Biophysics, Vol.42, No. 2, pp. 491-496, 1997
© 1997 Elsevier Science Ltd.
All rights reserved. Printed in Great Britain
(Biofizika, 42, No. 502-507, 1997.)
A SYSTEM FOR THE COMPUTER VISUALIZATION OF THE PROPAGATION OF EXCITATION WAVES IN THE MYOCARDIUM
A.V. Moskalenko*) and
In current research into the processes of propagation of excitation over the heart, together with traditional electrophysiological methods an ever increasing role is being played by autowave approaches based on the close analogy the myocardium and other active media of a biological, chemical and physical nature . In such approaches the myocardium is represented by a three-dimensional active medium with energy sources distributed over it. The propagation of excitation over the heart, which serves as the trigger mechanism of mechanical contraction, is regarded as an autowave which keeps its form, amplitude, velocity and other parameters constant thanks to the non -linearity of the medium and which pumps energy from distributed sources. Special points in the heart (pacemakers, ectopic foci and sites of the emergence of additional conduction pathways) are interpreted as autowave sources.
An experimental investigation of the propagation of excitation in the heart is now based on computer visualization (electrophysiological mapping) . This method used the computational power of a computer for rapid digital recording of the electrocardiosignals (electrograms) simultaneously at many points of the myocardium and the restoration, on the basis of them, of the course of propagation of the wave. The result of visualization is a map of the spread of the wave in the form of a set of isochrones corresponding to the successive positions of the wavefront.
An experimental investigation of autowave patterns in the heart involves special procedural complexities. These difficulties are largely due to the specific features of the ectopic excitation sources, such properties of them as wavelength, unsteadiness, the close dependence of the conditions of reproduction on the state of the myocardium and the short time of existence. A study of the patterns of excitation of the myocardium requires the solution of a number of problems, in particular, a considerable increase in the accuracy of the recording of the electrophysiological data, the development of methods for observing unstable, spontaneously evolving processes, and improvements in the visual representation of wave patterns.
The aim of the present research was to develop a method for the computer visualization of the autowave patterns on the surface of the heart tissue. We developed an algorithmic system and software for exploring the evolution of autowave patterns, making it possible to provide an adequate representation of the spread of excitation with complex trajectories and to distinguish the most important details of the behaviour of the excitation sources.
THE COMPUTER VISUALIZATION SYSTEM
From experience using the Volna apparatus constructed in the Institute of Biological Physics , we devised a new system for the computer visualization of the of the propagation of excitation waves in the myocardium. Part of the system realising the user interface, processing and storage of experimental data operates in the Smalltalk/V 286 program of Digitalk. The subroutines for servicing the equipment are written in Assembler language for an IBM PC. The system was designed using the Grady Booch methodology . The system is constructed on the basis of the graphic multiwindow user interface . It offers the following options.
In conducting the experiment it is possible to record simultaneously signals from 64 electrodes (32 electrodes each for the endocardium and the epicardium of the preparation). The sampling frequency for each electrode is 1 kHz. It is possible to use 64, 128 or 256 electrodes without updating the system. During the experiment the system continuously forms a base sequence of stimulation pulses on the stimulating electrode. The experimenter can arbitrarily fix their amplitude, period and duration. For data recording the system generates independent sequences of test (out-of-turn) pulses over one or two electrodes. Eim pulses are used to initiate tachyarrythmia. For each sequence the experimenter can indicate the number of pulses in it, the initial duration, amplitude and the period of the pulses and also the gains in amplitude and period.
The signals recorded during the course of the experiment are inspected in the windows of the electrograms (Fig. 1). An arbitrary number of such waves may be simultaneously opened. Processing of the electrograms consists in fixing tags marking the instants of activation. The program allows this operation to be performed automatically or manually. On the basis of the data on the position of the tags, maps of the spread of excitation are automatically plotted (Fig. 2). The operator may indicate the time interval between the isochrones.
To analyse the electrical activity of the preparation as a whole the system may compute and visualize the function E(t) (Fig.3). The pseudoECG E(t) is calculated by the program from the formula
cosQ D S
The main improvement over the previous version of the Volna apparatus is the use of an integrated medium based on a multiwindow interface enabling one to combine into one program such functions of the system as control of the conduct of the experiment, processing, presentation and storage of data. One of the deficiencies of the previous system was the limitation on the duration of the recorded electrograms (not more than 4-s use of 64 electrodes at a sampling frequency of 1 kHz).
In the new system this deficiency is eliminated and the recording time is determined only by the computer memory capacity. In addition, the set of regimes for generating the stimulation pulses of the preparation on data recording is greatly extended. The data processing of the experiments conducted using the system made it possible to demonstrate the link between the pseudoECG characteristic of polymorphous arrhythmias (Fig.4) and complex changes with time in the wave patterns recorded simultaneously from the epicardium and endocardium (Fig. 5). In some cases these changes were in the form of drift of the ectopic source of excitation. Figure 6 shows the drift of the core of the reverberator recorded in one experiment. The modified version of the Volna apparatus greatly extends the possibilities of the experiment with the aim of seeking new approaches to exploring the mechanisms of disturbance of cardiac rhythm.
This research was supported financially by the Soros International Foundation and the Russian Foundation for basic Research.