ABSTRACT
Ventricular fibrillation (VF) induced by an electrical stimulus in situ is normal hearts is initiated by a reentrant wave front of activation that is both functional and transient in nature. The characteristics of the induced functional reentry are compatible with the phenomenon of spiral wave of excitation, whose 2-D and 3-D (scroll-wave) dynamics and mechanisms have become the subject of intense recent experimental and simulation studies. The presence of an in-situ protective zone (i.e., termination of the induced in-situ scroll wave that heralds VF) by a single-point electrical stimulus strongly suggests the presence of only one or two counterrotating (‘‘figureeight’’) scroll waves in both ventricles at the onset of VF. The graded response hypothesis provides a cellular mechanism of a strong point electrical stimulus-induced reentry in the normal ventricular muscle. The induced reentry, with a relatively fast rotation period (around 100ms), has a transient lifespan. Within few cycles (1-3), the core of the reentrant wave front meanders (drifts), then breaks up into multiple wavelets, each of which then propagates with its own ‘‘independent’’ regime. The restitution
hypothesis provides an adequate explanation of the destabilization of the single scroll wave (meandering and breakup) that signals the transition from the tachysystolic Stage I VF (one or a pair of scroll waves) to the ‘‘convulsive incoordination,’’ Stage II VF (multiple independent wavelets). These two sequential dynamic stages of VF were elegantly described by Wiggers more than 60 years ago using electrocardiographic and cinematographic methods [1]. While stage I VF can be terminated by a timed, single-point electrical stimulus (‘‘protective zone’’), stage II VF cannot. For an electrical stimulus to terminate the Stage II VF, a critically large portion of both ventricles must to be engaged by the electrical shock. In this chapter we present recent experimental and simulation findings that provide insight into the dynamic scenarios of VF induced by an electrical stimulus. Termination of reentrant and nonreentrant wave fronts by an electrical stimulus (‘‘defibrillation’’) can also be adequately explained by the graded response (progressive depolarization) hypothesis of vulnerability to reentry. Not surprisingly, this hypothesis of the upper limit of vulnerability (ULV) in humans, which is characteristically identical to the defibrillation threshold (DFT).
