Implantable systems, and methods for use therein, perform at least one of a cardiac assessment and an autonomic assessment. Short-term fluctuations in PR intervals, that follow the premature contractions in the ventricles, are monitored. At least one of a cardiac assessment and an autonomic assessment is performed based on the monitored fluctuations in PR intervals that follow the premature contractions in the ventricles. This can include assessing a patient's risk of sudden cardiac death (SCD), assessing a patient's autonomic tone and/or detecting myocardial ischemic events based on the monitored fluctuations in PR intervals that follow the premature contractions in the ventricles.