Pacemakers activate depolarizations with electrical impulses and these appear as pacing spikes on the ECG. In ventricular pacing, a pacing spike will be followed by a broad QRS complex (because the depolarization is not conducted via the normal fast conduction pathways.)
When the atria are being paced via an atrial lead the pacing spike will be followed by a P wave.
This may be normally conducted via the AV node and followed by a normal QRS.
In dual chamber pacing the P wave will be followed by a pacing spike from the ventricular lead and a broad QRS complex will follow.
Failure of a pacing spike to be followed by depolarization indicates a problem with capture and the pacemaker should be checked by the manufacture.