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At least 1,477 records · Page 82Linked to original sources

PubMed2001

Effects of depressed myocardial contractility induced by microgravity on cardiovascular response to orthostatic stress: a computer simulation.

The aim of present study is to investigate the role played by the depression of myocardial contractility in the mechanism of cardiovascular deconditioning and orthostatic intolerance (OI) induced by space weightlessness. Based on our previous model, which was used to simulate cardiovascular response to lower body negative pressure (LBNP), we incorporated the factor of changes of myocardial contractility into the model by multiplying a coefficient to the time-varying elastance that represents the changes of cardiac contractility. By decreasing the coefficient progressively, we simulated the changes of heart rate (HR), blood pressure (BP), and cardiac output (CO) during LBNP after 0-30% of myocardial contractility depression combined with 12% decrease of the total blood volume. Simulation results indicate that depressed myocardial contractility induces more augment of HR, and more decrement of BP and CO during LBNP and suggest that the depression of myocardial contractility degenerate cardiovascular response to orthostatic stress.

Blood Pressure↗

PubMed2000

Different effects of blocked potassium channels on action potentials, accommodation, adaptation and anode break excitation in human motor and sensory myelinated nerve fibres: computer simulations.

Action potentials and electrotonic responses to 300-ms depolarizing and hyperpolarizing currents for human motor and sensory myelinated nerve fibres have been simulated on the basis of double cable models. The effects of blocked nodal or internodal potassium (fast or slow) channels on the fibre action potentials, early and late adaptations to 30-ms suprathreshold slowly increasing depolarizing stimuli have been examined. The effects of the same channels on accommodation after the termination of a prolonged (100 ms) hyperpolarizing current pulse have also been investigated. By removing the nodal fast potassium conductance the action potentials of the sensory fibres are considerably broader than those of the motor neurons. For both types of fibres, the blocked nodal slow potassium channels have a substantially smaller effect on the action potential repolarization. When the suprathreshold depolarizing current intensity is increased, the onset of the spike burst occurs sooner, which is common in the behaviour of the fibres. The most striking differences in the burst activity during early adaptation have been found between the fibres when the nodal fist potassium channels are blocked. The results obtained confirm the fact that the motor fibres adapt more quickly to sustained depolarizing current pulses than the sensory ones. The results also show that normal human motor and sensory fibres cannot be excited by a 100-ms hyperpolarizing current pulse, even at the threshold level. When removing the potassium channels in the nodal or internodal axolemma, the posthyperpolarization increase in excitability is small, which is common in the behaviour of the fibres. However, anode break excitation can be simulated in the fibres with simultaneous removal of the potassium channels under the myelin sheath, and this is more pronounced in the human sensory fibres than in motor fibres. This phenomenon can also be found when the internodal and some of the nodal (fast or slow) potassium channels are simultaneously blocked.

Action Potentials↗

PubMed1990

Dependence of cerebral capillary hematocrit on red cell flow separation at bifurcations: a computer simulation study.

The influence of preferential red cell entry into microvascular branches with higher flow on microvessel hematocrit distribution was studied by mathematical modeling in a reconstructed cerebrocortical microvascular network. More heterogeneous hematocrit distribution was obtained at stronger cell partitioning. Small variations in the cell separation parameter resulted in significantly different hematocrit distributions. The significance of these findings in vivo should be further evaluated.

Animals↗

PubMed2002

Modeling enzyme reactivity in organic solvents and water through computer simulations.

In this article, we review how molecular modeling techniques can be used to shed light on how water and organic solvents influence the reactivity of enzymes. The application of thermodynamics-based models allowed the first qualitative predictions on the selectivity of many reaction types. However, it was with the application of quantum mechanical/molecular mechanical (QM/MM) methods that quantitative models of actual reactivity patterns could be realistically formulated.

Computer Simulation↗

PubMedDate not supplied

[The importance of the blood transfusion factor in the epidemic process in hepatitis C in the patients of chronic hemodialysis units studied using a computer simulation model].

Examinations for the presence of antibody to hepatitis C virus (HCV) were carried out in 144 patients of chronic hemodialysis wards and 83 blood donors. The anti-HCV were found in 26.4% of the patients and only in 1.2% of the blood donors. A definite increase in the incidence of HCV infection in the patients of hemodialysis wards was established in relation to the duration of the treatment, namely from 17.4% in the patients treated for up to 1 year to 37.5% in those treated for 6 years or more. Significant differences were observed in the rate of anti-HCV findings in the patients with kidney transplantation and in those who had not experience this operation. The results obtained by an imitation computer model indicated that the hemotransfusion factor is not the only one determining the high rate of HCV infection in patients of chronic hemodialysis wards, however its influence on the intensity of the epidemic process in hepatitis C in these wards was sufficiently high.

Adult↗

PubMed1996

The role of pressure pulsatility in the carotid baroreflex control: a computer simulation study.

The role of pressure pulsatility in the arterial pressure control by the carotid baroreflex was investigated by means of a mathematical model. The model describes the main hemodynamic properties of the cardiovascular system in pulsating conditions, the static and dynamic components of the carotid baroreflex, and their effect on systemic arterial resistance, heart frequency and systemic venous capacity. Experimental findings on the role of pressure pulsatility in the carotid baroreflex were reproduced in terms of a non-linear interaction between the static and dynamic components. Simulations of physiological experiments (mild haemorrhage, carotid occlusion manoeuvres, genesis of self-sustained arterial pressure waves) reveal that the role of pressure pulsatility is meaningful in response to those perturbations (such as carotid occlusion manoeuvres) characterized by different alterations in the mean and pulsating components.

Baroreflex↗

PubMed1999

Optimal control of walking with functional electrical stimulation: a computer simulation study.

Bipedal locomotion was simulated to generate a pattern of activating muscles for walking using electrical stimulation in persons with spinal cord injury (SCI) or stroke. The simulation presented in this study starts from a model of the body determined with user-specific parameters, individualized with respect to the lengths, masses, inertia, muscle and joint properties. The trajectory used for simulation was recorded from an able-bodied subject while walking with ankle-foot orthoses. A discrete mathematical model and dynamic programming were used to determine the optimal control. A cost function was selected as the sum of the squares of the tracking errors from the desired trajectories, and the weighted sum of the squares of agonist and antagonist activations of the muscle groups acting around the hip and knee joints. The aim of the simulation was to study plausible trajectories keeping in mind the limitations imposed by the spinal cord injury or stroke (e.g., spasticity, decreased range of movements in some joints, limited strength of paralyzed, externally activated muscles). If the muscles were capable of generating the movements required and the trajectory was achieved, then the simulation provided two kinds of information: 1) timing of the onset and offset of muscle activations with respect to the various gait events and 2) patterns of activation with respect to the maximum activation. These results are important for synthesizing a rule-based controller.

Algorithms↗

PubMed2003

Properties of perimetric threshold estimates from full threshold, ZEST, and SITA-like strategies, as determined by computer simulation.

PURPOSE: To investigate the accuracy and precision of threshold estimates returned by two Bayesian perimetric strategies, staircase-QUEST or SQ (a Swedish interactive threshold algorithm [SITA]-like strategy) and ZEST (zippy estimation by sequential testing), and to compare these measures with those of the full-threshold (FT) algorithm. METHODS: A computerized visual field simulation model was developed to compare the performance (accuracy, precision, and number of presentations) of the three algorithms. SQ implemented aspects of the SITA algorithm that are in the public domain. The simulation was tested by using standard automated perimetry (SAP) visual field data from 265 normal subjects and 163 observers with glaucomatous visual field loss and by exploring the effect of response variability and response errors on algorithm performance. RESULTS: SQ was faster than FT or ZEST, with a comparable mean error when simulating field tests on patients. Point-wise analysis revealed similar error and standard deviation of error as a function of threshold for FT and SQ. If the initial estimate of threshold for either procedure was incorrect, the means and standard deviations of the error increased markedly. ZEST produced more accurate thresholds than did the other two strategies when the initial estimate was removed from the true threshold. CONCLUSIONS: When simulated patients made errors, the accuracy and precision of sensitivity estimates were poor when the initial estimate of threshold either overestimated or underestimated the true threshold. This was particularly so for FT and SQ. ZEST demonstrated more consistent error properties than the other two measures.

Algorithms↗

PubMedDate not supplied

Computer simulation of the receptor-ligand system.

A series of Monte Carlo simulations were carried out for receptor-ligand systems taken from the Protein Data Bank. The models of both objects were based on a simplified low-resolution model. The model chains consisted of alpha carbons and side groups represented by united atoms. The excluded volume effect and specific pairwise contact potential were introduced into the model. The process of docking was simulated step-by-step by random translations and reorientations of the ligand. The stability of complexes formed was investigated for a simple statistical potential between amino acid side groups. The feasibility of the model for finding the binding site on the receptor surface and the proper orientation of docked ligand were also studied.

Algorithms↗

PubMed1999

Comparison of three classes of snake neurotoxins by homology modeling and computer simulation graphics.

We present a systematic structure comparison of three major classes of postsynaptic snake toxins, which include short and long chain alpha-type neurotoxins plus one angusticeps-type toxin of black mamba snake family. Two novel alpha-type neurotoxins isolated from Taiwan cobra (Naja naja atra) possessing distinct primary sequences and different postsynaptic neurotoxicities were taken as exemplars for short and long chain neurotoxins and compared with the major lethal short-chain neurotoxin in the same venom, i.e., cobrotoxin, based on the derived three-dimensional structure of this toxin in solution by NMR spectroscopy. A structure comparison among these two alpha-neurotoxins and angusticeps-type toxin (denoted as FS2) was carried out by the secondary-structure prediction together with computer homology-modeling based on multiple sequence alignment of their primary sequences and established NMR structures of cobrotoxin and FS2. It is of interest to find that upon pairwise superpositions of these modeled three-dimensional polypeptide chains, distinct differences in the overall peptide flexibility and interior microenvironment between these toxins can be detected along the three constituting polypeptide loops, which may reflect some intrinsic differences in the surface hydrophobicity of several hydrophobic peptide segments present on the surface loops of these toxin molecules as revealed by hydropathy profiles. Construction of a phylogenetic tree for these structurally related and functionally distinct toxins corroborates that all long and short toxins present in diverse snake families are evolutionarily related to each other, supposedly derived from an ancestral polypeptide by gene duplication and subsequent mutational substitutions leading to divergence of multiple three-loop toxin peptides.

Amino Acid Sequence↗

PubMed2006

Linking dynamical perceptual decisions at different levels of description in motion pattern formation: computational simulations.

A two-level dynamical model of motion pattern formation is developed in which local motion/ nonmotion perceptual decisions are based on inhibitory competition between area V1 detectors responsive to motion-specifying versus motion-independent stimulus information, and pattern-level perceptual decisions are based on inhibitory competition between area MT motion detectors with orthogonal directional selectivity. The model accounts for the effects of luminance perturbations on the relative size of the pattern-level hysteresis effects reported by Hock and Ploeger (2006) and also accounts for related experimental results reported by Hock, Kelso, and Schöner (1993). Single-trial simulations demonstrated the crucial role of local motion/nonmotion bistability and activation-dependent future-shaping interactions in stabilizing perceived global motion patterns. Such interactions maintain currently perceived motion patterns by inhibiting the soon-to-be-stimulated motion detectors that otherwise would be the basis for the perception of an alternative pattern.

Computer Simulation↗

PubMed1988

Evaluation of the pacing rate response to treadmill exercise using computer simulation of a temperature-based, rate-adaptive algorithm.

A temperature-based, rate-adaptive, pacing algorithm was developed to benefit the patient. Rate-adaptive pacemakers use a physiologic parameter to identify the need for increased pacing rate. Parameters that have been clinically investigated include venous pH, Q-T interval, respiration, body motion, and blood temperature. The objective of this study was to provide pacing rates resembling normal heart rates in response to various levels and types of activity. A rapid response time (within 30 s of exercise onset) was also sought. Blood temperature, which reflects metabolic activity of all regions, was selected as the physiologic parameter. Right ventricular blood temperature was recorded in 25 patients with implanted Kelvin 500 pacemakers (Cook Pacemaker) during rest and treadmill exercise. The patient population included 16 men and 9 women, age 44-81 years (mean = 72). Indications for pacing were sinus node disease, atrioventricular block, and atrial fibrillation with slow ventricular response. The temperature changed with physical activity and emotional stress. Temperature typically dropped briefly at exercise onset, increased with continued exercise, and returned to the resting level after exercise. These components were employed in developing the temperature-based rate-adaptive algorithm, which was designed to use the rate of temperature change (dT/dt), temperature change (delta T), and baseline temperature (T). The temperature profiles were used to produce simulated pacing rates as determined by the algorithm. The drop in temperature at onset of activity was utilized to provide a rapid increase in pacing rate. As dT/dt became positive and delta T increased, pacing rate was further increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed2005

Computational simulation of penetrating trauma in biological soft tissues using the material point method.

The objective of this research was to develop realistic computational models for soft tissues subjected to finite deformation and failure, and to test these models in the context of numerical simulations of penetrating trauma injuries. A transversely isotropic hyperelastic model with strain-based failure criteria was used to represent the behavior of anisotropic soft tissue. The constitutive model was implemented into an existing numerical code based on the Material Point Method (MPM). The penetration of a low-speed bullet through a myocardium material slab was simulated and several wounding scenarios were analyzed and compared. The material symmetry, the type of contact modeled between the bullet and the soft tissue and the bullet speed were shown to have a significant influence on the wound profile.

Computer Simulation↗

PubMed1988

Evaluation of pulse-detection algorithms by computer simulation of hormone secretion.

A versatile method is presented for generating synthetic hormonal time series, containing peaks at known locations, to be used to objectively evaluate both the false-negative (F-) and false-positive (F+) statistical error rates of computerized pulse-detection algorithms. Synthetic data are generated by assuming hormone secretion to occur as a succession of instantaneous release pulses, distributed as Poisson events, separated by quiescent intervals. The pulses are convolved to simulate cumulation of consecutive events and clearance of the hormone. Randomly generated errors, corresponding in magnitude to typical experimental measurement error, are then added to the convolved series. The choice of different values for simulation parameters (e.g., frequency and amplitude of pulses) allows one to emulate some typical physiological patterns of hormone secretion for luteinizing hormone, growth hormone, and thyrotropin or other hormones. Various subsets can be extracted from a simulated time series to study the effect of sampling frequency on the detection of pulses. We show that in sampled series the "observable frequency" of pulses is less than the true nominal frequency. Methods for evaluating pulse-detection algorithms and expressing the results are presented. Simulations of LH secretion were analyzed with the program DETECT. We show that minimizing F+ error rates only might lead to excessively high F- rates. A proper choice of sampling frequency and program probability levels can be made to provide acceptable F+ and F- error rates for various patterns of hormone secretion.

Algorithms↗

PubMed1994

Analysis of intracerebral hematoma shapes by numerical computer simulation using the finite element method.

The distortion and stress distribution in the brain caused by putaminal hemorrhage were estimated by computer stimulation using the finite element method (FEM). The two-dimensional model of a single cerebral hemisphere contained cortex, white matter, caudate nucleus, lenticular nucleus, thalamus, falx, and lateral ventricle. Five types of intracerebral hemorrhage were modeled at different locations in the lenticular nucleus. The models generated putaminal hematomas of various shapes influenced by the location of the bleeding points. Hematomas caused deformation of the brain, collapse of the lateral ventricle, and destruction of the internal capsule. The stress distribution revealed various patterns influenced by the site of bleeding. The stress in the area of the internal capsule corresponded to the extent of destruction of the internal capsule. This study suggests that FEM modeling of putaminal hemorrhage can provide a useful simulation.

Biomechanical Phenomena↗

PubMed1992

Theoretical model and computer simulation of excitation-contraction coupling of mammalian cardiac muscle.

A mathematical model is developed to investigate the kinetics of electrical, mechanical and molecular processes in mammalian cardiac muscle. Isometric contractions at different muscle length and frequency of stimulation in response to a rhythmically applied clamp pulse or artificial action potential are simulated. Numerical results show that concentration of Ca2+ ions, bound to Ca(2+)-specific sites on protein troponin C, could be a regulatory factor in actin-myosin interactions and subsequent production of force in Huxley's mathematical approach for the sliding mechanism. The behavior of the model is compared to that of living cardiac muscle.

Animals↗

PubMed1993

A computer simulation study on the effects of input function measurement noise in tracer kinetic modeling with positron emission tomography (PET).

Tracer kinetic modeling with positron emission tomography (PET) requires measurements of the time-activity curves in both plasma (PTAC) and tissue (TTAC) to estimate physiological parameters, i.e. to fit the parameters of certain compartmental models using PTAC and TTAC as the model input and output functions, respectively. However, the estimation usually ignores the measurement noise in plasma tracer activity curves. The accuracy and reliability of the physiological parameters estimated by ignoring such noise are not well understood. In this paper, effects of noise in [18F] 2-fluoro-2-deoxy-D-glucose (FDG) tracer plasma concentration measurements on estimation of local cerebral metabolic rates of glucose (LCMRGlc) with PET is investigated systematically. The PTAC modeling approach used in this paper also provides a realistic means to filter out the noise and to improve the physiological parameter accuracy, which can be potentially used in model-based non-invasive measurements of PTAC.

Artifacts↗

PubMed2005

Understanding velocity of sound in trabecular bone via computer simulations.

Osteoporosis is a condition characterized by low bone mass and micro-architectural deterioration leading to non-traumatic fractures of the skeleton. It is a potentially debilitating condition especially for senior citizens. There is growing interest for quantitative ultrasound to measure bone mineral density. However, understanding of ultrasound-bone interaction is limited. Simulating ultrasound propagation of bone can help us better understand ultrasound-bone interaction and provide insight into bone architecture. In this study a mathematical model for the propagation of sound in bone is presented. Results demonstrate the suitability of the proposed modeling approach and the model's capability to reproduce conditions in the lab.

Animals↗