ELECTROCARDIOGRAPHY 



335 



LEAD/ VECTOR B 



FIG. 14. Mirror patterns of the ECG. Demonstration of 

 mirror patterns at opposite points of the thorax. The tracings 

 were recorded with unipolar electrodes at the points indicated 

 [From Duchosal & Sulzer (15).] 



pro.ximity potentials, but this view is denied by most 

 authors. The details of these proximity potentials 

 will be dealt with later. Neverthele.ss, it should be 

 stated here that even under ideal experimental condi- 

 tions, using an isolated cat heart in a homogeneous in- 

 finite field, proximity potentials are detectable in 

 unipolar leads with the exploring electrode lying at a 

 distance from the heart surface twice the diameter 

 of the heart (238). The distribution of proximity 

 potentials varies during the excitation process and is 

 different in depolarization and repolarization. This 

 leads to the fact that the error in a ''single fixed- 

 location dipole" concept can be diminished by 

 assuming that the dipole position shifts during the 

 cardiac cycle. [Migration of zero point, Nullpunkts- 

 wanderung (23, 307, 308).] After all, the assumption 

 of a migrating vector is nothing more than a hypoth- 

 esis to explain the incorrectness of a "single fixed- 

 location dipole" concept. The whole problem of 

 locating the heart vector on an "electric center" is 

 complicated (200) and only of theoretical interest. 

 Such a location is without a physical meaning if the 

 lead field is unknown. No one will deny that one 

 single vector of one fixed position is merely a simpli- 

 fication which never can be valid in a strict sense. 

 The attempt, therefore, to locate the electric center 

 by means of simple or corrected electrode systems 

 often leads to divergencies between the dipole location 

 and the anatomical mass center of the heart 

 (35'. 443)- 



The Image Surface 



Even if one accepts the "single fixed-location 

 dipole" concept, a number of difficulties remain in 



VECTOR A 



FIG. 15. Mirror patterns are observed if the lead vectors of 

 the two unipolar electrodes A and B are strictly opposite to 

 each other, as indicated. The heart vector HV projects itself 

 on the two lead vectors with the same amoimt PHV, but is 

 recorded with opposite polarity. 



the way of interpreting the ECG in a physically 

 correct manner. These difficulties derive from the 

 fact that the medium surrounding the heart is neither 

 homogeneous nor of a regular and geometrically 

 simple surface, nor is the dipole position centered. 

 This bears the consequence that the simple projection 

 laws are invalid, as mentioned above. The method 

 designed to overcome this situation was the construc- 

 tion of lead vectors. The lead vector reconciles the 

 anatomical data with the physical laws of projection: 

 a projective reconstruction of the heart vector as a 

 single fixed-location dipole becomes possible again in 

 an exact manner, if the lead vectors are experimen- 

 tally provided. 



The usual method of constructing lead vectors may 

 be briefly discussed. One first makes a model of the 

 medium in question (e.g., the thorax). The model is 

 filled with a homogeneous resistive medium (saline). 

 An artificial dipole is put into the model, at a point 

 at which the heart vector is supposed to lie, i.e., the 

 mass center of the heart. The artificial dipole is moved 

 into the three axes of space and the unipolar potential 

 recorded at P for each of these three positions. The 

 result of the record at P can now be easily translated 

 into the construction of a lead vector, so that the 

 dipole projection on this vector gives, in any case, 

 the recorded voltage at P (143, 197, 249, 298). 



To eliminate all the difficulties arising from the 

 irregular shape of the field and the eccentric position 

 of the heart, lead vectors are constructed for numer- 

 ous unipolar surface electrodes, for a given shape of 

 the thorax, and a presupposed position of the dipole. 

 These lead vectors are drawn from a single common 

 origin, which corresponds to the dipole location. The 

 tips of these lead vector arrows may be projected 

 onto a closed surface. This surface is called the 

 "image surface" of this model and this single dipole 

 location (143, 145, 197, 249, 298). This same image 



