An implantable medical device system operates in an atrial synchronized ventricular pacing mode and switches to an atrial-asynchronous pacing mode when pacing mode switching criteria are met. A control circuit of the system detects a cycle length change between two atrial cycle lengths determined from a cardiac signal that includes far-field atrial events. If the cycle length change is greater than a change, threshold the control circuit determines if the pacing mode switching criteria are satisfied subsequent to detecting the cycle length change.
