An implantable medical device system includes an extracardiac sensing device and an intracardiac pacemaker. The sensing device senses a P-wave attendant to an atrial depolarization of the heart via housing-based electrodes carried by the sensing device when the sensing device is implanted outside the cardiovascular system and sends a trigger signal to the intracardiac pacemaker in response to sensing the P-wave. The intracardiac pacemaker detects the trigger signal and schedules a ventricular pacing pulse in response to the detected trigger signal.
