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PubMed2005

Real time optimal guidance of low-thrust spacecraft: an application of nonlinear model predictive control.

Real time optimal guidance is considered for a class of low thrust spacecraft. In particular, nonlinear model predictive control (NMPC) is utilized for computing the optimal control actions required to transfer a spacecraft from a low Earth orbit to a mission orbit. The NMPC methodology presented is able to cope with unmodeled disturbances. The dynamics of the transfer are modeled using a set of modified equinoctial elements because they do not exhibit singularities for zero inclination and zero eccentricity. The idea behind NMPC is the repeated solution of optimal control problems; at each time step, a new control action is computed. The optimal control problem is solved using a direct method-fully discretizing the equations of motion. The large scale nonlinear program resulting from the discretization procedure is solved using IPOPT--a primal-dual interior point algorithm. Stability and robustness characteristics of the NMPC algorithm are reviewed. A numerical example is presented that encourages further development of the proposed methodology: the transfer from low-Earth orbit to a molniya orbit.

Journal Article↗

PubMed1995

Predicting common bile duct lithiasis: determination and prospective validation of a model predicting low risk.

BACKGROUND: The aim of this two-part prospective study was: (1) to identify simple, noninvasive, preoperative factors associated with low or very low risk of common bile duct lithiasis (CBDL); and (2) to test the validity of the statistical model obtained during Part One by the postcholecystectomy follow-up of patients classified into a low-risk group. PATIENTS AND METHODS: In Part One of the study, preoperative clinical, biologic, and ultrasonographic data, and intraoperative cholangiographic findings were collected from 503 consecutive patients undergoing cholecystectomy for symptomatic lithiasis from 1985 to 1989. Using the data obtained in Part One, a linear logistic model was used prospectively in Part Two to determine the prediction of absence of CBDL in 279 consecutive patients. No jaundice, normal transaminase levels, common bile duct (CBD) diameter < 8 mm, and no intrahepatic duct enlargement defined the low-risk group of CBDL. RESULTS: In Part One, CBDL was present in 84 (17%) of all patients. Five parameters were used to classify 73% of all patients as low risk of CBDL and 27% as high risk. In the low-risk groups, CBDL was present in 1% of 116 cases with acute gallbladder complications, and 5% of 250 cases with no acute gallbladder complications. In Part Two, 171 (61%) patients were classified in the low-risk group (Group 1), and CBD stones were not sought by any additional preoperative investigations or intraoperative cholangiography (IOC). One hundred eight patients (39%) were considered at risk of CBDL (Group 2). Mean follow-up was 20.6 months (median 19); 2 patients (1%) in the low-risk group presented a symptomatic retained stone. CONCLUSIONS: This study validated this simple model for predicting risk of CBDL and avoiding invasive preoperative investigations--as well as IOC--in more than 60% of symptomatic cholelithiases. In addition, this model seemed useful for defining patients in whom further exploration for CBDL was justified, since 42 (39%) of the 108 Group 2 patients were proved to have CBDL.

Analysis of Variance↗

PubMed2000

A comparison of neonatal mortality risk prediction models in very low birth weight infants.

BACKGROUND: Risk-adjusted severity of illness is frequently used in clinical research and quality assessments. Although there are multiple methods designed for neonates, they have been infrequently compared and some have not been assessed in large samples of very low birth weight (VLBW; <1500 g) infants. OBJECTIVES: To test and compare published neonatal mortality prediction models, including Clinical Risk Index for Babies (CRIB), Score for Neonatal Acute Physiology (SNAP), SNAP-Perinatal Extension (SNAP-PE), Neonatal Therapeutic Interventions Scoring System, the National Institute of Child Health and Human Development (NICHD) network model, and other individual admission factors such as birth weight, low Apgar score (<7 at 5 minutes), and small for gestational age status in a cohort of VLBW infants from the Washington, DC area. METHODS: Data were collected on 476 VLBW infants admitted to 8 neonatal intensive care units between October 1994 and February 1997. The calibration (closeness of total observed deaths to the predicted total) of models with published coefficients (SNAP-PE, CRIB, and NICHD) was assessed using the standardized mortality ratio. Discrimination was quantified as the area under the curve (AUC) for the receiver operating characteristic curves. Calibrated models were derived for the current database using logistic regression techniques. Goodness-of-fit of predicted to observed probabilities of death was assessed with the Hosmer-Lemeshow goodness-of-fit test. RESULTS: The calibration of published algorithms applied to our data was poor. The standardized mortality ratios for the NICHD, CRIB, and SNAP-PE models were.65,.56, and.82, respectively. Discrimination of all the models was excellent (range:.863-.930). Surprisingly, birth weight performed much better than in previous analyses, with an AUC of.869. The best models using both 12- and 24-hour postadmission data, significantly outperformed the best model based on birth data only but were not significantly different from each other. The variables in the best model were birth weight, birth weight squared, low 5-minute Apgar score, and SNAP (AUC =.930). CONCLUSION: Published models for severity of illness overpredicted hospital mortality in this set of VLBW infants, indicating a need for frequent recalibration. Discrimination for these severity of illness scores remains excellent. Birth variables should be reevaluated as a method to control for severity of illness in predicting mortality.

Female↗

PubMed2002

A prediction model for selecting patients undergoing in vitro fertilization for elective single embryo transfer.

OBJECTIVE: Construction of a prediction model to enable the selection of patients for elective single ET. DESIGN: Retrospective cohort study. SETTING: Fertility center in a tertiary referral university hospital. PATIENT(S): Six hundred forty-two women undergoing their first IVF treatment cycle in which no more than two embryos were transferred. INTERVENTION(S): Database analysis. MAIN OUTCOME MEASURE(S): Ongoing pregnancy and multiple pregnancy. RESULT(S): In multivariate analysis, the best predictors for ongoing pregnancy were female age, the number of retrieved oocytes, the developmental stage score and the morphology score of the two best embryos available for transfer, and the day of transfer. Younger age and high quality of transferred embryos were the best predictors for increased risk of multiple pregnancy. The resulting model enables the calculation of probabilities of pregnancy and twin pregnancy. Depending on embryo quality, there is a threshold age under which the chance of singleton pregnancy is higher if one embryo is transferred compared with two embryos. CONCLUSION(S): Application of this model may enable a reduction in the chance of twin pregnancy without compromising singleton pregnancy rates in a subgroup of patients undergoing IVF.

Adult↗

PubMed2000

Nerve stimulation with a multi-contact cuff electrode: validation of model predictions.

The recruitment characteristics of muscle selective nerve stimulation by a multi-contact nerve cuff electrode, as predicted by computer modeling, have been investigated in acute experiments on rabbits. A nerve cuff containing five or six dot electrodes was placed around the sciatic nerve in five rabbits. M-waves were recorded with wire electrodes from the lateral gastrocnemius, soleus, tibialis anterior, and extensor digitorum longus muscles. The muscle recruitment performances of three contact configurations (monopole, transverse bipole, transverse tripole) were compared. The selectivity was quantified by the recruitment of two muscles (one extensor and one flexor) in response to a particular stimulus. The results showed that only in a few cases, transverse bi- and tripolar stimulation provided a better selectivity than monopolar stimulation. Neither of the two extensors, nor of the two flexors could be stimulated separately. In accordance with the results of the modeling studies, bi- and tripolar stimulation required higher stimulus currents than monopolar stimulation, whereas maximum recruitment and slopes of recruitment curves were lower. The rabbit sciatic nerve appears to be a less suitable preparation for reproducible selectivity experiments, due to the variability in the number and size of the fascicles and their position in this nerve.

Animals↗

PubMed1991

Effects of cannabidiol in animal models predictive of antipsychotic activity.

The effects of cannabidiol (CBD) were compared to those produced by haloperidol in rats submitted to experimental models predictive of antipsychotic activity. Several doses of CBD (15-480 mg/kg) and haloperidol (0.062-1.0 mg/kg) were tested in each model. First, CBD increased the effective doses 50% (or) ED50 of apomorphine for induction of the sniffing and biting stereotyped behaviors. In addition, both CBD and haloperidol reduced the occurrence of stereotyped biting induced by apomorphine (6.4 mg/kg), increased plasma prolactin levels and produced palpebral ptosis, as compared to control solutions. However, CBD did not induce catalepsy even at the highest doses, in contrast to haloperidol. Such a pharmacological profile is compatible with that of an "atypical" antipsychotic agent, though the mechanism of action is uncertain and may not be identical to that of the dopamine antagonists.

Animals↗

PubMed2004

Solution of low-dimensional constrained model predictive control problems.

Large benefits are possible by utilizing the solution of the constrained optimization problem involved in model predictive control. For a special case of these problems, the solution can be obtained relatively easily from its relationship with the unconstrained optimum. In this paper, a visualization of the relationship between the constrained and unconstrained optimum is presented. Based upon this relationship, a method for finding the constrained optimum is proposed that is suitable for low-dimensional control systems. A comparison with a linear programming formulation on 2 x 2 and 3 x 3 problems shows that the computational effort can be 10-35 times lower. For such processes, the proposed approach may allow one to avail the benefits of optimization by using the small process control systems already present in many plants.

Journal Article↗

PubMed2003

Impact of cardiovascular risk factors in an urban sample of Portuguese adults according to the Framingham risk prediction models.

OBJECTIVES: To estimate the 10-year risk of coronary heart disease (CHD) according to the Framingham risk prediction models and to assess the impact of major risk factors in an urban sample of Portuguese adults. METHODS: We surveyed 938 non-institutionalized adults aged 40 to 74 years, recruited by random digit dialing. A fasting venous blood sample was collected to measure glucose, total cholesterol and high-density-lipoprotein (HDL) cholesterol. Blood pressure was measured and a resting 12-lead ECG registered. Sixty-eight individuals (7.2%) were excluded due to prevalent CHD, according to self-reported information and/or ECG. Data on the remaining 532 women and 338 men at risk were used for analysis. For each participant we calculated the 10-year risk using the Framingham risk tables. RESULTS: In this community-based sample, the expected 10-year cumulative incidence was estimated to be on average 19.3% for men and 9.5% for women. The factor with greatest impact was hypertension. Had this factor been altered such that individuals whose blood pressure was > or = 140/90 mmHg instead had high-normal blood pressure (130-139/85-89 mmHg), the risk of CHD would have been reduced to 15.7% in men and 7.3% in women. Thus, 18.6% and 23.2% of cases would have been avoided, respectively. The number of individuals in whom hypertension should be prevented to avoid one case of CHD was 15 among men and 25 among women. The impact of other risk factors expressed as a proportion of attributable cases was, in men and women respectively, 5.7% and 7.4% if none had diabetes; 12.4% and 3.2% if none smoked; 23.8% and 14.7% if total cholesterol were between 160 and 199 mg/dl instead of over 200 mg/dl; and 10.9% and 5.3% if HDL were above 45 mg/dl. If all participants did not smoke, did not have diabetes, had total cholesterol between 160 and 199 instead of over 200 mg/dl, HDL over 45 mg/dl and high-normal blood pressure instead of hypertension, the expected cumulative incidence would be 8.1% for men and 5.1% for women. This means that respectively 58.0% and 46.3% of cases would have been avoided. CONCLUSION: Blood pressure control in this population is the intervention with greatest impact on CHD prevention. Given the tendency for risk factors to cluster and to interact, we can expect to achieve even greater success with multifactorial interventions.

Adult↗

PubMedDate not supplied

The Nordic Back Pain Subpopulation Program: validation and improvement of a predictive model for treatment outcome in patients with low back pain receiving chiropractic treatment.

OBJECTIVE: The aim of this study was to develop a predictive model for treatment outcome in patients with low back pain (LBP) receiving chiropractic treatment. METHODS: This multicenter practice-based predictive validity study was conducted in private chiropractic practices in Sweden. Of 64 previously compliant chiropractors, 58 recruited a maximum of 30 consecutive patients with LBP each. Information was provided on 1061 patients, of which 1057 questionnaires were valid. Chiropractic treatment was decided by the treating chiropractor. The outcome variable was the self-reported "definite improvement" at the fourth visit. The predictor variables included model 1, 3 hypothesized prognostic groups (best, intermediate, and least favorable) based on clinical information collected at baseline and at the second visit; and 4 additional models based on the following variables: age, sex, pain intensity during past 24 hours, description of disability, duration and pattern of pain during present attack, total duration of pain, and pain pattern during the past 12 months. RESULTS: Three of our factors were best at predicting the absence of improvement by the fourth visit and were able to correctly classify 79% of patients and to cover 74% of the receiver operated characteristics curve. These were (1) no definite overall improvement by the second treatment, (2) presence of leg pain, and (3) the minimum total duration of pain over the last 12 months being 30 days. CONCLUSION: In this study, patients with LBP who also had leg pain and LBP occurring sufficiently frequently or having lasted sufficiently long to add up to at least 30 days in the past year, and who did not report definite general improvement by the second treatment were not good candidates for short-term recovery. It is suggested that patients who fit the criteria of potential nonresponders should be carefully monitored to allow a selective approach of care.

Adult↗

PubMed1987

A predictive model for determining asbestos concentrations for fibers less than five micrometers in length.

The controversy of whether small asbestos fibers are biologically significant has not been resolved. The present standard method for evaluating asbestos fiber concentrations in workroom air excludes fibers less than 5 micron long even though it has been shown that small fiber concentrations dominate in a dust cloud. This research project was conducted to develop a mathematical model whereby one could predict small (less than 5 micron length) asbestos fiber concentration based on the fiber count concentration determined by phase contrast microscope analysis. Dry chrysotile asbestos was aerosolized into a chamber and sampled by membrane filtration. Segments from each filter were analyzed by both the NIOSH technique using phase contrast microscopy (PCM) and scanning electron microscopy (SEM) at 2000 X for fiber concentrations. A linear relationship was found to exist between the natural logarithm of the SEM-determined concentration and the natural logarithm of the PCM-determined concentration (r = 0.852). Using these data, a mathematical model was developed to predict SEM concentrations based on PCM counts. This model may have application in retrospective epidemiological studies for estimating small fiber exposure levels to determine if small fibers play a role in disease production. The greatest utility would be in those retrospective studies where the only exposure information available is based on PCM counts.

Air Pollutants↗

PubMed2004

Comparison of mathematical model predictions to experimental data of fatigue and performance.

As part of the "Fatigue and Performance Modeling Workshop," six modeling teams made predictions for temporal profiles of fatigue and performance in five different scenarios. One scenario was based on a laboratory study of fatigue and performance during 88 h of extended wakefulness with or without nap opportunities. Another scenario was based on a field study of alertness in freight locomotive engineers. Two scenarios were based on laboratory studies with various conditions of chronic sleep restriction and recovery. There was a theoretical scenario for future ultra-long-range flight operations as well. Experimental data were available for all scenarios except the latter. The model predictions were compared with the experimental data; after linear scaling using mixed-effects regression, mean square errors were computed to quantify goodness-of-fit. The six models were also compared among each other on the basis of these mean square errors. The present paper provides detailed information about the results of these comparisons. The models were capable of predicting the data for some scenarios fairly well. However, predicting the data for the two scenarios involving chronic sleep restriction was more problematic. Differences among the predictions from the six models were relatively small, suggesting that these models have a broad common basis. More experimental research is needed to yield new insights for the further development of fatigue and performance models.

Adult↗

PubMedDate not supplied

Predictive models for mortality and length of hospital stay in an Egyptian burns centre.

Our aim was to obtain a statistical profile of survivors and deaths among burn victims and to develop predictive models for mortality and length of hospital stay. All patients admitted to the Burns Unit of Alexandria Main University Hospital over a 1-year period were included. Of 533 cases, mean length of hospital stay was 15.5 +/- 21.6 days and the mortality rate was 33%. Total surface area burnt, inhalation burns, age, sex, depth and degree of burn wounds were the significant independent predictors of mortality in multiple logistic regression analysis. The significant independent predictors of the length of hospital stay were clothing ignition, total surface area burnt, sex, degree and depth of burn and inhalation burns.

Adolescent↗

PubMed1991

A comparison of static and dynamic characteristics between rectus eye muscle and linear muscle model predictions.

The characteristics of a muscle model are analyzed using rectus eye muscle parameter values and compared to rectus eye muscle data. The muscle is modeled as a viscoelastic parallel combination connected to a parallel combination of active state tension generator, viscosity element, and length tension elastic element. Each of the elements is linear and their existence is supported with physiological evidence. The static and dynamic properties of the muscle model are compared to rectus eye muscle data. The length-tension characteristics of the model are in good agreement with the data within the operating region of the muscle. With the muscle model incorporated into a lever system to match the isotonic experiment paradigm, simulation results for this linear system yield a nonlinear force-velocity curve. Moreover, the family of force-velocity curves generated with different stimulus rates reported in the literature match the predictions of the model without parametric changes. The results of this paper are important in studies involving the oculomotor plant and oculomotor neural networks. Additionally, these results may be applicable to other muscles.

Isometric Contraction↗

PubMed2006

Estimating the combined toxicity by two-step prediction model on the complicated chemical mixtures from wastewater treatment plant effluents.

The toxicities of chemical mixtures containing 10 compounds, detected in wastewater treatment plant (WWTP) effluents, were investigated using Daphnia magna in a two-step prediction (TSP) model. The 10 chemicals determined by gas chromatography/ mass spectrometry in WWTP effluents included three groups: Three acetylcholinesterase inhibitors, six narcosis inhibitors, and one seedling root inhibitor. In the first step, a concentration addition (CA) model was used to predict the mixture toxicities for the three component groups with similar modes of action; in the second step, an independent action (IA) model was used for the newly developed concentration-response curves from the three CA predictions. The CA predictions did not show a statistically significant difference from the observed results with respect to the three groups of chemicals, whereas the IA model did not conform to the experimental results. Therefore, the concentration-response curves obtained from the mixture toxicity tests in each group was considered as a single curve and applied in the next step of the mixture toxicity prediction. However, the observed toxicity of the 10-chemical mixture showed large differences from the results of the IA and CA model predictions, whereas the TSP model predicted the toxicity well and with statistical significance (p = 0.0501, n = 17). This suggests that the TSP model would provide a valid prediction for a randomly selected chemical mixture having various modes of action if the concentration-response function for an individual component is obtained.

Animals↗

PubMed1998

Predictive models of carpal tunnel syndrome causation among VDT operators.

Carpal tunnel syndrome (CTS), a cumulative trauma disorder of the hand and wrist, is one of the most common disabling injuries experienced by video-display terminal (VDT) operators. The purpose of this study was to develop a theoretically based operational quantitative predictive model of the risk of work-related CTS among VDT operators. A total of 100 female VDT operators, who performed a variety of office functions, were studied at a major midwestern university. Data were collected on job exposure, anthropometry and posture factors using questionnaires, direct observation and video-recording. Discriminant analysis and logistic regression were performed to develop the operational models. The results of the study indicated the following: (1) percentage of workday working with a VDT was the most significant factor and accounted for 60% of the variance explaining the causation of musculoskeletal discomforts associated with CTS; (2) discriminant function with six variables (i.e. work duration, trunk incline, wrist extension, wrist ulnar deviation, overall anthropometric measure, weighted anthropometric measure) correctly classified 73% of the CTS group and 72% of the non-CTS group; (3) using the logistic regression model, the probabilities associated with changes in the predictive variables as affecting CTS risk are presented such that increasing the daily work duration from 1 h to 4 h increases the probability of CTS risk from 0.45 to 0.92. The results of the study suggest that the main causation of CTS is job design, the secondary (and lesser cause) is posture associated with the workplace design and the least contributing factor to CTS causation is the individual's anthropometric make-up.

Analysis of Variance↗

PubMed2004

Simple statistical models predict C-to-U edited sites in plant mitochondrial RNA.

BACKGROUND: RNA editing is the process whereby an RNA sequence is modified from the sequence of the corresponding DNA template. In the mitochondria of land plants, some cytidines are converted to uridines before translation. Despite substantial study, the molecular biological mechanism by which C-to-U RNA editing proceeds remains relatively obscure, although several experimental studies have implicated a role for cis-recognition. A highly non-random distribution of nucleotides is observed in the immediate vicinity of edited sites (within 20 nucleotides 5' and 3'), but no precise consensus motif has been identified. RESULTS: Data for analysis were derived from the the complete mitochondrial genomes of Arabidopsis thaliana, Brassica napus, and Oryza sativa; additionally, a combined data set of observations across all three genomes was generated. We selected datasets based on the 20 nucleotides 5' and the 20 nucleotides 3' of edited sites and an equivalently sized and appropriately constructed null-set of non-edited sites. We used tree-based statistical methods and random forests to generate models of C-to-U RNA editing based on the nucleotides surrounding the edited/non-edited sites and on the estimated folding energies of those regions. Tree-based statistical methods based on primary sequence data surrounding edited/non-edited sites and estimates of free energy of folding yield models with optimistic re-substitution-based estimates of approximately 0.71 accuracy, approximately 0.64 sensitivity, and approximately 0.88 specificity. Random forest analysis yielded better models and more exact performance estimates with approximately 0.74 accuracy, approximately 0.72 sensitivity, and approximately 0.81 specificity for the combined observations. CONCLUSIONS: Simple models do moderately well in predicting which cytidines will be edited to uridines, and provide the first quantitative predictive models for RNA edited sites in plant mitochondria. Our analysis shows that the identity of the nucleotide -1 to the edited C and the estimated free energy of folding for a 41 nt region surrounding the edited C are the most important variables that distinguish most edited from non-edited sites. However, the results suggest that primary sequence data and simple free energy of folding calculations alone are insufficient to make highly accurate predictions.

Arabidopsis↗

PubMed2002

Validation of a model predicting spontaneous pregnancy among subfertile untreated couples.

OBJECTIVE: To provide external validation of the Eimers model, which predicts spontaneous pregnancy among subfertile couples within the first year after the definitive establishment of the diagnostic category. DESIGN: Live birth rates predicted by an adapted version of the Eimers model were tested against observed live birth rates in a Canadian cohort study. SETTING: Fertility clinics in university medical centers. PATIENT(S): One thousand sixty-one couples consulting for subfertility due to cervical hostility, male subfertility, or unexplained subfertility. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): We measured the discriminative ability and reliability of the predictions from the model. RESULT(S): The live birth rate was lower in the Canadian population than in the Eimers population. Overall, the prognostic effect of the predictors did not differ significantly in both populations. The model showed moderate predictive power in the Canadian population. With adjustment of the average live birth rate, the reliability of the model was satisfactory. CONCLUSION(S): The Eimers model gave reliable spontaneous pregnancy predictions in the Canadian validation population after adjustment of the average live birth rate.

Adult↗

PubMed1986

Predictive model for prognosis in advanced diffuse histiocytic lymphoma.

The purpose of this study was to examine the validity of a predictive model for response to treatment and survival in advanced diffuse histiocytic lymphoma. One hundred twenty-seven consecutive patients with Ann Arbor stage II-IV diffuse histiocytic lymphoma, who completed treatment between 1974 and 1984 in one of four different Memorial Hospital combination chemotherapy protocols, were reviewed. The median follow-up time was 66.9 months for survivors (range, 21-153.1 months). Factors studied included: age; sex; Ann Arbor stage; prior therapy; B symptoms; serum lactic dehydrogenase (LDH); sites of initial disease; and tumor bulk. LDH was grouped accordingly (units/liter): low, less than 225; medium, 225-500; high, greater than 500. Each patient was assigned an overall level of site involvement (LSI) from the following mutually exclusive groups: group I, peripheral lymph node (PLN) (including +/- Waldeyer ring involvement, +/- spleen); group II, extranodal disease (EN) +/- PLN; group III, retroperitoneal lymph node (RLN) +/- PLN; group IV, bulky mediastinal disease (MED) +/- any other disease; group V, EN with RLN +/- PLN. The Ann Arbor staging system failed to dissect patient groups differing significantly in their prognosis. Serum LDH, LSI, and age were the only factors important for predicting response and survival after multivariate logistic regression and a parametric Weibull survival analysis. Using three levels of serum LDH and correlating them with the different LSI, four tentative "stages" differing significantly in their survival at 48 months were defined: stage I, low LDH, any LSI (80% alive); stage II, medium LDH, PLN, and/or EN (50% alive); stage III, high LDH, PLN, and/or EN or medium LDH, RLN +/- PLN +/- EN, and/or MED (35% alive); stage IV, high LDH, RLN +/- PLN +/- EN, and/or MED (15% alive). Identification of prognostic stages on the basis of LDH level and LSI will allow more accurate comparison of clinical trials for patients with advanced diffuse histiocytic lymphoma.

Adult↗