EXCITATION OF THE HEART 



313 



FIG. 37. The boundary between active and resting muscle 

 at four instants in time during depolarization of the ungulate 

 ventricle. A : an initial phase of activity is directed from left to 

 right at the left and apical portion of the junction between 

 septum and free wall. B: this left-to-right septal activity is 

 joined by activity from right to left in the septum, and there 

 is also simultaneous activation of a large portion of the wall, 

 between endocardium and epicardium. The activity in the 

 wall is moving towards both the epicardium and the endocar- 

 dium. C: very rapidly thereafter most of the wall is depolarized, 

 and only a small apical portion and a high basal portion of the 

 left wall remain to be depolarized in addition to the septal 

 region, which is being depolarized by a wave moving from 

 apex to base. D: the final phase of ventricular activation in- 

 volves a movement of the wave towards the base of the septum. 

 Leads V]o and AVF are shown in their approximate positions 

 with respect to the heart. [From Hamlin & Scher (51).] 



by a wave moving towards the base and somewhat 

 toward the right, since left-to-right activity pre- 

 ponderates in this portion of the septum as in its 

 more apical portion. Interestingly, the widespread 

 Purkinje fiber penetration in the goat is so efficient 

 in conducting the impulse that the QRS complex 

 may be of virtually normal duration when extra- 

 systoles are started at a single ventricular focus (52) 

 or when one bundle is cut. 



Derivation of the Ventricular Electrocardiogram 

 in the Ungulate 



Figure 37 shows the approximate position of the 

 boundary between depolarized and resting muscles 

 at four instants during ventricular activation in the 

 goat. Leads aVF and Vio are shown in approximate 



position with respect to these boundaries. The excita- 

 tion of the apical region of the septum and of the 

 left ventricle, which occurs during the first 5 msec 

 of the goat electrocardiogram, generates negative 

 potentials in a\'F and \'ui. The excitation of the in- 

 ter\cniricular septum that follows is directed from 

 both endocardial surfaces toward the center, and 

 the activation of the wall is directed both endo- 

 cardially and epicardially. There is so much cancella- 

 tion during this activity that onl>- a very small 

 potential or, generally, an isoelectric period is seen 

 in the electrocardiogram. During the next period the 

 septum is activated from both surfaces by a wave 

 which moves froin apex to base and results in basally 

 directed activity. At this time the wall has been 

 almost entirely activated, except for a small posterior 

 basal region on the left. In these regions, as in the 

 septum, activitv is moving toward the base of the 

 heart and positive potentials are seen in both aVF 

 and \'io. This late activation of the basal septum 

 (and wall) gives rise to a major deflection in the 

 electrocardiogram, because of the cancellation of 

 potentials generated during activation of much of 

 the apical portion of the heart. This same apical 

 portion gives rise to the major electrocardiographic 

 deflections in such animals as the dog and monkey, 

 and undoubtcdlv in man. 



ABNORM.\L EXCITATION 



Bundle Branch Block 



Bundle branch block results from failure of trans- 

 mission either in the right or left conduction bundles, 

 or in their terminal ramifications. The usual cause is 

 probably myocardial damage from infarction or 

 fibrosis from long-standing cardiac disease, although 

 right bundle branch block may occur in normal 

 young persons. The term "complete bundle branch 

 block" is an arbitrary designation for beats originating 

 in the A-V node but having a total QRS duration 

 of over 120 msec in man. 



The pattern of ventricular excitation in complete 

 left bundle branch block in the dog is shown in 

 figure 38; to produce this pattern, the left bundle 

 was cut immediately under the aortic valve. Figure 

 39 indicates the changes resulting from right bundle 

 branch block. Here the right bundle was cut in its 

 course down the right side of the interventricular 

 septum. As might be expected, after the main bundle 



