Narrow complex tachycardia with alternating atrial activation: What is the Mechanism?

Case Presentation: A 50-year male was referred for consideration of a redo ablation procedure. 2 years before this referral, he was diagnosed with an episode of pre-excited atrial fibrillation, and underwent an attempt at a left lateral pathway ablation. The index attempt with radiofrequency ablation did not eliminate the pathway and he was discharged on oral flecainide. The procedure was also complicated by a right-sided deep vein thrombosis and further attempts at a redo procedure were delayed. However, he developed recurrent episodes of palpitations 2 years after his index procedure. Hence, he was referred for a second attempt at ablation. His baseline 12-lead ECG did not show any evidence of manifest pre-excitation. During his EP study, pre-excitation was seen with atrial extrastimuli and with shorter intervals, a narrow complex tachycardia was induced (Figure [1](#fig-cap-0001)). However, the atrial activation appeared to alternate at times, despite the tachycardia being sustained. What is the mechanism of the alternating atrial activation?