A leadless cardiac pacemaker includes a housing and two or more electrodes secured relative to the housing. A controller is configured to receive electrical signals from the two or more electrodes indicative of a heartbeat of the patient's heart and determine a first measure of cardiac load based at least in part on a delay between the Q or R feature and the T feature for the heartbeat. An accelerometer is configured to sense an activity level of the patient and the controller is configured to determine a second measure of cardiac load based at least in part on the activity level of the patient. A pacing rate is based at least in part upon the first measure of cardiac load, with the second measure of cardiac load used as a governor to the determined pacing rate.
