A 48-year-oId-man was referred to our hospital because of sudden onset and offset palpitation. The 12-lead ECG during tachycardia presented a left bundle branch block and inferior axis. An electrophysiological study (EPS) revealed the retrograde left lateral accessory pathway (AP). Tachycardia was induced by programmed pacing repeatedly. The earliest activation site of atrium was left lateral AP site. His deflection was hidden in ventricular wave and right bundle branch (RB) potential preceded ventricular potential during tachycardia. The interval between RB and ventricular potential was same as during sinus rhythm. We conclude the tachycardia is atrioventricular reentrant tachycardia conducted via anterograde atriofascicular Mahaim fiber (MF) and retrograde left lateral concealed AP. After eliminating concealed AP, tachycardia was no longer induced and no delta wave was seen. Six months after ablation, the same QRS configuration of tachycardia recurred. At the second EPS, the mechanism of tachycardia was same as previous one; however, the retrograde AP was located at left posterior which was different from the first one. The conduction via MF was seen only in tachycardia either. After eliminating the retrograde AP, the conduction over MF was no longer seen. Latent Mahaim fiber conducted during only in tachycardia is rare.
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