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

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.