title: Quantification of shock-induced microscopic virtual electrodes assessed by subcellular resolution optical potential mapping in guinea pig papillary muscle.
abstract: BACKGROUND
  The primary objective of this study was the quantitative description of shock-induced, locally occurring virtual electrodes in natural cardiac tissue.
  
  
  RESULTS
  Multiscale optical potential mapping using 10x, 20x, and 40x 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 (tau = 0.92 +/- 0.65 ms, N = 64) than the average bulk polarization (tau = 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 approximately 3 peaks/mm2 in the range of approximately 30-65 microm. 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.
authors.affiliation: Institute for Biophysics, Center for Physiological Medicine, Medical University Graz, Graz, Austria. herbert.windisch@meduni-graz.at
authors.forename: Herbert
authors.initials: H
authors.lastname: Windisch
authors.forename: Dieter
authors.initials: D
authors.lastname: Platzer
authors.forename: Erek
authors.initials: E
authors.lastname: Bilgici
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D004558:Electric Stimulation / Q000379:methods; D006168:Guinea Pigs; D008839:Microelectrodes / Q000592:standards*; D055095:Optics and Photonics; D010210:Papillary Muscles / Q000166:cytology* / Q000502:physiology*