1. An implantable heart-monitoring device comprising: one or more leads for sensing electrical signals of a patient or for applying electrical energy to the patient; a therapy circuit for delivering electrical energy through one or more of the leads to the patient; and a monitoring circuit for monitoring heart activity of the patient through one or more of the leads, the monitoring circuit comprising: means for calculating a set of two or more intervals based on the electrical signals of the patient; means for determining a minimum interval based on the set of two or more intervals; means for calculating a range statistic based on a difference between a first subset and second subset of the intervals; means for calculating a dispersion index based on a subset of the intervals; means for calculating a number based on the minimum interval, the range statistic, the dispersion index; and means for comparing the number to a threshold to determine a therapy option.