Methods, systems, and apparatus for detecting a seizure in a patient. The determination is performed by collecting cardiac data； determining valid heart beats suitable for seizure detection from the cardiac data； calculating heart rate data of interest from the valid heart beats； and identifying a seizure event from the heart rate data. The medical device may then take a responsive action, such as warning, logging the time of the seizure, computing and storing one or more seizure severity indices, and/or treating the seizure. Also, methods for quantifying the quality of a fiducial time marker for a candidate heart beat, quantifying the quality of a candidate heart beat, or determining a time of beat sequence of the patient's heart. Also, methods of modifying constraints use in one or more beat validity tests.
