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At least 1,369 records · Page 76Linked to original sources
PubMed1996

Computer simulation of epicardial potentials using a heart-torso model with realistic geometry.

Previous cardiac simulation studies have focused on simulating the activation isochrones and subsequently the body surface potentials. Epicardial potentials, which are important for clinical application as well as for electrocardiographic inverse problems studies, however, have usually been neglected. This paper describes a procedure of simulating epicardial potentials using a microcomputer-based heart-torso model with realistic geometry. Our heart model developed earlier is composed of approximately 65,000 cell units which are arranged in a cubic close-packed structure. An action potential waveform with variable in duration is assigned to each unit. The heart model, together with the epicardial surface model constructed recently, are mounted in an inhomogeneous human torso model. Electric dipoles, which are proportional to the spatial gradient of the action potential, are generated in all the cell units. These dipoles give rise to a potential distribution on the epicardial surface, which is calculated by means of the boundary element method. The simulated epicardial potential maps during a normal heart beat and in a preexcited beat to mimic Wolff-Parkinson-White (WPW) syndrome are in close agreement with those reported in the literature.

Action Potentials↗
PubMed2005

On the Ewald artifacts in computer simulations. The test-case of the octaalanine peptide with charged termini.

The treatment of electrostatic interactions in molecular simulations is of fundamental importance. Ewald and related methods are being increasingly used to the detriment of cutoff schemes, which are known to produce several artifacts. A potential drawback of the Ewald method is the spatial periodicity that is imposed to the system, which could produce artifacts when applied in the simulation of liquids. In this work we analyze the octaalanine peptide with charged termini in explicit solvent, for which severe effects due to the use of Ewald sums were predicted using continuum electrostatics. Molecular Dynamics simulations for a total of 158 nanoseconds were performed in cells of different sizes. From the comparison of the results of different system sizes, no significant periodicity-induced artifacts were observed. It is argued that in current biomolecular simulations, the incomplete sampling is likely to affect the results to a larger extent than the artifacts induced by the use of Ewald sums.

Alanine↗
PubMed2003

Reconstruction of current density distributions in axially symmetric cylindrical sections using one component of magnetic flux density: computer simulation study.

In magnetic resonance current density imaging (MRCDI), we inject current into a subject through surface electrodes and measure the induced magnetic flux density B inside the subject using an MRI scanner. Once we have obtained all three components of B, we can reconstruct the internal current density distribution J = inverted triangle x B/mu0). This technique, however, requires subject rotation since the MRI scanner can measure only one component of B that is parallel to the direction of its main magnetic field. In this paper, under the assumption that the out-of-plane current density Jz is negligible in an imaging slice belonging to the xy-plane, we developed an imaging technique of current density distributions using only Bz, the z-component of B. The technique described in this paper does not require a subject rotation but the quality of reconstructed images depends on the amount of out-of-plane current density Jz. From numerical simulations, we found that the new algorithm could be applied to subjects such as human limbs using longitudinal electrodes.

Algorithms↗
PubMed1996

Optimal transport parameters of the inner medullary collecting duct in interaction between urine concentrating and urea excreting mechanisms: a computer simulation study.

Although the accumulation of urea in the renal medulla is essential for the formation of concentrated urine, it is also necessary for the kidney to excrete considerable amounts of urea into the urine as a waste product of protein degradation. Thus, the urine concentrating capacity is attained by the interaction with the efficiency of urea excretion. To seek the best condition for this phenomenon, we developed an objective function for evaluating urea excreting capacity relative to urine concentrating capacity by using a mathematical model consisting of components of the countercurrent multiplication system: the ascending thin limb, capillary network, and inner medullary collecting duct. The values of the objective functions were calculated as three-dimensional functions of transport parameters for the inner medullary collecting duct, including hydraulic conductivity, urea permeability, and reflection coefficient for urea. The results of the computer analysis revealed that the maximum value of the objective function was attained when values for transport parameters of the inner medullary collecting duct corresponded to those experimentally obtained values reported previously. We conclude that the maximum urine concentrating capacity is limited by the efficiency of urea excreting capacity of the kidney, and vice versa.

Biological Transport↗
PubMed1994

Computer simulation of pulsed field gel runs allows the quantitation of radiation-induced double-strand breaks in yeast.

A procedure for the quantification of double-strand breaks in yeast is presented that utilizes pulsed field gel electrophoresis (PFGE) and a comparison of the observed DNA mass distribution in the gel lanes with calculated distributions. Calculation of profiles is performed as follows. If double-strand breaks are produced by sparsely ionizing radiation, one can assume that they are distributed randomly in the genome, and the resulting DNA mass distribution in molecular length can be predicted by means of a random breakage model. The input data for the computation of molecular length profiles are the breakage frequency per unit length, alpha, as adjustable parameter, and the molecular lengths of the intact chromosomes. The obtained DNA mass distributions in molecular length must then be transformed into distributions of DNA mass in migration distance. This requires a calibration of molecular length vs. migration distance that is specific for the gel lane in question. The computed profiles are then folded with a Lorentz distribution with adjusted spread parameter gamma to account for band broadening. The DNA profiles are calculated for different breakage frequencies alpha and for different values of gamma, and the parameters resulting in the best fit of the calculated to the observed profile are determined.

Computer Simulation↗
PubMed1987

Impact of hepatitis B immunization strategies on infection rates in Italy: a deterministic computer simulation.

The impact of different immunization strategies on hepatitis B infection rates in Italy over a 20 year period is simulated by the means of a simple deterministic mathematical model. The anticipated effect of vaccination of health workers only, of newborns from HBsAg+ mothers, of all newborns, of the entire population are simulated. Immunization of newborns from HBsAg+ mothers is by far the strategy with the greater effectiveness per unit cost.

Computer Simulation↗
PubMed2002

Real-time computer simulations of excitable media: JAVA as a scientific language and as a wrapper for C and FORTRAN programs.

We describe a useful setting for interactive, real-time study of mathematical models of cardiac electrical activity, using implicit and explicit integration schemes implemented in JAVA. These programs are intended as a teaching aid for the study and understanding of general excitable media. Particularly for cardiac cell models and the ionic currents underlying their basic electrical dynamics. Within the programs, excitable media properties such as thresholds and refractoriness and their dependence on parameter values can be analyzed. In addition, the cardiac model applets allow the study of reentrant tachyarrhythmias using premature stimuli and conduction blocks to induce or to terminate reentrant waves of electrical activation in one and two dimensions. The role of some physiological parameters in the transition from tachycardia to fibrillation also can be analyzed by varying the maximum conductances of ion channels associated with a given model in real time during the simulations. These applets are available for download at http://arrhythmia.hofstra.edu or its mirror site http://stardec.ascc.neu.edu/~fenton.

Algorithms↗
PubMed1994

Realistic modeling of clinical laboratory operation by computer simulation.

An important objective of laboratory management is to adjust the laboratory's capability to the needs of patients' care as well as economy. The consequences of management may be changes in laboratory organization, equipment, or personnel planning. At present only one's individual experience can be used for making such decisions. We have investigated whether the techniques of operations research could be transferred to a clinical laboratory and whether an adequate simulation model of the laboratory could be realized. First we listed and documented the system design and the process flow for each single laboratory request. These input data were linked by the simulation model (programming language SIMSCRIPT II.5). The output data (turnaround times, utilization rates, and analysis of queue length) were validated by comparison with the current performance data obtained by tracking specimen flow. Congruence of the data was excellent (within +/- 4%). In planning experiments we could study the consequences of changes in order entry, staffing, and equipment on turnaround times, utilization, and queue lengths. We conclude that simulation can be a valuable tool for better management decisions.

Chemistry, Clinical↗
PubMed1997

An introduction to using computer simulation in healthcare: patient wait case study.

As healthcare continues to become more competitive, the ability to assess tradeoffs between resource utilization, service, and operating costs grows in importance, such as with respect to appointment access, waiting room delays, and telephone service. This paper discusses the use of simulation analysis for studying and improving these and other health systems. A case study concerning pediatric waiting times illustrates typical steps involved in a simulation study, possible types of analyses, and resources required. Other healthcare uses of stimulation, pitfalls to avoid, and software selection also are discussed briefly.

Algorithms↗
PubMed2002

Computer simulations of particle deposition in the lungs of chronic obstructive pulmonary disease patients.

Epidemiology data show that mortality rates for chronic obstructive pulmonary disease (COPD) patients increase with an increase in concentration of ambient particulate matter (PM). This is not seen for normal subjects. Therefore, the U.S. Environmental Protection Agency (EPA) has identified COPD patients as a susceptible subpopulation to be considered in regulatory standards. In the present study, a computer model was used to calculate deposition fractions of PM within the lungs of COPD patients. The morphology of COPD lungs was characterized by two distinct components: obstruction of airways (chronic bronchitis component), and degeneration of alveolar structure (emphysema component). The chronic bronchitis component was modeled by reducing airway diameters using airway resistance measurements in vivo, and the emphysema component was modeled by increasing alveolar volumes. Calculated results were compared with experimental data obtained from COPD patients for controlled breathing trials (tidal volume of 500 ml, respiratory time of 1 s) with a particle size of 1 microm. The model successfully depicts PM deposition patterns and their dependence on the severity of disease. The findings indicate that airway obstructions are the main cause for increased deposition in the COPD lung.

Aerosols↗
PubMed1999

[Computer simulation of cardiovascular response to lower body negative pressure].

OBJECTIVE: To simulate the cardiovascular response to lower body negative pressure (LBNP). METHOD: A computer model was developed. It had 7 subparts: the redistribution of blood, the filling of left ventricle, left ventricle, peripheral circulation, control of heart rate, control of peripheral resistance and control of venous tone. The heart rate and venous tone were controlled by high-pressure receptor baroreflex, while the peripheral resistance was controlled by high- and low-pressure receptor baroreflexes. RESULT: With the help of the model, cardiovascular response to LBNP up to -10.64 kPa (-80 mmHg) were simulated, including the changes of systolic blood pressure, mean blood pressure, heart rate and cardiac output. The time-dependent response to a LBNP profile was also simulated. The simulation results coincided well with human experiments. CONCLUSION: The model is valid and can accurately reproduce the short-term hemodynamic response to LBNP.

Aerospace Medicine↗
PubMed1992

Educational computer simulation of malignant hyperthermia.

An educational graphic simulator was developed to provide an interactive learning environment to practice the diagnosis and treatment of malignant hyperthermia. The program incorporates a set of dynamically interacting models to present the physiologic changes associated with malignant hyperthermia and the simulated patient's response to management. Cardiovascular, respiratory, and temperature changes are presented through a graphic display of the operating room monitors. Mouse-driven input is used to manage the airway, control ventilation, manage cardiovascular and rhythm disturbances, and control fluids, electrolytes, and temperature. Medications, including dantrolene, antidysrhythmics, diuretics, and sodium bicarbonate, can be administered. Four simulated patients with different presentations of malignant hyperthermia are included to illustrate variations in the syndrome. Two of these patients are described in detail.

Adult↗
PubMed1988

Preparative separation of peptide and protein samples by high-performance liquid chromatography with gradient elution. I. The Craig model as a basis for computer simulations.

The Craig model (assuming a Langmuir isotherm) has been used by us previously to successfully simulate isocratic high-performance liquid chromatographic (HPLC) separation in a mass-overload mode. Here we have extended this approach to the case of gradient elution for large samples. These simulations support our earlier conclusion that so-called "corresponding" isocratic and gradient separations provide similar sample resolution when the sample size is the same. "Corresponding" separations refer to the case where isocratic retention k' is equal to average gradient retention k, and where other conditions (column, flow-rate, etc.) are the same. Craig simulations reported here also provide further insight into the factors that affect preparative HPLC separations under mass-overload conditions.

Chromatography, High Pressure Liquid↗
PubMed2002

Nasal airflow diagnosis--comparison of experimental studies and computer simulations.

The lack of suited diagnostic tools providing insight into patient specific flow characteristics of the nasal airflow is one of the main problems in functional diagnosis. Diagnostic methods currently used do not provide the necessary information for flow analysis. But the flow distribution is essential for a physiological respiration, in particular for cleaning, moistening and tempering of the inhaled air as well as for the olfactory function of the nose. To overcome this current situation a cooperation project of the ENT surgeons and computer graphic engineers was established to develop the computer assisted planning system STAN (Simulation Tool for Airflow in the human Nose) combining Computer Fluid Dynamics (CFD) with advanced Computer Graphic Technology. The idea of the STAN system is to perform patient specific airflow simulations in the patient's nasal cavities. Therefore a geometrical model of the nasal airways is derived from the patient's tomography scans. A discretization of the surrounded flow volume is made by a computational grid. To establish the flow simulation Finite Element Methods are performed on the grid. A tailored visualization is offered to the surgeon that overlaps the flow pattern to the patient's tomography data shown in the coronal, sagittal and transversal plane. The surgeon can not only analyze the patient's current respiratory situation he has also the possibility to describe the planned surgical intervention. The goal is to simulate the flow distribution that can be expected after the surgical intervention and to offer a possibility to validate various surgical strategies. To verify the simulation results experimental investigations and measurements are made in nasal models. Silicon Models of patient's nose channels are made to analyze flow characteristics. The CT or MR scans of the same patients are used as input data for the simulation. The experimental outcome is compared to the simulation results to validate this diagnostic approach.

Computer Simulation↗