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HANDBOOK OF PHYSIOLOGY >^ CIRCULATION I 



endocardium (lo). Another theory considers the 

 repolarizing wave to "move" from apex to base 

 because of temperature differences, the apex being 

 warmer due to its proximity to the liver. 



Potentials within the right and left cavities of the 

 human heart are negative during repolarization. 

 Sodi-Pallares and co-workers (75) interpreted these 

 findings as indicating that the T wave normally 

 results from a spread of repolarization from the out- 

 side to the inside of the left wall. He believed further 

 that electrical forces from other portions of the ven- 

 tricles cancel one another, and that the right wall 

 and the septum are electrically silent during re- 

 polarization, i.e., have no clear-cut direction of re- 

 polarization but repolarize at random. Available 

 data do not allow complete acceptance of any theory 

 concerning ventricular repolarization, although the 

 normal "pathway" of repolarization is apparently 

 independent of, although statistically generally op- 

 posite to, the pathway of depolarization. In a careful 

 study Reynolds & Vander Ark (94) found a general 

 correlation between the direction of ventricular 

 recovery and the polarity of T waves recorded on the 

 epicardial surface. When the T wave was negative 

 or negligible, the epicardium recovered later than the 

 endocardium. When the T waves were positive, 

 recovery was delayed in the deeper layers. It should 

 also be noticed that acute injury produced by the 

 ligation of the coronary arteries caused an earlier 

 recovery in the deeper layers, sometimes extending to 

 the surface. (This is important in connection with 

 studies of myocardial injury discussed below.) 



Ventricular Activation in the Ungulate Heart 



The pathway of ventricular excitation in the im- 

 gulate heart has been determined in part by Durrer 

 & van der Tweel (41) and by Hamlin & Scher (51). 

 In the ungulate heart, as indicated previously, there 

 is a widespread penetration of Purkinje fibers into the 

 depth of the free wall. This leads to a nearly simul- 

 taneous activation of much of the depth of the free 

 wall and in this respect, particularly according to the 

 latter investigators, the ungulates are quite different 

 from the dog and monkey. In some experiments 

 Durrer found that even when extrasystoles were 

 started at the endocardium, the epicardial layers 

 were excited in an epicardial-endocardial direction, 

 pointing to a very extensive Purkinje penetration 

 (fig. 36). 



E5 



8 9 



JV, 



6-7 



L^A^ 



20 my 



FIG. 36. Records of normal (.V) and extrasystolic (ES) 

 activity in a goat's heart. During normal activity, deflections 

 on channels 5-6, 7-8, 8-9, and 9-10 are upright, indicating 

 movement from inside out (from endocardium to epicardium) 

 at these sites; downward deflection appears on channel 6-7. 

 With endocardial stimulation at low current, virtually this 

 pattern was reproduced; at a higher current (on the far right), 

 however, activity was entirely from inside out. [From Durrer 

 & van der Tweel (41).] 



Three-Dimensional Activation in the Goat (Fig. jy) 



During the earliest portion (5 msec) of the QRS 

 complex, activity is found in a cup-shaped zone 

 around the apex of the left ventricular cavity includ- 

 ing the endocardial portions of the septum and the 

 free wall. This activity is moving to the left in the 

 septum and, since it occurs in a buried area within 

 the free wall, is moving towards both the endocardium 

 and the epicarchum. Within the next 10 msec the 

 apical third of the septum is excited from both the 

 left and the right ventricular endocardial surface. It 

 is interesting to note that there is double envelopment 

 of the septum from both endocardial surfaces as in 

 the dog. Most of the free walls of the ventricles are 

 also activated during this period, in what has been 

 described as a "single burst" of depolarization. Only 

 a small apical and basal region of the free wall, 

 particularly of the left ventricle, remains to be excited. 

 This portion of the wall is excited during the next 3 

 to 5 msec, along with the middle third of the inter- 

 ventricular septum. The final 15 msec of the QRS 

 complex and some period beyond its end are oc- 

 cupied by activation of the basilar portion of the in- 

 terventricular septum. This basilar portion is excited 



