ELECTROCARDIOGRAPHY 



337 



FIG. 17. Ideal lead field penetrating the heart with equi- 

 distant flow lines. This field records all individual fiber po- 

 tentials with the same lead vector, i.e., with the projection of 

 the fiber dipole on the parallel flow lines. All fibers are therefore 

 recorded with the same relative amplitudes. 



parts of the heart considered individually. If we 

 reconsider figure 10, a correct projection of a vector 

 originated in the heart is possible as soon as we are 

 informed about the lead field of a given electrode 

 combination. Theoretically, one could interpret the 

 ECG of the total heart by knowing both the lead field 

 penetrating the heart as well as the distribution and 

 direction of the individual electromotive forces. In 

 reality, however, a lead field of complicated geo- 

 metrical structure will always inject practical diffi- 

 culties into the interpretation of an ECG record. 

 The only way to overcome this difficulty would be to 

 introduce lead fields with parallel and equidistant 

 flow lines. Using some simplifying assumptions in 

 such a field, and a multipole electrode combination, 

 we may achieve a simpler interpretation of the record 

 (fig. 17). Such a field records every single fiber dipole 

 in the heart with the same lead vector, i.e., with a 

 projection on one and the same lead direction, and 

 with a relative magnitude which is the same for all 

 parts of the heart. Such a system, therefore, would 

 allow determination of a single resultant \ector, 

 which would represent all fiber activities occurring 

 in the cardiac muscle. Only thus could the "heart 

 vector" be determined correctly. Unfortunately, such 

 a lead field can be realized only in an appro.ximate 

 manner. The obstacle is the inhomogeneous con- 

 ductivity of the thoracic media. (In fig. 17, this 

 inhomogeneity is taken into consideration only for 

 the vertebral column. The big inhomogeneities of the 

 lung are omitted.) We know that even big changes in 

 the conductivity of the media surrounding the heart, 

 e.g., metal or rubber shields at the heart's surface 

 (256, 276, 285, 325) or opening of the chest wall 

 (353), lead only to slight changes in the form of the 

 ECG. This fact is incompatible with the assumption 



that a parallel lead field may ever be obtained with 

 electrode combinations of such simple form as shown 

 in figure i 7. The flow lines seem to enter the heart 

 preferably through a very restricted area, mostly 

 in the direction of the mediastinum. The form of the 

 mediastinal conductor seems to indicate that a parallel 

 lead field might be achieved in the sagittal direction 

 with a minimum of distortion, if a multiple electrode 

 system were used. A suitable arrangement of this 

 kind has been described (382). 



The assumption of a homogeneous medium in the 

 thorax is invalidated also by the blood filling the 

 cavities of the heart. The conductivity of the blood 

 exceeds ten times that of the intrathoracic tissues. 

 This fact influences the potentials of the various 

 fibers in different ways, depending on their direction: 

 in radially directed fibers the dipole moment is 

 augmented; in fibers tangential to the cavities it is 

 diminished (128). A similar influence will be found 

 on the lead field ; the lead lines are compressed in the 

 cavities. A parallel, equidistant lead field, as could be 

 gained in models with a homogeneous medium, 

 principally cannot be achieved with the heart in situ. 



For the lead field concept, the same restrictions of 

 its applicability have to be made as for the lead vectors 

 and the image surface: we do not know any simple 

 method to determine lead fields in living man. Meas- 

 urements have not yet been made in corpses. In 

 living man, only one report is available of determina- 

 tion of the lead field of the retrocardial space in the 

 esophageal area (257). Thus our knowledge of the 

 lead fields is based merely on experiments with models 

 in which the flow lines of a certain electrode position 

 were determined by an analogous hydraulic model: 

 instead of a current, a fluid vi^as made to flow across 

 the torso, and the stream lines were photographed. 

 The flow lines were made visible by crystals of dye 

 [fluid mapper (343)]. The theory of the lead field has 

 been outlined in several papers (129, 245, 340-342, 

 382). 



Discussion of the l^ectorial Concept 



It seems necessary to emphasize the antagonism 

 Ijetween the lead field and the image surface. The 

 latter transforms the geometrical electrode site into a 

 "corrected" position, and is valid for any electrode 

 comljination, but only for one dipole of a fixed 

 location in a thorax of a given form. The lead field 

 however is valid only for one special electrode com- 

 bination and a given thorax form, but remains valid 

 for any dipole position or even for any combination 



