ELECTROCARDIOGRAPHY 



369 



leads I and II start with negative deflections, the Q 

 waves, or directly with R. In the frontal plane, the 

 normal form of QRS never shows a deep Q in any 

 lead. This means that the heart vector, as long as it 

 runs in an upward direction, never develops a high 

 voltage. If a deep Q is found, the differentiation 

 between normal and abnormal still offers considerable 

 difficulties (225, 338), but if Qui is very deep, the 

 spread of excitation may be regarded as abnormal. 

 By experience, it is known that posterior wall infarcts 

 and coronary sclerosis are the usual conditions 

 causing a deep Qm (Pardee-Q). The limit is usually 

 defined thus: Q should not exceed one-fourth of the 

 highest R wave. A deep S wave, however, is very 

 common and apparently without clinical significance 

 (192). An S in all leads points to late activation of 

 comparative large muscle masses in the posterior 

 portion of the Iwse (fig. 51). Only if S is extremely 



FIG. 51. Schematic drawing of the excitation waves pro- 

 ducing the S deflection. These waves occur in the last-activated 

 parts of the ventricular base. [From Schaefer (58).] ^ |^ 



deep and prevailing in all leads, the probability is 

 very high that the heart is abnormal (263, 267, 503). 



The Einthoven extremity leads record the pro- 

 jections of the heart vector on lead lines in the frontal 

 plane only. Wilson for the first time introduced leads 

 into electrocardiography which should be able to 

 detect the projections of the heart vector on a hori- 

 zontal plane, the unipolar precordial leads (fig. 52). 

 The electrode positions he chose are by no means 

 the best, because records from them contain fairly 

 high proximity potentials which are, however, too 

 small to give detailed information about local events. 

 The position of the electrodes was pictured in figures 

 26 and 27. The records are not uniform but resemble 

 each other in different persons much more than do 

 the extremity leads. The rule is that in Vi the rS 

 pattern prevails and that the R becomes larger and 

 dominates more and more as the electrode is shifted 

 to the left. There is a "transitional zone" usually 

 between V3 and V5, around the apex region, where 

 the positive and negati\e deflections are nearly equal. 

 After Vj, the deflection is converted into a predomi- 

 nantly positive one. Vj resembles, more or less, lead I, 

 since they have in common the horizontal position 

 of the lead vector. A quantitative description of the 

 relative voltages in the precordial leads is given in 

 figure 53. The average values are listed in table 3. 

 The S deflection indicates that, at that instant, 

 most of the "active cross section" is activated in a 

 direction away from the electrode. 



The normal values for amplitude, form, vector 

 position, and duration of QRS vary within wide 

 limits. Table 3 shows the averages to be expected in 

 normal hearts. Figures like these, however, are of a 

 restricted value, because the combination of certain 

 values for Q, R, and S characterize the ECG, and 

 this cannot be taken from such tables. We therefore 



FIG. 52. Normal chest leads. Vrj-Vrt are included. The calibration shows i mv. To the left are 

 the standard leads. 



