Magnetic field measurements above type-II superconductor films in a remanent magnetization state have been used to determine two dimensional surface current distributions. Using the Biot-Savart law ...for the surface current density, integral equations (of 2-D convolution type) for two components of current have been obtained. These equations have been solved by employing Tikhonov's method of generalized discrepancy, and the surface current pattern in superconductors as well as the magnetic field distribution on the film surface have been obtained. Current peculiarities related to various kinds of inhomogeneities, specific character of the film form, and a laser pulse effect have been observed. In particular, it was discovered that a laser pulse can signficantly redistribute the film currents without changing the total number of current vortices in the film.
1 Division of Cardiology, Department of Medicine,
Cedars-Sinai Medical Center, and 2 Division of Cardiology,
Departments of Medicine and Physiology and Physiological Science,
University of ...California School of Medicine, Los Angeles, California
90048; and 3 Department of Physics and Astronomy, Vanderbilt
University, Nashville, Tennessee 37235
Several different patterns of wave
break have been described by mapping of the tissue surface during
fibrillation. However, it is not clear whether these surface patterns
are caused by multiple distinct mechanisms or by a single mechanism. To
determine the mechanism by which wave breaks are generated during
ventricular fibrillation, we conducted optical mapping studies and
single cell transmembrane potential recording in six isolated swine
right ventricles (RV). Among 763 episodes of wave break (0.75 times · s 1 · cm 2 ), optical
maps showed three patterns: 80% due to a wave front encountering the
refractory wave back of another wave, 11.5% due to wave fronts passing
perpendicular to each other, and 8.5% due to a new (target) wave
arising just beyond the refractory tail of a previous wave. Computer
simulations of scroll waves in three-dimensional tissue showed that
these surface patterns could be attributed to two fundamental
mechanisms: head-tail interactions and filament break. We conclude that
during sustained ventricular fibrillation in swine RV, surface patterns
of wave break are produced by two fundamental mechanisms: head-tail
interaction between waves and filament break.
reentry; mapping; electrophysiology; action potentials; restitution
*
M.-H. Lee and Z. Qu contributed equally to this study.
Pump–probe spectroscopy studies on cobalt-doped spinel-based aluminosilicate glass–ceramics of various compositions have been made in the visible part of the spectrum. Stimulated emission from the
4T
...1(
4P) state to the
4A
2 one of tetrahedral Co
2+ ion has been observed for all glass–ceramic materials. The peak values of effective stimulated emission cross-sections for Co
2+ ions are estimated to be 4–5
×
10
−19
cm
2 depending on glass–ceramics composition and are found to be higher than in spinel single crystals.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The effect of effective refractory period (ERP) shortening on the vulnerability and characteristics of induced functional reentry in the ventricle remain poorly defined. We hypothesized that ERP ...shortening increases ventricular vulnerability to reentry and accelerates its rate, as is the case in the atrium.
The epicardial surfaces of 19 isolated and superfused canine right ventricular slices (4x4 cm and <2 mm thick) were mapped with 480 bipolar electrodes 1.6 mm apart. Vulnerability was tested during pacing at a cycle length (CL) of 600 ms and with a single premature stimulus of 5-ms duration at increasing current strength of 1 to 100 mA. Cromakalim (10 micromol/L), an ATP-sensitive potassium channel opener, caused a significant (P<0. 001) shortening of the ERP but had no effect on conduction velocity. Cromakalim increased (P<0.01) the vulnerability (product of current and the stimulus coupling interval) for reentry induction. Reentry had a significantly shorter CL and lasted for a longer duration (P<0. 001). The central core around which the wave front rotated became smaller, which caused shortening of the CL of reentry. A significant (P<0.001) linear correlation was found between core size and reentry CL. These effects of cromakalim were reversible. Two-dimensional simulation studies using the modified Luo-Rudy I model of cardiac action potential, in which the refractory period was variably shortened by a progressive increase of the time-independent potassium conductance, reproduced the experimental findings.
ERP shortening by an ATP-sensitive potassium channel opener increases ventricular vulnerability to reentry and accelerates its rate by decreasing the core size around which the wave front rotates.
The changes observed in the structure and phase composition of titanium-containing glasses in the magnesium aluminosilicate system at the equimolar ratio of MgO and Al2O3 during their heat treatment ...are investigated using the small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction analysis. Nickel oxide additives (from 0.5 to 5.0 mol %) are introduced into the glasses. It is shown that heat treatment of the studied glasses is accompanied by three-phase separation. As a result, two amorphous phases precipitate in the high-silica matrix. One phase (magnesium aluminotitanate) is enriched with magnesium, aluminum, and titanium oxides, whereas the other phase (magnesium aluminate) is enriched with aluminum and magnesium oxides. Nickel oxide is incorporated into both phases. Moreover, nickel oxide reduces the crystallization ability of the magnesium aluminotitanate phase and magnesium aluminotitanates do not crystallize in glasses containing 5 mol % NiO. During Subscriptsequent heat treatments, regions of the nickel magnesium aluminate phase serve as precipitation centers of the magnesium aluminate phase, and, consequently, nickel oxide affects the microinhomogeneous structure of glasses at the final stages of heat treatment. This leads in particular to a decrease in the size of spinel microcrystals. The temperature of the onset of crystallization in regions of existence of the nickel magnesium aluminate phase (spinel precipitation) is lower than that in glasses free from nickel oxide. It is assumed that spinel microcrystals are solid solutions of nickel aluminum and magnesium aluminum spinels.PUBLICATION ABSTRACT
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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