EXCITATION OF THE HEART 



305 



FIG. 27. Diagram showing excitation times along four inser- 

 tions into interventricular septum. Position of the electrodes is 

 indicated on sketch of heart at upper left. Times of local acti- 

 vation shown in block of tissue bounded by electrodes at bottom 

 of figure. Advance of wave front through tissue shown in three 

 small drawings at upper right. Hatched areas indicate volume 

 of tissue excited 5, 10, and 15 msec after time reference. Note 

 double envelopment of septum. [From Scher ti al. (121).] 



reverses many times, so that depolarization in the 

 wall has no clear direction and there is little if any 

 difference between the times of activation of points 

 on the endocardiuin or epicardium, or within the 

 wall (fig. 37). Depolarization of penetrating Purkinje 

 fibers in the goat leads to small "pips" superimposed 

 on the early portions of unipolar or bipolar intra- 

 mural records. When an electrode insertion showing 

 such pips is examined histologically, the Purkinje 

 fibers are found at the proper location. In the dog, 

 such pips are at times recorded, usually within 3 mm 

 of the endocardium; staining techniques which de- 

 lineate the Purkinje fibers show specialized tissue at 

 these recording sites. 



Activation of the Interventricular Septum 



This region has been studied in several laboratories, 

 the first detailed investigations being performed by 

 Rodriguez and her co-workers (98, gg). Certain of 

 their findings have been confirmed by Burchell and 

 co-workers (23), Scher and co-workers (121), and 

 Amer et al. (5). The following points are generally 

 agreed upon. Activation of the septum begins near 

 the terminations of the bundles in its central portion 

 (fig. 25). In the apicobasal direction, the wave moves 

 both toward the apex and toward the base, and the 

 base is activated last. It was noted in these studies 

 that the direction of activity was from left to right, 

 that a large movement was directed from apex to ba.se 

 and that the mean pathway of excitation was indeed 

 directed basally. Amer and colleagues (5) conducted 

 a detailed study of the excitation of the septal surface, 

 bilaterally, in the perfused dog heart. They found the 

 earliest activity to be nearly simultaneous on both 

 surfaces, just above the anterior papillary muscle on 

 the right and in a large region near the midline on 

 the left. 



Some studies have included exploration within the 

 muscular mass of the septum (121). These differ 

 somewhat in their final conclusions. In one case it is 

 claimed that the septum is excited virtually entirely 

 from the left, and that the right bundle activates only 

 the "right septal mass," which is maximally between 

 20 and 30 per cent of the total septal mass. It is further 

 suggested that the two electrical septal masses are 

 functionally independent and that the septal mass is 

 not syncytial from the electrical point of view. Sodi- 

 Pallares and his co-workers feel that at times the 

 impulse may take 20 msec to cross the "boundary" 

 between the right and left masses of the septum 

 (fig. 29). In the study by Scher and co-workers (121) 

 the septum was found to be predominately e.xcited 

 from the left, although at times as much as 45 per 

 cent of the canine septum was excited from right to 

 left (figs. 26, 27). Furthermore, when an extrasystole 

 was started on the septal surfaces, there was no region 

 of delay in the septum, there was no partition of the 

 septum into electrically separate masses, and the 

 septum was a functional syncytium. As a result of the 

 controversy existing on these points, special studies 

 were undertaken by Scher and Rodriguez (unpub- 

 lished observations), who found that the septum is 

 indeed a functional syncytium, that there is no parti- 

 tion, and that a sizable portion of the septum is 

 e.xcited by the right bundle. These data are sup- 

 ported by the studies of Amer et al. (5) discussed 



