Programmed stimulation, S1 600 ms. His and coronary sinus (CS) recordings during distal CS pacing. ( A ) S2 320 ms, ( B ) S2 260 ms. Evidence of pre-excitation on surface ECG following the extrastimulus. There is no His bundle signal preceding the pre-excited beat, confirming the conduction over the accessory pathway. The conduction over the pathway is decremental, as can be seen in this figure, where the stimulation–ventricular activation intervals get longer with shorter coupling intervals. His d, His distal; His p, His proximal; CSd, distal coronary sinus; CSp, proximal coronary sinus.