The excitation process within the septal myocardium has been described by many investigators. However, some of details of the process have been left as problems to be discussed. It is also the purpose of this study to investigate the details of the process. Method : Hearts of 11 adult mongrel dogs were used in situ. Flexible multipolar electrodes were inserted across the ventricular septum through the right ventricular free wall. A 3-channel oscillograph was used for recording. In normal sinus excitation, in RBBB, and in extrasystole, the arrival time of excitation was measured on direct lead ECGS which were led from many points in the septal myocardium. Reference electrode was set in the apical free wall. To produce BBB, right or left bundle branch was cut. To provoke extrasystole, electrical stimulation was put on the right or left ventricular free wall with a set delay after previous QRS complex. Results : A) In normal sinus excitation 1. On the septal surface, the excitation spread from apex to base. The mean conduction velocity of excitation was 620 cm/sec on left endocardial surface and 180 cm/sec on right endocardial surface. 2. Through the septum, the excitation spread from both (left and right) surface to medial layer of the septum in 15 of 21 electrodes. In other 6 electrodes, one-way (from left to right) excitation was observed. The mean conduction velocity was 31 cm/sec (right to left) and 33 cm/sec (left to right). 3. In 18 of 21 electrodes, excitation appeard on the left endocardium before on the right endocardium. B) In BBB, the septum excited from the non blocked side to the blocked side. The mean conduction velocity was 29 cm/sec (LBBB) and 26 cm/sec (RBBB). C) In extrasystole, the septal excitation spread from the stimulated side to the other side. The mean conduction velocity was 38 cm/sec (L-stimulation) and 51 cm/sec (R-stimulation).
