336 HANDBOOK OF PHYSIOLOGY ■^^ CIRCULATION 1 



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-^IG+H*UJ<-K) 



-30 mv 



FIG. 16. Image surface (space) of a set of thoracic electrodes and some of their combinations. 

 The anatomical picture of the electrode positions is shown on the left. On the right, these same 

 electrode positions are shifted so that, in the case of unipolar derivations, their connections with the 

 heart dipole form the lead vector of every electrode. In case of bipolar derivations, the lead \'ector 

 is equal to the line connecting the two electrodes on the image surface. The potentials recorded at 

 each electrode are the scalar (dot) product of the projection of the dipole moment on the lead 

 vectors and the length of the lead vectors, [From Heim (249) ] 



surface allows vectorial operations such as projections 

 for every electrode combination, e.g., for bipolar as 

 well as for unipolar leads. Even the effect on the 

 derived potential of multipolar lead connections can 

 be calculated (fig. i6). 



The mass center, however, is not always the "best" 

 location of the dipole, so that complicated methods 

 have been invented to improve this location, gaining 

 a "best" dipole position from a comljination of mirror 

 patterns of the human thorax and correcting factors 

 taken from the spatial image of a model of this same 

 thorax (200, 206). Another method consists of putting 

 an artificial dipole into the living body. In dogs, the 

 dipole can be introduced into the heart (116, 281). 

 In man, dipoles have been put into the esophagus 

 immediately behind the heart (78, 257), and in one 

 case even into the right ventricle (149). A third 

 method is to put the dipole at any point in the interior 

 of a human corpse (121, 527). It is evident that, in 

 all cases, the dimensions of the heart and the peculiar 

 distribution of its individual dipoles are completely 

 neglected. This is the only serious objection to the 

 validitv of the lead vector and image surface construc- 



tions. With the aid of such methods, a fairly correct 

 prediction of potentials recorded by any desirable 

 electrode position is possible. Nevertheless, in spite 

 of the great experimental effort put into such deter- 

 minations, there remains an error in such predic- 

 tions of ± 1 5 per cent. These errors occur because the 

 large diameter of the heart cannot be taken into 

 account by a procedure based on the "single fixed- 

 location dipole" concept. Consequently, it is probable 

 that no correcting system exists which is able to 

 compensate for all anatomical deviations from the 

 conditions of the "ideal" vector field, and which 

 consists in the application of lead vector projections. 



Lead Fields 



The preceding section showed the limits of certain 

 accepted interpretations of the ECG with the aid of 

 lead vectors and projections, on the basis of the single 

 fixed-location dipole concept. It therefore was a 

 decisive step forward to apply the Helmholtz theorem 

 to ECG analysis in such a form that the anatomical 

 peculiarities are taken into account and the various 



