Techniques are provided for use with a pacemaker or other implantable medical device for detecting arterial blood pressure. Briefly, the pacemaker detects aortic electrical resistance using sensing/pacing leads. Aortic electrical resistance pertains to the resistance to an electrical current passing through the aorta. The pacemaker then determines the arterial blood pressure of the patient based on the aortic resistance and a predetermined calibration value that relates aortic resistance to arterial pressure. The calibration value is updated monthly based on blood pressure values detected using an external blood pressure sensor employing a blood pressure cuff. Other techniques described herein pertain to the determination of other physiologic parameters such as stroke volume and cardiac output and to the detection of changes in hematocrit. Any of the various physiological parameters may then be used to trigger or control warning signals and responsive therapy.