Figure legends
Figure 1. A : Twelve-lead ECGs before and after the operation, and during the tachycardia. After the operation, QRS duration was prolonged to 132ms with slight left axis deviation which was noted by a deep S wave in lead II. The tachycardia showed a left bundle branch block (LBBB)-like morphology with superior axis deviation and the QRS width of 200 ms.
B : Induction of the tachycardia. At the baseline, an HV interval was 76 ms. Pacing from right ventricular apex (RVA) produced a spike potential (Spike, red arrows) followed by a fragmented potential (Fg, red underlines) (Spike-Fg) in a bipolar electrode (His 1-2) placed at the His bundle. When the stimulation interval was shortened, the fragmented component was further prolonged with reversal of the sequence to Fg-Spike, thereby inducing the tachycardia. During the tachycardia with a cycle length of 420 ms, the Fg to QRS interval was 189 ms and the QRS to Spike interval was 87 ms. Each dotted vertical line indicates onset of the QRS complex. Shown are the surface ECGs and intracardiac electrograms recorded from the low lateral right atrium (LLRA), His bundle (His) area, and right ventricular apex (RVA). Fg = fragmentated potential. Spike = spike potential. 1-2 = distal pairs of electrodes of the catheter. S = stimulus.
Figure 2. A : Pacing from RVA during the tachycardia. Post pacing interval of 624ms was much longer than the tachycardia cycle length of 430ms.
B : Twelve-lead ECGs during the burst pacing. Paced QRS morphologies from RVA with the cycle length of 400 ms during baseline (RVA(B)) and tachycardia (RVA(Tachy)) were identical, which indicated absence of constant and progressive fusion. The stimulation captured His bundle potential (i.e., Spike potential) (His p) and fragmented potential (Fg p). B = baseline rhythm. Tachy= tachycardia, His p= His bundle (i.e., Spike potential) pacing, Fg p= fragmented potential pacing.
C : Pacing at the His bundle area during the tachycardia. The pacing captured His bundle, i.e., Spike potential. The stimulus to QRS interval of 71 ms quasi-equaled the HV interval during the baseline rhythm. The post pacing interval was 578 ms and the interval from the last stimulation to fragmentation was 380 ms. The fragmented potential to QRS interval was 198 ms during the tachycardia (left panel). The pacing captured the fragmented potential (Fg) selectively with the stimulus to QRS interval of 162 ms and concealed fusion. The interval from the last stimulation to the fragmented potential was 479 ms (middle panel). At the pacing cycle length of 340 ms, the stimulus to QRS interval gradually increased and the tachycardia was terminated with a seeming Fg-Spike block. A direct ventricular capture immediately after termination produced a different QRS morphology from that during the tachycardia (right panel). Abbreviations are the same as in Figure 1.
Figure 3. A : A proposed mechanism of this tachycardia. AV node = atrioventricular node. RBB = right bundle branch. LBB = left bundle branch. Fg = fragmented potential. Spike = spike potential. See text for details.
B: Before the radiofrequency catheter ablation. A brief fragmented potential was recorded from bipolar electrodes of the ablation catheter (ABL). The electrogram was 49 ms earlier than the onset of QRS complex at the successful ablation site. At this site, concealed entrainment with the stimulus to QRS interval of 46 ms was observed. The His bundle catheter was displaced to a more distal (ventricular) site. ABL = ablation catheter. Other abbreviations are the same as in Figure 1