorders did not appreciably improve the R-squares for any of the outcome measures. When indicators were added for patients with severe SA who were in the most costly category, the explanatory ability of the models was modestly improved for all three outcomes. CONCLUSIONS: Modifying the DCG/HCC model with additional markers for SA modestly improved homogeneity and model prediction. Because considerable variation still remained after modeling, we conclude that health care systems should evaluate "off-the-shelf" risk adjustment systems before applying them to their own populations.

Adult↗

PubMed1992

Modeling the dishabituation hierarchy: the role of the primordial hippocampus.

We present a neural model for the organization and neural dynamics of the medial pallium, the toad's homolog of mammalian hippocampus. A neural mechanism, called cumulative shrinking, is proposed for mapping temporal responses from the anterior thalamus into a form of population coding referenced by spatial positions. Synaptic plasticity is modeled as an interaction of two dynamic processes which simulates acquisition and both short-term and long-term forgetting. The structure of the medial pallium model plus the plasticity model allows us to provide an account of the neural mechanisms of habituation and dishabituation. Computer simulations demonstrate a remarkable match between the model performance and the original experimental data on which the dishabituation hierarchy was based. A set of model predictions is presented, concerning mechanisms of habituation and cellular organization of the medial pallium.

Animals↗

PubMed2000

Risk adjusting capitation: applications in employed and disabled populations.

Risk adjustment may be a sensible strategy to reduce selection bias because it links managed care payment directly to the costs of providing services. In this paper we compare risk adjustment models in two populations (public employees and their dependents, and publicly-insured low income individuals with disabilities) in Washington State using two statistical approaches and three health status measures. We conclude that a two-part logistic/GLM statistical model performs better in populations with large numbers of individuals who do not use health services. This model was successfully implemented in the employed population, but the managed care program for the publicly insured population was terminated before risk adjustment could be applied. The choice of the most appropriate health status measure depends on purchasers' principles and desired outcomes.

Adolescent↗

PubMed1994

Young children's ability to understand a model as a spatial representation.

Children's ability to understand that a real environment can be represented in a symbolic form (i.e., by a model) is an important developmental achievement. Researchers have claimed that children who are just 3 years of age appreciate models as representations. This research was based on tasks that involved having young children use a model to locate a hiding place in an actual room. In this article, however, we point out the difficulties in interpreting previous model tasks, and we describe two studies that showed that young 3-year-olds could perform model tasks successfully when the hiding place they were looking for was a unique place in the model (and room). When the hiding place was unique, the children had to note only that place and they needed no further knowledge about the relationship between the model and the room. When the hiding place was one of two identical places, however, the children needed to take spatial relationships into account to distinguish the correct place, and young 3-year-olds were unable to do this. Four-year-olds were able to use spatial relationships to distinguish identical places when the model was aligned with the space it represented, but they had difficulty when the model was not aligned. Five-year-olds could use spatial relationships effectively between one model space and another whether or not the model was aligned.

Child↗

PubMedDate not supplied

Risk adjustment for measuring health outcomes: an application in VA long-term care.

An empirically derived risk adjustment model is useful in distinguishing among facilities in their quality of care. We used Veterans Affairs (VA) administrative databases to develop and validate a risk adjustment model to predict decline in functional status, an important outcome measure in long-term care, among patients residing in VA long-term care facilities. This model was used to compare facilities on adjusted and unadjusted rates of decline. Predictors of decline included age, time between assessments, baseline functional status, terminal illness, pressure ulcers, pulmonary disease, cancer, arthritis, congestive heart failure, substance-related disorders, and various neurologic disorders. The model performed well in the development and validation databases (c statistics, 0.70 and 0.68, respectively). Risk-adjusted rates and rankings of facilities differed from unadjusted ratings. We conclude that judgments of facility performance depend on whether risk-adjusted or unadjusted decline rates are used. Valid risk adjustment models are therefore necessary when comparing facilities on outcomes.

Aged↗

PubMed2000

Modeling selection for production traits under constant infection pressure.

This article presents a model describing the relationship between level of disease resistance and production under constant infection pressure. The model assumes that given a certain infection pressure, there is a threshold for resistance below which animals will stop producing, and that there is also a threshold for resistance above which animals produce at production potential. In between both thresholds animals will show a decrease in production, the size of decrease depending on the severity of infection and the level of resistance. The dynamic relationship between production and resistance when level of resistance changes, such as due to infection, is modeled both stochastically and deterministically. Selection started in a population with very poor level of resistance introduced in an environment with constant infection pressure. Mass selection on observed production was applied, which resulted in a nonlinear selection response for all three traits considered. When resistance is poor, selection for observed production results in increased level of resistance. With increasing level of resistance, selection response shifts to production potential and eventually selection for observed production is equivalent to selection for production potential. The rate at which resistance is improved depends on its heritability, the difference between both thresholds, and selection intensity. The model also revealed that when a zero correlation between resistance and production potential is assumed, the phenotypic correlation between resistance and observed production level increases for low levels of resistance and subsequently asymptotes to zero, whereas the phenotypic correlation between production potential and observed production asymptotes to 1.0. For most breeding schemes investigated, the deterministic model performed well in relation to the stochastic simulation results. Experimental results reported in literature support the model predictions.

Animal Husbandry↗

PubMed1998

Risk adjustment of mental health and substance abuse payments.

This study used 1992 and 1993 data from private employers to compare the performance of various risk adjustment methods in predicting the mental health and substance abuse expenditures of a nonelderly insured population. The methods considered included a basic demographic model, Ambulatory Care Groups, modified Ambulatory Diagnostic Groups and Hierarchical Coexisting Conditions (a modification of Diagnostic Cost Groups), as well as a model developed in this paper to tailor risk adjustment to the unique characteristics of psychiatric disorders (the "comorbidity" model). Our primary concern was the amount of unexplained systematic risk and its relationship to the likelihood of a health plan experiencing extraordinary profits or losses stemming from enrollee selection. We used a two-part model to estimate mental health and substance abuse spending. We examined the R2 and mean absolute prediction error associated with each risk adjustment system. We also examined the profits and losses that would be incurred by the health plans serving two of the employers in our database, based on the naturally occurring selection of enrollees into these plans. The modified Ambulatory Diagnostic Groups and comorbidity model performed somewhat better than the others, but none of the models achieved R2 values above .10. Furthermore, simulations based on actual plan choices suggested that none of the risk adjustment methods reallocated payments across plans sufficiently to compensate for systematic selection.

Actuarial Analysis↗

PubMed2001

Performance of a recirculating wetland filter designed to remove particulate phosphorus for restoration of Lake Apopka (Florida, USA).

Operation of a 14-km2 wetland filter for removal of total phosphorus (TP) from lake water is part of the restoration program for hypereutrophic Lake Apopka, Florida. This system differs from most treatment wetlands because 1) water is recirculated back to the lake, and 2) the goal is removal of particulate phosphorus (P), the dominant form of P in Lake Apopka. The operational plan for the wetland is maximization of the rate rather than the efficiency of P removal. The St. Johns River Water Management District operated a 2-km2 pilot-scale wetland to examine the capacity of a wetland system to remove suspended solids and particulate nutrients from Lake Apopka. TP in the inflow from Lake Apopka ranged from about 0.12 to 0.23 mg l(-1), and hydraulic loading rate (HLR) varied from 6.5 to 42 m yr(-1). The performance of the pilot-scale wetland supported earlier predictions. Mass removal efficiencies for TP varied between about 30% and 67%. A first-order, area-based model indicated a rate constant for TP removal of 55 m yr(-1). We compared actual removal of P with model predictions and used modeled performance to examine optimal operational conditions. Correspondence between observed and modeled outflow TP was not good with constant variable values. Monte Carlo techniques used to introduce realistic stochastic variability improved the fit. The model was used to project a maximal rate of P removal of about 4 g P m(-2) yr(-1) at P loading 10-15 g P m(-2) yr(-1) (HLR 60-90 m yr(-1)). Data from the pilot wetland indicated that actual rates of P removal may prove to be higher. Further operation of the wetland at high hydraulic and P loading rates is necessary to verify or modify the application of the model.

Conservation of Natural Resources↗

PubMed1990

Intentions to join HMOs: perceived relative performance versus satisfaction/dissatisfaction.

This study employed a national sample to investigate the determinants of consumers' intentions to join HMOs by considering different health care attributes. It employed two models, a performance model and a satisfaction model. The study found that the robustness of the performance model was substantially stronger in explaining intentions to join HMOs than the satisfaction model. More specific findings indicated that the health care attributes of cost and quality of care were important in both models, although to a far greater extent in the performance model, and that availability of physicians was also important in the performance model, albeit to a lesser degree. The study also examined demographic characteristics that may serve as a basis for segmenting the health care market. Of those considered, only two, age and urban/rural residence, were found to be significant. Age was considerably the more important of the two and younger respondents were far more interested in joining HMOs than their older counterparts. The pattern of results for the young subsample paralleled those found for the entire sample.

Adult↗

PubMed1997

Factors affecting atrazine fate in north central U.S. soils.

Atrazine persistence and fate are influenced by many factors, the interactions of which are difficult to predict. Several models, such as LEACHP (Wagenet and Hutson 1989), have been used as tools to estimate losses and identify variables that will impact the magnitude of loss. The LEACHP model was evaluated for predicting atrazine movement in sandy loam, silt loam, and clay loam soils during three consecutive years (two dry and one wet) in Minnesota (Khakural et al. 1995). Considering the broad range in soil properties and climatic conditions used in testing, the model performed well. However, these are only estimates, and additional field studies need to be conducted to verify model results. In a report by Fausey et al. (1995), the amount of atrazine found in groundwater throughout the Midwestern region was reported to be much below the MCL. However, specific sites in the Midwest may struggle with atrazine problems from both point and nonpoint sources of contamination. Some states, such as South Dakota, have created groundwater protection areas that alert growers and the public to sensitive areas where contamination may occur because of soil type, depth to groundwater, and distance to public wellheads. Wisconsin has developed a tiered managerial strategy, or zoning approach, in which restrictions are matched to pollution detections (Wolf and Nowak 1996). The USEPA has mandates for states to implement generic management plans to prevent pesticide contamination of groundwater. Chemical-specific plans by states will be required for at least five pesticides, one of which will be atrazine. Best management practices have been and are continuing to be developed to aid the grower in lessening the adverse impacts of atrazine. With continuing research into understanding the problem and developing solutions, and with adaptation of these recommendations by growers, the use of effective, inexpensive herbicides may continue with minimal off-site environmental effects.

Agriculture↗

PubMed1993

Evaluation of finite element analysis for prediction of the strength reduction due to metastatic lesions in the femoral neck.

Between 30 and 70% of almost one million new cancer patients diagnosed each year will develop osseous metastases. Clinicians are faced with the difficult task of determining which patients require prophylactic stabilization to prevent pathologic fracture. The objective of this study was to test the ability of macroscopic finite element models to predict the fracture strength of the proximal femur with a lesion in the femoral neck. Drill hole defects in human cadaver femora were used to simulate lesions that penetrate one cortex of the femoral neck. Based on the first of two series of in vitro experiments, the fracture strength of a femur with a lesion that penetrates either the inferior-medial or superior-lateral cortex of the neck is approximately 45% less than the fracture strength of the paired intact femur; based on the second series, the fracture strength with the inferior-medial lesion is approximately 20% less than the fracture strength with the superior-lateral lesion. A series of three-dimensional finite element models were used to predict the fracture strength for anterior and posterior lesions, as well as the inferior-medial and posterior-lateral lesions tested in vitro. Based on a direct comparison of the strengths predicted by the finite element models to the measured in vitro fracture strengths, the finite element models performed poorly. In particular, the application of an anisotropic strength criterion to the stresses predicted by the models resulted in a considerable underestimation of the percentage reduction in the in vitro fracture strength. This may reflect a fundamental inability of a linear, macroscopic continuum-based analysis to predict accurately the fracture strength of a bone structure as complex as the proximal femur. However, despite this lack of agreement in absolute fracture strength, the general trends for gait and stair ascent loading for the inferior-medial and superior-lateral lesions were consistent with the in vitro data. The greatest reduction in strength was predicted for the inferior-medial lesion, followed by the anterior lesion and then the superior-lateral lesion, and the least reduction in strength was predicted for the posterior lesion. Most importantly, the predicted strength ratio varied considerably as a function of the applied loads. Any metastatic lesions of the femoral neck may be especially sensitive to some particular activity, making it difficult to determine precisely the risk of fracture.

Aged↗