• defibrillation;
  • virtual electrodes;
  • secondary sources;
  • optical mapping;
  • heterogeneities

Introduction: The primary objective of this study was the quantitative description of shock-induced, locally occurring virtual electrodes in natural cardiac tissue.

Methods and Results: Multiscale optical potential mapping using 10×, 20×, and 40× magnifying objectives, achieving resolutions of 0.13, 0.065, and 0.033 mm, was performed when applying uniform shocks (±10 V/cm, 5 ms) during diastole and action potential plateau. A procedure was developed to identify local potential deviations as depolarizing or hyperpolarizing peaks and to quantify their occurrence and characteristic amplitudes, lateral extents, and dynamics. At shock onset, peaks of either polarity developed significantly faster (τ= 0.92 ± 0.65 ms, N = 64) than the average bulk polarization (τ= 2.25 ± 0.96 ms, P < 0.001) and appeared locally fixed, changing their polarity at shock reversal. The mean peak magnitude (21.2 ± 12 mV) and the amplitude distribution were essentially independent from the magnification. The peak density continuously increased with decreasing peak extent (taken at 70% of the amplitude), reaching a maximum of ∼3 peaks/mm2 in the range of ∼30–65 μm. There was no correlation between peak amplitude and size throughout. Potentially exciting peaks were found with a density of 0.04–0.2 peaks/mm2 corresponding to estimated 1–5 peaks/mm3.

Conclusions: Our results suggest that microscopic inhomogeneities form a substantial substrate for far-field excitation in natural cardiac tissue. Here, we effectively bridged the gap between the extensively studied myocyte cultures and larger heart preparations.