Implantable systems, and methods for use therein, perform at least one of a cardiac assessment and an autonomic assessment. Premature atrial contractions (PACs) are induced to thereby cause corresponding premature contractions in the ventricles. Short-term fluctuations in cardiac intervals, that follow the premature contractions in the ventricles caused by the induced PACs, are monitored. At least one of a cardiac assessment and an autonomic assessment is performed based on the monitored fluctuations in cardiac intervals that follow the premature contractions in the ventricles caused by the induced PACs. 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 cardiac intervals that follow the premature contractions in the ventricles caused by the induced PACs.