1. A non-transitory computer readable storage device storing instructions that, when executed by a processor, cause the processor to perform operations comprising: receiving data related to a beat sequence of a patient's heart; dynamically recalculating an XX value used to determine at least one NNXX value, wherein the at least one NNXX value corresponds to a number of adjacent normal R-R intervals in a sample that differ by more than XX milliseconds; determining the at least one NNXX value based on the beat sequence of the patient's heart; performing a first comparison of the at least one NNXX value to an NNXX threshold; and providing an output indicative of an unstable brain state, a brain state indicative of an elevated probability of a seizure event, a brain state indicative of an impending seizure, an aura, a seizure, or a combination thereof, wherein the output is determined based on the first comparison.