Case report

Figure 1. (A and B) (both speed 100 mm/s) showed the S1S2 atrial program stimuli during EP study. After atrial S1S2 stimuli in 450/310 ms, the A-V interval was 125 ms. When atrial S1S2 stimuli in 450/300 ms, A-V interval jumped to 285 ms, and tachycardia (TCL = 370 ms) was induced. The earliest A was from CS9,10 region. (C) (speed 100 mm/s) showed RS2 stimuli during his bundle effective refractory period (ERP) terminated tachycardia. (D) (speed 100 mm/s) showed a wide QSR tachycardia was induced by atrial S1S2 stimuli after AP ablation. The TCL of tachycardia was 220 ms. The earliest A wave was from his region and V: A was 1:1.

Figure 2. (Speed 100 mm/s) showed ventricular overpacing (S1S1 = 200 ms, shown as black stars) during wide QRS tachycardia (TCL = 220 ms). The continuous 3 beats of ventricular pacing did not capture atria. The last one successfully accelerated atria (A-A interval changed from 220 to 195 ms, shown as black arrow). After pacing, a V-A-V sequence was observed and tachycardia changed into narrow QRS. The V-A interval of tachycardia and the last ventricular pacing was 138 and 168 ms, respectively, and H-A interval of tachycardia and the last ventricular pacing were 158 and 163 ms, respectively (not shown in the figure).

Figure 3. The mechanisms of this complicated tachycardia case. (A) The SP-AP reentrant tachycardia (TCL = 370 ms), which was the first induced tachycardia in the paper; (B) The second FP-SP reentrant tachycardia (TCL = 220 ms), which showed a CRBBB morphology because of distal right bundle branch functional blocked (black double solid lines); (C) Ventricular pacing (black dotted arrow) facilitated the conduction of distal right bundle branch, downregulated the activation threshold of blocking position (black double dotted lines). Thus, the superior activation could pass through (black solid arrow) and QRS of tachycardia turned to narrow. AP = access pathway, AVN = atrioventricular node, FP = fast pathway, LB = left bundle branch, RB = right bundle branch, SP = slow pathway.