302 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



groove. These points were activated approximately 

 10 msec after the beginning of a positive potential in 

 the lead II electrocardiogram. The earliest points on 

 both the left and the right were near the origins of the 

 papillary muscles anteriorly. In their experiments 

 there was at times a tendency for both the base and 

 the apex to be activated later than the center of the 

 ventricles; however, on the anterior surface the apex 

 was activated later than the base, whereas on the 



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FIG . 2 1 . A scheme for the activation of the surface of the 

 dog ventricle presented by Sodi-Pallares. He finds the earliest 

 activity just to the right of the anterior descending branch of 

 the coronary artery. From this area there is a movement to the 

 right. There is also early depolarization in the apical area. 

 On the back, the earliest activity occurs, at between ao and 

 25 msec, near the apex, and the base of the heart is depolarized 

 last. [From Sodi-Pallares & Calder (126).] 



posterior surface the base was activated later than the 

 apex. These investigators found unexplained varia- 

 bility in their results when the measurements were 

 repeated in the same individual at intervals of days. 

 Groedel & Borchardt (50) recorded potential shapes 

 from several sites on the human heart, as did Nylin 

 & Crafoord f8o). 



Excitation of the ]'cntricular Walls 



In the late 1940's a number of laboratories (40, 41, 

 98, 99, 1 04, 117, 1 26) began to explore the inner 

 layers of the canine ventricular myocardium with 

 penetrating electrodes (referred to as needle elec- 

 trodes and plunge electrodes). Some of these elec- 

 trodes had one or two recording tips, and the elec- 

 trode was moved in and out of the wall to determine 

 the time of activity at various depths. In other studies, 

 multipolar electrodes which could record from the 

 various depths without being moved were used. 

 Recording equipment varied from pen-writers to 

 multichannel oscilloscopes. The results of many of 

 these studies are summarized in a symposium (54) 

 and in a text (iq6), although some must be read in 

 the original literature. Many of these papers stress 

 the finding that in the dog the impulse does not move 

 in a straightforward fashion from endocardium to 

 epicardium. The impulse may arrive first at a point 

 several millimeters within the wall and spread toward 

 both endocardium and epicardium, or it may show- 

 reversals of direction near the endocardium. In the 

 several studies, the amount of tissue excited from 

 within outward has been estimated as 20, 33,^3, and 



FIG. 22. Spread of surface excitation 

 in the monkey's heart. A: ventral sur- 

 face; B: dorsal surface. Numbers in- 

 dicate the time of excitation at the 

 marked points on the surface as com- 

 pared to the earliest point at vifhich 

 activity was recorded. Note that the 

 earliest activity on both sides is toward 

 the center of the anterior ventricle on 

 both the left and the right. Also, the 

 latest activity recorded on the anterior 

 surface is near the base of the heart. 

 The posterior base of the heart is ac- 

 tivated somewhat earlier. The base is 

 activated later than the apex anteriorly 

 and posteriorly. Note that all surface 

 points are excited within a very short 

 period of time, the maximum time 

 difference is 13 msec; the QRS of the 

 electrocardiogram has about three 

 times this duration. [From Harris (53).] 



