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At least 199 records · Page 11Linked to original sources

PubMed1988

Gap junction uncoupling and discontinuous propagation in the heart. A comparison of experimental data with computer simulations.

The effects of octanol on longitudinal propagation in guinea pig papillary muscles were measured by intracellular microelectrodes. These data were compared with alterations in conduction induced by stepwise removal of gap junction channels in computer simulations of propagation based on a discontinuous cable model. Octanol reduced the velocity (theta) of propagating action potentials (APs) from 53.2 +/- 3.5 to less than 6.6 +/- 2.1 cm/s before block occurred. The maximal rate of rise (Vmax) changed in a biphasic manner, increasing from 133.1 +/- 5.4 in controls to 201.7 +/- 11.0 V/s when theta was 20.5 +/- 2.8 cm/s, and then declining to less than 58.6 +/- 15.2 V/s just before block. The input resistance and time constant of the AP foot increased, and the ascending limb of phase-plane loops became increasingly nonlinear and notched during octanol treatment. All effects of octanol reversed upon washout. A strand of cardiac tissue was modeled as a discontinuous cable composed of 40 cells, each with 10 isopotential membrane segments described by Beeler-Reuter kinetics, and coupled by a variable number of gap junction channels (156 pS). Decreasing the number of channels from 40,000 to 400 to 60 slowed conduction from 62.6 to 16.4 to 3.1 cm/s. As noted in the experimental data, Vmax increased from 103 to 130 and then fell to less than 96 V/s. The AP foot increased and became nonexponential. Distinct notches developed during phase 1 of the APs at slower propagation velocities in the experiments and simulations. The close similarities between the experimental and theoretical data obtained in this study supports the applicability of a discontinuous cable model for describing longitudinal propagation in the heart.

1-Octanol↗

PubMed1992

[A computer simulated test for the long-time change of heterozygote. Analysis of possibility on preserving the heterozygote for long-time in population].

Under the conditions of random mating and selecting. The research by computer simulation analysed the possibility of long-time conserving heterozygote in colong. An important conclusion was obtained. It was impossible to conserve heterozygote for a long time in colong. Therefore it is not necessary to control inbreeding for conserving all genes of colong by reducing gene purity in the work of conserving breeds for farm animal and poultry.

Animals↗

PubMed1989

An evaluation of the effectiveness of a computer simulation of anaesthetic uptake and distribution as a teaching tool.

Gas Man is an interactive, microcomputer-based simulation and tutorial designed to teach the principles of anaesthetic uptake and distribution. Using two resident groups, one at a primary university-teaching hospital and the other at a medical school affiliated community hospital, the authors compared residents' knowledge of the subject prior to exposure to the tutorial and the average time spent using the program during a self-paced instruction period. Both groups not only significantly improved their knowledge of uptake and distribution 8 weeks after using this program, but wished to spend additional time with it, implying that it was a pleasant and satisfying way to learn. Computer simulation has the potential to be an important teaching tool in an anaesthesia residency training programme.

Anesthesiology↗

PubMed1976

Computer simulation of the precordial QRS complex: effects of simulated changes in ventricular wall thickness and volume.

The cardiac electric field generated by depolarization of the human ventricle is simulated with a computer model which utilizes 1,500 dipoles. The configuration of the ventricles utilized in the model assumed that the cross-sectional shape of the left ventricle was circular and the right ventricular free wall was a portion of an ellipse. The torso was assumed to be homogeneous and infinite. The activation sequence was based on the measurements of Durrer. The depolarizational wave was simulated by dipole layers. The output of the model is presented as a standard multilead precordial ECG. The ECG complexes generated by the model closely resemble the precordial QRS complexes of normal man. Simulated increases in wall thickness (1 to 2.2 X control) were associated with changes in the calculated precordial QRS complexes which were characteristic of left ventricular hypertrophy. Voltage (R in V5 or V6 and S in V1) and QRS duration increased linearly as a function of calculated left ventricular mass. Increases in ventricular activation time were related nonlinearly to changes in left ventricular mass and did not occur in the absence of a simulated increase in wall thickness. The effects of simulated changes in left ventricular volume (0.6 to 3.0 X control) on the QRS complex were mainly dependent on the resultant increase in left ventricular mass. This model may be useful in simulating the precordial QRS complexes that result from isolated or combined changes in ventricular volume or wall thickness or other disorders of the heart. Furthermore, it may be useful whenever a simulation of a QRS generator is needed.

Cardiac Volume↗

PubMed1991

Assessment of split-sample proficiency testing for cholesterol by use of a computer simulation model.

We developed a computer model to study the use of patients' specimens to assess compliance of cholesterol measurement performance with the 1992 goals of the Laboratory Standardization Panel of the National Cholesterol Education Program. The model uses Monte Carlo techniques to simulate cholesterol measurements that are subject to both systematic and random error. Split-sample measurements by a clinical laboratory and by a reference laboratory are compared by using linear regression to estimate clinical laboratory bias and imprecision; subsequently, according to specified decision limits, the performance of the clinical laboratory is classified as acceptable or deficient. We have quantified the influence of the bias and imprecision of the clinical laboratory, the imprecision of the reference laboratory, the number of split samples compared, and the decision limits on the accuracy of the classification of clinical laboratory performance. Unless the decision limits are carefully selected and a sufficient number of split samples are used, clinical laboratory performance will be frequently misclassified.

Cholesterol↗

PubMed1987

Computer simulation of human mitral valve mechanics and motion.

The human mitral valve is the left atrio-ventricular valve which is composed of several components including leaflets, chordae tendineae, papillary muscles, and the valve annulus. Any or all of these components may fail and contribute to various valvular diseases including mitral regurgitation and mitral valve prolapse. A computer simulation of mitral valve mechanics and motion was written in BASIC for micro-computers. This program allows valvular geometry and biomechanical parameters to be varied and records time varying motion of the valve and all components during systole including a graphic display of the valve leaflets.

Chordae Tendineae↗

PubMed1973

Kinetics of diffusion and convection in 3.2-A pores. Exact solution by computer simulation.

The kinetics of transport in pores the size postulated for cell membranes has been investigated by direct computer simulation (molecular dynamics). The simulated pore is 11 A long and 3.2 A in radius, and the water molecules are modeled by hard, smooth spheres, 1 A in radius. The balls are given an initial set of positions and velocities (with an average temperature of 313 degrees K) and the computer then calculates their exact paths through the pore. Two different conditions were used at the ends of the pore. In one, the ends are closed and the balls are completely isolated. In the other, the ball density in each end region is fixed so that a pressure difference can be established and a net convective flow produced. The following values were directly measured in the simulated experiments: net and diffusive (oneway) flux; pressure, temperature, and diffusion coefficients in the pore; area available for diffusion; probability distribution of ball positions in the pore; and the interaction between diffusion and convection. The density, viscosity, and diffusion coefficients in the bulk fluid were determined from the theory of hard sphere dense gases. From these values, the "equivalent" pore radius (determined by the same procedure that is used for cell membranes) was computed and compared with the physical pore radius of the simulated pore.

Cell Membrane Permeability↗

PubMed1991

Propagation of parallel fiber volleys in the cerebellar cortex: a computer simulation.

We have studied the dynamic changes occurring during the propagation of active volleys through the parallel fiber (PF) tract in a portion of molecular layer of the cerebellar cortex, by means of a realistic computer simulation of the fibers (random distribution of PF origin; correlation between length and conduction velocity throughout depth). Surface electrical stimulations as well as mossy fiber stimulations of the PFs were mimicked. From a quantitative analysis of the propagation of the different types of volleys it appears that the constraints enforced by the morphological and functional characteristics of the PFs lead to an important spatial and temporal dispersion of the action potentials during their displacement along the folium. In particular, the time relationship between activity in the deep and superficial fibers as well as the number and location of active fibers at a given time (i.e. locus) will considerably vary during propagation. Moreover, in the case of a mossy fiber stimulation, there exists a striking asymmetry of the volleys travelling on each side of the site of stimulation. Therefore any information coded by the conjunctive activation of more than a few parallel fibers will be spatially and temporally dispersed in such a manner that successive postsynaptic Purkinje cells will receive very different combinations of inputs. This seems in favor of the hypothesis that these neurons function as coincidence detectors.

Action Potentials↗

PubMed1992

Computer simulation of metoclopramide block of cardiac sodium channels.

Based on the gate-related receptor hypothesis, a quantitative study on kinetics and state-dependency of metoclopramide (Met) blockade of cardiac sodium channels was performed by computer simulation. The characteristics of Met, a sodium channel blocker modulated simultaneously by both activation and inactivation gates were described and analyzed for the first time. The time constant of recovery from blocking for Met at resting potential -85 mV was 2.82 s, which was increased during depolarization or hyperpolarization. Its apparent rate of onset of blocking was 0.49 AP-1 at a stimulation frequency of 2.0 Hz. A concentration-dependent shift of h infinity curve was found at doses higher than 100 mumol/L Met. This study provides a new test and verification for our gate-related receptor hypothesis.

Anti-Arrhythmia Agents↗

PubMed1979

Computer simulation of flow-dependent absorption in microperfused short Henle's loop of rats.

With computer simulation we examined the extent to which current theories and experimental data explain function of single microperfused superficial Henle's loops in rats. In the model standard phenomenological equations describe transport; two sets of transport parameters labeled rat and rabbit were taken from published experiments; Michaelis-Menten kinetics in the ascending thick limb were adjusted arbitrarily; tubular radius is either constant or depends on luminal pressure with compliance based on experimental observations; the interstitium is an infinite sink with salt and urea concentrations constant in the cortex and exponentially increasing in the outer medulla; concentrations resemble those found in hydropenic or saline diuretic rats. The following predictions were obtained. The model with rabbit parameters does not recirculate urea and will not operate with high medullary urea concentrations; with rat parameters too much urea recirculates an the results of perfusion with equilibrium solution are not reproduced. Using a compromise between rat and rabbit parameters, the model reproduces water absorption, salt reabsorption, and urea recirculation as observed in vivo in rat loops perfused at 5-40 nl/min. It also simulates perfusion with saline, equilibrium solution, saline plus furosemide, and 300 mM mannitol. When the model includes a short early distal segment, effluent salt concentration reaches a minimum at a 15 nl/min perfusion rate as observed in vivo; however, concentration at the macula densa is a monotonically increasing function of flow. When permeation rate is a function of wall surface area and thickness a better fit to experimental results is produced. However, the effect is small: water absorption alters by 4% or less and effluent salt concentration is reduced by up to 10% at low perfusion rates. Comparison of rigid and compliant loops shows no relationship between transit time per se and reabsorption.

Animals↗

PubMed1976

Implications of fertility patterns in the Republic of Korea: a study by computer simulation.

Implications of various fertility patterns in the Republic of Korea (ROK) are studied by computer simulation. These patterns include the factors: age of marriage, sex preference, abortion (induced), contraception and sterilisation. As a result of studying a large number of combinations of these factors, the following tentative conclusions, among others, are reached. If future cohorts of women in the ROK become limiters after two live births and marry according to an observed high level marriage distribution, at least two of the methods of birth control--abortion, contraception, and sterilisation--will have to be used extensively to yield a mean number of live births (MNLB) in these cohorts of the order 2-2. However, if these women marry later than the observed high level marriage distribution and, in addition, accept effective methods of contraception more widely and use them more effectively, then the MNLB experienced by these cohorts could move below 2-5 wihtout the extensive utilisation of abortion and sterilisation.

Abortion, Induced↗

PubMed1991

Diagnosis of a computer-simulated podiatric patient.

The authors discuss the use of computers in medicine. A patient presenting with a condition requiring surgery is represented by a data file in a computer program. The user obtains data from the file by selecting items from a menu. Data are presented on the screen in pop-up windows. By examining the data and making judgments, the user may determine the pathology and make first a provisional and then a definitive diagnosis.

Computer Simulation↗

PubMed1984

Optimal pacemaker sensing with respect to amplitude and slew rate of intracardiac electrograms: theoretical analysis by computer simulation.

The optimal amplitude and slew rate of intracardiac electrograms for pacemaker sensing were examined on a theoretical basis by computer simulation. The simulation was based on the concept that it is the voltage at the position of the pacing electrode in an electrical field of a moving electrical dipole. By changing the distance between the electrode and the myocardium and the moving velocity of the electrical dipole, simulated ECGs with arbitrary amplitudes and slew rates were generated by the computer and fed to a bandpass filter. This filter was equivalent to those assembled in some models of permanent pacemakers and had a center pass-band frequency of 50 Hz and a Q of 1.0. The outputs of the filter were measured. The results showed that, for pacemakers sensing, simulated intracardiac electrograms with high amplitude should have high slew rates and those with low amplitudes should have low slew rates, although the absolute values depend on the characteristics of the bandpass filter and the sensing threshold of the pacemaker.

Computers↗

PubMed1982

Histamine as a ligand in blood plasma. Part 5. Computer simulated distribution of metal histamine complexes in normal blood plasma and discussion of the implications of a possible role of zinc and copper in histamine catabolism.

Previously physiological experiments carried out on mice have proved that copper and zinc can interfere with the pharmacological effects of histamine that lead to anaphylactic shock. A quantitative study of the interactions between essential metal ions and histamine in plasma was thus undertaken. The progressive approach towards a reliable computer-simulated distribution of the histamine-containing plasma species necessitated a large series of physicochemical determinations of the formation constants of the binary and ternary metal complexes involved. The present paper deals with the determination of the formation constants in the zinc-serine, zinc-histamine-serine, zinc-histamine-lysine, copper-serine, copper-histamine-serine, and copper-histamine-valine systems, which were still necessary to reach the reliable simulation required. The subsequent final distribution of histamine in plasma has thus been computed, and interpreted in terms of a possible role for zinc in assisting the histamine catabolism process. Further computer calculations simulating the increase of the zinc concentration in human blood plasma support this interpretation. The antagonizing role of copper against that of zinc has also been examined.

Animals↗

PubMed1990

Computer simulation of tumor cell motility and proliferation.

Tumor growth is considered to depend on tumor cell proliferation and on tumor cell motility. The present study investigates in which way these two cellular properties influence the evolving morphological pattern. Computer simulations were performed, where cells were either dividing or moving for a variable distance at a present probability. The simulation parameters (probability of motility, maximum moving distance) were set interactively. The resulting patterns were evaluated by binary morphological criteria, 13 of 17 binary criteria showed a significant relationship with the simulation parameters (median test: p = less than 0.05). Discriminant analysis of two sets of simulations with different simulation parameters provided a correct classification with an efficiency of 100% (k-nearest-neighbour method; jack-knife-procedure). The results indicate that cell proliferation and motility affect morphological patterns in a reproducible way and that the patterns in turn provide morphological clues for the quantitative estimation of motility and proliferation.

Cell Division↗

PubMed1990

Delayed detection of hypoxic events by pulse oximeters: computer simulations.

There is a variable delay between a reduction in alveolar PO2 and the decrease in arterial oxygen saturation recorded on a pulse oximeter. The decrease in arterial oxygen saturation in response to disconnexion of a paralysed patient from the breathing system, oxygen supply failure with continued mechanical ventilation and disconnexion of the fresh gas supply to Mapleson D and circle absorption breathing systems were studied by simulations on the MacPuf computer model of the cardiorespiratory system. The simulations revealed that there were marked differences between the rate of arterial desaturation which resulted from each of the three types of oxygen supply failure and that arterial oxygen saturation may reach dangerous levels before a pulse oximeter alarm is activated.

Computer Simulation↗

PubMed1984

The slow inward current, isi, in the rabbit sino-atrial node investigated by voltage clamp and computer simulation.

The properties of the slow inward current, isi, in the sino-atrial (s.a.) node of the rabbit have been investigated using two microelectrodes to apply voltage clamp to small, spontaneously beating, preparations. Many of the experimental results can be closely simulated using the computer model of s.a. node electrical activity (Noble & Noble 1984) which has been developed from models of Purkinje fibre activity (Noble 1962; DiFrancesco & Noble 1984). Comparison of the computed reconstructions with experimental results provides a test of the validity of the modelling. Experiments using paired depolarizing clamp pulses show that inactivation of isi is calcium-entry dependent although, unlike the inactivation of Ca2+ currents in some other systems, it also shows some voltage-dependence. Re-availability (recovery from inactivation) of isi in s.a. node is much slower than inactivation at the same potential, showing that isi is not controlled by a single first order process. This very slow recovery from inactivation of isi in the s.a. node and the slow time course of its activation and inactivation at voltages near threshold (-40 to -50 mV) can be closely modelled by assuming that there are two components of 'total isi': a fast inward current, iCa,f' representing the 'gated' fraction and a second, slower, inward current component, iNaCa which, we propose, is caused by the sodium-calcium exchange that ensues when the initial Ca2+ -entry triggers the release of stored intracellular Ca2+. When repetitive trains of clamp pulses are given, a 'staircase' of isi magnitude is seen which can be increasing ('positive') or decreasing ('negative') according to the potential level and frequency of the pulse train given. When computer reconstructions of such staircases are made, it is found that the positive staircases (which, in contrast to negative staircases, imply that more complex processes than simple inactivation are present) can be closely simulated by a model which incorporates slower processes (suggested Na-Ca exchange current) in the total isi in addition to the gated current component.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1989

Computer simulation of respiratory impedance and flow transfer functions during high frequency oscillations.

The usefulness of measuring respiratory flow in the airway and at the chest wall and of measuring respiratory input impedance (Z) to monitor high frequency ventilation was investigated by computer simulation using a monoalveolar 10-coefficient model. The latter included a central airway with its resistance (Rc) and inertance (lc), a resistive peripheral airway (Rp), a lumped bronchial compliance (Cb), alveolar gas compliance (Cgas), lung tissue with its resistance (RL) and compliance (CL), and chest wall resistance (RW), inertance (lw) and compliance (Cw). Gas flow in the peripheral airway (Vp), shunt flow through Cb (Vb), gas compression flow (Vgas) and rate of volume change of the lung (VL) and of the chest (VW) were computed and expressed as a function of gas flow in the central airway (Vc). For normal values of the coefficients, Vp/Vc was found to decrease moderately with increasing frequency and was still 0.75 at 20 Hz. Peripheral airway obstruction (Rp x 5) considerably decreased Vp/Vc, particularly at high frequency. It did not change the relationship between the two measurable flows, Vc and Vw, but increased the effective resistance at low frequency and shifted the reactance curve to the right. A reduced lung or chest wall compliance produced little change in Vp/Vc and Z except at very low frequencies; however, it decreased the phase lag between Vw and Vc. Finally, an increased airway wall compliance decreased Vp/Vc, but had little effect on Z and Vw/Vc. It is concluded that measuring respiratory impedance may help in detecting some, but not all of the conditions in which peripheral flow convection is decreased during high frequency oscillations.

Airway Obstruction↗