This disclosure describes techniques for generating a risk stratification indicator based on HRT measurements computed using physiological parameters sensed by an implantable medical device (IMD). In some examples, the HRT measurements may be computed by the IMD based on the physiological parameters. In other examples, the IMD may sense the physiological parameters, and transmit data representative of the parameters to an external computing device, such as an IMD programmer, which then computes the HRT measurements. Exemplary physiological parameters include cardiac signals.