Prediction of tachyarrhythmia episodes

Ventricular tachyarrhythmia, in particular ventricular fibrillation, may be partially enabled by an imbalance within the autonomic nervous system. Decreased parasympathetic (or vagal) cardiac regulatory activity may be responsible for the development of conditions that allow ventricular fibrillation to develop and sustain itself. An analysis of R-R intervals immediately prior to episodes of ventricular fibrillation has yielded a modulation pattern of the R-R intervals, which was termed "vagal fatigue," that appears in a majority of ventricular fibrillation episodes detected and treated by an implantable defibrillator. When absent, this modulation pattern is typically associated with an aborted defibrillation shock, indicating that parasympathetic activity is sufficiently strong that it was able to overcome the tachyarrhythmia prior to therapy delivery. The tachyarrhythmia detector compares the instantaneous R-R interval with a target R-R. The resulting error signal was used to set the "vagal fatigue" indicator.