1. A system, comprising: a cardiac data collector adapted to collect cardiac data from a patient's heart, said cardiac data comprising a time series of fiducial time markers for candidate heartbeats; a first module adapted to identify valid beats from said candidate heartbeats by subjecting a plurality of candidate beats to at least one beat validity test, said at least one beat validity test comprising at least one beat interval test applied to a candidate beat interval derived from a candidate heartbeat and at least one preceding heartbeat; and accept as valid beats the candidate beats that pass said at least one beat validity test; a second module adapted to form a first window comprising a first valid beat and at least one prior beat, wherein said first window is selected from: a time window of from 1 to 10 seconds, bounded on the most recent end by said first valid beat; and a number of beats window comprising said first valid beat and a number of immediately preceding beats ranging from 2-15; test said first window with at least one window test, wherein said at least one window test comprises a dispersion test; and accept the beats in said window as suitable for seizure detection if said window has a dispersion less than a threshold level of dispersion; a third module adapted to, in response to said beats in said window being accepted by said second module as suitable for seizure detection, determine a foreground heart rate parameter comprising a statistical measure of central tendency of heart rate in said first window; form a second window larger than said first window; determine a background heart rate parameter comprising a statistical measure of central tendency of heart rate in said second window; determine a relative heart rate comprising at least one of a first ratio of said foreground to said background heart rate parameters and a second ratio of said background to said foreground heart rate parameters; compare said relative heart rate to at least one of a first seizure threshold value associated with said first ratio and a second seizure threshold value associated with said second ratio; and detect an epileptic seizure if at least one of said first ratio exceeds said first seizure threshold value, or said second ratio is below said second seizure threshold value; and a medical device adapted to take a responsive action in response to said third module detecting an epileptic seizure, wherein the responsive action is selected from providing a warning to the patient or a caregiver, notifying the patient or the caregiver, logging the time of occurrence of said epileptic seizure, storing one or more seizure severity indices relating to said epileptic seizure, or treating said epileptic seizure.