Approaches for determining threshold values for one or more arrhythmia rate zones and/or the number of rate zones are described. A probability function for heart rate is determined using collected and measured heart rate values. One or more heart rate probability values are selected. Thresholds for arrhythmia rate zones are determined from the probability function based on the selected probability values. Determining the rate zone thresholds may involve determining a threshold for a lower rate limit and/or determining one or more tachyarrhythmia rate zone thresholds. The number of rate zones may also be determined based on the probability function.