An apparatus for measuring a patient's heart rate response to respiration, for monitoring the patient's autonomic tone, includes an implantable system. The implantable system includes a sensor for generating sensed signals representing detected beat-to-beat heart rate. Further, the implantable system includes a sensor for generating sensed signals representing detected respiration. The implantable system also includes an element for estimating the sensitivity of heart rate to respiration, responsive to the sensed heart rate and respiration signals.