1. A method of operating a medical device system, the method comprising: receiving heart rate signals from a subject using a sensor coupled to the subject so as to detect the heart rate signals from the subject; processing the heart rate signals received by the sensor using a programmed computer processing unit to detect at least one pre-onset characteristic of an epileptic event and determine a likelihood of the epileptic event based at least in part on the detected at least one pre-onset characteristic, wherein said processing the heart rate signals comprises performing a high-power processing of a first set of the heart rate signals received by the sensor; and when the detected at least one pre-onset characteristic subsequently indicates a low likelihood of the epileptic event, performing a low-power processing of a second set of the heart rate signals received by the sensor subsequent to the first set of the heart rate signals.