To study the spread of excitation in ventricular heart preparations we have designed a fast, high-resolution recording and mapping system. Papillary muscles were dissected from the isolated guinea pig hearts. The preparation was fixed in a tissue bath and superfused with Tyrode solution. Linear and two-dimensional arrays of Ag/AgCl electrodes were made on glass with a thin-film technique. The transparent sensors with up to 24 electrodes (spaced 50, 90, or 180 microns apart) were positioned close to the surface of the preparation with a custom-designed three-dimensional micromanipulator. Extracellular signals were simultaneously recorded by a 24-channel data acquisition system with a 200 kHz per channel sample rate, with 12-bit amplitude resolution and a maximum data length of up to 3 MB. Digitized video images of the electrode array and the underlaying preparation were used to identify the locations of the recording sites. A UNIX-based computer system with a custom-designed data acquisition and database program was used to control the instruments and to manage the experimental data. This technique gave signals with excellent signal-to-noise ratios (up to 65 dB) and permitted accurate evaluation of the time and the site of the local activation with high resolution (to within 5 microseconds, 50 microns). We describe the spread of excitation within the area of a few cells and found a substantial dispersion of conduction velocities. Beat-to-beat comparison of activation patterns showed relatively small variations in the spread of excitation (a few microseconds).

In real electrophysiological experiments, irregularities in the extracellular excitation spread are believed to depend on cardiac tissue microstructure. An electronic hardware model was developed to analyze this dependence by placing some inhomogeneities (slow propagation areas) in the medium. The position of such inhomogeneities is correlated with abnormal delays and irregularities measured in signal propagation.

The investigation of the influence of microstructures in cardiac conduction disturbances requires a detailed analysis of the propagating process at a microscopic size scale. We present a system for recording extracellular potentials to measure the instant of the local depolarization with very high-resolution ( 3 0~s in time and 15 p m in space ). Micromaps of the extracellular activation indicated discontinuous conduction at a microscopic size scale. Time series of subsequent activation patterns showed an extraordinary small variability of all conduction times (STD < 20ps) under normal conditions. These microfluctuations are influenced by arrhythmogenic factors, antiarrhythmic drugs and uncoupling substances. INTRODUCTIONThe measurement of the excitation spread in cardiac muscle at a microscopic level is becoming of increasing practical importance. Recent experimental and theoretical findings confirm the hypothesis of the discontinuous nature of the propagation in heart tissue corresponding to the complex microstructure [1] [2]. To monitor micropatterns of excitation, measurements of high-resolution in time and space without injury of cells are required. We have developed a recording system to measure extracellular action potentials with the required resolution in time and space. With the described experimental setup sequences of the extracellular action potentials were analyzed and beat-to-beat microfluctuations of conduction velocities were studied.

In recent years there has been a remarkable progress in the knowledge of the microstructure of the cardiac tissue and its influence on the conduction of the cardiac impulse. The tissue domain can be thought as a discrete network of cells coupled electrically at stochastically distributed sites. During in-vitro experiments the tissue is surrounded by a conducting superfusate which represents a continuous domain. With electrode arrays, electrograms can be recorded simultaneously at many sites in this volume conductor and the spatio-temporal distribution of potentials can be obtained. In this paper we show, that despite the fact that the sensors were placed in a continuous medium, we were able to detect microscopic discontinuities of propagation with appropriate techniques.

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