A new approach for resolution of complex tissue impedance spectra in hearts
Andrew E. Pollard and Roger C. Barr
Volume: 60, Issue: 9, Page(s): 2494 – 2503
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 Hz to 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 (pt) from rabbit ventricular epicardium. Simultaneous acquisition of a signal proportional to the supplied current (Vstim) with uPD allowed identification of the voltage-to-current ratio needed for pt 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 high mean uPD signal-to-noise ratios (SNRs) measured with supplied currents as low as 10 nA. Comparable performance characteristics were identified during calibration of 9 arrays built with 0.25 mm 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 pt=134 ohm-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.