Recent findings about the mechanism of defibrillation

Summary form only given. Before the development of electrical and optical cardiac mapping techniques, it was thought that shocks near the defibrillation threshold in strength failed because they did not alter the transmembrane potential sufficiently throughout the myocardium to halt all of the activation fronts present during fibrillation. While this concept is true for extremely weak shocks, both electrical and optical mapping studies have indicated that shocks that are near the defibrillation threshold in strength halt all fibrillatory activation fronts (or, according to some investigators definition, reset them to another location) yet, the shock fails because the electric field created by the shock interacts with the myocardium to induce new activation fronts that cause fibrillation quickly to resume. However, the nature of this interaction and the mechanism by which it gives rise to new activation fronts is controversial. Experimental evidence suggests three different mechanisms by which the shock can reinduce fibrillation. (1) The shock electric field interacts with relatively refractory myocardium to form a "critical point" about which some tissue is directly excited while other tissue has its refractory period prolonged, leading to unidirectional block and reentry about the critical point. (2) The shock electric field creates "virtual electrodes" in which virtual cathodes depolarize tissue and adjacent virtual anodes hyperpolarize tissue. This hyperpolarized tissue is deexcited, allowing unidirectional propagation into it, causing reentry around another type of critical point formed by adjacent regions of depolarization and hyperpolarization. (3) The shock causes delayed after depolarizations that do not occur until tens of milliseconds after the shock that lead to rapid, repetitive firing which reinitiates fibrillation. While experimental results indicate that all three of these mechanisms can occur, the relative importance of each of them in the success or failure of defibrillation has not yet been determined.