In a device and method in a dual chamber pacing system operating in an atrial synchronized mode, the cardiac stimulator is connectable to a lead arrangement arranged for sensing atrial electrical and mechanical activity. Upon detection of an atrial arrhythmia based on either of sensed atrial mechanical activity, atrial electrical activity, or a combination thereof, a mode switch from an atrial synchronized ventricle stimulating mode to a non-atrial synchronized mode is triggered.
