A New Approach for Resolution of Complex Tissue Impedance Spectra in Hearts

Abstract

This study was designed to test the feasibility of using sinusoidal approximation in combination with a new instrumentation approach to resolve complex impedance (uCI) spectra from heart preparations. To assess that feasibility, we applied stimuli in the 10–4000 Hz range and recorded potential differences (uPDs) in a four-electrode configuration that allowed identification of probe constants (Kp) during calibration that were in turn used to measure total tissue resistivity ρt from rabbit ventricular epicardium. Simultaneous acquisition of a signal proportional to the supplied current (Vstim) with uPD allowed identification of the V –I ratio needed for ρt measurement, as well as the phase shift from Vstim to uPD needed for uCI spectra resolution. Performance with components integrated to reduce noise in cardiac electrophysiologic experiments, in particular, and provide accurate electrometer-based measurements, in general, was first characterized in tests using passive loads. Load tests showed accurate uCI recovery with mean uPD SNRs between 101 and 103 measured with supplied currents as low as 10 nA. Comparable performance characteristics were identified during calibration of nine arrays built with 250 µm Ag/AgCl electrodes, with uCIs that matched analytic predictions and no apparent effect of frequency (F = 0.12, P = 0.99). The potential ability of parasitic capacitance in the presence of the electrode–electrolyte interface associated with the small sensors to influence the uCI spectra was therefore limited by the instrumentation. Resolution of uCI spectra in rabbit ventricle allowed measurement of ρt = 134 ± 53 Ω·cm. The rapid identification available with this strategy provides an opportunity for new interpretations of the uCI spectra to improve quantification of disease-, region-, tissue-, and species-dependent intercellular uncoupling in hearts.

Sensor spacing affects the tissue impedance spectra of rabbit ventricular epicardium

“…All spectra were identified using the system documented in our recent report (17). Briefly, stimuli were delivered through an AC signal generation stage that supplied I stim between two outer electrodes that oscillated 180°out of phase from one another.…”

“…This issue is more managable when larger electrodes are used. Most investigators who have completed four-electrode measurements with whole heart preparations have used arrays with 250 m to 1 mm diameter electrodes separated on a millimeter to centimeter size scale (29,25,26,13,1,28,17). Although arrangements of this type limit the influence of instrumentation, spectra do not develop with traditional separations because I stim redistribution is equivalent at all frequencies.…”

“…To limit the influence of instrumentation, we relied heavily on active shielding shown previously to be effective in experiments using arrays with traditional electrode size and separation (17). With active shielding, supply of a replica of the measured signal to the outer conductor in a coaxial arrangement maintains a small voltage difference between the shield and measured signal that prevents formation of line capacitance on system interconnections (5,12).…”

An inductorless low-power tunable sinusoidal oscillator for micro-impedance spectroscopy

“…Although both DC and AC (single frequency) sources [4] are still widely used, many studies in which the electrical properties of tissues are presumed to be stable have started to use AC source with multiple frequencies [2] [5]. Micro-impedance spectroscopy using multi-frequency sinusoidal source can measure complex tissue impedances by delivering sinusoidal current signal with multiple different frequencies to the tissue and sensing the voltage variations.…”

“…With the help of impedance calculation algorithms, the magnitude and phase of the impedance can be calculated. Recently, sinusoidal approximation has been demonstrated to resolve complex impedance spectra from heart preparations [5].…”