During VF, as fibrillatory wavefronts propagate across the surface of a tissue, there are regions of tissue that can still be excited by external stimulation; these regions are known as excitable gaps.23'24 The concept of pacing during VF is premised on the use of low-energy pulses to capture the fibrillatory tissue, preferably during the excitable gaps,24-26 and enlargement of the captured region may eventually lead to VF termination. Defib-rillation studies have led to many attempts to design low-energy defibrillation or pacing strategies.23'27-29 These strategies may be categorized as either passive or interactive. The passive paradigm, such as overdrive pacing and antitachycardia pacing, delivers a constant frequency pulse train seeking to capture the rhythm and gain control. Overdrive pacing has been shown to be effective in capturing a small region of the heart;23'28 however, the limited success of this approach to VF termination may be attributable to the instability of VF frequency. Antitachycardia pacing is effective in terminating slow ventricular tachycardia (VT), but it is not effective in terminating faster VT or VF. On the other hand, the interactive paradigm, such as chaos control,20'30'31 seeks to deliver energy based on realtime feedback control. The stimuli are delivered irregularly based on the nonlinear dynamics of the heart. Application of nonlinear control has allowed termination of pacing-induced alternans32 and conversion of VF to a different "state" of arrhythmia,20 but spontaneous termination of VF has not been demonstrated.
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