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At least 91 records · Page 5Linked to original sources
PubMed1993

A computer simulation of the plasma leakage through a vascular prosthesis made of expanded polytetrafluoroethylene.

We explored the blood-retaining mechanism of a vascular prosthesis made of expanded polytetrafluoroethylene through analysis of its structure and physicochemical properties. Plasma leakage through this vascular prosthesis was simulated by computer to explore its etiology. These examinations disclosed that leakage is dependent upon the inner pressure and the density of fibers. In other words, the study revealed that the mean distance between fibers constituting the wall of the expanded polytetrafluoroethylene vascular prosthesis is increased by tension (that is, inner pressure), resulting in an increased probability of leakage. It was additionally found that a thin membrane is formed on the polytetrafluoroethylene surface if blood in contact with the surface is dried. This membrane was found to reduce the water-repelling property of polytetrafluoroethylene and to make it impossible to preserve the inter-fiber liquid surface, thus causing leakage through the expanded polytetrafluoroethylene vascular prosthesis.

Blood Vessel Prosthesis↗
PubMed1992

A method for computer simulation of ultrasound Doppler color flow images--II. Simulation results.

A computational method of simulating Doppler color flow images has been developed. It is based on a point-scattering model of moving blood and surrounding tissue and is capable of treating the entire flow image generation process. Simulated images of parabolic flow dynamics in a cylindrical vessel are presented to show the statistical nature of the map of velocity estimates and to demonstrate the effects of wall filters and different display schemes. Quantitative results of extracted velocity profiles are included and indicate the usefulness of the simulation method for studying the quantitative capabilities of flow imaging.

Blood Flow Velocity↗
PubMed1999

Computer simulations of global memory models.

Computer simulation models of memory--including the so-called global memory models--have had and continue to have a profound impact on current memory research and theory. Unfortunately, no memory textbooks published before 1998 present these models to students. The arguments for and against these types of models are summarized along with reasons why the models should be taught at both the undergraduate and graduate levels. A set of freely available computer programs that can facilitate classroom presentation is then briefly described.

Computer Simulation↗
PubMed1986

Computer simulations of macromolecular dynamics: models for vibrational spectroscopy and X-ray refinement.

The use of computer simulations to study the internal dynamics of globular proteins and nucleic acids has attracted a great deal of attention in recent years. These simulations constitute the most detailed theoretical approach available for studying internal motions and structural flexibility of biological macromolecules. In this paper we review recent work concerned with the use of computer simulations for the interpretation of, and comparison with, experimental probes of molecular dynamics. New methods for calculating vibrational spectroscopic lineshapes from computer simulations are discussed. A quasiharmonic approximation is described by which classical computer simulations on multidimensional potential surfaces can be used to estimate the effects of anharmonicity on vibrational spectra. A novel aspect of the method is the use of normal-mode eigenvectors as the independent coordinates for Monte Carlo sampling. Results for isolated small molecules and liquid water are reviewed. The construction of vibrational lineshapes from quantum computer simulations using path integral methods with new (quasiharmonic and variable quadratic) reference systems are discussed. Results for small model systems are presented and extension of the methods to large molecules is discussed. The recent use of molecular dynamics simulations to analyze X-ray refinement models for proteins and nucleic acids is also reviewed.

Biopolymers↗
PubMed2002

A comparison of the training value of two types of anesthesia simulators: computer screen-based and mannequin-based simulators.

UNLABELLED: In this study, we compared two different training simulators (the computer screen-based simulator versus the full-scale simulator) with respect to training effectiveness in anesthesia residents. Participants were evaluated in the management of a simulated preprogrammed scenario of anaphylactic shock using two variables: treatment score and diagnosis time. Our results showed that simulators can contribute significantly to the improvement of performance but that learning in treating simulated crisis situations such as anaphylactic shock did not significantly vary between full-scale and computer screen-based simulators. Consequently, the initial decision on whether to use a full-scale or computer screen-based training simulator should be made on the basis of cost and learning objectives rather than on the basis of technical or fidelity criteria. Our results support the contention that screen-based simulators are good devices to acquire technical skills of crisis management. Mannequin-based simulators would probably provide better training for behavioral aspects of crisis management, such as communication, leadership, and interpersonal conflicts, but this was not tested in the current study. IMPLICATIONS: We compared two different training simulators (computer screen-based versus full-scale) for training anesthesia residents to better document the effectiveness of such devices as training tools. This is an important issue, given the extensive use and the high cost of mannequin-based simulators in anesthesiology.

Anaphylaxis↗
PubMed2004

How enzymes work: analysis by modern rate theory and computer simulations.

Advances in transition state theory and computer simulations are providing new insights into the sources of enzyme catalysis. Both lowering of the activation free energy and changes in the generalized transmission coefficient (recrossing of the transition state, tunneling, and nonequilibrium contributions) can play a role. A framework for understanding these effects is presented, and the contributions of the different factors, as illustrated by specific enzymes, are identified and quantified by computer simulations. The resulting understanding of enzyme catalysis is used to comment on alternative proposals of how enzymes work.

Catalysis↗
PubMed2006

[Computer simulation methods of cardiac electrophysiology].

Computer simulation is one of the powerful protocols to study electrophysiology theoretically. In this paper,the algorithm of Rush and Larsen was used to solve the ordinary differential equations (ODE's) in the Luo-Rudy models of mammalian ventricular cell. The operator splitting and adaptive time step methods were used to solve the partial differential equations (PDE's) in cardiac tissue conduction models. Using these methods we accomplished the simulation programs of single ventricular cell model and two-dimensional (2-D) tissue model. The methods of initiating spiral waves were studied with this software. The data obtained from 2-D simulation can be used for further study on isopotential contour lines, spiral wave tip trajectories, and pseudo-ECG.

Algorithms↗
PubMed1994

Optimal location for a helicopter in a rural trauma system: prediction using discrete-event computer simulation.

A discrete-event computer simulation was developed using the C programming language to determine the optimal base location for a trauma system helicopter in Maine, a rural area with unevenly distributed population. Ambulance run reports from a one-year period provided input data on the times and places where major injuries occurred. Data from a statewide trauma registry were used to estimate the percentage of cases which would require trauma center care and the locations of functional trauma centers. Climatic data for this region were used to estimate the likelihood that a helicopter could not fly due to bad weather. The incidence of trauma events was modeled as a nonstationary Poisson process, and location of the events by an empirical distribution. For each simulated event, if the injuries were sufficiently severe, if weather permitted flying, if the occurrence were not within 20 miles of a center or outside the range of the helicopter, and if the helicopter were not already in service, then it was used for transportation. 35 simulated years were run for each of 4 proposed locations for the helicopter base. One of the geographically intermediate locations was shown to produce the most frequent utilization of the helicopter. Discrete-event simulation is a potentially useful tool in planning for emergency medical services systems. Further refinements and validation of predictions may lead to wider utilization.

Air Ambulances↗
PubMed2000

Progression criteria for cancer antigen 15.3 and carcinoembryonic antigen in metastatic breast cancer compared by computer simulation of marker data.

BACKGROUND: We investigated the utility of computer simulation models for performance comparisons of different tumor marker assessment criteria to define progression or nonprogression of metastatic breast cancer. METHODS: Clinically relevant values for progressive cancer antigen 15.3 and carcinoembryonic antigen concentrations were combined with representative values for background variations in a computer simulation model. Fifteen criteria for assessment of longitudinal tumor marker data were obtained from the literature and computerized. Altogether, 7200 different patients, each based on 50 measurements, were simulated. With a sampling interval of 4 weeks, the monitoring period for each event was approximately 3.8 years. RESULTS: Modulation of the background variation, the starting concentrations, and the cutoffs enabled identification of criteria that were robust against false-positive signals of progression. CONCLUSIONS: The computer simulation model is a fast, effective, and inexpensive approach for comparing the diagnostic potential of assessment criteria during clinically relevant conditions of steady-state and progressive disease. The model systems can be used to generate tumor marker assessment criteria for a variety of malignancies and to compare and optimize their diagnostic performance.

Breast Neoplasms↗
PubMed1995

Reoxygenation of hypoxic cells by tumor shrinkage during irradiation. A computer simulation.

PURPOSE: A 3-dimensional computer simulation was developed in order to estimate the impact of tumor shrinkage on reoxygenation of chronic hypoxic tumor cells during a full course of fractionated irradiation. MATERIAL AND METHODS: The growth of a small tumor situated in a vascularized stroma with 350 capillary cross-sections/mm3 which were displaced by the growing tumor was simulated. Tumors contained 10(4) cells when irradiation started, intrinsic radiosensitivity was set to either low (alpha = 0.3 Gy-1, beta = 0.03 Gy-2) or high (alpha = 0.4 Gy-1, beta = 0.04 Gy-2) values. Oxygen enhancement ratio was 3.0, potential tumor doubling time Tpot = 1.2 or 5 days. A simulated fractionated radiotherapy was carried out with daily fractions of 2.0 Gy, total dose 50 to 70 Gy. The presence or absence of factors preventing tumor cord shrinkage was also included. RESULTS: During the growth phase, all tumors developed a necrotic core with a hypoxic cell fraction of 25% under these conditions. During irradiation, the slower growing tumors (Tpot = 2 to 5 days) showed complete reoxygenation of the hypoxic cells after 30 to 40 Gy independent from radiosensitivity, undisturbed tumor shrinkage provided. If shrinkage was prevented, the hypoxic fraction rose to 100% after 30 to 50 Gy. Local tumor control, defined as the destruction of all clonogenic and hypoxic tumor cells increased by 20 to 100% due to reoxygenation and 50 Gy were enough in order to sterilize the tumors in these cases. In the fast growing tumors (Tpot = 1 day), reoxygenation was only observed in the case of high radiosensitivity and undisturbed tumor shrinkage. In these tumors reoxygenation increased the control rates by up to 60%. CONCLUSION: The simulation results suggests that shrinking of a homogeneous tumor during irradiation with restoration of the normal vascular architecture can contribute significantly to reoxygenation, which in turn has a major effect on tumor control.

Capillaries↗
PubMed1990

Blood or urine glucose-based insulin therapy and control of glycemia. Computer-simulation study.

Adjustment algorithms for conventional insulin therapy must be tested for safety and efficacy before clinical implementation. We did this by computer simulation. Accordingly, a computer simulator of human intermediary metabolism created 10 randomly chosen diabetic subjects for study. All were well defined with respect to compliance (i.e., medication and diet) and life-style (i.e., physical and emotional stress). Insulin-adjustment algorithms that were tested calculated daily insulin dosages for these computer-simulated patients based on either blood or urine glucose concentrations self-measured 4 times/day before breakfast, lunch, dinner, and bedtime snack. The twofold purpose of the simulation study was to determine the ability of the adjustment algorithms to improve initially poor metabolic control and to compare the outcomes when either blood or urine glucose measurements were the basis on which glycemic control was implemented. A significant improvement in metabolic control could be achieved with either blood or urine glucose measurements as input to the algorithms. Detailed comparisons between blood and urine glucose-based treatments showed no significant advantage of blood glucose-based algorithms at breakfast (122 +/- 21 vs. 131 +/- 16 mg/dl) and dinner (117 +/- 27 vs. 130 +/- 23 mg/dl), whereas mean glycemia at lunch (122 +/- 24 vs. 164 +/- 21 mg/dl) and bedtime (117 +/- 25 vs. 150 +/- 21 mg/dl) after 120 days of simulation did differ significantly (P less than 0.01). Hypoglycemia was not provoked by either treatment. Total daily insulin doses evolved by blood glucose-based algorithms were significantly (P less than 0.05) higher than the doses used by urine glucose-based algorithms (53 vs. 47 U).(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1985

Computer simulation of aqueous biomolecular systems.

Computer simulation techniques are increasingly being used to predict structural and thermodynamic properties of large heterogeneous macromolecule and solvent assemblies. We discuss, with examples from our own studies, some problems we and others have experienced in using these techniques, which were originally devised for simple liquids. In particular, we consider the problems which arise from the large size and heterogeneity of macromolecule water systems, comparisons with experimental data and equilibrium and sampling procedures.

Computer Simulation↗
PubMed1987

The aggregation of reverse micelles. A computer simulation.

We have developed a computer simulation to model the formation of reverse micelles in two dimensions. Several of the qualitative results obtained from these calculations agree with experimental observations. Specifically, we have shown that the chain length has a large influence in determining the size and shape of the aggregate. We predict the existence of a critical tail length: Chains below this value will form an extended lamella-like structure, whereas chains longer than this value will form clusters that appear ellipsoid or circular in cross-section. Finally, we obtained a scaling law that relates the aggregation number (N) to the length of the tail (L): N approximately L-1.14. A physical model to account for the observed exponent will be developed in a future paper.

Biophysical Phenomena↗
PubMed2005

Formation of the chick primitive streak as studied in computer simulations.

We have used a computer simulation system to examine formation of the chick primitive streak and to test the proposal (Wei and Mikawa Development 127 (2000) 87) that oriented cell division could account for primitive streak elongation. We find that this proposal is inadequate to explain elongation of the streak. In contrast, a correctly patterned model streak can be generated if two putative mechanisms are operative. First, a subpopulation of precursor cells that is known to contribute to the streak is assigned a specific, but simple, movement pattern. Second, additional cells within the epiblast are allowed to incorporate into the streak based on near-neighbor relations. In this model, the streak is cast as a steady-state system with continuous recruitment of neighboring epiblast cells, egress of cells into deeper layers and an internal pattern of cell movement. The model accurately portrays elongation and maintenance of a robust streak, changes in the composition of the streak and defects in the streak after experimental manipulation.

Animals↗
PubMed1988

The costs of employee smoking. A computer simulation of hospital nurses.

A computer simulation model of the economic impact of employee smoking on an employer was developed with a population of hospital nurses as the example. Net economic impact was calculated by estimating cumulative costs borne by the employer under various scenarios and comparing them with projected costs estimated from baseline data. The model included the following: baseline number of staff, baseline percentage of smokers, annual employee turnover rate, number of smokers interested in quitting, the cost of a smoking cessation program, expected success rate of a voluntary cessation program, smokers quitting spontaneously without a program, and employer-borne costs related to employee smoking. A variety of scenarios were constructed to generate a range of employer net economic impact figures and resulting percentages of smoking employees. Results showed that the benefits of a cessation program would be eliminated over several years, unless the prevalence of smoking in incoming employees was reduced. The most favorable scenario, combining a hospital cessation program and reduced smoking among new employees, generated cumulative savings, discounted at 5%, of $358,000 to $684,000 over an eight-year period.

Computer Simulation↗
PubMed1984

The ionic currents underlying pacemaker activity in rabbit sino-atrial node: experimental results and computer simulations.

The membrane currents underlying the pacemaker depolarization have been investigated in rabbit s.a. node preparations using the two-microelectrode voltage clamp technique. Many of the experimental results have been simulated using a computer model of s.a. node electrical activity. Changes of three time-dependent membrane currents which could contribute to pacemaker depolarization are found to occur in the relevant potential range: decay of the potassium current, iK, and activation of the inward current, if, and of the slow inward current, isi. The contribution of if activation to the pacemaker depolarization ranges from nil to an appreciable part depending on the preparation; when Cs (1 mM) blocks if, it nevertheless does not prevent pacemaking. In the model, holding the if activation variable at zero slows but does not stop pacemaking; doubling if conductance and shifting its activation curve by 15 mV in the positive direction causes a 15% faster rate of pacemaking. The slow time course of re-availability of isi must be allowed for when determining the isi threshold. A voltage clamp protocol designed to mimic as closely as possible an action potential followed by a pacemaker depolarization gives an estimate of isi threshold at the potential level of the last third of the pacemaker depolarization. This has been confirmed in experiments in which the voltage clamp was switched on at different points in the pacemaker depolarization. In the computer simulation, 'blocking' isi depolarizes the membrane to the zero current level (close to the potential reached at the end of a pacemaker depolarization) and stops the generation of action potentials. The decay of iK contributes to the pacemaker depolarization; with both our own model and that of K. Yanagihara, A. Noma and H. Irisawa, Jap. J. Physiol. 30, 841-857 (1980) 'blocking' iK decay abolishes pacemaker activity. Computations of extracellular K+ concentration changes compared with iK decay in a cylindrical model allow re-assessment of the interpretation of K+ concentration measurements during pacemaking made by J. Maylie, M. Morad and J. Weiss, J. Physiol., Lond. 311, 167-178 (1981).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2004

Full scale computer simulators in anesthesia training and evaluation.

PURPOSE: With the advent of competency-based curriculum, technology such as full scale computer simulators have acquired an increasingly important role in anesthesia both in training and evaluation. This article reviews the current role of full scale computer simulators in teaching and evaluation in anesthesia. SOURCE: This review draws from existing anesthesia and medical education literature in order to examine and assess the current role of full scale computer simulators in anesthesia education today. PRINCIPAL FINDINGS: The last decade has witnessed a major increase in the use of full scale computer simulators in anesthesia. Many applications have been found for these simulators including teaching and training, evaluation and research. Despite the increasing use and application of full scale computers in anesthesia in the area of teaching and training, definitive studies evaluating its cost effectiveness, its efficacy compared to traditional training methods or its impact on patient outcome are still pending. Although there is some preliminary evidence of reliability and validity in using the simulator to evaluate clinical competence, development in this area has not progressed enough to justify its use in formal, summative evaluation of competence in anesthesia at this time. CONCLUSIONS: As technology acquires an increasingly important role in medical education, full scale computer simulators represent an exciting potential in anesthesia. However, the full potential and role of simulators in anesthesia is still in development and will require a dovetailing of clinical theory and practice with current research in medical education.

Anesthesiology↗
PubMed1991

Testing life-cycle theory by computer simulation--II. Bet-hedging revisited.

Analytical and computer models were used to reexamine bet-hedging, the reduction in fecundity that is evolutionarily advantageous in conditions of greater variation in juvenile survivorship or less variation in adult survivorship. The computer simulation models represent diploid one-locus genetic systems with semidominance. Schaffer's (1974) predictions proved remarkably robust when variations were symmetrical, and a simple modification allowed successful prediction for the asymmetries that occurred in the computer simulations when variations were large.

Animals↗