An implantable medical device (IMD), such as an implantable pacemaker, cardioverter, or diagnostic device, generates an EGM signal, e.g., a far field EGM signal, samples the EGM signal to obtain a single T-wave amplitude value for each T-wave over a plurality of beats, and stores the T-wave amplitude values in memory. The IMD creates a time series of the T-wave amplitude values stored in memory, calculates the power spectral density for the times series, and selects a power spectral density of a particular frequency, e.g., 0.5 cycles per beat, as the TWA value. The IMD may periodically determine TWA values for the patient and store the values in memory. The TWA values may be presented to medical personnel, e.g., as a trend. The IMD may deliver or modify therapy, or provide an alert, based on the TWA values.