340 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



FIG. 19. A rectangular coordinate system delining three 

 planes commonly used in electrocardiography. The axes of 

 this figure are used throughout this monograph. [From Frank 

 (198).] 



Ill) consists of the following electrode combinations: 

 component x combines, with equal weight, lead I 

 and a lead at the height of the fifth interspace at the 

 sternum, which reaches from a surface point of an 

 azimuth of 60° to an azimuth of 300° (nearly point 

 B and H in fig. 16). The component y is recorded 

 Ijetween the head and left leg, the component z is de- 

 ri\ed by a weighted combination of four chest and 

 four back electrodes, the details of which may be seen 

 in figure 2 1 . Somewhat simpler systems have been 

 described by Frank (202) and many others (72, 251, 

 271, 382, 492, 518). 



UNIPOLAR SYSTEMS OF NONLOCAL CHARACTER. A uni- 

 polar lead may serve as a heart vector lead, if it is re- 

 mote from the heart. The condition which permits it 

 to act as a total lead is that the lead field penetrating 

 the heart must be approximately parallel and uniform. 

 Numerous unipolar lead positions have been de- 

 scribed, the value of which is often dubious. There is 

 really only one practical argument for choosing leads 



differing from the orthogonal orientation. If the par- 

 tial vector of a certain group of heart fibers is di- 

 rected so that the angles between the fiber and the 

 orthogonal axes are maximal and equal (which cor- 

 responds to an angle of 55°), this vector projects it- 

 self with a factor cosine of 0.575. ^^ such a case, only 

 about 58 per cent of the vector is represented in each 

 of the lead records.^ As the ECG is always the result 

 of superimposing fields, it might happen that such 

 minimally recorded potentials would be masked by 

 others. For every heart vector position there exists an 

 "optimal" lead vector running in the same direction 

 as the heart vector (23). This may be the reason why 

 the unipolar limlj leads are so often used clinically as 

 a supplement to the standard Einthoven leads (24). 

 In "unipolar limb leads" the elevational angles of the 

 lead vectors lie in between those of the Einthoven 

 leads. These leads are commonly marked as \'R, \'L. 

 and VF (V means voltage and is used as the symbol 

 for all unipolar leads with CT as reference electrode). 

 The angles of elevation are: \'R —150° and 30°, 

 VL —30° and 150°, VF 90° and —90° reading from 

 positive to negative polarity in these leads, whereas 

 the Einthoven leads have elevational angles of: I 0° 

 and 180°, II 60° and -120°, III 120° and -60°, 

 again reading from positive to negative polarity of 

 the leads. [For details see (24, 305, 450).] 



Goldberger proposed a procedure named "aug- 

 mented unipolar limb lead," which invohes connect- 

 ing one lead (e.g., V'R) with the combination of the 

 two others (V'L and VF). These leads are symbolized 

 with a\'R (\'R against VL + \T), aVL and aVF.^ 

 The lead vectors of these are given in table la. They 

 are useful even though their basic theory incorrectly 

 assumes the presence of an ideal field. 



The most commonly used unipolar leads are those 

 of Wilson, with the classical electrode positions stand- 

 ardized by various national cardiological societies 

 (529). Most of these leads can be regarded as heart 

 vector leads, although the records from them usually 

 deviate to some extent from the vector loop recorded 

 with an orthogonal corrected system (15, 200), and 



' It may be mentioned that, in a planar system, the minimal 

 projection corresponds to an angle of 45° with a cosine of about 

 0.708. Therefore, the minimal projection is still 70 per cent of 

 the vector. As can be seen, the derivations in space may record 

 a much smaller percentage of a vector than derivations which 

 are in the same plane with the vector. 



' "Augmented" meams that these leads record a potential 

 which is 5o9() larger than that recorded with the use of a CT 

 as indifferent electrode. However, the method introduces 

 .serious deviations from the simple projection laws, because the 

 "reference" electrode combination no longer has zero potential 



