Epicardial excitation of the ventricles in a patient with wolff-Parkinson-White syndrome ( type B).

Epicardial excitation was explored by means of an exploring electrode during operation on a patient with a large atrial septal defect of the secundum type, whose ECG indicated a Wolff-Parkinson-White syndrome of type B. Very early excitation occurred, 10 msec after the end of the P wave, at the right lateral border, near the atrioventricular sulcus, an area which is located a relatively large distance from the atrioventricular node. Because the epicardial region closest to this node did not show early excitation, it was concluded that in this heart the node was not involved, but -that a muscular bypass between the right atrial muscle and the closely adjacent right ventricular surface was responsible. The location corresponds with that described in several extensive anatomic studies of hearts with a WPW syndrome. The right ventricle was activated predominantly or completely by an excitation wave originating in this area; the excitation pattern of the left ventricle did not differ significantly from the normal.