A rare case of narrow QRS tachycardia with "hidden" concealed left-sided accessory pathway.

A 54-year-old-woman with narrow QRS tachycardia (heart rate, 210 beats per minute) was admitted to the authors' hospital. Tachycardia terminated after administration of adenosine. The 12-lead electrocardiogram after conversion to sinus rhythm was unremarkable. During electrophysiological study, tachycardia could be easily induced by programmed atrial stimulation without jump phenomenon. The cycle length was 260 ms. During tachycardia, the atrial activation sequence in the coronary sinus (CS) was distal-to-proximal. However, during decremental ventricular pacing, retrograde conduction showed exclusively proximal-to-distal activation in the CS. What is the underlying mechanism?