An intracardiac ventricular pacemaker, comprising: a pulse generator configured to generate and deliver ventricular pacing pulses to a ventricle of a patient's heart via electrodes coupled to the pacemaker; a motion sensor configured to produce a motion signal; a control circuit coupled to the motion sensor and the pulse generator; a housing implantable within a heart chamber and enclosing the pulse generator, the motion sensor and the control circuit; wherein the control circuit is configured to: detect atrial systolic events from the motion signal; operate in a selected one of an atrial tracking ventricular pacing mode and a non-atrial tracking ventricular pacing mode, wherein operating in the atrial-tracking ventricular pacing mode comprises setting an atrioventricular pacing interval in response to detecting an atrial systolic event from the motion signal and controlling the pulse generator to deliver a ventricular pacing pulse in response to the atrioventricular pacing interval expiring; while operating in the selected one of the atrial-tracking ventricular pacing mode and the non-atrial tracking ventricular pacing mode, determine from the motion signal at least one motion signal metric; compare the at least one motion signal metric to pacing mode switching criteria; and responsive to the pacing mode switching criteria being satisfied, switch from the selected one of the non-atrial tracking pacing mode and the atrial tracking pacing mode to the other one of the non-atrial tracking pacing mode and the atrial tracking pacing mode for controlling the pacing pulses delivered by the pulse generator; and a cardiac electrical signal sensing circuit enclosed by the housing and configured to sense intrinsic ventricular electrical events; wherein the control circuit is further configured to determine a ventricular event rate from a plurality of at least one of the sensed intrinsic ventricular events and the delivered ventricular pacing pulses; and establish a first switching threshold of the pacing mode switching criteria based on at least the ventricular event rate. a pulse generator configured to generate and deliver ventricular pacing pulses to a ventricle of a patient's heart via electrodes coupled to the pacemaker; a motion sensor configured to produce a motion signal; a control circuit coupled to the motion sensor and the pulse generator; a housing implantable within a heart chamber and enclosing the pulse generator, the motion sensor and the control circuit; wherein the control circuit is configured to: detect atrial systolic events from the motion signal; operate in a selected one of an atrial tracking ventricular pacing mode and a non-atrial tracking ventricular pacing mode, wherein operating in the atrial-tracking ventricular pacing mode comprises setting an atrioventricular pacing interval in response to detecting an atrial systolic event from the motion signal and controlling the pulse generator to deliver a ventricular pacing pulse in response to the atrioventricular pacing interval expiring; while operating in the selected one of the atrial-tracking ventricular pacing mode and the non-atrial tracking ventricular pacing mode, determine from the motion signal at least one motion signal metric; compare the at least one motion signal metric to pacing mode switching criteria; and responsive to the pacing mode switching criteria being satisfied, switch from the selected one of the non-atrial tracking pacing mode and the atrial tracking pacing mode to the other one of the non-atrial tracking pacing mode and the atrial tracking pacing mode for controlling the pacing pulses delivered by the pulse generator; and detect atrial systolic events from the motion signal; operate in a selected one of an atrial tracking ventricular pacing mode and a non-atrial tracking ventricular pacing mode, wherein operating in the atrial-tracking ventricular pacing mode comprises setting an atrioventricular pacing interval in response to detecting an atrial systolic event from the motion signal and controlling the pulse generator to deliver a ventricular pacing pulse in response to the atrioventricular pacing interval expiring; while operating in the selected one of the atrial-tracking ventricular pacing mode and the non-atrial tracking ventricular pacing mode, determine from the motion signal at least one motion signal metric; compare the at least one motion signal metric to pacing mode switching criteria; and responsive to the pacing mode switching criteria being satisfied, switch from the selected one of the non-atrial tracking pacing mode and the atrial tracking pacing mode to the other one of the non-atrial tracking pacing mode and the atrial tracking pacing mode for controlling the pacing pulses delivered by the pulse generator; and a cardiac electrical signal sensing circuit enclosed by the housing and configured to sense intrinsic ventricular electrical events; wherein the control circuit is further configured to determine a ventricular event rate from a plurality of at least one of the sensed intrinsic ventricular events and the delivered ventricular pacing pulses; and establish a first switching threshold of the pacing mode switching criteria based on at least the ventricular event rate. determine a ventricular event rate from a plurality of at least one of the sensed intrinsic ventricular events and the delivered ventricular pacing pulses; and establish a first switching threshold of the pacing mode switching criteria based on at least the ventricular event rate.