An irregular supraventricular tachycardia with the earliest atrial activation at the coronary sinus ostium region: What is the mechanism?

A 57-year-old woman with frequent palpitations was referred for radiofrequency ablation. The baseline 12-lead electrocardiogram and echocardiography were normal. The baseline atrio-His (AH) and His-ventricular (HV) intervals were 70 and 35 ms, respectively. Dual atrioventricular (AV) nodal physiology was not observed during programmed atrial stimulation. Ventriculoatrial conduction showed a decremental and concentric pattern, with the earliest atrial activation recorded in the His bundle region. There was no evidence of a jump-up phenomenon during programmed ventricular stimulation. Supraventricular tachycardia (SVT) with variable AH and His-atrial (HA) intervals was induced by extra atrial stimulation without any jump-up in the AH interval. The earliest atrial activation during SVT was recorded at the coronary sinus (CS) 7,8 region, corresponding to the CS ostium, which differed from the atrial activation sequence during ventricular pacing. The atrial-atrial (AA) intervals demonstrated variability exceeding 50 ms without any alteration in the atrial activation sequence (Figure 1A). Ventricular entrainment pacing was performed during SVT (Figure 1B). Transient 2:1 AV conduction was observed during SVT (Figure 1C). Based on these findings, what is the underlying mechanism of the tachycardia?