EXCITATION OF THE HEART 



307 



wall rather than from inside out and that there 

 appears to be some barrier to inside-out movement 

 of the wave in these basal regions. 



Alap of Ventricular Activation 



Ventricular activation [(fig. 30) (117, 119)] ap- 

 parently commences somewhat earlier on the left 

 than on the right. As indicated above, the left bundle 

 separates into two separate branches and it is possible, 

 although by no means proved, that activity com- 

 mences in two separate areas supplied by the anterior 

 and posterior branches of the bundle, respectively. On 

 the right, activity begins earliest at the septal termina- 

 tion of the right bundle, in the region of the anterior 

 papillary muscle of the right ventricle. In addition 

 to tlie fact that earliest activity is usually found on 

 the left, a greater volume of tissue is usually activated 

 early on the left. Septal activity on the left moves 

 toward the center of the septum, as does that on the 

 right as well. These two waves of activation thus 

 tend to cancel. The preponderance of activity moving 

 from left to right is important in the production of 

 the initial deflections of the electrocardiographic 

 complex. Branches of the Purkinje system run from 



the septal terminations of both bundles to the free 

 wall. On the left these bundles are rather close to the 

 endocardium; on the right, one or more branches of 

 the bundle cross the cavity. The activity moves out 

 along these branches to the endocardium and the 

 periendocardial regions of the free walls almost 

 immediately after the beginning of septal excitation. 

 Since most of the apical mural endocardium is now 

 excited, the major direction of movement available 

 to the depolarizing impulse is toward the epicardium. 

 The impulse moves toward the epicardium through- 

 out tiie major portion of the QRS complex. If we 

 think of this impulse as a wave directed from the in- 

 side out in the wall and remember that the termina- 

 tions of the Purkinje fibers on the right are somewhat 

 anterior, we can see that initially the wave will be 

 directed to the right and anteriorly at this region. On 

 the left, however, there is activity directed anteriorly, 

 apically, and posteriorly in the apical portion of the 

 left wall. Because the right wall is thin, the wave of 

 activity breaks through to its surface early in the 

 QRS complex and, for this reason, the right wall 

 ceases to be electrocardiographically important. The 

 thicker left wall, particularly posteriorly, and the 

 posterior right wall, however, are not activated so 



NORMAL 



10 20 30 MSEC 



FIG. 30. Pathway of normal ventricular excitation in dog as plotted by noting extent of depolari- 

 zation o, 5, 12, 18, and 25 msec after beginning of QRS complex. Simultaneous lead II electro- 

 cardiogram is shown. At o msec, small amount of muscle bordering left cavity is active. This volume 

 of muscle is too small to give a deflection in peripheral electrocardiogram at this amplification. .At 5 

 msec after beginning of QRS, an incomplete and irregular cone of activity surrounds left cavity, 

 mostly on septal aspect, and a smaller cone surrounds right cavity. By 12 msec after beginning of 

 QR.S, these two cones have united in lower three sections and have joined slightly in upper section. 

 Heart now contains a cone of depolarized muscle within an incomplete cone of muscle which is 

 still in resting state. Notice breakthrough of electrical activity anteriorly on right. This leaves activity 

 in posterior and leftward portion of ventricles unopposed. This pattern of excitation continues 

 during next 6 msec. Picture at 18 msec is generally unchanged, although amount of muscle de- 

 polarized is, of course, larger; the fraction of posterior and left portions of the ventricle in resting 

 state has become smaller. At 25 msec after beginning of QRS complex, only a small amount of 

 muscle in posterior and lateral portion of left wall and of basal septum remains to be e.xcited. Dark 

 lines on the first drawing indicate the positions of electrodes used to plot this pattern of activity. 

 [From Scher (114a).] 



