ELECTROCARDIOGRAPHY 



347 



FIG. 29. A dipole moves along 

 a myocardial fiber directly 

 below the epicardial surface. 

 The field is asymmetrical, and 

 a pair of closely spaced elect- 

 rodes is supposed to pick up the 

 potential gradients of the field. 

 Instead of mosing the dipole, 

 we may move the electrodes 

 across the fixed dipole field. The 

 potential recorded is drawn at 

 the top of the figure. Each point 

 on the record corresponds to a 

 certain electrode position. If 

 the dipole moves below the fixed 

 electrodes, the same picture is 

 obtained, with the abscissa 

 indicating velocities instead of 

 distances. 



of the dipole below the electrodes, and a tiiird deflec- 

 tion being the receding phase (160). Records of this 

 type permit determination of the moment when the 

 center of the dipole (or, in our case, the center of the 

 depolarization process) passes the midpoint of the 

 electrode distance : it is the summit of the recorded 

 spike. The smaller the electrode distance, the more 

 correct is the determination, because fewer fibers 

 contribute to the recorded potential. This bipolar 

 technique with "contiguous" electrodes is the only 

 one apt to record local excitation directly below the 

 electrodes. Distances between the electrodes of less 

 than I to 2 mm are desirable to obtain correct results 

 (412). 



The unipolar potential recorded in close contact 

 with the surface is much more difficult to interpret. 

 Here we never get a preferably local potential, since 

 the duration of the QRS complex is always nearly 

 identical with that recorded by total lead s\'stems. 

 There is not even a marked spike of short duration 

 superimposed, so that the total mass of the heart 

 participates in the potential to nearly the same extent 

 as the proximal parts of the heart. There is no strictly 

 local derivation possible with unipolar electrodes ! 

 This is easy to understand if one considers the theory 

 outlined earlier: if the lead field is spherically radiate 

 (fig. 22), the local potential depends upon the solid 

 angle under which the active cross sections of e.xcited 

 fibers appear. Since many fibers are active simul- 

 taneously, the momentary distribution of these 

 excitatory processes governs the potential pattern. 

 The sum of all individual solid angles toi, under 

 which the dififerent active fibers i appear, is a solid 

 angle il as well, regardless of the irregularity of its 



shape. The potential recorded is then 

 Vp = m • 21" i = m • 12 



Let us call this solid angle 12 the "weighted active 

 cross section," weighted, because each fiber con- 

 tributes proportionately less the more remote it is 

 from the electrode. The polarity depends upon the 

 direction of the excitatory wave, whether it runs 

 toward the electrode or away from it. Parts of the 

 heart, the fibers of which run in opposite directions, 

 develop potentials of opposite polarity and cancel 

 each other. If a definite polarity is recorded, it 

 represents the influence of the prevailing direction 

 of the weighted active cross section. When the unipolar 

 potential is positive, as against the CT or any other 

 indifferent electrode, the fibers run predominantly 

 toward the electrode during depolarization. (During 

 repolarization, of course, all polarities are of opposite 

 sign.) A bundle of nearby fibers prevails over a remote 

 bundle because of its larger solid angle. But the 

 effect of a small amount of very near fibers may be 

 counterbalanced easily by a large mass of remote 

 fibers with a great solid angle Q. The unipolar record 

 therefore reveals always the prevalent direction in 

 the activation of fibers, plus the effects of mutual 

 cancellations of waves running in opposite directions. 

 If under comparable field conditions the ECG of a 

 big (hypertrophic) heart is compared with that of a 

 smaller one, the hypertrophied heart has a somewhat 

 larger potential (219), because it appears under a 

 somewhat larger solid angle. If the electrode is put 

 on the heart's surface, no such effect can be demon- 

 strated (56), because now the solid angle under which 



