Basic rhythm in this ECG is a paced rhythm, with pacing spikes (pacemaker artefacts) seen immediately before each QRS complex. The QRS complexes have a left bundle branch block pattern with a superior axis, indicating right ventricular apical pacing. An ectopic beat with right bundle branch block (RBBB), left anterior hemiblock morphology has occurred in between, causing the pacemaker to be inhibited, indicating that it is a demand pacemaker working in VVI mode. In VVI mode, the chamber paced and sensed is the ventricle and the response to a sensed beat is inhibition of pacing. After the regular pacing cycle, the next spike (S) occurs, but the QRS has a morphology intermediate between the spontaneous ectopic beat and the paced beat, with a slightly narrower QRS than the paced beat. This is a fusion beat between a paced beat and a spontaneous ectopic beat which has occurred just after the onset of the pacing spike. The ectopic beat could either have a left ventricular origin due to the RBBB morphology or a supraventricular origin with aberrant conduction.