The Impact of Adjacent Isotropic Fluids on Electrograms from Anisotropic Cardiac Muscle A Modeling Study Downloaded from

SUMMARY. Recent studies have reported good agreement between extracellular potentials recorded at the surface of a tissue bath preparation and calculated potentials derived from intracellular action potentials assuming interstitial space is unbounded, homogeneous, and isotropic. In fact, heart muscle is electrically anisotropic. To investigate the effect of interstitial anisotropy, we developed a computer model in which the heart muscle and the perfusate are represented by a three-dimensional grid of resistors. Electric sources are related to transmembrane cardiac action potentials through a bisyncytial model of the heart which considers intracelluiar space and interstitial space to be interpenetrating domains or syncytia. The sources are affected by intracellular anisotropy. The model study gave the following results: there is good agreement between the model and potentials reported by others for a tissue bath preparation and for epicardial potentials on an insulated heart; to a good approximation, interstitial anisotropy can be ignored in calculating extracellular potentials at the surface of the tissue when it is immersed in perfusate, although there are differences between calculated and experimental values consistent with those observed; interstitial anisotropy becomes important when the electrode penetrates the tissue or when the fluid level drops below 1 mm; potentials are significantly affected by the presence of a very thin layer of fluid; the scale factor for potential amplitude is consistent with a theoretical model previously derived. 1982) RECENT studies by In this paper we will show why it is reasonable to neglect extracellular (interstitial) anisotropy in the tissue bath preparation. The problem is approached using a digital computer model in which interstitial space is characterized by a finite element model comprising a resistor grid. Calculations from this model, which incorporates anisotropy and finite extent of the volume conductor, are compared with calculations assuming an unbounded isotropic volume conductor. In addition, effects of variation of certain parameters of the preparation, such as height of the fluid above the tissue or depth of penetration of the recording electrode, are evaluated. The method of analysis was based on the reciprocity theorem. The overall objective of these calculations was to examine the effects of interstitial anisotropy on extracellular waveforms computed from hypothesized (but realistic) cellular action potentials. We were interested in the waveforms themselves, their variation with changes in conductivity and geometry characteristics, and in the relation of the model results to experimental results of Theory We will first sketch the theory that relates bioelectric sources in a volume conductor to action potentials. We will then develop the theory based on reciprocity for relating potentials in the volume conductor to these sources. Bisyncytial Model Consider that heart muscle consists of two interpenetrating domains: an intracellular space and an extracellular or interstitial spac