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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



of such a recording electrode consists of verv short 

 and strongly curved flow lines, the density of which 

 diminishes rapidly as the distance increases at a 

 ratio of about i:r'' (r = distance) (58). Such an 

 electrode records very small amplitudes, probably 

 from very small regions. Therefore, the total duration 

 of the QRS is distinctly lower than that of standard 

 leads, and the latencies vary from position to position. 

 However, there has been almost no experimental 

 work done with this electrode. 



STANDARD UNIPOLAR LEADS OF "LOCAL" CHARACTER. 



The only commonly used electrodes which may have 

 a certain local efi^ect are the Wilson electrodes on the 

 precordial part of the chest (528). We therefore dis- 

 cuss the theory of these electrodes in detail. Wilson's 

 first aim in introducing unipolar precordial electrodes 

 apparently was to avoid the potential variations of 

 the second electrode, which might, he believed, 

 interfere with its own potential pattern. This belief 

 was based on the single dipole concept. He therefore 

 put the second "indifferent" electrode on a very 

 remote part of the body, e.g., on one leg. The inven- 

 tion of the CT as reference point was a step toward a 

 more correct solution of the problem. But soon the 

 lead field concept proved such derivations to be of a 

 local character (see fig. 23). Because the heart is 

 imbedded in the mediastinum, the flow lines of the 

 lead field do not have a good chance to diverge; 

 rather they are forced by the lung resistance to run 

 along the mediastinal borders. Therefore it is not 

 surprising to hear that even in precordial leads the 

 influence of the proximity potentials is comparatively 

 small. The local character of such electrodes is re- 

 stricted by still another factor. Figure 23 shows the 

 flow lines in the sagittal plane being bent very 

 strongly along their way through the heart. There is 

 no one prevailing direction in the flow lines, not 

 even that one directed strictly toward the electrodes. 

 Therefore, no single direction of the excitation waves 

 is favored. From a bundle of muscle fibers which run 

 parallel to the chest wall, only minimal potential is 

 recorded. There is only one kind of excitation prefera- 

 bly recorded : waves which run directly toward the 

 electrode or directly away. Assuming the heart to 

 consist of a bulk of fibers running "at random," 

 such fibers would be selected. Provided the unipolar 

 electrode is positive against the CT, the majority of 

 these fibers develop excitation waves running to the 

 electrode. In case of negativity, the waves run away 

 from the electrode. If an electrode is attached close 

 to the heart, the recorded potential pattern usually 



FIG. '^6. Standard positions of the Wilson chest leads Vi-Ve. 



is determined by the interference of approaching and 

 receding waves which run in the direction of the lead 

 field and near the electrode. 



Considering the standard positions of unipolar 

 leads, as Wilson proposed them, only some of these 

 leads can contain a noticeable amount of a local 

 derivation. The standard positions are shown in 

 figure 26. Evaluation of their proximity potentials 

 may be given in the following manner. Potentials to 

 be recorded from various parts of the heart are 

 easily calculated for the ideal conditions of a perfect 

 spherically radiate lead field, assuming "random"' 

 distribution of fiber directions for all parts of the 

 heart. As figure 27 demonstrates, the mass of the 

 ventricle can be subdivided into spheric sectors of a 

 given solid angle 9 \iewed from the electrode, and 

 a certain thickness d. Similar distribution of fiber 

 directions assumed, the potential to be recorded from 

 such spheric sectors can be calculated. The rough 

 estimation of figure 27 leads to the conclusion that 

 from the Wilson point \'5, the left ventricle is re- 

 corded with 72 per cent of the total potential, the 

 right ventricle only with 28 per cent. From point Vj, 

 the relative percentages are 46 and 54 percent. Thus, 

 even in a precordial electrode quite close to the right 

 ventricle, this part of the heart contributes only a lit- 

 tle more than one-half of the potential of the record. 

 In some extremely fa\oral)le electrode positions the 

 small muscle mass of the right ventricle may be re- 

 corded with the same relative weight as the left ven- 

 tricle, but, considering the relative masses of the ven- 

 tricles, no real local derivation can be gained under 

 normal circumstances (58). 



