A medical device system including a pacemaker implantable in an atrial chamber of a patient's heart is configured to sense near field atrial events from a cardiac signal received by a sensing module of the pacemaker and to sense far field ventricular events. The pacemaker is configured to establish an atrial lower rate interval to control a rate of delivery of atrial pacing pulses, determine a rate of the far field ventricular events sensed by the sensing module, determine an atrial event rate, compare the rate of the sensed far field ventricular events to the atrial event rate, and adjust the atrial lower rate interval in response to the comparison.