Figure 4.

Wide QRS complex tachycardia. A: Induction of wide complex tachycardia (WCT) with rapid atrial pacing. Following the fourth atrial paced event from the left, His-to-ventricular conduction time (HV) block is noted. Gradual HV prolongation is evident on the previous 2 atrial paced beats. The subsequent fifth atrial paced complex is associated with a shortened HV interval and possible pre-excitation. Given the dissociation of atrial and His electrograms from the ventricle on ensuing atrial paced events, it is probable that the tachycardia started on the fourth QRS complex, following the HV block. B: Induction of the WCT during delivery of triple ventricular extrastimuli. The third ventricular paced complex was delivered in the ventricular refractory and does not capture the ventricle. Following the second ventricular extrastimuli 2 beats of atrioventricular node reentry occur, which facilitates the occurrence of the WCT. C: Ventricular pacing at a cycle length = 230 ms in the region of the proximal right bundle branch resulted in concealed entrainment with a post-pacing interval of 15 ms (recovery interval = 264 ms). D: QRS morphology during pacing at the fascicular potential site elicits QRS morphology similar to the WCT (compare with Figure 2B). Format of electrocardiographic tracings is similar to Figure 3.