Tachyarrhythmia thresholds in a three-dimensional computer model of a heart

A three-dimensional computer model of the ventricles of a heart is used to study induction of fibrillation under both healthy and ischemic conditions. In these simulations, fibrillation was initiated by administering multiple premature stimuli to an ectopic site on the heart, and ischemia was modeled as a regional reduction in conduction velocities. For the healthy heart, a well-defined 17-msec `window of vulnerability´ was located completely within the ectopic site´s relative refractory period. This cardiac model indicates that a localized reduction in conduction velocities influences the heart´s vulnerability to ectopic stimuli through two separate mechanisms: (a) it decreases the thresholds required to achieve fibrillation, and (b) it increases the fraction of the cardiac cycle susceptible to these stimuli, thereby increasing the probability that randomly timed stimuli will initiate fibrillation