Multisite interstitial stimulation for cardiac micro-impedance measurements.

On theoretical grounds, interstitial current injected and removed using electrodes in close proximity does not cross the membrane, while equilibration of intracellular and interstitial potentials occurs distant from electrodes widely separated. Multisite interstitial stimulation should therefore give rise to interstitial potential differences recorded centrally that depend on intracellular and interstitial micro-impedances, allowing independent measurement. We tested the feasibility of completing such measurements using simulations of multisite stimulation with fine and wide spacing in models that included Luo-Rudy dynamic (LRd) membrane equations. Using two-dimensional models, test data (delta phi o) were generated with isotropic interstitial and intracellular micro-impedances prescribed for one set of simulations, and with anisotropic micro-impedances including unequal ratios (intracellular/interstitial) along and across fibers prescribed for another set of simulations. Micro-impedance measurements were then obtained by making statistical comparisons between delta phi o values and interstitial potential differences from passive bidomain simulations (Delta phi o) in which a wide range of possible micro-impedances were considered. Our findings suggest development of microfabricated devices to implement the multisite stimulation procedure would facilitate routine measurement as a component of cardiac electrophysiologic study.