Virtual Electrode–Induced Reexcitation

Abstract

Abstract-Mechanisms of defibrillation remain poorly understood. Defibrillation success depends on the elimination of fibrillation without shock-induced arrhythmogenesis. We optically mapped selected epicardial regions of rabbit hearts (nϭ20) during shocks applied with the use of implantable defibrillator electrodes during the refractory period. Monophasic shocks resulted in virtual electrode polarization (VEP). Positive values of VEP resulted in a prolongation of the action potential duration, whereas negative polarization shortened the action potential duration, resulting in partial or complete recovery of the excitability. After a shock, new propagated wavefronts emerged at the boundary between the 2 regions and reexcited negatively polarized regions. Conduction velocity and maximum action potential upstroke rate of rise dV/dt max of shock-induced activation depended on the transmembrane potential at the end of the shock. Linear regression analysis showed that dV/dt max of postshock activation reached 50% of that of normal action potential at a V m value of Ϫ56.7Ϯ0.6 mV postshock voltage (nϭ9257). Less negative potentials resulted in slow conduction and blocks, whereas more negative potentials resulted in faster conduction. Although wavebreaks were produced in either condition, they degenerated into arrhythmias only when conduction was slow. Shock-induced VEP is essential in extinguishing fibrillation but can reinduce arrhythmias by producing excitable gaps. Reexcitation of these gaps through progressive increase in shock strength may provide the basis for the lower and upper limits of vulnerability. The former may correspond to the origination of slow wavefronts of reexcitation and phase singularities. The latter corresponds to fast conduction during which wavebreaks no longer produce sustained arrhythmias. (Circ Res. 1999;85:1056-1066.)

Acute His Bundle Injury during Permanent His Bundle Pacing: Mechanistic Insights into Intra-Hisian Block

“…The ability to correct HV block in both these scenarios suggests that the intra-Hisian lesion causing HV block is fairly discrete, and virtual electrode polarization created during pacing is able to capture the His beyond the site of block. 22,23 In the series of HBP in infranodal AV block, we reported two patients with split His potentials. A similar clinical scenario was noted during the acute HV block induced during HBP in two patients where split His potentials developed following lead fixation ( Figure 4).…”

Modeling Defibrillation

“…The virtual cathode serves as an electrical stimulus eliciting a regenerative depolarization and a propagating wave in the newly created excitable area. Whether or not break excitations arise depends on whether the transmembrane potential gradient across the border spans the threshold for regenerative depolarization (Cheng, Mowrey, Van Wagoner, Tchou & Efimov, 1999). The finding of break excitations, combined with the fact that positive VEP can result in "make" excitations (where "make" refers to the onset of a shock) in regions where tissue is at or near diastole, resulted in a novel understanding of how a strong stimulus can trigger the development of new activations.…”

“…All factors depend, ultimately, on shock strength. Increasing shock strength results in higher voltage gradients across borders between regions of opposite polarity, leading to more break-excitations (Cheng, Mowrey, Van Wagoner, Tchou & Efimov, 1999) which then start to traverse the post-shock excitable gap earlier (Skouibine et al, 2000a) and at a faster velocity (Cheng, Mowrey, Van Wagoner, Tchou & Efimov, 1999), as well as extending refractoriness to a larger degree (Knisley et al, 1994).…”

Defibrillation Shock Amplitude, Location and Timing

“…Experimental studies demonstrated that shock might induce virtual electrodes (Kinsley et al, 1994;Wikswo JP et al, 1995;Fast et al, 2002;Sharifov et al, 2004). The picture was completed with the computer simulation methods (Efimov et al, 1997;Efimov et al 1998;Cheng et al, 1999;Efimov et al, 2000a;Efimov 2000b;Zemlin et al, 2006). Virtual electrode can create singularity points by closing an electrical circle through electrically conducting tissue.…”