3o8 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



early and the mean direction of activity thus sweeps 

 to the left and posteriorly. 



Eventually, the wave completely depolarizes all 

 apical portions of the right and left ventricles, leaving 

 unexcited a basal and posterior portion of the left 

 ventricle. In addition, there is a change in the direc- 

 tion in which the wave of activation in the septum 

 moves. The apical portions of the septum are de- 

 polarized from both surfaces, but the leading edge of 

 the boundary between resting and active tissue is 

 directed basally. Also, there appears to be less 

 Purkinje tissue on the right than in the left, so that 

 the basal portion of the septum is often excited by a 

 wave moving from left to right and toward the base 

 of the heart late in the QRS complex. The latest 

 activity is thus directed posteriorly, slightly leftward, 

 and basally in the wall and basally in the septum. 

 The velocity of conduction across the septum and in 

 those portions of the wall where there is no evidence 

 of Purkinje penetration has been calculated by 

 measuring the time and distance between successive 

 positions of the wave front. The velocity averages 

 0.37 m per sec. Along the endocardium the velocity 

 of spread cannot be calculated, since many endo- 

 cardial points are simultaneously activated by the 

 higUy branched Purkinje system. A detailed picture 

 of the activation as directly measured in the dog is 

 gi\ en in figure 30. 



Conduction During Ventricular Extrasystoles 



The data concerning excitation of the canine 

 ventricles already discussed are explained by the 

 anatomy of the conduction system — the locations of 

 its terminations and its extensive branching — and 

 by the conduction velocities of i to 2 m per sec and 

 0.3 to 0.4 m per sec for Purkinje fibers and m)o- 

 cardial fibers, respectively. Studies of conduction 

 during extrasystoles provide a means of checking this 

 explanation. Scher & Young (118) stimulated various 

 ventricular points at a rate slightly higher than the 

 sinus rate. The conduction velocity was about i.o m 

 per sec near the endocardium and parallel to it in the 

 apical regions of the heart, and about 0.3 m per sec 

 within much of the thickness of the walls and septum, 

 and along the basal endocardium. The pathway of 

 excitation was such that it "was explained by the 

 endocardial and mural velocities stated, and the 

 electrocardiographic complexes recorded in lead II 

 were in accord with the pathway (fig. 31). 



Durrer and \an der Tweel (41) found that with 

 epicardial stimulation the wave traveled with con- 

 stant velocity to the endocardium. With endocardial 

 stimulation, they generally observed a straight inside- 

 out spread, even in regions where reversals of polarity 

 during normal beats had indicated Purkinje fiber 

 penetration. During some extrasystoles the normal 



CORONAL SECTION 



-20 -10 



FIG. 31. Successive positions of depolarizing wave front for coronal section of heart in normal 

 beat (left) and in premature systole which originated at apical endocardium (right). Electrode 

 terminals i mm apart. .Arrows above and below each drawing indicate the approximate position of 

 the line connecting the lead II terminals. [From .Seller & Young (i i8).] 



10 20 U 40 50 60 70 



