328 HANDBOOK OF PHYSIOLOGY -^ CIRCULATION I 



ZERO AREA 



FIG. 3. Equipotential lines in a spherical, limited electric 

 dipole field of homogeneous conductivity. The dipole is indi- 

 cated by the two circles and lies in the center of the sphere. 

 The sign for the values of one side is opposite that for the 

 other. [From Schaefer (58).] 



recorded at a similar distance from the dipole from 

 an interior point in an infinite medium. Calculations 

 for special dipoles in a tank model are given bv NeLson 



(357). 



The physics of the field are those of a stationary 



current. There are apparently no whirlpools detect- 

 able, since the algebraic sum of derivations forming a 

 closed circle is zero (18). There is no evidence that 

 the potential distribution is distorted either by 

 impedances or bv nonlinear resistance properties 



(«7)- 



The meaning of "potential" V,, in equations 2 and 

 5 of this section needs a more detailed explanation. 

 The value of \^,, is identical with the potential differ- 

 ence between the reference electrode and a very 

 remote point of the field. As may be seen from figure 

 4, the potential of a very remote point is identical 

 with a potential to be found at the point between the 

 dipole charges. This potential may be called the 

 "mid-dipole potential," but the term "zero potential" 

 is commonly used. The term "zero potential" may be 

 misunderstood. It does not mean a "potential" of zero 

 value in the meaning of the potential theory (140). 

 The use of the word "zero" in connection with the 

 ECG indicates that an electrode lies in a certain area 

 of the potential field reaching from a point of mid- 

 dipole potential between the poles or discs to the 



FIG. 4. Definition of a zero area in an infinite dipole field. 

 (See text.) 



infinitely remote boundaries of the field (204). If, for 

 practical purposes, "zero" is assumed at any point 

 of the field where the potential difference against the 

 plane of symmetry is less than a certain percentage 

 (e.g., o. i) of the dipole potential difference, we find a 

 "zero area" which may be symbolized in figure 4 

 as a shaded area. Every electrode lying in this area 

 may be assumed to be at zero potential in this practical 

 sense. 



Every recording system consists of two electrodes, 

 the potential differences of which are led to the input 

 of an amplifier. If the two electrodes are both put on 

 arbitrary points of the field or of the body surface, the 

 record measures the difference Vpi — V'po of the 

 potentials at the points Pi and P2. Such a record is 

 called "bipolar." 



If one of the two recording electrodes is put on 

 the zero area of the field, the potential difference 

 recorded between this and a second, "different" 

 electrode, is called a "unipolar potential." 



Zero electrodes may easily be defined in an ideal 

 field of infinite extension. In the practice of electro- 

 cardiography, however, the use of unipolar leads is 

 complicated, because we never know the site of the 

 zero area, and this area changes its position within 

 the cardiac cycle. Also the distances within the body 

 are too short to develop sufficiently large zero areas. 

 The difficulties in defining zero potentials of the 

 entire heart will be discussed later. There are several 

 possible ways to solve the zero potential proijlem, at 

 least approximately (57). Under greatly simplified 

 assumptions, a zero electrode may be constructed, 

 if the three electrodes of the classical Einthoven lead 

 system (R, L, F) are combined over equal resistances 



