A 46-year-old man was referred for an invasive electrophysiological study with a view to ablation, for a history of classic sudden onset-offset palpitations. The patient’s son had recently survived an out of hospital cardiac arrest, and was found to have an accessory pathway (details unknown) at another institute, which was ablated. Our patient’s 12 lead electrocardiogram (ECG) showed sinus rhythm with no evidence of pre-excitation. Echocardiography revealed a structurally normal heart. An electrophysiological study was performed with a quadripolar catheter positioned at the high right atrium (HRA), a steerable decapolar catheter in the coronary sinus and quadripolar catheters along the His bundle and at the right ventricular apex. Baseline atrio-His (AH) and His-ventricular (HV) intervals measured 60ms and 40ms respectively. Programmed atrial extra-stimulus testing revealed decremental AH intervals, before tachycardia was reproducibly induced. Figure 1 shows the tachycardia on a 12-lead ECG. Figure 2 shows the induction of tachycardia with atrial pacing. Based on the findings within the figures, what is the mechanism of the tachycardia?