Mechanism of Electrical Defibrillation: Current Status and Future Perspective

Ventricular fibrillation (VF) is currently a major cause of sudden cardiac death (SCD). To cure VF, electrical defibrillation is the only therapy. However, strong energy is required. Thus, to reduce the energy or develop a new method is desired. The mechanism of how the electric shock sweeps VF is still controversial. In this article, we summarize evidence and remaining problems of this topic. There are three issues in time sequence of VF: how to initiate, how to continue, and how to terminate. Many investigations to achieve VF-free heart have been reported, but there are currently no definite methods to prevent VF. Thus, to terminate VF is one of the big challenges to prevent SCD. There are two strategies to improve electrical defibrillation: elucidate the substantial mechanism and reduce the energy. (1) Substantial mechanism proposed: In a failed defibrillation episode, at the energy level of the near defibrillation threshold, the initial activation site is related to the repolarization phase of the location. However, it is still not clear whether it is part of the continuous VF activity or initiation of re-VF. It is well known that strong field electric shock (including cathodal and anodal stimuli) has many effects on the cardiac tissue, such as electroporation, virtual electrode effects, and electrophysiological responses, which are influenced by tissue geometry (including fiber orientation and bifurcation of tissues). These phenomena should modify the defibrillation effect. Finally, the characteristics of dynamic spiral wave (SW; the sources of continuity of re-entries) influence the continuity of VF. (2) Efforts to reduce the defibrillation energy: To reduce the defibrillation energy, biphasic pulse, regional cooling, modified stimuli programs, and automated local stimuli to SW are proposed. The superiority of biphasic pulse to monophasic pulse was established in the late 20th century; however, the mechanism is still not well understood. Cooling of some region of the heart ventricles widens the route of SW trajectory and terminates SW. Programming high frequency stimulus or double stimuli according to computer simulation of the heart model could reduce the defibrillation threshold. Automated local stimulus to the site between the tail of SW activation and the next activation front could terminate the SW.