Fibrillation and defibrillation: the odd couple?

Shock-Induced Epicardial and Endocardial Virtual Electrodes Leading to Ventricular Fibrillation via Reentry, Graded Responses, and Transmural Activation

INTRODUCTION

The mechanism of ventricular fibrillation (VF) induction by T wave shocks has been attributed to reentry, propagated graded responses (PGR), and triggered activity. The limitation of recording transmembrane potential (V(m)) from only a single surface has hampered efforts to elucidate the relative role of these phenomena and their relationship to shock-induced virtual electrodes.

METHODS AND RESULTS

V(m) patterns from epicardial and endocardial surfaces of isolated sheep right ventricles were recorded with two CCD cameras for monophasic (M) and biphasic (B) shocks delivered at various coupling intervals (CI) from a unipolar mesh electrode on the epicardium. VF was induced via (1) the formation of reentry following make or break excitation; (2) propagated graded responses during apparent isoelectric window; and (3) breakthrough activation patterns coincident with endocardial-to-epicardial gradients in V(m). M shocks depolarized both surfaces at long CIs and polarized epicardial and endocardial surfaces oppositely at short CIs. At intermediate CIs, postshock V(m) patterns could lead to reentry on one surface or endocardial-to-epicardial gradients resulting in breakthrough. B induced VF less than M for short and intermediate CIs due to more homogeneous end-shock V(m) patterns. However, at long CIs these homogeneous patterns resulted in more VF induction because B left the tissue closer to the V(m) threshold for propagation.

CONCLUSION

Postshock activity occurred either immediately via epicardial or endocardial reentry, or after a delay caused by transmural propagation or propagated graded responses. These findings could explain the isoelectric window and focal activation patterns observed on the epicardium following VF induction shocks.

Effect of sotalol and acute ventricular dilatation on action potential duration and dispersion of repolarization after defibrillation shocks

Ventricular dilatation shortens action potential duration and increases the defibrillation threshold, whereas sotalol prolongs action potential duration and may decrease the defibrillation threshold. Whether these action potential changes remain after defibrillation shocks, and how they relate to defibrillation success, is not known. In this study, eight monophasic action potentials were recorded simultaneously during electrical defibrillation (shock strength: 20%-200% of the defibrillation threshold) in 16 normal and acutely dilated isolated rabbit hearts at baseline and after addition of sotalol (2 x 10-5 M). Post-shock action potential duration (PS-APD) and dispersion of PS-APD [Disp(PS-APD)] of monophasic action potentials were analyzed after 322 defibrillation shocks at different repolarization levels and related to defibrillation success. Acute ventricular dilatation shortened PS-APD, whereas sotalol prolonged PS-APD. Successful defibrillation was associated with lower Disp(PS-APD) at all repolarization levels in the normal and dilated heart at baseline and with sotalol (mean difference: 33%-46%, all P < 0.005). Minimal PS-APD was longer (mean difference: 5%-11%), while maximal PS-APD was shorter (mean difference: 2%-16%) after successful defibrillation shocks than after failing defibrillation shocks. Therefore, sotalol prolongs action potential duration after defibrillation shocks. Synchronization of repolarization, caused by both prolongation of short PS-APD and shortening of long PS-APD, is associated with successful defibrillation in the normal, acutely dilated, and sotalol-treated heart.

New concepts in transthoracic defibrillation

The transition of biphasic waveforms from ICDs to external defibrillators constitutes a significant technological advances for transthoracic defibrillation. Impedance compensation has enabled the delivery of defibrillating current adapted to each patient and each shock in the same patient. Optimally designed biphasic waveforms have been shown clinically to have greater efficacy in the termination of VF when compared with monophasic waveforms, and because peak current delivery is less, these waveforms are likely to be less injurious to myocardial function. Advances in the understanding of the mechanisms of fibrillation and defibrillation have identified the electrophysiologic events that initiate and sustain VF and the effects of defibrillation shocks on those events. Definition of the role of VEP and postshock excitation has clarified the mechanisms by which shocks can either fail or succeed. The ability of the second phase of optimal biphasic waveform shocks to exploit recruited sodium channels in negatively polarized areas and thus induce rapid propagation of postshock excitation assures uniform depolarization and prevention of re-entry. This appears to be the major mechanism of greater efficacy of biphasic waveforms. It seems certain that continuing investigation of virtual electrodes will enhance our understanding of defibrillation and optimal waveforms. At the same time, much more needs to be known regarding translation of these experimental observations to mechanisms of defibrillation in human hearts with long-standing underlying structural heart disease, which often arises of multiple factors. This represents a major challenge in defibrillation research.