Widening of the QRS complex during atrial pacing: what is the mechanism?

An electrophysiologic procedure was performed in a 50-year-old man with a 20-year history of paroxysmal tachycardia. During sinus rhythm, the atrial-His (AH) interval was 70 msec, the His-ventricular (HV) interval was 45 msec, and the QRS width was 80 msec. During atrial pacing at a cycle length of 290 msec, there was widening of the QRS complex (Fig. 1). The transition from a narrow to wide QRS complex is shown at a more rapid paper speed in Figure 2. Upon cessation of pacing, sinus rhythm was present. What accounts for widening of the QRS complex during atrial pacing?