[Parameters of cardiac muscle repolarization on the electrocardiogram when changing anatomical and electric position of the heart].

While discussing the diagnostic value of the single channel electrocardiogram a set of theoretical considerations emerges inevitably, one of the most important among them is the question about dependence of the electrocardiogram parameters from the direction of electrical axis of heart. In other words, changes in what of electrocardiogram parameters are in fact liable to reflect pathological processes in myocardium, and what ones are determined by extracardiac factors, primarily by anatomic characteristics of patients. It is arguable that while analyzing electrocardiogram it is necessary to orient to such physiologically based informative indexes as ST segment displacement. Also, symmetry of the T wave shape is an important parameter which is independent of patients anatomic features. The results obtained are of interest for theoretical and applied aspects of the biophysics of the cardiac electric field.

[Computer simulation for localization and extensiveness of myocardial ischemia].

Features of genesis of the electrocardiosignal in conditions of the myocardial ischemia under changing localization and extensiveness of the ischemia foci, as well as an impact of this process on the repolarization parameters of the electrocardiosignal were investigated using a system for 3D simulation of the heart electrical activity. The model of the ventricles of the heart is a cellular automaton with the matrix about of 10(5) elements of the "Myocardium", "His", and "Purkinje" types and with prescribed rules of their interaction. In process of the study a developed earlier method of segmentation of the modeled ventricles was used. For estimating the contribution of the "ischemic" segments to the total signal of different leads of the electrocardiograms the previously suggested method of the partial curves was used. The ischemia foci were simulated by changing the characteristics of the transmembrane action potential in selected areas of the ventricles, such as the resting potential, transmembrane action potential amplitude, duration of the plateau phase, and the steepness of the final phase of rapid repolarization in accordance with literature data. Electrocardiosignals generated by the model were processed to obtain previously selected parameters, characterizing repolarization process. The influence of size and localization of the myocardial ischemia on the parameters obtained are examined, as well-as some hypotheses about their relation with an electrophysiological state of the heart muscle are discussed.

[Electrocardiographic image of myocardial ischemia: real measurements and biophysical models. Part II].

By means of biophysical models of the heart electrical activity some alterations of the ST-T interval of an electrocardiogram were considered under changes of electrophysiological parameters of the cardiac muscle, characteristic for ischemia development. A computer program "The model of electrical activity of ischemic heart" developed in our Lab was used as a "Generator" for selection of model electrocardiosignals (ECS) when changing parameters of the transmembrane action potential (TAP) in "ischemic" area which was characterized in the model terms by its location and extent. The obvious development of ischemia was triggered by the deflection of TAP parameters from the reference values accepted for "conventional norm". For trapezoidal approximation of the TAP form chosen at the first stage of the work the following changed TAP parameters were used: (a) modification of duration of the horizontal plateau phase at the constant phase 3 slope; (b) a change in the amplitude of the plateau phase at the constant duration of this phase and all TAP; (c) modification in the slope of the fast repolarization phase at the constant duration of the plateau phase (in this case, the total duration of TAP also changes); (d) reduction in the duration of phase 2 at constant TAP duration, that, naturally, entails modification of the slope for the fast repolarization phase. We studied (out-of-scenarios developed before) separate effects of these parameters on changes of the T wave shape and the ST segment level. A number of new parameters for description of the changes observed in the ST-T interval pattern are offered and investigated. Computer experiments were performed using a system for 3D-modeling of the cardiac electrical activity. With the selected real and simulated ECSs a comparative study for diagnostic possibilities of some electrocardiographic criteria for detecting myocardial ischemia is carried out. The results obtained at this stage of work are of interest for identification and prediction of an electrophysiological state of the heart.

[Electrocardiographic image of myocardial ischemia: real measurements and biophysical models].

The peculiarities of the genesis of electrocardiosignal under myocardial ischemia in connection with an increase of its electrical inhomogeneity, as well as the electrophysiological mechanisms of morphological changes of the T wave of the electrocardiogram, including its "symmetrization" have been considered. A systemic approach to the problem has been used, which combines the mathematical, computer, and physiological modeling of the cardiac electrical activity with studies of the electric field of the human heart in terms of biophysical models. A database for the repolarization parameters of experimental electrocardiosignals ("Norm" and "Ischemia" samples) has been formed. The parameters of the ST-T interval and T wave, which could characterize the symmetry of the latter, and some additional properties of the repolarization process have been obtained. The methods of mathematical modeling were used also. Computer experiments were carried out on a system for 3-D modeling of the cardiac electrical activity at different structural levels of the object. By the results of preliminary analysis, the betaT index, which is calculated as the ratio of two maximum absolute values of the derivative of the cardiosignal at the left and right of the T wave apex, has been chosen as one of the main diagnostic markers of ischemia. There is reason to believe that the use of the betaT index allows one to recognize those deviations from the norm that are usually hidden from a physician in traditional ECG analysis. The ratios of repolarization intervals inside of the generalized QT interval have also appeared potentially informative. With the purpose to test and correct the hypotheses for conducting further investigations, some preliminary experiments on a low-resolution model for several alternatives of the degree, localization, and extensiveness of ischemia have been carried out. The results obtained at the first stage of the team-work are essential to the understanding of mechanisms of the genesis of an electrocardiographic image for myocardial ischemia and of interest for the biophysically sound development of new algorithms for computer cardiodiagnostics.

[Realization of biophysical models for the cardiac electrical activity].

Some principles for the realization of biophysical models of electrical activity of heart ventricles, presented by a double electrical layer along the surface of the electrically active myocardium and the boundary surfaces that divide model compartments with different electrophysiological properties, have been considered. The parameters of the model are the electrophysiological and anatomical characteristics of the heart, such as the geometry of ventricles and the His-Purkinje specialized conductive system, the velocity of depolarization propagation in the myocardium, the ratio of excitation velocities in "Myocardium", "His", and "Purkinje" model elements, the ,shape of transmembrane action potentials on boundary surfaces, the orientation of the own heart anatomical axes relative to the initial coordinate system, and others. The model has been included as a basic unit into a computer modeling system containing databases of simulated and real electrocardiosignals.

[Effect of sex on informative power of various electrocardiographical criteria of left ventricular hypertrophy in patients with excessive body mass].

Aim of the study was to analyze dependence of various voltage parameters of QRS complex on increase of left ventricular myocardial mass (LVMM) in samples of men and women with excessive body mass or obesity. We included data from 223 patients with excessive body mass and diagnosis of stage I - II arterial hypertension. ECG was registered in 12 standard leads. Left ventricular hypertrophy (LVH) was certified if according to echoCG data LVMM exceeded 125 g/m2 in men and 110 g/m2 in women. Depending on sex and presence of LVH all patients were divided into 4 groups: M1 (men with LVH, n=74), M2 (men without LVH, n=74), W1 (women with LVH, n=55), anb W2 (women without LVH, n=20). We analyzed amplitudes of all waves of the QRS complex as well as Sokolow-Lyons voltage parameters and the Cornell index. The following intergroup differences were most significant: between groups M1 and M2 - in amplitudes of S waves in chest leads V3, V4; between groups W1 and W2 - in amplitudes of R-waves in limb leads I and aVL, and amplitudes of S-waves in lead III. Increases of the Cornell voltage index were observed both in men and women with LVH. The following criteria had greatest sensitivity at 95% specificity: in men - SV4 > 1,1 mV (34%) and RaVL+SV3 > 2,3 mV (32%); in women - RaVL > 0,8 mV (56%) and RI+SIII > 1,5 mV (56%). Informative power of electrocardiographical diagnosis of LVH can be augmented by the use of different voltage criteria in groups of men and women. In men most informative are chest leads (SV1 - V3, RaVL) while in women - limb leads (RI, RaVL, and SIII). The use of combination parameters RaVL+SV3 > 2,3 mV (in men) and RI+SIII > 1,5 mV (in women) allows to augment sensitivity with unchanged specificity. In patients with excessive body mass voltage the Sokolow-Lyons criterion is not informative. Most significant component of the Cornell voltage criterion in groups of men with excessive body mass is amplitude of SV3, in groups of women - amplitude of RaVL.

[A relationship between the QRS maximal vector and myocardium mass of the left ventricle during the development of hypertrophy: computer modeling and real measurements].

It has been demonstrated on an model of the cardiac electrical activity that, during the course of proportional homothetic transformations of the heart and His-Purkinje system, the values of the spatial heart vector change, as a first approximation under comparable conditions, in proportion to the extension of the heart ventricle surface rather than the myocardium mass, since the magnitude of the electrocardiosignal is determined at every instant by the area of depolarized regions on the epicardium and endocardium, which corresponds to the model of a double electrical layer on the surface of the electrically active myocardium.

[Biophysical models of the heart electrical activity].

Based on the fundamental knowledge of the space-temporal organization of extracellular electrical fields of the myocardium, a system for 3-D computer modeling of the cardiac electrical activity at different structural levels of the object is being developed at the Institute of Theoretical and Experimental Biophysics. The system is based on the earlier proposed and modified biophysical model of the electrocardiosignal genesis represented by a double electrical layer along the surface of the electrically active myocardium. The system combines the model for activation and repolarization of the heart ventricles; the advanced model for the evaluation of parameters of the cardiac electric field, which makes it possible to derive model electrocardiosignals both in the direct regime of calculation of the potentials and in the regime of calculation of electrocardiosignals from preliminarily determined components of the multipole equivalent electrical heart generator; a database for the model parameters and their combinations in the form of cards of simulated "patients", and a database of modeled electrocardiosignals. In the present paper (first from three within the framework of the problem), simulation methods in electrocardiology are briefly described and a biophysical model of the heart electrical activity is presented, which has made up the basis of the system for computer modeling of forward and inverse problems of the cardiac electric field. The parameters of the model are electrophysiological, anatomical, and biophysical characteristics of the heart.

[Slow excitation waves and mechanisms of polymorphic ventricular tachycardia in an experimental model: isolated walls of the right ventricle from rabbit and squirrel]. / Medlennye volny vozbuzhdeniia i mekhanizmy polimorfnoi zheludochkovoi takhikardii v éksperimental'noi modeli: izolirovannye stenki pravogozheludochka krolika i suslika.

The mechanism of polymorphic disturbances of the heart rhythm is studied on an experimental model, isolated ventricular preparations of ground squirrel and rabbit. Polymorphic arrhythmias are identified from habitus of the isolated preparation pseudoECGs mathematically derived from electrograms registered simultaneously at 32 endocardial and 32 epicardial points. The same electrograms allow one to visualize the excitation wave propagation along each of the preparation surfaces. The comparison of excitation wave pictures and corresponding pseudoECGs enabled us to reveal the conditions necessary and sufficient for polymorphism in heart rhythm disturbances. Polymorphic arrhythmias are due to changes in wave pictures in the regions of retarded excitation propagation.

Computer simulation of the heart magnetic field dynamics.

A personal-computer based model, which enables the investigation of the correlation between the temporal dynamics of the magnetic field distribution and electrophysiological processes during ventricular depolarisation, has been tested. The activation sequence following a stimulus to the atrioventricular node or directly to the ventricular myocardium can be simulated in isotropic conditions and visualised in a three-dimensional presentation. The corresponding behaviour of the distribution of the normal component of the magnetic field is calculated using a simple algorithm. Colour animation is used to observe the simulated magnetic field dynamics and to compare them with measured data.