Case 1
A 56-year-old male with mild preexcitation was referred for EP study who had several episodes of palpitation recently. All recorded ECGs during tachycardia were in left bundle branch block type. Maximal preexcitation pattern was consistent with a left AP. Retrograde conduction was over distal coronary sinus (CS) when pacing from right ventricle (RV). Tachycardia was induced by an atrial extrastimulus blocked in the AP, conducting over the AV node and causing left bundle branch block.
A late-delivered premature ventricular contraction (PVC) was able to advance the next A and reset the tachycardia, which was diagnostic for ORT with aberrancy. In the meantime, left bundle branch conduction was restored which narrowed the next QRS complex. Tachycardia was then terminated by the advanced wavefront reaching the ventricular insertion site of AP which was refractory (Figure 1). Mapping of the AP was performed using transseptal approach during RV pacing. The earliest atrial activation site was located at anterolateral mitral annulus (MA) with a sharp potential, where radiofrequency application successfully blocked the AP. The patient has been free from the tachycardia for 1 year.
Case 2
A 64-year-old gentleman presented with very frequent tachycardia which always terminated spontaneously within minutes. The only ECG from emergency room showed tachycardia with left bundle branch block type. His tortuous femoral veins made it difficult for catheterization, therefore only an RV catheter was inserted with CS catheter placed from jugular vein for EP study. The earliest atrial activation site was CS3-4 during RV pacing, indicating a left lateral AP. Atrial pacing and extrastimulus showed no preexcitation or intraventricular block.
The clinical tachycardia could only be induced by RV extrastimulus with isoproterenol, which could not sustain for more than 10 beats, compatible with ORT with aberrancy. Termination of the tachycardia was always preceded by a narrow QRS complex, with the V and subsequent A at MA advanced, suggesting anterograde block in the AV junction (Figure 2). The pathway was ablated at anterolateral MA with retrograde approach, after which no tachycardia could be induced. The patient has been free from the symptom for more than 3 years.