[Mechanism of Formation of Cardiac Arrhythmia Due to Pathological Distribution of Myocardium Conductivity].

Two mechanisms responsible for the emergence of arrhythmia are known: a change of part of the cells to a self-oscillatory mode and generation of circulating waves. In this paper, we investigate the generation mechanism of the circulating waves using the unidirectional block. One of the variants of its realization is a narrow gap between two non-conducting regions. Implementation of this mechanism in the human heart turns out to be impossible, since in the heart in which the duration of cardiac action potential lasts 0.3 s and the velocity of wave propagation is equal to 33 cm/s, the minimal length of the pathway for wave circulation is approximately 10 cm, while the distance between the ventricular apex and atrioventricular septal is, on the average, 8 cm. Therefore, that inhomogeneity cannot exist at the scale of human heart. To adapt this mechanism to the size of the human heart, we introduce into the scheme the regions with low conductivity, which provide slow propagation of the wave. The value of conductivity is chosen based on the results of evaluation of the "conductivity-wave velocity" correlation. The analysis of wave propagation through the boundary between two regions with different conductivities has shown that the refractory period depends on the conductivity ratio. To minimize this dependence we introduce the transition zone, in which conductivity changes linearly from some normal value to a reduced one. This allowed us to generate a 12-mm inhomogeneity area, provoking the appearance of the circulating wave.

[Numerical Simulation of Propagation of Electric Excitation in the Heart Wall Taking into Account Its Fibrous-Laminar Structure].

The propagation of excitation wave in the inhomogeneous anisotropic finite element model of cardiac muscle is investigated. In this model, the inhomogeneity stands for the rotation of anisotropy axes through the wall thickness and results from a fibrous-laminar structure of the cardiac muscle tissue. Conductivity of the cardiac muscle is described using a monodomain model and the Aliev-Panfilov equations are used as the relationships between the transmembrane current and transmembrane potential. Numerical simulation is performed by applying the splitting algorithm, in which the partial differential solution to the nonlinear boundary value problem is reduced to a sequence of simple ordinary differential equations and linear partial differential equations. The simulation is carried out for a rectangular block of the cardiac tissue, the minimal size of which is considered to be the thickness of the heart wall. Two types of distribution of the fiber orientation angle are discussed. The first case corresponds 'to the left ventricle of a dog. The endocardium and epicardium fibers are generally oriented in the meridional direction. The angle of fiber orientation varies smoothly through the wall thickness making a half-turn. A circular layer, in which the fibers are oriented in the circumferential direction locates deep in the cardiac wall. The results of calculations show that for this case the wave form strongly depends on a place of initial excitation. For the endocardial and epicardial initial excitation one can see the earlier wave front propagation in the endocardium and epicardium, respectively. At the intramural initial excitation the simultaneous wave front propagation in the endocardium and epicardium occurs, but there is a wave front lag in the middle of the wall. The second case refers to the right ventricle of a swine, in which the endocardium and epicardium fibers are typically oriented in the circumferential direction, whereas the subepicardium fibers undergo an abrupt change in the angle of orientation. For this case the dependence of the wave front on the location of initial excitation is weak. One can see the earlier wave front propagation in the middle of the wall. However, the wave front formation rate is different: with highest velocity for intramural initial excitation and with lowest one during excitation on the endocardial surface.

Vertically polarizing undulator with the dynamic compensation of magnetic forces for the next generation of light sources.

A short prototype (847-mm-long) of an Insertion Device (ID) with the dynamic compensation of ID magnetic forces has been designed, built, and tested at the Advanced Photon Source (APS) of the Argonne National Laboratory. The ID magnetic forces were compensated by the set of conical springs placed along the ID strongback. Well-controlled exponential characteristics of conical springs permitted a very close fit to the ID magnetic forces. Several effects related to the imperfections of actual springs, their mounting and tuning, and how these factors affect the prototype performance has been studied. Finally, series of tests to determine the accuracy and reproducibility of the ID magnetic gap settings have been carried out. Based on the magnetic measurements of the ID Beff, it has been demonstrated that the magnetic gaps within an operating range were controlled accurately and reproducibly within ±1 µm. Successful tests of this ID prototype led to the design of a 3-m long device based on the same concept. The 3-m long prototype is currently under construction. It represents R&D efforts by the APS toward APS Upgrade Project goals as well as the future generation of IDs for the Linac Coherent Light Source (LCLS).

Some aspects of achieving an ultimate accuracy during insertion device magnetic measurements by a Hall probe.

An extensive test of a new Senis 2-axis Hall probe was done at the Advanced Photon Source using the Undulator A device and calibration system. This new probe has clear advantages compared with previously used Bell and Sentron Hall probes: very stable zero offset (less than the noise of 0.026 G) and compensated planar Hall effect. It can be used with proper calibration even for first and second field integral measurements. A comparison with reference measurements by long stretched coil shows that the difference in the first field integral measurement results for a 2.4-m-long Undulator A device is between 17 G cm for the best of four Hall probes used for the test and 51 G cm for the worst of them for all gap ranges from 10.5 mm to 150 mm.

Characterization of a high-gain harmonic-generation free-electron laser at saturation.

We report on an experimental investigation characterizing the output of a high-gain harmonic-generation (HGHG) free-electron laser (FEL) at saturation. A seed CO2 laser at a wavelength of 10.6 microm was used to generate amplified FEL output at 5.3 microm. Measurement of the frequency spectrum, pulse duration, and correlation length of the 5.3 microm output verified that the light is longitudinally coherent. Investigation of the electron energy distribution and output harmonic energies provides evidence for saturated HGHG FEL operation.

Exponential gain and saturation of a self-amplified spontaneous emission free-electron laser.

Self-amplified spontaneous emission in a free-electron laser has been proposed for the generation of very high brightness coherent x-rays. This process involves passing a high-energy, high-charge, short-pulse, low-energy-spread, and low-emittance electron beam through the periodic magnetic field of a long series of high-quality undulator magnets. The radiation produced grows exponentially in intensity until it reaches a saturation point. We report on the demonstration of self-amplified spontaneous emission gain, exponential growth, and saturation at visible (530 nanometers) and ultraviolet (385 nanometers) wavelengths. Good agreement between theory and simulation indicates that scaling to much shorter wavelengths may be possible. These results confirm the physics behind the self-amplified spontaneous emission process and forward the development of an operational x-ray free-electron laser.

High-gain harmonic-generation free-electron laser

A high-gain harmonic-generation free-electron laser is demonstrated. Our approach uses a laser-seeded free-electron laser to produce amplified, longitudinally coherent, Fourier transform-limited output at a harmonic of the seed laser. A seed carbon dioxide laser at a wavelength of 10.6 micrometers produced saturated, amplified free-electron laser output at the second-harmonic wavelength, 5.3 micrometers. The experiment verifies the theoretical foundation for the technique and prepares the way for the application of this technique in the vacuum ultraviolet region of the spectrum, with the ultimate goal of extending the approach to provide an intense, highly coherent source of hard x-rays.

Unsaturated fatty acids enhance low density lipoprotein uptake and degradation by peripheral blood mononuclear cells.

The precise mechanism by which unsaturated fatty acids lower low density lipoprotein cholesterol is not known. Because cis-unsaturated fatty acids incorporated in cell membranes increase membrane fluidity and can thereby dramatically alter membrane-dependent cellular functions, we examined the effect of linoleate and oleate incorporation in peripheral blood mononuclear cell membranes on the physical properties of the membrane and concomitantly on low density lipoprotein uptake and degradation. We found that membrane enrichment with linoleate increased the rate of low density lipoprotein degradation in both freshly isolated and derepressed mononuclear cells. Enrichment with oleate led to similar increases in degradation. "Specific" low density lipoprotein uptake by derepressed cells was also enhanced by linoleate and oleate incorporation. Enrichment with both of these fatty acids produced an increase in membrane fluidity, as indicated by a reduction in the steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene incorporated in the membrane. In contrast, stearate enrichment had little effect on uptake or degradation of low density lipoprotein, nor did it affect membrane fluidity. These data point to a novel mechanism for the reduction in low density lipoprotein produced by unsaturated fatty acids that involves their physical effects on cell membranes as it relates to metabolism of the lipoprotein.