An implantable medical device, IMD, comprises atrial and ventricular sensing units for sensing atrial or ventricular electric events. The IMD also comprises atrial and ventricular pulse generators for generating atrial or ventricular pacing pulses. The ventricular sensing unit is connectable to a multi-electrode lead to individually sense electric events in a ventricle using multiple electrode pairs implanted at different ventricular sites. A controller blanks the ventricular sensing unit during a blanking period following delivery of an atrial stimulating pulse by the atrial pulse generator and activates the ventricular sensing unit at the expiry of the blanking period. Due to the lower propagation speed of PVC depolarization waves and the multi-site sensing, a PVC depolarization wave initiated at a ventricular site during the blanking period can be detected by the IMD.