[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.

[Structural and functional changes in human erythrocytes and leukocytes during a 7-day immersion hypokinesia]. / Strukturnye i funktsional'nye izmeneniia éritrotsitov i leikotsitov cheloveka v usloviiakh 7-sutochnoi immersionnoi gipokinezii.

Changes in blood parameters (red and white blood cell counts) were examined in 6 healthy male volunteers during an acute stage of adaptation to the exposure simulating physiological effects of weightlessness. The parameters varied, remaining, however, within the physiological limits.