Intramural Virtual Electrodes During Defibrillation Shocks in Left Ventricular Wall Assessed by Optical Mapping of Membrane Potential

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Abstract
Background— It is believed that defibrillation is due to shock-induced changes of transmembrane potential (ΔV m ) in the bulk of ventricular myocardium (so-called virtual electrodes), but experimental proof of this hypothesis is absent. Here, intramural shock-induced ΔV m were measured for the first time in isolated preparations of left ventricle (LV) by an optical mapping technique. Methods and Results— LV preparations were excised from porcine hearts (n=9) and perfused through a coronary artery. Rectangular shocks (duration 10 ms, field strength E ≈2 to 50 V/cm) were applied across the wall during the action potential plateau by 2 large electrodes. Shock-induced ΔV m were measured on the transmural wall surface with a 16×16 photodiode array (resolution 1.2 mm/diode). Whereas weak shocks (E≈2 V/cm) induced negligible ΔV m in the wall middle, stronger shocks produced intramural ΔV m of 2 types. (1) Shocks with E>4 V/cm produced both positive and negative intramural ΔV m that changed their sign on changing shock polarity, possibly reflecting large-scale nonuniformities in the tissue structure; the ΔV m patterns were asymmetrical, with ΔV − m >ΔV + m . (2) Shocks with E>34 V/cm produced predominantly negative ΔV m across the whole transmural surface, independent of the shock polarity. These relatively uniform polarizations could be a result of microscopic discontinuities in tissue structure. Conclusions— Strong defibrillation shocks induce ΔV m in the intramural layers of LV. During action potential plateau, intramural ΔV m are typically asymmetrical (ΔV − m >ΔV + m ) and become globally negative during very strong shocks.

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