EXCITATION OF THE HEART 



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FIG. 20. Activation time over surface of right and left ventricles seen from the sides; two different 

 dogs. Lejt: the earliest activity was at the right border of the cardiac outline; latest activity at the 

 base of the right ventricle (upper left portion of fig.). This figure shows predominantly the free wall 

 of the right ventricle. Right: the distribution over the left ventricle. The earliest activity occurred 

 just to the right of the anterior descending branch of the coronary artery and near the ape.\ of the 

 ventricle, and the latest activity was generally near the base. Activity tended to be later on the 

 posterior and lateral portions of the heart. [From Lewis (72).] 



ventricular myocardium, respectively. These estima- 

 tions were close to those later measured directly. 



Excitation of the I 'entricular Surface 



Several early investigators attempted to plot the 

 sequence of excitation of the ventricles of small hearts 

 (24, 26, 70, 71). Lewis found the earliest excitation in 

 the toad \entricle (fig. 18) in its central region and 

 the wave appeared to spread from there toward both 

 apex and base. A similar pattern is illustrated in 

 figure 19. In all cases the apex and base were ac- 

 tivated within 20 msec (26). The problem of activa- 

 tion of the ventricular surface (43, 45, 145) has been 

 difficult to study in small hearts and has been most 

 easily resolved in the large canine heart. A study in 

 guinea pigs has, however, been conducted by Dower 

 & Osborne (38), who found that the surface of the 

 right ventricle was activated earlier than the surface 

 of the left. A number of excellent studies have been 

 carried out on the surface excitation of the ventricle 

 in large mammals. Particularly important are the 

 studies by Lewis (71), Harris (53), Sodi-Pallares & 

 Calder (126) and Schaefer & Trautwein (113, 114) 

 [see also Katz (61)]. 



Lewis' plot of the surface activation of the dog 



ventricle (fig. 20) is similar to that for the toad. Some 

 points in the center of the heart, particularly over 

 the trabecular region of the right ventricle, become 

 active earlier than any other surface points. The base 

 of the heart tends to be excited later than the central 

 region and than the apical regions, but in some hearts 

 5 msec separates the earliest point on the apex from 

 the last on the base. A somewhat different picture is 

 presented by Sodi-Pallares (fig. 21). From these 

 small time differences, certain authors did attempt to 

 derive a complete pattern of ventricular activation. 



Figure 22 shows a plot of ventricular surface excita- 

 tion in the monkey from Harris' work (53). The time 

 differences are roughly the same as, or perhaps 

 slightly greater than, those for the dog. The earliest 

 area is in the center of the trabecular region on the 

 right with a similar area on the anterior central left 

 ventricle. Surrounding these earliest points are points 

 activated later, which extend down to the apex and 

 laterally in the right ventricle. The latest points 

 activated are at the basal regions of the left and right 

 ventricle. In the posterior section there are much 

 smaller time differences, but again the base is later. 



Barker et al. (13) studied surface activation in the 

 human heart. They found the earliest points on the 

 anterior right ventricular surface near the A-V 



