An implantable medical device system, comprising: a pacemaker implantable in an atrial chamber of a heart of a patient, comprising: a sensing module configured to sense near field atrial events from a cardiac signal received by the sensing module and configured to sense far field ventricular events, a pulse generator configured to generate and deliver atrial pacing pulses via a pair of electrodes, and a control module coupled to the sensing module and the pulse generator and configured to: establish an atrial lower rate interval to control a rate of delivery of the atrial pacing pulses; characterised by the control module being further configured to: determine a rate of the far field ventricular events sensed by the sensing module by determining ventricular-ventricular intervals between the sensed far field ventricular events, determine an atrial event rate by determining atrial-atrial intervals, 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. a pacemaker implantable in an atrial chamber of a heart of a patient, comprising: a sensing module configured to sense near field atrial events from a cardiac signal received by the sensing module and configured to sense far field ventricular events, a pulse generator configured to generate and deliver atrial pacing pulses via a pair of electrodes, and a sensing module configured to sense near field atrial events from a cardiac signal received by the sensing module and configured to sense far field ventricular events, a pulse generator configured to generate and deliver atrial pacing pulses via a pair of electrodes, and a control module coupled to the sensing module and the pulse generator and configured to: establish an atrial lower rate interval to control a rate of delivery of the atrial pacing pulses; characterised by the control module being further configured to: determine a rate of the far field ventricular events sensed by the sensing module by determining ventricular-ventricular intervals between the sensed far field ventricular events, determine an atrial event rate by determining atrial-atrial intervals, 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. establish an atrial lower rate interval to control a rate of delivery of the atrial pacing pulses; characterised by the control module being further configured to: determine a rate of the far field ventricular events sensed by the sensing module by determining ventricular-ventricular intervals between the sensed far field ventricular events, determine an atrial event rate by determining atrial-atrial intervals, 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. determine a rate of the far field ventricular events sensed by the sensing module by determining ventricular-ventricular intervals between the sensed far field ventricular events, determine an atrial event rate by determining atrial-atrial intervals, 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.