Fig. 1.

(A) These results are consistent with WPW syndrome with the right accessory pathway. (B) Normalization of both PR and QRS intervals during sinus tachycardia. (C) Both of the atrial-His and atrial-Mahaim by atrial extra pacing demonstrated decremental conduction property by atrial extra pacing. Atrial-Mahaim block was observed at 280 ms and A-His block was observed at 210 ms. (D) Mahaim block was observed at 280 ms. (E) The distal His potential appeared earlier than the proximal His potential. This suggested that the His bundle was excited retrogradely. (F) A sharp potential appeared earlier than the His potential. We diagnosed this potential as Mahaim potential.