A medical device includes a housing configured for implantation within a body of a patient. Detection circuitry is disposed in the housing and coupled to an electrode arrangement. The detection circuitry is configured to sense a cardiac electrical signal from the patient. A processor is coupled to the detection circuitry and configured to compute a first measure of heart rate variability (HRV) using the cardiac electrical signal, and compute a second measure of HRV using the cardiac electrical signal, the second measure of HRV differing from the first measure of HRV. The processor is also configured to produce an index of patient status derived from a ratio of the first and second measures of HRV, such that the index is a normalized HRV metric personalized to the patient. The processor or a remote system can use the index to assess acute and chronic changes in patient status.
