The role of electroporation in defibrillation

Artifacts, assumptions, and ambiguity: Pitfalls in comparing experimental results to numerical simulations when studying electrical stimulation of the heart

Insidious experimental artifacts and invalid theoretical assumptions complicate the comparison of numerical predictions and observed data. Such difficulties are particularly troublesome when studying electrical stimulation of the heart. During unipolar stimulation of cardiac tissue, the artifacts include nonlinearity of membrane dyes, optical signals blocked by the stimulating electrode, averaging of optical signals with depth, lateral averaging of optical signals, limitations of the current source, and the use of excitation-contraction uncouplers. The assumptions involve electroporation, membrane models, electrode size, the perfusing bath, incorrect model parameters, the applicability of a continuum model, and tissue damage. Comparisons of theory and experiment during far-field stimulation are limited by many of these same factors, plus artifacts from plunge and epicardial recording electrodes and assumptions about the fiber angle at an insulating boundary. These pitfalls must be overcome in order to understand quantitatively how the heart responds to an electrical stimulus. (c) 2002 American Institute of Physics.

Mechanisms of shock-induced arrhythmogenesis during acute global ischemia

Little is known about the mechanisms of vulnerability and defibrillation under ischemic conditions. We investigated these mechanisms in 18 Langendorff-perfused rabbit hearts during 75% reduced-flow ischemia. Electrical activity was optically mapped from the anterior epicardium during right ventricular shocks applied at various phases of the cardiac cycle while the excitation-contraction decoupler 2,3-butanedione monoxime (BDM; 15 mM) was used to suppress motion artifacts caused by contraction of the heart. During ischemia, vulnerable window width increased [from 30-90% of the action potential duration (APD) in the control to -10 to 100% of the APD in ischemia]. Moreover, arrhythmia severity increased along with the reduction of APD (176 +/- 9 ms in control and 129 +/- 26 ms in ischemia, P < 0.01) and increased dispersion of repolarization (45 +/- 17 ms in control and 73 +/- 28 ms in ischemia, P < 0.01). Shock-induced virtual electrode polarization was preserved. Depolarizing (contrary to hyperpolarizing) response time constants increased. Virtual electrode-induced wavefronts of excitation had much more tortuous pathways leading to wavefront fractionation. Defibrillation failure at all shock strengths was observed in four hearts. Optical mapping revealed that the shock extinguished the arrhythmia; however, the arrhythmia self-originated after an isoelectric window of 339 +/- 189 ms. In conclusion, in most cases, virtual electrode-induced phase singularity (VEIPS) was responsible for shock-induced arrhythmogenesis during acute global ischemia. Enhancement of arrhythmogenesis was associated with an increased dispersion of repolarization and altered deexcitation. In four hearts, arrhythmogenesis could not be explained by VEIPS.

A dynamic action potential model analysis of shock-induced aftereffects in ventricular muscle by reversible breakdown of cell membrane

To elucidate the subcellular mechanism underlying the aftereffects of high-intensity dc shocks, a small pore, which mimics reversible breakdown of the cell membrane (electroporation), was incorporated into the phase-2 Luo-Rudy (L-R) model of ventricular action potentials. The pore size was set to occupy 0.15%-4.25% of the total cell membrane during the 10-ms shock. The pore was assumed to decrease after the shock exponentially with a time constant of 100-1,400 ms to simulate resealing process. In normal myocytes, the pore formation results in a delay of repolarization of the shocked action potential, which is followed by prolonged depolarization and oscillation of membrane potential like early afterdepolarization (EAD). Time- and voltage-dependent changes in the delayed rectifier K+ currents (IKr, IKs) in combination with those of L-type Ca2+ current (ICa,(L)) and ion flux through the pore (I(pore)) are responsible for the potential changes. Spontaneous excitation from the oscillation depends on activation of ICa,(L). In myocytes overloaded with Na+ and Ca2+ secondary to 90% inhibition of Na+-K+ pump, the pore formation results in a delay of repolarization of the shocked action potential, which is followed by slower cyclic depolarization in response to spontaneous release of Ca2+ from the sarcoplasmic reticulum (SR). This delayed afterdepolarization-type oscillation is abolished by complete block of Ca2+ release from the SR. These findings suggest that high-intensity electric field application will cause arrhythmogenic responses through a transient rupture of sarcolemma with different subcellular events in ventricular cells under normal and pathological conditions.