Published Online
on July 22, 2002

A more recent version of this article appeared on August 20, 2002

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Submitted on February 12, 2002
Revised on May 22, 2002
Accepted on May 23, 2002

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

Vladimir G. Fast PhD^*, Oleg F. Sharifov PhD, Eric R. Cheek MS, Jonathan C. Newton PhD, and Raymond E. Ideker MD, PhD

From the Department of Biomedical Engineering (V.G.F., O.F.S., E.R.C., R.E.I.) and Department of Medicine (J.C.N., R.E.I.), University of Alabama at Birmingham, Birmingham, Ala.

^* To whom correspondence should be addressed. E-mail: fast{at}crml.uab.edu.

Background—It is believed that defibrillation is due to shock-induced changes of transmembrane potential (V_m) in the bulk of ventricular myocardium (so-called virtual electrodes), but experimental proof of this hypothesis is absent. Here, intramural shock-induced V_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 V_m were measured on the transmural wall surface with a 16x16 photodiode array (resolution 1.2 mm/diode). Whereas weak shocks (E2 V/cm) induced negligible V_m in the wall middle, stronger shocks produced intramural V_m of 2 types. (1) Shocks with E>4 V/cm produced both positive and negative intramural V_m that changed their sign on changing shock polarity, possibly reflecting large-scale nonuniformities in the tissue structure; the V_m patterns were asymmetrical, with V^-_m>V⁺_m. (2) Shocks with E>34 V/cm produced predominantly negative V_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 V_m in the intramural layers of LV. During action potential plateau, intramural V_m are typically asymmetrical (V^-_m>V⁺_m) and become globally negative during very strong shocks.

Key words: arrhythmia • defibrillation • excitation • mapping

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